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 microscop y and transmission electron microscopy. The female system is tubular, made up of a thin-walled proximal seminal receptacle/ootype and a dist al uterus, separated by a complex cellular region. Both parts have a c ontinuous syncytial cytoplasmic lining. Maturing oocytes in the semina l receptacle/ootype are in intimate contact with the receptacle lining . The uterus cytoplasmic lining completely surrounds the developing em bryo, 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 organell es including star-shaped configurations of electron-dense membranes an d 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 re mains continuous and in intimate contact with the embryonic tegument ( at least until the near-term embryo begins independent movement). Nume rous 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 an d embryo. Putative vitelline cells in the posterior of the body contai n 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 uteru s appears to regulate oocyte/embryo nutrition. Similar syncytial layer s occur in rotifers, but are unlike the nutritive epithelia of most ot her viviparous organisms.