Myofiber growth and extracellular matrix (ECM) accumulation after damage differ in young and aged muscles. (Top) In young muscles in response to injury, the satellite cells under the basal lamina can be activated by environmental cues released by the neighboring cells (a local milieu composed of fibroblasts, interstitial cells, resident macrophages, fibro/adipogenic progenitors (FAPs) and microvasculature-associated cells) to eventually form new fibers almost indistinguishable to the pre-existing ones. (Bottom) In aged muscles, the repair process will result in reduced size of newly formed myofibers and thickening of the basal lamina by enhanced deposition of ECM components, which could potentially be ascribed to the increased presence and activity of resident fibroblasts (and/or FAPs), and to decreased satellite-cell myogenic potential. Conversion of myogenic into fibrogenic cells could also contribute to ECM accumulation. These new microenvironment conditions within the satellite-cell niche will impede efficient satellite-cell functions and muscle repair.