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histological diagram of sclera choroid and retina

histological diagram of sclera choroid and retina


It is a thick and tough layer of Irregular, tough, dense and white fibrous tissue (thus It is opaque) and also contains few elastic fibres. The matrix is non-homogeneous (possessing varying refractive indices) containing greater amount of water. It is composed of five distinct layers from without inwards as follows.
1.    The external surface of sclera is called eplsclera. It Is formed of loose C.T. It consists of loose collagenous and elastic fibers and blood vessels. It provides attachment to the extraocular muscles. It is covered by conjunctive.
2.   Tenon's capsule (fascia bulbl): A dense layer of C.T.
3.    Tenon's space: it Is a loose space at which the movements of eyeball can take place easily.
4.    Sclera proper: It consists of dense framework of flat bundles of collagen fibers,
which are not only arranged parallel but also Intersect each other. Some elastic fibres, ground substance and elongated fibroblasts are also seen. It Is poorly supplied with blood capillaries.
5.    Lamina Fusca (Suprachoroldal lamina): It consists of small collagenous bundles of loose C.T., numerous elastic fibers and melanophores. Melanophores are connective tissue cells in cytoplasm of which black granules of melanin pigment are seen.

It is black in color and is formed of following structures arranged from without inwards.
1.   SUPRACHOROIDAL LAYER: it is formed of C.T. layers containing melanocytes, fibrocytes, % macrophages, lymphocytes, mast cells, plasma cells, collagen fibres, ganglion cells and nerve fibres. It is attached to sclera by elastic fibres.
2.    VASCULAR LAYER: It is highly vascular containing tortuous choroidal arteries and veins and has same structure as suprachoroidal layer.
3.    CHORIO CAPILLARY LAYER: It Is a single layer of fenestrated blood capillaries in a
stroma of fine collagen and elastic fibres.
4.    bruch'S membrane: Briefly, It is formed of two basement membranes sandwiched by collagen and elastic fibres. It is comprised of five layers from without inwards as follows.
a.   Basal lamina of capillaries of chorlocapillary layer
b.   Layer of collagen fibres
c.   Layer of elastic fibres
d.    Layer of collagen fibres
e.    Basal lamina of pigment epithelium of retina







RETINA

 
Retina is composed of ten layers from without inwards:
1.      pigmented epithelium: It is formed of simple cubical or columnar pigmented epithelium (rich in melanin) the cells contain basal rounded nuclei, smooth endoplasmic reticulum and many mitochondria. The basal regions adhere firmly to the Bruch's membrane and present many basal invaginations. The apical surface of cells bear microvilli which interdigitate with apices of rods and cones.
2.   LAYER OF RODS AND CONES: Each rod and cone is formed of: (a) an outer segment; (b) an inner segment; (c) nuclear region and; (d) synaptic region. Rods and cones layer contains outer and inner segments of both rods and cones.
3.    external limiting membrane: It is a dark line, which contains junctional complexes between glial cells of Muller and the bases of rods and cones.
4.   OUTER NUCLEAR LAYER: It contains cell bodies (nuclear regions of rods and cones) The
nuclear regions of cones are present nearer to the outer limiting membrane in the form of a single line while cell bodies of rods are present in the whole thickness of this layer.
5.    OUTER PLEXIFORM LAYER: This contains the synapses between the synaptic processes of rods and cones and the dendrites of bipolar cells and horizontal cells.
6.    INNER NUCLEAR LAYER: it contains the cell bodies of bipolar cells, horizontal cells, amacrlne cells and Muller cells.
Bipolar neurons: Each cell has.- (a) a single dendrite which synapses with the synaptic processes of rods and cones in outer plexiform layer and; (b) an axon which synapses with dendrites of ganglion cells.
Horizontal cells: These are branched cells which interconnect the synaptic terminals of
rods and cones
Amacrlne cells: These are the multipolar anaxonic cells, which establish connections between axons of bipolar neurons and dendrites of ganglion cells.
Muller Cells: These are the large and extensively ramified neuroglial cells whose processes extend between outer and Inner limiting membranes. In addition astrocytes and microglia are also present.

7.    INNER PLEXIFORM LAYER: It contains synapses between axons of bipolar neurons and dendrites of ganglion cells. It also contains the synapses of amacrine cells.
8.      LAYER OF GANGLION CELLS: It contains ganglion cells. Each cell has a vesicular euchromatic nucleus and highly basophilic cytoplasm due to abundant Nlssl substance. Numerous retinal vessels are present here. Their dendrites synapse with the axons of bipolar neurons while axons form the fibres of optic nerve.
9.   optic nerve fibers layer: These are the axons of ganglion cells, which pass at right angle to form the optic nerve, which are not covered by myelin sheath initially but acquire myelination after having traversed the sclera.
10.     internal limiting membrane: It is a dark line formed by terminal processes of Muller cells.
Two areas on the retinal surface (fovea centralis and optic disc) have indifferent structure than the typical ten-layered structure of the retina and are given below.
fovea centralis: It is a shallow central depression of the macula lutea (yellow spot) on retina. It provides maximum visual acuity. Here, the retina Is very thin. The histology of this area differs from the rest of retina as follows*
      There is absence of retinal vessels
      No rods are seen. Only cones are present

      Presence of only three layers instead of typical ten layers. These are: (a) cones layer (elongated cones); (b) outer limiting membrane and; (c) outer nuclear layer.
      The bipolar and ganglion cells are displaced to the periphery of fovea.
optic disc: It is a blind spot due to absolute absence of both rods and cones. It consists of only unmyelinated optic nerve fibres and blood vessels.


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