Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 19

The anterior margins of the optic cup advance beneath the surface ectoderm and adjacent neural crest mesenchyme after lens vesicle detachment (day 25 in the dog). The surface ectoderm overlying the optic cup (i.e., the presumptive corneal epithelium) secretes a thick matrix, the primary stroma. Mesenchymal neural crest cells migrate between the surface ectoderm and the optic cup, using the basal lamina of the lens vesicle as a substrate. This loosely arranged mesenchyme fills the future anterior chamber and gives rise to the corneal endothelium and stroma, anterior iris stroma, ciliary muscle, and most structures of the iridocorneal angle (ICA). The presence of an adjacent lens vesicle is required for induction of corneal endothelium, identified by their production of the cell adhesion molecule, N‐cadherin. Patches of endothelium become confluent and develop zonulae occludentes during days 30–35 in the dog, and during this period, Descemet's membrane also forms.

Neural crest migration anterior to the lens forms the corneal stroma and iris stroma also results in formation of a solid sheet of mesenchymal tissue, which ultimately remodels to form the anterior chamber. The portion of this sheet that bridges the future pupil is called the pupillary membrane. Vessels within the pupillary membrane form the TVL, which surrounds and nourishes the lens. These vessels are continuous with those of the primary vitreous (i.e., hyaloid). The vascular endothelium is the only intraocular tissue of mesodermal origin; even the vascular smooth muscle cells and pericytes, which originate from mesoderm in the rest of the body, are of neural crest origin. In the dog, atrophy of the pupillary membrane begins by day 45 of gestation and continues during the first two postnatal weeks. Separation of the corneal mesenchyme (neural crest cell origin) from the lens (surface ectoderm origin) results in formation of the anterior chamber.