Anatomy of the Retina

Othman Al-Abbadi, M.D

Introduction • It’s the internal layer of eyeball.• A thin transparent membrane having a purplish-red color.• Thickness>> 0.56mm near the optic disc to 0.1mm at the

ora serrata… while it’s thinnest at the fovea centralis.• Continuous posteriorly with the optic nerve, while

anteriorly with the ciliary body & iris epithelium.• Bordered by the vitreous internally & the Bruch’s

membrane externally.• Firmly attached at the margins of optic disc and at its

anterior termination at the ora serrata .• Approximate landmark on outside of the eye; medially

insertion of medial rectus, laterally insertion of lateral rectus (closer to the limbus nasally)

• It consists of an : - outer retinal pigment epithelium. (RPE) (derived from outer layer of optic cup). - inner neurosensory layer. (derived from inner layer of optic cup).• Macula lutea: an oval ,yellowish area ,at the center of

the posterior part of retina… It has central depression called the fovea centralis.• The optic disc: a 3-mm depression of the retina where

the optic nerve leaves… pierced by the central retinal artery & vein… Blind spot.

Retinal pigment epithelium

• Hexagonally arranged single layer of narrow & tall cells from the margin of the optic nerve flattening gradually towards the ora serrata.• The basal end of each cell is infolded and rests on a

B.M which forms a part of the Bruch's membrane.• Apically shows microvili (5-7um long ), which

project between & adhesively bind with rods & cones but with no specialized attachments.• The adjacent cell membrane are bound together in

basal region by zonula adherens, and in apical region by zonula occludens.

Melanin granules

• Functions: - Absorption of light. - Turnover of the outer segments of the rods & cones &

formation of rhodopsin & iodopsin by vitamin A. - Prevention of scattering by absorption of reflected light.

Keep in mind that the remainder of the optic vesicle is still a potential space that can separate the RPE from the retina resulting in Detached Retina

Neural RetinaPhoto receptors*Rods: - 110-125 milion . - denser at the periphery (absent at

fovea) - responsible for vision in dim light

producing images of varying shades of black & white.

* Cones: - number about 6.3-6.8milion . - highest density at the fovea - responsible for color vision in bright light.

Bipolar cells

• First order neuron in visual pathway.• one or more dendrites pass outward to synapse with

photoreceptor cell terminal .• The single axon is directed inward to synape with

ganglion cell and amacrine cells.• Types: - Rod bipolar cells: connecting rods to ganglion cells, - Flat or diffuse bipolar cell: connect cones to ganglion

cells, - Midget bipolar cell: connect a single cone cell to a single

midget ganglion cell.

Ganglion cells

• Second order neuron in visual pathway.• Only 100 million ganglion cells in the retina; meaning

more than 100 photoreceptors per ganglion cell.• Single layer in periphery and increase towards the

macula while completely absent at the fovea.• Nonmyelinated intraocular & become myelinated just

after leaving the lamina cribrosa.• The myelin sheath is formed by oligodendrocytes.• If intraocular part gets myelinated, it results in a blind

spot.

• Horizontal cell:• They have one long & several short processes.• Inhibitory function by releasing GABA.• Have a role in sharpening contrast.

Amacrine cells

• No axons.• Stimulated by the Bipolar cells & then they excite

the Ganglion cells.

Muller cells

• A supporting cell.

Layers of the Retina

1. RPE2. Rods & Cones3. External limiting membrane .4. Outer nuclear layer.5. Outer plexiform layer.6. Inner nuclear layer.7. Inner plexiform layer.8. Ganglion cell layer .9. Nerve fiber layer.10. Internal limiting membrane.

Blood supply

• Dual supply:• The outer laminae (down to the outer nuclear) by the

choroidal capillaries• The inner laminae by the cental retinal artery & vein

• The integrity of the retina depends on both of these circulations, neither of which alone is sufficient & there’s No anastomosis.