CHAPTER 11
Retina and Retinal Pigment Epithelium
Topography
The retina and the retinal pigment epithelium (RPE) make up 2 distinct layers that together line the inner two-thirds of the globe:
1.The RPE is a pigmented layer derived from the outer layer of the optic cup.
2.The neurosensory retina is a delicate, transparent layer derived from the inner layer of the optic cup.
Anteriorly, the RPE becomes continuous with the pigmented epithelium of the ciliary body, and the retina becomes continuous with the nonpigmented ciliary body epithelium. Posteriorly, the RPE terminates at the optic nerve, just prior to the termination of the Bruch membrane. The nuclear, photoreceptor, and synaptic layers of the retina gradually taper at the optic nerve head, and only the nerve fiber layer (NFL) continues on to form the optic nerve. See BCSC Section 12, Retina and Vitreous, for additional discussion.
Neurosensory Retina
The topographic variation in the structures of the retina is striking, with regional variation in the neural structures as well as the retinal vasculature. The neurosensory retina has 9 layers (Fig 11-1). Beginning on the vitreous side and progressing to the choroidal side, they are
1.internal limiting membrane (ILM; a true basement membrane synthesized by Müller cells)
2.nerve fiber layer
3.ganglion cell layer
4.inner plexiform layer
5.inner nuclear layer
6.outer plexiform layer
7.outer nuclear layer (nuclei of the photoreceptors)
8.external limiting membrane (ELM; not a true membrane but rather an apparent membrane formed by a series of desmosomes between Müller cells and photoreceptors)
9.photoreceptors (inner and outer segments) of the rods and cones
Figure 11-1 Normal retinal layers. From vitreous to choroid: ILM = internal limiting membrane, NFL = nerve fiber layer, GCL = ganglion cell layer, IPL = inner plexiform layer, INL = inner nuclear layer, OPL = outer plexiform layer, ONL = outer nuclear layer, P = photoreceptors (inner/outer segments) of rods and cones. RPE = retinal pigment epithelium. Bruch membrane, arrowhead; choroid, asterisk. The external limiting membrane (ELM) is not shown in this figure.
(Courtesy of Robert H. Rosa, Jr, MD.)
The arrangement of the retina (in tissue sections oriented perpendicular to the retinal surface) is vertical from outer to inner layers, except for the NFL, where the axons run horizontally toward the optic nerve head. Consequently, deposits and hemorrhages in the deep retinal layers have a round appearance clinically when viewed on edge, whereas those in the NFL have a feathery appearance.
The blood supply of the retina comes from 2 sources, with a watershed zone inside the inner nuclear layer. The retinal blood vessels supply the NFL, ganglion cell layer, inner plexiform layer, and inner two-thirds of the inner nuclear layer. The choroidal vasculature supplies the outer one-third of the inner nuclear layer, outer plexiform layer, outer nuclear layer, photoreceptors, and RPE. Because of this division of the blood supply to the retina, ischemic choroidal vascular lesions and ischemic lesions attributed to the retinal vasculature produce different histologic pictures. Ischemic retinal injury produces inner ischemic atrophy of the retina (see Fig 11-13), and choroidal ischemia produces outer ischemic retinal atrophy (see Fig 11-12).
Histologically, the term macula refers to that area of the retina where the ganglion cell layer is thicker than a single cell (Fig 11-2). Clinically, this area corresponds approximately with the area of the retina bound by the inferior and superior vascular arcades. The macula is subdivided into the foveola, the fovea, the parafovea, and the perifovea. Only photoreceptor cells appear in the central foveola; the ganglion cells, other nucleated cells (including Müller cells), and blood vessels are not
present. The concentration of cones is greater in the macula than in the peripheral retina, and only cones are present in the fovea.
Figure 11-2 A, The normal macula is identified histologically by a multicellular, thick ganglion cell layer and an area of focal thinning, the foveola. Note the nerve fiber layer (arrowhead) in the nasal macular region and the oblique orientation of the nerve fiber layer of Henle (outer plexiform layer, asterisk). Clinically, the macula lies between the inferior and superior vascular arcades. B, Spectral domain optical coherence tomography (SD-OCT) of the macula showing in vivo histologic assessment with tremendous details of the lamellar architecture of the retina. Note the nerve fiber layer (arrowhead) in the nasal macular region, the nerve fiber layer of Henle (outer plexiform layer, asterisk), and the external limiting membrane (arrow). C, In the region of the foveola, the inner cellular layers are absent, with an increased density of pigment in the RPE. Note the external limiting membrane (ELM). The incident light falls directly on the photoreceptor outer segments, reducing the potential for distortion of light by overlying tissue elements. (Part B courtesy of Robert H. Rosa, Jr, MD.)
Nerve fibers in the outer plexiform layer (nerve fiber layer of Henle) of the macula run obliquely (Fig 11-2A). This morphologic feature results in the flower-petal appearance of cystoid macular edema (CME) observed on fluorescein angiography and the star-shaped configuration of hard exudates observed ophthalmoscopically in conditions that cause macular edema. Xanthophyll pigment gives the macula its yellow appearance clinically and grossly (macula lutea), but the xanthophyll dissolves during tissue processing and is not present in histologic sections.