Influence of genotype on contractile protein differentiation in different bovine muscles during foetal life

The purpose of this work was to compare muscle fibre differentiation in two genetic types: "normal Charolais" and double-muscled (DM) "INRA 95" cattles displaying muscle hypertrophy. Six muscles with different contractile and metabolic characteristics in adult animal: Masseter, Diaphragma (Di), Biceps femoris (BF), Longissimus thoracis, Semitendinosus and Cutaneus trunci (CT) were excised from 60 to 260-day-old foetuses of both genotypes. These muscles present different degrees of hypertrophy in DM animals. Fibre types were characterised by immunohistochemistry using monoclonal antibodies raised against different myosin heavy chain isoforms. They were also studied by SDS-PAGE electrophoresis, immunoblotting and ELISA methods. In both genotypes, foetal muscle development involved at least two generations of fibres but their contractile differentiation was more or less delayed in double-muscled animals according to the muscle type. CT muscle, the most hypertrophied in adults DM, showed the most important differences in the appearance of the primary and secondary fibres in DM foetuses. It showed a delay of differentiation which was made up during the last third of foetal life. Other muscles did not differ in the speed of appearance of the two generations of fibres. They were originated from a higher proliferation of secondary fibres. Moreover, all DM muscles presented similar characteristics at 210 days of foetal life. These characteristics seem to be genetically determined. Just before birth, muscles of double-muscled foetuses were composed of a greater proportion of fast type Il fibres than normal animals. The most important differences between genotypes were observed in BF and Di muscles.