FOLLISTATIN AND ACTIVIN IN BONE - EXPRESSION AND LOCALIZATION DURING ENDOCHONDRAL BONE-DEVELOPMENT

The involvement of activin and follistatin, an activin-binding protein , in endochondral bone development was examined by sc implantation of demineralized bone matrix in rats. Immunoreactive follistatin was loca lized in proliferating chondrocytes and round osteoblasts, whereas it was not detected in hypertrophic chondrocytes and osteoblasts surround ing bone marrow. Western blot analysis also revealed that immunoreacti ve follistatin was higher during the initial stages of chondrogenesis (day 5) and osteogenesis (days 11 and 14) and lower during tile conver sion from cartilage to bone (day 9). These results suggest that follis tatin is produced by proliferating cells, and the expression decreases with differentiation of the cells. Implants injected with follistatin on days 9 and 10 contained lower calcium levels on day 14 than those injected with rat albumin. Furthermore, the follistatin-injected impla nts were still mainly composed of cartilage, ting that the disappearan ce of follistatin is necessary for the conversion of cartilage to bone . In contrast, immunoreactive activin beta(A) (55-60 kDa) was continuo usly detected in implants on days 7-14. The content of C propeptide of type II procollagen was increased and cartilageous area was enlarged on day 7 by activin A injections on days 5 and 6. suggesting a chondro genic effect of activin in the initial stage of cartilage formation. T hese results indicate that proliferating chondrocytes and round osteob lasts produce follistatin, and that the activity of activin is regulat ed by changes in the expression of follistatin at the stages of chondr ogenesis and transition from cartilage to bone.