Ciliary resorption and membrane retrieval during morphogenesis in the ciliateEufolliculina uhligi

Abstract

In this study, two processes of ciliary resorption in the sessile heterotrich ciliateEufolliculina uhligi are presented. (1) Prior to division,E. uhligi resorbs its oral apparatus. This occurs by the incorporation of axonemes of single cilia into the cytoplasm, and by the formation of a large vacuole, in which the membranellar cilia of the buccal cavity are enclosed. The cilia inside the vacuole are continuously resorbed. After some minutes, this vacuole divides into several smaller ones which accumulate in the opisthe. They are characterized by a progressive lamination and infolding of their membrane; the cilia inside the vacuole degenerate. Morphological similarities and dissimilarities to digestive vacuoles are discussed. (2) Transformation of the motile swarmer into the sessile cell includes lorica secretion as well as resorption of the anterior membraneller cilia. The lorica material is secreted by exocytosis. The vesicular membrane is incorporated into the plasma membrane. The cilia are resorbed by retraction of the axonemes or by lateral fusion with the plasmalemma. The fate of the additional plasma membrane has been studied using cationized ferritin as an exogeneous marker. It could be demonstrated, that at the sites of ciliary resorption and exocytosis plasma membrane is retrieved endocytotically via coated pits. Endocytosis is not restricted to the parasomal sacs, but also occurs at the septae between adjacent alveoli. The internalized membrane moves as small vesicles and/or flattened cisternae from the periphery toward the center of the cell. No fusion of these membraneous elements with secondary lysosomes has been observed 1 h after application of ferritin. The further fate of the internalized membrane is discussed.