Cornea and anterior chamber
Surface ectoderm closes over the lens pit and gives rise to the corneal epithelium (see Fig 4-6D). This is followed by 3 successive waves of migration of neural crest–derived cells (Figs 4-15, 4-16; see also Fig 4-6E). The first wave of ectomesenchymal cells, passing between the surface ectoderm and the anterior lens vesicle, gives rise to the corneal endothelium. The second wave, consisting of cells of neural crest and mesodermal origin, contributes to the iris and part of the pupillary membrane. The third wave, consisting of ectomesenchymal cells, migrates into the space between the endothelium and the epithelium to give rise to the keratocytes of the corneal stroma. The corneal endothelial cells meet with developing iris, forming the angle recess. The trabecular meshwork and the Schlemm canal develop from mesenchymal cells posterior to the recess. The endothelial cells that line the canal and collector channels are derived from adjacent capillaries, which will eventually form the episcleral venous plexus. The resultant aqueous vein receives aqueous humor and delivers it to the venous circulation (see Chapter 2, Figs 2-17, 2-18). The trabecular beams undergo further maturation and stratification to form the layered trabecular meshwork. Alteration in this process has been implicated in the development of congenital glaucoma and anterior chamber dysgenesis. The scleral spur forms between the trabecular meshwork and the ciliary muscle as the anterior chamber angle develops (see Fig 4-13C, D).
Figure 4-15 Three successive waves of neural crest cell migration are associated with differentiation of the anterior chamber. 1, First wave forms the corneal endothelium. 2, Second wave forms the iris and part of the pupillary membrane. 3, Third wave forms keratocytes.
(Illustration by Paul Schiffmacher.)
Figure 4-16 Development of the cornea in the central region. A, At day 39, 2-layered epithelium rests on the basal lamina and is separated from the endothelium (single layer) by a narrow acellular space. B, At week 7, neural crest–derived ectomesenchymal cells from the periphery migrate into the space between the epithelium and the endothelium. C, Mesenchymal cells (future keratocytes) are arranged in 4–5 incomplete layers by 7½ weeks of gestation; a few collagen fibrils are present among the cells. D, By 3 months, the epithelium has 2–3 layers of cells, and the stroma has approximately 25–30 layers of keratocytes that are arranged more regularly in the posterior half. Thin, uneven Descemet membrane lies between the most posterior keratocytes and the single layer of endothelium.
(Illustration by Cyndie C. H. Wooley.)
The sclera is formed from mesodermal (temporal sclera) and neural crest–derived ectomesenchymal elements. The sclera joins the developing cornea near the equator of the eye but continues to develop and expand to surround the developing optic cup. The scleral spur and Tenon capsule form later, at the time of extraocular muscle insertion.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.