FIGURE 1-9A-B. (A) Drawing of a cross section through forebrain and optic sulci of 23- to 26-day-old embryo, during the period of neural tube closure. The optic sulci are lined by neural ectoderm (shaded cells); the surface of the forebrain is covered with surface ectoderm (clear white cells). As the optic sulci (neural ectoderm) evaginate towards the surface ectoderm (hollow arrows), the edges of the brain vesicles move together to fuse, thus closing the neural tube (solid arrows). (B) Drawing of a cross section through a 26-day-old embryo at the level of the optic vesicle. The neural tube has closed, the surface ectoderm now covers the exterior of the forebrain, and the neural ectoderm is completely internalized. The surface ectoderm cells overlying the optic vesicles thicken to form the early lens placode. (From Cook CS, Sulik KK. Scanning Electron Microsc 1986;III:1215-1227, with permission).

forebrain vesicles. Expansion of the optic sulci toward the surface ectoderm and fusion of the forebrain vesicles create the optic vesicles (Figs. 1-9, 1-10) by approximately day 25 to 26 (embryo size, 3 mm). Closure of the neural tube and expansion of the optic vesicles occur through the mechanical influences of the cytoskeletal and extracellular matrix and localized proliferation and cell growth.91

The mesencephalic neural crest cells populate the region around the optic vesicle and ultimately give rise to nearly all the connective tissue structures of the avian eye, and the same can be presumed for the mammalian eye (see Table 1-1).55,64 An external bulge indicating the presence of the invaginating optic vesicle can be seen at approximately 25 days human gestation (see Fig. 1-9). The optic vesicle appears to play a significant role in the induction and size determination of the palpebral fissure and orbital and periocular structures.56

At approximately 27 days gestation, the surface ectoderm that is in contact with the optic vesicle thickens to form the lens placode (Figs. 1-9, 1-10, 1-11). The lens placode and underlying neural ectoderm invaginate through differential growth (Fig. 1-10). The invaginating neural ectoderm folds onto itself as the optic vesicle collapses, creating a double layer of neural

FIGURE 1-10. Drawing of a transection through a 28-day-old embryo shows invaginating lens placode and optic vesicle (arrows), thus creating the optic cup. Note the orientation of the eyes 180° from each other; this corresponds to the SEM view shown in Figure 1-12C.

Lens vesicle

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