The anterior margins of the optic cup advance beneath the surface ectoderm and its subjacent mesenchyme following lens vesicle detachment at approximately day 33 of gestation. The surface ectoderm overlying the optic cup and lens represents the presumptive corneal epithelium-, it secretes a thick matrix producing the primary cornea stroma.43 This acellular material consists of collagen fibers, hyaluronic acid, and glycosaminoglycans. Neural crest cells migrate between the surface ectoderm and
optic cup using the basal lamina of the lens vesicle as a substrate or scaffold.11 Hydration of hyaluronic acid helps to create the space for cellular migration.105 This loosely arranged neural crest cell-derived mesenchyme initially fills the future anterior chamber and gives rise to the corneal stroma, corneal endothe-lium, the anterior iris stroma, the ciliary muscle, and most of the structures of the iridocorneal angle. Separation of the corneal mesenchyme (neural crest cell origin) from the lens (surface ectoderm origin) results in formation of the anterior chamber. Mesenchymal tissue surrounds the lens and forms the tunica vasculosa lentis and is continuous anteriorly with the pupillary membrane. Capillaries within the tunica vasculosa lentis anastomose with the hyaloid vascular system. The vascular endothe-lium appears to be the only component of the anterior segment that is of mesodermal origin, as even the vascular smooth muscle cells and pericytes are of neural crest origin.55,64
The anterior corneal stroma remains acellular and gives rise to Bowman's membrane, which underlies the corneal epithelium. Although the corneal epithelium is of surface ectodermal origin, Bowman's membrane is a condensation of anterior corneal stroma that is of neural crest cell origin. Type I collagen fibrils and fibronectin secreted by the developing keratocytes (neural crest cell origin) form the secondary corneal stroma. Subsequent dehydration of the corneal stroma results in loss of much of the fibronectin and a 50% reduction in thickness of the stroma.44,65 The endothelium plays an important role in the dehydration of the stroma. Patches of endothelium become confluent during the early part of the fourth month of gestation and develop zonulae occludentes at their apices by the middle of the fourth month of gestation.115 By the sixth month of gestation, Descemet's membrane and endothelium are structurally and functionally present and, at this time, the cornea achieves relative transparency. Proteoglycans containing keratan sulfate chains play a role in generating and maintaining corneal transparency.34
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