There is considerable variation in retinal thickness (Figs 2-41, 2-42). The retina is thickest in the papillomacular bundle near the optic nerve (0.23 mm) and thinnest in the foveola (0.10 mm) and ora serrata (0.11 mm).
Clinically, retina specialists tend to regard the macula, which is 5–6 mm in diameter, as the area between the temporal vascular arcades. Histologically, it is the region with more than 1 layer of ganglion cell nuclei (Fig 2-43; see also Figs 2-32, 2-39, 2-40). See BCSC Section 12, Retina and Vitreous, for further detail.
The name macula lutea (which means yellow spot) derives from the yellow color of the central retina in dissected cadaver eyes or in eyes with retinal detachment involving the macula. This color is due to the presence of carotenoid pigments, located primarily in the Henle fiber layer. Two major pigments—zeaxanthin and lutein—have been identified whose proportions vary with their distance from the fovea. In the central area (0.25 mm from the fovea), the lutein-to-zeaxanthin ratio is 1:2.4, and in the periphery (2.2–8.7 mm from the fovea), the ratio is greater than 2:1. This variation in pigment ratio corresponds to the rod-to-cone ratio. Lutein is more concentrated in rod-dense areas of the retina; zeaxanthin is more concentrated in cone-dense areas.
Figure 2-40 Henle fiber layer. A, Histologic section through the fovea. Note that only the outer nuclear layer and photoreceptors are present centrally. Oblique photoreceptor axons extend to the outer plexiform layer. These radial fibers are known as the Henle fiber layer.B, Electron micrograph showing the Henle fiber layer.
(Reproduced with permission from Spalton D, Hitchings R, Hunter P. Atlas of Clinical Ophthalmology. 3rd ed. Oxford: Elsevier/Mosby; 2005:405–406.)
The fovea is a specialized portion of the macula that appears as a central retinal depression. At approximately 1.5 mm in diameter, it is comparable in size to the optic nerve head (see Fig 2-39). Its margins are clinically inexact, but in younger eyes, the fovea is evident ophthalmoscopically as an elliptical light reflex that arises from the slope of the thickened ILM of the retina. From this point inward, the basal lamina rapidly decreases in thickness as it dives down the slopes of the fovea toward the depths of the foveola, where it is barely visible, even by electron microscopy.
The foveola is a central depression in the floor of the fovea, located approximately 4.0 mm temporal and 0.8 mm inferior to the center of the optic nerve head. It is approximately 0.35 mm in diameter and 0.10 mm thick at its center. The borders of the foveola merge imperceptibly with the fovea. The nuclei of the photoreceptor cells in the region of the foveola bow forward toward the ILM to form the fovea externa (see Fig 2-40). Usually, only photoreceptors, Müller cells, and other glial cells are present in this area.
Figure 2-41 Regional differences in the thickness of retinal layers. A, Papillomacular bundle, which has the thickest ganglion cell layer. B, Macula with 2-cell-thick ganglion cell layer. C, Peripheral retina with single-cell ganglion cell layer and thinner inner and outer nuclear layers. D, Fovea, in which only the outer nuclear layer and photoreceptors are present.
(All parts courtesy of Thomas A. Weingeist, PhD, MD.)
The photoreceptor layer of the foveola is composed entirely of cones, whose dense packing accounts for the high visual acuity and color vision for which this small area is responsible. The foveal cones are shaped like rods but possess all the cytologic characteristics of extramacular cones. The outer segments are oriented parallel to the visual axis and perpendicular to the plane of the RPE. In contrast, the peripheral photoreceptor cell outer segments are tilted toward the entrance pupil.
The location of the foveal avascular zone (FAZ), or capillary-free zone (Fig 2-44; see also Fig 2-39), is approximately the same as that of the foveola. Its appearance in fundus fluorescein angiograms varies greatly. The diameter of the FAZ ranges from 250 to 600 μm or greater; often, a truly avascular, or capillary-free, zone cannot be identified. This area of the retina is entirely perfused by the choriocapillaris and can be severely affected when retinal detachment involves the FAZ. Around the fovea is the parafovea, which is 0.5 mm wide and is where the GCL, the INL, and the OPL are thickest. Surrounding this zone is the most peripheral region of the macula, the 1.5-mm-wide perifovea.
Figure 2-42 OCT images demonstrating the regional differences in retinal layer thickness that are described in Figure 2-41. A, Papillomacular bundle. B, Macula. C, Peripheral retina. D, Fovea.
(Courtesy of Vikram S. Brar, MD.)
Figure 2-43 OCT image through the fovea. International consensus on segmentation of the normal retina on spectral-domain OCT.
(From Staurenghi G, Sadda S, Chakravarthy U, Spaide RF; International Nomenclature for Optical Coherence Tomography (IN•OCT) Panel. Proposed lexicon for anatomic landmarks in normal posterior segment spectral-domain optical coherence tomography: the IN•OCT consensus. Ophthalmology. 2014;121(8):1572–1578.)
Figure 2-44 Foveal avascular zone. A, Scanning electron micrograph of a retinal vascular cast at the fovea, showing the foveal avascular zone (FAZ) and underlying choriocapillaris, the sole source of oxygen to the retina at this location. B, Fluorescein angiogram of the FAZ, obtained during the peak venous phase. Fluorescence from the choriocapillaris is blocked by the RPE.
(Part B courtesy of Vikram S. Brar, MD.)
Orth DH, Fine BS, Fagman W, Quirk TC. Clarification of foveomacular nomenclature and grid for quantitation of macular disorders. Trans Sect Ophthalmol Am Acad Ophthalmol Otolaryngol. 1977;83(3 Pt 1):OP506–514.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.