Chapter 15: Development of the Eye

Eye development initiates with the neurectoderm forming a broad anterior eye field, a potential domain established through the activation of transcriptional regulators such as PAX6 and SIX3, and the repression of Wnt signaling. This field is subsequently divided into bilateral optic domains by the midline expression of the secreted protein sonic hedgehog (SHH); loss of SHH function can result in holoprosencephaly or cyclopia. Beginning around 28 to 29 days postfertilization, the optic primordium evaginates from the diencephalon to form the optic vesicles, which project laterally toward the surface ectoderm. The optic vesicle induces the formation of the lens placode. Coordinated invagination of both structures leads to the lens placode pinching off to form the lens vesicle, while the optic vesicle invaginates to become the optic cup, a two-layered structure. The inner layer of the cup differentiates into the sensory (neural) retina, and the outer layer forms the retinal pigmented epithelium (RPE). Failure of the optic fissure, an opening on the ventral surface of the cup, to close can result in congenital coloboma. Within the retina, a common multipotential progenitor cell gives rise to all seven retinal cell types in a conserved sequence, with early development including ganglion and cone photoreceptor cells, and later differentiation yielding rod photoreceptors and Müller glial cells. The lens vesicle cells differentiate into transparent primary and secondary lens fibers, filling the internal cavity with crystallin proteins, a process sustained by the proliferation of cells in the anterior epithelium. Vascularization is sequential, involving the choroidal vasculature, the temporary fetal hyaloid vasculature, and finally the definitive retinal vasculature, which develops by a combination of vasculogenesis and angiogenesis. The anterior segment structures—the iris, ciliary body, and cornea—form largely from migrating neural crest cells and the peripheral extensions of the optic cup. The cornea itself is unique, with its epithelium derived from surface ectoderm and its stroma and endothelium derived from neural crest mesenchyme. Postnatal development is characterized by rapid growth and the maturation of visual pathways, which is profoundly influenced by external visual stimuli during a critical period up to postnatal year 3. Clinical implications include congenital anomalies like anophthalmia (absence of the eye), microphthalmia (small eyes), and conditions affecting infants, such as retinopathy of prematurity in preterm newborns, and amblyopia, resulting from improper patterning of the visual cortex.