[Action of crystals of Bacillus thuringiensis var. israelensis on the midgut of Aedes aegypti L. larvae, studied by electron microscopy].

Ingestion of Bacillus thuringiensis var. israelensis crystals by Aedes aegypti larvae is followed by midgut epithelium disruption. Earliest ultrastructural changes consist of an enlargement of intra- and intercellular spaces in the basal region of the cell. Endoplasmic reticula disintegrate by forming spherical structures which increase in size during intoxication. Mitochondria are transformed at first into a condensed form, then become swollen with the disappearance of internal cristae. In the cardia cells, which secrete the peritrophic membrane, the Golgi apparatus may produce electron-dense secretion vesicles; in this event, the peritrophic membrane assumes an abnormal configuration. Before complete breakdown, a cellular hypertrophy is observed: few cells become balloon-like. At the same time, the microvilli decrease in size, widen and then disappear after a few hours exposure to crystal delta-endotoxin.