Ultrastructural characters of the spermatozoon of the liver fluke Opisthorchis viverrini (Poirier, 1886) (Opisthorchiidae).

The present study records the ultrastructural organization of the mature spermatozoon of Opisthorchis viverrini by means of transmission electron microscopy. The spermatozoon of O. viverrini is a filiform cell, tapered at both extremities. It exhibits the characteristics of type IV spermatozoon of digeneans, namely with two axonemes of the 9+'1' trepaxonematan pattern, external ornamentation of the plasma membrane associated with cortical microtubules that are in the posterior part of the anterior region of the sperm cell, and with two mitochondria. The maximal number of cortical microtubules is in the anterior part of the spermatozoon and arranged into two bundles. Other characteristics are spine-like bodies and a posterior extremity with only the second axoneme. Ultrastructural characters of the spermatozoon of O. viverrini are compared with those of other known digeneans belonging to the Opisthorchioidea, with particular emphasis on representatives of the family Opisthorchiidae. The main differences between O. viverrini and its congener Opisthorchis felineus are the spine-like bodies (present and absent, respectively) and the posterior spermatozoon character (axoneme and nucleus, respectively).