Electron microscopic studies of planarian regeneration. III. Degeneration and differentiation of muscles

Planarians were surgically decapitated, and morphological changes in their muscles during regeneration were studied with the electron microscope. Many damaged muscles were found in the vicinity of the wound surface immediately following decapitation. These stain darker than normal muscle and show degenerative changes in organelles as early as 1 hour. Some damaged muscles were observed to fall of f the cut surface into the external medium during the first 16 hours, but most were phagocytized by reticular cells mobilized to the wound area during this period. These were the only fates observed for damaged muscle in the vicinity of the wound. Detailed searches of large areas of wound region, at various times from 1 to 72 hours following decapitation, revealed no evidence of de‐ and redifferentiation of preexisting muscle cells. Newly differentiating muscle cells, identifiable by small nascent myofibril bundles in the cytoplasm close to the nuclear envelope or rough endoplasmic reticulum, are first discernible in the cellular aggregate of the new regeneration blastema on the third day. Vestiges of nuclear satellite material in the perinuclear cytoplasm of such cells indicate their derivation from neoblasts. Further histogenesis involved migration of these newly formed muscle cells to the apical perimeter of the blastema 4 to 6 days after decapitation. Neuromuscular junctions were observed to form during these later stages.