Hymenolepis diminuta: two morphologically distinct tegumental secretory mechanisms are present in the cestode

The free-surface of the tapeworm's tegument was examined for morphological evidence of secretion after fixation by rapid freezing-freeze substitution and alternatively by immersion in low concentrations of glutaraldehyde maintained at room temperature. After low-aldehyde fixation, omega profiles were at the bases of tegumental microvilli, arguing for the participation of some of the ectocytoplasm's vesicles in secretion of their contents to the intestinal lumen. The almost instantaneous fixation provided by the rapid freezing-freeze substitution technique documents the constitutive production of 0.03 to 0.075-micron microvesicles from outpocketings from the plasma membrane of the tegumental brush border. Observed in secretory epithelial cells of other species, microvesicles are recognized as a secretory pathway for constituent molecules of surface membranes. We conclude that in addition to the primary route of merocrine exocytotic secretion provided by the fusion of the Golgi-derived, ectocytoplasmic vesicles at the bases of the brush border microvilli, tegumental microvesicles provide a second secretory pathway for endogenous macromolecules across the tegumental free surface.