The influence of intermittent compressive force (ICE') and of continuous compressive force (CCF) on calcification of growth plate cartilage was investigated, using organ cultures of fetal mouse cartilaginous long bone rudiments. Sixteen-day-old metatarsal rudiments, still consisting of uncalcified cartilage, were isolated and cultured for 5 days. Initial calcification of hypertrophic Cartilage occurred under control conditions (atmospheric pressure), and under the influence of ICF or CCF by intermittently or continuously compressing the gas phase above the culture medium. Calcification was monitored by means of 45Ca and 32P incorporation into calcium-phosphate mineral and by morphometric methods. Both ICF and CCF increased cartilage calcification, but ICF was about twice as effective as CCF. Killed rudiiments did not calcify during the culture period, nor did ICF or CCF increase the incorporation of label. The effects of ICF and CCF on calcification could not be mimicked by increasing the Po, and Pcoz levels in the g s phase. The length of the central zone of calcified cartilage was significantly increased by ICF and CCF.W e conclude that hypertrophic chondrocytes respond directly to ICF and CCF by an increased deposition of calcium-phosphate mineral in the matrix. Discontinuous mechanical stimulation evokes a higher cellular response than does continuous stimulation.

The use of hydrostatic pressure to apply mechanical stress to bone organ cultures is reviewed. Ossifying long bones and calvarial rudiments are sensitive to this type of stress. Intermittent hydrostatic compression of near physiologic magnitude (ICF) has anabolic effects on mineral metabolism in such rudiments, and continuous hydrostatic stress of high magnitude (CCP) has catabolic effects. The effects of ICF may be ascribed to shear stress generated at tissue interphases of different chemical and mechanical properties. Local factors, such as prostaglandins and growth factors, seem to be involved in the tissue response to mechanical stress.

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