Number, frequency, self-renewal, and expansion of osteoprogenitor cells (CFU-O) in subcultured female rat vertebral cell populations.

The present study was undertaken to determine whether the frequency and/or number of dexamethasone- and progesterone-responsive osteoprogenitors in cell populations derived from vertebrae of 6-week-old female rats could be increased relative to that of other progenitors. Frequencies and numbers of both progenitor types were determined for up to six subcultures using continuous subculturing, limiting dilution analysis, and colony assays. In dexamethasone-containing medium, subculturing resulted in an eightfold increase in the total number of dexamethasone-responsive osteoprogenitors and a 14-fold increase in progesterone-responsive osteoprogenitors in second subculture cells over first subculture cells without a significant increase in the frequency of these progenitors. From the third subculture onward, the frequency of both classes of osteoprogenitors decreased in a linear manner and none were observed after six subcultures. Similar results were obtained in progesterone-containing medium. Limiting dilution analysis in the presence of dexamethasone indicated that 2.61 % of cells represented a colony forming unit-fibroblast and 0.28 % represented an osteoprogenitor in first subculture cells, while in second subculture cells, these frequencies increased to 5.56 % and 0.40 %, respectively. Results show that while the frequency of colony forming unit-osteoprogenitor is not increased in the second subculture over the first, the total number of osteoprogenitors is greatly increased because of expansion of the total progenitor cell pool.