Heterogeneity of multipotent mesenchymal stromal cells: from stromal cells to stem cells and vice versa.

Discovered more than 40 years ago, the biological features of multipotent mesenchymal stromal cells (MSC) were progressively compared first with hematopoietic stem cells (HSC) and, more recently, with embryonic stem cells (ESC). Although these comparisons have been crucial in helping to clarify their nature, there is now a robust amount of data indicating that MSC in vitro represent an independent and heterogeneous group of progenitors with distinct self-renewal properties and established differentiation potentials. However, research developments both in humans and animals have progressively revealed the limits that MSC may face in vivo. To recognize these issues and challenge MSC stemness may seem to be a step backward. Nevertheless, it might also represent the beginning of a phase in which the introduction of novel preclinical approaches could provide better characterization and standardization of the in vivo factors influencing cell engraftment and survival, allowing a more successful impact of mesenchymal progenitors in several clinical settings.