Myosin expression in semitendinosus muscle during fetal development of cattle: immunocytochemical and electrophoretic analyses.

The pattern of expression of different types of myosin and the development of different muscle cell populations were studied in the semitendinosus muscle of cattle from 39 d of gestation to 30 d of post-natal life. Monoclonal antibodies specific to different myosin heavy chains were used. Two cell generations were identified during myogenesis. They appeared successively and were characterized by different patterns of expression of myosins. The first population, which was present from the first stage studied (39 d of gestation), gave rise to type I fibers, which, in the mature animal, express only slow myosin. A second generation became differentiated at about 120 d of fetal life and then developed into type II fibers (IIa, IIb or IIc). The beginning of differentiation was characterized in all the cell populations by the expression of specific types of embryonic or fetal myosins. A comparison of these results with findings from previous works shows a marked similarity between species in the pattern of myogenesis but great differences in the length of the different stages of development. In this respect, myogenesis in cattle closely resembles that in man.