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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Ref ID : 7752

Maria A. Jareno; [Etude Ultrastructurale de L'enkystement et du dekystement chez Onychodromus acuminatus (Ciliate, Hypotrichida)]. Protistologica XXI(3):313-321, 1985

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The main ultrastructural events associated with encystment and excystment are described in Onychodromus acuminatus: differentiation of the multilayered wall and cytoplasmic and nuclear changes. Cells in various stages of encystment were selected according to their morphological state. Excystment was induced, so that cells at definite stages could be recovered after various delays following induction. The cyst wall is made of three distinct layers; an additional inner layer, rather irregular in aspect and thickness, becomes prominent during excystment. Autophagic vacuoles are generated at a high rate during the first stages of encystment, ensuring lysis of many mitochondria, cytoplasmic fragments, inclusions and those among cyst wall precursors which remain unused. So far, neither basal bodies nor cilia have been observed inside autophagosomes. Major macronuclear changes occur during the first 90 minutes following induction of excystment. During encystment as well as at the beginning of excystment, the density of nucleopores is low, and many chromatin aggregates are adherent to the inner part of the nuclear envelope; in later stages, the number of nucleopores strongly increases, while inside the macronucleus the chromatin is decondensed and looses its connections with nuclear membrane, and nucleoli regress. Microtubules are observed in shape-pointed projections of the macronucleus: their function remains unknown. The micronuclei are rather stable: interphasic during encystment an prophasic during excystment. Basal bodies and cilia, in more or less altered states, can be seen up to the end of synthesis of mesocyst, but neither at later stages of encystment or at the beginning of excystment.