Anatomy and Physiology of the Retina

PARS PLANA

The pars plana is the flat posterior aspect of the ciliary body, extending between the pars plicata of the ciliary body and the ora serrata of the neurosensory retina. The pars plana is located 3.5 mm from the limbus and is 3.5 to 4.5 mm in width. The pars plana consists of a double layer of epithelial cells: the inner, nonpigmented epithelium, which is contiguous with neurosensory retina; and the outer, pigmented epithelium, which is contiguous with the retinal pigment epithelium. The apices of the nonpigmented and pigmented epithelium are fused by tight junctions. The pars plana provides surgical access to the vitreous and retina.

ORA SERRATA

The ora serrata is the anterior border of the neurosensory retina. Topographically, it corresponds to the insertion of the medial and lateral rectus muscles. It has a smooth appearance temporally but is serrated nasally. Ora bays are rounded extensions of the pars plana whereas dentate processes are “teeth-like” extensions of neurosensory retina. There are approximately 20 to 30 dentate processes per eye.

MACULA

The macula is recognized clinically as the region within the temporal vascular arcades. Histologically, the macula is defined as having the following: two or more layers of ganglion cell nuclei; the presence of xanthophyll pigment; and taller, more pigmented retinal pigment epithelial cells.

FOVEA, FOVEOLA, AND UMBO

The fovea is a 1500- m area (roughly the size of the optic disc) located slightly inferior and temporal to the optic disc. It has a concave surface due to displacement of the inner retinal layers including the nerve fiber layer, ganglion cell layer, inner plexiform layer, and inner nuclear layer. The central 500 m of the fovea is devoid of blood vessels (foveal avascular zone [FAZ ]). Within the fovea there is a central depression called the foveola. The foveola contains only specialized cones and Müller cells. The center of the foveola is known as the umbo.

The clinical macula is the region within the temporal vascular arcades. Histologically, the macula is characterized by multiple ganglion cell nuclei, xanthophyll pigment, and taller, more pigmented retinal pigment epithelium.

The foveal avascular zone is the central 500-µm area of the fovea devoid of blood vessels. Note the relative hypofluorescence of the central macula.

NEUROSENSORY RETINA

The neurosensory retina is derived from neuroectoderm. The nine layers of the neurosensory retina include the internal limiting membrane (ILM), nerve fiber layer (NFL), ganglion cell layer (GCL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer (ONL), external limiting membrane (OLM), and photoreceptors.

The neurosensory retina includes three types of cells: neurons, glial cells, and vascular cells. The neurons include the photoreceptors, bipolar cells, and ganglion cells for vertical integration of electrical signals from light stimulation. Horizontal and amacrine cells provide horizontal integration and coordination between the other neurons. Glial cells—consisting of Müller cells, astrocytes, and microglia—provide metabolic support for blood vessels and neurons. They play a major role in regulation of the extracellular environment of the retina. Müller cells traverse the entire retinal thickness and contribute to the internal and external limiting membranes. Astrocytes invest blood vessels and neurons in the ganglion cell layer and microglia act as resident macrophages within the retina. The vascular cells include the retinal vascular endothelial cells.

PHOTORECEPTORS

The photoreceptors are specialized neuroepithelial cells derived from neuroectoderm. The two types of photoreceptors are rods and cones. There are approximately 100 million rods and 5 million cones (rod to cone ratio equals 20:1). Rods mediate dim-light vision and have great sensitivity. Cones function in bright light and are responsible for color vision and pattern recognition. Cone density is maximal in the fovea and decreases across the macula and peripheral retina. Conversely, rod density is greater in the peripheral retina and decreases in the macula. It is important to note that although the cone density is greatest in the fovea, the macula is still rod dominated due to the high ratio of rods to cones.

The inner segments of the photoreceptors contain metabolic machinery whereas the outer segments contain visual pigment. Light energy is converted into electrical signals in the outer segments. In cones, the outer segment discs are attached to the cell membrane. In rods, the outer segment discs are arranged like a “stack of coins” with no attachment to the cell membrane.

RETINAL PIGMENT EPITHELIUM

The retinal pigment epithelium (RPE) consists of a monolayer of cuboidal-shaped cells derived from neuroectoderm. The RPE extends from the margin of the optic disc to the ora serrata where it is contiguous with the pigmented epithelium of pars plana. The RPE has several features: tight junctional complexes including zonula occludens and zonula adherens; apical microvilli; basement membrane infoldings; melanin granules; and phagosomes. The functions of the RPE include the outer blood-retinal barrier; promotion of retinal adhesion; synthesis of extracellular matrix; degradation of photoreceptor outer segments; retinol uptake and transport; absorption of scattered light; supporting the nutritional requirements of outer retina; and response to disease (atrophy, hyperplasia).

RETINAL BLOOD FLOW

Blood travels from the internal carotid artery through the ophthalmic artery before reaching the central retinal artery. Blood traveling through the central retinal artery and its branches supplies oxygen and nutrition to the inner retinal layers from the nerve fiber layer to the inner third of the inner nuclear layer. The retinal vessels lack internal elastic lamina and smooth muscle cells. The retinal vascular endothelial cells are nonfenestrated and contain tight junctions. Pericytes surround the endothelial cells and contribute to the inner blood-retinal barrier. Cilioretinal arteries derived from the ciliary circulation are present in approximately 20% of individuals.

Schematic drawing of the nine layers of the neurosensory retina, retinal pigment epithelium, Bruch’s membrane, choroid, and sclera.

Cone

Pedicle Myoid

Spherule Ellipsoid Cillium

Schematic drawing of the fovea. The normal foveal depression results from displacement of the inner retinal layers including the nerve fiber layer, ganglion cell layer, inner plexiform layer, and inner nuclear layers.

Microvilli

Apex

Base

Choriocapillaris

The ratio of rods to cones is 20:1. In cones, the outer segment discs are attached to the cell membrane whereas in rods the discs are arranged like a “stack of coins.”

The retinal pigment epithelium is a monolayer of cuboidal-shaped cells derived from neuroectoderm.

The retinal vascular endothelial cells are nonfenestrated with tight junctions. This forms the inner blood-retinal barrier. Note that fluorescein does not leak from normal retinal vessels.

Approximately 20% of individuals have a cilioretinal artery. Cilioretinal arteries arise from the ciliary circulation and supply various portions of the peripapillary retina and macula.