THE EFFECTS OF CYTOKINES AND GROWTH-FACTORS ON OSTEOBLASTIC CELLS

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 ost eoblast differentiation and mineralized bone matrix formation, Of cour se, these proposals must be assessed in the light of the limitations o f the models, which probably represent a simplification of the complex and different ways in which normal mammalian bone is formed at differ ent sites, Nevertheless, it is likely that the same general types of c ontrol mechanisms are active in each of the different types of bone fo rmation, In adult humans, bone formation predominantly occurs by remod eling, the process by which bone which has recently been resorbed by o steoclasts is replaced by teams of osteoblasts, Other types of bone fo rmation such as endochondral bone formation and appositional bone form ation are also important, particularly during growth and adolescence, The end results of each of these processes are the same, namely a comp lex mineralized proteinaceous bone matrix, These processes are modulat ed by systemic hormonal influences, which are particularly important w ith respect to pituitary hormones and sex steroids during growth and a dolescence, 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 formati on process occurring during normal bone remodeling.