Demonstration of the presence of independent pre-osteoblastic and pre-adipocytic cell populations in bone marrow-derived mesenchymal stem cells

Mesenchymal stem cells (MSC) are defined as plastic-adherent, clonal cells that are common progenitors for osteoblasts and adipocytes. An inverse relationship between bone and fat has been observed in several clinical conditions and has been suggested to be caused by re-directing MSC differentiation into one particular lineage. However, this inverse relationship between bone and fat is not consistent and under certain in vivo conditions, bone and fat can change independently suggesting separate precursor cell populations. In order to test for this hypothesis, we extensively characterized two plastic-adherent clonal MSC lines (mMSC1 and mMSC2) derived from murine bone marrow. The two cell lines grew readily in culture and have undergone more than 100 population doublings with no apparent differences in their growth rates. Both cell lines were positive for the murine MSC marker Sca-1 and mMSC1 was also positive for CD13. Both cell lines were exposed to in vitro culture induction of osteogenesis and adipogenesis. mMSC1 and not mMSC2 were only able to differentiate to adipocytes evidenced by the expression of adipocyte markers (aP2, adiponectin, adipsin, PPARγ2 and C/EBPa) and the presence of mature adipocytes visualized by Oil Red O staining. On the other hand, mMSC2 and not mMSC1 differentiated to osteoblast lineage as demonstrated by up-regulation of osteoblastic makers (CBFA1/RUNX2, Osterix, alkaline phosphatase, bone sialoprotein and osteopontin) and formation of alizarin red stained mineralized matrix in vitro. Consistent with the in vitro results, mMSC2 and not mMSC1, were able to form bone in vivo after subcutaneous implantation in immune-deficient (NOD/SCID) mice. Our data suggest that contrary to the current belief, bone marrow contains clonal subpopulations of cells that are committed to either osteoblast or adipocyte lineage. These cell populations may undergo independent changes during aging and in bone diseases and thus represent important targets for therapy.