In vitro evidence that bone formation may be coupled to resorption by release of mitogen(s) from resorbing bone.

Bone-derived proteins have been shown to stimulate the proliferation of bone-forming cells and to increase the rate of embryonic bone formation in vitro. The current studies were intended to determine the tissue distribution of bone cell-active mitogen(s) in the embryonic chick, to determine the cellular origin and the target cell specificity of the bone cell-active mitogen(s) in embryonic chick bone, to determine whether the release of mitogenic activity from embryonic chick tibiae was proportional to bone resorption, and to compare mitogenic activities prepared from different skeletal sources, with respect to Mr, chemical stability, and mitogen activity kinetics. A bone cell-active mitogen(s) was identified in extracts of bone and cartilage but not in extracts of muscle, liver, intestine, or brain. (Mitogenic activity was determined as increased incorporation of 3[H]-thymidine into DNA in serum-free, calvarial cell cultures.) Together, the following three observations indicate an osteoblastic origin for the bone cell-active mitogen(s) in chick bone. First, the mitogen content of embryonic chick tibiae increased 4.5-fold, during eight days of serum-free in vitro growth (P less than .005). Second, conditioned medium (CM) from serum-free monolayer cultures of calvarial cells contained bone cell-active mitogen(s), but CM from parallel cultures of skin, liver, and intestinal cells did not. And, finally, the amount of bone cell-active mitogen(s) in calvarial cell CM was correlated with the amount of alkaline phosphatase (ALP) activity per cell, ie, an index of osteoblastic differentiation (r = .92, P less than .005).(ABSTRACT TRUNCATED AT 250 WORDS)