Fibre hybrids in type group . An investigation of human muscle biopsies

Change of fibre type caused by reinnervation implies change in a series of metabolic processes. As long as these changes are in progress the histochemical pattern in muscle fibres may demonstrate deviations from the normal characteristics. The present histochemical study was undertaken to evaluate in human neurogenic muscle disease the completeness of conversion of presumably reinnervated muscle fibres. At least a number of muscle fibres in type groups is reinnervated. Type grouping of non-atrophic fibres was found in 27 of 42 muscle biopsies from patients with denervating diseases. The myosin ATPase activity in these groups was often strikingly even. In a varying degree and in a varying number of muscle fibres myosin ATPase-uniform groups showed intermediate capacity of aerobic and/or glycolytic metabolism; this finding was considered compatible with conversion due to reinnervation. Two main kinds of fibre hybrids were observed. One kind showed low myosin ATPase activity and an apparently low capacity for aerobic metabolism. The other kind also showed low myosin ATPase activity but the capacity of glycolytic metabolism was high, aerobic metabolism in these fibres being intermediate or high. It has been suggested that low capacity of aerobic metabolism in fibre hybrids of the first kind is related to hypertrophy of the muscle fibres. The appearance of fibre hybrids of the second kind would be conceivable as a stage in a process of conversion, if at least changes in the capacities of the 2 metabolic pathways can develop at a markedly different pace. However, groups or fascicles of fibre hybrids of this kind are present is some cases. These configurations point to a steady state rather than to a dynamic process of conversion; a lack of plasticity in the muscle fibre apparently prevents completion of conversion