Initial intramembraneous osteogenesis in vitro

Calvariae of fetal mice 12 to 13 days in utero were placed in Rose chambers and cultured in BGJb medium supplemented with 20% fetal calf serum and antibiotics. After periods of 7 and 12 days, the explants were harvested and processed for light and electron microscopy. The mesenchymal cells, after a brief lag time, differentiated into osteoblasts which produced woven bone. Light and electron microscopic observations showed that in vitro, as well as in vivo, growth and development share all of the same characteristics of initial intramembraneous osteogenesis: (1) migration and differentiation of mesenchymal cells into osteoblasts, (2) the subsequent appearance of matrix vesicles in the extra‐cellular space, (3) crystallization of hydroxyapatite within and about these vesicles, (4) growth of hydroxyapatite crystals into spheroidal nodules of bone, and (5) the subsequent fusion of these nodules into seams of woven bone. Thus cellular involvement in initial osteogenesis has been observed in a system where differentiation into osteoblasts and initial calcification takes place in vitro, suggesting that the events responsible for these phenomena have occurred within the cells prior to their morphological differentiation.