Gene expression during vascular pericyte differentiation.

Pericytes, an integral part of the microvasculature, are involved in a number of different processes, including angiogenesis. Many of the early studies on these cells are descriptive and concentrate on the location of pericytes in vivo, surrounding the endothelial cells in the microvessels. These studies led to the proposals that pericytes have a function in maintaining blood flow and contribute to the mechanical strength of the microvessels. However, with the advancement of tissue culture techniques and molecular technology it has been shown that these cells also have the ability to differentiate into a variety of different cell types, including osteoblasts, chondrocytes, adipocytes, fibroblasts, and smooth muscle cells. This review concentrates on the differentiation of pericytes along the osteogenic pathway. Pericytes behave like osteoblasts in vitro, by forming a mineralized matrix and expressing a number of genes that are also expressed by osteoblasts. These cells also form a well-defined matrix of bone, cartilage, and fibrous tissue in vivo, although it is not clear under what circumstances pericytes express osteogenic potential in situ. This review highlights the potential functional importance of pericytes in the growth, maintenance, and repair of the skeleton and in diseases involving ectopic ossification and calcification.