Development of retinal vessels in the rat.

The development of retinal vessels in the rat from birth to 5 days of age is described. Various techniques including India ink injections into the vascular system, retinal digestion, PAS stained whole mounts, and combinations of these methods were utilized. The first stage in retinal vascularization is the ingrowth of an undifferentiated mesh or syncytium of mesenchymal cells into the peripapillary retina. Later a primitive capillary net forms from the initial meshwork. In a third stage larger vessels, i.e., early arterioles and venules, make their appearance, developing out of the capillary network and possibly from the syncytium directly. The capillary-free zone around arterioles is a late development in vasculogenesis and occurs by retraction of mesenchymal strands and primitive capillaries from around the developing arteriole.