Ultrastructure of the rostellar tegument of Echinococcus granulosus with special reference to biogenesis of mitochondria

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Jha R. K. and Smyth J. D. 1970. Ultrastructure of the rostellar tegument of Echinococcus granulosus with special reference to biogenesis of mitochondria. International Journal for Parasitology, 1: 169–177. On the rostellum of Echinococcus granulosus the tegumental microtriches are long and thin, and often branched, hooked, barbed or curved—a pattern which probably provides firmer adhesion to the host gut and increases the absorptive area. Mitochondria are almost absent from the rostellar tegument in a freshly evaginated protoscolex, although dumb-bell shaped membranous structures are present. After 24–28 h in vitro culture, the latter structures disappeared and active mitochondrial biogenesis was seen to be taking place. It is speculated that the occurrence of this process during the early phases of the protoscolex-strobila transformation may be triggered by transfer from the environment of a tissue site (in the intermediate host) to that of an intestinal site (in the dog) and presumably results in a ‘switch’ in metabolism. This phenomenon also appears to be associated with the ‘secretory’ activity in the nuclei of the rostellar gland cells. An analogy with the biogenesis of mitochondria in trypanosomes is made. In an established and growing strobilate phase of Echinococcus, mitochondria are abundant at all levels of the distal cytoplasm. This and the occurrence of ‘concentric’ mitochondria suggests a higher metabolic activity in this region than in other parts of the worm. Elongated mitochondria with longitudinal cristae, restricted only to the rostellar tegument, were also found. In addition, unusual membrane-bound ‘mitochondrial sacs’, containing numbers of mitochondria, were present. Some of these sacs opened to the outer surface, and displayed mitochondria apparently in the process of ‘extrusion’. Such extrusion of mitochondria would explain the frequent occurrence of free ‘intact’ mitochondria on the surface of the distal cytoplasm—a phenomenon which may be related to the process of ‘membrane digestion’ believed to occur in this region. ‘Lamellar bodies’ of unknown function, were also found often in the distal cytoplasm. ‘Secretion droplets’, which appeared externally on the rostellum of in vitro cultured specimens, were found to be bubble-like outgrowths of distal cytoplasm containing vesicles, mitochondria and vacuoles of varying sizes.