Histochemical changes in ATPase activity during regeneration of adult skeletal muscle fibers

Developing skeletal muscle fibers (myotubes) and fully differentiated (adult) muscle fibers can be distinguished according to their histochemical “myofibrillar” ATPase reaction. Myotubes stain darkly for the ATPase reaction at pH 9.4 following preincubation of sections in either alkali (pH 10.4) or acid (pH 4.35). This staining is exclusively localized to the myofibrils of these embryonic fibers. In the soleus muscle of the guinea pig, a progressive conversion to the adult staining pattern is completed 1 month following birth; fibers then stain lightly after alkaline-preincubation and darkly following acid-preincubation when staining is intermyofibrillar as well as myofibrillar. Thus the staining reaction can be used to follow developmental alterations in muscle fibers, and in the present study it was used to determine whether the regenerating fibers in an injured, fully differentiated muscle repeat the developmental changes. Fibers were transected in fully differentiated soleus muscles of guinea pigs, and at periodic intervals these injured muscles were examined histochemically. The sequence of histochemical and morphological changes in the regenerating muscle fibers essentially repeated normal development. Since regenerating fibers go through the same stages as during normal development, the restoration process probably involves the elaboration and fusion of undifferentiated myogenic cells or dedifferentiated muscle cells.