Macrophage Plasticity and the Role of Inflammation in Skeletal Muscle Repair

Figure 1

Inflammation and macrophage polarization in skeletal muscle injury and repair. Satellite cells are muscle-resident stem cells which are located underneath the basal lamina of myofibers and are normally quiescent (top right). Upon muscle injury, satellite cells get activated, start to proliferate as myoblasts, and subsequently fuse and differentiate into myotubes that later grow thereby replacing damaged muscle. Several cell types influence the outcome of regeneration, in particular inflammatory cells released from the blood (top left). Proinflammatory monocytes and neutrophils (not shown) extravasate shortly after damage, invading the injured areas where they differentiate into proinflammatory macrophages that phenotypically resemble M1 macrophages. These cells clear the damage and release a number of cytokines that stimulate myoblast proliferation. M2-like macrophages are present locally at later stages of regeneration acting as promoters of myoblast differentiation and fusion. Other cell types such as mast cells and lymphocytes also have less defined roles in muscle repair (not shown).