In 2010, two groups characterized a population of skeletal muscle-resident progenitors with bipotent fibro/adipogenic potential [28, 29]. Using fluorescence-activated cell sorting on digested mouse muscle preparations, Joe et al.  isolated fibro/adipogenic progenitors (FAPs) based on CD45−/CD31− (lineage-negative), α7 integrin−, Sca1+ and CD34+ cell-surface antigen presentation. Similarly, Uezumi et al.  isolated a functionally and phenotypically equivalent population of mesenchymal progenitors based on CD45−/CD31−, SM/C2.6− and PDGFRα+ expression. FAPs  and mesenchymal progenitors  readily entered adipocyte and fibroblast differentiation spontaneously in vitro in bulk cultures as well as in clonal assays, producing both αSMA-expressing fibroblasts and perilipin/peroxisome proliferator-activated receptorγ-positive adipocytes. Both groups demonstrated that this cell population was capable of in vivo adipogenic differentiation when transplanted into glycerol-injected skeletal muscle (a fatty degeneration model) [28, 29]. The fibrogenic potential of this PDGFRα+ population has also been verified in vivo following transplantation of genetically labelled cells (PDGFRα–GFP) into γ-irradiated skeletal muscle after cardiotoxin injury . Here, GFP-labelled cells accumulated in areas of fibrosis within the muscle interstitium, presumably consistent with differentiation into collagen type I-producing cells. Recent work by Wosczyna et al.  revealed further developmental potency of this progenitor population in vivo. Lineage tracing of muscle-resident cells based on a Tie2-driven Cre-dependent GFP reporter revealed a significant contribution of this cell type to cartilage and bone formation in a model of heterotopic ossification. Analysis of cell-surface antigen expression in lineage-negative, Tie2–GFP+ cells revealed that ~ 90% were PDGFRα+ Sca1+, while the PDGFRα− Sca1− fraction did not contribute to bone or cartilage formation . These results provide evidence that an equivalent or closely related skeletal muscle progenitor as described by Joe et al.  and Uezumi et al.  is also capable of osteogenic and chrondogenic differentiation in vivo.
Immunohistochemistry demonstrates that these progenitors are localized to the muscle interstitium and adjacent to myofibre-associated blood vessels [28-31], although they do not express markers such as neuro-glial 2 proteoglycan , defining a cell population distinct from pericytes. Although provisionally labelled as FAPs, evidence highlighting the perivascular localization and capacity to differentiate down multiple skeletal lineages in vivo (as determined by the micro-environment they are transplanted into) suggests these cells may be best recognized as skeletal muscle-resident mesenchymal progenitors/stromal cells (MSCs).
In response to muscle injury, skeletal muscle mesenchymal progenitors become activated and expand rapidly [28, 32]. However, unlike satellite cells, which enter myogenic differentiation and repair damaged myofibres, mesenchymal progenitors do not contribute directly to regenerative myogenesis. Transplantation of genetically labelled FAPs (lineage-negative, α7 intrigin−, Sca1+, CD34+) or mesenchymal progenitors (lineage-negative, SM/C2.6−, PDGFRα+) into regenerating muscle made little or no contribution to the regenerating myofibres . Similarly, no myosin heavy chain-positive (MyHC+) myotubes were observed in clonal assays or following stimulation by low serum conditions in vitro, indicating minimal myogenic capacity of these cells [28, 29]. Instead, Joe et al.  demonstrated that mesenchymal progenitors play an important non-cell autonomous role in facilitating myogenesis. Using co-culture assays in vitro, mesenchymal progenitors were shown to promote myotube formation and differentiation of muscle progenitors . Although the precise ‘pro-myogenic’ signals/factors released from mesenchymal cells during muscle injury remain under investigation, interleukin-6 was significantly up regulated and remains an obvious candidate. The important role of non-myogenic mesenchymal cells was further highlighted by Murphy et al.  and Mathew et al. . Here, creation of Tcf4-Cre/CreERT2 alleles used in combination with the Cre-responsive ablator allele R26-DTA allowed the authors to investigate the consequences of ablation of Tcf4+ (transcription factor 7-like 2, Tcf7L2) connective tissue fibroblasts on developmental and regenerative myogenesis in vivo. During development, muscle-resident fibroblasts were shown to be important for slow MyHC expression and the maturation of fetal myofibres . In the adult, despite effective deletion of only 40% of Tcf4+ fibroblasts, skeletal muscle regeneration was significantly impaired following injury . Loss of skeletal muscle fibroblasts altered the proliferative kinetics of satellite cells and induced premature differentiation as determined by the early appearance of embryonic MyHC+ myofibres only 3 days after injury. Although regeneration still proceeded with a depleted fibroblast population in Tcf4-CreERT2/R26-DTA mice, regenerated myofibres were smaller in diameter and cross-sectional area. Using the same genetic strategy, Murphy et al.  also ablated satellite cells under the Pax7 locus (Pax7CreERT), and found that Tcf4+ fibroblasts failed to expand effectively in response to injury and did not decrease to pre-injury levels 28 days after injury. These results demonstrate nicely the important reciprocal interactions that occur between satellite cells and other progenitor populations during skeletal muscle regeneration. Interestingly, premature myogenic differentiation in the absence of fibrogenic cells differs from the results obtained by co-culture experiments by Joe et al. , who showed that mesenchymal cells provide a micro-environment that supports myotube formation in vitro. Such differences in findings probably reflect the differing models, cellular systems and precise cellular populations examined. Indeed, precise phenotypic and functional distinctions, if any, between ‘fibroblasts’ and MSCs in skeletal muscle and other tissues remain largely uncharacterized. In a classical model, a fibroblast perhaps represents a more lineage-committed cell type capable of producing extracellular matrix proteins such as collagen and other connective tissue components. However, an MSC may be distinguished by its broadened potency, being capable of differentiating into bone or fat as well as connective tissues, and perhaps, although yet to be shown, capable of self-renewal. Such controversies and conceptual ideas are the subject of an excellent recent review .
The above evidence highlights that skeletal muscle comprises, in addition to satellite cells, populations of tissue-resident mesenchymal progenitors with multi-potent lineage potential in vivo. Although not directly contributing to regenerative myogenesis in normal settings, such progenitors represent a critical component of the cellular niche required for effective satellite cell-mediated regeneration. Indeed, the paracrine effect of MSCs in assisting tissue regeneration has provided the underlining rationale for a plethora of clinical trials investigating stem cell transplantation therapies in various tissues, such as the heart [35, 36]. However, it seems unlikely that such progenitor populations exist solely to provide the correct environmental cues to facilitate myogenesis. Little is know about the contribution of mesenchymal progenitors to normal muscle repair processes. Do these progenitors differentiate into collagen-producing cells to restore damaged extracellular matrix and repair muscle architecture needed for myofibre/myofibril support? Are these cells capable of self-renewal? What are the molecular networks involved in their regulation? Answering of such questions will provide a more complete picture of the mechanisms and processes involved in muscle regeneration in the future.