No identical 'mesenchymal stem cells' at different times and sites: Human committed progenitors of distinct origin and differentiation potential are incorporated as adventitial cells in microvessels

A widely shared view reads that 'MSCs' are ubiquitous in human connective tissues, can be defined by a common in vitro phenotype, share a skeletogenic potential as assessed by in vitro differentiation assays, and coincide with the ubiquitous 'pericytes.' Using stringent in vivo differentiation assays and transcriptome analysis, we show here that human cell populations from different anatomical sources, which would all be regarded as 'MSCs' based on these criteria and assumptions, actually differ widely in their transcriptomic signature and in vivo differentiation potential. In contrast, they share the capacity to guide the assembly of functional microvessels in vivo, regardless of their anatomical source, or in situ identity as perivascular or circulating cells. This analysis further reveals that muscle 'pericytes,' which are not spontaneously osteo-chondrogenic as previously claimed, may indeed coincide with an ectopic perivascular subset of committed myogenic cells similar to satellite cells. Cord blood-derived stromal cells, on the other hand, display the unique capacity to form cartilage in vivo spontaneously, in addition to an assayable osteogenic capacity. These data suggest the need to revise current misconceptions on the origin and function of so-called 'MSCs,' with important applicative implications. The data also support the view that rather than a uniform class of 'MSCs,' different mesoderm derivatives include distinct classes of tissue-specific committed progenitors, likely of different developmental origin.