Development of mineralized nodules in fetal rat mandibular osteogenic precursor cells: Requirement for dexamethasone but not for beta-glycerophosphate

We have reported that a cell population obtained from fetal rat mandible wi th neutral protease (Pro I) has a unique differentiation sequence in which the elevation of alkaline phosphatase (ALPase), calcium accumulation, and c ollagen synthesis occurs simultaneously. In this report, we further charact erized Pro I-released population of cells by studying the effect of dexamet hasone (Dex) or beta-glycerophosphate (beta-GP on the formation of bone nod ules. The formation of bone nodules in Pro I-released population of cells ( ProIRPC) was augmented by the addition of Dex (10(-7) M) from days 3 to 14, suggesting that Pro IRPC contained osteoprogenitor (OP) cells. A 24-hour p ulse treatment of ProIRPC released population of cells with Dex on days 9 a nd 12 resulted in an increase in the number of nodules but treatment on day s 3, 6, or 15 did not. The number of bone nodules formed in Pro IRPC pulse treated with Dex on day 9 was comparable with that in Pro IRPC treated with Dex from days 3 to 14. Dex caused an earlier elevation of ALPase, in which maximal expression was observed on day 10. beta-GP caused a prolonged elev ation of ALPase, but did not affect the formation of bone nodules. Unlike P ro I-released population of cells, rat calvarial cells did not form mineral ized nodules without beta-GP, and showed that a Dex-responsive period on bo ne nodule formation in rat calvarial cells was at preconfluency (days 0 and 1). Thus, it appeared that the Dex-induced differentiation of early OP cel ls in Pro IRPCs occurred during the limited period from day 9 to day 12. Pr o IRPC was found to have an unique characteristic that bone nodule formatio n was not affected by beta-GP.