Ultrastructural adaptations for viviparity in the female reproductive system of gyrodactylid monogeneans.

The female reproductive system of viviparous monogeneans (Gyrodactylus and Macrogyrodactylus) has been examined using fluorescence microscopy and transmission electron microscopy. The female system is tubular, made up of a thin-walled proximal seminal receptacle/ootype and a distal uterus, separated by a complex cellular region. Both parts have a continuous syncytial cytoplasmic lining. Maturing oocytes in the seminal receptacle/ootype are in intimate contact with the receptacle lining. The uterus cytoplasmic lining completely surrounds the developing embryo, and is continuous with anterior and posterior cell bodies which fluoresce strongly when stained with bisBenzimide. This lining is most extensive around small embryos, when it contains specialised organelles including star-shaped configurations of electron-dense membranes and multilamellate bodies. Pits in the uterus wall bridged by membranous structures connect the cytoplasmic lining to parenchyma or digestive cells. The cytoplasmic lining regresses as the embryo develops, but remains continuous and in intimate contact with the embryonic tegument (at least until the near-term embryo begins independent movement). Numerous ribosomes, membranes and mitochondria in the uterine cytoplasmic layer indicate a high metabolic rate, and exo/endocytotic vesicles in the F1 tegument suggest transfer of materials occurs between parent and embryo. Putative vitelline cells in the posterior of the body contain abundant RNA, ribosomes and membrane-bound secretory bodies, and are filled with an electron-lucent secretion. However, there are no ducts associated with these cells, and their function remains unknown. The cytoplasmic lining of both the seminal receptacle/ootype and the uterus appears to regulate oocyte/embryo nutrition. Similar syncytial layers occur in rotifers, but are unlike the nutritive epithelia of most other viviparous organisms.