Fine Structure, Development, and Senescence of the Uterine Epithelium of Mesocestoides lineatus (Cestoda: Cyclophyllidea)

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The fine structure of the uterine epithelium of Mesocestoides lineatus was studied at three stages of development and in three distinct uterine regions. Observations by transmission electron microscopy revealed both temporal and spatial variation in some features of the epithelium. In early mature proglottids, nuclei of the posterior tubular uterus (U1) were juxtaluminal, whereas those of the anterior saccate uterus (U2) were withdrawn into the medullary parenchyma. In late mature and early gravid proglottids, some spatial variations were seen in the number of microvilli and in the location and abundance of mitochondria, ribosomes, and endomembrane components. Spatial and temporal variation in the epithelium of U2 was associated with the formation of the thick parenchymal paruterine organ wall around the posterior end of U2. The basis for structural variation in U2 appeared to be related to epithelial senescence in that region, which differentiated into the paruterine organ lining. Thus, I was able to demonstrate that earlier reports of paruterine organs lacking an epithelial lining were based upon examination of late gravid proglottids in which epithelial senescence was complete. Several cyclophyllidean taxa are characterized by gravid proglottids that possess thick-walled paruterine organs containing the eggs. The best known of these are species of the genus Mesocestoides, which is common and has a cosmopolitan distribution. Conn et al. (1984) studied the fine structure of the fully gravid paruterine organ of Mesocestoides lineatus (Goeze, 1782), using proglottids collected from the host's feces. They demonstrated a parenchymal origin for the paruterine organ wall, but showed no structure resembling a uterine epithelium associated with the wall. The uterus, as described by Byrd & Ward (1943) for Mesocestoides variabilis, consists of two parts. The posterior tubular uterus (U1) is difficult to discern in whole mounts. It receives ova from the ootype and passes them to the prominent anterior saccate uterus (U2). Developing eggs are passed to the paruterine organ from U2, but the structural and developmental relationships between these two organs are not known. Chandler (1946) also described the bipartite nature of the uterus in Mesocestoides latus, but referred to U1 and U2 as the uterine duct and uterus, respectively. Although the reproductive system is the most complex system in cestodes, few studies have been made specifically of the fine structure of its components. The resulting paucity of information has been pointed out recently in reviews by Davis & Roberts (1983), Lumsden & Hildreth (1983), and Lumsden & Specian (1980). The uterus has been generally neglected as the focal point of major studies, although the report of uterine capsule fine structure by Conn & Etges (1984) is an exception. The present report describes the fine structure of the uterus of M. lineatus TRANS. AM. MICROSC. Soc., 106(1): 63-73. 1987. ? Copyright, 1987, by the American Microscopical Society, Inc. This content downloaded from 207.46.13.21 on Tue, 27 Sep 2016 03:59:43 UTC All use subject to http://about.jstor.org/terms TRANS. AM. MICROSC. SOC. and its structural and developmental relationships to the paruterine organ wall. Hopefully, this work will provide a baseline for further detailed studies on the comparative fine structure of eucestode reproductive systems. MATERIALS AND METHODS Mesocestoides lineatus tetrathyridia (metacestodes) were removed from the body cavities of naturally infected lizards, Anolis carolinensis, from Louisiana, U.S.A., and fed immediately to hamsters by pipette. On day 42 post-exposure, gravid adults were removed from the hamsters' small intestines, rinsed quickly in Earle's balanced salt solution at room temperature and immersed whole in 3% glutaraldehyde in sodium cacodylate buffer (pH 7.4, 375 mOsm/liter) at 4?C. Worms were cut into sections (1-3 mm thick) immediately after immersion, fixed for 8 h, and post-fixed in 1% osmium tetroxide in cacodylate buffer at 4?C for 2 h. Tissues were dehydrated in ethanol, infiltrated with propylene oxide, embedded in Pelco Medcast epoxy (Ted Pella, Inc., Tustin, California, U.S.A.) on a rotation mixer at room temperature, and polymerized at 60?C. Sections were cut at 60-90 nm, mounted on uncoated copper grids, stained with lead citrate and uranyl acetate, and examined with a Zeiss 9S-2 or Hitachi HU-11E-1 transmission electron microscope at 40 or 75 kV, respectively. Sections were selected for study from three general strobilar regions: (1) early maturity with first sign of embryo accumulation in the uterus; (2) late maturity with U2 fully packed with developing embryos, but with few eggs in the paruterine organ; and (3) early gravid with U2 and paruterine organ packed with eggs, none of which possessed fully developed embryophores. The epithelium of U1 was studied only in early mature proglottids, but that of U2 was examined in all three regions.