Programmed cell death—II. Endocrine potentiation of the breakdown of the intersegmental muscles of silkmoths

The abdominal intersegmental muscles of saturniid pupae are well preserved during the entire pupal period and throughout the three weeks required for the development of the adult moth. Then, within 48 hr after the emergence of the moth, the muscles break down and disappear. The breakdown is a result of developmental reactions within the muscles themselves; their dissolution, in this sense, is a carefully timed, final episode in their own metamorphosis. Thus, the endocrine conditions obtaining at the onset of adult development (the presence of ecdysone and the absence of juvenile hormone) dictate both the formation of all adult structures, including muscles, and the eventual death of the intersegmental muscles. If juvenile hormone is injected, the breakdown does not occur in the intersegmental muscles or in any of the other tissues which are normally destroyed during the pupal-adult transformation. The final signal for the initiation of breakdown does not appear to be endocrine in character. Thus, when pairs of pupae were joined in parabiosis so that they shared the same blood, the breakdown of muscle in one individual did not hasten or retard the breakdown in the other individual. This finding points to some further signalling system which triggers the initiation of cell death in the endocrinologically potentiated muscles.