MITOSIS IN THE AQUATIC FUNGUS RHIZOPHYDIUM SPHEROTHECA (CHYTRIDIALES)

The structure of centric, intranuclear mitosis and of organelles associated with nuclei are described in developing zoosporangia of the chytrid Rhizophydium spherotheca. Frequently dictyosomes partially encompass the sides of diplosomes (paired centrioles). A single, incomplete layer of endoplasmic reticulum with tubular connections to the nuclear envelope is found around dividing nuclei. The nuclear envelope remains intact during mitosis except for polar fenestrae which appear during spindle incursion. During prophase, when diplosomes first define the nuclear poles, secondary centrioles occur adjacent and at right angles to the sides of primary centrioles. By late metaphase the centrioles in a diplosome are positioned at a 40° angle to each other and are joined by an electron-dense band; by telophase the centrioles lie almost parallel to each other. Astral microtubules radiate into the cytoplasm from centrioles during interphase, but by metaphase few cytoplasmic microtubules are found. Cytoplasmic microtubules increase during late anaphase and telophase as spindle microtubules gradually disappear. The mitotic spindle, which contains chromosomal and interzonal microtubules, converges at the base of the primary centriole. Throughout mitosis the semipersistent nucleolus is adjacent to the nuclear envelope and remains in the interzonal region of the nucleus as chromosomes separate and the nucleus elongates. During telophase the nuclear envelope constricts around the chromosomal mass, and the daughter nuclei separate from each end of the interzonal region of the nucleus. The envelope of the interzonal region is relatively intact and encircles the nucleolus, but later the membranes of the interzonal region scatter and the nucleolus disperses. The structure of the mitotic apparatus is similar to that of the chytrid Phlyctochytrium irregulare.     

Mitosis and autospore formation in the green algaKirchneriella lunaris

Summary Asexual reproduction inKirchneriella lunaris involves autospore formation. After an initial mitosis, the curved cell cleaves to a variable extent, and then the nuclei divide again; finally the cytoplasm is partitioned into four around each nucleus. Rudimentary centrioles appear prior to the first mitosis; centriole complexes then become associated with a developing sheath of extranuclear microtubules at prophase; fenestrae appear at the poles through which both microtubules and centrioles migrate, preceding intranuclear spindle formation. The nucleus meanwhile is enveloped by a perinuclear layer of endoplasmic reticulum which is also interposed between the golgi body and nuclear envelope. Chromosome separation is accompanied by considerable spindle elongation. Finally the reforming nuclear envelope excludes both centriole complex and interzonal spindle apparatus from daughter nuclei. Cleavage is preceded by i) nuclear movement to the cell center, ii) movement of centriole complexes around daughter nuclei until they are opposite one another, and iii) the concurrent formation of a system of transverse microtubules extending across the cell. Other microtubules encircle the cell predicting the cleavage plane. A septum then appears amongst these cytokinetic microtubules, possibly derived from the plasmalemma; it extends across the cell too, through the cleaving peripheral chloroplast. Secondary mitoses follow (as above) during which this septum may be partially resorbed. Finally this septum is reformed, if necessary, and two other septa appear (as above) to quadripartition the cell. Mitotic and cytokinetic structures in this algae are briefly compared with some others.     

An unusual mitotic mechanism in the parasitic protozoan Syndinium sp

Syndinium and related organisms which parasitize a number of invertebrates have been classified with dinoflagellates on the basis of the morphology of their zoospores. We demonstrate here that with respect to chromosome structure and chemistry as well as nuclear division, they differ fundamentally from free-living dinoflagellates. Alkaline fast green staining indicates the presence of basic proteins in Syndinium chromosomes. Chromatin fibers are about 30 Å thick and do not show the arrangement characteristic of dinoflagellate chromosomes. The four V-shaped chromosomes are permanently attached at their apexes to a specific area of the nuclear membrane through a kinetochore-like trilaminar disk inserted into an opening of the membrane. Microtubules connect the outer dense layer of each kinetochore to the bases of the two centrioles located in a pocket-shaped invagination of the nuclear envelope. During division kinetochores duplicate, and each sister kinetochore becomes attached to a different centriole. As the centrioles move apart, apparently pushed by a bundle of elongating microtubules (central spindle), the daughter chromosomes are passively pulled apart. During the process of elongation of the central spindle, the cytoplasmic groove on the nuclear surface which contains the central spindle sinks into the nuclear space and is transformed into a cylindrical cytoplasmic channel. A constriction in the persisting nuclear envelope leads to the formation of two daughter nuclei.     

Ultrastructure of the cytoplasm of the lower holotrichous infusorian Tracheloraphis prenanti]

The ciliature of T. prenanti Dragesco 1960 (forma oligocineta Raikov et Kovaleva, 1968) consists of 14-18 ventral and lateral longitudinal kineties with paired kinetosomes, carrying either two cilia or one cilium per kinetosome pair (in the latter case, the nonciliated kinetosome is always the posterior one). The ectoplasmic fibrillar system belongs to the postciliary type. A pair of kinetosomes shares a common basal plate. The anterior kinetosome gives rise to a short ribbon of transverse microtubules, the posterior one, to a poorly developed kinetodesmal filament and to a strong ribbon of postciliary microtubules. The latter proceeds backwards along 8 to 12 kinetosome pairs, being incorporated into a laminated postciliodesma which accompanies each kinety on its right side. Rows of Golgi elements, sending secretory vesicles and channels towards the body surface, exist beneath the kinetosome bases. Each kinety is accompanied on its left by a microfibrillar myoneme, surrounded by perimyary vesicles and underlain by a row of mitochondria. The median part of the dorsal surface is nonciliated; the cytoplasm here is rich of membrane systems, contains peripheral, electron-dense, extrusible inclusions and sometimes also bacteria. The electron-dense inclusions develop in the endoplasm, in close contact with mitochondria. The endoplasm contains also large microfibrillar spheres of unknown nature.     

Centrioles in the cell cycle. I. Epithelial cells

A study was made of the structure of the centrosome in the cell cycle in a nonsynchronous culture of pig kidney embryo (PE) cells. In the spindle pole of the metaphase cell there are two mutually perpendicular centrioles (mother and daughter) which differ in their ultrastructure. An electron-dense halo, which surrounds only the mother centriole and is the site where spindle microtubules converge, disappears at the end of telophase. In metaphase and anaphase, the mother centriole is situated perpendicular to the spindle axis. At the beginning of the G1 period, pericentriolar satellites are formed on the mother centriole with microtubules attached to them; the two centrioles diverge. The structures of the two centrioles differ throughout interphase; the mother centriole has appendages, the daughter does not. Replication of the centrioles occurs approximately in the middle of the S period. The structure of the procentrioles differs sharply from that of the mature centriole. Elongation of procentrioles is completed in prometaphase, and their structure undergoes a number of successive changes. In the G2 period, pericentriolar satellites disappear and some time later a fibrillar halo is formed on both mother centrioles, i.e., spindle poles begin to form. In the cells that have left the mitotic cycle (G0 period), replication of centrioles does not take place; in many cells, a cilium is formed on the mother centriole. In a small number of cells a cilium is formed in the S and G2 periods, but unlike the cilium in the G0 period it does not reach the surface of the cell. In all cases, it locates on the centriole with appendages. At the beginning of the G1 period, during the G2 period, and in nonciliated cells in the G0 period, one of the centrioles is situated perpendicular to the substrate. On the whole, it takes a mature centriole a cycle and a half to form in PE cells.     

Ultrastructural studies of the cell cycle in a multicentriolar form ofBracteacoccus minor (Chlorophyceae,Chlorellales)

Summary The ultrastructure of mitosis and cytokinesis is studied in the typical and a multicentriolar form of the multinucleate green algaBracteacoccus minor (Chodat) Petrovà. These processes are essentially identical in both forms, and are similar to those in other uni- and multinucleate chlorellalean algae. The mitotic spindle is closed and centric, and a fragmentary perinuclear envelope is present. In multinuclear cells mitosis is synchronous and may occur at the same time as cytokinesis. Cleavage is simultaneous and centrifugal, starting near the nucleus-associated centrioles and apparently mediated by phycoplast microtubules of the trochoplast type. Flagellated wall-less spores are usually formed. In the typical form ofB. minor, each interphase nucleus is associated with two mature centrioles (= one set) which function as centrosomal markers. At the onset of mitosis these centrioles duplicate and segregate and eventually establish the two poles of the spindle, where polar fenestrae develop in the nuclear envelope. In the multicentriolar form, however, each interphase nucleus generally is associated with two or three sets of centrioles. Consequently, during mitosis each half-spindle is associated with two or three sets. These centrioles are not necessarily all associated with the fenestrae at the spindle poles, but one or more sets are frequently associated with the nuclear membrane, more or less remote from the nuclear poles. However, the spindle in this multicentriolar form remains essentially bipolar. Cleavage generally results in zoospores with two, four or six flagella. The behaviour of the extra centrioles during the cell cycle and their possible relationship with centrosomes are discussed.     

Ultrastructural evidence for the biflagellate origin of the uniflagellate fungal zoospore

Summary The morphological similarities between the kinetosome and the second centriole of the zoospores of Phlyctochytrium kniepii and P. punctatum (Chytridiomycetes) suggest that the second centriole in the chytrid zoospore is a vestigial flagellum base. It is suggested that the term vestigial kinetosome may also be used when referring to the structure which is presently termed the second centriole of the chytrid zoospore. Morphological similarities between the chytrid zoospores of P. kniepii and P. punctatum and the zoospores of Rhizidiomyces apophysatus (Hyphochytridiomycetes) are noted. The possible biflagellate origin of fungi with uniflagellate zoospores is discussed. The third fiber (C fiber) of the kinetosome triplet is shown to form as an outgrowth of the B fiber of the kinetosome doublet.     

Transmission Electron Microscopical Observations on Stomatogenesis during Metamorphosis of Eufolliculina uhligi (Ciliophora: Heterotrichida)1

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.     

Daughter centrioles assemble preferentially towards the nuclear envelope in Drosophila syncytial embryos

Centrosomes are important organizers of microtubules within animal cells. They comprise a pair of centrioles surrounded by the pericentriolar material, which nucleates and organizes the microtubules. To maintain centrosome numbers, centrioles must duplicate once and only once per cell cycle. During S-phase, a single new ‘daughter’ centriole is built orthogonally on one side of each radially symmetric ‘mother’ centriole. Mis-regulation of duplication can result in the simultaneous formation of multiple daughter centrioles around a single mother centriole, leading to centrosome amplification, a hallmark of cancer. It remains unclear how a single duplication site is established. It also remains unknown whether this site is pre-defined or randomly positioned around the mother centriole. Here, we show that within Drosophila syncytial embryos daughter centrioles preferentially assemble on the side of the mother facing the nuclear envelope, to which the centrosomes are closely attached. This positional preference is established early during duplication and remains stable throughout daughter centriole assembly, but is lost in centrosomes forced to lose their connection to the nuclear envelope. This shows that non-centrosomal cues influence centriole duplication and raises the possibility that these external cues could help establish a single duplication site.     

The Fine Structure of Microgametogenesis in Haemoproteus columbae Kruse*

SYNOPSIS. The structural changes leading to the formation of motile microgametes from a single immobile intracellular gametocyte have been examiued in the electron microscope. After pigeon blood infected with Haemoproteus columbae was exposed to the air at room temperature for a few minutes axonemes appeared in the parasite's cytoplasm and the cytoplasm itself appeared less dense. The axonemes were connected with bundles of intranuclear microtubules that were perhaps spindle fibers. No conventional kinetosomes or centrioles have been observed. After the microgametocyte left the erythrocyte, it assumed the shape of a polarized slug or a dumb-bell. Half of the organism was surrounded by a single membrane and filled by part of the nucleus. The other half was surrounded by the remains of the multiple membranes of the gametocyte and contained pigment granules, mitochondria, axonemes and nuclear extensions. The axonemes and nuclear extensions were segregated at the periphery of the cell, exterior to the gametocyte's inner membrane, and were assembled in situ into microgametes. The mature microgamete appeared to peel off from the gametocyte, leaving a residual body.     

The Resorption of Cilia in Cyathodinium piriforme*

SYNOPSIS. The fine structure of the cilium, kinetosome, kinetodesmal fiber, and basal microtubules has been described in Cyathodinium piriforme. The ciliary axoneme is encased in an electron-dense jacket termed the axonemal jacket. This jacket surrounds the axoneme and is found midway between the axoneme and the ciliary membrane when viewed in cross section. Before division or reorganization the cilia are withdrawn into the cell. Intact cilia surrounded by their jackets are found in the cytoplasm during the early phases of retraction. Degradation of the axonemal microtubules precedes the dissolution of the axonemal jacket. Profiles of the jackets are observed after the microtubules have been resorbed. The cilia appear to detach from the kinetosomes. Barren kinetosomes are seen below the cell surface frequently with kinetodesmal fibers still attached. Whether all or some of these barren kinetosomes contribute to the formation of the new ciliary anlage cannot be ascertained.     

MITOSIS IN THE FUNGUS THRAUSTOTHECA CLAVATA

The ultrastructure of mitosis is described in Thraustotheca clavata, an oömycete fungus. An intranuclear spindle develops between differentiated regions of the nuclear envelope which move apart, each associated with 180° oriented centriole pairs. The spindle contains low numbers of continuous and interdigitating microtubules in addition to chromosomal microtubules. Each kinetochore is attached to only one microtubule. Serial section analysis shows that at meiosis there are probably 12 chromosomes in the diploid nucleus, yet at mitosis the methods utilized in the present study suggest that there may be less than 12 kinetochores connected to each pole. At mitosis many of the kinetochores within a given spindle are not arranged in opposite pairs. The behavior of the spindle microtubules during mitosis is comparable to that of higher organisms but the rarity of short intertubular distances appears to preclude significant force generation by means of intertubular bridge mechanisms. Evidence is presented for a nuclear envelope-microtubule interaction which is capable of generating shear forces during both mitosis and interphase nuclear movements.     

THE ULTRASTRUCTURE OF CRUCIFORM NUCLEAR DIVISION IN SOROSPHAERA VERONICAE (PLASMODIOPHOROMYCETE)

Vegetative nuclear divisions in cystosoral Plasmodia from the shoot system of Sorosphaera veronicae Schroeter were studied with standard transmission electron microscopy. Each metaphase nucleus forms a cruciform configuration as the persistent nucleolus elongates perpendicularly to chromatin aligned on the equatorial plate. The nuclear envelope remains intact during metaphase and anaphase. Each spindle pole consists of a fenestrated nuclear envelope with an exteriorly situated centriole and closely associated endoplasmic reticulum. Intranuclear membranous vesicles occur within metaphase and anaphase nuclei and are closely associated with chromatin and nuclear envelope. Microtubules pass from centrioles into the nucleus and are also attached to chromatin at kinetochores.     

Pericentriolar processes of photoreceptor cell basal bodies in the mammalian retina

Pericentriolar processes (arm-like fibers) of the migrating centrioles (diplosome) in differentiating retinal photoreceptor cells were examined in six mammalian species (hamster, vole, rat, rabbit, ferret, cat). These processes emanate in a radial fashion from one end of the centrioles comprising the photoreceptor diplosome. The pericentriolar processes of the basal body are first observed as the diplosome migrates toward the apical plasmalemma, suggesting that centrioles are committed early-on to developing such processes. One pericentriolar process arises from each set of microtubular triplets comprising the centriole and are apicoexternally oriented at an angle of between 30 and 60 degrees with the centriolar axis. Prior to the arrival of one of the centrioles at the apical plasmalemma these processes connect with an electron-dense portion of a centriole-associated vacuole. The diplosome migrates to the apical plasmalemma where one centriole (the presumptive basal body) orients perpendicularly to the apical plasmalemma. The centriole-associated vacuole appears to fuse with the plasmalemma. The pericentriolar processes appear to attach to this fusion site on the plasmalemma which is a region of the membrane characterized by increased electron density (the basilar plate). Invagination of the apical membrane, which occurs at this same site, is accompanied by a lengthening of the microtubules forming a cilium and is observed as an outpouching of the plasmalemma within the aforementioned invagination. The associated vacuole apparently becomes continuous with the apical plasmalemma. These pericentriolar processes appear to be functionally involved in ciliogenesis and offer structural stability between the basal body, the plasmalemma and indirectly the cilium.     

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.     

Fine structure of the dorsal bristle complex and pellicle of Euplotes

The fine structure of the dorsal bristle complex and pellicle of non-developing Euplotes eurystomus is described in detail by scanning and transmission electron microscopy. The bristle-pit unit is a highly differentiated complex of organelles. The bristle complex is composed of a pair of kinetosomes (basal bodies) joined by a connective. The anterior kinetosome bears the bristle cilium, which contains a polarized network of particles (“lasiosomes”). The posterior kinetosome bears a very short, knob-like “condylocilium,” and has an associated striated fiber. Accessory ribbons of microtubules are also associated with the kinetosome couplets. Parasomal sacs, a septum connecting the bristle cilium to the anterior wall of the pit, core granules of the kinetosomes, and large membranous ampules are described. The organization of the bristle complex bears many similarities to the somatic ciliature of other ciliates. The pellicle of Euplotes is composed of a continucus outer cell membrane subtended by membranous alveoli, which contain a “fibrous mat.” Two sheets of subpellicular microtubules (longitudinal and transverse) are located just beneath the alveoli. The “epiplasm” seen in some other ciliates is apparently absent in Euplotes. The texture of the cell surface is a pattern of folds or rugae composed of the outer cell membrane and the upper membrane of the alveolus. The pattern of rugae probably defines the “silverline-system” of light microscopy.     

Ultrastructure of segmentation mitoses in cleaving newt eggs after blocking of centrospheres by quinoline

In quinoline treated blastomeres of Triturus helveticus Raz. and Pleurodeles waltlii Michah., microtubules disappear and around the centrioles markedly enlarged dense bodies accompanied by striated bodies are accumulated, from prophase beyond metaphase. It is concluded that these bodies in untreated cells play a role in the formation of the spindle microtubules. — During metaphase the chromosome are smooth-surfaced and show a pronounced tendency to stickiness. During ana-telophase they become surrounded by nuclear membranes and form caryomeres. — Quinoline does not interfere with centriole replication, but prevents the separation if diplosomes that have been already formed, if it acts before that separation. — Some centrioles exhibit different degrees of ultrastructural disarrangement.     

MITOSIS,CYTOKINESIS, AND COLONY FORMATION IN THE GREEN ALGA SORASTRUM1

Synchronous mitotic divisions produce multi-nucleate cells of Sorastrum. Perinuclear envelopes of endoplasmic reticulum and a virtually intact nuclear envelope enclose mitotic nuclei. Cytoplasmic cleavage, which shirts before the last round of Synchronous mitoses, gives rise to uninucleate fragments which differentiate to form zoospores. These zoospores are released into a spherical vesicle, presumably derived from the inner layer of the parental cell wall, in which they swarm actively before aggregating as a spherical colony. The roughly conical shaped zoospores apparently adhere laterally before withdrawing their flagella and extending horns and a stipe, which, following wall deposition, interconnects the cells at the center of the colony. The probable role of the microtubules, which underlie the plasmalemma of aggregating cells, in determining the shape of both the cells and the colony itself is discussed.     

Sperm morphology in the black coral Cirrhipathes sp. (Anthozoa, Antipatharia)

Abstract. Male polyps of the antipatharian Cirrhipathes sp., collected along the coral reef of Siladen Island (Sulawesi, Indonesia), were studied in order to gain an insight into the reproductive biology. Spermatocysts (maximum size 120 μm) are located within the primary gametogenic mesenteries and are separated by mesenteric cell cytoplasmic extensions. Sperm, maturing along radial rows, have a fairly round shape and contain a series of electron-dense vesicles in the apical nuclear region. A single mitochondrion flanks the nucleus. A peculiar cup-like electron-dense body, edged with regularly spaced electron-dense granules, is interposed between the nucleus and the tail, and delimits a central region that includes two centrioles. Cross-sections of the cup-like body reveal that the distal centriole has a pericentriolar system, consisting of nine arms arranged in a radial pattern. Each arm branches into three processes that are connected to the electron-dense granules. Indirect evidence of spawning is derived from the accumulation of sperm in the gastric cavity. This process takes place through the lysis of the cells bordering the mesenteries. Intact cells of this bordering layer appear to be involved in the phagocytosis of non-expelled gametes.