Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 88

The tight junctions between the retinal pigmented epithelial cells comprise the bulk of the epithelial portion of the BRB. The nonfenestrated retinal capillary endothelium with tight junctions between the cells comprises the endothelial portion of the BRB. The most permeable point of the BRB is the optic nerve head, at which substances from the choroid can pass into the nerve. The choroidal capillaries are highly permeable to permit passage of all low molecular weight compounds and proteins. Thus, nutrients from the choroidal blood supply pass readily into the retinal pigment epithelium, where numerous transport systems account for the selectivity of the barrier and elaborate transcellular pathways exist to pass them on into the retina. This high protein permeability of the choroidal vessels also elevates osmotic pressure, which helps fluid to pass out of the retina. The transport of water from the retina to the choroid is driven by the active transport of chloride to prevent water accumulation in the subretinal space. No significant barrier exists between the vitreous body and the retina.