Ultrastructure of spermatogenesis of Atriaster heterodus (Platyhelminthes, Monogenea, Polypisthocotylea).

The development of spermatozoa in the Polyopisthocotylea Atriaster heterodus was studied by transmission electron microscopy. Spermatogonia were undifferentiated, with irregular nuclei and little cytoplasm. Primary spermatocytes had sparse chromatin and typical synaptonemal complexes. The nuclear chromatin of secondary spermatocytes was in patches along the nuclear envelope and throughout the nucleoplasm. The complete process of fusion of the early spermatids to a common cytoplasmic mass forming a rosette was elucidated. Nuclei migrated to the center of the mass and changed from round to lamellar or tubular in shape. At the borders of the common cytoplasmic mass, the irregular zones of differentiation had microtubules, mitochondria, nuclei, centrioles, and intercentriolar bodies that give rise to 2 flagella. The spermatozoa presented a continuous row of cortical microtubules surrounding 2 parallels axonemes of the 9+1 type.