Cytological studies of muscle dedifferentiation and differentiation during limb regeneration of the newt Triturus

During limb regeneration of the newt Triturus , muscle first dedifferentiates giving rise to mesenchymal cells which, subsequently, differentiate to reform normal muscle. In dedifferentiating muscle, myofibrils and elements of the sarcoplasmic reticulum decrease gradually in number. Myofilaments become less distinct and seem to be supplanted by zones of amorphous material. Large masses of glycogen accumulate in dedifferentiating cells and mitochondria show a transient enlargement. In the later stages of dedifferentiation, glycogen decreases in amount while cytoplasmic ribosomes and cisternae of rough‐surfaced endoplasmic reticulum appear. Mesenchymal cells are formed by disappearance of all traces of myofilaments, by further elaboration of rough endoplasmic reticulum, and increase in number of Golgi complexes. During the earliest stages of differentiation, muscle has less endoplasmic reticulum and many more free ribosomes which occur in clusters or linear chains. Thin (−70 A) and thick (−135 A) filaments appear in the cytoplasm, often in association with ribosomal olusters, and shortly thereafter aggregate into small fibrils. In early fibrils, the thin filaments overlap the thick to produce I and A bands and the free ends of the thin filaments converge and cross to mark the site of the Z line. Fibrils enlarge by addition of filaments along their sides and at their ends. In larger fibrils, the H band and M line are apparent. Initially, the transverse tubular system develops by the confluence of vesicular inpocketings of the surface plasma membrane. The smooth sarcoplasmic reticulum is continuous with rough‐surfaced membranous cisternae. Some of the smooth tubules, probably sarcoplasmic reticulum, contain intracisternal masses of dense granular material and are situated adjacent to the Z line. When dense material occurs in the membranous structures, material of similar density and texture appears within the adjacent Z line. Normal muscle fibers are formed by fusion of myoblasts, increase in number and size of myofibrils, organization of transverse tubules and sarcoplasmic reticulum into triads, loss of ribosomes, and appearance of glycogen.