Ultrastructure of the intrauterine eggs of Didymobothrium rudolphii (Monticelli, 1890) (Cestoda, Spathebothriidea, Acrobothriidae) and its phylogenetic implications

Abstract

The intrauterine polylecithal eggs of the spathebothriidean cestode Didymobothrium rudolphii (Monticelli, 1890) were examined by means of transmission electron microscopy (TEM). Each unembryonated egg is composed of a fertilised oocyte or ovum and several vitelline cells, all surrounded by a newly formed shell. The lumen of the proximal uterus is packed with unutilised vitelline material and eggs at different stages of shell formation. In the proximal region of the uterus, the fertilised oocytes, initially surrounded by dense, discontinuous islands of eggshell material and containing long axonemes of spermatozoa in their cytoplasm, were frequently observed. Sperm axonemes also remain in the oocyte cytoplasm of eggs surrounded by a thick electron-dense shell until the sperm nucleus is transformed into a male pronucleus. Despite the fact that the two-pronuclei stage and cell divisions within the eggs of D. rudolphii were never observed, individual eggs containing several blastomeres of different sizes were seen in the middle and distal regions of the uterus. This provides indirect evidence that at least a few initial cleavage divisions must take place in the intrauterine eggs and direct evidence that the early embryonic development of D. rudolphii starts in utero. The several vitellocytes present in each egg contain nutritive reserves for the developing embryos; these are composed mainly of numerous lipid droplets and a moderate amount of glycogen. In the eggs containing early embryos composed of several blastomeres, the cytoplasm of the degenerating vitellocytes exhibits the presence of so-called 'foci of cytoplasmic degradation', which appear to be involved in the autolytic process of the vitellocyte cell components and inclusions, such as a high accumulation of lipids and glycogen. This progressive degeneration of the vitellocytes, considered as an example of programmed cell death or apoptosis, likely contributes towards the resorption of nutritive reserves by the developing embryo. Some of the results of this study are commented upon in relation to the affiliation of the spathebothriideans with other lower cestode groups.