Comparison of the foetal development of fibre types in four bovine muscles.

The pattern of expression of different types of myosin heavy chains and the development of different generations of muscle cells during foetal life were studied in four bovine muscles with widely varying characteristics, the Masseter, Longissimus thoracis, Cutaneus trunci and Diaphragma. Different complementary techniques were performed: immunocytochemistry, electrophoresis, immunoblotting and ELISA. Monoclonal antibodies against different myosin heavy chain isoforms were used. The results confirmed the existence of at least two generations of cells during foetal development in cattle. A first generation, which appeared at a very early stage, gave rise to adult type I fibres. A second generation, made up of different cell populations, gave rise to adult fast type IIA and IIB fibres, and to type IIC. In the slow muscles, it also seemed to give rise to type I fibres. The beginning of myogenesis was characterized in the different cell generations by the expression of transitory myosin forms that are not found in the adult. Type 1 myosin heavy chain was observed from 90 days whereas the fast types, 2a and 2b, were present from 210 to 230 days, at which stage the foetal form disappeared. Muscles that have greatly different contractile characteristics in the adult exhibit also different profiles of differentiation: the Diaphragma was the first to develop, followed by Cutaneus trunci, Longissimus thoracis and Masseter.