Site-specific regulation of osteogenesis: maintenance of discrete levels of phenotypic expression in vitro.

Intrinsic differences in bone formation rate, cell numbers, and the percentages of cells expressing alkaline phosphatase activity were studied in explants of chick calvaria periosteum cultured for 4 days and 6 days. Proliferation, differentiation, and bone production were examined in radioautographs of plastic sections and by using whole-culture biochemical assays of protein and alkaline phosphatase. Ectocranial explants at both 4 days and 6 days exhibited more alkaline phosphatase-positive cells and significantly more bone formation than endocranial cultures. There were no detectable differences in cell numbers or 3H-thymidine labeling indices. The volume of bone synthesized per osteoblast was significantly higher in the ectocranial group. Examination of bone stripped of periostea and then cultured for 4 days revealed that large areas of bone were covered by osteoblasts, indicating that the periosteal explant cultures were composed almost exclusively of osteoprogenitor cells and fibroblasts. The data suggest that the level of expression of predetermined osteogenic phenotypes can be maintained in vitro for 6 days following explantation and that variations in the rate of osteogenesis are programmed into progenitor cells prior to their differentiation into osteoblasts.