Satellite cells comprise adult muscle stem cells and committed myogenic procursors; they are involved in muscle growth after birth and in muscle regeneration after muscle damage. A prevalent view is that satellite cells first become activated, and then divide and differentiate before fusing to existing myofibers to initiate muscle growth. M-cadherin (Mcad), a cell-cell adhesion factor of satellite cells, is thought to be important for the fusion process, but the exact underlying molecular details are not clear. To better understand satellite cell activation and function, Juan Carlos Izpisua Belmonte and colleagues (p. 5116) follow satellite cells of newborn mice by electron and confocal microsopy, and also in vitro in cell culture. They find that activated satellite cells initiate fusion with myofibers while they are still in mitosis; in contrast to the proposed model of fusion of divided and differentiated satellite cells. In addition, stimulation of satellite cells in vitro with Mcad promotes their cell cycle progression, thereby increasing the number of activated cells and accelerating cell proliferation. Conversely, inhibition of Mcad by addition of anti M-cadherin antibodies results in fewer satellite cells with slower proliferation rates. Taken together, these results suggest that Mcad-mediated cell-cell interactions have a role in satellite cell division that could be used in strategies aimed at enhancing muscle turnover and regeneration in muscle dystrophies.