The need to revisit the definition of mesenchymal and adult stem cells based on their functional attributes

Abstract

A debate is ongoing about the 'stem cell' status of mesenchymal stem cells (MSCs). This can easily be resolved based on the definition of a stem cell. 'True' stem cells are expected to undergo asymmetric cell divisions (ACD) whereby they divide to self-renew and give rise to a slightly bigger, differentiated cell. However, MSCs like any other adult tissue-specific stem cells, including hematopoietic (HSCs), spermatogonial (SSCs) and ovarian (OSCs) stem cells, do not undergo ACD; rather they undergo rapid symmetrical cell divisions. The true stem cells in adult tissues are possibly the pluripotent stem cells termed very small embryonic-like stem cells (VSELs), which were recently shown to undergo ACD to give rise to tissue-specific stem cells 'progenitors' (currently termed 'adult stem cells') that in turn undergo rapid symmetric cell divisions and clonal expansion (sphere formation with incomplete cytokinesis) followed by differentiation into tissue-specific cell types. MSCs can be cultured from any tissue source and are an excellent source of growth factors/cytokines and thus could provide a niche for proper functioning of the stem/progenitor cells.