Are osteoblastic cells required for the control of osteoclast activity by parathyroid hormone?

Osteoclasts cultured from the endosteal surface of marrow appear to require osteoblasts for PTH to stimulate their resorptive activity, but the contribution of preosteoclasts to these observations has not been ruled out. Other data indicate that an osteoblast-derived factor can influence osteoclast activity, or confer sensitivity of osteoclasts to PTH. Acid phosphatase content of osteoclasts formed in osteoblast-free cultures of marrow cells is stimulated by PTH, but resorption is not. Also, there are conflicting data on the presence of a PTH receptor on osteoclasts. Thus, while PTH may act directly on preosteoclasts, or even on osteoclasts, to enhance resorptive capability, it seems evident that an osteoblastic factor mediates the PTH stimulation of osteoclastic resorptive activity.