Assessment of myogenic activity in muscles of mdx and WT mice. (A) Protocol for labelling cell participation in muscle regeneration. Mice are given drinking water containing 0.8 mg/ml BrdU for 7 days and maintained on normal drinking water for a further 7 days before being euthanased for analysis. (B) Section from a gastrocnemius muscle taken from a mdx mouse subjected to the regime in (A). Seven days after the end of BrdU administration, it shows extensive labelling of both interstitial cells and centrally located myonuclei in a spontaneous mdx muscle lesion (green or yellow). Virtually all of the centrally placed myonuclei are labelled, showing that the BrdU had not been diluted to subliminal levels by proliferation of the myogenic cells that had repaired this particular lesion. (C) Gastrocnemius of a WT mouse subjected to this same labelling regimen. An occasional interstitial nucleus is stained, but there are no centrally placed myonuclei and no identifiable myonucleus is stained. Thus, the cessation of myonuclear-based growth at 3 weeks, identified in the isolated fibres of the WT EDL muscles, is a generalized phenomenon extending to the gastrocnemius muscle and to other limb muscles examined.