| Vascular development involves the formation of endothelial cell (EC) tubes, the recruitment and subsequent differentiation of smooth muscle cells (SMC), and the ultimate establishment of vascular cell quiescence. hi an in vitro model of SMC differentiation, coculture of multipotent, mesenchymal 10T1/2 cells with bovine aortic EC (BAE) led to a dramatic downregulation of the platelet-derived growth factor α receptor (PDGFαR) in 10T1/2 cells while simultaneously inducing them to differentiate toward a differentiated SMC phenotype. 10T1/2 cells expressing PDGFαR under the control of a constitutively active promoter, when cocultured with BAE, were no longer able to upregulate SM α-actin, a marker of differentiated SMC. These results suggest that mesenchymal cells of the forming vasculature downregulate the PDGFαR in order to decrease their responsiveness to PDGF and to allow a more complete differentiation toward a SMC phenotype.;Angiopoietins are recently-identified factors that have been implicated in SMC recruitment during vascular development. Neither angiopoietin-1 (Ang1) nor -2 (Ang2), however, could increase EC expression of platelet-derived growth factor (PDGF)-B or heparin-binding EGF-like factor, both of which are chemotactic factors for SMC. Unexpectedly, (Ang1) had significant mitogenic activity for both large and small vessel EC, but not for SMC. Ang2, although it had no mitogenic effect on EC or SMC, inhibited the mitogenic effect of Ang1 on EC.;Once EC and SMC have associated to form a vessel, both cell types cease to proliferate, although the precise mechanisms that lead to vascular cell quiescence are not yet fully understood. To determine if manipulating the microenvironment of EC in vitro would lead to a more quiescent, differentiated phenotype, BAE were cultured on floating collagen gels. BAE cultured on floating gels displayed mitotic indices nearly identical to those of quiescent endothelium in vivo. BAE cultured in vitro, as opposed to their in vivo counterparts, expressed high levels of PDGF-B and fibronectin mRNAs, and low levels of the connexin 40 message. Culture of BAE cells on floating gels restored the expression of these genes to the patterns seen in vivo. |
