While local muscle resident MSCs are a logical candidate as HO progenitors, other cells have been proposed. Some studies have implicated vascular endothelial cells as a potential source for HO progenitors [8]. Constitutively activated ACVRI in FOP change the morphology of endothelial cells to mesenchymal-like

cells and induce the co-expression of mesenchymal markers in vitro, a process that Tyrosine Kinase Inhibitor Library concentration resembles the endothelial–mesenchymal transition [8]. Moreover, endothelial marker Tie2 has been histologically observed in heterotopic lesions from patients with FOP. In addition, lineage tracing studies using Tie2-Cre reporter mice indicated that these cells generate approximately half the chondrocytes and osteoblasts found in skeletal muscle lesions [8] and [9]. However, Tie2 is not specific to endothelial cells and is also expressed in a number of non-endothelial cell types, including perivascular cells [10] and [11]. It has also

been shown in vivo that the endothelial fraction of murine Tie2 cells (Tie2+CD31+) does not participate Rucaparib nmr in HO whereas the non-endothelial fraction of Tie2 cells (Tie2+CD31−) does [12]. These recently published findings strongly suggest that the Tie2 progenitors observed in HO are not of endothelial origin [7]. Indeed, more than 90% of Tie2+CD31− cells are also PDGFRα+Sca1+, pointing to a mesenchymal rather than an endothelial origin [12], which supports the findings of Leblanc et al., who showed that

a Sca1+CD31− muscle resident stromal cell population contributes to HO [2]. In humans, PDGFRα has been reported to be a specific marker for interstitial mesenchymal progenitors that are distinct from CD56+ myogenic cells and that possess adipogenic and fibrogenic potentials [13]. While human skeletal muscle PDGFRα+ cells display osteogenic potential in vivo [14], the confirmation of their osteogenic activity came from subcutaneous-implanted cell-loaded PLGA-hydroxyapatite blocks, which are not likely representative of the HO environment. In addition, their osteogenic activity was comparable to CD56 myogenic Lepirudin cells [14], suggesting that PDGFRα may not be a marker that is exclusive to osteogenic progenitors. Other human studies have shown that a fraction of skeletal muscle adherent cells can give rise to osteoblasts and that this potential is greatly increased following trauma [15] and [16]. A multipotent myo-endothelial cell population in human skeletal muscle has been characterized based on the presence of myogenic (CD56) and endothelial (CD34, CD144) cell surface markers and the ability to differentiate into mesenchymal lineages [17]. Interestingly, the brown adipogenic potential of these putative HO progenitors has not been investigated, although it has been shown that brown adipocytes can promote endochondral ossification in an HO mouse model by regulating oxygen availability and inducing a hypoxic microenvironment [18] and [19].