The two layers of the optic cup (neural ectoderm origin) consist of an inner nonpigmented layer and an outer pigmented layer. Both the pigmented and nonpigmented epithelia of the iris and ciliary body develop from the anterior aspect of the optic cup whereas the retina develops from the posterior optic cup. The optic vesicle is organized with all cell apices directed to the center of the vesicle (see Figs. 1-10, 1-11). During optic cup invagination, the apices of the inner and outer epithelial layers become adjacent. Thus, the cells of the optic cup are oriented apex to apex.
A thin periodic acid-Schiff-(PAS) positive basal lamina lines the inner aspect (vitreous side) of the nonpigmented epithelium and retina (inner limiting membrane). At approximately 4.5 months, both the pigmented and nonpigmented epithelial cells show apical cilia that project into the intercellular space. There is also increased prominence of Golgi complexes and associated vesicles within the ciliary epithelial cells.12 These changes and the presence of "ciliary channels" between apical surfaces probably represent the first production of aqueous humor.113
The iris develops by an anterior growth of the optic cup. The iris stroma develops from the anterior segment mesenchymal tissue of neural crest cell origin. The iris epithelium, including the pupillary sphincter and dilator muscles, originates from the neural ectoderm of the optic cup.51,62,63,104 The smooth muscles of the pupillary sphincter and dilator muscles represent the only muscles in the body of neural ectodermal origin. In avian species, however, the pupillary muscles are striated and originate from stromal mesenchymal (neural crest) cells that migrate into the muscle bundles to become skeletal muscle cells.116,117
The ciliary body develops as the neuroectoderm of the anterior optic cup folds, and underlying mesenchyme differentiates into the ciliary muscles. Tertiary vitreous in the area of the ciliary body folds develops into lens zonules.
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