PDGF, TGF-BETA, AND HETEROTYPIC CELL-CELL INTERACTIONS MEDIATE ENDOTHELIAL CELL-INDUCED RECRUITMENT OF 10T1 2 CELLS AND THEIR DIFFERENTIATION TO A SMOOTH-MUSCLE FATE/

We aimed to determine if and how endothelial cells (EC) recruit precur sors of smooth muscle cells and pericytes and induce their differentia tion during vessel formation. Multipotent embryonic 10T1/2 cells were used as presumptive mural cell precursors. In an under-agarose cocultu re, EC induced migration of 10T1/2 cells via platelet-derived growth f actor BE. 10T1/2 cells in coculture with EC changed from polygonal to spindle-shaped, reminiscent of smooth muscle cells in culture. Immunoh istochemical and Western blot analyses were used to examine the expres sion of smooth muscle (SM)-specific markers in 10T1/2 cells cultured i n the absence and presence of EC. SM-myosin, SM22 alpha, and calponin proteins were undetectable in 10T1/2 cells cultured alone; however, ex pression of all three SM-specific proteins was significantly induced i n 10T1/2 cells cocultured with EC. Treatment of 10T1/2 cells with TGF- beta induced phenotypic changes and changes in SM markers similar to t hose seen in the cocultures. Neutralization of TGF-beta in the cocultu res blocked expression of the SM markers and the shape change. To asse ss the ability of 10T1/2 cells to contribute to the developing vessel wall in vivo, prelabeled 10T1/2 cells were grown in a collagen matrix and implanted subcutaneously into mice. The fluorescently marked cells became incorporated into the medial layer of developing vessels where they expressed SM markers. These in vitro and in vivo observations sh ed Light on the cell-cell interactions that occur during vessel develo pment, as well as in pathologies in which developmental processes are recapitulated.