14.6: Nervous Tunic
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The retina is the most complex of all the ocular tissues. The classic description of the retina includes 10 layers that, from the outside in, are as follows:
- Retinal pigment epithelium (RPE)
- Photoreceptor outer segments – Rods and Cones
- Outer limiting membrane
- Outer nuclear layer
- Outer plexiform layer
- Inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer
- Nerve fiber layer
- Inner limiting membrane
Embryologically, the retina is neuroectodermal in origin and forms first as an optic vesicle and later invaginates to form the optic cup. The posterior wall of the optic cup gives rise to the retinal pigment epithelium (RPE), while the anterior wall gives rise to the remaining 9 layers of the retina. The space between these layers that was present embryologically remains as a potential space and it is here, between the RPE and the sensory retina, that retinal detachments usually occur. In humans, the retina is mature at birth but, in many other species, the development of the retina continues postnatally. In the dog and cat, the retina is not mature until approximately 3-5 weeks centrally and 8-9 weeks peripherally.
Retinal pigmented epithelium (RPE)
The RPE is the homolog of the epithelium of the choroid plexus of the brain. It is a monolayer of hexagonal cells which is continuous anteriorly with the pigmented epithelial layer of the ciliary body and the anterior epithelium of the iris. External to the RPE is Bruch’s membrane and the choriocapillaris. The inner apical surface of the RPE has numerous long villous processes that surround the outer segment of the photoreceptors. There is no attachment between these cells, but an acid mucopolysaccharide ground substance exists between them. The lateral RPE cell margins, near the apical surface, have terminal bar attachments (zonula occludens, zonula adherens) which are one portion of the blood-retinal barrier. As the name suggests, the RPE contains melanin granules. These are found predominantly in the inner third of the cytoplasm. An important exception to this is seen in animals possessing a tapetum where the RPE overlying the tapetal portion of the choroid is non-pigmented.
The sensory retina is thickest near the optic nerve and thins towards the periphery. External to the sensory retina is the RPE and internally is the vitreous humor. The photoreceptors are in the outer portion of the sensory retina and can be divided into the cell body, inner segment, and outer segment. In addition, photoreceptors are divided into rods and cones based on their morphology, physiology, and sensitivity. As suggested by their name, the rods are long and slender, while the cones tend to have a shorter, wider appearance. The cones are further divided according to their spectral absorption characteristics into the red-sensitive (570 nm), green-sensitive (540 nm), and the blue-sensitive (440 nm) cones.
Photoreceptors (first neuron layer)
This is the outermost cells of the sensory retina and is also known as the rod and cone layer.
In this layer, a photochemical process takes place to convert light to an electrical impulse occurs in the outer segment. The nuclei of the photoreceptor cell forms the outer nuclear layer and the axons of the photoreceptors synapse with processes of the bipolar neurons in the outer plexiform layer
The rods are highly sensitive to light. They provide minimal detail and are inactive during daylight vision. The rods predominate in all portions of retina (except the fovea of birds and primates)
The cones are less sensitive to light. They provide more detailed vision, color vision and predominant only in the fovea.
Integrating neurons (second neuron layer)
The integrating neurons receive input from the photoreceptors and transmit it to the ganglion cells. The nuclei of these cells form the inner nuclear layer.
Ganglion cells (third neuron layer)
The ganglion cells receive input from the integrating neurons and transmit impulses to the brain. The axons of the ganglion cells form the nerve fiber layer and converge at the posterior pole of the eye to form the optic nerve
These are the supporting cells of the retina, however they do not participate directly in the electrical impulse transmission. The nuclei of Muller cells lie in the inner nuclear layer among the nuclei of the bipolar neurons
Retinal blood vessels
The microanatomy of retinal blood vessels varies according to species. Some species lack retinal blood vessels entirely. The retinal blood vessels are located in the nerve fiber layer and nourish the only inner third of the retina. The outer 2/3 of retina nourished by choroidal vasculature. The pattern of retinal vessels varies among species, but in all species retinal vessels originate from or directly adjacent to the optic nerve
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