Ontogeny of factors associated with proliferation and differentiation of muscle in the ovine fetus.

The number of muscle fibers within a muscle has been found to be of high importance for the growth potential of an animal, and this number is set during fetal development. The objective of this study was to identify the ontogeny of muscle cell differentiation and fiber formation by observing the changes in expression of factors known to influence myoblast proliferation and differentiation. Twenty-one Swaledale x Leicester Blue Face ewes carrying twins were allotted to this trial. From d 40 of gestation, three ewes were killed every 15 d until term. At each time point, the fetuses were located, removed, and total muscle from both hind limbs was dissected from each fetus and snap frozen in liquid N2. Ribonuclease protection assays were used to quantify transcripts for IGF-I, IGF-II, GH receptor (GHR), and myostatin genes in the muscle samples, whereas quantitative real-time PCR was used to quantify myogenin transcripts. Histological sections also were taken from the fetal muscle samples and observed for evidence of muscle differentiation resulting in fiber formation. The abundance of mRNA for ovine IGF-II and ovine myogenin peaked at d 85 of gestation (P < 0.001). The abundance of ovine IGF-I transcripts peaked at d 100 of gestation, whereas the abundance of ovine GHR mRNA increased throughout gestation (P < 0.05). No change (P = 0.87) in the abundance of myostatin mRNA was observed. The histological sections from the muscle samples demonstrated a clear change in the appearance of the muscle tissue at each time period. Major fiber formation was observed around d 85. The results obtained from the analysis of gene expression and the histological sections suggest that the majority of muscle differentiation and fiber formation takes place around d 85, with myoblast proliferation mainly occurring before this time. It may be possible to manipulate the number of muscle fibers formed by targeting treatments during this proliferation stage immediately before the period of major fiber formation.