Fig. 1.

Populations of satellite cells, FAPs and myonuclei in uninjured skeletal muscle. Genetic studies support the existence of at least two subpopulations of quiescent satellite cells. Stem quiescent satellite cells have been identified by expression of high levels of Pax7 (Rocheteau et al., 2012) and by absence of expression of Myf5 (Kuang et al., 2007). Stem cells resistant to stress express Pax3 (Der Vartanian et al., 2019; Scaramozza et al., 2019). Myonuclei adjacent to neuromuscular junctions (NMJ) and myotendinous junctions (MTJ) are distinct from myonuclei along the main body of the myofiber (Jacobson et al., 2001; Koch et al., 2004; Petrany et al., 2020). There is currently no consensus on whether there are multiple subpopulations of quiescent FAPs. Muhl et al. (2020) identified FAP subpopulations localized to different muscle regions, including adjacent to the motor neurons. ECM, extracellular matrix.