Mdx muscles show excess lysosomal activity prior to disease and massive turnover after onset. (A) Cryostat sections of TA muscles from 2-week-old mdx and WT muscles immunostained for laminin (red) to show fibre outlines and Lamp1 (green) and DAPI (blue) to show the strong representation of lysosomes within mdx fibres as a possible contributory factor to the hypotrophic condition of these fibres. (B) Immunostains for BrdU (green) and laminin (red) counter-stained with propidium iodide (red) of the section of TA muscles from mice injected twice daily with BrdU during the first week of life and euthanased at 16 or 42 days of age. At 16 days, prior to the onset of myonecrosis, muscle from both strains showed similar frequencies of labelled nuclei. N = 4 muscles. Data are shown as mean ± SD. In the WT muscle, the BrdU label had been retained at 42-day muscle, both by identifiable myonuclei (arrowed) and by interstitial cells but had been lost, apart from the occasional rare (1 to 2 per whole section) centrally placed labelled myonucleus in the mdx muscles.