Programmed cell death. electrophysiological and ultrastructural correlations in metamorphosing muscles of lepidopteran insects.

Individual spontaneously degenerating fibers of moths have been examined successively by electrophysiological and ultrastructural techniques. The correlations observed by this method have revealed two phases of lysis. The first lytic phase begins at or before the ecdysis of the moth. It is characterized by a probable but occult respiratory lesion, normal movement of the ions across the plasma membrane, and limited degradation of myofilaments and autophagy of mitochondria. There are nevertheless early degenerative changes in the Z-band and a slight swelling of the T system. The second lytic phase is introduced, at approximately 15 hr after ecdysis, by collapse of the tracheal airways. The collapse of the tracheae is followed within 2 or 3 hr by a drastic reduction of input resistance, depolarization of the fiber, and loss of contractility. The rate of proteolysis increases, and the remaining myofilaments lose all structural identity by the twentieth hour. During this period of rapid change, the electron density of the reticular portion of the dyads decreases before a compactive process increases their opacity. Neural elements degenerate secondarily. The early lytic phases are apparently non-lysosomal for the myofilaments, although organelles such as mitochondria, ribosomes, and glycogen are destroyed in autophagic vacuoles. Elimination of oxygen and collapse of the resting potential, with the presumed equilibration of intracellular ionic content, provokes a rapid dissolution of the myofilament, with continued autophagic elimination of the organelles.