Roles of endothelial cell migration and apoptosis in vascular remodeling during development of the central nervous system

Objective: To examine the roles of apoptosis, macrophages, and endothelial cell migration in vascular remodeling during development of the central ner vous system. Methods: The terminal deoxynucleotide transferase-mediated dUTP nick end la beling (TUNEL) technique was combined with Griffonia simplicifolia isolecti n B4 histochemistry ro detect apoptotic endothelial cells in retinal whole- mount preparations derived from rats at various stages of postnatal develop ment as well as from rat pups exposed to hyperoxia. Macrophages were detect ed by immunohistochemistry with the monoclonal antibody. ED1, and prolifera ting endothelial cells were identified by incorporation of bromodeoxyuridin e. Results: Only small numbers of TUNEL-positive endothelial cells were detect ed during normal development of the retinal vasculature, with the apoptotic cell density in the inner plexus peaking during the first postnatal week a nd decreasing markedly during the second week, at a time when vessel retrac tion was widespread. Neither apoptotic endothelial cells nor macrophages we re apparent at sites of initiation of vessel retraction. TUNEL-positive end othelial cells were observed in vessels destined to remain. Hyperoxia induc ed excessive vessel withdrawal, resulting in the generation of isolated vas cular segments containing apoptotic endothelial cells. A topographical anal ysis showed low numbers of proliferating endothelial cells at sires of angi ogenesis whereas vascular proliferation was increased in the adjacent inner plexus. Conclusions: Endothelial cell apoptosis and macrophages do not initiate ves sel retraction. but lather contribute to the removal of redundant cells thr oughout the vasculature. We suggest that vessel retraction is mediated by e ndothelial cell migration and that endothelial cells derived from retractin g vascular segments are redeployed in the formation of new vessels. Only wh en retraction results in compromised circulation and redeployment is not po ssible, does endothelial cell apoptosis occur.