Human Sternal Mesenchymal Stem Cells: Isolation, Characterization and Cardiomyogenic Differentiation

Background: Bone marrow mesenchymal stem cells (MSCs) have multiple potentials and represent an attractive cell population for regenerative medicine. Here, we isolated human sternal MSCs (hMSCs) from patients undergoing cardiac surgery and investigated their growth properties, immortalization and the cardiomyogenic differentiation. Methods: Bone marrow from sternum during cardiac surgery was collected (n = 20). hMSCs were isolated through immuno-depletion, density gradient centrifugation and selective adhesion. Their growth properties were determined. Sternal hMSCs were characterized by surface antigens, immortalized by HPV16 E6/E7 retroviral vector and then exposed to 5-azacytidine. Results: Sternal liMSCs showed a gradual loss of ability to proliferate during cell passage, accompanied by a morphological conversion of senescence tendency. They uniformly lacked antigens typically identifying haematopoietic cells. They were positive for CD90, CD44, CD29 and other MSCs markers (SH2, SH3 and SH4). The HPV16 E6/E7 retroviral vector successfully immortalized the sternal hMSCs. After induction of 5-azacytidine, primary culture, early passage and immortalized E6/E7 hMSCs, but not late passaged cells, demonstrated the aborted cardiomyogenic differentiation by activating a cardiac-specific gene, cardiac troponin-I without a recognizable morphologically differentiated phenotype. Conclusion: Expanded sternal hMSCs from patients undergoing cardiac surgery showed senescent tendency with decreased ability of proliferation and differentiation in cardiomyogenic lineage. Specifically immortalized hMSCs may regain the ability of cardiomyogenic differentiation and serve as a useful cell source for elucidation of cardiomyogenic differentiation.