Muscle differentiation in the bovine fetus: a histological and histochemical approach.

The chronology of muscle fiber differentiation was analysed in 37 fetal calves of 69 to 266 days of age. Semitendinosus muscle weight was measured throughout the experimental period and biochemical, histological and histochemical investigations were made to determine respectively the protein and DNA content of the muscle, the size and the number of the fibers and their ATPase and SDH activity. The relative growth of all the quantitative characteristics (muscle weight, protein and DNA content) was much greater in the early stages of gestation than in the new-born animal. In the younger fetuses DNA relative growth was faster than protein relative growth, whereas at the end of gestation the reverse progression was observed. Before 90 days, the muscle tissue was composed of myotube-like cells without any clear organization. The organization of muscle tissue into clear bundles occurred around 120 days of age, and about 30 days later the large myotubes transformed into myofibers. The myotubes reacted positively for acid-ATPase activity, whereas the large population of smaller cells which developed in parallel did not. The number of muscle cells increased up to 240 days of age, as did the percentage of fibers positive for acid-ATPase activity. Finally, oxidative differentiation occurred around 260 days of age, with the appearance of a population of cells characterized by increased SDH activity. A comparison of these results with previous findings suggests that the muscular tissue differentiates through similar stages in various species, but over different lengths of time. The percentage of mature weight might provide a better inter-species time scale than chronological age.