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.     

Occurrence of microtubules in the hyphae of Schizophyllum commune during intercellular nuclear migration

Summary During the intercellular nuclear migration of the basidiomycete Schizophyllum commune cytoplasmic microtubules were frequently observed scattered in the hyphae around interphase nuclei and connected with a semiglobular structure at the poles of mitotic and postmitotic nuclei. Thus it seems possible that microtubules, which have been demonstrated to participate in the intracellular nuclear movements in the dikaryotic hyphae of the basidiomycetes, are also involved in the intercellular nuclear movements of these fungi. During hyphal fusion microtubules close to an interphase nucleus were connected with electron-dense structures. It is suggested that these structures are centers for the assembly of microtubules necessary for nuclear movements not associated with nuclear divisions.Abbreviations KCE kinetochore equivalent - ch chromatin - cw cross wall of septum - ge semiglobular end of KCE - gm grey material - m mitochondrion - mp middle plate of KCE - mt microtubules - n nucleus - ne nuclear envelope - nu nucleolus - s electron-dense structure connected with microtubules     

Ultrastructural Features of Mitosis in Anisonema sp. (Euglenida)1

The fine structure of stages in mitosis in a colorless euglenoid, Anisonema sp., reveals that chromosomes remain condensed throughout the life cycle and are attached to the nuclear envelope at interphase. The onset of mitosis is marked by the anterior migration of the nucleus towards the base of the reservoir and by elongation of the nucleolus. The nuclear envelope persists throughout mitosis. Microtubules are generated in the peripheral nucleoplasm adjacent to the envelope and attach to the chromosomes while they are still associated with the envelope. The region of microtubular contact develops into a distinct layered kinetochore as the developing spindle with attached chromosomes separates from the nuclear envelope and moves into the nucleoplasm. The mature spindle consists of a number of subspindles each containing about 8–10 microtubules and a few associated chromosomes. Both chromosomal and non-chromosomal microtubules are present in each subspindle and extend towards the envelope terminating at or near the nuclear pores. Chromosomal segregation is concomitant with nuclear elongation. By late division, an interzonal spindle develops in the dumbbell-shaped nucleus and nucleolar separation occurs. Continued invagination of the nuclear envelope in the region of the interzonal spindle eventually separates the daughter nuclei. A remnant of the interzonal spindle persists in the cytoplasm until cytokinesis.     

Ultrastructural features of mitosis inLeishmania adleri

Summary The interphase nucleus ofLeishmania adleri has clumps of chromatin associated with the nuclear envelope and a large centrally located nucleolus. Prior to mitosis the basal bodies replicate at the cell anterior. Subsequently, dense plaques appear in the equatorial region of the nucleus at the time of spindle development. Microtubules appear in the nucleus adjacent to the nuclear envelope and embedded in the matrix of the plaques. A central spindle composed of a single bundle of microtubules develops and spans the nucleus. Plaques and nucleolar components laterally associate with the spindle and migrate towards the poles. The central spindle elongates to three to four times its original length separating the forming daughter nuclei and producing an interzonal spindle. A remnant of the interzonal spindle remains attached to each of the daughter nuclei until late into cytokinesis. The kinetoplast does not divide until after the completion of mitosis.     

Nuclear and cell division inSorodiplophrys stercorea,a labyrinthulid-like protist

Summary Nuclear division in the labyrinthulid-like protist,Sorodiplophrys stercorea was studied and compared to the mitotic processes reported in other protistan forms.S. stercorea was found to have centrioles with nine peripheral singlet microtubules and a tubular core 300–400 Å in diameter. The nuclear envelope remains intact throughout division, with microtubules passing through nuclear pores. The nucleolus disappears during nuclear division. After intranuclear division, cells cleave by introgressive cleavage while pronucleoli appear in the nuclei and fuse to form the single nucleolus characteristic of the interphase nucleus.     

Ultrastructure of mitosis inAmoeba proteus

Summary The fine structure ofAmoeba proteus nuclei has been studied during interphase and mitosis. The interphase nucleus is discoidal, the nuclear envelope is provided with a honeycomb layer on the inside. There are numerous nucleoli at the periphery and many chromatin filaments and nuclear helices in the central part of nucleus.In prophase the nucleus becomes spherical, the numerous chromosomes are condensed, and the number of nucleoli decreases. The mitotic apparatus forms inside the nucleus in form of an acentric spindle. In metaphase the nuclear envelope loses its pore complexes and transforms into a system of rough endoplasmic reticulum cisternae (ERC) which separates the mitotic apparatus from the surrounding cytoplasm; the nucleoli and the honeycomb layer disappear completely. In anaphase the half-spindles become conical, and the system of ERC around the mitotic spindle persists. Electron dense material (possibly microtubule organizing centers—MTOCs) appears at the spindle pole regions during this stage. The spindle includes kinetochore microtubules attached to the chromosomes, and non-kinetochore ones which pierce the anaphase plate. In telophase the spindle disappears, the chromosomes decondense, and the nuclear envelope becomes reconstructed from the ERC. At this stage, nucleoli can already be revealed with the light microscope by silver staining; they are visible in ultrathin sections as numerous electron dense bodies at the periphery of the nucleus.The mitotic chromosomes consist of 10 nm fibers and have threelayered kinetochores. Single nuclear helices still occur at early stages of mitosis in the spindle region.     

Mitosis in the dermatophyteMicrosporum canis as revealed by freeze substitution electron microscopy

Summary Mitosis in the dermatophyteMicrosporum canis was studied by freeze substitution and electron microscopy, and analyzed by three dimensional reconstruction from serial sections of the mitotic nuclei. The interphase nucleus has associated nucleus-associated organelle (NAO) on a portion of the outer surface of the nuclear envelope, subjacent to which there was dense intranuclear material. The NAO divided and separated on the envelope, and a spindle was formed. The spindle was composed mostly of microtubules extended between opposite NAOs. Pairing of kinetochores was observed in the spindle from an early stage of development, when chromosomes were not so condensed, and remained unchanged while chromosome condensation proceeded until metaphase. Before the completion of nuclear division, daughter nuclei were connected by a narrow spindle channel, and then the nucleolus, whose structure underwent minimal change during mitosis, was eliminated into the cytoplasm.     

THE ULTRASTRUCTURE OF MITOSIS IN THE MARINE RED ALGA MEMBRANOPTERA PLATYPHYLLA1,2

Gametophyte germlings from unialgal cultures of Membranoptera platyphylla were examined with the electron microscope. The events of mitosis were observed in dividing cells near the thallus apex. In prophase the nucleus is spindle-shaped and surrounded by microtubules and a layer of endoplasmic reticulum. A unique organelle, the polar ring, is present at each pole; its junction is not clear. At metaphase the nuclear envelope is intact except for fenestrations at the poles. Spindle microtubules are attached to distinct kinetochores on the chromosomes and continuous pole-to-pole microtubules are present. The nucleolus has dispersed but, its granular components are still evident in the nucleoplasm. As the chromosomes separate, the nucleus elongates and finally constricts in the middle to produce 2 daughter nuclei.     

Mitosis in the coenocytic green algaBoergesenia forbesii (Harvey) Feldmann (Siphonocladales,Ulvophyceae)

Mitosis in vegetative cells of the siphonocladalean algaBoergesenia forbesii (Harvey) Feldmann was investigated mainly by electron microscopy. The mitotic spindle was centric and closed. The interphase nucleus contained a spherical nucleolus. The nucleolus was slightly dispersed at prophase, but nucleolar materials remained during nearly all stages of mitosis. Kinetochores were evident on chromosomes. The polar regions of nuclear envelope had no fenestrae during mitosis. Anaphase separation of the chromosomes was asynchronous. Elongation of interzonal spindle at telophase separated the two daughter nuclei widely. The ultrastructural features of mitosis inB. forbesii revealed by the present investigation are compared with those of other siphonous and siphonocladous algae in the Ulvophyceae.     

CELL DIVISION IN CHLAMYDOMONAS MOEWUSII1

Cell division in Chlamydomonas moewusii is described. The cells become immobile with flagellar abscission prior to mitosis. The basal bodies migrate toward the nucleus and become intimately associated with the nuclear membrane which is devoid, of ribosomes where adjacent to the basal bodies. The basal bodies replicate at preprophase. The nucleolus fragments at this stage. By prophase the basal body pairs have migrated, to the nuclear poles. Spindle fibers become prominent in the nucleus. The nuclear membrane does not fragment. The nucleus assumes a crescent-form by metaphase. Polar fenestrae are absent. Kinetochores appear at anaphase. An interzonal spindle elongates as the chromosomes move to the nuclear poles. Daughter nuclei become abscised by an ingrowth of nuclear membrane, leaving behind a separated, degenerating interzonal spindle. Ribosomes reappear on the outer nuclear membrane at late telophase. Nucleoli reform early in cytokinesis. The cleavage furrow, associated microtubules, and endoplasmic reticulum comprise the phycoplast. Cytokinesis proceeds rapidly after the completion of telophase. The basal body-nucleus relationship becomes reorganized into the typical interphase condition late in cytokinesis. Specific and predictable organelle rearrangements during mitosis have been described. Cell division in C. moewusii is compared with other algae, especially C. reinhardi.     

Spindle formation in Aspergillus is coupled to tubulin movement into the nucleus

In many important organisms, including many algae and most fungi, the nuclear envelope does not disassemble during mitosis. This fact raises the possibility that mitotic onset and/or exit might be regulated, in part, by movement of important mitotic proteins into and out of the nucleoplasm. We have used two methods to determine whether tubulin levels in the nucleoplasm are regulated in the fungus Aspergillus nidulans. First, we have used benomyl to disassemble microtubules and create a pool of free tubulin that can be readily observed by immunofluorescence. We find that tubulin is substantially excluded from interphase nuclei, but is present in mitotic nuclei. Second, we have observed a green fluorescent protein/alpha-tubulin fusion in living cells by time-lapse spinning-disk confocal microscopy. We find that tubulin is excluded from interphase nuclei, enters the nucleus seconds before the mitotic spindle begins to form, and is removed from the nucleoplasm during the M-to-G1 transition. Our data indicate that regulation of intranuclear tubulin levels plays an important, perhaps essential, role in the control of mitotic spindle formation in A. nidulans. They suggest that regulation of protein movement into the nucleoplasm may be important for regulating mitotic onset in organisms with intranuclear mitosis.     

The ultrastructure of mitosis inChroomonas salina (Cryptophyceae)

Summary A detailed account of the ultrastructure of mitosis in a member of theCryptophyceae is given for the first time. The initial indication of mitosis is the duplication of the flagellar bases. The nucleus migrates towards the anterior of the cell and its envelope and nucleolus break down. The chromatin which at interphase is in the form of scattered clumps, condenses into a solid mass through which run narrow tunnels. Each tunnel allows the passage of one to four microtubules. At metaphase the dense plate of chromatin is situated on the equator and the spindle has a rectangular shape. Individual chromosomes cannot be recognized and no morphologically differentiated kinetochores have been observed. The flagella remain functional, their bases stay at the anterior side of the nucleus and do not move to the poles. At anaphase two plates of chromatin separate and these move apart until they come to lie against the ER sheath surrounding the chloroplasts. The new nuclear envelope starts to form on the opposite side of the daughter nucleus. Cytokinesis may commence early in mitosis and consists of a constriction of the parent cell, starting from the posterior end, followed by separation of the two daughters. The present work supports earlier views that only one chromosome is evident during the nuclear division of these organisms. The mitosis is completely different from that of theDinophyceae with which theCryptophyceae were formerly linked.     

A light and electron microscopic study of mitosis inBullera alba and the histochemistry of some cytoplasmic substances

Summary Mitosis in the imperfect yeast-like basidiomyceteBullera alba was studied by comparative light and electron microscopy. During mitosis the chromatin containing part of the nucleus moved into the progeny cell, and the nucleolus containing part of the nucleus remained in the parent cell. The two portions of the nucleus then separated and the nucleolar part degenerated. Metaphase and anaphase took place in the progeny cell. Subsequently one mass of chromatin returned to the parent cell, and two new nuclei were formed. The study concentrated on the nuclear envelope, nucleolus, spindle pole body, chromatin, spindle, and cytoplasmic microtubules. Mitosis inB. alba was compared with reports of mitosis in other basidiomycetes, theUredinales, and theAscomycotina and was deemed closest to the heterobasidiomycete yeasts.Histochemical evidence for the presence of lipid, glycogen, and polyphosphate in the cytoplasm was presented.     

Mitosis in the free-living flagellate Bodo saltans strain PS+ (Kinetoplastidea, Bodonida)

The mitosis in the free-living flagellate Bodo saltans Ps+ with prokaryotic cytobionts in perinuclear space has been studied. The nuclear division in B. saltans Ps+ occurs by closed mitosis type without condensation of chromosomes. Two spatially separated mitotic spindles begin to form consistently at the initial stages of nuclear division. The spindle including about 20 microtubules appears first and later the second spindle with half the number of microtubules comes at the angle of 30-40 degrees. Both spindles rest their ends against the inner nuclear membrane and form 4 distinct poles. The microtubules of the first spindle are associated with 4 pairs of kinetochores, the microtubules of the second one are associated with 2 pairs of kinetochores. The divergence of the kinetochores towards the poles occurs independently in each spindle. The equatorial phase is not revealed in B. saltans Ps+. The poles of both spindles unite in pairs at the elongation phase of mitosis and form the integrated bipolar structure. At this stage of the nuclear division, the kinetochores reach the poles of subspindles and become indistinguishable. Then the nucleus takes the shape of a dumbbell. The inner nuclear membranes of just formed nuclei have layers of condensed chromatin characteristic of the interphase nuclei of kinetoplastidea. The daughter nuclei separate at the phase of reorganization. There are 1-2 prokaryotic endocytobionts in the perinuclear space of the interphase nuclei in B. saltans Ps+. The symbionts multiply during mitosis and their number reaches more than 20 specimens par nucleus.     

Nuclear envelope dynamics.

The nuclear envelope (NE) provides a semi permeable barrier between the nucleus and cytoplasm and plays a central role in the regulation of macromolecular trafficking between these two compartments. In addition to this transport function, the NE is a key determinant of interphase nuclear architecture. Defects in NE proteins such as A-type lamins and the inner nuclear membrane protein, emerin, result in several human diseases that include cardiac and skeletal myopathies as well as lipodystrophy. Certain disease-linked A-type lamin defects cause profound changes in nuclear organization such as loss of peripheral heterochromatin and redistribution of other nuclear envelope components. While clearly essential in maintenance of nuclear integrity, the NE is a highly dynamic organelle. In interphase it is constantly remodeled to accommodate nuclear growth. During mitosis it must be completely dispersed so that the condensed chromosomes may gain access to the mitotic spindle. Upon completion of mitosis, dispersed NE components are reutilized in the assembly of nuclei within each daughter cell. These complex NE rearrangements are under precise temporal and spatial control and involve interactions with microtubules, chromatin, and a variety of cell-cycle regulatory molecules.     

高等动物细胞核膜和核纤层结构、功能及动态变化调控机制

细胞核是真核细胞中最大的细胞器．高等动物细胞核主要由双层核膜、核孔复合体、核纤层、染色质和核仁等组成．在细胞有丝分裂期,细胞核呈现去装配和再装配等动态变化．在细胞分裂间期,核膜、核孔复合体和核纤层构成细胞核的外周结构,为遗传物质在染色质和核仁中的代谢提供了一个相对稳定的环境,同时调控细胞核内外的物质转运,在细胞增殖、分化、个体发育和细胞衰老等许多方面发挥着重要作用．本文主要对高等动物细胞核膜和核纤层结构、功能及动态变化调控机制等方面的研究进展进行简要综述．     

A UNIQUE MITOTIC VARIATION IN THE MARINE DINOFLAGELLATE OXYRRHIS MARINA (PYRROPHYTA)1

Mitosis is described in the flagellate Oxyrrhis marina Dujardin and is compared in related genera. Dense plaques develop in the nuclear envelope at prophase and give rise to an intranuclear spindle. Some of the microtubules associate with the chromosomes while others extend across the nucleus. The basal bodies migrate toward the poles early in division and retain a position lateral to the nuclear poles throughout mitosis. Microtubules are not present between the nucleus and the basal bodies. The nucleolus is persistent and elongates throughout anaphase and telophase. Chromosomal separation is accomplished by sliding of non-chromosomal microtubules and by elongation of the nuclear envelope rather than by shortening of the spindle microtubules. The nuclear envelope begins to constrict in the center early in anaphase. Continued constriction of the envelope and elongation of the nucleus leads to the formation of a dumbbell-shaped nucleus by late telophase. Mitosis culminates by the constriction of the nucleus into two daughter nuclei. The taxonomic position of Oxyrrhis marina is discussed in light of these findings.     

Aspects of Interaction of Putative Anchorosomal Protein p68 with the Nuclear Envelope during the Cell Cycle

In the interphase nucleus, the chromatin associated with the nuclear envelope is represented by a layer of anchorosomes, granules with a diameter of 20–25 nm. Biochemically, the fraction of chromatin directly associated with the nuclear envelope is characterized by resistance against decondensing influences, a low level of DNA methylation, and presence of specific acid-soluble proteins. However, the mechanisms lying at the base of chromatin-nuclear envelope interaction have been insufficiently studied. Specifically, it is unknown whether anchorosomes are constant structures or subject to reversible disassembly, when the contacts between chromatin and nuclear envelope are destroyed. We obtained immune serum recognizing a 68 kDa protein from the nuclear envelopes fraction and studied the localization of this protein in interphase and mitotic cells. We show that this protein, present in the NE/anchorosomal fraction, does not remain bound with chromosomes during mitosis. It dissociates from chromosomes at the beginning of the prophase and then can be identified again at the periphery of the newly forming nuclei in the telophase.     

Fine structure of division in ciliate protozoa. I. Micronuclear mitosis in Blepharisma

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.     

Features of interactions of p68 anchorosomal protein with the nuclear envelope in the cell cycle

Chromatin associated with the nuclear envelope appears in the interphase nuclei as a layer of anchorosomes, granules 20-25 nm in diameter. The fraction of chromatin directly associated with the nuclear envelope is resistant to decondensation, shows a low level of DNA methylation, and contains specific acid-soluble proteins. However, mechanisms underlying the interaction of chromatin with the nuclear envelope are not fully understood. Specifically, it is not known whether anchorosomes are permanent structures or if they undergo reversible disassembly during mitosis, when contacts between chromatin and the nuclear envelope are destroyed. We obtained immune serum recognizing a 68 kDa protein from the nuclear envelopes fraction and studied the localization of this protein in interphase and mitotic cells. We show that this protein present in the NE/anchorosomal fraction does not remain bound with chromosomes during mitosis. It dissociates from chromosomes at the beginning of the prophase and then can be identified again at the periphery of the newly forming nuclei in the telophase.