Cellular mechanisms in retinal vascular development.

Since the pioneering work of Ashton and others, the primate retina has been thought to vascularize by a vasculogenic linkage of endothelial precursor cells. Recent investigations using specific histologic and morphologic criteria question the contribution of vasculogenesis to retinal development. Instead, in primates and mice cells previously designated as retinal angioblasts have been identified as astrocytes that form a vascular-like plexus preceding vessel invasion. Further, in primates and mice retinal vascularization proceeds via angiogenic sprouting from pre-existing vessels in all regions and stages. However, the developing retinal vasculature may utilize novel sources of endothelial cells, such as recruitment of circulating stem cells and redeployment of mural cells from regressing vessel segments. These results provide a framework for study of retinal vascular development, validate the common use of perinatal retinal models in angiogenesis research, and clarify the cellular basis of retinopathy of prematurity.