Bone marrow cells obtained from rat femora were subjected to primary culture with 15% fetal bovine serum in the presence of
$10^{-8}$
M dexamethasone, and following trypsin treatment 5 days later were seeded on Petriperm
$^{{\rm TM}}$
dishes which have a flexible bottom. After a 2‐day subculture, a cyclic stress consisting of a 1 s stretch (0.3% strain, 0.5 Hz) and a 1 s relaxation for 30 min every day was started. Culture tissue was removed on day 2 of the subculture (immediately prior to start of stimulation), and then on days 5 and 8 (3 and 6 days after the start of stimulation, respectively), at which times dry weight, DNA, alkaline phosphatase (ALP) activity, and bone Gla protein (BGP, osteocalcin) were measured. Both the dry weight and DNA showed a significant increase in the stimulated group by day 8, while the ALP activity showed a significant increase by day 5. The BGP began to increase in the stimulated group on day 5 in contrast to the control group in which it only increased on day 8. These results support the contention that mechanical stimulation promotes the differentiation of osteogenic cells and enhances bone formation. Since in this experimental model the acceleration of bone formation by mechanical stimulation can be reproduced in vitro, it is extremely useful for investigating the mechanisms underlying mechanical stimulation.
