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The ultrastructure of the uterus proper of the aspidogastrean Aspidogaster limacoides Diesing, 1835 and two digenean species, Phyllodistomum angulatum Linstow, 1907 (Plagiorchiida, Gorgoderidae) and Azygia lucii (Müller, 1776) (Strigeida, Azygiidae), was examined by transmission electron microscopy (TEM). The uterine epithelial lining of these species is thin, except for the perinuclear region of the epithelial cells. Septate junctions occur between adjacent epithelial cells within the uterine wall. The luminal surface of the cells is elevated into microlamellae, which project into the uterine lumen and cover the entire epithelial lining. Basally the uterine epithelium is attached to a basal matrix, and its supporting layers of muscle fibres are weak and composed of scattered circular muscles. Despite the marked similarity in the ultrastructural pattern of the aspidogastrean and two digeneans studied, there is some degree of variation in the secretory activity of their uterine epithelium. The high level of vesicular exocytotic activity in the epithelial cytoplasm of A. lucii may be associated with differences in egg emission and the subsequent life cycle involving a non-ciliated, non-swimming and non-free-living miracidium, as opposed to the free-swimming miracidium of P. angulatum. The similar nature of the uterine epithelium in all three species studied represents an ultrastructural marker possibly supporting a close phylogenetic relationship between the Aspidogastrea and the Digenea.
Fine structural features of the vitellarium of two digeneans, Phyllodistomum angulatum and Azygia lucii, are documented and compared with those of other digenean species. The cytodifferentiation of immature vitelline cells (vitellocytes) assumes the production and subsequent accumulation in their cytoplasm of several inclusions. Mature vitelline cells of P. angulatum are characterized by the presence of vitelline clusters (∼2.7 μm in diameter, with ∼100 vitelline globules of ∼0.35 μm in diameter) and osmiophobic, saturated lipid droplets (∼2-3 μm in diameter), and in A. lucii vitelline clusters of the same diameter include much fewer vitelline globules (∼50 globules of ∼0.5 μm in diameter), osmiophilic lipid droplets and α-glycogen. In both P. angulatum and A. lucii, interstitial cells are also present within the vitellarium. Two types of contact sites (septate and tight junctions) between adjoining interstitial cells also occur in both digenean species. Judging from the present and previous ultrastructural studies, it is suggested that there are three potential discriminatory characters of the digenean vitellarium (the number of different types of cell components within the vitellarium, the presence and type of junctional complexes between these cells, and the isolation of the vitellarium from the surrounding tissue) which may prove useful for a better understanding of the biology and evolutionary history of the different digenean groups.
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