The eyes begin to develop as a pair of outpocketings that will become the optic vesicles on each side of the forebrain at the end of the fourth week of development. The optic vesicles contact the surface ectoderm and induce lens formation. When the optic vesicle begins to invaginate to form the pigment and neural layers of the retina, the lens placode invaginates to form the lens vesicle. Through a groove at the inferior aspect of the optic vesicle, the choroid fissure, the hyaloid artery (later the central artery of the retina) enters the eye (Figs. 17.2 and 17.3). Nerve fibers of the eye also occupy this groove to reach the optic areas of the brain. The cornea is formed by (a) a layer of surface ectoderm, (b) the stroma, which is continuous with the sclera, and (c) an epithelial layer bordering the anterior chamber (Fig. 17.7).
PAX6, the master gene for eye development, is expressed in the single eye field at the neural plate stage. The eye field is separated into two optic primordia by SHH, which up-regulates PAX2 expression in the optic stalks while down-regulating PAX6, restricting this gene's expression to the optic cup and lens. Epithelial-mesenchymal interactions between prospective lens ectoderm, optic vesicle, and surrounding mesenchyme then regulate lens and optic cup differentiation (Figs. 17.9 and 17.10).
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