Retinal Vessels

The fetal fissure along the optic stalk closes around the hyaloid artery, and the portions of the vessel within the stalk become the central retinal artery.3 A branch of the primitive maxillary vein located within the optic stalk is the likely precursor of the central retinal vein.26 Early in the fourth month of development, primitive retinal vessels emerge from the hyaloid artery near the optic disc and enter the developing nerve fiber layer. Signals from biomolecular agents called guidance molecules guide the growth and pathway of neurons and likely also guide the growth of these retinal vessels.41,42 The vessels of the retina continue to develop, gradually forming the arterioles, venules, and capillary beds, but all vessel structure is not completed until approximately 3 months after birth, with the vessels to the nasal periphery completed before those to the temporal periphery.3

Clinical Comment: Retinopathy

of Prematurity

Premature infants who are exposed to a high concentration of oxygen can develop retinopathy of prematurity (also called retrolental fibroplasia). The immature retinal

blood vessels respond to the high concentration of oxygen with vasoconstriction and cease to develop. On removal

of the oxygen, vasoproliferation occurs; however, the new vessel growth is composed of “leaky” vessels with

poorly formed endothelial tight junctions. Potential serious complications include neovascular invasion of the vitreous and development of vitreoretinal adhesions, which may be followed by hemorrhage and retinal detachment.43

CORNEA

Induction by the developing anterior epithelium of the lens signals multiple steps in corneal development, at about the time the lens vesicle separates from the surface ectoderm (day 33).12,17,44 One or two layers of epithelial cells from surface ectoderm become aligned and will form the corneal epithelium. During the sixth week, zonula occludens are evident.1 The first component of the anchoring system is the basal lamina, which is evident by week 9, and hemidesmosomes are present by week 13.45 By the fifth or sixth month, all the cellular layers of the corneal epithelium are present.46 Corneal endothelium, formed from the first wave of mesenchyme that migrates into the space between the corneal epithelium and the lens, is one to two cells thick by week 6.3 At 4 months the endothelium is a single row of flattened cells with a basal lamina, the first evidence of Descemet’s membrane.3,47 By the middle of the fourth month, tight junctions are apparent in