Although skeletal muscles appear superficially alike at different anatomical locations, in reality there is considerably more diversity than previously anticipated. Heterogeneity is not only restricted to completely developed fibers, but is clearly apparent during development at the molecular, cellular and anatomical level. Multiple waves of muscle precursors with different features appear before birth and contribute to muscular diversification. Recent cell lineage and gene expression studies have expanded our knowledge on how skeletal muscle is formed and how its heterogeneity is generated. This review will present a comprehensive view of relevant findings in this field.

Skeletal myogenesis, like hematopoiesis, occurs in successive developmental stages that involve different cell populations and expression of different genes. We show here that the transcription factor nuclear factor one X (Nfix), whose expression is activated by Pax7 in fetal muscle, in turn activates the transcription of fetal specific genes such as MCK and beta-enolase while repressing embryonic genes such as slow myosin. In the case of the MCK promoter, Nfix forms a complex with PKC theta that binds, phosphorylates, and activates MEF2A. Premature expression of Nfix activates fetal and suppresses embryonic genes in embryonic muscle, whereas muscle-specific ablation of Nfix prevents fetal and maintains embryonic gene expression in the fetus. Therefore, Nfix acts as a transcriptional switch from embryonic to fetal myogenesis.

Skeletal muscle development occurs asynchronously and it has been proposed to be dependent upon the generation of temporally distinct populations of myogenic cells. This long-held hypothesis has not been tested directly due to the inability to isolate and analyze purified populations of myoblasts derived from specific stages of prenatal development. Using a mouse strain with the GFP reporter gene targeted into the Myf5 locus, a cell-sorting method was developed for isolating embryonic and fetal myoblasts. The two types of myoblasts show an intrinsic difference in fusion ability, proliferation, differentiation and response to TGFbeta, TPA and BMP-4 in vitro. Microarray and quantitative PCR were used to identify differentially expressed genes both before and after differentiation, thus allowing a precise phenotypic analysis of the two populations. Embryonic and fetal myoblasts differ in the expression of a number of transcription factors and surface molecules, which may control different developmental programs. For example, only embryonic myoblasts express a Hox code along the antero-posterior axis, indicating that they possess direct positional information. Taken together, the data presented here demonstrate that embryonic and fetal myoblasts represent intrinsically different myogenic lineages and provide important information for the understanding of the molecular mechanisms governing skeletal muscle development.

We have previously observed that Wnt signaling activates a fibrogenic program in adult muscle stem cells, called satellite cells, during aging. We genetically labeled satellite cells in a mouse model of Duchenne muscular dystrophy to follow their fate during the progression of the disease. We observed that a fraction of satellite cells had a reduced myogenic potential and showed enhanced expression of profibrotic genes compared to age-matched controls. By combining in vitro and in vivo results, we found that expression of transforming growth factor–β2 (TGFβ2) was induced in response to elevated canonical Wnt signaling in dystrophic muscles and that the resulting increase in TGFb activity affected the behavior of satellite cells in an autocrine or paracrine fashion. Indeed, pharmacological inhibition of the TGFb pathway in vivo reduced the fibrogenic characteristics of satellite cells. These studies shed new light on the cellular and molecular mechanisms responsible for stem cell dysfunction in dystrophic muscle and may contribute to the development of more effective and specific therapeutic approaches for the prevention of muscle fibrosis.

Satellite cells, the adult stem cells responsible for skeletal muscle regeneration, are defined by their location between the basal lamina and the fiber sarcolemma. Increasing evidence suggests that satellite cells represent a heterogeneous population of cells with distinct embryological origin and multiple levels of biochemical and functional diversity. This review focuses on the rich diversity of the satellite cell population based on studies across species. Ultimately, a more complete characterization of the heterogeneity of satellite cells will be essential to understand the functional significance in terms of muscle growth, homeostasis, tissue repair, and aging.