Kristensmith Taylor

Eye Eyelid Lacrimal Gland Eye

The eye is the organ of vision. It is spherical and has the following three coats:

• Fibrous coat – Sclera and cornea
• Vascular coat – Choroid, ciliary body and iris
• Nervous coat – Retina

The transparent structures in the path of light are the cornea, aqueous humour (of the anterior chamber), the lens and the vitreous body (a gel-like substance in the posterior chamber).

1. The sagittal section of the eye shows Its gross structure.
2. Schematic diagram showing sclerocorneal Junction, ciliary body and process, Iris and lens.

Fibrous Coat

Sclera

The sclera is a tough, fibrous connective tissue layer. It consists of irregularly arranged dense connective tissue, i.e., flat bundles of type 1 collagen fibres, a network of elastic fibres and fibroblasts. Its thickness ranges from 0.6 mm to 1 mm.

Cornea

The cornea is located at the front of the eye and is continuous with the sclera at the limbus. It is a transparent portion of the eye, hence, avascular.

It is about 1 mm thick and consists of five layers. From superficial to deep these are:

1. Section of the human cornea showing various layers.
2. Section of the cornea at low magnification.
3. Photomicrograph at high magnification.

Corneal epithelium: It consists of stratified squamous, non-keratinized epithelium. Sensory nerves supply it. The cornea is very sensitive and any irritation or injury causes severe pain, excessive lacrimation, photophobia and blinking of eyelids.

Bowman’s membrane: It is about 10-12 mm thick, structureless, homogeneous lamina consisting of randomly oriented thin fibrils of collagen.

• This layer is also known as an anterior limiting membrane. It acts as a barrier to the spread of infection. It is believed that Bowman’s membrane is synthesized by both the corneal epithelium and underlying stroma.
• If damaged, it cannot regenerate. It heals by the formation opaque scar, which interferes with vision.

Corneal stroma: It constitutes the bulk of the cornea (90% of thickness). It is made up of thin lamellae of mostly type I collagen fibres, which are arranged in many layers.

• The bundles of collagen fibres, within a lamella, are highly ordered. In the adjacent lamellae, the bundle of collagen fibrils are arranged at a right angle to each other.
• The cornea is transparent because of the orderly arrangement of collagen fibrils. The flattened fibroblasts are located between the lamellae.
• The canal of Schlemm is present at the sclerocomeal junction (limbus). It is the site of the outflow of the aqueous humour from the anterior chamber of the eye into the venous system.

Descentet’s membrane: This layer is also known as a posterior limiting membrane. It is an acellular, homogeneous basement membrane consisting of interwoven meshwork of collagen fibrils. It is a product of endothelium.

Endothelium: It is a single layer of squamous or low cuboidal cells on the posterior surface of the cornea, resting on Descemet’s membrane.

• All the metabolic exchange between the cornea and aqueous humour takes place through the endothelium. It is responsible for the synthesis of proteins, which are necessary for maintaining Descemet’s membrane.
• It also reabsorbs excessive fluid from the stroma to keep it relatively dehydrated, which helps in the refractive quality of the cornea.

Fibrous Coat Remember

The cornea is a transparent avascular membrane. It consists of two noncellular layers (Bowman’s membrane and Descemet’s membrane) and three cellular layers (corneal epithelium, corneal stroma and endothelium).

If Bowman’s membrane is damaged, it cannot regenerate. It heals by the formation of an opaque scar, which interferes with vision. Descemet’s membrane is an unusually thick basement membrane for a very thin endothelial lining.

Fibrous Coat Clinical Applications

Corneal Transplants

• If a person has a defective cornea (presence of opacity), it interferes with normal vision. This cornea may be removed and a normal cornea obtained from a dead person (through eye donation) can be transplanted.
• Corneal transplants are the most common and successful organ transplants. This is because the cornea is an avascular tissue.
• Hence, antibodies that may cause rejection of transplanted cornea do not enter the transplanted tissue.

Glaucoma

The failure of drainage of aqueous humour from the anterior chamber of the eye leads to a prolonged increase in the intraocular pressure. The condition is known as glaucoma. It is an important cause of blindness.

Vascular Coat

The vascular coat consists of a choroid, ciliary body and iris.

Choroid

The choroid is the posterior portion of the middle coat of the eyeball. It is the thin, highly vascular layer that lies between the sclera and retina. It consists of four layers:

• Suprachoroid layer: This layer consists of fine collagenous fibres, elastic fibres and pigment cells (chromatophores).
• Vascular layer: This layer consists of large blood vessels between loose connective tissue and pigment cells.
• Chorio-capillary layer: This layer consists of a capillary network, which is essential for the nutrition of the retina.
• Bruch’s membrane: This membrane separates the choroid from the retina. It is a thin (1-4 mm) refractile membrane. It also acts as the basal lamina for the pigment cell layer of the retina.

Vascular Coat Remember

The choroid is a pigmented vascular layer, which lies between the sclera and the retina. It is separated from the retina by Bruch’s membrane.

Ciliary Body

The ciliary body is made up of ciliary muscle (smooth muscle) and ciliary process. The smooth muscle of the ciliary body is oriented in longitudinal, radial and circular directions.

• The ciliary muscle alters the shape of the lens for near and far vision. From the ciliary body, there come out 60-70 short ciliary processes with suspensory ligaments of the lens.
• The ciliary processes are covered by bi-laminar epithelium and have a core of loose connective tissue and blood vessels.
• The outer cell layer is the non-pigmented columnar epithelium, whereas the inner cell layer is composed of a pigmented simple columnar epithelium. The ciliary epithelium produces aqueous humour.

Ciliary Body Remember

The ciliary body is the anterior part of the vascular coat. It is located between the iris and the choroid. The ciliary body is made up of ciliary muscle (smooth muscle) and ciliary process.

The smooth muscle of the ciliary body is oriented in longitudinal, radial and circular directions.

Iris

The iris arises from the ciliary body and lies between the cornea and the lens. It consists of smooth muscle fibres in the connective tissue stroma (fine collagen fibres), melanocytes and blood vessels.

• It has no epithelium on its anterior surface, but the posterior surface of the iris is lined by ciliary epithelium. This layer is heavily pigmented and the colour of the eye depends on its pigmentation.
• The smooth muscle of the iris is arranged in two layers, i.e., dilator pupillae and sphincter pupillae. The dilator pupillae fibres are radially oriented.
• While sphincter pupillae are oriented circumferentially near the pupil. Iris acts as a diaphragm, which controls the amount of light entering the eye.

Iris Remember

The iris is the anterior extension of the vascular coat. It acts as a diaphragm, which controls the amount of light entering the eye.

Nervous Coat

Retina

This is the inner coat of the eyeball and lines its posterior surface. The retina contains photoreceptors (rods and cones), which are essential for vision. The retina has a specialized area where vision is most acute, the fovea centralis or macula.

• This area contains only cones, which are essentially bare (the overlying layers are pushed to the side). The retina also has a ‘blind spot,’ the optic disc.
• Where the optic nerve leaves the eye and there are no photoreceptor cells. The retina has several layers from the outside in the following.

1. The microscopic structure of the retina.
2. Diagrammatic representation of principal cell types of the retina and their connections.
3. Photomicrograph of the retina at high magnification.

Pigment Epithelium

It is the outermost layer of the retina, which is separated from the choroid by Bruch’s membrane. This epithelium consists of low cuboidal cells, which contain melanin granules.

• Epithelial cells have long apical processes that occupy spaces between the outer segment of rods and cones.
• The melanin pigment of epithelium absorbs light, thus preventing its reflection from the outer coats of the eye. The pigment epithelium also phagocytizes the tip of rod and cone cell processes.

Layer of Rod and Cone Cell Processes

This layer consists of outer segments of rod and cone cells. The processes of rod cells are cylindrical and cones are thicker, cone-shaped. This layer is a photoreceptor in function.

• Rods and cones are photoreceptor cells. They transfer the light energy into receptor potential. Both types of cells are long slender cells but the outer segments of rods are cylindrical or rodshaped
• Whereas those of cones are tapered or cone-shaped. The rod cells are much more in number [120 million] compared to cone cells [6 to 7 million]. The parts of rod and cone cells are shown.
• The rod consists of the outer segment, inner segment, inner fibre and spherule. The parts of the cone cell are almost the same except the terminal part is called the pedicle, instead of the spherule.

Outer Nuclear Layer

This layer consists of nuclei belonging to rods and cones. The nuclei of these cells are arranged in several layers. This layer is darkly stained.

• Between the second and third layers, there is the presence of a pink linear marking called an outer limiting membrane or lamina.
• This results because of zonula adherens of the glial cells (Muller cells) with the cell bodies of photoreceptor cells.
• The Muller cells are supporting cells of the retina. They have long slender bodies that are radially oriented in the retina.

Outer Plexiform Layer

This layer stains lightly. The layer contains synapses between rods and cones with the dendritic processes of bipolar cells and horizontal cells.

Inner Nuclear Layer

It consists of cell bodies and nuclei of bipolar cells, horizontal cells, amacrine cells and Muller’s cells. The bipolar cells are oriented perpendicular to the layers of the retina.

• They have synaptic contact with rod or cone cells at the outer (dendritic) end and with ganglion cells at the inner (axonal) end. The horizontal cells are oriented parallel to the layers of the retina.
• Their processes form synaptic contact with rod and cone cells in the outer plexiform layer.
• The amacrine cells are situated in the inner portion of this layer and have synaptic contact with axonal processes of bipolar cells and dendrites of ganglion cells in the inner plexiform layer.

Inner Plexiform Layer

In this layer, the axons of bipolar cells synapse with dendrites of ganglion cells and amacrine cells.

Ganglion Cell Layer

Mainly consists of the body and nuclei of large multipolar neurons (ganglion cells). They receive input from bipolar cells and their axons form optic nerve.

Nerve Fibre Layer

It consists of central processes (axons) of ganglion cells, which gather at the optic papilla (disc) and leave the eye as the optic nerve. As soon as these fibres leave the eyeball, they become myelinated.

The blood vessels of the retina are present only in the nerve fibre layer. However, sometimes they may reach as deep as the outer nuclear layer.

Nerve Fibre Layer Remember

The retina consists of ten layers of cells and their processes. Road and cone cells of the retina are specialized photoreceptors. Roads are specialized to perceive dim light, while cone cells perceive bright light and colour.

Nerve Fibre Layer Clinical Application

Detachment of Retina

As the apical ends of rods and cone cells are not firmly attached to the pigment epithelium, the retina may get separated from the pigment epithelium. This condition is called detachment of the retina.

• This is a common but serious condition. It may be treated by laser surgery. If it remains untreated, rods and cones are damaged, leading to blindness.
• This is because rod and cone cells get their nutrition from a chorio-capillary layer of choroid.

Lens

The lens is a transparent, flexible, biconvex disc. The lens is involved in near and distant vision, which is achieved by changing its curvature. The lens consists of a lens capsule, anterior epithelium and lens substance.

Lens Capsule

The lens is covered by a lens capsule, which is a homogeneous basal lamina coat. The capsule is made up of type 4 collagen and proteoglycans.

Anterior Epithelium

Beneath the capsule, simple cuboidal epithelium covers the anterior surface of the lens.

Lens Substance

The anterior epithelial cells toward the equator become columnar. The columnar cells of the equatorial region further elongate to form long fibres.

• These long fibres are about 7-10 mm in length, 8-12 pm wide and 2 pm thick. In the cross-section, their shape is like a hexagon.
• Although these fibres are called lens fibres, they are in a true sense modified elongated epithelial cells. The younger fibres are nucleated, whereas old fibres lose their nuclei.

Lens Substance Clinical Application

• Cataract
  • The entire lens substance consists of transparent lens fibres. However, with increasing age or metabolic disorders (diabetes), the lens may become opaque.
  • This interferes with clear vision. This condition is called a cataract. In this condition, the lens is removed from its capsule and replaced with a plastic lens.
• Presbyopia
  • The lens is highly elastic. However, the elasticity of the lens is gradually lost due to increasing age. This leads to difficulty in accommodation for near objects (reading, etc).
  • This is usually observed after 40-45 years of age. The condition (presbyopia) is corrected by wearing glasses with a convex lens.

Eyelid

The eyelids protect the eye. From external to internal surface eyelid consists of many layers, i.e., skin, loose connective tissue, orbicularis oculi muscle, tarsal plate and palpebral conjunctiva.

• The skin of the eyelid is thin, loose and elastic.
• The loose connective tissue is present deep in the skin, which is devoid of fat.
• The third layer of the eyelid is formed by bundles of skeletal muscle (orbicularis oculi). The tarsal plate is made up of dense connective tissue and forms the skeleton of the eyelid.
• The inner surface of the lid is lined by palpebral conjunctiva. The palpebral conjunctiva consists of stratified columnar epithelium with goblet cells.

There are three types of glands present in eyelids.

• Tarsal glands (Meibomian glands) are long sebaceous glands, which are present in the tarsal plate.
• They produce sebaceous secretion, which forms an oily layer on the surface of the ear, thus preventing an early evaporation of tears.
• Glands of Zeis (sebaceous gland of the eyelid) are small modified sebaceous glands, which are connected with the follicles of eyelashes.
• Glands of Moll are small sweat glands that pour their secretion into eyelashes. They are simple spiral tubular glands.

1. Histological structure of eyelid.
2. Section of eyelid.

Eyelid Remember

The main function of eyelid is to protect the eye. On its outer surface, it is covered by skin and its inner surface is covered by palpebral conjunctiva. Three types of glands are present in the eyelid.

Lacrimal Gland

The lacrimal gland is a compound tubuloacinar serous gland. It consists of many separate lobes, which pour their secretion (tears) in the superior conjunctival fornix through small ducts.

• The lacrimal gland acini has the features of a serous salivary gland. These acini are lined by light-staining low columnar cells.
• The acini are large and have a wider lumen compared to the serous salivary gland.

Myoepithelial cells are located within the lamina of acini and help in the release of tears.

1. Section of lacrimal gland.
2. Lacrimal gland as seen under low magnification.
3. Magnified view of lacrimal gland

The Central Nervous System

The central nervous system consists of the brain and spinal cord. The cut surface of any part of the central nervous system consists of grey matter and white matter.

• The cerebrum and cerebellum have a cortical layer of grey matter on the surface deep to which is white matter. However, in the spinal cord, grey matter is inside, surrounded by white matter.
• Out of the various parts of the brain, we should study only the cerebral cortex and cerebellar cortex which are of histological significance.

Some Important Definitions

• Grey matter: It is a collection of neurons, neuroglial cells, the processes of these cells lying adjacent to the cell body, and blood vessels inside the central nervous system (brain and spinal cord).
• White matter: It consists of bundles of nerve fibers; such as sociated neuroglial cells and blood vessels. The Myelin sheath that surrounds the nerve fibers gives it a white appearance.
• Nuclei: These are islands of grey matter situated deep inside the cerebrum, cerebellum brain stem, and spinal cord For Example., the thalamus, basal nuclei, etc.
• Ganglion: These are collections of nerve cells (grey matter) outside the brain and spinal cord For Example., dorsal root ganglia and sympathetic trunk ganglia.

Cerebral Cortex

Most of the cerebral cortex (except the cortex of hippocampal formation and piriform lobe) is described as having six layers. However, the distinction between these layers is not well-marked.

• These layers are distinguished based on the predominance of cell type and arrangement of fibers.
• The arrangement of fibers can be visualized only by special stains and not by H and E. From superficial to deep, the following are the layers of the cerebral cortex.

1. In a section of the cerebral cortex, it is difficult to distinguish various layers.
2. Schematic diagram to show neurons in various layers of the cerebral cortex.
3. Section of the cerebral cortex. Six layers of the cerebral cortex are ill-defined as they merge.

Molecularor Plexiform Layer

This layer is situated just beneath the pia mater. It consists predominantly of fibers and neuroglial cells. A few horizontal cells of Cajal are also present.

• The peripheral portion consists largely of fibers, which travel parallel to the surface. In its deeper part lie the horizontal cells of Cajal.
• Their cell body and processes are disposed of horizontally. This layer shows the blood capillaries and many nuclei of neuroglial cells.

External Granular Layer

This layer consists of two types of neurons, i.e., small pyramidal and granular or stellate cells. Their apical dendrites extend in the first layer and the axon ends in the deeper layer.

External Granular Layer Further Details

Pyramidal Neurons

These are triangular and their size ranges from 10 to 120 μm. Their apical dendritic end faces the surface of the cerebral cortex.

• Dendrites take origin from all three angles and synapse with the fibers in other layers. The axon is given off from the base of the cell and extends to deeper layers.
• The large pyramidal cells (Betz cells) are seen in the motor cortex and their axons form pyramidal (corticospinal) fibers.
• The stellate or granular cells are star-shaped small neurons (8 μm). Their processes extend only into neighboring areas.

External Pyramidal Layer

The layer consists of medium pyramidal cells. This layer also consists of a few stellate cells and cells of Martinotti. The axons ofpyramidal cells form association and commissural fibres.

The Martinotti cells are small, triangular, or polygonal cells seen almost in all the layers of the cortex. Their axons travel toward the surface of the cortex.

Internal Granular Layer

This layer consists of densely packed granular cells with a white horizontal fiber layer called the external band of Baillarger. This layer provides connections between neurons of different layers.

Internal Pyramidalor Ganglionic Layer

This layer consists of large pyramidal cells (cells of Betz) and a few cells of Martinotti. The horizontal fibers in the deeper part are called the internal band of Baillarger.

Fusiform Layeror Layer of Polymorphic Cells

This layer predominantly contains spindle-shaped fusiform cells and a few stellate and Martinotti cells. The fusiform cells are located in the deeper layer of the cerebral cortex.

These cells lie perpendicular to the surface with axons coming out from the center of the cell and dendrites from both ends. Deep in the sixth layer of the cerebral cortex lies white matter.

Fusiform Layeror Layer of Polymorphic Cells Remember

The cerebral cortex is composed of 14-16 billion nerve cells approximately. The cortex consists of 6 layers of cells. The two principal neurons are pyramidal cells and granular (stellate) cells. Large-sized pyramidal cells may measure up to 120 μm.

Fusiform Layeror Layer of Polymorphic Cells Clinical Application

Alzheimer’s Disease

The disease is of unknown cause and occurs in old people. To begin with, they suffer from loss of memory, but later all their intellectual capabilities are also lost (dementia).

In this disease, the neurons of the cerebral cortex accumulate late tangled masses of filaments in the cytoplasm and later degenerate. The motor system remains unaffected.

Fusiform Layeror Layer of Polymorphic Cells Further Details

Histologically, one can differentiate motor and sensory cortex. The motor cortex consists predominantly of pyramidal cells in layers 3 and 5. These pyramidal cells are densely packed and large.

On the other hand, the sensory cortex shows very few pyramidal cells in layers 3 and 5 and most of the layers contain small granular cells.

Cerebellar Cortex

The histological structure of the cerebellar cortex is uniform throughout the cerebellum. It consists of three layers, i.e., molecular layer, Purkinje cell layer, and granular layer. The molecular layer is situated just beneath the pia mater.

1. The structure of two adjacent cerebellar folia shows outer grey and inner white matter.
2. The grey matter of the cerebellar cortex consists of three layers, i.e., molecular, Purkinje, and granular layer.
3. Photomicrograph of cerebellar cortex at low magnification, cerebellar cortex is folded and presents fissures and folia.
4. Photomicrograph of cerebellar cortex showing three layers, i.e., molecular layer, Purkinje cell layer, and granular cell layer.

Molecular Layer

It stains lightly with eosin and is featureless as it consists of a few cells and more myelinated and unmyelinated fibers.

• This layer consists of a few scattered stellate cells in the superficial part and a few basket cells in the deeper part.
• However, the dendritic processes of Purkinje cells and the axonal processes of granular cells (that run parallel to the surface of the cortex) occupy most of the molecular layer.
• The axons of granular cells come in synaptic contact with the dendrites of several Purkinje cells and basket cells. The molecular layer also contains the terminal part of climbing fibers.

Purkinje Cell Layer

These cells are arranged in a single row between molecular and granular layers. Purkinje cell (Golgi type F) is the large pyriform or flask-shaped neuron, that sends numerous dendrites into the molecular layer.

These dendrites synapse with axons of granular cells and climbing fibers. Purkinje cells give a single thin axon, which passes through a granular layer to end in deeper nuclei of the cerebellum.

Granular Layer

The granular layer stains deeply with the hematoxylin because it is densely packed with granular cells. Granular cells are small neurons with round nuclei surrounded by a thin rim of cytoplasm.

• These cells receive impulses from various parts of the CNS through Mossy fibers. The Mossy fibers in the granular layer end as the dilated terminal.
• On which the dendrites of granule cells and axons of Golgi cells (type 2) synapse to form light-stained areas called glomeruli.
• Granular cells send their axons into the molecular layer where they branch in the form of T and come in synaptic contact with dendrites of various Purkinje cells and basket cells.
• At the junction of molecular and granular layers, Golgi type 2 cells are found. Their vesicular nuclei are larger than granule cells. They contain chromophil substances present in the molecular layer.
• However, their axons form synaptic contact with glomeruli in the granular layer. Deep to the granular layer the cerebellar cortex lies in contact with white matter.

Granular Layer Remember

The cerebellar cortex consists of three layers, i.e., the molecular layer, the Purkinje cell layer, and the granular layer. The histological structure of the cerebellar cortex is uniform throughout the cerebellum.

Purkinje cells are large pyriform or flask-shaped neurons arranged in a single row between the molecular and granular layers.

Granular Layer Further Details

Neuronal Circuit of Cerebellum

• The input to the cerebellum is from different parts of CNS and is in the form of Mossy and climbing fibers
• The climbing fibers ascend to the molecular layer where they form synaptic contact with the dendritic arborization of Purkinje cells.
• The Mossy fibers end in glomeruli and form synaptic contact with granular cells in the granular layer.
• The granular cells convey the impulse received by Mossy fibers to the dendrites of Purkinje cells.
• The output of the cerebellum is in the form of axons of Purkinje cells, which synapse with the intracerebellar nuclei.
• The other neurons like Golgi cells, basket cells, and stellate cells interconnect the intracortical circuit to modify the outgoing impulses.

Spinal Cord

The human spinal cord is about 45 cm long and has cervical, thoracic, and lumbosacral parts. A cross-section of the spinal cord shows a central canal lined by ependyma made up of simple ciliated columnar cells.

• The central canal is surrounded by grey matter, which contains neurons, nerve fibers, neuroglial cells, and blood vessels.
• The grey matter is surrounded by white matter, which consists of bundles of nerve fibers and neuroglial cells. The surface of the spinal cord is covered with pia mater.

Grey Matter

The grey matter of the spinal cord appears roughly in the form of an H. It has anterior and posterior grey columns or horns. The anterior grey column contains large-size multipolar motor neurons.

• The nucleus of the motor cell is a large, spherical, light-staining structure with intensely staining nucleolus. The cytoplasm contains clumps of dark staining basophilic Nissl substance.
• The Nissl substance extends into the dendritic processes neuron but not into the axon. The axons of motor cells form the ventral spinal root.
• The neurons of the posterior grey column are much smaller than the anterior horn cells. Within the grey matter, besides the sensory and motor nerve cells, there are numerous neuroglial cells and blood vessels.

1. Drawing to show the arrangement of the grey and white matter of the thoracic spinal cord (transverse section).
2. The section of a part of the spinal cord shows the motor neurons in the anterior grey column and fibers in adjacent white matter.
3. Section of spinal cord (at low magnification) showing centrally placed grey matter and peripherally placed white matter.

White Matter

The white matter is composed primarily of myelinated nerve fibers, but also neuroglial cells and blood vessels. The nerve fibers are comprised of ascending and descending tracts.

• Nerve fibers are surrounded by a myelin sheath, which in turn is surrounded by a fine connective tissue sheath called epineurium. As the myelin sheath gets dissolved during the preparation of H and E sections.
• The dark-staining axons (in a transverse section of the spinal cord) are surrounded by a clear space, which had been occupied by myelin.

White Matter Remember

A cross-section of the spinal cord shows a central canal surrounded by grey matter. The grey matter is surrounded by white matter. The grey matter of the spinal cord is in the form of an H and has anterior and posterior grey columns or horns.

• The anterior grey column contains large-size multipolar motor neurons, while neurons of the posterior grey column are much smaller than anterior horn cells and are sensory neurons.
• The white matter of the spinal cord consists of ascending and descending nerve fibers, which are mostly myelinated.

White Matter Clinical Application

Multiple Sclerosis

It is the most common disorder of the nervous system affecting young adults. In this condition, myelinated nerve fibers of the brain and spinal cord are progressively damaged due to the destruction of the myelin sheath.

• This affects the sensation, movements, body functions, and balance. Damage to the optic nerve may cause blurred vision. If nerve fibers in the spinal cord are affected.
• It may cause weakness or heaviness in the limbs. Damage to the fibers in the brain stem may affect balance. The demyelination is thought to result from an autoimmune disease with inflammatory features.

Guillain-barre syndrome

ln this disease, there occurs the demyelination of peripheral nerves and motor nerves arising from the ventral roots. The person suffers from muscle weakness and difficulty in respiration.

Peripheral Nervous System

All nervous tissues other than the brain and spinal cord are classified as the peripheral nervous system. The peripheral nervous system consists of nerves (made up of bundles of nerve fibers) and ganglia (collection of neurons and nerve fibers outside CNS).

Nerve

A nerve is defined as the collection of nerve fibers, which may be myelinated and or unmyelinated, and held together by connective tissue.

• A nerve consists of nerve fibers (axons or dendrites), supporting neuroglial cells (Schwann cells) and connective tissue.
• A nerve fiber is first surrounded by Schwann cells (which may form the myelin sheath around it), then it is surrounded by a thin layer of connective tissue called endoneurium.
• The endoneurium is made up of delicate collagen fibers and a few fibroblasts.
• At the light microscopic level, the endoneurium shows the nuclei of fibroblasts, which are difficult to distinguish from the nuclei of Schwann cells.
• Numerous fibers are held together to form a bundle of nerve fibers, which is surrounded by a sheath, called perineurium.

The perineurium is a cellular sheath made up of 3-4 layers of squamous-shaped cells and extracellular material. These cells show basal lamina and are contractile.

1. A part of a transverse section of a nerve showing nerve fibers (axons) arranged in bundles and covered by a connective tissue sheath (perineurium).
2. A drawing of the transverse section of a few nerve fibers is shown in an enlarged view.
3. Under microscope.

1. At low magnification.
2. At medium magnification
3. Photograph of a longitudinal section of nerve at high magnification.
4. Micrograph of a longitudinal section of nerve fibers (Sliver Stain).

Differences between dorsal root ganglion and sympathetic ganglion

The perineurium forms a semi-permeable barrier. Thus, the cells of perineurium are not in true sense connective tissue cells, which comprise epineurium and endoneurium.

• These cells are more like an epithelioid tissue. Many bundles of nerve fibers are finally covered by a dense connective tissue sheath called epineurium.
• The epineurium not only forms the outermost covering of a nerve, but it also goes inside between perineurial bundles, to bind them together. The epineurium contains blood vessels and adipose tissue.
• The fine branches of blood vessels after penetrating the perineurium reach the endoneurium to supply it and nerve fibers.

Nerve Remember

Peripheral nerves are covered by three different connective tissue sheathes, i.e., epineurium, perineurium, and endoneurium.

Dorsal Root Ganglion

It is also known as sensory ganglion. This ganglion is the collection of sensory neurons on the dorsal root of the spinal nerves.

• Each ganglion is surrounded by a connective tissue capsule, which is the epineurium of the dorsal root. Beneath the capsule, the ganglion contains large cell bodies arranged in groups.
• Also, between and around the groups of neurons there are bundles of myelinated nerve fibers. Most nerve fiber bundles are seen in the central part of the ganglion and groups of nerve cells are seen in the peripheral part.
• The neurons of the dorsal root ganglion have large, spherical bodies with large pale-staining euchromatic nuclei and dark-staining nucleoli. Each neuron is surrounded by the satellite cells.
• The satellite cells, which form a sheath around the neuron, are much smaller compared to neurons. They are flattened or low cuboidal neuroglial cells and form an inner capsule around each neuron.
• The fibroblast and fibers form an outer capsule surrounding the inner capsule formed by satellite cells. The satellite cells prevent unwanted depolarization of sensory neurons.

Dorsal Root Ganglion Remember

The dorsal root (sensory) ganglion is a collection of pseudo-unipolar, rounded sensory neurons on the dorsal root of the spinal nerve.

1. Section of dorsal root ganglion (spinal ganglion) showing spherical nerve cells with euchromatic nuclei and prominent nucleolus.
2. Section of dorsal root ganglion (at low magnification) consisting of large-size pseudo-unipolar neurons.
3. Photomicrograph at high magnification.

Sympathetic Trunk Ganglion

The sympathetic ganglion is the ganglion of the autonomic nervous system and lies along the sympathetic trunk. They contain cell bodies of postsynaptic motor neurons of the autonomic nervous system.

• The ganglion is covered with a thin capsule of connective tissue. It consists of small, irregular neurons dispersed between the nerve fibers. Cells are multipolar and, therefore appear irregular in shape.
• They contain eccentrically placed nuclei with prominent nucleoli. The cytoplasm contains small Nissl bodies. The satellite cells are less in number than in dorsal root ganglion cells.
• In between nerve cells, there is supportive connective tissue, blood vessels, and bundles of nerve fibers (both myelinated preganglionic and unmyelinated postganglionic).

Sympathetic Trunk Ganglion Remember

Autonomic ganglion houses cell bodies of postganglionic autonomic nerves.

1. Section of sympathetic ganglion showing small irregular (multipolar) nerve cells
2. Photomicrograph of sympathetic ganglion showing multipolar neurons, scattered between nerve fibers.

Epithelium Tissue

What is Histology?

The subject of histology deals with the microscopic and ultramicroscopic structure of cells, tissues and organs of the body.

• An aggregation of similar types of cells (and material surrounding them) is defined as tissue (For Example., epithelial tissue, connective tissue, muscular tissue and nervous tissue).
• All the cells of a tissue work in a collective manner to perform a particular function. Various tissues combine to form an organ of the body (For Example., the intestine, kidney, liver, etc).
• The intestine is formed by the combination of various types of tissues like epithelial (epithelium lines the mucous membrane), connective (forms lamina propria, submucosa and serosa), muscular (forms muscularis mucosae and muscle coats) and nervous (forms Meissner’s and myenteric nerve plexus).
• Similarly, various organs of our body aggregate to form a system of the body (For Example., the urinary system is formed by the kidney, ureter, bladder and urethra).

A system of the body consists of related organs that have a common function. All the systems of the body function synchronously to form one living person.

1. Cells aggregate to form various kinds of tissues (epithelial and
   muscular tissue)
2. Various tissues aggregate to form organs (kidney and ureter)
3. Various organs form an organ system (urinary system).

Epithelium

The epithelium is the basic tissue of the body. It consists of cells arranged as continuous sheets, in either single or multiple layers. Cells forming epithelium, on their lateral surfaces, are in close apposition with one another and are held tightly together by various kinds of cell junctions.

One of the surfaces of epithelial cells is a free surface that is exposed to the body cavity, lumen of an internal organ or external surface of the body. The basal surface adheres to a thin, continuous, supporting layer called basal lamina or basement membrane.

Epithelium Remember

The epithelium is defined as a tightly bound continuous sheet of cells, covering the free surface (inner or outer surface) of the body.

What are epithelioid tissues?

At certain places in the body, cells are found aggregated in close apposition with one another like epithelial cells but do not present a free surface. These kinds of issues are called epithelioid tissues.

For Example., parenchyma of the adrenal gland, Leydig cells of the testis, islets of the Langerhans, pituitary gland, parathyroid gland and luteal cells in the ovary.

epithelioid tissues Remember

Epithelial cells when showing the absence of a free surface are called epithelioid tissue.

• Epithelial tissue is avascular but derives its nutrition from capillaries present in the connective tissue layer beneath the basement membrane. Although epithelial tissue is devoid of blood supply, it is supplied by nerves.
• Epithelial tissue is subjected to constant wear and tear and, hence has a high capability to regenerate. The secretory portion of glands and cells lining the ducts are epithelial as the glands are epithelial in origin.

Functions Of Epithelium

• Protection: As epithelial tissue forms a boundary layer between the body and external environment (epithelium of skin), and lines the body cavities, it is protective.
• Absorption: The epithelia of organs like kidneys and intestines are absorptive.
• Acts as barrier: Epithelium may also act as a barrier, as in the skin and urinary bladder, which resists the absorption of water and toxic substances.
• Excretion: Epithelium of a certain portion of the nephron of the kidney is involved in the excretion of harmful metabolites.
• Secretion: Epithelium may also act as secretory as in intestine, For Example., goblet cells, and gastric and intestinal glands.
• Detection of sensations: Epithelial tissue may combine with nervous tissue to form special organs of sense, For Example., organs of hearing, smell and vision.

Classification Of Epithelia

Epithelia are classified according to the number of cell layers, the shape of cells, the arrangement and the specialization of their free surface.

• Epithelium with a single layer of cells is called as simple. While epithelium consisting of multiple layers of cells is called stratified.
• A third type of epithelium that has the appearance of being stratified is classified as pseudo-stratified.
• The fourth class of epithelium is called transitional because it can change its shape when stretched.

Epithelia Remember

Classification of epithelia is based on the shape and arrangement of the epithelial cells.

Classification Of Epithelia

• Simple (single layer of cells)
  • Squamous
  • Cuboidal
  • Columnar
• stratified (multiple layers of cells)
  • St. squamous
  • St. cuboidal
  • St. columnar
• Pseudostratified (epithelium appears as if stratified)
• Transitional (shape of epithelium is not fixed)

Based on the shape of the cells, simple epithelium is further classified as simple squamous, simple cuboidal and simple columnar.

Similarly, the stratified epithelium is further classified as stratified squamous, stratified cuboidal and stratified columnar depending on the shape of the cells on its uppermost layer (free surface).

Simple Epithelium

Simple Epithelium Squamous

Squamous Description

• Consists of a single layer of fat cells.
• The nucleus is oval or flat, situated in the centre of the cell.
• On the surface view, cells appear to be arranged like floor tiles.

Squamous Location

• It lines the heart, blood vessels and lymphatics. Here, it is called the endothelium.
• It also lines the serous membranes of body cavities and the surface of viscera. Here, it is called mesothelioma.
• It is present in lung alveoli, a parietal layer of Bowman’s capsule, certain tubules of the kidney and at certain places on the inner aspect of the tympanic membrane.

Squamous Functions

It helps in rapid transport of substances, filtration of fluids, diffusion of gases and osmosis.

1. Three-dimensional arrangement.
2. Sectional view.
3. Squamous epithelium lining the lumen of blood vessels.
4. Section of a venule lined with simple squamous epithelium called endothelium (arrowheads). Few blood cells are seen in the lumen of the vessel (arrows).
5. Endothelium of arteriole.
6. Squamous epithelium lining the parietal layer of Bowman’s capsule.

Simple Epithelium Cuboidal

Cuboidal Description

• In sectional view, cells appear cuboidal in shape.
• Nuclei are round and centrally placed. All nuclei are arranged at the same level.
• When viewed from the top, cells are hexagonal

Cuboidal Location

• Epithelium lining the follicles of the thyroid gland
• Ducts of exocrine glands
• Pigmented epithelium of the retina
• Lens capsule
• Surface of ovary
• Certain ducts and tubules of the kidney

Cuboidal Function

Secretion and absorption.

1. Three-dimensional view.
2. As seen in sectional view.
3. This epithelium is present in the thyroid gland.
4. Collecting ducts of the kidney lined by simple cuboidal epithelium.
5. Lining the thyroid follicles.
6. Lining the duct of a glands

Simple Epithelium Columnar

Columnar Description

• Cells of the epithelium are much taller compared to their width.
• Nuclei are elongated and located in the lower half of the cells. All nuclei are placed at the same level in neighbouring cells).
• On their free surface, modifications like microvilli or cilia may be seen. When viewed from the top, cells look hexagonal.
• It may also contain goblet cells.

Columnar Location

Epithelium lining gall bladder, ducts of glands, gastrointestinal tract (from the stomach to anus), uterine tube, uterine cavity, cervical canal and central canal of the spinal cord.

Columnar Function

• Secretion and absorption.
• Ciliary action moves mucus in the respiratory tract and ovum in the uterine tube.

1. Three-dimensional view.
2. Simple columnar (without any modification on their apical surface, Example., stomach).
3. With microvilli, Example., small intestine.
4. With cilia.
5. The simple columnar epithelium with cilia is seen in the uterine tube.
6. Simple columnar epithelium from the intestine.
7. Photomicrograph showing simple columnar epithelium with microvilli taken from the lining of the gall bladder.

Simple Epithelium Remember

In cross-section, the cells of the simple cuboidal and simple columnar epithelia are shaped like hexagonal solids.

Stratified Epithelium

Stratified epithelium is named based on the shape of the cells lining the topmost layer (free surface).

Stratified Squamous (non-keratinized) Description

• Cells are arranged in many layers.
• The basal layer is attached to the basement membrane and is usually columnar, cuboidal or rounded in shape.
• Intermediate cells are irregularly polyhedral in shape and become increasingly flattened as they move towards the superficial layer.
• The superficial layer consists of thin squamous cells.
• Basal cells replace surface cells as they are shed off.

StratifiedSquamous (non-keratinized) Location

• Epithelium lining the oral cavity, tongue, part of epiglottis, oesophagus and vagina.
• StratifiedSquamous (non-keratinized) Functions
• Protection of deeper tissue.

1. Three-dimensional view.
2. Sectional view.
3. As seen in the mucosal lining of the vagina.
4. Stratified squamous non-keratinized epithelium taken from the lining of the oesophagus.
5. Para-keratinized epithelium from the tongue. Parakeratin is an intermediate stage between non-keratinized to keratinized epithelium.
6. The photomicrograph shows the stratified squamous keratinized epithelium from the epidermis of the skin.

Stratified Squamous (keratinized) Description

• In this type of epithelium, superficial cells become dead, dehydrated andnon-nucleated like scales.
• These dead cells become hard (comified) as they are filled with keratin.
• Stratified Squamous (keratinized) Location
• Epithelium of the skin

Stratified Squamous (keratinized) Functions

• Protection of deeper structure.
• It prevents the absorption of water.
• Keratin prevents dehydration of underlying tissue.

Stratified Squamous (keratinized) Remember

In keratinized epithelium, cells lining the free surface are dead and filled with keratin. Nuclei are also absent in these superficial cells.

Stratified Cuboidal Description

• The epithelium consists of two or more layers of cells.
• Cells of the superficial layer are cuboidal in shape

Stratified Cuboidal Location

• Ducts of sweat glands.

Stratified Cuboidal Functions

• Provides passage to the secretion and acts as a barrier.

1. Three-dimensional view.
2. Sectional view.
3. As seen in the duct of the sweat gland.
4. The stratified cuboidal epithelium is present in the excretory duct of an exocrine gland.

Stratified Columnar Description

• Two or more layers of cells.
• Cells of the superficial layer are columnar

Stratified Columnar Location

Epithelium lining large ducts of some glands, fornix of the conjunctiva and cavernous urethra.

Stratified Columnar Functions

Provides passage to the secretion and acts as a barrier.

Stratified Columnar Remember

The stratified epithelium is composed of more than one layer. It is classified based on the shape of cells forming the uppermost layer (surface layer).

1. Three-dimensional view.
2. Sectional view.
3. As seen in the duct of the salivary gland.
4. image
5. The Duct of the pancreas,
6. Epithelium lining the duct of the serous salivary gland.

Pseudostratified Epithelium

Columnar Description

• It is not a true stratified epithelium but appears to be stratified.
• All cells are attached to the basement membrane but are of different heights. Hence, not all reach the apical surface. Because of this, the nuclei of cells are at different levels.
• The epithelium may be ciliated or non-ciliated and may contain goblet cells.

Columnar Location

• The non-ciliated epithelium is found in the large excretory ducts, auditory tube and male urethra.
• The ciliated epithelium is found in the upper respiratory tract.

Columnar Function

• Protection of underlying tissue.
• Ciliary movements remove mucus, while goblet cells secrete mucus.
• Pseudostratified Epithelium Remember
• Although pseudostratified epithelium looks as if cells are arranged in many layers all the cells of this epithelium are attached to basal lamina.

1. Three-dimensional view
2. Sectional view
3. Trachea
4. The Pseudostratified Ciliated Columnar epithelium is taken from the lining of an intrapulmonary bronchus.

Transitional Epithelium (urothelium) Description

• The appearance of epithelium varies during stretched and relaxed conditions. When this epithelium is stretched then it looks like stratified squamous epithelium.
• But when the epithelium is in a relaxed condition, it appears stratified cuboidal. Due to this apparent change in the shape this epithelium is called transitional epithelium.
• This stratified epithelium is made up of 2-3 layers of cells as seen in a distended urinary bladder. However, when the bladder is relaxed many layers of cells are seen due to the folding of epithelial cells.
• These changes in shape are confined to the urinary bladder only and not observed at any other place in the urinary tract. The deeper cells are cuboidal or polyhedral, while superficial cells are large and have rounded free surfaces (domeshaped or umbrella-shaped cells).
• The apical surface of dome-shaped cells is thickened and more eosinophilic. This is due to the presence of plaques of intramembranous glycoprotein particles. Some cells of superficial cells may show two nuclei.

Transitional Epithelium (urothelium) Location

Epithelium lining the urinary tract.

Transitional Epithelium (urothelium) Functions

• The presence of occluding junctions and intramembranous plaques forms an effective barrier, i.e., prevents the absorption of toxic substances in urine.
• Distention

Transitional Epithelium (urothelium) Remember

Transitional and pseudostratified epithelia are a special class of epithelium.

1. Transitional epithelium
2. Sectional view.
3. Urinary bladder.
4. The transitional epithelium is taken from the urinary bladder.

The Basement Membrane

The basement membrane is the thin supporting layer placed between the basal surface of the epithelium and underlying connective tissue.

• This membrane is not seen very clearly in most of the organs in H and E (haematoxylin and eosin) preparations. However, when stained with the PAS (periodic acid Schiff) technique.
• It appears as a well-defined magenta(pink) layer. This colour reaction is due to the presence of carbohydrates (sugar)in the basement membrane.

The Basement Membrane Structure

When seen under an electron microscope, the basement membrane appears to be made up of two layers, i.e., basal lamina and reticular lamina.

The basal lamina is associated closely with the basal layer of the cell surface and the reticular lamina is near the connective tissue layer. The reticular lamina consists of reticular fibres.

Basal Lamina

The basal lamina is further divided into lamina lucida and lamina densa. The lamina lucida is a layer of very low density and lies immediately beneath the epithelium.

• The lamina densa is the outer layer of greater density, facing the underlying reticular lamina. Lamina lucida is almost featureless while lamina densa contains a network of extremely fine (3-4 nm) filaments (type 4 collagen) in an amorphous matrix.
• Anchoring fibrils are type 7 collagen, which links basal lamina to reticular lamina. Chemically the basal lamina consists of proteoglycans, laminin and typeIV collagen.
• Lamina lucida consists of molecules of glycoproteins and aminin while a network of type 4 collagen is Present in lamina densa. The type 4 collagen of basal lamina is produced by epithelial cells.
• The basal lamina and basement membrane are sometimes used as interchangeable terms. However, the term basement membrane is mostly used in light microscopy and basal lamina in electron microscopy. Basal lamina in non-epithelial cells is known as external lamina.

Reticular Lamina

It is placed between the basal lamina and underlying connective tissue. It is manufactured by fibroblasts and is composed of type 1 and type 3 collagen.

These collagen fibres are bound to anchoring fibrils (type 7 collagen) of the lamina reticularis.

The Basement Membrane Remember

Basement membrane, as seen by a light microscope, is made up of two layers, i.e., basal lamina and lamina reticularis.

The Basement Membrane Functions

• It provides support to the epithelium.
• The epithelium is attached to the underlying connective tissue with the help of basal lamina.
• It provides support to the epithelium.
• The epithelium is attached to the underlying connective tissue with the help of basal lamina.
• It acts as a mechanical barrier preventing malignant cells from invading deeper tissues.
• It provides a selective filtration barrier. Molecules of certain shapes, sizes and electrostatic charges only are allowed to pass through the basal lamina.
• In some diseases of the kidney, the basal lamina of glomerular capillaries is thickened.

Intercellular Contacts

The cells of epithelia on their lateral surfaces are in contact with each other. These contacts provide adhesion and communication between adjoining cells.

• The plasma membranes of two adjacent cells are usually separated from each other by a 15-20 nm gap. This gap is occupied by cell adhesion molecules (CAM), which are glycoproteins in nature.
• Besides these adhesion molecules, opposing membranes also show some specialization for intercellular contacts that are called junctional complexes (cell junction). The following four junctional complexes are described.

Zonula Occludens (Tight Junction)

This junction is located near the apical part of cells of tissues that line the body cavities. It is in the form of a circumferential (belt-like) band or ring that encircles the entire circumference of the cell.

In this junction, surrounding each cell near its apex, the outer surfaces of adjacent plasma membranes arc fused by a web-like strip of proteins. These transmembrane junctional proteins are called claudins and occludins.

Zonula Occludens (Tight Junction) Remember

The zonula occludens is present near the apical end of epithelial cells. This intercellular cell junction consists of localized sealing of the plasma membrane of adjacent epithelial cells.

Zonula Occludens (Tight Junction) Functions

It is a barrier device. As the adjacent plasma membranes are fused, it prevent the passage of large and small water-soluble molecules between cells.

• It also prevents the leak of contents of organs into the blood or surrounding tissues, For Example., urine from the urinary bladder.
• This kind of junctional complex is common in kidney tubules, intestines and urinary bladder.

Zonula Adherens

This junctional complex is present immediately below the zonula occludens. This complex also completely encircles the cell. There is a gap of 15-20 nm between opposing cell membranes.

• In this complex, the electron-dense material is found along the cytoplasmic side of the membrane of each cell. The microfilaments (actin) are seen embedded in this electron-dense area.
• These actin microfilaments are continuous with the filaments of the terminal web situated near the apex of the cell.
• A specific glycoprotein i.e., the cell adhesion molecule (CAM) is present in the gap between two opposing membranes. This helps membranes to maintain their adherence to one another.

Zonula adherens Functions

It provides strong adhesion between adjacent cells. It gives stability to the terminal web of the cell. Terminal webs of adjacent cells are interconnected by zonula adherens.

Zonula Adherens Remember

Zonula adherens is present below occludentes and provides encircling band-like lateral adhesion between epithelial cells.

Terminal Web

The terminal web is a network of actin filaments present horizontally in the apical part of the cytoplasm just beneath the microvilli. The actin filaments of the terminal web are cross-linked and contractile.

• On the periphery, this network is attached to the intracellular density of zonula adherens. The terminal web also gives attachment to the actin filaments of microvilli.
• The contraction of the terminal web causes the microvilli to spread apart, thus increasing the space between microvilli thereby exposing more surface for absorption.

Desmosomes (Macula Adherens)

This type of junctional complex is not in the form of an entire cling band but is in the form of discs scattered over the lateral surface of cells. The opposing membranes are separated from each other by 30 nm distance.

• This junction shows the presence of a dense disc (attachment plaque) on the cytoplasmic side of opposing membranes. The intermediate filaments of the cytoplasm converge and terminate on this dense disc.
• The transmembrane linker proteins extend between opposing dense discs across the intercellular space. These linkerproteins are cadherins, For Example., desmogleins, desmocollins, etc.
• Desmosomes are predominantly present in epithelia that are subjected to abrasion and physical stress (For Example., stratified squamous epithelium of epidermis).

Desmosomes (Macula Adherens) Functions

• It provides stability to epithelium as a whole by linking cytoskeletons of adjacent cells.
• It provides strong adhesion between cells.

Desmosomes (Macula Adherens) Remember

The desmosomes are spot-like (disc-like) junctions between epithelial cells, while hemidesmosomes are cell junctions between the basal cell membrane and underlying basal lamina.

Hemidesmosomes

It looks like half a desmosome and connects cells to the basal lamina. Thus, they are found at the basal surface of cells. The basal lamina that faces the hemidesmosome is usually thick and connecting strands (integrins) extend between it and the plasma membrane.

Gap Junction (nexus)

Gap junctions allow ions and small molecules to pass from the cytosol of one cell to another. The two adjacent plasma membranes come close to each other and a gap of 3 nm is observed between the two.

• These intercellular gaps are bridged by transmembrane protein channels called connexons. These six rod-like protein subunits (connexins) are so arranged around a central pore that they form a minute fluid-filled tunnel.
• The diameter of this tunnel (central pore) is about 1.5-2 nm. The connexons of opposite membranes face each other and project 1.5 nm into the intercellular gap where they are linked end to end. Channels thus formed connect the cytoplasm of neighbouring cells.
• The opening and closing of gap junction is regulated. An increase in calcium concentration or decrease in cytoplasmic pH closes gap junctions. While channels are open when there is a decrease in Ca⁺ concentration or an increase in pH in the cytoplasm.

Gap Junction Functions

1. It coordinates the activities of cells in the epithelium, heart and smooth muscles.
2. Gap junctions enable nerve or muscle impulses to spread rapidly between cells.
3. It coordinates the activities in embryonic cells by distributing signalling molecules throughout the cell mass.

Gap Junction Remember

The nexus or gap junctions allow communication between adjacent epithelial cells by allowing the passage of small molecules, ions, hormones, amino acids and vitamins

Gap Junction Clinical Applications

Tumours of Epithelial Cells

• A tumour is a swelling that results due to excessive proliferation of cells. Epithelial cells can give origin to both benign (harmless, non-cancerous) and malignant (cancerous) tumours.
• Epithelial cells are a common site of origin of malignant tumours (cancers) and are called carcinomas. Cancers derived from glandular epithelial tissues are called adenocarcinomas.

Specialization Of The Free Surface Of Cell

Following modifications may occur on the free surface of epithelial cells to perform duties related to tissue function:

• Microvilli
• Stereocilia
• Cilia

Microvilli

Under the light microscope, the epithelia lining the tubules of the kidney and intestine show fine vertical striations near their free surface.

• This surface modification in intestinal absorptive cells is called a striated border and brush border for kidney tubule cells.
• Under an electron microscope, this border is found to consist of cytoplasmic finger-like protrusions from the apical cell surface.
• These cell processes arc closely packed and measure about 1-2 pm in length and about 80-90 ran in diameter. These processes are called microvilli. Microvilli are covered by a fuzzy coat called glycocalyx.
• Each microvillus contains a bundle of 25-35 actin filaments. These actin filaments at one end arc attached to the terminal web in the apical cytoplasm and at the other end to the membrane at the tip of the microvillus.
• In epithelia not involved in absorption, microvilli are few and lack actin filament.

Specialization Of The Free Surface Of Cell Functions

The formation of microvilli is a special modification of the cell surface to increase the surface area of the membrane. It achieves a 15-30-fold increase in the area.

• Increased surface area is an adaptation to increase the capability for absorption. Microvilli are non-motile processes.
• However, in some microvilli oscillatory, contractile movements may be seen due to the presence of actin filaments. This helps in the process of absorption.
• Microvilli are small finger-like cytoplasmic projections from the free cell surface. It contains the core of actin filaments.

1. Seen as a brush or striated border under a light microscope.
2. Seen as small projections in an enlarged view.
3. Simple columnar epithelium showing striated border (microvilli).
4. Electron micrograph of microvi li from the apical surface of cell lining intestinal villi. The microvilli are finger-like protrusions measuring about 1 to 2 pm.

Stereocilia

Stereocilia are found on the apical surface of the epithelium lining epididymis, ductus deferens and on the sensory hair cells of the cochlea of the inner ear.

• Stereocilia are long, non-motile cytoplasmic processes measuring about 100-120 pm in length. Their structure resembles that of microvilli and is therefore called a large microvilli.
• Their function is not known exactly. It is believed that they increase cell surface area and serve as an absorptive device. In the hair cells of the ear, they probably function in signal generation.

Stereocilia Remember

Stereocilia are very long non-motile microvilli. Their core consists of actin filaments.

Cilia

Cilia are structural modifications of the cell surface that are capable of rapid, regular and synchronous to-and-fro movements. Cilia covering the epithelium move as waves in a grass field.

• Cilia are short (5-10 pm in length, 0.3 pm in diameter), fine, hair-like structures arranged in rows on the apical cell surface. The shaft of each cilium is covered by the cell membrane.
• The core of it consists of a central pair of microtubules (which are separated from each other) with nine pairs of evenly spaced microtubules around them, The outer pairs of tubules are connected to the central pair.
• At the base of cilia are basal bodies that are believed to have originated from the centriole. Each basal body is similar in structure to a centriole and is made up of nine triplet microtubules around the periphery.
• The 9+2 microtubules of cilia extend between the tip of the cilium to its base where the basal body is situated. Each of the paired microtubules of the cilium is continuous with the two inner microtubules of the triplet of the basal body.

1. Under the light microscope, cilia are seen as hair-like structures.
2. In an enlarged view, they are seen as finger-like projections.
3. Photomicrograph shows the ciliated cells of the respiratory epithelium (trachea). A single cell may have up to 300 cilia.

Cilia Remember

Cilia are hair-like motile structures arranged on the api¬ cal surface of the cell. The core of cilium contains longitudinal microtubules arranged in a 9+2 organization called the axoneme.

Cilia Functions

• The wave-like (rapid back-and-forth) movements of cilia on the surface of bronchial and tracheal epithelium help to move the mucus in one direction (towards the pharynx).
• Cilia are responsible for the movements of ova in the oviduct.

1. Schematic diagram showing cross-section of a cilium.
2. To one of the peripheral microtubules, protein dynein is attached.
3. Electron micrograph showing a cross-section of the cilium.

Glandular Epithelia

A gland is an organ that consists of specialized secretory cells. The material secreted by the gland is usually a liquid (enzyme, hormone, mucus or fat).

Glands are epithelial in origin. They may be unicellular and multicellular. The unicellular gland consists of a single cell distributed among non-secretory cells, For Example., the goblet cells.

The multicellular glands are formed by the invagination of surface epithelium in deeper tissue. These are classified into the following two types depending on how their secretion is released.

Exocrine glands – These types of glands remain in contact with the surface epithelium by the ducts and pour their secretions on its surface.

Endocrine glands – On the other hand, some glands lose their epithelial contact because of the disappearance of ducts. These types of glands pour their secretion directly into the blood.

They secrete hormones that act on target cells, which are usually situated some distance away from the gland.

Glandular Epithelia Remember

Exocrine glands secrete their product through a duct. Endocrine glands are ductless and secrete directly into the blood bloodstream or lymphatic system. Paracrine gland secretion remains locally and affects the surrounding cells.

1. Formation of exocrine gland.
2. Formation of the endocrine gland.

Exocrine Glands

Classification of Exocrine Glands

1. Classification based on the shape and branching pattern of the duct

The classification of exocrine glands is based on the shape of secretory units (tubular or alveolar) and the branching pattern of their ducts. If the ductofa gland is unbranched, it is called as simple while glands with branched ducts are called compound.

• Simple Glands: Simple tubular (For Example., crypt of Lieberkuhn); simple coiled tubular(For Example., sweat glands); simple branched tubular (where only the secretory part is branched,
  • For Example., fundic glands stomach); simple alveolar (mucous glands of the urethra); simple branched alveolar (where only secretory units are branched, For Example., meibomian gland).
• Compound Glands: Compound tubular (Brunner glands); compound alveolar (For Example., mammary gland); compound tubulo-alveolar (submandibular gland).

1. Various types of simple glands were duct unbranched.
2. Various types of compound glands

2. Classification based on the mode of release of their product

Exocrine glands secrete their products by three different methods.

• Merocrine: Secretion is released by the exocytosis of secretory granules (For Example., pancreas and parotid gland. Here, neither cell membrane nor cytoplasm becomes part of the secretion).
• Apocrine: In this process of secretion, the apical portion of the cell along with the secretory product is pinched off (For Example., the lipid component of milk from the mammary gland is secreted by this method. However, the protein component of milk is secreted by merocrine method).
• Holocrine: In this process of secretion, the whole cell is shed along with secretory products (For Example., the sebaceous gland). As the secretory cell matures, it dies and becomes the secretory product.

1. Merocrine (secretion through exocytosis).
2. Apocrine (an apical portion of the cell is pinched off).
3. Holocrine (cell dies and becomes secretory product)

3. Classification based on the nature of their secretion

• Mucous Glands: Mucous glands secrete mucus.
• Serous Glands: They secrete thin watery secretion rich in enzymes.
• Mixed Glands: They contain both mucous and serous secretory units. Sometimes, most of the secretory units are mucous acini and serous cells form crescentic caps on the acini. These crescentic caps are called serous demilunes.

1. Simple squamous
2. Simple cuboidal
3. Simple columnar
4. Pseudostratified columnar
5. Stratified squamous and
6. Transitional.

Cartilage

we have seen that cartilage is classified as a specialized connective tissue. It consists of all the three components of a connective tissue, i.e., ground substance, fibres and cells.

Cartilage differs from other connective tissues by the presence of substances in the intercellular matrix that give firmness to it.

The firm pliable matrix is capable of resisting mechanical stresses. Three types of cartilage are found in the body:

• Hyaline cartilage
• Elastic cartilage
• Fibrocartilage

Although all the above cartilages consist of cells (chondrocytes), ground substance and fibres, they differ from each other because of the type and amount of fibres present in them.

• Cartilage is not supplied with the blood vessels(it is avascular). However, it is usually surrounded by a membrane called as perichondrium, which is rich in blood vessels.
• The chondrocytes receive nutritive substances from blood vessels in the perichondrium by diffusion through the ground substance. P
• Erichondrium is present in most of the hyaline cartilage, and elastic cartilage but absent in fibro cartilage

Cartilage Remember

Cartilage is a special kind of connective tissue consisting 1 of chondrocyte cells, connective tissue fibres and specialized ground substances. The firm pliable matrix is capable of resisting mechanical stresses.

Hyaline Cartilage

Hyaline Cartilage Location

It is the most abundant type of cartilage in the human body. This
cartilage is found in the fetal skeleton, ends of adult bones(articular cartilage), nose, costal cartilage, trachea, bronchi and larynx.

Hyaline Cartilage Description

It consists of a homogeneous, transparent and amorphous intercellular matrix. The matrix consists of collagen fibres and ground substance.

Throughout the matrix, cartilage cells (chondrocytes) are present in small spaces called lacunae. Hyaline cartilage is surrounded by perichondrium.

1. Schematic diagram showing features of hyaline cartilage
2. As seen under a microscope.
3. Photomicrograph of hyaline cartilage perichondrium in the upper part chondrocytes are present in lacunae
4. Photomicrograph of hyaline cartilage perichondrium in the lower part chondrocytes are present in lacunae

1. Fibres

The ground substance of hyaline cartilage contains fine type II collagen fibres that are about 15-40 nm in diameter. The thin fibres do not show cross striations and form a three-dimensional network in the ground substance.

• The thicker fibres show cross-striations and are arranged in a direction that can resist the strain to which cartilage is subjected. Collagen fibres provide stability and strength to the cartilage.
• These fibres constitute about 40% of the dry weight of cartilage. These fibres are not seen in the histological section because they have the same refractive index as that of ground substance.
• Some other types of collagen fibres(types 6,9,10 and 11) are also present in this cartilage in small amounts.

2. Ground Substance

It is a featureless(homogeneous) gel-like substance that stains blue with a basic dye(haematoxylin). The main constituent of ground substance is sulphated proteoglycans (aggrecan).

1. The basophilia of the matrix is due to the presence of chondroitin and keratan sulfate which are acidic. The highest concentration of these substances is around the lacuna as it is newly formed.
2. This dark blue staining around the lacuna is called a capsule or territorial matrix. As we go away from the lacuna the concentration of sulphated proteoglycans becomes less and less.
3. Thus the matrix does not show that intense blue staining that is seen in a capsule. This matrix is called an inter-territorial matrix.

Hyaline Cartilage Remember

The matrix of hyaline cartilage consists of type 2 collagen fibres and sulfated proteoglycans, glycoproteins and water.

The matrix is homogenous as fibres are not seen in the histological section because they have the same refractive index as that of ground substance.

1. A portion of hyaline cartilage showing lacunae containing shrunken chondrocytes.
2. Photomicrograph of a portion of hyaline cartilage

Hyaline Cartilage Further Details

Chemical Composition of Ground Substance

The principal constituent of ground substance is proteoglycan which is present in high concentration and is responsible for the firmness of matrix.

• The proteoglycan molecule consists of a core protein from which many glycosaminoglycan molecules radiate in a bottlebrush configuration. There are two types of glycosaminoglycans in the proteoglycans of hyaline cartilage.
• These are chondroitin sulphate and keratan sulphate. About 80-100 molecules of proteoglycans are joined to a long molecule of hyaluronic acid to form large hyaluronate proteoglycan aggregates.
• These aggregates are then bound to the thin collagen fibres present in the matrix. The glycosaminoglycan side chains form electrostatic bonds with the collagen.

Thus, the ground substance and collagen form a cross-linked molecular framework that resists tensile forces.

• The ground substance also contains other types of proteoglycans that do not form aggregates. It also contains various other types of glycoproteins.
• The ground substance also can collect large amounts of water molecules.
• Because of a high degree of hydration hyaline cartilage solutes can diffuse easily through the matrix and it acts as an effective weight-bearing cartilage (For Example., articular cartilage).

1. A proteoglycan subunit is formed with a core protein to which about 100 glycosaminoglycan units(chains) are joined in bottlebrush configuration.
2. About 80-100 molecules of proteoglycans are joined to a long molecule of hyaluronic acid with the help of a link protein.
3. These hyaluronate proteoglycan aggregates bound to thin collagen fibres present in the matrix.

3. Cells

Only one type of cell(chondrocyte) is seen in the cartilage. These cells occupy lacunae in the matrix. In H and E preparation, these cells are usually shrunken and the gap is seen between the cell and the margin of a lacuna.

• However, in elec tron microscopy cell appears elliptical and occupies a complete lacuna. Chondrocytes are responsible for the synthesis of collagen fibres and ground substances.
• In young cartilage, chondrocytes show mitotic cell division and give rise to daughter cells. These newly formed chondrocytes produce fibres and ground substances around themselves.
• Groups of daughter cells from one chondro cytes remain in close relationship and form cell nests. As newly divided chondrocytes produce a matrix, this surrounds them, and the chondrocytes move away from each other.

1. The hyaluronate proteoglycan aggregates bound to two thin collagen fibrils.
2. Enlarged view of area enclosed in a rectangle. The chondroitin sulphate side chains of proteoglycan form an electrostatic bond with the collagen.

Perichondrium

Hyaline cartilage, on its free surface, is always covered with a fibrovascular membrane called perichondrium. The perichondrium is absent in the cartilage where it forms.

• The free surface as in joint cavity articular cartilage) and where cartilage makes direct contact with bone (i.e., costal cartilage making direct contact with rib; epiphyseal cartilage making contact with metaphyses and epiphyses in a developing bone).
• Perichondrium consists of two layers, i.e., an outer fibrous layer(made up of dense irregular fibrous connective tissue) and an inner cellular layer(made up predominantly of cells which may convert to chondrocytes when the cartilage is growing.
• In adult cartilage, only a fibrous layer is present. Although cartilage itself is avascular, the fibrous layer of the perichondrium has blood capillaries.
• These capillaries provide nutrition to cartilage cells by diffusion through the matrix. The hyaline cartilage displays both appositional and interstitial growth.

In the case of appositional growth, there is the addition of new cartilage at its surface, while in the case of interstitial growth new cartilage is formed within the existing cartilage by division and differentiation of chondrocytes.

Perichondrium Functions

• Articular cartilage provides cushioning and a smooth surface for movements at joints.
• Although it is flexible, it provides support because of its firmness (For Example., as in costal cartilage).
• Firmness of cartilage keeps the lumen of the trachea and bronchi patent.
• Most long bones of the fetus, to begin with, are cartilage models. Later this cartilaginous model is replaced by bone.

Elastic Cartilage

Elastic Cartilage Location

The elastic cartilage is present in the pinna of the ear, epiglottis, tips of arytenoids, corniculate and cuneiform cartilages of the larynx, external auditory meatus and auditory tube.

Elastic Cartilage Description

This cartilage is highly elastic. It looks yellow in the fresh state and is hence sometimes called yellow elastic cartilage. The elastic cartilage also consists of ground substance fibres, cells and perichondrium.

Fibres

Elastic cartilage contains a meshwork of branching and anastomosing elastic fibres that give it a yellow appearance. The elastic fibres are more heavily concentrated in the centre of cartilage than near the perichondrium.

This cartilage also contains a few type 2 collagen fibres. Elastic fibres are not seen in H and E stains but are visualized by special staining methods for elastic fibres(orcein stain).

Ground Substance

As in hyaline cartilage, it also contains proteoglycans.

Cells

• Chondrocytes are present in lacunae. These cells are bigger than cells present in hyaline cartilage and are present singly or in groups of two.
• Cells are closely placed as the intercellular ground substance is much less than in hyaline cartilage.
• Although the matrix of hyaline cartilage calcifies in old age, the matrix of elastic cartilage does not calcify with age.

Perichondrium

It is the same as described in hyaline cartilage.

Elastic Cartilage Remember

Elastic cartilage is differentiated from hyaline cartilage as it contains elastic fibres in the matrix as well as in the perichondrium.

Elastic Cartilage Functions

It provides shape and support to the organ, with elasticity.

1. Schematic diagram of elastic cartilage.
2. under microscope.
3. Low power view of elastic cartilage from epiglottis.
4. Photomicrograph of elastic cartilage

Fibrocartilage

It is also known as fibrocartilage it contains bundles of thick collagen fibres. The histological structure of fibro-cartilage resembles dense regular connective tissue. One may confuse it with the section of a tendon. However, it can be identified because chondrocytes are seen between collagen bundles.

1. Schematic diagram of fibrocartilage.
2. Microscope.
3. Section of fibrocartilage from intervertebral disc.

Fibrocartilage Location

Fibrocartilage is found in intervertebral discs (annulus fibrosus), pubic symphysis and manubriostemal joint.

• The menisci of the knee joint and articular disc of tempo roman tabular and sternoclavicular joints are also fibrocartilage nous. Similarly, the glenoidal and acetabular labrum is also made up of fibrocartilage.
• Sometimes, this cartilage is seen at the site of insertion of tendon in bone.

Fibrocartilage Description

Fibres

In fibrocartilage, all the collagen fibres are of type 1 and type 2 varieties. The proportion of type 1 and type 2 fibres varies in different types of fibrocartilage. In the intervertebral disc, both type 1 and type 2 are in equal proportion.

Ground Substance

A minimal amount of basophilic ground substance is seen around the chondrocyte.

cells

Very few chondrocytes are seen, which are oriented between large collagenous fibre bundles. Chondrocytes are either present in a row or scattered singly between bundles of fibres.

Perichondrium is absent in fibrocartilage.

Fibrocartilage Functions

Fibrocartilage is capable ofresisting compressive and shear forces, i.e., deformation, For Example., intervertebral disc.

Fibrocartilage Remember

Fibrocartilage consists of abundant type 1 collagen fibres and minimal ground substance. The perichondrium is absent in this type of cartilage.

Fibrocartilage Clinical Applications

• Pseudoachondroplasia
  • It is due to a defect of cartilage oligomeric matrix protein (COMP) in the joints and is characterized by a more typical development of the head and face.
  • In this condition, there occurs the reduced proliferation of cartilage cells in the epiphyseal plate of long bones. This causes shortening of limbs, resulting in short stature.
  • However, the head and face are normal. The achondroplasia is inherited as an autosomal dominant disease. Shown in the accompanying photograph are seven pseudochondroplastic members of the Ovitz family.
  • A family of Romanian Jews who toured Eastern Europe as a musical troupe before World War 2 (their taller siblings working backstage), survived imprisonment at Auschwitz, and the family immigrated to Israel. They were photographed on arrival in Haifa in 1949.
• Osteoarthritis
  • Osteoarthritis is a disease of old age. In this condition, the articular cartilage of interphalangeal joints of the hand, hip joint and knee joint progressively become thinner and ultimately break.
  • This exposes the bone beneath the cartilage. It is a highly painful condition and leads to a walking disability.

Comparison Between different types of cartilages

1. Healthy diseased
2. Knee joints.

Bone Tissue

Bone like cartilage, is also a specialized type of connective tissue. Similar to all other connective tissues, it also consists of ground substances, fibres and cells.

• However, the bone is classified as specialized connective tissue because of the presence of minerals (calcium salts) in its intercellular matrix.
• The presence of calcium salts makes bone tissue hard, which is suited for providing support and protection to the vital organs(lungs, heart and brain).
• The hardness of bone is responsible for locomotion as it forms a skeletal framework and also provides attachment to muscles on its surface.
• Although bone is the hardest tissue of the body, it constantly changes shape about stress is applied.
• If pressure is applied to the bone, it leads to resorption, whereas tension applied to it results in the formation of new bone. These facts are used by orthodontists to treat malocclusion of teeth.

Before we study the histological structure of the bone, it would be useful to understand the gross structure and composition of bone.

Bone Tissue Remember

Bone is a specialized connective tissue. It is special because its intercellular matrix is mineralized making it hard. The hardness of the bone enables it to perform various functions associated with it.

The Macroscopic(Gross) Structure Of A Long Bone

A longitudinal section of a long bone consists of two knobby ends(epiphyses), joined by a long shaft(diaphyses). The expanded portion of the bone between epiphysis and diaphysis is called metaphysis.

• Diaphysis consists of a thick wall of dense bone in which no spaces are visible to the naked eye exi amination; hence, called compact bone. Diaphysis encloses a central cavity known as the marrow cavity.
• Both the ends of a long bone(epiphyses) are covered by a thin layer of compact bone and filled internally by the meshwork of thin and small rods and curved plates.
• This meshwork looks like a sponge; therefore, this kind of bone is called spongy or trabecular bone. The articular areas of epiphyses(which are in contact with another bone to form a joint) are covered by hyaline cartilage(articular cartilage) in living bone.
• Articular cartilage provides a smooth area to facilitate the movements between two bones forming a joint.

The entire outer surface of bone, except the area covered by articular cartilage, is covered by a connective tissue membrane called periosteum.

Similarly, the marrow cavity and spaces of spongy bone are also lined with a membranous layer called an endosteum. The marrow cavity and spaces of spongy bone are filled by bone marrow, which is highly vascular tissue.

• In adults, bone marrow tissues are of two different kinds, i.e., yellow marrow and red marrow. The red marrow is present at the ends of the bone and is involved in the formation of blood cells, while yellow marrow is in the shaft of long bone and is predominantly made UP of adipose tissue.
• From the above description, it is evident that bone tissue can be macroscopically classified into two distinct types, i.e., compact and spongy.
• Students should note that the above description is of a living bone (bone present in a living person). The bones, which are handled in the classroom, are dry and devoid of many structural components of a living bone.
• For example, a dry bone is not covered by hyaline cartilage at its epiphyseal ends. Similarly, it is also devoid of periosteum, endosteum, bone marrow, blood vessels and nerves.

Bone is not only a living tissue but it is also a dynamic tissue. It is continuously engaged in building new bone and breaking down old bone.

• Each living bone is not just a bone tissue, but somewhat similar to an organ. It is evident by the fact that a bone consists of not only bone.
• Tissue proper also consists of many other tissues like fibrous membranes(periosteum and endosteum), cartilage (articular cartilage), bone marrow (adi pose and haemopoietic tissues), nerves and blood vessels.
• Similar to any other organ of the body, bone is also involved in various functional activities(locomotion, support and protection of delicate organs, formation of blood and storage of calcium).

Composition Of Bone Tissue

Like any other connective tissue, bone tissue is also comprised of the following three basic components: cells, fibres and ground substance. In addition to this, the intercellular matrix(fibres and ground substance) of bone tissue is mineralized.

1. Cells

There are four types of cells in bone tissue, i.e., osteogenic cells, osteoblasts, osteocytes and osteoclasts.

Osteogenic Cells

These cells are present in the cellular layer of the periosteum, endosteum and Haversian canals(see below). These are stem cells which, after cell division, give origin to osteoblasts. These cells are derived from embryonic mesenchymal cells.

Osteoblasts

These are bone-forming cells. They synthesize and secrete matrix(collagen fibres and ground substance). They are also responsible for the calcification of the matrix.

• Thus, an osteoblast has a well-developed rough endoplasmic reticulum, Golgi complex and mitochondria that are needed for synthesis and secretion of matrix.
• These cells also possess receptors for parathyroid hormones. During active bone formation, osteoblasts secrete a high level of alkaline phosphatase, thus its level increases in blood. By measuring blood alkaline phosphate levels, one can monitor bone formation.

Osteocytes

These are the main cells of bone tissue. They are formed from osteoblasts that have become entrapped in matrix secretion at the time of formation of new bone.

• They occupy lacunae within the matrix and send cytoplasmic processes into the canaliculi, where they form gap junctions with the processes of adjacent osteocytes.
• The cytoplasm and nucleus of an osteocyte show the features of a resting cell. Osteocytes not only play a role in the maintenance of the surrounding matrix.
• But they also respond to various pressures and tensions applied to bone. They release osteocalcin and insulin-like growth factors, which help in the remodelling of bones.

Osteoclasts

These cells are involved in bone resorption. They are huge cells containing up to 50 nuclei and measuring about 50-150 pm in diameter.

• The cytoplasm shows many lysosomes containing acid phosphatase. The osteoclast’s plasma membrane shows deep foldings(ruffled border) towards the site that comes in contact with bone.
• Here, the cell releases powerful lysosomal enzymes and acids that help in the destruction of the mineralized matrix. As a result, a shallow depression (Howship’s lacuna) can be observed in the bone immediately below the osteoclast.
• It is considered that osteoclasts arise from the fusion of many monocyte cells in the blood. In both origin and function, osteoclasts are closely related to macrophages.
• However, according to new evidence, osteoclasts have bone marrow precursors in common with monocytes termed mononuclear phagocyte system.

Cells Remember

• Bone Cells: Bone tissue consists of four different types of cells, i.e., osteogenic cells, osteoblasts, osteocytes and osteoclasts.
• Osteocytes are mature bone cells derived from osteoblasts that have become entrapped in matrix secretion at the time of formation of new bone.
• Osteoclasts are multinucleated large phagocytic cells that play a role in bone resorption.

1. Osteogenic cell
2. Osteoblast
3. Osteocyte
4. Osteoclast
5. Photograph of a multinucleated osteoclast

2. Fibers

Bone consists of type 1 collagen fibres that are synthesized by osteoblasts. These fibres are embedded in the ground substance. The collagen fibres are responsible for providing tensile strength to the bone.

• The collagen fibres within a lamella (see below) are oriented parallel to each other, but the fibres in one lamella are at an angle to those in an adjacent lamella.
• Although the major structural component of bone matrix is type 1 collagen, to a lesser extent type 5 collagen is also found.

3. Ground Substance

It consists of a small amount of amorphous ground substance rich in proteoglycans. The proteoglycans of bone compared with the cartilage-have shorter core protein and fewer side chains.

• The glycosaminoglycans are of three different types, i.e., hyaluronic acid, keratan sulphate and chondroitin sulphate.
• Several multiadhesive glycoproteins are also present in the bone matrix. these are osteocalcin ( this captures calcium from circulation and stimulates osteoclasts in bone remodelling).
• Osteonectin osteopontin and sialoprotein 1 and 2, are responsible for the attachment of collagen fibres to mineralized ground substances.

The ground substance and fibres form the organic part of the matrix, 90% of which is fibres and 10% ground substance. The inorganic component in the bone matrix consists of mineral salts.

Minerals

The principal constituent of the inorganic matrix is crystals of calcium phosphates. However, it also contains calcium carbonate, calcium fluoride, citrate, magnesium and sodium.

Most of the bone minerals are in the form of rod-like crystals that are arranged along the length of collagen fibres. A layer of water and ions surrounds each crystal.

Further Details

In bone, the matrix consists of about 25% water, 25% collagen fibres and 50% mineral salts. The hardness of bone is due to crystallized inorganic mineral salts which are responsible for the compressive strength of bone.

• The flexibility of bone depends upon the presence of collagen fibres and is also responsible for its tensile strength.
• If a long and thin bone-like fibula is treated with a weak acid the inorganic mineral salts will be removed from the bone but collagen fibres will remain.
• The bone thus will lose its rigidity and will become so flexible that it can be tied into a knot.
• On the other hand, burning fibula will destroy the organic matrix (collagen fibres) but mineral salts will remain in the bone and thus the shape of the bone will be maintained.
• This bone will become as brittle as chalk. Thus bony hardness is due to inorganic salts while strength and flexibility are due to collagen fibers.

Ground Substance Remember

The predominant organic component of bone matrix is type 1 collagen, while inorganic components are crystals of calcium hydroxyapatite.

Microscopic Structure Of Bone

Bone tissue is made up of lamellae. It would be useful to understand the lamellar organization of bone before we study the histology of compact and spongy bone.

Lamellar Organization of Bone

As stated earlier, on naked eye examination, two different types of bones are observed, i.e., spongy and compact. Both these types of bones are made up of bone tissue layers or lamellae.

• A lamella(layer) is the basic unit of adult bone. It would be useful to understand the lamellar organization of bone before we study the histology of compact and spongy bone.
• A lamella is a thin plate of bone and is made up of collagen fibres and mineral salts embedded in a gelatinous ground substance. These lamellae are arranged one upon another.
• The orientation of collagen fibres in each lamella is parallel to each other, but the collagen fibres in adjacent lamellae are oriented almost at a right angle to one another.

Small spaces are seen between adjacent lamellae that are called lacunae. Each lacuna is occupied by an osteocyte.

• The adjacent lacunae are interconnected with one another and with a central canal with the help of numerous canaliculi that radiate from each lacuna.
• Each lacuna is present between two adjacent lamellae, but the canaliculi travel through lamellae. Canaliculi arc minute canals that contain cytoplasmic processes of osteocytes and are filled with extracellular fluid.
• This lamellar arrangement Of bone is found in both spongy and compact bones. Even a small spicule or trabeculae of spongy bone consists of several lamellae placed over one another.

Microscopic Structure Of Bone Remember

Bone tissue (both compact and spongy) is made up of lamellae. A lamella is a thin plate (layer) of bone made up of collagen fibres and mineral salts embedded in a gelatinous ground substance.

These lamellae are arranged one upon the other. A lamella (layer) is the basic unit of adult bone.

1. The bone lamellae are arranged one upon another. Osteocytes are present in lacunae and send their processes to canaliculi.
2. Enlarged view of lacuna, osteocytes and canaliculi. The lacuna is present between two adjacent lamellae.

Structure of Spongy Bone

Spongy bone tissue is present at the epiphyses of long bones. A11 short flat and irregular bones also consist of spongy bones. Similarly, a thin rim around the marrow cavity of the diaphyses of long bone consists of spongy bone.

• On naked eye observation, a spongy or cancellous bone is made of a three-dimensional meshwork of trabeculae or spicules. These trabeculae are made up of inter-anastomosing.
• Thin, curved plates and rods. Between bone tissues (plates and rods) numerous interconnecting spaces are filled with bone marrow.
• If we examine the cross-section of a trabecular rod or P’ate under a microscope, it appears to be made of several lamellae placed over one another.

Between these lamellae are small spaces(lacunae) containing osteocytes. Radiating from these lacunae are canaliculi, which are occupied by the processes of osteocytes.

• The osteocytes situated in lacunae derive their nutrition through the canalicular system from blood vessels present in the bone marrow.
• As trabecular rods and plates are not more than 0.4 mm in thickness, there is no need to have blood vessels within bone tissue. (However, in the case of compact bone, blood vessels are present within the bone tissue, i.e., in the Haversian canal.)

Structure of Spongy Bone Remember

Although the spongy bone also consists of lamellae, these lamellae are not organized in the form of Haversian canal systems (osteons).

1. Spongy bone trabeculae.
2. Section of a trabecula.
3. A section of spongy bone.
4. Section of spongy (cancellous) bone showing network of bony trabeculae.

Structure of Compact Bone

Compact bone forms the bulk of the diaphysis of long bones. It also forms a thin layer on the external surface of all other bones in which the core is made up of spongy bone.

• For Example., short, flat and irregular bones. This bone is called as compact because no space is visible on naked eye examination.
• On its outer and inner surface, it is covered by periosteum and endosteum, respectively.

The compact bone is also made up of lamellar bones. However, here lamellae are present in three different patterns:

• Haversian system of lamellae
• Interstitial lamellae
• Circumferential lamellae

Haversian System of Lamellae

• It is also known as osteon.
• In this system, 4-15 concentric lamellae are arranged around a central canal that is called as Haversian canal. This canal contains a small amount of loose connective tissue, capillaries, nerves and lymphatics.
• Between the concentric lamellae are lacunae that contain osteocytes.
• The radiating canaliculi from lacunae connect the Haversian canal with all the lacunae present in an osteon.
• This canalicular route facilitates the passage of nutrients and oxygen to reach the osteocytes from the blood capillaries in the Haversian canal.
• The blood vessels and nerves from the periosteum go inside the compact bone through Volkmann’s canal and communicate with Haversian canals.
• Volkmann’s canals not only communicate periosteal vessels with vessels in Haversian canals but also with vessels in adjacent Haversian canals and with vessels in the marrow cavities.
• Volkmann’s canals are usually identified based on two features: firstly, they are horizontally or obliquely placed concerning the long axis of bone and secondly, are not surrounded by concentric lamellae.
• The Haversian system is cylindrical with its long axis parallel to the long axis of bone. This is because osteons in a long bone are oriented in the direction of the line of stress.
• Surrounding each osteon there is a thin layer of mineralized bone matrix called a cement line.
• It is distinctly visible compared with surrounding lamellae due to the absence of collagen fibres in it.

1. Schematic diagram.
2. As seen in the ground section of a dried compact bone.
3. The Haversian canal is surrounded by concentric lamellae.

Interstitial Lamellae

As stated earlier, older bone tissue is constantly being replaced by new bone tissue. Because of this, the fragments of older osteons are seen in areas between osteons.

• These areas also show the lamellar arrangement of bone, i.e., between lamellae are lacunae, which are occupied by osteocytes.
• Radiating from lamellae are canaliculi. These kinds of lamel are called interstitial lamellae.

1. Ground section of compact bone (transverse section)
2. Ground section of compact bone (longitudinal section).

Circumferential Lamellae

The circumferential lamellae are of two different kinds. The outer circumferential lamellae are present on the outer surface of bone just beneath the periosteum.

They completely encircle the bone. Similarly, the inner circumferential lamellae encircle the marrow cavity.

Structure of Compact Bone Remember

The compact bone consists of four different types of lamellar systems, i.e., outer and inner circumferential, Hav Persian system (osteon) and interstitial lamellae.

Periosteum

Like perichondrium covering the cartilage, the bone on its external surface is also covered with a membrane called periosteum.

• It consists of two layers: an outer fibrous layer made up of collagen fibres and an inner cellular layer made up of cells. These cells in a young developing bone are osteogenic.
• In an adult(developed) bone, the cellular layer is not well developed and may have a few cells called periosteal cells. These cells may convert to osteoblasts when the need arises, i.e., in case of repair of bone after a fracture.
• The blood vessels of the periosteum pass into compact bone, through Volkmann’s canals, to supply the nutrients to the outer layers of bone.
• When a tendon is attached to bone, the collagenous fibres of the tendon after passing through the periosteum continue into the outer layers of bone. These fibres are called as fibres of Sharpey.

Endosteum

The endosteum is the thin lining of the bone that faces the marrow cavity and the spaces of spongy bone. This layer is mostly one cell thick and consists of cells that are concerned with bone formation.

Bone resorption or resting cells. The resting cells are flat or squamous in shape and may change to osteoblasts when the need arises.

Endosteum Remember

The periosteum is made up of two layers, i.e., outer fibrous and inner cellular. The inner cellular layer consists of osteogenic and osteoblast cells.

Endosteum Clinical Applications

Some Important Bone Diseases

Scurvy

As seen certain amino acids and vitamin C are necessary for collagen synthesis. Scurvy is caused by a dietary deficiency of vitamin C.

• This leads to the synthesis of inadequate amounts of normal collagen and organic matrix.
• In a patient suffering from scurvy, spongy bone consists of a reduced number of trabeculae. In the case of compact bone, the cortex of long bone is thinner than normal.

Rickets

This disease is due to inadequate mineralization of bone matrix in young individuals. This is due to an inadequate dietary supply of calcium and phosphorus.

1. Vitamin D is needed for the absorption of calcium. Thus, if there is a deficiency of vitamin D, calcium absorption would be affected.
2. Most cases of rickets are due to inadequate intake of vitamin D in infancy or childhood. Patients with rickets suffer from the bowing of long bones.
3. This is because of loss of rigidity in weight-bearing long bones, as there is inadequate mineralization of bone matrix.

Osteomalacia

This disease is seen in adults and is due to an inadequate supply of minerals or vitamin D. The deformities due to osteomalacia are the same as seen in rickets.

Osteoporosis

Normally a balance is maintained between bone formation and bone resorption. When bone resorption becomes higher than bone formation, the bone becomes thin and unable to resist stress.

• This leads to frequent fractures. This disease is due to poor calcium or phosphorus ratios, which is seen in persons above 50 years of age.
• Osteoporosis is more common in women after menopause. After menopause, there is a reduction in estrogen levels which is considered the cause of osteoporosis.

Osteogenesis Imperfecta

This is a genetic disease. Here, there is a mutation of genes responsible for the synthesis of collagen. Due to inadequate and abnormal collagen, bones become weak and brittle.

• Persons suffering from this disease are prone to frequent fractures. The formation of collagen begins as procollagen molecules.
• Each rope-like procollagen molecule is made up of three chains, two alpha-1 chains and one alpha-2 chain.
• Alpha 1 chain is produced by the gene COL1A1 and alpha 2 chains by COL1A2 gene. The COL1A1 gene is located on the long arm of chromosome 17 and the COL1A2 gene is located on chromosome number 7.
• Mutation of these gene(s) leads to osteogenesis imperfecta. No genetic cure is available at present. Putting metal rods in long bones can prevent fractures of long bones.

Comparison Between Bone And Cartilage

What is the ground section of a bone?

Ground sections of a bone are prepared from dry bones. Thin slices of dried bone are cut with the help of a saw and then further reduced in thickness on a grinding stone.

• A ground specimen must be so thin that light can easily pass through it. These sections are then mounted on glass slides usually unstained.
• As the ground sections of a bone are unstained, it is difficult to observe the details of a section under a microscope. These sections are best observed under minimal light.
• For this, bring the condenser completely down and regulate the light passing through the diaphragm(from completely open to closed position) till you start seeing lacunae, canaliculi and lamellae.
• You should also recollect that a dried bone is devoid of endosteum, periosteum, osteocytes in lacunae and blood vessels and nerves in Haversian canals. Hence, all the above structures will not be seen in a ground section.

All the spaces in a ground section(Haversian canals, lacunae and canaliculi) are filled with dust particles, during the preparation of the section, and therefore appear black.

1. Photomicrograph of a transverse unstained ground section of the compact bone.
2. TS ground section of compact bone(magnified view).
3. Longitudinal section of compact bone at low magnification.
4. Canaliculi originating from lacunae.

Connective Tissue Proper

Classification Of Connective Tissue

1. Embryonic connective tissue
   • Mesenchyme
   • Mucous connective tissue(areolar tissue)
2. Connective tissue proper
   • Loose connective tissue(areolar tissue)
   • Adipose tissue
   • Reticular tissue
   • Dense connective tissue
     1. Irregular dense connective tissue
     2. Regular dense connective tissue
     3. Elastic connective tissue
3. Specialized connective tissue
   • Cartilage tissue
   • Bone tissue
   • BIood

In the previous chapter, we studied the basic components of general connective tissue, i.e., ground substance, fibers, and cells. All three components are present in all types of connective tissues.

• However, the proportions of various fibrous and amorphous components vary from one connective tissue type to the other.
• For example, dense connective tissue consists predominantly of fibers, but a very small amount of ground substance and few cells.
• On the other hand, loose connective tissue consists predominantly of ground substances and cells but very few fibers. Following is a brief description of various connective tissues.

Embryonic Connective Tissue

Embryonic connective tissue is classified into two subtypes, i.e., mesenchyme and mucous connective tissue.

Mesenchyme

It consists of small spindle-shaped cells with many processes. Processes of neighboring cells are in contact with each other through a gap junction.

Space between cells is occupied by ground substance and reticular fibers. Mesenchy mal cells are found in the embryo.

Mucous Connective Tissue

It consists of spindle-shaped cells with long and thin cytoplasmic processes. Large intercellular spaces are occupied by jelly-like matrix and thin collagen fibers.

The ground substance of the umbilical cord is also called Wharton’s jelly.

Loose Connective Tissue

Loose Connective Tissue Description

Loose or areolar connective tissue is the most widespread
of all the connective tissues. It consists of a loosely woven network of fibers (all types of fibers are present-collagen, elastic, and reticular).

• Almost all kinds of connective tissue cells(fibroblasts, macrophages, plasma cells, fat cells, white blood cells, and mast cells) are present.
• Fibers and cells are embedded in a semi-fluid ground substance that consists of hyaluronic acid and chondroitin sulfate.

Loose Connective Tissue Location

• It lies beneath the epithelium as lamina propria.
• It is present in superficial fascia along with the adipose tissue.
• It surrounds blood vessels, nerves, viscera muscle, etc.
• It is also present in the mesentery.
• It surrounds the parenchyma of glands.
• It lies below the mesothelium of body cavities.

Loose Connective Tissue Functions

It provides support to the epithelium. It acts as packing material around various structures and thus provides strength, support, and elasticity.

• There occurs the secretion of histamine by mast cells in inflammatory conditions. Histamine causes increased permeability of blood capillaries.
• This leads to a collection of excessive fluid in loose connective tissue resulting in swelling or edema.

Loose Connective Tissue Remember

Loose connective tissue consists of a loosely arranged network of fibers and almost all kinds of connective tissue cells embedded in a gel-like ground substance.

1. Low magnification photomicrograph of oesophagous.
2. Magnified view of loose connective tissue.

Adipose Tissue

Adipose tissue or fat can be classified into two types, i.c., white adipose tissue(unilocular fat) and brown adipose tissue (multilocular fat).

White Adipose Tissue Description

The color of adipose tissue(fat) is white or yellowish. This tissue is formed by the aggregation of adipocytes(fat cells). Fat cells arc polyhedral or oval.

• Their size is quite large(up to 100 pm), Within a fat cell there is the presence of a single large droplet of lipid (unilocular) that pushes the nucleus to one side and the cytoplasm is present in the form of a thin rim around it.
• Adipocytes are surrounded by the reticular fibers; while groups of adipocytes arc surrounded by connective tissue septa. Adipose tissue is rich in blood vessels. Adipose tissue stores the nutritional calories.

White Adipose Tissue Location

Adipose tissue is present in superficial fascia deep to the skin, bone marrow, omentum, mesentery, and around some viscera(heart, kidney, eyeball, etc).

White Adipose Tissue Functions

It serves as an energy reserve that supports and protects the structures, which surround and prevent heat loss through the skin.

1. Adipose cells are placed close to each other. These cells are separated by a small amount of connective tissue in which blood vessels run
2. An adipose(fat cell) appears as an empty cell with a flattened nucleus compressed in the peripheral rim of the cytoplasm
3. Multilocular adipose cells of brown adipose tissue
4. In this photomicrograph of adipose tissue(H and E) lipid contents of fat cells are extracted during tissue processing.

Brown Adipose Tissue

The brown adipose tissue of fat is found only at certain places in the newborn human body, i.e., the posterior triangle abdominal wall. Here, it prevents the excessive heat loss from the body surfacing.

The cells of brown fat differ from the white or yellowish fat in two ways, i.e., the nucleus is not flat but may be pushed to one side, and fat contained in the cytoplasm is in the form of small drops (Multilocular).

Brown Adipose Tissue Remember

Brown fat is not present in mature humans. It is present only in newborns and children and is gradually replaced by white fat (thus changing from multilocular droplets to unilocular droplets of fat within adipose cells).

Brown Adipose Tissue Functions

The function of the brown adipose tissue is to maintain normal body temperature in newborns. This is regulated by the thermogenin protein present in the mitochondria.

Reticular Connective Tissue

Reticular Connective Tissue Description

The reticular tissue consists of a mesh-like network of reticular fibers and reticular cells. Reticular fibers are type 3 collagen.

These cells tend to be star-shaped with radiating processes. Some consider that these cells are not distinct types but fibroblasts. Reticular tissue also consists of macrophages.

Reticular Connective Tissue Location

This tissue forms the architectural framework in the spleen, lymph nodes, liver, glands, bone marrow, reticular lamina of the basement membrane, and around smooth muscle cells.

Reticular Connective Tissue Functions

• It provides structural support to various organs by forming stroma or reticular framework.
• Reticular cells may be phagocytic.
• It binds smooth muscle cells.

Reticular Connective Tissue Remember

It is believed by some that reticular cells are not different from fibroblasts.

Dense Connective Tissue

Dense connective tissue contains a large amount of fibers and few cells as compared to loose connective tissue. This connective tissue may be subdivided into three types, i.e., dense irregular, dense regular, and elastic connective tissues.

Dense Irregular Connective Tissue Description

This tissue consists predominantly of collagen fibers. In this tissue, the bundles of collagen fibers are randomly(irregularly) oriented.

Fiber bundles are coarse and form a compact network with little spaces occupied by few connective tissue cells and little ground substance. Fibroblasts are present between bundles.

Dense Irregular Connective Tissue Location

It is present in the dermis of the skin, dura mater, epimysium, epineurium, pericardium, periosteum, tunica albuginea of testis, sclera, capsule of various organs, and submucosa of the intestinal tract.

Dense Irregular Connective Tissue Functions

It provides strength to the tissue.

Dense Regular Connective Tissue Description

This tissue also consists predominantly of collagen fibers (type 1). Bundles of collagen fibers are arranged regularly and parallel to the direction of stress, i.e., as in the case of tendons and aponeuroses.

Fibroblasts are present in rows between bundles of collagen fibers. Bundles of collagen fibers are so closely packed that very little space is left for ground substance and loose connective tissue through which run small blood vessels.

Dense Regular Connective Tissue Location

This tissue is present in tendons and aponeuroses of muscles and ligaments.

Dense Regular Connective Tissue Functions

Tendons, aponeuroses, and ligaments provide great resistance to pulling force but at the same time are flexible.

1. Longitudinal section of tendon showing bundles of collagen fibers arranged parallel to each other.
2. Transverse section of a tendon. Between the collagen bundles note the presence of loose connective tissue and blood vessels.

Elastic Tissue Description

Few elastic fibers are found in almost all types of connective tissues. However, in certain situations, elastic fibers are predominantly present.

• In these situations, connective tissue is called clastic connective tissue. This type of connective tissue consists of layers of freely branching elastic fibers.
• Fibroblasts and ground substances are present in small spaces intervening between fibers.

Elastic Tissue Location

Elastic tissue is found in the fascia of the anterior abdominal wall; a wall of the aorta and large arteries; trachea and bronchi; vocal cords; ligamentum nuchae; ligamentum flavum and in the suspensory ligament of the penis.

Elastic Tissue Functions

K provides elasticity to tissue.

Elastic Tissue Clinical Applications

• Obesity
  • Obesity in adults may result due to the accumulation of excessive fat in white adipose cells so that cells become larger than usual. Sometimes, an increase in the number of adipose cells may also occur.
• Lipoma
  • The most common adipose tissue tumor is called lipoma. They are usually found in the subcutaneous tissue of middle-aged persons. These are benign tumors (not cancerous).
• Liposarcoma
  • It is a malignant tumor of adipose tissue, which is quite rare.

Introduction to Connective Tissue Components

What is Connective Tissue?

Connective Tissue Definition:

Connective tissue is a term that is used for a widely dispersed group of tissues performing a variety of functions. As the name suggests, connective tissues serve as a connecting system binding, supporting, and strengthening all other tissues together.

• It protects and insulates internal organs by forming capsules, sheaths, or septas. Blood, a fluid connective tissue, acts as a major transport system within the body.
• Fat is stored in the superficial fascia, which is a major site for the storage of energy reserves and provides insulation against heat loss from the skin. Blood vessels and nerves enter or leave an organ by passing through the connective tissue only.

General Features Of Connective Tissue

Connective tissue consists of two basic components: cells and matrix. The extracellular matrix itself is made up of two components, i.e., fibers and ground substance.

• Cells in the connective tissue are widely separated from each other because of the presence of an abundant extracellular matrix. Usually, the matrix consists of a gel-like ground substance in which fibers are embedded.
• In some, connective tissue matrices may be fluid, semi-fluid, gelatinous, fibrous, or calcified. The fibers and ground substance (matrix) are synthesized by the cells of connective tissue.
• A tissue can be designated as connective tissue only when it has all three components, i.e., ground substance, fibers, and cells.

1. Ground Substance

The ground substance is colorless, sol to gel in consistency, It is highly hydrated. Fibers are embedded in it. The cells of the connective tissues are surrounded by it.

• Ground substance supports cells and provides a medium through which substances are exchanged between blood and cells. It is made up of complex molecules of polysaccharides and proteins.
• It contains many types of proteoglycans, multi-adhesive glycoproteins (laminin and fibronectin), and glycosaminoglycans (dermatan sulfate, keratan sulfate, hyaluronic acid, etc).

Ground Substance Remember

• A tissue can be designated as connective tissue only when it has all three components, i.e., ground substance, fibers, and cells.
• The extracellular matrix provides mechanical and structural support for tissue and helps in extracellular communication.

2. Fibers

Fibers in the matrix provide strength and support to the connective tissue. Three different types of fibers may be found in the matrix: collagen, elastic, and reticular.

Collagen Fibres

Collagen fibers are the main fibers of the connective tissue. They are found in abundance in bone, cartilage, tendon, and ligament. These fibers are strong, and inelastic but flexible (20 to 300 nm in diameter).

• Collagen fibers mostly occur in bundles which may branch and anastomose with neighbouring bundles. However, an individual fiber does not branch.
• A bundle measures about 0.5 to 1 0 pm in diameter and is of indefinite length. A collagen flber shows faint transverse striations indicating that it is made up of smaller subunits.
• Collagen fibers run in bundles, which may branch and anastomose with neighboring bundles.
• Elastic fibers run singly and not in bundles. They branch and join together to form a network.
• Reticular fibers are much thinner than collagen fibers and form a branching network.
• Chemically, collagen fibers are made up of protein collagen that in turn is made of tropocollagen molecules. They are synthesized by fibroblasts.

At present, 28 different types of collagen fibers have been identified. They are designated as type 1, type 2, type 3, type 4 to 28, etc.

Further Details

Distribution of commonly occurring collagen fibers in the body

Following is a brief description of the most commonly occurring types of collagen fibers.

Type 1 Collagen

It is the most common of all the collagen types (90% of body collagen is type 1). These fibers show classical cross striations and are of large diameter.

These fibers are found in dense and loose connective tissue, bone, tendon, fascia aponeuroses, ligaments, skin, cornea, and dentine.

Type 2 Collagen

It consists of thin fibers showing faint cross striations. These types of fibers are found in hyaline cartilage, the vitreous of the eye, nucleus pulposus of the intervertebral disc.

Type 3 Collagen

These types of collagen form reticular fibers (see reticular fibers). It is present in connective tissue of organs (spleen, lung, liver, etc).

Type 4 Collagen

This type of collagen forms meshwork in the basal lamina (lamina densa) of epithelia. It is also present in the kidney, glomeruli, and lens capsule.

Type 5 Collagen

It is present in the placenta and is associated with type 1 collagen.

Type 7 Collagen

Present in anchoring fibrils that attach the basal lamina to the lamina reticularis.

Types 1,2 and 3 collagen can be seen by light microscope, while the remaining collagen types are detectable only by the use of specific antibodies.

Synthesis of Collagen Fibres

Fibroblasts synthesize fibers and ground substance components of connective tissue. Besides fibroblasts, many other cells of the body also synthesize collagen fibers.

• These cells are mesenchymal cells, perineurial cells, cementoblasts, odontoblasts, cartilage cells, and smooth muscle cells. Type 4 collagen, which is found in basal lamina, is synthesized by epithelial cells.
• The synthesis of collagen fibers by fibroblasts takes place in two different steps: intracellular and extracellular.

Synthesis of Collagen Fibres Remember

Synthesis of collagen fibers occurs both inside and outside the fibroblast.

Intracellular Synthesis

• With the help of mRNA, amino acids are arranged sequentially to form alpha polypeptide chains.
• These chains are then transferred to the lumen of the rough endoplasmic reticulum.
• The polypeptide chains undergo the following modifications in the rough endoplasmic reticulum:
  • Proline and lysine amino acids of the chain are converted to hydroxyproline and hydroxylysine.
  • Hydroxylysine combines with sugar groups.
  • Three polypeptide chains now form a helix (triple helix). However, both the terminal ends of these chains remain uncoiled.
  • This molecule is now called procollagen. [Vitamin C is necessary for the formation of procollagen. Therefore, the deficiency of vitamin C leads to non-healing of wounds, and the formation of bone is impaired.]
• These procollagen molecules now move from the rough endoplasmic reticulum to the Golgi complex. There occurs the packaging of soluble procollagen in secretory vesicles.
• From here these molecules are secreted in the extracellular space through secretory vesicles.

Extracellular Synthesis

• Various enzymes act on the terminal uncoiled portion of procollagen molecules and cleave it from the rest of the coiled portion. These enzymes (procollagen peptidase) are secreted by fibroblasts themselves.
• After cleavage of the procollagen molecule, the remaining molecule is now called a collagen molecule or tropocollagen molecule.
• The tropocollagen molecules aggregate in an orderly manner to form the collagen fibrils.
• The collagen fibrils assemble to form microscopically visible collagen fiber.

Extracellular Synthesis Remember

Collagen is not only synthesized by fibroblasts but also by many other cells of the body, i.e., chondroblasts, osteoblasts, epithelial cells resting on the basement membrane, mesenchymal cells, perineurial cells, cementoblasts, odontoblasts, and smooth muscle cells.

Elastic Fibres

The diameter of elastic fibers is 0. 1 to 0.2 pm. They run singly and branch to form a network in loose areolar tissue but are present in bundles in ligamentum flava and ligamentum nuchae.

• Chemically, elastic fibers consist of a protein called elastin. The fiber is surrounded by a glycoprotein called fibrillin. Fibrillin plays an important role in organizing elastin into fibers.
• Absence of fibrillin results in the formation of elastin sheets or lamellae, instead of elastic fibers, as found in blood vessels.
• Elastic fibers are strong and can be stretched up to 150% of their relaxed length. Their elasticity is due to the protein elastin. These fibers are stained dark brown with orcein and Verhoeffs methods.
• They are synthesized by fibroblasts and smooth muscle cells of blood vessels. Elastic fibers are mainly found in skin, blood vessels, and lung tissue.

Elastic Fibres Remember

• In relaxed conditions, the molecules of elastin are coiled and cross-linked to each other to form a network. The stretching leads to the uncoiling of these molecules leading to an increase in the length of the elastic ligament or membrane or lamella, etc.
• When the stretching force is withdrawn, the elastin molecules return to a relaxed condition (coiled condition).

Elastic Fibres Clinical Applications

Marfan Syndrome

• It is a disorder, which is due to the abnormal development of elastic fibers. Marfan’s syndrome is inherited as autosomal dominant and is due to a mutation of the fibrillin gene(FBN1).
• Tissues rich in elastic fibers, i.e., large arteries, I periosteum (a membrane covering the bone), and the ligaments that suspend the lens of the eye are weakened.
• This results in blurred vision due to displacement of the lens. Long bones become abnormally large and there occurs the weakening of the wall of the aorta. The weakened aorta may suddenly rupture leading to sudden death.

Comparison of the properties of connective tissue fibers

Reticular Fibres

• These are fine delicate strands (20-40 nm in diameter), that form a supportive network for many tissues.
• They do not run in bundles.
• Chemically, they consist of collagen (type 3) and have a coating of glycoprotein.
• They can be stained black by the silver impregnation method. Because of their affinity to silver salts, these fibers are called argvrophilic. These fibers are also PAS-positive.
• These fibers are also synthesized by fibroblasts. The other cells, which produce reticular fibers are reticular cells in lymphatic tissues, Schwann cells in endoneurium, and smooth muscle cells in blood vessels and the intestine.
• Reticular fibers provide support and strength and form a supporting framework around fat cells, nerve fibers, and smooth muscles. They also form the framework of the spleen, lymph nodes, bone marrow, liver, and glands.
• These fibers also help to form the basement membrane.

Reticular Fibres Remember

Reticular fibers are arranged in the form of a network (meshwork), which is necessary to provide support to glandular and epithelioid tissues.

3. Cells of Connective Tissue

The cells of connective tissue are of two different types: fixed cells and free cells. The fixed cells are long-lived. These include fibroblasts, myofibroblasts, pericytes, fat cells, mast cells, and pigment cells.

The free cells arc short-lived wandering cells that are continually replaced by cells of the blood. These cells include eosinophils, neutrophils, monocytes, lymphocytes, mast cells, and plasma cells.

Fibroblasts

• They are the most numerous connective tissue cells and are present in almost all types of connective tissues.
• Fibroblasts are large, fiat, spindle-shaped cells that have many branching processes.
• Fibroblasts are responsible for the synthesis of extracted lularmatrix (secretion of ground substance and all types of connective tissue fibers).
• Inactive fibroblasts are called fibrocytes. Active fibroblasts have a large quantity of basophilic cytoplasm and euchromatic nuclei.
• They become very active during wound repair and synthesize collagen fibers. Here, they lie parallel to the long axis of the fibers.

Fibroblasts Remember

Fibroblasts are the most numerous connective tissue cells and are involved in the synthesis of all types of connective tissue fibers and ground substances.

1. Active fibroblast
2. Inactive fibrocyte
3. Fat cell
4. Mast cell
5. Plasma cell
6. Lymphocyte and
7. Macrophage.

Myofibroblast

The myofibroblast is a cell showing features of both fibroblast and smooth muscle cells. In appearance, these cells resemble fibroblasts but contain actin and myosin filaments in large amounts.

• The myofibroblasts possess contractile properties and behave like smooth muscle cells. However, a myofibroblast differs from a smooth muscle cell in that it is not surrounded by basal lamina.
• Their activity is responsible for wound closure after tissue injury due to the contraction of the wound. They are also found in periodontal ligaments probably helping in tooth eruption.

Pericytes

Pericytes surround the endothelial cells of capillaries and venules. They are surrounded by their basal lamina, which fuses with the basal lamina of endothelium.

Pericytes show features of both endothelial and smooth muscle cells. They contain actin and myosin filaments and, thus are involved in contraction.

Fat Cells

Fat cells (adipose cells) synthesize and store large quantities of lipids. A fat cell is spherical. The nucleus is flattened and displaced to one side.

• The lipids occupy almost the whole of the cell, pushing the cytoplasm as a thin rim around it. The cell may occur singly as in loose areolar tissue or they may occur in groups as in adipose tissue.
• Fat cells are stained orange with Sudan 3 stain. In H and E, stains fat is dissolved, so cells appear empty (un-stained). Their appearance is like a signet ring. Fat cells are fully differentiated and do not undergo cell division.

Pigment Cells

Pigment cells are capable of synthesizing pigment melanin which is a dark brown pigment. These cells are called melanocytes.

These cells are found in the skin, iris, some areas of the brain, and the choroid. These cells are star-shaped with branching processes. In the skin, they protect the tissue from the harmful effects of ultraviolet rays.

Macrophages (histiocytes)

Macrophages develop from monocytes of blood. Macroph ages are capable of eating bacteria and cellular debris by the process of phagocytosis. They have irregular shapes with short branching projections.

• They have an indented nucleus. Macrophages are of two types: fixed macrophages and wandering macrophages. When the need arises macrophages may fuse to form giant cells (Langhans cells).
• The Langhans cells are very large and may contain up to 100 nuclei. They can engulf large foreign bodies.

Mast Cells

Mast cells are found mostly close to the blood vessels in connective tissue. These cells are small and oval. The nucleus is centrally placed and the cytoplasm contains many granules.

• These cytoplasmic granules take purple to red color when stained with toluidine blue stain, while the nucleus is stained blue. This kind of color reaction is called metachromasia.
• These granules contain heparin which acts as an anticoagulant of blood. Mast cells also release histamine that in turn causes contraction of smooth muscles (mainly of bronchioles) and dilates small blood vessels as part of allergic reaction.
• Leukotricnc C released by mast cells causes bronchospasm. Mast cells also produce TNF-a (tumor necrosis factor-a), interleukins, growth factors, and prostaglandins.
• The release of chemicals from mast cells promotes allergic reactions (For Example., immediate hypersensitivity reactions or anaphylactic shock).

Mast Cells Remember

Mast cells contain numerous granules in their cytoplasm These granules contain heparin, histamine, prostaglandin din, and many other chemicals involved in the inflammatory process.

Plasma Cells

Plasma cells arise from B-lymphocytes. They synthesize antibodies against antigens. Thus these cells are found more in connective tissue at the time of infection. Plasma cells are ovoid.

• The cytoplasm is basophilic due to the presence of abundant RER. There is a clear area of cytoplasm near the nucleus for the Golgi complex. The nucleus is round and eccentrically placed.
• The chromatin pattern is unique, giving it a cartwheel appearance. All the above features indicate that plasma cells are actively involved in protein (antibody) synthesis. The toxic antigen may be neutralized when it combines with the respective antibody.

Plasma Cells Remember

Plasma cells originate from B lymphocytes and synthesize antibodies.

White Blood Cells

Three different types of white blood cells are seen in connective tissues, i.e., lymphocytes, monocytes, and eosinophils. Lymphocytes play an important role in the defense of the body against microorganisms (bacteria etc.).

Monocytes act as macrophages, while eosinophils play an important role in allergic reactions. Thus all these cells function towards the defense of the body.

Plasma Cells Clinical Applications

Mast Cells and Anaphylactic Shock

Some persons when exposed to an antigen (substances foreign to the body, For Example. ( bee venom after a bee sting, some food, or certain drugs) becomes oversensitive to the antigen.

• This means that they produce antibodies as allergic reactions against antigens. These antibodies remain attached to the surface of mast cells.
• When they are exposed to the same antigen (bee venom, food, or drug) for the second time the antigen-antibody reaction takes place on the surface of the mast cells.
• This triggers the release of the contents of mast cell granules (i.e., heparin, histamine, and other chemicals) within a few minutes after the bee sting or taking the drug.
• Histamine causes contraction of smooth muscles (mainly of the bronchiole leading to wheezing and difficulty in breathing); the dilatation of blood vessels leading to the swelling of the face and sudden fall in blood pressure.
• This condition is called an anaphylactic shock. Anaphylactic shock is a serious condition and if not treated urgently may lead to death.

Functions Of Connective Tissue

The following main functions can be attributed to the connective tissue:

• Support: Connective tissue provides support to the epithelium, For Example., lamina propria.
• Strength: It provides tensile strength to those areas that are subjected to mechanical stress, For Example., the dermis of the skin, ligament retinaculum, etc.
• Storage: Fat cells store fat while ground substances store water, ions, and inorganic materials.
• Transport: Water, ions, and inorganic materials are transported from blood to the various tissues of the body through a connective tissue matrix.
• Packing: Connective tissues act as packing material as they fill spaces, For Example., loose connective tissue and adipose tissue. Connective tissue also forms capsules around organs.
• Repair: Connective tissue helps in wound healing.
• Defense: The cells of the connective tissue (plasma cells, lymphocytes, macrophages, monocytes, and eosinophils) function toward the defense of the body.