Local control of bone formation by osteoblasts.

Although bone formation traditionally has been difficult to study in vitro, recent improvements in cell culture techniques have made it possible to study primary cultures of fetal rat calvarial osteoblasts, and to examine specific gene events associated with osteoblast proliferation, differentiation, and mineralization of extracellular matrix to form bone nodules. Results of studies show that extracellular matrix proteins such as Type I collagen and osteocalcin are expressed. Also expressed are growth regulatory factors such as the bone morphogenetic proteins, which are presumably secreted and stored in the bone matrix or involved in additional osteoblast differentiation in an autocrine/paracrine role. The bone matrix is 1 source of these growth regulatory factors for bone formation; another is those animal and human tumors associated with osteoblastic metastases. In some of these tumors, an extended form of basic fibroblast growth factor has been identified, and various bone morphogenetic proteins have been found in others.