Autophagocytosis in the larval midgut cells of Pieris brassicae during metamorphosis.

We observed three types of cells in the epithelial layer of the midgut of last instars of Pieris brassicae. The columnar and goblet cells degenerate during the second part of the last larval stage while the undifferentiated basal cells proliferate during this period and create the epithelium of the pupal midgut. The first morphological sign of involution is the formation of autophagic vacuoles and dense bodies in the cytoplasm of columnar and goblet cells which begins on day 4 of the stage. The number and size of autophagic vacuoles and dense bodies increase during the spinning period (85-96 h). Finally, at the end of the stage, the columnar and goblet cells become displaced by the growing pupal epithelium and reach the lumen where they disintegrate. Autophagocytosis was not seen in the cells during the feeding period (0-72 h). However, we observed many autophagic vacuoles in the columnar and goblet cells of 50-h-old instars 3 h after the administration of 30 μg/g body weight of 20-hydroxyecdysone. The hormone treatment elevated by 100% the incorporation of H-leucine into the proteins of the midgut. Inhibitors of protein synthesis, cycloheximide and puromycin, in doses that supressed the incorporation of the amino acid by 60-70% either in hormone treated or untreated larvae, exerted diverse effects on the autophagic process. Puromycin did not block the hormone-induced formation of autophagic vacuoles while cycloheximide prevented it. Possible explanations for this diversity are discussed. [ABSTRACT FROM AUTHOR]