Isolation and Characterization of Multipotent Mesenchymal Stem Cells Adhering to Adipocytes in Canine Bone Marrow

The ceiling culture method has been used to isolate mature adipocytes from adipose tissue that can be dedifferentiated into fibroblastic cells, also known as dedifferentiated fat (DFAT) cells that self-renew and are multipotent, with much higher homogeneity and colony-forming efficiency than those of adipose tissue-derived mesenchymal stem cells. We cultured adipocytes from canine bone marrow using this technique, with the expectation of obtaining DFAT cells. However, contrary to our expectations, continuous monitoring of ceiling cultures by time-lapse microscopy revealed many small cells adhering to adipocytes that proliferated rapidly into cells with a fibroblastic morphology and without any dedifferentiation from adipocytes. We named these cells bone marrow peri-adipocyte cells (BM-PACs) and demonstrated the multipotent properties of BM-PACs compared to that of conventionally cultured canine bone marrow mesenchymal stem cells (BMMSCs). BM-PACs showed significantly greater clonogenicity and proliferation ability than BMMSCs. An in vitro trilineage differentiation assay revealed that BM-PACs possess adipogenic, osteogenic, and chondrogenic capacities superior to those of BMMSCs. Flow cytometric analysis revealed that the expression of CD73, which plays an important role in cell growth and differentiation, was significantly higher in BM-PACs than in BMMSCs. These results indicate that canine BM-PACs have stem cell characteristics that are superior to those of BMMSCs, and that these mesenchymal stem cells (MSCs) appear to be a feasible source for cell-based therapies in dogs.