Naturally occurring cell death during postnatal development of rat skeletal muscle.

Naturally occurring cell death has been extensively analyzed in many tissues, but little data exist regarding its occurrence in developing skeletal muscle. We investigated its occurrence and time course in rat hindlimb skeletal muscles during the first 3 weeks of postnatal development, its morphological and biochemical features, and the concomitant expression of Bax, Bcl-2, and Bcl-x(L). Myofibers displaying morphological features of apoptosis were found during the first 9 postnatal days. Terminal transferase (TdT)-mediated dUTP-biotinylated nick end labeling (TUNEL)-positive nuclei were present at all days examined and peaked between postnatal days 5 and 7. Total genomic DNA extracted from muscles at postnatal days 5, 7, and 9 showed internucleosomal fragmentation after Southern hybridization. Constitutive levels of Bax, Bcl-2, and Bcl-x(L) were detected by means of reverse transcriptase-polymerase chain reaction (RT-PCR) analysis at all ages examined, with a moderate increase around the period of maximal apoptosis. The results show that apoptosis and a concurrent expression of some genes of the Bcl-2 family, occur postnatally in rat skeletal muscle. This information is relevant to studies addressing the mechanisms of developmental muscle injuries.