An ultrastructural study of the egg wall surrounding the miracidium of the digenean Brandesia turgida () (Plagiorchiida: Pleurogenidae), with the description of a unique cocoon-like envelope |
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| Affiliation: | 1. W. Stefański Institute of Parasitology, Polish Academy of Sciences, 00-818 Warsaw, Poland;2. Department of General Biology and Parasitology, Medical University of Warsaw, 5 Chałubińskiego Street, Warsaw, Poland;3. I.D. Papanin Institute for the Biology of Inland Waters, Russian Academy of Sciences, 152742 Borok, Yaroslavl Province, Russia;4. Laboratori de Parasitologia, Departament de Microbiologia i Parasitologia Sanitàries, Facultat de Farmàcia, Universitat de Barcelona, Av. Joan XXIII, sn, E08028 Barcelona, Spain;5. Institut de Recerca de la Biodiversitat, Facultat de Biologia, Universitat de Barcelona, Av. Diagonal, 645, E08028 Barcelona, Spain;6. Department of Life Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK;1. Physikalisches Institut, University of Bern, Sidlerstr. 5, CH-3012 Bern, Switzerland;2. Laboratoire d’Astrophysique de Marseille, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France;3. DLR, Institute of Planetary Research, Rutherfordstr. 4, Berlin-Adlershof, Germany;1. Laboratoire de Planétologie et Géodynamique, UMR 6112, CNRS et University of Nantes, Faculté des Sciences et Techniques, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France;2. Centre National d’Études Spatiales, 2 Place Maurice Quentin, 75039 Paris, France;1. SUBATECH, Ecole des Mines de Nantes, Université de Nantes, CNRS/IN2P3, Nantes, France;2. GIP Arronax, 1 rue Aronnax, 44817 Saint-Herblain, France;1. Department of Oceanography, University of Cape Town, South Africa;2. Sorbonne Universités (UPMC, Univ Paris 06)-CNRS-IRD-MNHN, LOCEAN Laboratory, Paris, France;3. LMI ICEMASA, Laboratoire de Physique des Océans, UMR 6523 (CNRS, Ifremer, IRD, UBO), Plouzané, France;4. Instituto Nacional de Hidrografia e Navegação, Maputo, Mozambique |
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| Abstract: | The intrauterine eggs of the pleurogenid trematode Brandesia turgida (Brandes, 1888), exhibiting advanced stages of miracidial differentiation and fully formed, ciliated miracidia, were examined by means of transmission electron microscopy (TEM). Each embryonated egg is composed of a mature miracidium surrounded by a four-layered egg wall: (1) an outer, anucleate layer external to the eggshell, which forms a thick cocoon; (2) the operculate egg-shell; (3) a small remnant of the compact, granular cytoplasm of the outer embryonic envelope (sensu stricto); and (4) a relatively distinct cellular remnant of the inner embryonic envelope. Layers enveloping the egg apparently play an important role in the protection, metabolism and storage of nutritive reserves for the developing miracidium. The outer, anucleate layer, or cocoon, situated externally to the eggshell and composed of a transparent, electron-lucent substance with numerous dense, osmiophilic islands attached to its peripheral membrane, has never previously been seen in TEM studies of the eggs of parasitic platyhelminths. The origin, formation, functional ultrastructure and chemical composition of this peculiar layer remain enigmatic, although its function appears to be protective. The thick, electron-dense eggshell resembles that of other trematodes, exhibiting a characteristic fissure zone around the operculum. The very small, indistinct remnants of the outer embryonic envelope appear in the form of a very thin, compact, granular cytoplasm closely attached to the inner surface of the eggshell. Conversely, the inner embryonic envelope is frequently apparent at one or both poles of the developed egg as a syncytial envelope formed by the fusion of mesomeres. This envelope, even in eggs containing a fully formed miracidium, still has the features of a metabolically active layer with an energy storage capability. Lysosome-like structures observed in some eggs may be involved in the autolysis of the embryonic envelopes. |
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| Keywords: | Digenea Egg Egg envelopes Cocoon Eggshell Operculum Miracidium Ultrastructure |
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