Myogenesis in small and large ovine fetuses at three stages of pregnancy

Perturbations of the prenatal environment may influence fetal muscle development. This study investigated muscle cellularity and mRNA abundance of myogenic genes in fetal sheep divergent in their patterns of growth. Muscle samples were obtained from small and large fetuses on Days 50, 92 and 133 of pregnancy. Number of myofibres in the semitendinosus muscle increased between Day 92 and 133 of pregnancy, but did not differ between small and large fetuses at either stage of pregnancy. The semitendinosus of small fetuses had smaller cross-sectional areas of myofibres than did those of their large counterparts on Day 133 of pregnancy. The semitendinosus of small fetuses also had lower DNA concentration on Day 92 and lower protein concentration on Day 133 than did those of large fetuses. The mRNA levels of the myogenic regulatory factors (MRFs), myostatin, the insulin-like growth factors and embryonic myosin in fetal muscles varied with the stage of development, but no differences occurred in response to divergent fetal growth. Myostatin mRNA was more abundant in the semitendinosus than in the supraspinatus muscle on Days 92 and 133, as were myogenic regulatory factors, myf-5, myf-6 and follistatin mRNA on Day 133. The results indicated that muscle growth but not the number of myofibres in fetal sheep is modified by restricted fetal growth, and that genes that regulate muscle development are affected by the stage of development in an anatomical muscle-specific manner.