PERICYTE DIFFERENTIATION

Pericytes are defined in vivo by their location: They are embedded wit hin the basement membrane of microvessels. They form an integral part of the microvascular wall and are believed to participate in angiogene sis, although their precise role is not clear. Pericytes derived from the retinal microvasculature have been cultured and identified by a se ries of phenotypic characteristics that clearly distinguishes them fro m other stromal cells such as smooth muscle cells. Pericytes in vitro form multicellular nodules rich in extracellular matrix. This matrix b ecomes mineralized in the presence of growth medium containing serum, without exogenous beta-glycerophosphate. These results indicate that p ericytes represent primitive mesenchymal cells able to differentiate i nto an osteogenic phenotype. Pericyte differentiation also is defined by alterations in their response to transforming growth factor beta 1 and changes in the synthesis and/or deposition of various extracellula r matrix proteins such as laminin, Type IV collagen, tenascin, Type X collagen osteonectin, and thrombospondin-1. Angiogenesis is associated commonly with mineralization. These data suggest that pericytes may c ontribute to mineralization in vivo.