Cell sorting enriches osteogenic populations in rat bone marrow stromal cell cultures

The presence of multiple cell types in bone marrow stromal populations complicates interpretation of cytokine and hormone effects on the osteoprogenitors present, indicating a need for a method for purification of the osteoprogenitor population. Flow cytometric sorting of 7 day primary rat bone marrow stromal cell cultures was performed on the basis of alkaline phosphatase (AP) expression with an antibody against AP (RBM 211.13). The resultant AP(high), AP(low), or control cells were plated to determine osteoprogenitor, macrophage, and adipocyte distribution and frequency. Approximately 50% of osteoprogenitor/bone nodule-forming cells were lost during processing/sorting when compared with unsorted controls. Nevertheless, within the AP(high) fraction, the numbers of AP-positive colonies and osteoprogenitors (bone nodules) were significantly enriched compared with the unfractionated control; the increase in osteoprogenitor frequency ranged from approximately 2 to 100-fold. There were few assayable osteoprogenitors in either the AP(high) or AP(low) fractions in the absence of dexamethasone (dex), suggesting that RBM stroma contains largely dex-dependent osteoprogenitor populations, and that dex may regulate osteoprogenitors subsequent to the upregulation of AP. Osteoprogenitor/bone nodule numbers in either the AP(high) or AP(low) fraction did not follow a linear relationship with decreasing plating density. The AP(high) fraction of cells was depleted for adipocyte and macrophage colonies. In contrast, within the AP(low) fraction of cells, adipocyte and macrophage colonies were consistently enriched. We conclude that flow-cytometric sorting of RBM stromal populations according to high or low AP expression is an effective technique for enrichment of AP-positive colonies and osteoprogenitors/bone nodule-forming cells.

Dexamethasone alters the subpopulation make-up of rat bone marrow stromal cell cultures

Bone marrow stromal cells comprise a heterogeneous population including fibroblastic, adipocytic, hemopoietic, and osteogenic cells. Although the conditions under which different lineages are regulated have not been fully elucidated, dexamethasone clearly stimulates osteogenic expression in stromal cultures. The purpose of this study was to begin to elucidate and quantify some of the subpopulations present when rat bone marrow stromal cells are grown with or without dexamethasone under conditions favoring bone formation. Bone marrow stromal cells from young adult rats were cultured with ascorbic acid, beta-glycerophosphate, and with or without dexamethasone for various periods of time. Culture dishes were then analyzed for cell counts, or stained with either histochemical or immunohistochemical stains, and colony types were quantitated, or cells were processed for flow cytometry. Dexamethasone significantly increased the number of alkaline phosphatase (AP) positive colonies, von Kossa positive bone nodules, alpha-naphthylbutyrate esterase positive colonies, and ED2 positive (macrophage) colonies. The number of adipocytic foci was largely unaffected in these experiments. Flow cytometry confirmed colony counts and showed stimulation by dexamethasone of AP positive cells and macrophages, and in addition, the reduction of hemopoietic cells expressing leukocyte common antigen. These data show conclusively that when rat bone marrow stromal populations are grown under conditions stimulating osteoprogenitor differentiation and bone formation, the stromal subpopulation make-up, including expression of hemopoietic lineages, is markedly altered.