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In Eufolliculina uhligi and other folliculinid ciliates, a territory has been identified that differs ultrastructurally from other areas of the cell, and that is especially sensitive to mechanical stimuli. This territory is located around the anterior oral apparatus of the loricate trophont and posterior to the membranellar spiral of the swarmer. Each cilium in this territory is closely apposed to a small membrane-covered pin that is supported by transverse microtubules of the cilium. In front of the pin, the base of the cilium bulges out; the ciliary membrane is interconnected with the axoneme by filamentous material. Freeze-fractured cilia show a large rectangular particle array at the site of the basal swelling. Only scattered particles have been observed in the pin membrane. It is suggested that the cilium and the pin act as a unit, which has therefore been named the ciliumpin-complex. Comparison with ciliary organelles of unicellular and multicellular organisms indicates that, because of their polar organization, the complexes are involved in the transduction of oriented, presumably mechanical, stimuli.

Arabidopsis thaliana contains three genes with high homology to potato p24 which was described as a member of the Whirly family of nuclear transcriptional activators. Computer-based analysis revealed that all Arabidopsis Whirly (Why) proteins contain targeting sequences for either plastids or mitochondria. The functionality of these sequences was demonstrated by in vitro import assays into isolated organelles. Transient expression of GFP fusion proteins in protoplasts and onion epidermal cells confirmed the localisation of these proteins in plastids or mitochondria, respectively. The possession of organellar targeting sequences seems to be conserved among Why proteins of higher plant species, including potato p24.

Microtubules are present in all eukaryotic cells where they are involved in a broad range of processes: as cytoskele- tal structures they maintain the cell shape; they are the basis of motility phenomena, and they play a role in morphoge- netic processes as nuclear and cell division (Dustin 1984). However, although the knowledge about microtubules and their cellular functions has extremely increased, their role in pattern formation is little understood.

Different fixation techniques were employed to obtain satisfactory fixation of the endoplasm ofStentor coeruleus for ultrastructural investigations. The nuclei ofS. coeruleus are surrounded by a flattened fenestrated cisterna. The space between the nuclear envelope and the cisterna (= perinuclear space) is continuous with the cytoplasm via channels. The envelopes of both, micronucleus and macronucleus, are connected to the fenestrated cisterna by filamentous material. This organization accounts for the close association between micronucleus and macronucleus inStentor coeruleus. The fenestrated cisterna is compared to similar structures occurring in other organisms, and its possible function is discussed.

Summary The epiplasm is an intrinsic element of the cortex of ciliated protozoa, lying immediately beneath the inner alveolar membrane. In Pseudomicrothorax dubius this layer is about 150 nm thick. Glycerination of the cells for up to 48 hours has shown that the epiplasm is not affected in its structure whereas most of the other organelles are broken down by this treatment. So, in addition to its known function during morphogenesis, the epiplasm appears to have a cytoskeletal role in Pseudomicrothorax dubius. In this species the epiplasm may be equivalent to the cortical networks of filaments which in many other ciliates act as skeletal structures.

Summary Unlike most other ciliates, folliculinids divide into two physiologically and morphologically different cells: the anterior cell part (proter) becomes a non-feeding motile swarmer, the posterior part (opisthe) remains sessile and develops a new oral apparatus. After settling, the swarmer transforms into a sessile cell (metamorphosis). Division of the loricate ciliate Eufolliculina uhligi has been studied by scanning electron microscopy (SEM). Stomatogenesis in many aspects resembles that during metamorphosis. Eight stages can be distinguished. Stage 0 is characterized by the resorption of the old oral apparatus. New cilia arise from somatic kineties in a single, well-defined area. Two primordia of different sizes appear. The long, hook-shaped primordium of the opisthe gives rise to the AZM around the peristomial wings and to the paroral kinety. While the opisthe develops a functioning oral apparatus, the proter forms neither a buccal cavity nor a cytostome, and its oral ciliature is reduced. Stomatogenesis during cell division and during metamorphosis in E. uhligi is compared to that in motile heterotrichs, especially Stentor.

Our freeze-fracture study has revealed the presence of two types of intramembrane particle (IMP) aggregates in the somatic plasma membrane of Eufolliculina uhligi: rectangular particle arrays and rosettes. Their morphology and distribution differ in the motile and in the lorica secreting stage. Rectangular particle arrays occur to the left of the kineties. They are not very obvious in the swarmer, but increase in size and number during lorica formation. We discuss whether they are involved in the contractility of this ciliate. Rosettes occur to the right of the kinetics where they mark attachment sites of extrusive vesicles at the plasma membrane of the swarmer. No annulus-like structure as e.g. in Tetrahymena mucocysts and Paramecium trichocysts has been found in the membrane of these vesicles. The rosettes consist of 10-nm particles and lack an outer ring. Vesicle membrane and plasma membrane appear to remain separated during exocytosis. In the course of lorica secretion, the number of rosettes continuously decreases, following an anterior-posterior gradient. This indicates that new insertion of extrusive vesicles from intracellular stores occurs preferably at the anterior of the cell.

The structure and ultrastructure of the chitinous lorica of Eufolliculina sp. are described. The lorica is produced from precursor material secreted by the motile swarmer immediately after settling. This material is located in numerous vesicles found in the cortical region of the cells and is secreted by exocytosis. Initially, material is secreted from the ventral part of the cell to produce the attachment plate of the lorica. After this, exocytosis occurs over most of the body surface as the ampulla part of the lorica is constructed. During the later stages of lorica formation, secretion is mainly limited to the anterior of the cell as the neck is formed. The lorica is shaped mainly by the action of the cilia and by the behavior of the cell. While the neck is being formed, the anterior part of the cell is deformed by a local accumulation of cytoplasmic vacuoles. This deformation is employed in shaping the neck. No changes were detected in the organization of the cortical infraciliature during the first stages of lorica formation, but they do occur after the neck has been produced and as the swarmer develops into the sessile form.

Stomatogenesis during metamorphosis of the marine loricate ciliate, Eufolliculina uhligi, was observed by transmission electron microscopy. Kinetosome proliferation in the stomatogenic territory leads to the formation of an anarchic field. This separates into the left adoral and the right paroral primordia. Both primordia consist of pairs of kinetosomes. One kinetosome of a pair is associated with one transverse and two postciliary microtubules; the other has one transverse microtubule. The postciliary microtubules of the adoral kinetosomes become divergent; those of the paroral kinetosomes become convergent. The adoral kinetosomes arrange in promembranelles. Then a third row of kinetosomes is produced anteriorly to each promembranelle. This third row is short at the peristome but longer in the buccal area. The paroral kinetosomes form a stichodyad. The buccal part of the paroral primordium is resorbed during formation of the buccal cavity. Stomatogenesis ends with the development of a functioning cytostome. During this process, the postciliary microtubules of the buccal adoral membranelles elongate and become associated with cytopharyngeal vesicles. Fusion of these vesicles with the cytostome has been observed some time after the completion of the oral structures.

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.

Identification of protists can be made by their light microscopic anatomy. This statement seems to be trivial. However, protists need skeletal elements to maintain their shape. This shape may be rather complex or even bizarre. The reasons why they have such shapes is not discussed, rather this chapter deals with how the intra- and extracellular cortical organization is responsible for the appearance of protists. For, convenience, in this chapter protists will be divided into the (algal and protozoan) Flagellata, Sarcodina, Sporozoa, and Ciliata.