Histochemical sub-types of three fibre-types of avian skeletal muscle.

The soleus muscle of adult chicken was studied histochemically. Succinic dehydrogenase (SDH) and myofibrillar ATPase reactions, with or without preincubation in K2-EDTA salt, were compared in serial frozen sections. Based upon the distributions of the above reactions, the three major fibre types distinguished were "Type I red", "Type II red" and "Type II white". On the basis of non preincubated ATPase reaction alone two sub-types of type I red fibres could be distinguished. However, following preincubation in a "Cold" solution of K2-EDTA, Type II red fibres fell into two sub-types and Type II white fibres fell into three sub-types. Amalgamating the two already existing classifications, a more elaborate classification is presented for characterizing these different sub-types. The presence of two different or a spectrum of staining variations in a seemingly homogeneous population of muscle fibres in a given fibre-type emphasizes the possible correlation between this histochemical data and the heterogeneity of contraction times of the different motor units.