The Fine Structure of the Centriolar Apparatus and Associated Structures in the Flagellates Deltotrichonympha and Koruga. I. Interphase

SYNOPSIS. An electronmicroscopic study was made of the centriolar apparatus in the rostrum of Deltotrichonympha operculata and Koruga bonita , 2 closely related hypermastigote flagellates from the Australian termite, Mastotermes darwiniensis. In interphase flagellates, the centriolar apparatus consists of 2 similar parts with a mutually perpendicular orientation. Each part contains a large, club-shaped centriolar body consisting of fibrillar and granular material, without recognizable internal symmetry or microtubules. The anterior centriolar body extends from the inner rostral wall, which is structurally related to the fibrous wall surrounding the posterior centriolar body. The 2 centriolar bodies are joined by connecting branches, which meet at 3 barren kinetosome-like structures located inside the rostrum. Thus, an interphase flagellate has 2 centriolar bodies oriented at a 90° angle to each other, like a pair of typical centrioles in an interphase metazoan cell.     

THE AMEBA-TO-FLAGELLATE TRANSFORMATION IN TETRAMITUS ROSTRATUS : II. Microtubular Morphogenesis

Tetramitus exhibits independent ameboid and flagellate stages of remarkable morphological dichotomy. Transformation of the ameba involves the formation of four kinetosomes and their flagella. The arrangement of these kinetosomes and associated whorls of microtubules extending under the pellicle establishes the asymmetric flagellate form. While no recognizable kinetosomal precursors have been seen in amebae, and there is no suggestion of self-replication in dividing flagellates, developmental stages of kinetosomes have been identified. These are occasionally seen in association with the nucleus or with dense bodies which lie either inside of or close to the proximal end of the prokinetosome. Outgrowth of flagella involves formation of an axoneme and a membrane. From the distal tip of the kinetosome microtubules grow into a short bud, which soon forms an expanded balloon containing a reticulum of finely beaded filaments. The free ends of the microtubules appear unraveled; they are seen first as single elements, then as doublets, and finally are arranged into a cylinder. Growth in length is accompanied by a reduction in the diameter of the balloon. The concept that the formation of the kinetic apparatus might involve a nuclear contribution, followed by a spontaneous assembly of microtubules, is suggested.     

Drosophila Bld10 Is a Centriolar Protein That Regulates Centriole, Basal Body, and Motile Cilium Assembly

Cilia and flagella play multiple essential roles in animal development and cell physiology. Defective cilium assembly or motility represents the etiological basis for a growing number of human diseases. Therefore, how cilia and flagella assemble and the processes that drive motility are essential for understanding these diseases. Here we show that Drosophila Bld10, the ortholog of Chlamydomonas reinhardtii Bld10p and human Cep135, is a ubiquitous centriolar protein that also localizes to the spermatid basal body. Mutants that lack Bld10 assemble centrioles and form functional centrosomes, but centrioles and spermatid basal bodies are short in length. bld10 mutant flies are viable but male sterile, producing immotile sperm whose axonemes are deficient in the central pair of microtubules. These results show that Drosophila Bld10 is required for centriole and axoneme assembly to confer cilium motility.     

Flagellum retraction and axoneme depolymerisation during the transformation of flagellates to amoebae inPhysarum

Summary The transformation ofPhysarum polycephalum flagellates to myxamoebae is characterised by disappearance of the flagellum. This transition, from the flagellate to the myxamoeba was observed by phase contrast light microscopy and recorded by time lapse video photography to determine whether flagellates shed their flagella or they are absorbed within the cell. In addition, the kinetics of flagellum disappearance were also studied. Our observations indicate that the flagellum was absorbed within the cell; the process occurred within seconds. Flagellum resorbtion was preceded by typical morphological cell changes. The shape of the nucleus altered and its mobility within the cell decreased. It was not possible to observe the flagellum within the cell with phase contrast video recordings. Thin section electron microscopy was used to study this intracellular phenomenon. Several stages of flagellum dissolution could be identified within the cell. The two most important stages were: an axoneme surrounded by the flagellar membrane within a plasma membrane lined pocket or vacuole and the naked axoneme without its membrane, free within the cell cytoplasm. The existence of cytoplasmic microtubules prevented identification of any further dissolution stages of the flagellum. A group of microtubules adjacent to the flagellum but within the cytoplasm was observed in flagellates and also in those cells which possesed enveloped axonemes. The flagellum did not dissociate from the kinetosomes before resorbtion.Immunofluorescence studies with the 6-11-B-1 monoclonal antibody indicated that acetylated microtubules exist in myxamoebae after transformation from flagellates for up to 40 min. Acetylated tubulin is not limited to the centrioles in these cells.     

Feeding spectra and pseudoconoid structure in predatory alveolate flagellates

It is revealed experimentally that predatory alveolate flagellates Colpodella angusta Simpson et Patterson, C. edax Simpson et Patterson, and Voromonas pontica (Mylnikov, 2000) Cavalier-Smith, 2004 eat its prey by sucking out the insides through the front of its own body (rostrum). Feeding spectra of these predators include bodonids, percolomonads and colorless chrysomonads, while colorless euglenoids, cryptomonads, ciliates and naked amoebas are unsuitable food for them. Predators’ rostrum contains a microtubular structure (pseudoconoid) complemented by microtubular bands, micronemas or rhoptries. It is shown that the pseudoconoid begins near the kinetosomes of flagella and passes along the flagellate pocket into the rostrum. The pseudoconoid forms an unlocked cone in Colpodella angusta and Voromonas pontica, while, in Colpodella edax, it coils significantly without forming a cone. The similarity between the pseudoconoid and associated structures of predatory flagellates with analogous structures in other colpodellids, percinzoids, dinoflagellates and sporozoans is discussed.     

Ultrastructure and ATPase activity of the centriolar body at the mitotic stage in neoblasts of the planarianDugesia sp.

The fine structure and ATPase activity of the mitotic spindle in neoblasts of planaria were examined. In neoblasts, the cells have a large nucleus and nucleolus. Mitochondria are aggregated around the nucleus with chromatoid bodies adjacent. The cytoplasm contains little endoplasmic reticulum (ER) and few Golgi bodies but many free ribosomes, forming polysomes, can be seen throughout the cytoplasmic and spindle ground areas. In addition, centriolar bodies, atypical centrioles, can also be recognized in the cytoplasm. Cells in the G₂ stage contain a pair of electron-dense bodies, both consisting of fibrogranules but differing from each other in fine structure and, in the mitotic stage, only one fibrogranular body can be recognized at each pole. ATPase activity was detected in the centriolar bodies in the G₂ and mitotic stages and in the ground area of the cytoplasm and spindle apparatus filled by free ribosomes. The activity associated with the microtubules differed with the developmental stage.     

Ultrastructure of the flagellate Cruzella marina (Kinetoplastidea)

The ultrastructure of a marine, free-living heterotrophic kinetoplastid Cruzella marina was investigated with special attention being paid to the mitochondrion and flagellar organization. The flagellates have a polykinetoplastidal mitochondrion. Two flagella emerge from the pocket; one of these turns anteriorly being forward-directed, while the other is posteriorly directed to be adjacent to the ventral cell surface. The transition zone of both the flagella includes central filaments. The cytostome opens on the tip of the rostrum. The cytostome leads to the channel of cytopharynx, which penetrates the rostrum and proceeds into the flagellate body cytoplasm. The comparison of the relevant morphological and molecular data suggest that C. marina may arise early in the Kinetoplastidea lineage, before divergence of the majority taxa of the kinetoplastid flagellates.     

Ultrastructure of the dinoflagellate Polykrikos. II. The nucleus and its connections to the flagellar apparatus

Electron microscopy of the colonial dinoflagellate Polykrikos kofoidi revealed a nuclear cortex formed of two electron-dense cortical layers directly beneath the nuclear envelope. Nuclear pores were confined to vesicular outpocketings of the nuclear envelope over circular discontinuities in the cortical layers. A conspicuous fibrous ribbon extended from the nucleus to the flagellar apparatus of each zooid. The ribbons resembled in their structure and position the attractophores of termite flagellates. Each flagellar apparatus consisted of two flagella, two elongate axial kinetosomes, an oblique kinetosome, and two roots of markedly different periodicities.     

The Fine Structure of the Centriolar Apparatus and Associated Structures in the Flagellates Deltotrichonympha and Koruga. II. Division

SYNOPSIS. At division of Deltotrichonympha operculata and Koruga bonita from the Australian termite, Mastotermes darwiniensis , the 2 centriolar bodies separate, each becoming a mitotic center. Spindle microtubules develop from the lower end of each centriolar body and radiate towards the elongating nucleus. A new rostrum is formed in association with each centriolar body. Thus, centriolar bodies which lack the structure of typical centrioles can nevertheless function as division centers during mitosis.     

Ultrastructure of the amoeboid flagellate <Emphasis Type="Italic">Thaumatomonas zhukovi</Emphasis> Mylnikov et Mylnikov (Thaumatomonadida (Shirkina) Karpov, 1990)

The ultrastructure of the amoeboid flagellate Thaumatomonas zhukovi sp. is presented. The cell is covered by cell body scales that formed on the surface of mitochondria. Capturing bacteria, the pseudopodia emerge from the ventral groove, which is supported by two longitudinal microtubular bands. The heterodynamic flagella emerge from the small flagellar pocket. Both flagella are covered by cone-shaped scales and thin twisted mastigonemes. The transitional zone of the flagella contains a thin-walled cylinder. The transversal plate of the flagella rises above the cell surface. The kinetosomes lie parallel to each other. The flagellar root system consists of three microtubular bands and a fibrillar rhizoplast. The vesicular nucleus and the Golgi apparatus have typical structures. The cytoplasm contains microbodies and food vacuoles. Mitochondria contain tubular cristae. Extrusive organelles (kinetocysts), which contain amorphous material and a capsule, were found in the cytoplasm. The capsule consists of a theca and a cylinder. The resemblance of Thaumatomonas zhukovi to other thaumatomonads is discussed.     

Ciliogenesis and centriole formation in the mouse embryonic nervous system. An ultrastructural analysis

Serial ultrathin sections were used to study the formation of the primary cilium and the centriolar apparatus, basal body, and centriole in the neuroepithelial primordial cell of the embryonic nervous system in the mouse. At the end of mitosis, the centrioles seem to migrate toward the ventricular process of the neuroepithelial cell, near the ventricular surface. One of these centrioles, the nearest to the ventricular surface, begins to mature to form a basal body, since its tip is capped by a vesicle probably originating in the cytoplasm. This vesicle fuses with the plasmalemma and the cilium growth by the centrifugal extension of the 9 sets of microtubule doublets. These 9 sets invade the thick base of the cilium which is initially capped by a ball-shaped tip with the appearance of a mushroom cilium. The secondary extension of 7, then 5, and finally 2 sets of microtubule doublets contribute to form the tip of the mature cilium, which is associated with a mature centriolar apparatus formed by a basal body and a centriole. Centriologenesis occurs before mitosis and is concomitant with the progressive resorption of the cilium. The daughter centriole, or procentriole, begins to take form near the tips of fibrils that extend perpendicularly and at a short distance from the wall of the parent centriole. Osmiophilic material accumulates around these fibrils, and gives rise to the microtubules of the mature daughter centriole. These centrioles formed by a centriolar process are further engaged in mitosis, after the total resorption of the cilium. This pattern of development suggests that in the primordial cells of the embryonic nervous system, centriologenesis and ciliogenesis are 2 independent phenomena.     

Structure of the flagellar apparatus of heterotrophic flagellate Klosteria bodomorphis Nikolaev et al., 2003 (Kinetoplastea Excavata)

The structure of the flagellar apparatus of excavate flagellate Klosteria bodomorphis was considered. Two naked heterodynamic flagella covered with a dense layer of glycocalyx emerge from a single flagellar pocket. The kinetosomes are parallel or at an acute angle to each other. The dorsal and ventral rootlets run from the kinetosomes and produce dorsal and ventral bands which are not connected to each other. The MTR band begins in the wall of the flagellar pocket. The long cytopharynx is reinforced with an MTR band and additional microtubules. A small fibril and a horseshoe-shaped structure lie in the anterior part of the cytopharynx. The vesicular nucleus and Golgi apparatus have the typical structure. The mitochondrion has discoid cristae. The kinetoplast as a compact formation was not found. The similarity between K. bodomorphis and other free-living kinetoplastids is discussed.     

Localization of Tubulin and a Centrin-Homologue in Vegetative Cells and Developing Gametangia of Ectocarpus siliculosus (Dillw.) Lyngb. (Phaeophyceae,Ectocarpales)

The distribution of tubulin and centrin in vegetative cells and during gametogenesis of Ectocarpus siliculosus was studied by immunofluorescence. In interphase cells bundles of microtubules are focused on the centriolar region near the nuclear surface. Some of the bundles ensheath the nucleus while others traverse the cytoplasm in various directions, sometimes reaching the cell cortex. Evaluation of serial optical sections by confocal laser scanning microscopy (CLSM) revealed that the perinuclear and “cytoplasmic” microtubule bundles presumably constitute a single complex. In interphase cells centrin is localized as a single bright spot in the centriolar region. In dividing cells duplication and separation of the microtubular complex and the centrin spot takes place. In post-mitotic cells with two nuclei, the centrioles are located at opposite cell poles, short microtubule bundles emanate from them and partially encompass the nucleus. During gametogenesis a gradual transformation of the vegetative cytoskeleton to the gametic flagellar apparatus occurs.     

Structure of the cell of the amoeboid flagellate <Emphasis Type="Italic">Thaumatomonas coloniensis</Emphasis> Wylezich et al., 2007 (Thaumatomonadida (Shirkina) Karpov, 1990)

The ultrathin structure of the amoeboid flagellate Thaumatomonas coloniensis Wylezich et al. has been studied. The cell is surrounded by somatic scales forming on the surface of the mitochondria. The heterodynamic flagella emerge from the small flagellar pocket. Both flagella are covered by pineal scales and thin twisted mastigonemes. The kinetosomes lie parallel to each other. The transitional zone of the flagella carries the thin-walled cylinder. The transversal plate of the flagella is above the cell surface. The flagellar root system consists of three microtubular bands and a fibrillar rhizoplast. The vesicular nucleus and Golgi apparatus are of the usual structure. The mitochondria contain tubular cristae. The extrusive organelles (kinetocysts) contain amorphous material and a capsule; they are located in cytoplasm. The capsule consists of a muff and cylinder. Osmiophilic bodies of various shapes contain crystalloid inclusions. The pseudopodia capturing the bacteria emerge from the ventral groove. The groove is armored by the two longitudinal groups of the close situated microtubules. Microbodies and symbiotic bacteria have not been discovered. The resemblance of Th. coloniensis with other thaumatomonads is discussed.     

Spermiogenesis and the spermatozoon ultrastructure of Robphildollfusium fractum (Digenea: Gyliauchenidae), an intestinal parasite of Sarpa salpa (Pisces: Teleostei)

Spermiogenesis in Robphildollfusium fractum begins with the formation of a differentiation zone containing: two centrioles, each bearing striated rootlets, nucleus, several mitochondria and an intercentriolar body constituted by seven electron-dense layers. The two centrioles originate two free flagella growing orthogonally to the median cytoplasmic process. Later, the free flagella rotate and undergo proximodistal fusion with the median cytoplasmic process. Nuclear and mitochondrial migrations occur before this proximodistal fusion. Finally, the young spermatozoon detaches from the residual cytoplasm after the constriction of the ring of arched membranes. The spermatozoon of R. fractum exhibits two axonemes of different length of the 9 + ‘1’ trepaxonematan pattern, nucleus, two mitochondria, two bundles of parallel cortical microtubules, external ornamentation of the plasma membrane, spine-like bodies and granules of glycogen. Additionally, a shorter axoneme, which does not reach the nuclear region, the presence of an electron-dense material in the anterior spermatozoon extremity and the morphologies of both spermatozoon extremities characterize the mature sperm of R. fractum.     

CENTRIOLE REPLICATION : II. Sperm Formation in the Fern, Marsilea, and the Cycad, Zamia

Sperm formation was studied in the fern, Marsilea, and the cycad, Zamia, with particular emphasis on the centrioles. In Marsilea, the mature sperm possesses over 100 flagella, the basal bodies of which have the typical cylindrical structure of centrioles. Earlier observations by light microscopy suggested that these centrioles arise by fragmentation of a body known as the blepharoplast. In the youngest spermatids the blepharoplast is a hollow sphere approximately 0.8 µ in diameter. Its wall consists of closely packed immature centrioles, or procentrioles. The procentrioles are short cylinders which progressively lengthen during differentiation of the spermatid. At the same time they migrate to the surface of the cell, where each of them puts out a flagellum. A blepharoplast is found at each pole of the spindle during the last antheridial mitosis, and two blepharoplasts are found in the cytoplasm before this mitosis. Blepharoplasts are also found in the preceding cell generation, but their ultimate origin is obscure. Before the last mitosis the blepharoplasts are solid, consisting of a cluster of radially arranged tubules which bear some structural similarity to centrioles. In Zamia, similar stages are found during sperm formation, although here the number of flagella on each sperm is close to 20,000 and the blepharoplast measures about 10 µ in diameter. These observations are discussed in relation to theories of centriole replication.     

Ultrafine organization of Leptomonas peterhoffi flagellates cultured in broth and solid nutrient media

The morphology of in vitro grown lower trypanosomatids L. peterhoffi was studied by means of electron microscopy. The flagellates from both liquid and solid culture media are represented by uninucleate cells of two structural types. Type I flagellates are characterized by dense cytoplasm enriched with numerous ribosomes. Type II flagellates are most abundant in the cultures; they display a less dense cytoplasm and fewer ribosomes. The flagella of L. peterhoffi of both types form enlargements, which are most expressed at the outlet of the flagellar pocket. The nuclei of some cells contain twisted threads about 10 nm in diameter. L. peterhoffi from the liquid media usually possess long, narrow and curved flagellar pockets. On the solid medium, amoeboid and hemispherical colonies composed of both uninucleate and giant multinucleate cells are formed. In these cells the flagellar pockets are usually short and straight. Outside the flagellar pocket, the axoneme often becomes looped in the flagellar enlargements of the colonial uninucleate cells.     

BASAL BODIES, BUT NOT CENTRIOLES, IN NAEGLERIA

Amebae of Naegleria gruberi transform into flagellates whose basal bodies have the typical centriole-like structure. The amebae appear to lack any homologous structure, even during mitosis. Basal bodies are constructed during transformation and, in cells transforming synchronously at 25°C, they are first seen about 10 min before flagella are seen. No structural precursor for these basal bodies has been found. These observations are discussed in the light of hypotheses about the continuity of centrioles.     

Ultrastructure and development of the amoebo-flagellate cells of the protostelidProtosporangium articulatum

Summary Amoebo-flagellate cells develop upon spore germination in the protostelidProtosporangium articulatum. The germling may emerge flagellate or as an amoeba. In either case the cell undergoes mitosis within an hour of germination. The spindle is open and centric, and typically has several pairs of kinetosomes at the poles. During telophase, the kinetosomes are found at the surface of the cell and flagella and flagellar rootlets begin to develop. Some flagella remain in close association with the nucleus, the nucleus-associated flagella; others are located away from the nucleus, the supernumerary flagella. The flagellar apparatus is identical for both nucleus-associated flagella and supernumerary flagella. However, only the nucleus-associated flagella are able to generate the jerking, helical swim typical of amoebo-flagellates with a swarm cell-like morphology.