Bone formation in osteoporotic rats, reduced effect of bone morphogenetic protein(BMP) ascribed to the suppressed osteoblast differentiation.

The bone-inducing efficiency in rat tissues of bone morphogenetic protein (BMP) was compared between the normal and ovariectomized (OVX) animal. In the calvaria of OVX rats, BMP induced a smaller amount and a slower process of bone formation than that of the control rats (sham operated). In the subcutaneous tissue of the OVX rats, BMP induced no bone formation in terms of alkaline phosphatase activity and calcium content, while in the same tissue of the control rats, BMP induced active bone formation. In an attempt to clarify the mechanism behind the reduced effect of BMP in the OVX rats, bone -marrow stromal cells of the OVX and SHAM rats were compared for their abilities to differentiate into osteoblasts. Using a culture system, it was shown that the bone-marrow stromal cells from the OVX rats exhibited much less alkaline phosphatase activity and osteocalcin synthesis than those from control rats. The results indicate that ectopic response to BMP is reduced in OVX rats, although still significantly active in isotopic response for bone formation. This change may be ascribed to the impaired recruitment of osteoblasts compared with normal animals.