Kinetics of skeletal muscle regeneration after mild and severe muscle damage induced by electrically-evoked lengthening contractions.

Post-injury skeletal muscle regeneration requires interactions between myogenic and non-myogenic cells. Our knowledge on the regeneration process is mainly based on models using toxic, chemical, or physical (e.g., based on either muscle freezing or crushing) injury. Strikingly, the time course and magnitude of changes in the number of cells involved in muscle regeneration have been poorly described in relation to mild and severe muscle damage induced by electrically-evoked lengthening contractions. We investigated for the first time the kinetics and magnitude of changes in mononuclear cells in relation to the extent of muscle damage. Mild and severe injury were induced in vivo in the mouse gastrocnemius muscle by 1 and 30 electrically-evoked lengthening contractions, respectively. Several days after muscle damage, functional analysis of maximal torque production and histological investigations were performed to assess the related cellular changes. Torque recovery was faster after mild injury than after severe muscle damage. More necrotic and regenerating myofibers were observed after severe muscle damage as compared with mild injury, illustrating an association between functional and histological alterations. The kinetics of changes in muscle stem cells (total, proliferating, and differentiating), endothelial cells, fibro-adipogenic progenitors (FAPs), and macrophages in the regenerating muscle was similar in mild and severe models. However, the magnitude of changes in the number of differentiating muscle stem cells, hematopoietic cells, among which macrophages, and FAPs was higher in severe muscle damage. Collectively, our results show that the amount of myogenic and non-myogenic cells varies according to the extent of skeletal muscle injury to ensure efficient skeletal muscle regeneration while the kinetics of changes is independent of muscle tissue alterations. The possibility to experimentally modulate the extent of muscle damage will be useful to further investigate the cellular and molecular events involved in muscle regeneration.