Ultrastructure of the ovary of Amphilina japonica Goto & Ishii, 1936 (Cestoda) and its implications for phylogenetic studies.

The ultrastructure of the ovary of the amphilinidean cestode Amphilina japonica Goto & Ishii, 1936 from the body-cavity of the American sturgeon Acipenser transmontanus Richardson is described using transmission electron microscopy. The characters of the ovary of Amphilina japonica are different from those of all other cestodes. The most important difference is in the nature of the relationship between the germ and accessory cells within the ovary. In A. japonica the oocytes and accessory cells form numerous different intercellular contacts (desmosome-like junctions and zonulae adherentes). Gap junctions are present between the narrow cytoplasmic processes of the accessory cells. Numerous micropinocytotic vesicles and vacuoles from the accessory cells discharge their content into spaces between the oocytes and the accessory cells. The accessory cells are closely associated with the oocytes during the early and middle stages of oogenesis. As the volume of oocytes increases, the accessory cells gradually lose their association with the oocyte surfaces. Peripherally located individual accessory cells of A. japonica give rise to a cellular epithelial layer of irregular shape and thickness which breaks down via numerous invaginations of the basal membrane and underlying basal matrix. The different arrangements of the interconnection of cell components in the Amphilinidea compared with the Gyrocotylidea and Eucestoda (the absence of specialised cell contacts and the syncytial nature of the accessory 'interstitial' cells) are evidence suggesting the presence of unrelated groups within the Cestoda. The nature of the association of the accessory and germ cells in ovary of A. japonica more closely resembles the ovary of non-platyhelminth invertebrates rather than that of other neodermatans.