Ultrastructure of spermatozoa in the seminal receptacle of the liver fluke Opisthorchis felineus (Rivolta, 1884).

The spermatozoon ultrastructure in the seminal receptacle of the liver fluke Opisthorchis felineus (Digenea, Opisthorchiidae), the agent of human opisthorchiasis endemic to Russia and Eastern Europe, was examined. The bean-shaped seminal receptacle of O. felineus has a wall consisting of epithelial and muscle layers. Mature spermatozoa are located in the interior of the seminal receptacle, whereas vacuoles containing degenerating spermatozoa are detectable at the periphery. The mature spermatozoon of O. felineus has two axonemes of a 9 + "1" pattern, a nucleus, two mitochondria, a lamellar body, two bundles of parallel cortical microtubules and an external ornamentation of the plasma membrane in the anterior area of the sperm. The lamellar body is likely to be the third mitochondrion of a small size. The cytoplasm of the spermatozoon is filled with numerous electron-dense granules of storage polysaccharides. Additionally, the ultrastructural characteristics of the seminal receptacle and spermatozoa of O. felineus were compared to available published data on other trematode species. The functional roles of the observed structures of this spermatozoon are discussed.