Evidence for the coupling of bone formation to bone resorption in vitro

An in vitro organ culture system of embryonic chick tibiae was used to examine bone formation and bone resorption under conditions of stimulation with 10 −12 M parathyroid extract. The bones were cultured in a serum-free medium to alleviate any potential interference by serum growth factors. Evidence is presented showing that this bone culture system responds to parathyroid extract with an increase in both bone resorption and bone formation. Although initially there was a decrease in formation, this was followed after 12–16 h by an increase in formation which was correlated with resorption (r = .96, p < .05). These results suggest that the increase in formation was some consequence of the increase in resorption, and as such are consistent with the interpretation that coupling can occur in vitro. Un système de culture d'organes in vitro de tibias embryonnaires de poulets a été utilisé pour étudier la formation et la résorption osseuses dons des conditions de stimulation par de l'extrait parathyroidien a 10 −12M. Les os ont été cultivés dans un milieu sans sérum pour supprimer toute possible interférence avec des facteurs de croissance sériques. Il est démon-tré que ce système de culture osseux répond à l'extrait parathyroïdien par une augmentation à la fois de la résorption et de la formation osseuses. Toutefois, après diminution initiale de la formation, il y a une augmentation après 12 à 16 heures qui est corrélée à l'augmentation de la résorption (r = 0.96, p < 0.05). Ces résultats suggèrent qua l'augmentation de la formation est conséquence de l'augmentation de la résorption, ce qui est compatible avec l'intérpretation selon laquelle le coupling peut intervenir in vitro

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