Adipose cells have been recognized as an integral component of the bone marrow hematopoietic microenvironment in vivo and as an essential cell type required for in vitro maintenance of stem cells. Four stromal cell lines obtained from the adherent cell population of murine bone marrow cultures have been enriched and purified by multiple trypsinizations. We noted that these cell lines exhibited an accumulation of vacuoles of lipid, the extent of which varied between cell lines in response to a change from medium containing 10% fetal calf serum to medium containing 20% horse serum. The lipid was lost when the cell lines were transferred back into the medium supplemented with fetal calf serum. In light of the reported lipogenic and antilipolytic effects of insulin on fibroblasts and adipocytes, we investigated the ability of insulin to induce adipocyte transformation of these bone marrow stromal cell populations. Three cell lines were exposed to bovine insulin at concentrations ranging from to the extent of accumulation and the insulin concentration at which maximum lipid content was attained were population specific. One cell line (MCJ responded fully at physiological levels of insulin M), whereas the other two showed lipid accumulation only at pharmacological concentrations. The initial growth of MC1 was inhibited in the presence of M insulin which is compatible with the observed differentiation to adipocytes. The growth of MC, was unaltered in the presence of physiological concentrations of insulin, whereas that of MC, was accelerated. Grafts of organ cultures of the cell lines under the kidney capsule of syngeneic mice developed specific characteristics representative of the different cell lines. In particular, the majority of the grafts of MC1 consisted primarily of fat cells which were not observed in the grafts of MC, and MC,. These data strongly suggest that these cell lines comprise cells with different potentialities and that the MC, line represents a preadipocyte stromal cell of bone marrow.
M. All three cell lines responded to the insulin by accumulating lipid, but