Dexamethasone recruitment of self-renewing osteoprogenitor cells in chick bone marrow stromal cell cultures

Bone marrow stromal cells are a mixed population that contribute to the formation of the hematopoietic microenvironment. The osteogenic lineage includes populations of cells that, in culture, form discrete nodules of mineralized tissue when grown in the presence of ascorbic acid and sodium beta-glycerophosphate. We have used nodule formation to assay for the self-renewal capacity of osteoprogenitor cells in chick bone marrow cultures. To examine the regulatory influence of dexamethasone (Dx), first subcultures were grown continuously or split 1:1 at repeated subculture. Cells in continuous culture exhibited less than two population doublings, while cellular proliferation and alkaline phosphatase area were inhibited by 10(-8) mol/L Dx. Cells in split (redistributed) cultures exhibited up to 14 population doublings and cellular proliferation was also inhibited by Dx. In contrast with continuous cultures, redistributed cultures treated with Dx had increased alkaline phosphatase area and 15-fold larger amounts of mineralized tissue formation than controls. Osteogenesis was sustained for up to four subcultures and the ratio of mineralized tissue area to alkaline phosphotase positive cell area was at most 0.55. These data indicate that the osteogenic lineage of bone marrow stromal cells contains self-renewing progenitors that are recruited by Dx in culture and that at a maximum, only 55% of the alkaline phosphatase-positive cell population contributes to osteogenesis.