Immunohistochemical detection of myogenic cells in muscles of fetal and neonatal lambs.

Previous studies have implied that myonuclei accumulation in a muscle is more important than myofibre number in the determination of muscle size in fetal/neonatal lambs. However, due to the lack of a reliable marker, the role of myogenic precursor nuclei (satellite cells) in myofibre hypertrophy in late fetal and postnatal life is not well understood. In this study, MyoD was shown to be a useful marker for actively proliferating satellite cells in both fetal and neonatal lambs. MyoD was used to determine whether there were differences in the number of actively proliferating satellite cells between single and twin fetuses/neonates, which may explain at least some of the difference in myofibre size observed near birth. Eighteen single-bearing and 9 twin-bearing Coopworth ewes were randomly assigned to one of three slaughter groups (100, 120 and 140 days of gestation). The remaining ewes were kept on pasture until 20 days postpartum at which time 4 single and 4 twin lambs were sacrificed. Twin fetuses/neonates had lower body weights and muscle weights compared to singles. Lower muscle weights in the twins were associated with smaller myofibre cross-sectional areas and lower total nuclei numbers and myogenic precursor cell numbers per muscle in selected hind-limb muscles. These results indicate that myofibre hypertrophy in late gestation and early postnatal life is related to myogenic precursor cell number which may have important implications for growth potential of the growth-restricted fetus.