We already reported that ecdysone (E) has a specific effect on the morphological change of midgut epithelial cells that is distinct from 20-hydroxyecdysone (20E) in precocious fifth-instar larvae induced by 20E. Silkworm larvae, Bombyx mori, were fed an artificial diet supplemented with 20E during the fourth-instar to promote premature molting. At the onset of the fifth instar, these precocious fifth-instar larvae were fed diets supplemented with either E or 20E to determine the effects of the two ecdysteroids on the morphology of midgut epithelial cells. In the larvae that ingested 20E, the epithelial cells disrupted without regeneration because of the degeneration of the regenerative cells, and the failure of the larvae to renew midgut epithelial cells may lead to premature death during the fifth instar. On the other hand, in the larvae that ingested E, the regenerative cells began to proliferate and differentiate very rapidly and the epithelial cells were replaced to new ones 24 hours before the larvae entered apolysis, then most of the larvae molted to the sixth-instar. However, regeneration of midgut epithelial cells normally occurs only during the molting period. Such a rapid regeneration was also induced by an oral or a hemolymph injection of E. The regenerative cells began to differentiate within 2 hrs after the injection of E. These results results suggest that E has a specific effect on the regenerative cells that differ from that of 20E in the precocious fifth-instar larvae obtained with 20E.

Index terms:Bombyx mori, midgut, regeneration, ecdysone, 20hydroxyecdysone.

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