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.
Highlights
A debate is ongoing about the ‘stem cell’ status of mesenchymal stem cells (MSCs)
Boregowda et al [2] disagreed with the concept proposed by Caplan and suggested that defining MSCs as stem cells will better define their potential since the stem cell properties and paracrine functions of MSCs are interdependent
Ratajczak and Ratajczak [3] discussed the regenerative potential of MVs, which is being tested in various animal models. How do these MVs derived from MSCs act and do they preclude a role for stem/progenitor cells in regenerative medicine? I discuss this based on our studies [4, 5] wherein chemoablated mouse testes were regenerated on transplanting MSCs
Summary
A debate is ongoing about the ‘stem cell’ status of mesenchymal stem cells (MSCs). This can be resolved based on the definition of a stem cell. I discuss this based on our studies [4, 5] wherein chemoablated mouse testes were regenerated on transplanting MSCs. It is well known that busulphan treatment depletes the adult mouse testes of sperm and germ cells in the seminiferous tubules whereas Sertoli cells survive. We reported that a novel population of pluripotent stem cells, termed very small embryonic-like stem cells (VSELs), survives in the chemoablated testis [4, 5] and similar stem cells were detected in azoospermic, human testicular biopsies collected from survivors of childhood cancers [6].
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