Skeletal growth factors.

Growth factors are polypeptides with important actions on the replication and differentiated function of cells. Skeletal cells synthesize a variety of growth factors, which are believed to act in an autocrine or paracrine fashion. These growth factors include platelet-derived growth factor, fibroblast growth factor 1 and 2, insulin-like growth factor I and II, transforming growth factors beta 1, 2, and 3, and selected bone morphogenetic proteins. Skeletal cells also synthesize specific binding proteins for selected growth factors. In addition, bone marrow cells synthesize a variety of cytokines known to have important actions in bone remodeling. Fibroblast growth factors and platelet-derived growth factors are, for the most part, mitogenic for skeletal cells, whereas insulin-like growth factors and transforming growth factor beta enhance the differentiated function of the osteoblast. Growth factors also modify osteoclast recruitment and function and as such, bone resorption. Skeletal growth factors can be regulated at the level of synthesis, activation, binding proteins, and receptor binding, and, as a result, their activity can be modified by exogenous agents.