Contractile differentiation of foetal cattle muscles: intermuscular variability.

Contractile differentiation was studied in six foetal muscles exhibiting different contractile characteristics in adult cattle: the Masseter, Diaphragma, Biceps femoris, Longissimus thoracis, Semitendinosus and Cutaneus trunci. These muscles were excised from foetuses aged 60-260 days. Fibre types were identified by immunohistochemistry using three monoclonal anti- bodies raised against types 1, 2a, 2b (or 2x) and foetal myosin heavy chains. The different myosin iso- forms were also separated by electrophoresis, identified by immunoblotting and quantified by ELISA. At least two generations of cells were observed in all the muscles studied. The primary, early dif- ferentiated one, gave rise to type II fibres in Cutaneus trunci and type I fibres in all remaining mus- cles. The secondary generation of cells differentiated later than the first generation of cells. Its pat- tern of differentiation was more complex in particular from 150 to 210 days. It formed slow fibres in slow adult muscles, fast fibres in fast adult muscles and both types in mixed muscles. Precocity of dif- ferentiation was muscle-type dependent and related to muscle function at birth. © Inra/Elsevier,