IDENTIFICATION OF NOVEL ENDOTHELIAL CELL DYNAMICS DURING ANGIOGENESIS
Understanding angiogenesis increases comprehension of pathological conditions such as tumor growth and peripheral artery disease. Angiogenesis is the growth of new vessels from pre-existing vessels and commonly associated with two modes: capillary sprouting and capillary splitting. Past observations in our laboratory provide evidence of vascular islands in the adult microcirculation. Vascular islands are defined as endothelial cell segments disconnected from nearby networks. The two objectives of this specific aim were to (1) determine if vascular islands are involved in angiogenesis during microvascular network growth, and (2) determine whether vascular islands associated with microvascular regression are involved in microvascular remodeling. Mesenteric tissues were harvested from adult male Wistar rats according to the experimental groups: unstimulated, post stimulation (3, 10 and 70 days), and 70 days post stimulation + restimulation (3 and 10 days). Stimulation was induced by mast cell degranulation via intraperitoneal injections of compound 48/80. Tissues were immunolabeled for PECAM (endothelial cells), NG2 (pericytes), collagen IV (basement membrane), and BrdU (proliferation). On day 3, the percentage of islands with at least one BrdU-positive cell increased compared to the unstimulated level and was equal to the percentage of capillary sprouts with at least one BrdU-positive cell. At day 10, the number of vascular islands per vascular area dramatically decreased compared to unstimulated and day 3 levels. Data collected independently for both aims showed that percent vascular area per tissue area and length density increased by day 10 post stimulation compared to the unstimulated group. At day 70, vascular area and length density were then decreased, indicating vascular regression compared to the day 10 levels. During regression at day 70, the number of islands increased. The disconnected endothelial cells were commonly bridged to surrounding networks by collagen IV labeling. NG2-positive pericytes were observed both along the islands and the collagen IV tracks. At 3 days post restimulation, vascular islands contained BrdU-positive cells. By day 10 post restimulation, when vascular area and length density were again increased, and the number of vascular islands was dramatically reduced. The result of this study suggest that (i) segment proliferation correlates with sprouting and network remodeling, (ii) blood vessel segments could provide a novel mode of endothelial cell presence in angiogenesis, (iii) blood vessel segments may be a reserve to connect with existing vasculature, and (iv) vascular islands originating during microvascular regression are capable of undergoing proliferation and incorporation into nearby networks during network regrowth.