RAT AORTIC SMOOTH-MUSCLE CELLS BECOME PERICYTES DURING ANGIOGENESIS IN-VITRO

BACKGROUND: We previously reported that the intimal endothelium of the rat aorta switches to a microvascular phenotype during angiogenesis i n vitro. The microvessels formed by the rat aortic endothelium are coa ted with pericytes. The purpose of this study was to evaluate the rela tion of the pericytes to the angiogenic process and to identify the si te of origin of these cells in the aortic wall. EXPERIMENTAL DESIGN: R ings of rat aorta were cultured in collagen gel under serum-free condi tions. The formation of a pericyte coating around aorta-derived microv essels was evaluated by counting pericytes and microvessels in the liv ing cultures. Pericytes and endothelial cells were studied by immunohi stochemistry, lectin labeling, electron microscopy, H-3-thymidine labe ling followed by autoradiography, and time-lapse video microscopy. The capacity of aortic smooth muscle cells to differentiate into pericyte s was studied by coculturing intimal or medial-derived smooth muscle c ells with endothelial cells in a collagen gel overlay assay that induc ed reorganization of endothelial cells into microvessels. RESULTS: Mic rovessels during the early stages of angiogenesis were composed primar ily of endothelial cells. As vascular proliferation decreased, the mic rovessels became coated with pericytes. The pericytes migrated from th e root to the tip of the microvessels using the endothelium as a surfa ce for attachment, proliferation, and contact guidance. The pericytes were continuous with the myointimal endothelial cells of the cultured aorta. Pericytes and myointimal cells were positive for alpha-smooth m uscle actin and vimentin and were actively engaged in DNA synthesis. T reatment of the cultures with heparin caused a marked reduction in the number of pericytes. Smooth muscle cells isolated from the intimal as pect of the rat aorta migrated toward the endothelium and differentiat ed into pericytes when cocultured with microvessels formed by isolated endothelial cells in a collagen gel overlay assay. Conversely, smooth muscle cells isolated from the deep layers of the media had no signif icant endothelial tropism and failed to differentiate into pericytes. CONCLUSIONS: This study demonstrates that the rat aorta contains a sub population of intimal/subintimal smooth muscle cells that differentiat e into pericytes during angiogenesis in vitro. These cells have a dist inct endothelial tropism and respond to endothelial cues by contributi ng to the differentiation and maturation of microvessels. Smooth muscl e cells of rat aortic intimal/subintimal origin can be used as a sourc e of pericytes for the in vitro assembly of histotypic microvessels.