FIGURE 1-11A,B. Drawings show the formation of the lens vesicle and optic cup. Note that the optic fissure is present as the optic cup is not yet fused inferiorly. Mesenchyme (M) surrounds the invaginating lens placode. The optic stalk is continuous with the forebrain. Note that the optic cup and optic stalk are neural ectoderm. RPE, retinal pigment epithelium.

ectoderm, the optic cup (Fig. 1-11). The optic cup will eventually differentiate into neurosensory retina (inner layer) and retinal pigment epithelium (RPE) (outer layer) (Fig. 1-11). Local apical contraction112 and physiological cell death91 have been identified during invagination of the lens placode and formation of the optic cup. In the mouse embryo, Msx2, a homeobox-containing transcription factor, is expressed only in the cells of the optic cup that are destined to become neural retina. In vitro Msx2 has been shown to suppress RPE differentiation and may be involved in the initial patterning of the optic cup.48 Abnormal differentiation of the outer layer of the optic cup to form aberrant neural retina has been demonstrated in several mutant mouse strains.21,26,109 The area of future retinal differentiation demonstrates the greatest concentration of vimentin (a cytoskeletal protein) in the optic cup.53 Regionally, within the optic cup, spatial orientation is predicted by expression of the transcription factor, vax2, which defines the ventral region (area of the optic fissure).10 The PAX6 gene has been demonstrated within cells of neural ectodermal origin (optic cup and, later, in the ciliary body and retina), surface ectoderm (lens), and neural crest (cornea).74 The widespread distribution of this gene supports its involvement in many stages of ocular morphogenesis.

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