Immunocytochemical analysis of fibre type differentiation in developing skeletal muscle.

Two monoclonal antibodies (McAbs) reactive with 'fast' and 'slow' adult myosin heavy chains have been produced and used in the analysis of human and rat muscle fibre type development. Expression of adult 'slow' myosin heavy chain was detected in human foetal muscle fibres as early as 14 weeks of gestation and in 1 day newborn rat muscle fibres. The standard histochemical stains used to show muscle fibre type do not distinguish fast from slow fibres at this early stage of development. These McAbs should therefore be of value in identifying factors involved in the differentiation of myotubes into fast and slow muscle fibres.