Disassembly of the nucleus in mitotic extracts: membrane vesicularization, lamin disassembly, and chromosome condensation are independent processes

We describe a stable cell-free mitotic extract derived from Xenopus eggs that contains activities necessary for nuclear envelope breakdown and chromosome condensation during mitosis. Using these cell-free extracts, we have demonstrated that nuclear envelope vesicularization, lamina solubilization, and chromosome condensation are independent and separable biochemical processes. We present evidence indicating that during mitosis nuclear membrane breakdown may involve the binding of a coating protein, lamin solubilization is enzymatically driven, and chromosome condensation involves both binding proteins and enzymatic activities including topoisomerase II. These results provide a coherent framework for investigating structural modification of the nucleus during mitosis at the biochemical level.     

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.     

Events associated with the initiation of mitosis in fused multinucleate HeLa cells

Large multinucleate (LMN) HeLa cells with more than 10–50 nuclei were produced by random fusion with polyethylene glycol. The number of nuclei in a particular stage of the cell cycle at the time of fusion was proportionate to the duration of the phase relative to the total cell cycle. The fused cells did not gain generation time. Interaction of various nuclei in these cells has been observed. The nuclei initially belonging to the G1-or S-phase required a much longer time to complete DNA synthesis than in mononucleate cells. Some of the cells reached mitosis 15 h after fusion, whereas others required 24 h. The cells dividing early, contained a larger number of initially early G1-phase nuclei than those cells dividing late. The former very often showed prematurely condensed chromosome (PCC) groups. In cells with a large number of advanced nuclei the few less advanced nuclei could enter mitosis prematurely. On the other hand, the cells having a large number of nuclei belonging initially to late S-or G2-phase took longer to reach mitosis. These nuclei have been taken out of the normal sequence and therefore failed to synthesize the mitotic factors and depended on others to supply them. Therefore the cells as a whole required a longer period to enter mitosis. Although the nuclei became synchronized at metaphase, the cells revealed a gradation in prophase progression in the different nuclei. At the ultrastructural level the effect of advanced nuclei on the less advanced ones was evident with respect to chromosome condensation and nuclear envelope breakdown. Less advanced nuclei trapped among advanced nuclei showed PCC and nuclear envelope breakdown prematurely, whereas mitotic nuclei near interphase or early prophase nuclei retained their nuclear envelopes for a much longer time. PCC is closely related to premature breakdown of the nuclear envelope. Our observations clearly indicate that chromosome condensation and nuclear envelope breakdown are two distinct events. Kinetochores with attached microtubules could be observed on prematurely condensed chromosomes. Kinetochores of fully condensed chromosomes often failed to become connected to spindle elements. This indicates that the formation of a functional spindle is distinct from the other events and may depend on different factors.     

Mitosis in the Hemoflagellate Trypanosoma cyclops*

SYNOPSIS. The ultrastructure of interphase and mitotic nuclei of the epimastigote form of Trypanosoma cyclops Weinman is described. In the interphase nucleus the nucleolus is located centrally while at the periphery of the nucleus condensed chromatin is in contact with the nuclear envelope. The nucleolus fragments at the onset of mitosis, but granular material of presumptive nucleolar origin is often recognizable in the mitotic nucleus. Peripheral chromatin is in contact with the nuclear envelope throughout mitosis, and it seems reasonable to assume that the nuclear envelope is involved in its segregation to the daughter nuclei. Spindle microtubules extend between the poles of the dividing nucleus and terminate close to the nuclear envelope. The basal body and kinetoplast divide before the onset of mitosis and do not appear to have any morphologic involvement in that process. Spindle pole bodies, kinetochores, and chromosomal microtubules have not been observed.     

Microtubule rearrangements during mitosis in multinucleate cells

The peroxidase-antiperoxidase (PAP) method for the detection of polymerized tubulin has been used to study the microtubule rearrangements during mitosis in PtK1 and HeLa multinucleate cells obtained by polyethyleneglycol (PEG)-mediated fusion. We demonstrate here that the transition of the microtubular cytoskeleton from interphase to mitosis is an inducible event and independent of the factor(s) responsible for chromatin condensation and nuclear envelope breakdown. However, for the induction of the microtubule rearrangements nuclear envelope breakdown is required. At midprophase, cytoskeletal microtubule rearrangements start for multinucleate PtK1 cells, whereas in HeLa cells such changes are delayed, and a more abrupt transition is observed here. After complete nuclear envelope breakdown (prometaphase) mitotic asters and spindles but no cytoplasmic (interphase) microtubuli can be observed in both systems. Metaphase is characterized by an interaction between the different mitotic poles which show the form of bipolar spindles, but individual separated mitotic poles far removed from the chromatin can also be seen.     

A nucleolar auto-antigen is part of a major chromosomal surface component

Several nucleolar antigens are defined by human autoantibodies. These antigens can therefore be used to follow the fate of nucleolar components through mitosis when this major nuclear structure disintegrates and becomes reassembled in G₁-phase. We found that fibrillarin leaves the nucleolus before complete breakdown of this structure and attaches to chromosomes before nuclear envelope breakdown. In mouse, fibrillarin attaches over the chromosomal surface except for the excluded centromeric region. The antigen is transported to the new nucleus via the chromosomes and is last seen on chromosomal surfaces facing the cytoplasm during nuclear envelope reformation. Lamin B reappears on the same chromosomal surfaces before the nucleolar antigen is removed and aggregates for new nucleolar reformation in G₁-phase cells. From our observations, we postulate that the antigen acts in concert with other proteins as a nuclear envelope equivalent by forming a protective sheath around the chromosome, that it excludes larger molecules, and helps to separate the chromosomes, in addition to segregation of the ribonucleoprotein (RNP) back to the nucleus for nucleolar reconstruction. We also suggest that the selective retention of these antigens from certain areas on individual chromosomes together with specific lamin B attachment over these chromosomal surfaces allows for a nonrandom positioning of chromosomes in the nucleus.     

Fine structural studies of early mitotic stages in untreated and nocodazole-treated HeLa cells

When cells in mitosis are treated with nocodazole, a microtubule-disrupting drug, it can be shown in comparison to untreated cells that microtubules are responsible for the polarized formation of indentations, folds, tubes, and crypts of the nuclear envelope in prophase nuclei. No translocation of chromosomes within the nucleus takes place. Microtubules are not necessary for chromosome condensation, nuclear envelope breakdown, the formation of trilaminar kinetochores, and the orientation of sister-kinetochores within one chromosome in relation to each other. The orientation of kinetochores in relation to the mitotic poles, however, is mediated by microtubules. The data shown here support the working hypothesis about chromosome translocation in prophase nuclei which was presented in an earlier paper.     

Integration of murine leukemia virus DNA depends on mitosis.

In synchronized rat or mouse cells infected with Moloney murine leukemia virus (MLV), integration of viral DNA and production of viral proteins occur only after the cells traverse mitosis. Integration is blocked when cells are prevented from progressing through mitosis. Viral nucleoprotein complexes isolated from arrested cells contain full-length viral DNA and can integrate this viral DNA in vitro, showing that the block to integration in arrested cells is not due to a lack of mature integration machinery. When infected cells traverse mitosis, there is a sharp increase in nuclear accumulation of viral DNA. The dependence of integration on mitosis may therefore be due to a requirement for mitosis and nuclear envelope breakdown for entry of the viral integration complex into the nucleus.     

1,2-Dioctanoylglycerol accelerates or retards mitotic progression in Tradescantia stamen hair cells as a function of the time of its addition

We have treated living, intact stamen hair cells from the spiderwort plant, Tradescantia virginiana, with 0.5 microgram/ml or 60 micrograms/ml 1,2-dioctanoylglycerol, a potent and permeant activator of protein kinase C, and have observed the rates of progression of mitosis from prophase through anaphase. We have found that in addition to the concentration used, the time of initial treatment with 1,2-dioctanoylglycerol defines the response by the cells. The cells rapidly undergo nuclear envelope breakdown when this diglyceride is added in very late prophase, 0 to approximately 8 min prior to the time of normal nuclear envelope breakdown. Anaphase onset occurs 28 min after nuclear envelope breakdown, rather than after the 33 min interval observed in untreated cells. Rapid progression through metaphase is also observed if cells are treated with 0.5 microgram/ml 1,2-dioctanoylglycerol during prometaphase, up to 15 min after nuclear envelope breakdown. The addition of 0.5 microgram/ml 1,2-dioctanoylglycerol in late metaphase, approximately 26 min after nuclear envelope breakdown, results in sister chromatid separation slightly ahead of its normal time, 33 min after nuclear envelope breakdown, and in precocious cell plate vesicle aggregation, 3-5 min earlier than that observed in untreated cells. Treatment of cells with 60 micrograms/ml of 1,2-dioctanoylglycerol at any point during the interval from 0 to approximately 5 min prior to nuclear envelope breakdown results in precocious entry into anaphase. If cells are treated with either 0.5 microgram/ml or 60 micrograms/ml 1,2-dioctanoylglycerol earlier than 20 min before nuclear envelope breakdown, they do not enter mitosis, but instead revert to interphase without dividing. When 1,2-dioctanoylglycerol is added at other times during mitosis, the rate of subsequent mitotic progression is dramatically slowed; the cells require greater than 55 min to progress from nuclear envelope breakdown to anaphase onset, though once in anaphase, the cells progress onward to cytokinesis at normal rates. Treatments o of cells with 1,3-dioctanoylglycerol at any point during prophase, prometaphase, or metaphase are without effect on the rate of subsequent mitotic progression. The shifts in response by cells treated at specific times with 1,2-dioctanoylglycerol during mid- and late metaphase may be indicative of the existence of one or more regulatory switch points (i.e., checkpoints) just prior to anaphase onset.     

Parvoviruses Cause Nuclear Envelope Breakdown by Activating Key Enzymes of Mitosis

Disassembly of the nuclear lamina is essential in mitosis and apoptosis requiring multiple coordinated enzymatic activities in nucleus and cytoplasm. Activation and coordination of the different activities is poorly understood and moreover complicated as some factors translocate between cytoplasm and nucleus in preparatory phases. Here we used the ability of parvoviruses to induce nuclear membrane breakdown to understand the triggers of key mitotic enzymes. Nuclear envelope disintegration was shown upon infection, microinjection but also upon their application to permeabilized cells. The latter technique also showed that nuclear envelope disintegration was independent upon soluble cytoplasmic factors. Using time-lapse microscopy, we observed that nuclear disassembly exhibited mitosis-like kinetics and occurred suddenly, implying a catastrophic event irrespective of cell- or type of parvovirus used. Analyzing the order of the processes allowed us to propose a model starting with direct binding of parvoviruses to distinct proteins of the nuclear pore causing structural rearrangement of the parvoviruses. The resulting exposure of domains comprising amphipathic helices was required for nuclear envelope disintegration, which comprised disruption of inner and outer nuclear membrane as shown by electron microscopy. Consistent with Ca⁺⁺ efflux from the lumen between inner and outer nuclear membrane we found that Ca⁺⁺ was essential for nuclear disassembly by activating PKC. PKC activation then triggered activation of cdk-2, which became further activated by caspase-3. Collectively our study shows a unique interaction of a virus with the nuclear envelope, provides evidence that a nuclear pool of executing enzymes is sufficient for nuclear disassembly in quiescent cells, and demonstrates that nuclear disassembly can be uncoupled from initial phases of mitosis.     

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．     

Entry into Mitosis in Vertebrate Somatic Cells Is Guarded by a Chromosome Damage Checkpoint That Reverses the Cell Cycle When Triggered during Early but Not Late Prophase

When vertebrate somatic cells are selectively irradiated in the nucleus during late prophase (<30 min before nuclear envelope breakdown) they progress normally through mitosis even if they contain broken chromosomes. However, if early prophase nuclei are similarly irradiated, chromosome condensation is reversed and the cells return to interphase. Thus, the G₂ checkpoint that prevents entry into mitosis in response to nuclear damage ceases to function in late prophase. If one nucleus in a cell containing two early prophase nuclei is selectively irradiated, both return to interphase, and prophase cells that have been induced to returned to interphase retain a normal cytoplasmic microtubule complex. Thus, damage to an early prophase nucleus is converted into a signal that not only reverses the nuclear events of prophase, but this signal also enters the cytoplasm where it inhibits e.g., centrosome maturation and the formation of asters. Immunofluorescent analyses reveal that the irradiation-induced reversion of prophase is correlated with the dephosphorylation of histone H1, histone H3, and the MPM2 epitopes. Together, these data reveal that a checkpoint control exists in early but not late prophase in vertebrate cells that, when triggered, reverses the cell cycle by apparently downregulating existing cyclin-dependent kinase (CDK1) activity.     

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.     

Fragmentation of polyploid nuclei in the giant cells of the rat trophoblast. I. The ultrastructure of the nuclear fragments

Electron microscope study of the nuclear fragments in the rat trophoblast has demonstrated that the division of the trophoblast giant nucleus results first in the formation of a multinuclear cell. Each nuclear fragment is covered with its own nuclear envelope made of two membranes with numerous pore complexes. The chromatin in these nuclear fragments is condenced with various degrees of condensation, which depends on the step of placenta development, cell differentiation and the degree of nuclear fragmentation. The nuclear ultrastructure in nuclear fragments also depends on the degree of nuclear fragmentation and on the level of chromatin condensation. The nucleolus has no granular component. On large fragments, with lower chromatin condensation the nucleolus is not homogenous being made of fragments of more and of less electron dense fibrilles. Small light lacunae are seen in the nucleolus where chromatin threads and strands pass on. With a high chromatin condensation in the nucleus, round small nucleoli look homogenous being made of moderately electron dense fibrilles. Products of chromosome activity have been found in the nuclear fragments: accumulations of minute granules (d = 15--20 nm), perichromatinous granules (d = 35--40 nm), and fibrillar nucleolus-like bodies. In the multinuclear cell, made as the result of fragmentation of the initially giant nucleus, all the small nuclei are first arranged very close to each other, so that the contours of the neighbouring nuclei coincide.     

Active cyclin B1-Cdk1 first appears on centrosomes in prophase

Cyclin B1-Cdk1 is the key initiator of mitosis, but when and where activation occurs has not been precisely determined in mammalian cells. Activation may occur in the nucleus or cytoplasm, as just before nuclear envelope breakdown, Polo-like kinase1 (Plk1) is proposed to phosphorylate cyclin B1 in its nuclear export sequence (NES), to trigger rapid nuclear import. We raised phospho-specific antibodies against cyclin B1 that primarily recognise the active form of the complex. We show that cyclin B1 is initially phosphorylated on centrosomes in prophase and that Plk1 phosphorylates cyclin B1, but not in the NES. Furthermore, phosphorylation by Plk1 does not cause cyclin B1 to move into the nucleus. We conclude that cyclin B1-Cdk1 is first activated in the cytoplasm and that centrosomes may function as sites of integration for the proteins that trigger mitosis.     

Inhibiting MAP kinase activity prevents calcium transients and mitosis entry in early sea urchin embryos

A transient calcium increase triggers nuclear envelope breakdown (mitosis entry) in sea urchin embryos. Cdk1/cyclin B kinase activation is also known to be required for mitosis entry. More recently, MAP kinase activity has also been shown to increase during mitosis. In sea urchin embryos, both kinases show a similar activation profile, peaking at the time of mitosis entry. We tested whether the activity of both kinases is required for mitosis entry and whether either kinase controls mitotic calcium signals. We found that reducing the activity of either mitotic kinase prevents nuclear envelope breakdown, despite the presence of a calcium transient, when cdk1/cyclin B kinase activity is alone inhibited. When MAP kinase activity alone was inhibited, the calcium signal was absent, suggesting that MAP kinase activity is required to generate the calcium transient that triggers nuclear envelope breakdown. However, increasing intracellular free calcium by microinjection of calcium buffers or InsP(3) while MAP kinase was inhibited did not itself induce nuclear envelope breakdown, indicating that additional MAP kinase-regulated events are necessary. After MAP kinase inhibition early in the cell cycle, the early events of the cell cycle (pronuclear migration/fusion and DNA synthesis) were unaffected, but chromosome condensation and spindle assembly are prevented. These data indicate that in sea urchin embryos, MAP kinase activity is part of a signaling complex alongside two components previously shown to be essential for entry into mitosis: the calcium transient and the increase in cdk1/cyclinB kinase activity.     

Removal of cellular water prevents the reformation of the interphase nucleus

The mature snRNP (small nuclear ribonucleoprotein) particles are localized quantitatively in the interphase nucleus. Like many nuclear antigens, they distribute throughout the cytoplasm after the nuclear envelope breaks down during mitosis and then return to the newly formed daughter nuclei in early G1. Their abundance and stability and the availability of monoclonal antibodies that recognize them, make the snRNP particles a useful model system for studying the reformation of the nucleus at the completion of mitosis. A wide variety of metabolic inhibitors and alterations in normal culture conditions were investigated for their ability to interfere with the return of the snRNP particles to daughter nuclei after mitosis. None of the well-characterized cytoskeletal inhibitors, biosynthetic inhibitors, calcium antagonists, nor ionophores were effective in interfering with this return. However, the removal of cellular water by exposure of cells to hypertonic medium during mitosis blocked the reformation of the nucleus and trapped the snRNP particles in the cytoplasm. In medium of twice the normal tonicity, the function of the mitotic spindle and the cleavage furrow are inhibited, however, the cells reattach to the substratum as if returning to interphase. The chromatin stays condensed and does not form a normal interphase nucleus and the snRNP particles stay dispersed throughout the cytoplasm. This condition is reversible and after return to normal medium the nucleus reforms and the snRNP particles collect in the new nuclei. After gentle extraction of metaphase cells, about 30% of the snRNP particles are soluble, however, the remainder are associated with an insoluble remnant. These data are consistent with the notion that the snRNP particles accumulate in the nucleus due to both preferential solubility and specific binding sites in the interphase nucleus.     

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.     

Morphological alterations in mammalian neurons in vitro in response to hypertonic solutions

Summary Neurons in cultures of central nervous tissue exhibited marked structural changes when exposed to hypertonic solutions. Cellular reactions were described in living neurons as well as after fixation and staining in preparations observed with both the light and electron microscope. The structures involved in these changes were mainly the nucleolus, the nucleus and the Nissl substance.Nucleolus In living neurons, observed with phase contrast optics, the nucleolus became invisible in hypertonic medium. This change occurred within a few seconds, and it was reversible when the cells were brought back to isotonic solutions. Fixation of the cells while exposed to hypertonic solution caused the nucleolus to reappear as a granular body. In stained preparations it appeared as a more irregular body in contrast to the smoothly outlined nucleolus in normal cells. In electron microscopic preparations of neurons which were fixed while exposed to hypertonic solutions the nucleolus was visible only as nucleolar shadow, overlaid by a few small irregular bodies of higher electron density than other nuclear contents.Nucleus The nuclear membrane of living neurons exposed to hypertonic media lost much of its sharp definition and became rather hazy in outline. The nuclear diameter increased about 10% in hypertonic medium, and the nuclear space became somewhat denser when observed with the phase contrast microscope. In Nissl stained preparations the nuclear space was filled with many small granular or rod-shaped bodies in contrast to the clear vesicular appearance of the nuclei of untreated cells. In electron microscopic preparations the nuclear space exhibited a spotty appearance due to the presence of electron dense and light areas.Nissl Substance In living neurons immersed in hypertonic solutions the Nissl substance showed a slight increase in phase density, especially after repeated changes between hypertonic and isotonic solutions. Sometimes a distinct striation in the Nissl substance appeared. In Nissl stained preparations there was no marked change observed in comparison with normal cells. However, in the electron microscope, the Nissl substance of hypertonically treated cells exhibited a marked structural change. The membrane-bound spaces of the endoplasmic reticulum assumed a rather precise orientation parallel to the cell membrane so that in extreme cases a concentric arrangement of endoplasmic cisternae was observed. The normal arrangement of ribosomal granules in rosettes and clusters became disturbed and the granules were more uniformly distributed.The cells as whole units showed a distinct shrinkage in hypertonic solution which may account for the more crowded appearance of various organelles such as mitochondria and Golgi complexes. There was also a marked increase in agranular reticulum profiles and small membrane bound vesicles in treated cells. Vacuoles appeared frequently in the cytoplasm of treated cells; they disappeared upon re-immersion in isotonic medium.This investigation was supported by USPHS Grants NB 03114-04, NB 00690-11 and 5 T 1 GM 495 from the National Institutes of Health, Bethesda, Maryland.Acknowledgement. Mrs. Eleanor W. Morris and Mr. Edwin E. Pitsinger, Jr. gave indispensible aid with the management of the cultures and with photographic procedures.     

多头绒泡菌细胞核周期的电镜研究

多头绒泡菌PhysarumpolycophalumSchw的营养生长阶段为没有细胞壁的原生质团（合胞体）,内部众多的细胞核进行着同步的核内有丝分裂,本文电镜下研究了细胞核在有丝分裂周期中的结构变化。有丝分裂前期,染色质经松散改组和集缩形成染色体,核仁由中央移向边缘,并在近核膜处解体;中期核膜不消失,在核内形成纺锤体,核仁解体后的物质是不规则状散在于核内;有丝分裂后核膜的破裂处重新愈合,染色体解集缩成染色质,分散的核仁物质逐渐合并形成新的核仁。