Mechanisms of neovascularization. Vascular sprouting can occur without proliferation of endothelial cells.

The kinetics of endothelial cells during microvascular growth were studied using a model of inflammation-induced neovascularization of the rat cornea. Inflammation was produced by central silver nitrate cauterization, cellular proliferation was assessed by tritiated-thymidine autoradiography and nuclear counts on plastic sections, and formation of new vessels was studied on whole-mount preparations after vascular perfusion with colloidal carbon. The 3H-thymidine-labeling index of endothelial cells was significantly higher than normal at 1 day following cauterization, although neither mitotic figures nor vascular sprouts were present. The labeling index reached a peak at 2 days, when cell division was evidenced by mitotic figures and doubling of the number of nuclei per section. Actual vascular sprouting also began during the 1- to 2-day interval. To determine whether vascular sprouting was dependent upon endothelial cell division, proliferation was suppressed by X-irradiation (2000 or 8000 rads) prior to cauterization. In irradiated corneas displaying no cellular proliferation, vascular sprouting at 2 days was similar to that in contralateral shielded corneas. Vascular growth continued in irradiated corneas between 2 and 4 days, but at 4 days the length of the vascular ingrowth was reduced to 66.7 and 53.4% of control after 2000 and 8000 rads, respectively. Vascular ingrowth did not progress between 4 and 7 days. This study demonstrates that initial vascular sprouting does not require proliferation of endothelial cells, although under ordinary circumstances DNA synthesis has been stimulated and is in progress at the time of sprouting. After initial sprouting without proliferation, limited vascular growth can continue for about 2 more days but subsequently ceases. Ultrastructural evaluation suggested that migration and redistribution of existing endothelial cells from the limbal vessels enable vascular sprouting and elongation without cellular proliferation.