In the present study we analyzed the lineal relations between cells associated with myofibers and MSCs. MSCs were cloned from the bone marrow and myogenic and non-myogenic cells were cloned from individual myofibers. Our analyses suggest, as depicted in , that primordial stem cells that are the common ancestors of myofiber-associated myogenic and non-myogenic stem cells, first migrate to individual muscles and then part into myogenic and non-myogenic precursors. The latter two populations have a similar, but not identical, developmental path which is different from the lineage of bone marrow MSCs.
Although muscles are known to contain MSCs as part of their interstitium, and determining their lineage relations with myofiber-associated progenitors would have been informative, doing so in our current experimental setting was not practical, as isolating these MSCs requires digesting many muscles 
, and therefore we included in the lineage analysis bone marrow MSCs instead. Determining the lineage relations between bone-marrow MSCs and muscle-adhering MSCs, as well as the lineage relations between the latter and muscle-fiber derived progenitor cells remains a challenge for future research. Still, we note that: (i) MSCs from virtually all post-natal organs and tissues, including muscle interstitium, share very similar characteristics 
; and (ii) bone-marrow contains circulating MSC cells that reach other/distant organs in the body 
Satellite cells (all or a subpopulation) as well as several other types of cells, such as MSCs or white fat cells were suggested to account for such non-myogenic cells 
. The most attractive candidates are the MSCs as they are present in the muscle tissue and were shown to give rise to cells of several lineages including adipocytes and myoblasts 
Additionally, by means of multiple immuno-staining of clones derived from mice myofibers, we previously showed that the composition of the non-myogenic clones resembles that of MSC progeny 
. Our data clearly show that myofibers harbor myogenic and non-myogenic progenitors with a similar (but not identical) developmental path that is significantly different than the lineage of MSCs.
Potential candidate cells for a shared ancestor of the myogenic and non-myogenic founders maybe the mesoangioblasts/pericytes. Pericytes are cells that surround endothelial cells in capillaries and microvessels and are thought to include progenitors of different cell types including skeletal muscle cells 
. Specifically, Dellavalle and colleagues 
indicated that pericytes are myogenic precursors, distinct from satellite cells, which are associated with the microvascular walls in human skeletal muscles. Authors also pointed that these cells may represent a correlate of embryonic mesoangioblasts. In another study mesoangioblasts were isolated from embryonic dorsal aorta and shown to participate in postnatal muscle myogenesis 
. Albeit we cannot rule out the possibility that mesoangioblasts/pericytes are the common ancestors of the founder of MA-NM and MA-M, our past and present studies do not favor this explanation for the following three reasons. First, unlike cultures of pericytes/mesoangioblasms that possess extensive myogenic capacity in-vivo and in-vitro, the MA-NM clones did not contain myogenic cells even after long time culture and albeit supplemented in growth medium that favors myogenesis 
or upon co-culture with myogenic cells (Shefer, G. and Yablonka-Reuveni, Z unpublished data). Second, recent studies suggest that MSCs are of pericyte origin 
whereas present results indicate that MA-NM and MA-M founders share a common ancestor which is different from that of the MSCs. Third, if accepting that pericytes are the common ancestor of the MA-NM and MA-M founder, it would have been expected that MA-NM cells from the masseter would have been of a very different lineage than MA-NM from the limb muscles. This is because pericytes in the cephalic region are derived from neural-ectoderm, and not from mesoderm. Nevertheless, our results do not point to a significant lineage difference between clones from the masseter to clones derived from the limb muscles.
Whether the primordial stem cells or the myogenic progenitors represent the satellite cells pool cannot be conclusively established based only on the current lineage analyses. If the primordial stem cells represent the satellite cell population, then upon their population of target muscles they give rise to myogenic and non-myogenic cells. The notion that satellite cells can give rise to non-myogenic cell types accords with studies with rat and mice myofibers 
and a study with newts 
. In the latter study, an in-vivo
approach was taken to determine the fate of satellite cells during limb regeneration after amputation. Data showed that some of the re-introduced labeled satellite-cell derived clones adopted non-myogenic fates. Alternatively, if the myofiber-associated myogenic progenitors represent the satellite cells pool, then satellite cells are homogenous with regards to their differentiation breadth and are only in close, but not same, lineal relations with the ancestors of non-myogenic cells. This accords with evidence implying that satellite cells are uni-potent cells, giving rise to myogenic cells only 
Regardless of their origin, the finding that muscle fibers consistently harbor a distinct lineage of non-myogenic stem cells immediately raises the question on the biological function of such cells in health and disease. One may speculate that the presence of non-myogenic progenitors within the muscle maybe of advantage. For example, there may be an advantage in the immediate availability of fibroblasts in case of injury as such cells are needed to synthesize extracellular matrix proteins that take part in scar formation which is necessary for adequate myofiber-repair 
. The down side of an intimate source of non-myogenic cell may be reveled when the muscle niche is disturbed and encourages the proliferation of non-myogenic rather than the proliferation of myogenic cells. In such cases enhanced proliferation of non-myogenic cells may account for the fibrosis and adipose accumulation characteristics of myopathic diseases and aging 
In any event, this current study clearly demonstrates that myofiber-associated non-myogenic progenitors are not the progeny of MSCs. We thus conclude that at least some of the inter-muscular adipocytes and/or fibroblasts are the progeny of myofiber-associated progenitors rather than MSCs. This does not necessarily contradict with recent findings that MSCs also contribute to adipogenesis and fibrogenesis in skeletal muscles 
. Considering the finding that myogenic and non-myogenic progenitors share similar developmental path we postulate that cells of different source such as fat cells 
cannot be the source of all non-myogenic cells that develop in myofiber cultures.
In summary, the combination of computation and biological approaches allowed analyzing three different cell types at the same time, a commodity that is not available by any other means up to date. This allowed better understanding the differentiation dynamics of the pools of these stem cells and the lineal relationships between the subpopulations.