The retina develops from neural ectoderm, with the retinal pigment epithelium (RPE) developing from the outer layer of the optic cup and the neurosensory retina developing from the inner layer of the optic cup (Figs. 1-11, 1-12, 1-13, 1-14, 1-18). As with the ciliary epithelium, invagination of the optic vesicle causes the apices of the inner nonpigmented layer to be directed

FIGURE 1-18A-C. (A) At approximately 38 days, the hyaloid vasculature surrounds the lens (L) with capillaries that anastomose with the tunica vasculosa lentis. Axial migration of mesenchyme forms the corneal stroma and endothelium (C). The retina (R) is becoming stratified while the pigment epithelium (PE) remains cuboidal. (B) By day 41, the retina has segregated into inner (IN) and outer (ON) neuroblastic layers. The ganglion cells are the first to differentiate, giving rise to the nerve fiber layer (arrowhead). The pigment epithelium has become artifactually separated from the neural retina in this specimen. (From Cook CS, Sulik KK. Scanning Electron Microsc 1986;III:1215-1227, with permission.) (C) Differentiation of the retina progresses from the central to the peripheral regions. At this time, the inner (IN) and outer (ON) neuroblastic layers are apparent at the posterior pole but, peripherally, the retina consists of outer nuclear and inner marginal zones. Between the inner and outer neu-roblastic layers is a clear zone, the transient fiber layer of Chievitz. PE, pigment epithelium; arrowhead, nerve fiber layer. (From Cook CS, Sulik KK. Scanning Electron Microsc 1986;III:1215-1227, with permission.)

outward, to face the apices of the outer pigmented layer, which are directed inward. Thus, the apices of these two cell layers are in direct contact. Primitive RPE cells are columnar, but by 5 weeks they change shape to form a single layer of cuboidal cells that exhibit the first pigment granules in the embryo. Bruch's membrane, the basal lamina of the RPE, is first seen during this time (optic cup stage) and becomes well developed by 6 weeks when the choriocapillaris is starting to form. By 4 months, the RPE cells take on a hexagonal shape on cross section and develop microvilli that interdigitate with projections from photorecep-tors of the nonpigmented layer.

By the sixth week postfertilization, the nonpigmented inner layer of the optic cup differentiates into an outer nuclear zone and an inner marginal zone. Cell proliferation occurs in the nuclear zone with migration of cells into the marginal zone. This process forms the inner and outer neuroblastic layers (Fig. 1-18B,C), separated by their cell processes, which make up the transient fiber layer of Chievitz. With further realignment of cells, this layer is mostly obliterated by 8 to 10 weeks gestation. The ganglion cells of the inner neuroblastic layer are the first to differentiate (7th week), giving rise to a primitive nerve fiber layer (Fig. 1-18B,C, arrow).

By the 16th week, mitosis has nearly ceased and retinal differentiation commences, as does synaptic contact between retinal neurons.99 Cellular differentiation progresses in a wave from inner to outer layers and from central retina to peripheral retina (Fig. 1-18C). The ganglion cells give rise to a more defined nerve fiber layer that courses to the developing optic nerve. Cell bodies of the Mueller and amacrine cells differentiate in the inner portion of the outer neuroblastic layer; bipolar cells are found in the middle of the outer neuroblastic layer, with horizontal cells and photoreceptors maturing last, in the outermost zone of the retina.99 Early in development, retinal cells demonstrate neurite regeneration in vitro. This regenerative capability decreases with age and is lost postnatally in the rat at a time that corresponds to the time of eye opening and retinal maturation (equivalent to the eighth month of human gestation).106 Thy-1, the most abundant surface glycoprotein found in the retina, is primarily associated with ganglion cells and appears to regulate neurite outgrowth.97

Macular differentiation occurs relatively late, beginning in the sixth month.46 First, multiple rows of ganglion cells accumulate in the central macular area. At this time, the immature cones are localized in the central macular area while the rods develop in the periphery. At 7 months, the inner layers of the retina (including ganglion cells) spread out to form the central macular depression or primitive fovea. The cones in the foveal area elongate, allowing denser cone populations and enhanced foveal resolution. These changes in foveal cones continue until after birth. At birth, the fovea is fairly well developed and consists of a single row of ganglion cells, a row of bipolar cells, and a horizontal outer plexiform layer of Henle. It is not until several months postpartum that the ganglion cells and bipolar cells completely vacate the fovea centralis.

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