scholarly journals The ultrastructure and timing of events in the nuclear cycle of the fungus Entomophaga aulicae

1988 ◽  
Vol 90 (3) ◽  
pp. 501-516
Author(s):  
FAYE MURRIN ◽  
WILLIAM NEWCOMB ◽  
I. BRENT HEATH
Keyword(s):  
Nuclear Envelope ◽  
Linear Array ◽  
Nuclear Division ◽  
Metaphase Plate ◽  
Serial Sections ◽  
Free Zone ◽  
Nuclear Cycle ◽  
Full Complement ◽  
Spindle Microtubules ◽  
Spindle Elongation

The ultrastructure of the mitotic nuclear division cycle of the fungus Entomophaga aulicae was studied from serial sections of hyphal tips and protoplasts. The extranuclear bar-shaped nucleus- associated organelle (NAO) remained associated with the persistent nuclear envelope throughout. Prior to spindle formation, a patch of intranuclear NAO-associated chromatin detached from the nuclear envelope to yield a chromatin free zone containing fine filaments and a linear array of presumptive kinetochores. Early metaphase spindles less than 1μm in length were characterized by a ‘fused’ metaphase plate consisting of kinetochore-associated chromatin and a full complement of at least 15 kinetochore microtubules per half-spindle, while most of the chromatin was remote from the intranuclear spindle. Analysis of the distribution of antiparallel spindle microtubules indicated that polar separation and concomitant spindle elongation through metaphase were not accompanied by intermicrotubule sliding. Anaphase exhibited extensive decondensation of the large patches of condensed chromatin characteristic of all other stages. In a logarithmically growing protoplast population all nuclei contained spindle microtubules, with metaphase occupying approximately 66% of the nuclear cycle time. The calculated genome size of 4.3 pg, and average DNA content per chromosome of 0.3 pg, are extremely high for fungi.

Nuclear division in the marine dinoflagellate Oxyrrhis marina

1986 ◽  
Vol 85 (1) ◽  
pp. 161-175
Author(s):  
X.P. Gao ◽  
J.Y. Li
Keyword(s):  
Nuclear Envelope ◽  
Phylogenetic Relationships ◽  
Nuclear Division ◽  
Metaphase Plate ◽  
Active Role ◽  
Ultrastructural Changes ◽  
Interphase Nuclei ◽  
Oxyrrhis Marina ◽  
Two Stages ◽  
Anaphase B

The nuclear division of Oxyrrhis marina is a very distinct one among the mitoses of dinoflagellates that have been studies. Using synchronized populations, we have investigated the ultrastructural changes in this nuclear division. In interphase, nuclei can be classified into two groups on the basis of the shapes of the chromosomes. Y- and U-shaped chromosomes have been observed in both types of interphase nuclei. By prophase the nucleus becomes oval, many nuclear plaques appear on the nuclear envelope, and many microtubules radiate from these nuclear plaques within the nucleus. Metaphase can be identified by the characteristic arrangement of the chromosomes; an equatorial metaphase plate is absent. As in many higher organisms, anaphase includes two stages: anaphase A and anaphase B. During anaphase A the nucleus does not apparently elongate and the chromosomes migrate towards the poles by a combination of the shortening of the chromosome-associated microtubules and the elongation of those located between daughter chromosomes. During anaphase B the nucleus elongates to about twice its former length. This elongation may result from growth of the interzonal nuclear envelope. Dividing nucleoli are associated with microtubules, which suggests that microtubules may play an active role in the division of the nucleolus. The evolution of mitosis and the phylogenetic relationships between Oxyrrhis, typical dinoflagellates and Syndinium are discussed.

Ultrastructural changes in nuclear membranes and organelle associations during mitosis of the aquatic fungus Entophlyctis sp.

1975 ◽  
Vol 53 (7) ◽  
pp. 627-646 ◽  
Author(s):  
Martha J. Powell
Keyword(s):  
Endoplasmic Reticulum ◽  
Nuclear Envelope ◽  
Nuclear Division ◽  
Ultrastructural Changes ◽  
Aquatic Fungus ◽  
Electron Microscopic ◽  
Mitotic Apparatus ◽  
Inner Membrane ◽  
Nuclear Membranes ◽  
Spindle Microtubules

Electron microscopic observations on an endobiotic chytrid, Entophlyctis sp., have revealed a mitotic apparatus which is presently unique among fungi. Daughter nuclear envelopes are reconstituted from cisternae apparently proliferated by the inner membrane of the nuclear envelope. Before nuclear division, centrioles replicate and migrate to the poles of the nucleus. Large pores appear at this time in a depression of the nuclear envelope opposite the paired centrioles. This region of the envelope fragments and leaves polar fenestrae as spindle microtubules appear in the nucleus. The inner membrane of the nuclear envelope then invaginates and proliferates cisternae until a layer of inner membrane cisternae lines the original nuclear envelope at late metaphase. Connections between the inner membrane of the original nuclear envelope and the cisternae persist until telophase. As the spindle elongates and the inner membrane cisternae fuse centripetally to form a reticulum around the chromatin mass, the original nuclear envelope opens more at the poles. The reticulum becomes the nuclear envelope of the new daughter nuclei. When the original envelope finally disperses, it is distinguishable from the endoplasmic reticulum only by the presence of pores. Microbodies are consistently associated with the original nuclear envelope and appear adjacent to the new daughter envelopes at the end of telophase. Densely staining arms project from the sides of the primary centrioles toward the polar mitochondria.

scholarly journals Spindle microtubule differentiation and deployment during micronuclear mitosis in Paramecium.

1985 ◽  
Vol 101 (5) ◽  
pp. 1966-1976 ◽  
Author(s):  
J B Tucker ◽  
S A Mathews ◽  
K A Hendry ◽  
J B Mackie ◽  
D L Roche
Keyword(s):  
Nuclear Envelope ◽  
Early Stage ◽  
Microtubule Assembly ◽  
Membrane Association ◽  
Paramecium Tetraurelia ◽  
Spindle Microtubule ◽  
Spatial Control ◽  
Spindle Microtubules ◽  
Spindle Elongation ◽  
Anaphase B

Spindles underwent a 12-fold elongation before anaphase B was completed during the closed mitoses of micronuclei in Paramecium tetraurelia. Two main classes of spindle microtubules have been identified. A peripheral sheath of microtubules with diameters of 27-32 nm was found to be associated with the nuclear envelope and confined to the midportion of each spindle. Most of the other microtubules had diameters of approximately 24 nm and were present along the entire lengths of spindles. Nearly all of the 24-nm microtubules were eliminated from spindle midportions (largely because of microtubule disassembly) at a relatively early stage of spindle elongation. Disassembly of some of these microtubules also occurred at the ends of spindles. About 60% of the total microtubule content of spindles was lost at this stage. Most, perhaps all, peripheral sheath microtubules remained intact. Many of them detached from the nuclear envelope and regrouped to form a compact microtubule bundle in the spindle midportion. There was little, if any, further polymerization of 24-nm microtubules after the disassembly phase. Polymerization of microtubules with diameters of 27-32 nm continued as spindle elongation progressed. Most microtubules in the midportions of well-elongated spindles were constructed from 14-16 protofilaments. A few 24-nm microtubules with 13 protofilaments were also present. The implications of these findings for spatial control of microtubule assembly, disassembly, positioning, and membrane association, that apparently discriminate between microtubules with different protofilament numbers have been explored. The possibility that microtubule sliding occurs during spindle elongation has also been considered.

scholarly journals Analysis of the distribution of spindle microtubules in the diatom Fragilaria.

1978 ◽  
Vol 79 (3) ◽  
pp. 737-763 ◽  
Author(s):  
D H Tippit ◽  
D Schulz ◽  
J D Pickett-Heaps
Keyword(s):  
Spatial Arrangement ◽  
Spindle Pole ◽  
Serial Sections ◽  
Late Prophase ◽  
Central Spindle ◽  
Sources Of Error ◽  
Late Anaphase ◽  
Spindle Microtubules ◽  
Spindle Elongation ◽  
Intercellular Variability

The spindle of the colonial diatom Fragilaria contains two distinct sets of spindle microtubules (MTs): (a) MTs comprising the central spindle, which is composed of two half-spindles interdigitated to form a region of "overlap"; (b) MTs which radiate laterally from the poles. The central spindles from 28 cells are reconstructed by tracking each MT of the central spindle through consecutive serial sections. Because the colonies of Fragilaria are flat ribbons of contiguous cells (clones), it is possible, by using single ribbons of cells, to compare reconstructed spindles at different mitotic stages with minimal intercellular variability. From these reconstructions we have determined: (a) the changes in distribution of MTs along the spindle during mitosis; (b) the change in the total number of MTs during mitosis; (c) the length of each MT (measured by the number of sections each traverses) at different mitotic stages; (d) the frequency of different classes of MTs (i.e., free, continuous, etc.); (e) the spatial arrangement of MTs from opposite poles in the overlap; (f) the approximate number of MTs, separate from the central spindle, which radiate from each spindle pole. From longitudinal sections of the central spindle, the lengths of the whole spindle, half-spindle, and overlap were measured from 80 cells at different mitotic stages. Numerous sources of error may create inaccuracies in these measurements; these problems are discussed. The central spindle at prophase consists predominantly of continuous MTs (pole to pole). Between late prophase and prometaphase, spindle length increases, and the spindle is transformed into two half-spindles (mainly polar MTs) interdigitated to form the overlap. At late anaphase-telophase, the overlap decreases concurrent with spindle elongation. Our interpretation is that the MTs of the central spindle slide past one another at both late prophase and late anaphase. These changes in MT distribution have the effect of elongating the spindle and are not involved in the poleward movement of the chromosomes. Some aspects of tracking spindle MTs, the interaction of MTs in the overlap, formation of the prophase spindle, and our interpretation of rearrangements of MTs, are discussed.

Unorthodox Mitosis in Trichonympha Agilis: Kinetochore Differentiation and Chromosome Movement

1973 ◽  
Vol 13 (2) ◽  
pp. 511-552
Author(s):  
DONNA F. KUBAI
Keyword(s):  
Nuclear Envelope ◽  
Direct Contact ◽  
Direct Interaction ◽  
Stage Iv ◽  
Spindle Pole ◽  
Stage Ii ◽  
Nuclear Surface ◽  
Chromosome Movement ◽  
Serial Sections ◽  
Spindle Microtubules

Changes in rostral structures and the nuclear events which occur in dividing cells of Trichonympha agilis (obtained from experimentally refaunated termites) were studied by means of electron microscopy of serial sections. It is possible to characterize 5 stages of division: Stage I. During this earliest recognizable division stage, the bilaterally symmetrical hemirostra have begun to separate and spindle microtubules appear in the intervening space. As in interphase, the kinetochore regions of chromosomes are distinguishable as fibrillar masses underlying the intact nuclear envelope; and, in individual sections, they are often seen to occur in pairs. These pairs are taken to be sister kinetochores. Stage II. The extranuclear spindle has become established between the posterior ends of well separated hemirostral tubes. Elaboration of daughter rostral structures begins and will continue through the subsequent stages of division. Kinetochores differentiate, becoming bipartite structures consisting of a fibrillar element underlain by a dense disk. The fibrillar kinetochore element is associated with the still-intact nuclear envelope which lies between kinetochores and cytoplasmic spindle microtubules. Reconstruction from serial sections shows all kinetochores to be disposed in pairs which are distributed randomly over the nuclear surface. Stage III. The fibrillar elements of kinetochores are enclosed in evaginations of the nuclear envelope, while the disk elements have come to lie in the plane of the nuclear surface. Kinetochores remain separated from the extranuclear spindle microtubules by the intact nuclear envelope. The distribution of kinetochores has changed relative to that seen in stage II: kinetochores no longer appear to be paired, and they are confined to that hemisphere of the nuclear surface closest to the spindle. Stage IV. The nuclear envelope opens at the sites of kinetochores, leaving the dense disk kinetochore element inserted in pore-like discontinuities of the nuclear envelope and the fibrillar element in the cytoplasm. Direct interaction between fibrillar kinetochore element and extranuclear spindle microtubules is, however, not yet established. Stage V. The cytoplasmically situated fibrillar elements of ‘inserted’ kinetochores are now in direct contact with spindle microtubules. As seen in reconstructions of the nucleus from serial sections, kinetochores have become segregated in 2 groups on the nuclear surface, one near each spindle pole. It is during this stage that final elaboration of rostral structures takes place. On the basis of the observed changes in kinetochore distribution which occur between stages II and III while the intact nuclear envelope prevents any direct interaction between intra-nuclear kinetochores and extranuclear spindle microtubules, it is suggested that kinetochore-membrane interaction is involved in early chromosome movement in Trichonympha agilis. Only during stage V, when direct contact between kinetochores and spindle microtubules is established, may the microtubules assume their usual role in chromosome movement.

scholarly journals DIVISION IN THE DINOFLAGELLATE GYRODINIUM COHNII (SCHILLER)

1969 ◽  
Vol 40 (2) ◽  
pp. 508-528 ◽  
Author(s):  
Donna F. Kubai ◽  
Hans Ris
Keyword(s):  
Nuclear Envelope ◽  
Electron Micrographs ◽  
Cylindrical Channel ◽  
Nuclear Division ◽  
Three Dimensional ◽  
Serial Sections ◽  
Complex Process ◽  
Dividing Cells ◽  
Cytoplasmic Microtubules

Dinoflagellates are of interest because their chromosomes resemble the nucleoplasm of prokaryotes both chemically and ultrastructurally. We have studied nuclear division in the dinoflagellate Gyrodinium cohnii (Schiller), using cells obtained from cultures undergoing phasic growth. Electron micrographs of serial sections were used to prepare three-dimensional reconstructions of nuclei and chromosomes at various stages of nuclear division. During division, a complex process of invagination of the intact nuclear envelope takes place at one side of the nucleus and results in the formation of parallel cylindrical cytoplasmic channels through the nucleus. These invaginations contain bundles of microtubules, and each of the bundles comes to lie in the cytoplasm of a cylindrical channel. Nuclear constriction occurs perpendicular to these channels without displacement of the microtubules. There are no associations between chromosomes and the cytoplasmic microtubules. In dividing cells most chromosomes become V-shaped, and the apices of the V's make contact with the membrane surrounding cytoplasmic channels. It is proposed that the membrane surrounding cytoplasmic channels in the dividing nucleus may be involved in the separation of daughter chromosomes. Thus, dinoflagellates may resemble prokaryotes in the manner of genophore separation as well as in genophore chemistry and ultrastructure.

Chromosome order — possible implications for development

Development ◽  
1984 ◽  
Vol 83 (Supplement) ◽  
pp. 51-73
Author(s):  
J. S. Heslop-Harrison ◽  
M. D. Bennett
Keyword(s):  
Cell Cycle ◽  
Cell Differentiation ◽  
Nuclear Envelope ◽  
Metaphase Plate ◽  
Grass Species ◽  
Meiotic Pairing ◽  
Serial Sections ◽  
Spatial Domains ◽  
Species Evolution ◽  
Nuclear Behaviour

Chromosomes are arranged in ordered haploid sets around the centre of the metaphase plate at mitosis in several grass species and hybrids. Each chromosome is in a fixed mean position relative to other, heterologous chromosomes, this order can be predicted using Bennett's model, and is clearly demonstrated from reconstructions of electron micrographs of serial sections (see Heslop-Harrison & Bennett, 1983a, b, c). The nucleus contains spatial domains of genes with similar functions. Chromosomes with major effects on nuclear behaviour — division or meiotic pairing — may be at special positions in the order. Changing spatial relationships of chromosomes with respect both to each other and the nuclear envelope (during the cell cycle and during development) may affect cell differentiation and gene activity. Chromosome order may have implications for the control of development within the nucleus and the organism. Order may constrain karyotype and hence species evolution.

scholarly journals FINE STRUCTURE OF DIVISION IN CILIATE PROTOZOA

1967 ◽  
Vol 34 (2) ◽  
pp. 463-481 ◽  
Author(s):  
R. A. Jenkins
Keyword(s):  
Electron Microscopy ◽  
Nuclear Envelope ◽  
Osmium Tetroxide ◽  
Distinctive Features ◽  
Chromosome Alignment ◽  
Mass Cultures ◽  
Spindle Microtubules ◽  
Spindle Elongation ◽  
And Function ◽  
Ciliate Protozoa

The mitotic, micronuclear division of the heterotrichous genus Blepharisma has been studied by electron microscopy. Dividing ciliates were selected from clone-derived mass cultures and fixed for electron microscopy by exposure to the vapor of 2% osmium tetroxide; individual Blepharisma were encapsulated and sectioned. Distinctive features of the mitosis are the presence of an intact nuclear envelope during the entire process and the absence of centrioles at the polar ends of the micronuclear figures. Spindle microtubules (SMT) first appear in advance of chromosome alignment, become more numerous and precisely aligned by metaphase, lengthen greatly in anaphase, and persist through telophase. Distinct chromosomal and continuous SMT are present. At telophase, daughter nuclei are separated by a spindle elongation of more than 40 µ, and a new nuclear envelope is formed in close apposition to the chromatin mass of each daughter nucleus and excludes the great amount of spindle material formed during division. The original nuclear envelope which has remained structurally intact then becomes discontinuous and releases the newly formed nucleus into the cytoplasm. The micronuclear envelope seems to lack the conspicuous pores that are typical of nuclear envelopes. The morphology, size, formation, and function of SMT and the nature of micronuclear division are discussed.

scholarly journals Cell Division in Oedogonium II. Nuclear Division in O. Cardiacum

1970 ◽  
Vol 23 (1) ◽  
pp. 71 ◽  
Author(s):  
JD Pickett-Heaps ◽  
LC Fowke
Keyword(s):  
Cell Division ◽  
Granular Material ◽  
Nuclear Envelope ◽  
Nuclear Division ◽  
Complex Structure ◽  
Metaphase Plate ◽  
Spindle Axis ◽  
Large Numbers

Nuclear division in O. cardiacum is described. Before division, the nucleus enlarges considerably. At prophase, the nucleolus starts dispersing and kinetochores appear on the condensing chromatin, situated and oriented apparently at random in the nucleus. By prometaphase, the kinetochore pairs become aligned along the spindle axis before moving into the metaphase-plate configuration; this supports an earlier theory explaining metakinesis. During prophase and metaphase particularly, the nuclear envelope at the poles forms channels that extend for some distance into the cytoplasm; these may also bifurcate. The nucleolus disperses but remains in the intranuclear spindle throughout division as a loosely knit skein of granular material. The kinetochores have a complex structure, up to seven distinct layers being detectable; the kinetochore pairs split, and then migrate polewards at anaphase with the rest of the chromosome trailing behind. Large numbers of microtubules run from the kinetochore into evaginations of the nuclear envelope which increase in size during anaphase. The spindle grows in length considerably during anaphase, this coinciding with a proliferation of interzonal microtubules, first seen amongst the trailing chromosome arms. The nuclear envelope enclosing the spindle becomes severely stretched at this stage; it contracts closely around each of the daughter nuclei, isolating them from the rest of the spindle (including microtubules and the remains of the nucleolus). The spindle then collapses; the nuclei come together and then flatten against one another; between them, vesicles and other components of the septum collect amongst a large number of transversely oriented micro tubules.

Spindle Microtubules in the Dividing Nuclei of Trypanosomes

1970 ◽  
Vol 6 (2) ◽  
pp. 365-383 ◽  
Author(s):  
K. VICKERMAN ◽  
T. M. PRESTON
Keyword(s):  
Electron Microscope ◽  
Nuclear Envelope ◽  
Nuclear Division ◽  
Sleeping Sickness ◽  
Condensed Chromatin ◽  
Division Process ◽  
Spindle Microtubules

Stages in nuclear division have been identified in sections of bloodstream and cultured trypanosomes examined with the electron microscope. In the sleeping sickness trypanosome Trypanosoina rhodesiense at division the nuclear envelope and nucleolus-like endosome persist and become stretched along an axis. An acentric spindle of microtubules encases the elongating endosome As division proceeds the endosomal material fragments In bloodstream forms condensed chromatin (chromosomal material) appears to be associated with the nuclear envelope during the phase of nuclear elongation but to fall away from the envelope late in the phase of nuclear constriction. In culture forms the chromatin is not so abundant The discrete chromosomes envisaged by some light microscopists in stained preparations have not been identified using the electron microscope. The spindle may contain only continuous microtubules. It is suggested that the spindle serves to push the two halves of the nucleus apart, and that the nuclear envelope of each half may act as a vehicle in separation of the daughter genomes. In culture forms of the elasmobranch parasite T. raiae, the endosome appears to disintegrate as the spindle is formed, and from then onwards it becomes difficult to distinguish endosomal material from what might be chromatin. There is no noticeable association between chromatin-like material and the nuclear envelope. Some of the spindle tubules converge on kinetochore-like plaques and the presence of chromosomal microtubules cannot be ruled out. These preliminary studies indicate that the nuclear division process of trypanosomes is not closely akin to eukaryote mitosis, though it may bear some resemblance to nuclear division in Euglena. Within the genus Trypanosoma, moreover, the nuclear division process may vary from species to species and, possibly, even from one strain to another within a species.

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