One of the most fascinating questions in regenerative biology is why some animals can regenerate injured structures while others cannot. Skeletal muscle has a remarkable capacity to regenerate even after repeated traumas, yet limited information is available on muscle repair mechanisms and how they have evolved. For decades, the main focus in the study of muscle regeneration was on muscle stem cells, however, their interaction with their progeny and stromal cells is only starting to emerge, and this is crucial for successful repair and reestablishment of homeostasis after injury. In addition, numerous murine injury models are used to investigate the regeneration process, and some can lead to discrepancies in observed phenotypes. This review addresses these issues and provides an overview of the some of the main regulatory cellular and molecular players involved in skeletal muscle repair.Key words: skeletal muscle, regeneration, stem cells, evolution, quiescence, injury
IntroductionThe ability to regenerate tissues and structures is a prevalent feature of metazoans although there is significant variability among species ranging from limited regeneration of a tissue (birds and mammals) to regeneration involving the entire organism (cnidarians, planarians, hydra). The intriguing evolutionary loss of regenerative capacity in more complex organisms highlights the importance of identifying the underlying mechanisms responsible for these diverse regenerative strategies. One of the most studied tissues that contributes to new appendage formation is skeletal muscle, thereby making it a major focus of regeneration studies during evolution. The emergence of new lineage-tracing tools in different animal models has permitted the identification of specific progenitor cell populations and their contribution to tissue repair.Skeletal muscles allow voluntary movement and they play a key role in regulating metabolism and homeostasis in the organism. In mice and humans this tissue represents about 30-40% of the total body mass. This tissue provides an excellent tractable model to study regenerative myogenesis and the relative roles of stem and stromal cells following a single, or repeated rounds of injury. Although muscle regeneration relies mainly on its resident muscle stem (satellite) cells (MuSCs) to effect muscle repair, interactions with neighbouring stromal cells, by direct contact or via the release of soluble factors, is essential to restore proper 3 function. Each step of the myogenic process is regulated by specific regulatory factors including extrinsic cues, yet the nature and source of these signals remain unclear. This review will address these issues and discuss the different experimental models used to investigate the regenerative process.
Prenatal and postnatal skeletal muscle developmentIn amniotes, skeletal muscles in the limbs and trunk arise from somites through a series of successive waves that include embryonic and foetal phases of myoblast production (Biressi et al., 2007;Comai and Tajbakhsh, 201...