Retina, RPE & Choroid Anatomy

Overview

The retina is a stratified, ten-layer neural tissue sandwiched between the vitreous and the choroid. The choroid beneath it is a distinct, three-tiered vascular structure in its own right, not a single uniform layer. The outer retina, including the metabolically demanding photoreceptors, has no blood supply of its own; it depends entirely on diffusion across Bruch's membrane from the choroid's innermost vascular layer. That dependency is why so many chorioretinal diseases, age-related macular degeneration among them, involve both the RPE and the choroid together rather than the retina in isolation. This page covers layer structure and blood supply anatomy. Phototransduction mechanism itself is covered separately, on the phototransduction and photoreceptor physiology page.

Retinal Layers

The ten layers are numbered here from outermost, against Bruch's membrane and the choroid, to innermost, against the vitreous. Light passes through the transparent inner layers first to reach the photoreceptors in layer 2. The visual signal then travels back inward, layer by layer, and leaves the eye as ganglion cell axons in layer 9.

1. Retinal pigment epithelium (RPE): a single layer of pigmented cells on Bruch's membrane. It phagocytoses photoreceptor outer segments and recycles retinal. The other nine layers together form the neurosensory retina, so a retinal detachment separates layer 1 from layer 2.
2. Photoreceptor layer: the outer and inner segments of the rods and cones, where light is absorbed. The ellipsoid zone seen on OCT lies in this layer.
3. External limiting membrane: a row of junctions between photoreceptor inner segments and Müller cell processes rather than a true membrane. Its integrity on OCT, alongside that of the ellipsoid zone, predicts visual recovery after successful macular hole surgery.
4. Outer nuclear layer: the cell bodies of the rods and cones.
5. Outer plexiform layer: photoreceptors synapse here onto bipolar and horizontal cells.
6. Inner nuclear layer: the cell bodies of bipolar, horizontal, and amacrine cells, the interneurons that relay and modulate the signal as it moves inward.
7. Inner plexiform layer: bipolar cell axons synapse here onto ganglion cell dendrites, with amacrine cells adding lateral connections.
8. Ganglion cell layer: the cell bodies of the ganglion cells.
9. Nerve fiber layer: unmyelinated ganglion cell axons converging on the optic disc, where they exit through the lamina cribrosa and become the optic nerve, myelinated only once past the globe.
10. Inner limiting membrane: not a cellular layer but a basement membrane, built from the footplates of Müller cells and the basal lamina they secrete. It is the true boundary between neural retina and vitreous. During macular hole and epiretinal membrane surgery, vital dyes such as brilliant blue G stain it so it can be identified and peeled.

The names follow a pattern that makes the order easier to hold. A "nuclear" layer holds cell bodies and a "plexiform" layer holds synapses, and moving inward they alternate: outer nuclear, outer plexiform, inner nuclear, inner plexiform, ganglion cell bodies. Each plexiform layer is the synapse between the cell bodies on either side of it.

Macula and Fovea

The macula is the retina's central, high-acuity region. The fovea, a shallow depression within it, carries the highest concentration of cone photoreceptors anywhere in the retina, which is what makes it responsible for maximal visual acuity and for reading and fine detail. The foveola, at the very center of the fovea, is smaller still and carries the single highest cone density of all.

At the very center of the fovea, retinal capillaries stop altogether, leaving a capillary-free foveal avascular zone that depends entirely on the choriocapillaris for oxygen. OCT angiography visualizes the zone directly, and clinicians track its size to flag capillary dropout in diabetic retinopathy.

Ora Serrata and Peripheral Retina

The ora serrata is the serrated anterior termination of the neurosensory retina, marking the junction where the multilayered sensory retina gives way to the simple, bilayered non-photoreceptive epithelium of the pars plana. Embryologically this is where the inner layer of the optic cup changes from neural retina to the non-neural epithelium that continues forward over the ciliary body and iris (see the posterior segment and optic nerve embryology page). Clinically, retinal tears cluster at the posterior edge of the vitreous base, which straddles the ora serrata, because vitreous traction concentrates there. Lattice degeneration typically lies between the equator and the posterior border of the vitreous base, not at the ora itself.

Retinal Blood Supply and the Choroid

Two circulations feed the retina, one from each side. The inner two-thirds receives its blood from the central retinal artery, a branch of the ophthalmic artery. The outer third, the photoreceptor layer and RPE, gets none of it and depends entirely on the choroid beneath it.

Moving outward from Bruch's membrane, the choroid has three vascular layers. The choriocapillaris is the innermost, fenestrated capillary layer, sitting immediately outside Bruch's membrane; its fenestrations let it nourish the avascular outer retina, especially the photoreceptors and RPE, by diffusion alone. External to that is Sattler's layer, built of medium-caliber vessels, and outermost is Haller's layer, built of the largest-caliber vessels. The progression from small vessels near the tissue to large feeding and draining vessels farther out is the same organizing principle capillary beds follow throughout the body.

External to Haller's layer, the choroid meets the sclera at the suprachoroidal space. In a healthy eye this is only a potential space: intraocular pressure and fine connective attachments keep the choroid and sclera pressed together, leaving no actual gap. Fluid or blood can force the two tissues apart. Serous choroidal effusions collect there after a sudden drop in eye pressure, while suprachoroidal hemorrhage arises intraoperatively or postoperatively from torn short or long posterior ciliary vessels, a rare but sight-threatening surgical complication.

Arterial blood enters through the short posterior ciliary arteries, which penetrate the sclera around the optic nerve. Through the peripapillary anastomosis known as the circle of Zinn-Haller, that same region supplies both the choroid and the optic nerve head. Optic nerve head perfusion and choroidal circulation are therefore usually discussed together, not as separate vascular territories.

Venous drainage leaves through four vortex veins that pierce the sclera posterior to the equator, one per quadrant. Each collects blood from the choriocapillaris, Sattler's layer, and Haller's layer together. The superior pair drains into the superior ophthalmic vein, and the inferior pair into the inferior ophthalmic vein. Surgeons working near the equator, during a scleral buckle or oblique muscle surgery, stay alert to their location so they don't disrupt choroidal venous outflow.

Blood-Retinal Barrier and Müller Cells

The blood-retinal barrier mirrors those two circulations, so it sits at two tissue planes. The inner blood-retinal barrier is formed by tight junctions of the retinal capillary endothelium, the vessels within the retina itself. The outer blood-retinal barrier is formed by tight junctions (zonula occludens) joining adjacent RPE cells, which regulate fluid and solute exchange between the fenestrated choriocapillaris and the retina.

Müller cells are the retina's principal glial cell type, spanning its full thickness from the inner to the outer limiting membrane. Their job is structural and metabolic: maintaining ion and water homeostasis and supporting retinal neurons, not phagocytosis. Outer segment phagocytosis belongs to the RPE alone, a distinction worth keeping straight since both cell types sit in close contact with the photoreceptors.

High-Yield Pearls

• Photoreceptor nuclei live in the outer nuclear layer; their synapses onto bipolar/horizontal cells are in the outer plexiform layer.
• Fovea = highest cone density overall; foveola = the single highest-density point within it.
• Inner blood-retinal barrier = retinal capillary endothelium; outer blood-retinal barrier = RPE tight junctions.
• Inner 2/3 of retina: central retinal artery. Outer 1/3 (photoreceptors, RPE): choroid, by diffusion only.
• Ora serrata = anterior retinal termination. Tears cluster at the posterior edge of the vitreous base; lattice typically lies between the equator and that edge.

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