STIMULATION OF THE DIFFERENTIATION OF OSTEOGENIC RAT BONE-MARROW STROMAL CELLS BY OSTEOBLAST CULTURES

The effect of conditioned medium (CM) from rat calvaria (RC) cell cultures on the growth and differentiation of osteogenic cells in rat bone marrow stromal cell (BMSC) cultures was investigated. Control cultures received either CM from periodontal ligament fibroblast cultures or fresh medium. RCCM stimulated the formation of nodules of bonelike tissue in bone marrow stromal cell cultures in a dose-dependent manner, and the maximal stimulation was associated with the osteoblast-enriched cell populations of the RC cultures. Ultrafiltration demonstrated that activity was confined to a CM fraction of 10- to 30-kilodalton molecular size. The activity was sensitive to boiling and trypsin treatments, but was not affected by neutralizing antibodies to transforming growth factor beta or insulin-like growth factor I or II. RCCM was found to initially increase the number and proportion of cells that expressed alkaline phosphatase activity, although the proportion of alkaline phosphatase-positive cells subsequently declined. These data were consistent with an initial stimulation of proliferation of a subpopulation of osteoprogenitor cells within the cultures, followed by their differentiation. The results suggest that mature osteoblasts may produce a paracrine growth factor that can stimulate the differentiation of osteoblasts from precursor cells.