Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 20

The two layers of the optic cup (neuroectoderm origin) consist of an inner, nonpigmented layer and an outer, pigmented layer. Both the pigmented and nonpigmented epithelia of the iris and the ciliary body develop from the anterior aspect of the optic cup; the retina develops from the posterior optic cup. The optic vesicle is organized with all cell apices directed to the center of the vesicle. During optic cup invagination, the apices of the inner and outer epithelial layers become adjacent. Thus, the cells of the optic cup are oriented apex to apex.

A thin, periodic acid–Schiff (PAS)‐positive basal lamina lines the inner aspect (i.e., vitreous side) of the nonpigmented epithelium and retina (i.e., inner limiting membrane). By approximately day 40 of gestation in the dog, both the pigmented and nonpigmented epithelial cells show apical cilia that project into the intercellular space. These changes probably represent the first production of aqueous humor.

The iris stroma develops from the anterior segment mesenchymal tissue (neural crest cell origin), and the iris pigmented and nonpigmented epithelia originate from the neural ectoderm of the optic cup. The smooth muscle of the pupillary sphincter and dilator muscles ultimately differentiate from these epithelial layers, and they represent the only mammalian muscles of neural ectodermal origin. In avian species, however, the skeletal muscle cells in the iris are of neural crest origin, with a possible small contribution of mesoderm to the ventral portion.

Differential growth of the optic cup epithelial layers results in folding of the inner layer, representing early, anterior ciliary processes. The ciliary body epithelium develops from the neuroectoderm of the anterior optic cup, and the underlying mesenchyme differentiates into the ciliary muscles. Extracellular matrix secreted by the ciliary epithelium becomes the tertiary vitreous and, ultimately, develops into lens zonules.

The three phases of iridocorneal angle (ICA) maturation include (i) the separation of anterior mesenchyme into corneoscleral and iridociliary regions (i.e., trabecular primordium formation), followed by differentiation of ciliary muscle and folding of the neural ectoderm into ciliary processes; (ii) the enlargement of the corneal trabeculae and development of clefts in the area of the trabecular meshwork; and (iii) the postnatal remodeling of the drainage angle, associated with cellular necrosis and phagocytosis by macrophages, resulting in opening of clefts in the trabecular meshwork and outflow pathways.

In species born with congenitally fused eyelids (i.e., dog and cat), development of the anterior chamber continues during this postnatal period before eyelid opening. At birth, the peripheral iris and cornea are in contact with maturation of pectinate ligaments by three weeks and rarefaction of the uveal and corneoscleral trabecular meshworks to their adult state during the first eight weeks after birth.