We previously identified a minimal surface

We previously identified a minimal surface phenotype suited not only to enrich the archetypal human “MSCs” in uncultured BM cell suspensions, but also to correlate their ex vivo-assayed clonogenic capacity with their in situ identity and in vivo fate following transplantation (Sacchetti et al., 2007). As applied to the study of BMSCs, this led to identification of “MSCs” as subendothelial, perivascular CD146+ purchase 4-ethylphenyl sulfate on BM sinusoids, and also provided evidence for their self-renewal in vivo, which had long been the missing evidence to support the claim that BMSCs indeed include a subset of bona fide stem cells, rather than multipotent progenitors (Bianco et al., 2013; Sacchetti et al., 2007). Using an identical approach to prospectively isolate “MSCs” from a variety of non-BM tissues, Crisan and co-workers later reported that a ubiquitous population of highly myogenic and skeletogenic CD146+ cells, coinciding with “MSCs,” is found in association with microvessels of skeletal muscle and other tissues, lending support to the view of pericytes as a uniform, widely distributed population of cells that can be explanted and cultured as “MSCs” (Caplan, 2008; Caplan and Correa, 2011; Crisan et al., 2008). However, striated muscle and skeletal lineages such as bone, cartilage, and marrow fat diverge early in development, and no common progenitor of bone and muscle is found in prenatal life past the time of sclero-myotome specification in somites (Applebaum and Kalcheim, 2015). The notion of a common postnatal progenitor of bone and muscle, therefore, would be at odds with established tenets in developmental biology (Bianco and Robey, 2015). We show here that MCAM/CD146-expressing stromal cells from different human tissues diverge radically from their BM counterparts in differentiation potency and transcriptional profile, reflective of their different developmental origin. While BM-derived “MSCs”/pericytes are natively skeletogenic but not myogenic, muscle-derived “MSCs”/pericytes are inherently myogenic but not natively skeletogenic, and appear to represent a subset of cells with functional features of satellite cells, but not their characteristic anatomical location. We further show that prenatal, cord blood-borne “MSCs” in turn exhibit a distinct transcriptional and potency profile, and an inherent cartilage commitment, which diverge markedly from that of postnatal BM-derived “MSCs.” Finally we show that, irrespective of the postnatal tissue source of these perivascular cells or from fetal blood, these committed progenitors of mesoderm derivatives can associate with nascent blood vessels (BVs) in vivo and be recruited to a mural cell fate. However, a system of committed and self-renewing progenitors with distinct native potency, and not a uniform, equipotent class of “MSCs” is associated with microvascular walls in postnatal mesoderm-derived tissues as reported previously for bone/marrow (Sacchetti et al., 2007), and as shown herein for muscle. Pericyte recruitment from preexisting local progenitors is a simple developmental process that explains the very existence of such progenitors in postnatal life and their tissue-specific properties.
Results
Discussion A widely accepted view in the literature holds that “MSCs” can be defined by rather loose and non-specific in vitro properties, and exhibit identical functional and differentiation properties regardless of their tissue source. This view, complemented by the notion that “MSCs” would coincide with ubiquitous pericytes, was recently reinforced by the claim that MU-derived pericytes would be both myogenic and skeletogenic, and exhibit an in situ surface phenotype similar to the one characteristic of BMSCs (i.e., of a skeletal stem cell origin) (Bianco, 2014). We have shown here that CD34−/CD45−/CD146+ “MSCs” isolated from different tissues have inherently distinct transcriptomic signatures and differentiation capacities. By gauging their native differentiation potential with a variety of stringent differentiation assays, we demonstrated that human BM CD146+ cells are inherently geared to generate bone and BM stroma that support hematopoiesis and include adipocytes, but are not myogenic, and are not spontaneously chondrogenic in vivo; MU “MSCs” with an identical cell-surface phenotype are not inherently skeletogenic, and are inherently myogenic; and CB “MSCs” are not myogenic, but are chondro-osteoprogenitors most likely due to their fetal origin (Bianco and Robey, 2015) and are the only kind of human “MSCs” ever shown to actually form cartilage consistently in open heterotopic transplants in vivo, independent of any ex vivo induction to cartilage differentiation as applied in the widely used pellet culture assay. CB cells are unlike other cells that are found in the umbilical cord, based on their cell-surface markers, their ability to differentiate into adipocytes and cartilage, and expression of associated markers, and they have a distinct expression pattern of HOX genes compared with other umbilical cord derivatives (Bosch et al., 2012; Liedtke et al., 2010, 2016). BM- and MU-derived clonogenic progenitors of mesoderm derivatives are associated with BV walls in situ and include cells in a position characteristic of mural cells/pericytes, but also include vascular-wall cells that do not necessarily exhibit anatomical features of pericytes proper, for example, adventitial cells in muscular veins. More strikingly, fetal chondro-osteoprogenitors that circulate in CB, by definition, are neither associated with BV walls nor represent pericytes. The origin of these cells remains to be elucidated. It is conceivable, however, that skeletal progenitors located at sites of active skeletal growth can accidentally spill over into the bloodstream. Nonetheless, independent of their origin, native differentiation potencies, gene-expression profiles, and in situ anatomical positioning, perinatal and postnatal mesoderm progenitors of different origins can dynamically associate with, and organize, nascent BVs as shown here using a simple in vivo assay.