AbstractMicro‐environment seems to exert an important influence on human mesenchymal stem cell (MSC) differentiation and proliferative capacity in bone marrow as well as in culture ex vivo. Oct‐4, Rex‐1, and TERT genes are well‐known for the maintenance of pluripotentiality differentiation and the proliferative capacity of embryonic stem cells. Some previous data report expression of these embryonic factors in selected clones from bone marrow adult stem cells. Our goal was to study expression of Oct‐4, Rex‐1, and TERT in primary cultured human MSC according to the serum concentration. In addition, we have studied the expression of Gata‐4 since this factor plays a key role in organogenesis. We hypothesized that low serum concentration with appropriate growth factors may induce an undifferentiated status with a re‐expression of embryonic factors and extend differentiation capacity. Thus, using a defined culture medium, we report on the increased expression of Oct‐4, Rex‐1, and Gata‐4 in human MSC. We have correlated this expression to an increase in differentiation efficiency towards osteogenic and adipogenic phenotypes. Our data suggest that the culture medium used permits the emergence of adult stem cells with a high differentiation capacity and expression of embryonic factors. These cells may have important implications for cell therapy. J. Cell. Biochem. 101: 271–280, 2007. © 2007 Wiley‐Liss, Inc.