Vascular smooth muscle cell lineage during arteriolar network remodeling

Despite its central role in growth and development and during a wide range of cardiovascular disorders, very little is known about the fundamental processes governing arteriolar remodeling. In particular, the source of new smooth muscle cells contributing to new contractile blood vessel formation, a key component of vascular remodeling, has remained elusive. The objective of this study was to determine the role of the fibroblast as a potential smooth muscle progenitor cell and the role of pre-existing pericyte or smooth muscle cell proliferation during microvascular network formation in vivo . This project proceeded in four separate phases to elucidate the contribution of these cellular populations to microvascular network growth. In the first phase, we tracked the fate of fluorescently labeled cultured fibroblasts in three models of vascular growth: the rat gracilis muscle during maturation or prazosin stimulated growth, dermal punch wounds in the rat, and in the rat mesentery during Compound 48/80 induced inflammatory microvessel formation. Labeled fibroblasts were found to be closely associated to microvessels in all the models we investigated. A small number (1.65 ± 1.74 cells per mesenteric window) of cultured mesenteric fibroblasts were found to be expressing smooth muscle myosin heavy chain, a marker of mature smooth muscle cells, after inflammation-induced microvessel formation in the mesentery, suggesting the potential of fibroblast differentiation to a smooth muscle cell fate. In the second phase of this project, we developed a new technique to label native fibroblasts within the mesentery. Following Compound 48/80-induced remodeling, a small number of labeled fibroblasts (approximately 2 per window) were also found to be differentiated into smooth muscle cells, confirming our earlier findings. In the third phase, we developed a new technique to determine the ability of platelet derived growth factor (PDGF), a cytokine thought to mediate arteriolar formation, to recruit fibroblasts via a concentration gradient in an intravital mesentery preparation. PDGF-BB infusion elicited a chemotactic response evident by a significant increase in labeled cell number within 100μm of the infusion pipette after 75 minutes, whereas PDGF-AA and control groups exhibited no recruitment of fibroblasts. In the fourth phase, we determined the contribution of proliferating pericytes and smooth muscle cells to microvascular network formation. To elucidate the contribution of these preexisting vascular cells to network formation, we counted the number of proliferating smooth muscle (SM) α-actin positive cells and total SM α-actin positive cells at four time points during the peak phase of Compound 48/80-induced vascular growth in the rat mesentery. We calculated the maximum percentage of new vascular cells resulting from existing smooth muscle cell or pericyte proliferation to be 70 ± 33%. These results are the first report of the contribution of proliferation of pericytes and smooth muscle cells to a developing microvascular network, and suggest that proliferation of preexisting vascular wall cells plays an important role in providing new smooth muscle cells for arteriolar formation. Our study has indicated that fibroblasts may be recruited to sites of arteriolar growth possibly by PDGF-BB mediated mechanisms and that some may indeed differentiate into mature smooth muscle cells. However, smooth muscle cell and pericyte proliferation appears to be an important mechanism leading to increases in smooth muscle cell number and the resultant formation and enlargement of new contractile blood vessels during microvascular remodeling in the postnatal animal.