The frequency of common progenitors for adipocytes and osteoblasts and of committed and restricted adipocyte and osteoblast progenitors in fetal rat calvaria cell populations.

In fetal rat calvaria (RC) cell populations, adipocyte differentiation is stimulated by both dexamethasone (Dex) and 1,25 dihydroxyvitamin D3 [1,25(OH)2D3], whereas osteoblast differentiation is stimulated by Dex but inhibited by 1,25(OH)2D3. We examined whether the osteoblastic and adipocytic colonies were derived from a common progenitor, from committed and restricted adipocyte and osteoblast progenitors, or from both and whether the adipocyte progenitors stimulated by 1,25(OH)2D3 constitute a population of progenitors that is different from that stimulated by Dex. RC cells were isolated by sequential enzyme digestion yielding five populations designated I-V. In population I the effect of Dex on adipocyte formation was greater than that of 1,25(OH)2D3, whereas the effect of 1,25(OH)2D3 was greater than that of Dex in populations III-V. We next applied replica plating techniques to further investigate the response characteristics of individual osteoprogenitors and adipocyte progenitors by looking at the fate of duplicate colonies derived from the same progenitor under different culture conditions. RC cells were plated at 1,000-1,500 cells/100 mm culture dish and a 17-microm mesh polyester membrane overlaid onto master dishes on day 4 or day 5 and removed on day 11 or day 12. Then, replicas and master dishes were cultured separately in medium containing either Dex, 1,25(OH)2D3, or Dex plus 1,25(OH)2D3 for a further 17-21 days and then fixed and stained with both Sudan IV and the von Kossa technique. Nine hundred twenty-seven matched colonies present on both master dishes and replica membranes were screened and colonies were classified as either adipocytic, osteoblastic (bone or osteoid), or fibroblastic. Results show convincingly that most of the osteoprogenitors present in fetal RC cells are committed and restricted to the osteoblastic cell lineage (95.29%); that the 1,25(OH)2D3-responsive adipocyte progenitors are different from the Dex-responsive adipocyte progenitors, but both are restricted to form adipocytes and finally; and that a common osteoblastladipocyte progenitor is present in a low frequency (4.71% of osteoprogenitors).