Steinhauser and Lee Pericyte Progenitors and Tissue Fibrosis 231 pathways to the various perivascular progenitor populations have not been definitively described. In addition to myofibroblasts, lineage-tracing studies have shown in vivo differentiation of pericytes into various terminally differentiated cell types, including adipocytes9 and myocytes. 10 Viewed collectively, the terminally differentiated products of pericyte lineage-tracing studies functionally overlap with the differentiated progeny of MSCs. MSCs can be isolated from multiple tissues other than bone marrow, and these cells can display multilineage differentiation potential into adipocytes, chondrocytes, osteoblasts, and myocytes. 11 Although the potency of MSCs suggests the possibility of linking several pericyte progenitors with a common ancestor, the existence of MSCs in tissues outside the bone marrow remains controversial because their potency has been largely defined in narrow culture conditions or with in vivo cell transplantation studies that include prolonged ex vivo expansion steps. 12 A specific genetic marker of MSCs remains elusive. Until such a marker is found or new approaches are developed to prospectively track endogenous cell fate in vivo, developmental links between pericyte populations and the existence of a multipotent MSC-like progenitor in the adult perivasculature will remain debated.