The FACS purified and cultivated pericytes described in

The FACS-purified and cultivated pericytes described in the present study along with those described by others who documented MSC-like behavior of pericytes do not appear to be the same as mesodermal progenitor Phenyl sulfate (MPCs) (Caplan, 2008; Crisan et al., 2008c), a subset of MSCs in human bone marrow-derived populations that were shown to exhibit angiogenic potential (Petrini et al., 2009). Localization of our ex vivo cultured pericytes alongside branching endothelial cells in vitro is characteristic of other pericytic populations (Mayo and Bearden, 2015) and distinguishes our aortic-derived pericytes from MPCs. Furthermore, the capacity of these aortic adventitial-derived pericytes to form spontaneous spheroids and subsequent sprouting has not been previously reported for similar cell types. A lack of CD34 and CD31 expression on our sorted adventitia-derived pericytes defines them as a population altogether distinct from MPCs, MSCs derived from CD34+ adventitial cells (Corselli et al., 2011; West et al., 2016; Zimmerlin et al., 2010), and CD34+ supra-vasa cells described herein by our group. Several vascularized tissues of fetal and adult origin such as white adipose, lung, pancreas, muscle, and placenta were all found to harbor cells capable of multi-lineage differentiation (Caplan, 2008; Corselli et al., 2011; Crisan et al., 2008c); and a recent study challenges the notion of a universal MSC by conversely showing that populations of cells in multiple tissues and organs with similar surface marker profiles differ broadly in their in vivo potential (Sacchetti et al., 2016). Since the aortic adventitia is associated with peri-adventitial adipose and human pericytes isolated from stromal vascular fraction of white adipose tissue (Zimmerlin et al., 2010) both exhibited high adipogenic potential, one could reasonably question whether the cells described in our study are adipose stem cells (ASCs). ASCs exhibit basal and TGF-β1-inducible expression of α-SMA (Lee et al., 2006), and pericytes may represent progeny of a subset of ASCs (Baer, 2014). In contrast to our sorted pericytes, ASCs have also been described in situ as CD34-expressing cells that lack expression of α-SMA, CD146, and CD105 (Baer, 2014). Therefore, the CD34+/CD146− cells we classified as supra-vasa are the more likely candidates to be considered ASCs, and populations from the stromal vascular fraction with identical reported surface proteomes were previously shown to be adipogenic, albeit not as potently as the pericyte fraction (Zimmerlin et al., 2010). Indeed, as summarized in Table 2, similar populations in human adult specimens of blood vessels and other vascularized tissues have been shown to have multi-lineage potential, like MSCs. Several highly analogous cell populations in other human adult blood vessels have been shown to be SMC/pericyte progenitors and exhibited propensity for adipogenic, chondrogenic, and osteogenic lineage progression (Klein et al., 2011). Furthermore, several studies of human fetal tissues (Invernici et al., 2007) and in large (Zaniboni et al., 2014) and small animal models (Howson et al., 2005; Hu et al., 2004) have contributed important knowledge regarding the identity, location, and function of MSCs and other perivascular cell subsets. Curiously, recent fate-mapping interrogations in mouse could not find any appreciable contribution of pericytes and SMCs to other lineages during aging or induced disease states (Guimarães-Camboa et al., 2017). On the contrary, others revealed that adventitial cells co-expressing Sca-1, CD34, and Gli1 contribute to atherosclerotic lesions in the intima in ApoE−/− mice (Hu et al., 2004). Likewise, Kramann et al. (2016) showed through inducible lineage tracing that CD34+/Sca1+/Gli1+ adventitial cells differentiate to mature SMCs, migrate to intimal atherosclerotic lesions, and localize in close proximity to regions of vascular calcification in vivo. Since Gli1+ cells co-express CD34 and lack CD31, they appear to be distinct from our pericytes, but isolated cells may include a subpopulation of pericytes or other non-endothelial adventitial cells that partially share these markers. Future in vivo studies investigating the fate of our pericytic population in physiological and pathological settings are required to better appreciate the potential contribution of vasa vasorum-associated pericytes to other lineages.