Postnatal growth dynamics of EDL myofibres. Measuring myofibre cross-sectional area from mid-belly EDL muscle sections showed that there was a significant (ANOVA: p < 0.0001; F = 850.6) and highly linear (R2 = 1.0) increase with age (a), which was accompanied by a significant increase in the length of isolated EDL myofibres (b). Myonuclei were counted on isolated EDL myofibres from Myf5nlacZ/+ and/or 3F-nlacZ-E mice and there was a 4-fold increase in myonuclei number per myofibre between P3 and P14 from ~50 to ~200 (c). The adult complement of myonuclei (~250) was reached by P21. Mean total myofibre volume was calculated by multiplying mean length by mean cross-sectional area and increased in a linear fashion between P7 to P56 (blue line - d). By then dividing mean total myofibre volume by the mean number of myonuclei per myofibre, we estimated myonuclear domain during postnatal development (dashed red line - d). Data shown are mean ± SEM, except for (d), which is mean; trend line in (c) depicts the moving average [(a) n > 100 myofibres from each of at least 3 mice at each age; (b) n > 19 myofibres from each of at least 3 mice at each age; (c) n > 30 myofibres from each of at least 2 mice at each age, see Tables 1 and 2 for data sets].
White et al. BMC Developmental Biology 2010 10:21 doi:10.1186/1471-213X-10-21