Numbers of Pax7-positive satellite cells and of fibre branches in mdx and WT muscles. (A) Numbers of satellite cells on myofibres extracted from EDL muscles of 1- to 28-week-old WT and mdx mice, shown as Pax7-positive cells per fibre (mean ± SEM). Up to week 4, mdx fibres carried markedly fewer satellite cells than those of WT. Beyond 6 weeks of age, the numbers of satellite cells on mdx fibres increased progressively and significantly exceeded the WT number at 14 and 28 weeks.(B) Pax7-positive cells on fibres extracted from EDL muscles of 1- to 28-week-old WT and mdx mice, normalized to total myofibre nuclei (mean ± SEM). Again, during the first 4 weeks, the percentage of satellite cells on mdx fibres is markedly lower than that on WT fibres but, at 6 weeks and beyond, the numbers are closely similar, suggesting a link between myonuclear density and satellite cell number in mature muscle. (C) Numbers of Pax7-positive cells per myofibre profile in frozen sections of muscle from 2-week-old mdx and WT mice (three individual mice of each strain). In the sectioned muscle, Pax7-positive cells are equally frequent in WT and mdx mice at this age. (D) Frequency of fibres bearing one to four branches in muscles of mdx mice at 6, 14 and 28 weeks of age. No branches were seen on fibres extracted from three batches of mdx mice younger than 6 weeks or from WT mice at any age examined, but the frequency and complexity of branching increased progressively with age of the mdx mouse.