Changes in microvilli and Golgi-associated membranes of lepidopteran cells induced by an insecticidally active bacterial δ-endotoxin

The δ-endotoxin from Bacillus thuringiensis var. kurstaki was fed to late larvae of the tobacco hornworm, Manduca sexta, to determine its effect on the anterior midgut epithelium, which was examined ultrastructurally at intervals thereafter. The cells of the midgut are primarily of the columnar and goblet variety and the effects of the toxin were more pronounced on the former. Tissues examined after only 1–5 min of exposure to the toxin already revealed fine-structural alterations. These were most notably changes in the microvilli and membranes associated spatially with the Golgi complex: vacuoles associated with its maturing face became enlarged. This effect was intensified with more-extensive exposure to the toxin, resulting in an increase in both vacuoles and the number of lysosomal bodies, many containing myelin-like formations; some of these arose as autophagic vacuoles. There seemed to be no consistent increase in endocytotic activity at the apical border, however. The intramembranous particle population of the microvilli of the columnar cells showed some slight changes with toxin treatment; alterations in microvillar contours also occured. The intercellular septate and gap junctions on the lateral borders were sometimes disrupted and with time often became internalized. It seems, then, that the toxin initially modifies the microvillar membranes and subsequently the Golgi-associated saccules are affected, giving rise to vacuoles and lysosomes.