Satellite cell number steadily decreases from P6, to reach the adult level by P21. Satellite cells were identified and counted on isolated Myf5nlacZ/+ and 3F-nlacZ-E myofibres and pooled at each age to give a mean ± SEM (a). The number of satellite cells per myofibre decreased by 0.66 satellite cells per day (R2 = 0.98) to reach the adult level by P21, when it remained unchanged until 10 months, our oldest age analysed. The same data with satellite cells expressed as a percentage of total myofibre nuclei (myonuclei plus satellite cells) (b). Due to the rapidly increasing myonuclear number, there is an exaggerated fall from P6 until P21. Data shown are mean ± SEM (n > 30 myofibres from at least 6 mice at P7, P14, P21 and P56, and at least 2 mice per age at all other stages).