The Retina of eye or Nervous tunic forms the innermost tunic of the eyeball and extends from the entrance of the optic nerve to the iris. It consists of three parts; a posterior part the essential region containing the special neuroepithelium, the pars optica retinae. This terminates around the ciliary body where it ends in a regular circular line, the ora ciliaris retinae (ora serrata in man). Hereafter, the retina loses its nervous elements. The part of the retina over the ciliary body is the pars ciliaris retinae and that over the iris is the pars iridica retinae. Pars iridica retinae are a layer of pigmented cells that covers the posterior surface of the iris. The area centralis retinae is a rounded spot situated above the optic papilla and corresponds to the macula lutea of man. The entrance of optic nerve forms a definite oval area -the optic papilla or blind spot. The optic nerve fibres converge from all parts of the pars optica to the papilla, where they collect into bundles that traverse the lamina cribrosa of the chorioid and sclera, and constitute the optic nerve.
The pars optica retinae is that part of the eye which transforms the stimulus of light into nerve impulses, resulting in the sensation of vision. Embryologically, the retina arises from the two layers of ectoderm that form the optic cup. The outer layer gives rise to the pigment epithelium; the inner layer forms the remainder of the retina. In pathological detachment of the retina, or as generally occurs in the fixed and preserved specimens, the primary embryonic cavity between the two layers become reformed and the two parts of the retina gets separated, the pigment epithelium remaining adherent to the chorioid. The strictly nervous portion is firmly attached to the underlying structures at only two regions, at the ora ciliaris retinae and at the optic disc. The retina detached from the pigment epithelium is a thin, delicate membrane, which in life is transparent.
STRUCTURE OF RETINA
- Structure: Histologically the retina consists of ten layers, except at the optic disc, optic papilla, the area centralis and extreme periphery. These layers from without inward are:
- Pigment epithelium.
- Layer of rods and cones.
- External limiting membrane.
- Outer plexiform (molecular or synaptic) layer.
- Inner nuclear (granular) layer.
- Inner plexiform (molecular or synaptic) layer.
- Ganglion cell layer.
- Nerve fibre layer.
- Internal limiting membrane.
- The pigment epithelium is a single layer of nearly cuboidal cells, broad and hexagonal in surface view. The outer portion of the cell contains a spherical nucleus with a moderate amount of rounded pigment granules. The inner portion filled with dark needle shaped pigment granules, which also extent into fine cytoplasmic processes arising from the inner surface of the cell and reaching down between the outer membranes of the visual cells. The melanin pigment here is a known as fuscin.
- The nature and significance of the remaining layers of the retina will be understood if it is realized that the stratification depends upon the location of three sets of neurons and their relation to each other. These neurons, together with certain supportive elements of ectodermal origin make up the bulk of the retina. Considering them in the order of their conduction of nerve impulse, we find that the first neuron is represented by the photoreceptors, the rod and cone cells. These form not only the layer of that name but also the outer nuclear layer that consists of the nuclei and much reduced cell bodies of the neurons. From each cell a fibre is continued inward in the outer plexiform layer, where it makes a synaptic junction with the dendrite of a second neuron, the bipolar cell. The nuclei of the bipolar cells lie in the inner nuclear layer; their axons pass inward to make up the inner plexiform layer, where they effect synapse with the dendrites of the ganglion cells. The relatively large cell bodies of the latter form the ganglion cell layer; their long axons course in the nerve fibre layer to the optic disc, where all the axons converge to form the optic nerve. Thus the retina consists mainly of layers of nerve cells alternating regularly with layers formed by their processes. It is further evident that the true receptors, the rods and cones lie further most removed and turned away from the light stimulus, which is to affect them must pass through all the intervening layers (except at the centre of area centralis retinae). Thus the retina is an upside-down organ. The inverted retina is characteristic of all vertebrates.
- In addition to nerve cells certain supportive elements, the supporting fibres of Muller, contribute to the formation of several layers.
- Layer of rods and cones lies between the external limiting membrane and the pigment epithelium, facing the latter. The rods and cones or visual cells are the outer light sensitive portions of this layer arranged in parallel fashion, perpendicular to the surface.
- Rods: These are long and slender. Each rod shows an outer and an inner segment. The inner segment is slightly thicker. From the inner end of the inner segment, a slender filament passes as the rod fibre through the external limiting membrane to the outer nuclear layer. Here it enlarges to accommodate the nucleus and is then continued as a fibre into the outer plexiform layer. The retina from the dark-adapted eye appears purplish red. This colour is due to rhodopsin or visual purple in the outer segments of the rod. This visual purple bleaches in light and regenerates in the dark. Rods function under conditions of low light intensities.
- Cones: The cones are bottle or flask shaped cells, which consist of inner and outer segments. The outer segment is small and conical. The inner segment has a bulbous form. The inner segment is continuous with the cone fibre that passes through the external limiting membrane. Just under this membrane the fibre enlarges to include the nucleus and is then continued as a slightly short fibre into the outer plexiform layer. Cones function under high intensities of illumination and are responsible for colour vision.
- External limiting membrane: This is a thin sieve-like membrane formed by the Muller’s fibers.
- Outer nuclear layer: It appears to consist solely of the nuclei of the visual cells. However, a delicate mantle of cytoplasm can be made out around each nucleus. The rod nuclei are smaller, round in shape and stain intensely. The cone nuclei are slightly larger, oval in shape and stain less intensely. The cone nuclei are situated close to the external limiting membrane.
- Outer plexiform layer: It is consists of the meshwork formed by the terminal fibres of rods and cones and the arborizations of the dendrites of the bipolar cells. The fibres of several rods are here related to the dendrite ending of one bipolar cell. In the case of cones, however a single cone fibre is associated with a single bipolar cell.
- Inner nuclear layer: It is thinner than the outer nuclear layer but is similar in appearance. It contains the nuclei of bipolar cells, certain association neurons and the nuclei of Muller’s fibre. The association neurons are the horizontal and amacrine cells; they serve to interconnect the various regions of the retina. The nuclei of this layer arranged in three zones: an outer one or horizontal cell nuclei, middle one or bipolar cell nuclei and an inner one in which the nuclei of amacrine cells predominates.
- Inner plexiform layer: This consists of the processes of amacrine cells, the axons of bipolar cells and the profusely branched dendrites of ganglion cells.
- Ganglion cell layer: It is composed of single layer of typical multipolar ganglion cells, among which are scattered neuroglia cells. Branches of regional blood vessels are also seen.
- Nerve fibre layer: It consists of the axons of ganglion cells. The non-medullated fibres are arranged in bundles, which run parallel to the surface of the retina and converge at the optic papilla to form the optic nerve. Between these bundles, rows of Muller’s fibres and retinal blood vessels also pass.
- Internal limiting membrane: It is a thin membrane of hyaline nature, formed by the opposition of expanded bases of Muller’s fibres. It separates the retina from the vitreous body.
- Blood supply: Central artery of retina
