Extracts of bone contain a potent regulator of bone formation

We prepared aqueous extracts of whole femorae and tibiae of embryonic chicks. An amount of extract containing 25 μg of protein resulted in a 500% increase in DNA synthesis in calvarial cell cultures, and significant effects were detected with 5 μg (55%). The time course for stimulation of DNA synthesis showed a peak occurring 16–20 h after addition of the extract. This matrix factor is nondialyzable, and fractionation on a column of Sephadex G-100 indicated a molecular weight of 60–80 000. At the maximum dose used, [ 3H]proline incorporation into total protein of calvarial cells was increased by 55%, and thus far, all fractions active in promoting DNA synthesis have been found to increase collagen synthesis in culture chick tibiae. These data are consistent with an effect on osteoblasts as well as bone precursor cells. Extracts prepared from tibiae of 2-day-old chicks, from which the marrow had been removed, also stimulated DNA synthesis (280% increase), thus ruling out the possibility that the factor is a relatively nonspecific mitogen from the hematopoietic cell line. We conclude that bone matrix contains a substance which could regulate bone formation in vitro by control of mitosis in osteogenic precursors and/or stimulation of osteoblast activity