Chapter 20: Eye

The process begins around day 22 with the formation of optic grooves that deepen into optic vesicles, which subsequently contact the surface ectoderm to induce the formation of the lens placode. A crucial invagination process converts these structures into the double-walled optic cup and the lens vesicle, while the choroid fissure temporarily allows the hyaloid artery to supply the developing interior before fusing to form the pupil. The summary details the differentiation of the optic cup layers, where the outer layer becomes the pigmented retina and the inner neural layer develops into the pars optica retinae containing photoreceptive rods and cones, as well as the mantle and fibrous layers. Anteriorly, the cup forms the pars ceca retinae, which contributes to the iris and ciliary body, while the sphincter and dilator pupillae muscles develop from the underlying ectoderm. The text further explores the formation of the lens through the elongation of primary and secondary lens fibers and describes how the surrounding mesenchyme differentiates into the choroid and sclera. Distinct attention is given to the anterior chamber, where vacuolization creates the iridopupillary membrane and the cornea, and the dynamics of aqueous humor circulation through the canal of Schlemm are explained alongside glaucoma risks. The transformation of the optic stalk into the optic nerve, enclosing the central artery of the retina, is also covered. From a molecular perspective, the chapter highlights PAX6 as the master regulatory gene, the role of SHH in separating the eye fields, and the signaling pathways involving FGFs, TGF-beta, MITF, and CHX10 that guide retinal differentiation. Finally, the summary addresses clinical correlates such as coloboma resulting from fissure closure failure, congenital cataracts associated with rubella, microphthalmia, aniridia linked to PAX6 mutations, and severe midline defects like cyclopia and synophthalmia often associated with holoprosencephaly.