Contractile differentiation of foetal cattle muscles: intermuscular variability

Contractile differentiation was studied in six foetal muscles exhibiting di fferent contractile characteristics in adult cattle: the Masseter, Diaphrag ma, Biceps femoris, Longissimus thoracis, Semitendinosus and Cutaneus trunc i. These muscles were excised from foetuses aged 60-260 days. Fibre types w ere identified by immunohistochemistry using three monoclonal antibodies ra ised against types 1, 2a, 2b (or 2x) and foetal myosin heavy chains. The di fferent myosin isoforms were also separated by electrophoresis, identified by immunoblotting and quantified by ELISA. At least two generations of cell s were observed in all the muscles studied. The primary, early differentiat ed one, gave rise to type II fibres in Cutaneus trunci and type I fibres in all remaining muscles. The secondary generation of cells differentiated la ter than the first generation of cells. Its pattern of differentiation was more complex in particular from 150 to 210 days. It formed slow fibres in s low adult muscles, fast fibres in fast adult muscles and both types in mixe d muscles. Precocity of differentiation was muscle-type dependent and relat ed to muscle function at birth. (C) Inra/Elsevier, Paris.