Development of the primate retinal vasculature

Human and macaque retinae have similar retinal vascular anatomy. The general features of the retinal vascular anatomy of these two primates have much in common with more widely studied animal models such as rat and cat. However, primates are unique amongst mammals in having a region in temporal retina specialized for high visual acuity, which includes the fovea centralis (or 'fovea'). Several features distinguish the fovea from other parts of the retina, including a very high local density of cone photoreceptors, a high density of inner retinal cells during development, and an absence of retinal blood vessels. The retinal vascular complex comprises a number of cell types, in addition to vascular endothelial cells, including pericytes, microglia, astrocytes-none of which is intrinsic to the retina. In addition, amacrine-like cells make bouton-like associations with retinal vessels and may be involved in the autoregulation of blood flow. During development endothelial cells 'invade' the retina, accompanied by a population of microglial cells; glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes are also seen associated with the developing vasculature, and are in advance of the vascular front by a few hundred microns. Recent findings indicate that astrocytes at the vascular front proliferate in response to factors released by endothelial cells, including leukemia inhibitory factor. Better understood is the role of GFAP-immunoreactive astrocytes just in advance of the developing vessels. These astrocytes are sensitive to hypoxia and in response release vascular endothelial growth factor (VEGF) which in turn promotes the migration, differentiation and proliferation of vascular endothelial cells. This hypoxia/VEGF-mediated process of migration, proliferation and differentiation appears common to the retinae of a variety of species, including human. However, in human and macaque retina, different mechanisms appear to govern the development of the retinal vessels growing along the horizontal meridian of the retina towards the central area, which contains the fovea. Despite the relatively advanced state of differentiation and maturation of cells in the central area compared with the periphery, the growth of retinal vessels into the central area has been described as 'retarded', and the incidence of cell proliferation associated with these vessels is lower than in peripheral vessels. Furthermore, neither retinal vessels nor their accompanying astrocytes grow into a circumscribed region which, at a later stage, develops into the foveal depression. These observations suggest that molecular markers define the foveal region and inhibit cell proliferation and vascular growth at the fovea and, perhaps, along the horizontal meridian. The findings also suggest that at the fovea, the retina is adapted morphologically to its blood supply, since in the vicinity of the fovea, the development of retinal vessels is retarded or inhibited. The limitations on vascularization of central retina has implications for its vulnerability to degenerative changes, as seen in age-related macular degeneration.