Increased bone formation and decreased bone resorption in fetal mouse calvaria as a result of intermittent compressive force in vitro.

We have shown earlier that hypertrophic chondrocytes of growth plate cartilage in vitro react to an intermittent compressive force (ICF) of physiological magnitude by an increased calcification of the matrix. In this communication, we report the influence of ICF on bone metabolism, i.e., osteoblastic and osteoclastic activity, using fetal mouse calvaria in vitro. Seventeen-day-old calvaria were cultured for 5 days under control conditions (atmospheric pressure), or under the influence of ICF. ICF was generated by intermittently compressing the gas phase above the culture medium (130 mbar, 0.3 Hz). Osteoblastic activity was monitored by measuring alkaline phosphatase (AP) activity and 45Ca incorporation into the bone mineral. Osteoclastic resorption of the mineral phase was monitored by measuring the release of 45Ca from prelabeled bone rudiments. In addition, the total mineral content (Ca and Pi) of the calvaria was determined. Exposure to ICF resulted in a significant increase in bone formation, indicated by an enhanced alkaline phosphatase activity and increased incorporation of 45Ca, as well as a decreased bone resorption. The combined effects led to a net increase in mineral content per calvarium of some 16%. We conclude that both osteoblasts and osteoclasts are affected by intermittent compressive force. Osteoblasts are stimulated, and osteoclasts are inhibited in their activity and/or growth. The effect of ICF on osteoblasts is comparable with the effect on fetal growth plate chondrocytes; both cell types respond to ICF by an increase in calcium-phosphate mineral deposition in the matrix. The lower bone resorption may be a direct effect of ICF on osteoclasts, but it is also possible that osteoblasts play an intermediate role.