PROGRAMMED CELL DEATH--I. CYTOLOGY OF DEGENERATION IN THE INTERSEGMENTAL MUSCLES OF THE PERNYI SILKMOTH.

Abstract In the silkworm, Antheraea pernyi , the intersegmental muscles of the abdomen degenerate within 48 hr after the ecdysis of the moth from the old pupal cuticle. The process of breakdown was studied by light and electron microscopy. At about the 5th hour after ecdysis, the first trace of degeneration is seen as scattered areas of erosion of the myofibrils, accompanied by a slight contraction of the mitochondria. By the 10th hour after ecdysis, one can detect larger areas of degeneration and vacuolization, and an apparent extrusion of sarcoplasm. Electron micrographs at the 10th hour show degenerative myelin figures, shrunken mitochondria, and deposits of lipid near the mitochondria. Meanwhile, small, darkly staining bodies, presumed to be lysosomes, undergo swelling and apparent dissolution; in their vicinity, the myofilaments are eroded and destroyed. By the 15th hour after ecdysis, the muscle is flaccid, non-contractile, and reduced in volume by about one-third; the myofibrils and cross-striations become disorganized and disappear, except for the persistent Z-bands, to which remnants of the myofilaments are attached. The nuclei are pycnotic, the mitochondria greatly shrunken and degenerate, and the lysosomelike bodies rarely visible. The muscle has completely disappeared by the 48th hour. Meanwhile, the pre-synaptic portion of the neuromuscular junctions remains intact. The breakdown of the muscles can be accounted for by cytolysis brought about by what we interpret to be the rupture of lysosomes in the apparently viable tissues. This event is followed by the erosion and eventual loss of the myofibrils and by the degeneration of the intracellular organelles.