4. Anatomy of the retina.

Anatomy of the retina

Retina is the inner most layer of the eye. Embryologically it is derived from neuroectoderm. Borders: Retina is a thin membrane extending from the optic disc posteriorly to the ora serrata anteriorly, terminating at the epithelium of the ciliary body. Externally it’s covered by choroid and internally by the vitreous fluid.

STRUCTURE

It is composed of two main layers with a potential space in between the layers:

1. a) The outer retinal pigment epithelium which absorbs the light
2. b) The inner neural layer containing sensory rod and cone cells, which detect different frequencies of light and send this information to the CNS through the optic tract for an image to be formed.
3. c) Thickness from 0.4 mm near the optic nerve to 0.15 mm anteriorly at the ora serrata.
4. d) Other prominent structures include the optic disc, Ora Serrata, Macula, and Fovea.

• Optic Disc: Part of retina with no sensory cells and therefore called the blind spot. It is made up of the bundle of nerve fibres from the retina’s sensory cells which penetrate through lamina cribrosa to form the optic tract.
• Ora Serrata: It is the anterior termination of the retina where it is continuous with the epithelium of the ciliary body.
• Macula Lutea (Yellow Spot): It is an area 1.5 mm in diameter situated at the posterior pole, about 3 mm lateral to the optic disc.
• Fovea Centralis: It is a small depression in the center of the macula. The cones predominate in this area. The fovea is the most sensitive part of the retina, where the sharpest image is formed.

Histologically, retina consists of ten layers namely:

1. Layer of pigment epithelium: A single layer of hexagonal cells containing melanin pigment is situated on the outer aspect of retina.
2. Layer of rods and cones
3. External limiting membrane: Lies between rods and cones and outer nuclear layers.
4. Outer nuclear layer: Consists of nuclei of rods and cones.
5. Outer plexiform layer: Consists of the axons of rods and cones nuclei with the dendrites of the bipolar cells.
6. Inner nuclear layer: It consists of nuclei of bipolar cells.
7. Inner plexiform layer: It consists of synapses of the axons of the bipolar cells with the dendrites of the ganglion cells.
8. Layer of ganglion cells: Large ganglion cells are present in this layer.
9. Nerve fibre layer: These are axons of the ganglion cells. These fibres are non-medullated and are continued as optic nerve fibres.