Degeneration of the midgut epithelium in Allacma fusca L. (Insecta, Collembola, Symphypleona): apoptosis and necrosis.

Apoptotic and necrotic changes in the midgut epithelium cells of Allacma fusca (Collembola, Symphypleona) are described at the ultrastructural level. The morphological sign indicating the beginning of the apoptotic process in these cells is their shrinkage and the transformation of their mitochondria. The nucleus assumes a lobular shape and finally undergoes fragmentation. The intercellular junctions between an apoptotic cell and adjacent epithelial cells gradually disappear. Apoptotic cells are discharged into the midgut lumen just beneath the peritrophic membrane, where they are initially distributed singly but ultimately form a single layer. No phagocytosis was observed, so no apoptotic bodies are formed. Only young midgut epithelium shows apoptosis; as cells age, necrosis accompanies apoptosis, and necrosis finally completely replaces apoptosis.