Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 21

Infolding of the neuroectodermal optic vesicle results in a bilayered optic cup with the apices of these two cell layers in direct contact. Primitive optic vesicle cells are columnar, but by 20 days of gestation in the dog, they form a cuboidal layer containing the first melanin granules in the developing embryo. The neurosensory retina develops from the inner nonpigmented layer of the optic cup and the retinal pigment epithelium (RPE) originates from the outer, pigmented layer. Bruch's membrane (the basal lamina of the RPE) is first seen during this time, and becomes well developed over the next week, when the choriocapillaris is developing. By day 45, the RPE cells take on a hexagonal cross‐sectional shape and develop microvilli that interdigitate with projections from photoreceptors of the nonpigmented (inner) layer of the optic cup.

At the time of lens placode induction, the retinal primordium consists of an outer, nuclear zone and an inner, marginal (anuclear) zone. This process forms the inner and outer neuroblastic layers, separated by their cell processes that make up the transient fiber layer of Chievitz. Cellular differentiation progresses from inner to outer layers and, regionally, from central to peripheral locations. Peripheral retinal differentiation may lag behind that occurring in the central retina by three to eight days in the dog. Retinal ganglion cells develop first within the inner neuroblastic layer, and axons of the ganglion cells collectively form the optic nerve. Cell bodies of the Müller and amacrine cells differentiate in the inner portion of the outer neuroblastic layer. Horizontal cells are found in the middle of this layer; the bipolar cells and photoreceptors mature last, in the outermost zone of the retina.

Significant retinal differentiation continues postnatally, particularly in species born with fused eyelids. At birth, the canine retina has reached a stage of development equivalent to the human at three to four months of gestation. In the kitten, all ganglion cells and central retinal cells are present at birth with continued proliferation in the peripheral retina continuing during the first two to three postnatal weeks in dogs and cats.