In this short review, some regulatory mechanisms that are involved in the control of normal bone formation are proposed, based on several in vivo and in vitro models our group has utilized recently to study osteoblast differentiation and mineralized bone matrix formation. Of course, these proposals must be assessed in the light of the limitations of the models, which probably represent a simplification of the complex and different ways in which normal mammalian bone is formed at different sites. Nevertheless, it is likely that the same general types of control mechanisms are active in each of the different types of bone formation. In adult humans, bone formation predominantly occurs by remodeling, the process by which bone which has recently been resorbed by osteoclasts is replaced by teams of osteoblasts. Other types of bone formation such as endochondral bone formation and appositional bone formation are also important, particularly during growth and adolescence. The end results of each of these processes are the same, namely a complex mineralized proteinaceous bone matrix. These processes are modulated by systemic hormonal influences, which are particularly important with respect to pituitary hormones and sex steroids during growth and adolescence, and by local cellular microenvironmental differences. The former will not be discussed here. Rather, we will concentrate on the local events and factors which are likely involved in the bone formation process occurring during normal bone remodeling.

Osteoclasts are terminally differentiated cells that express tartrate-resistant acid phosphatase (TRAP) at a higher level than other normal cells. Therefore, in an attempt to develop immortalized osteoclasts, we produced two lines of transgenic mice in which expression of the simian virus 40 T antigen oncogene was targeted to osteoclasts using the TRAP gene promoter. Osteoclasts were increased in number in bones from both lines. More than 50% of them appeared morphologically transformed, 2-5% were mitotic, but, unexpectedly, 5% were apoptotic. Osteoclast tumors were observed occasionally in one line of mice (line 4), and sheets of TRAP-positive cells (tumorlets) developed in most mice in both lines. Although cells isolated from these tumorlets formed multinucleated TRAP-positive cells that resorbed bone in vitro, to date we have been unable to develop an immortalized osteoclast cell line from them. Osteoclasts from one line (line 5) had reduced ruffled border formation and a higher level of T-antigen expression than osteoclasts in the other line (line 4), and these features were associated with the presence of osteopetrosis. However, osteoclasts from these osteopetrotic mice and from line 4 mice resorbed bone normally when the mice were treated with interleukin-1. These findings indicate that T antigen can be targeted to osteoclasts in transgenic mice and causes osteoclast transformation, tumors, mitosis, and apoptosis. When T antigen is expressed at high levels, functional impairment of osteoclasts can be detected. Furthermore, these results suggest that T antigen is insufficient on its own to immortalize cells in the osteoclast lineage.