MyoD^−/− myogenic precursor cells are resistant to apoptosis in vivo. (A) Before CTX injection (day 0), no Pax7⁺ TUNEL⁺ apoptotic satellite cells were detected in wild-type and MyoD^−/− TA muscles. 2 d after CTX injection, Pax7⁺ TUNEL⁺ apoptotic myogenic precursor cells (arrows) were detected in wild-type muscles. Bar, 20 µm. (B) Number of Pax7⁺ TUNEL⁺ apoptotic myogenic precursor cells was quantified. (C) 1 d after CTX injection, myoblasts prepared from Rosa26 or MyoD^−/−:Rosa26 mice were intramuscularly injected into regenerating TA muscle. By 2 d after cell injection, lacZ⁺ Pax7⁺ donor-derived myogenic precursor cells were examined for activated caspase-3 expression (arrows). Arrowheads denote lacZ⁻ Pax7⁺ activated caspase-3⁺ endogenous myogenic precursor cells. Bar, 10 µm. (D) The number of lacZ⁺ Pax7⁺ activated caspase-3⁺ apoptotic myogenic precursor cells was measured. Nuclei were counterstained with DAPI (blue). *, P < 0.05; **, P < 0.01. Error bars indicate SEM.