Pericardial cell ultrastructure in the tobacco hornworm

The ultrastructure of the pericardial athrocytes of fifth instar tobacco hornworm Manduca sexta L. (Lepidoptera: Sphingidae) was examined by transmission electron microscopy. These cells are highly specialized for the maintenance of hemolymph homeostasis by synthesis and secretion of some proteins, and uptake and degradation of others. We observed a striking radial zonation with numerous labyrinthine channels extending into the cell providing a large surface area for enhanced efficiency of endocytosis. Granular material was imported into the endosomal compartment by fusion of endocytotic vesicles from the basal region. Anatomical evidence supports the hypothesis that proteins are transported from the endosome to the lysosome, by maturation of the early endosome to form the late endosome/lysosome, as opposed to vesicular transport. The presence of catalase in athrocyte lysosomes is reported for the first time. Endocytosis in the athrocytes of insects is compared with endocytosis in vertebrate systems.