THE FINE STRUCTURE OF MITOSIS AND CELL DIVISION IN THE CHRYSOPHYCEAN ALGA OCHROMONAS DANICA1

At the ultrastructural level, cell division in Ochromonas danica exhibits several unusual features. During interphase, the basal bodies of the 2 flagella replicate and the chloroplast divides by constriction between its 2 lobes. The rhizoplast, which is a fibrous striated root attached to the basal body of the long flagellum, extends under the Golgi body to the surface of the nucleus in interphase cells. During proprophase, the Golgi body replicates, apparently by division, and a daughter rhizoplast, appears. During prophase, the 2 pairs of flagellar basal bodies, each with their accompanying rhizoplast and Golgi body, begin to separate. Three or 4 flagella are already present at this stage. At the same time, there is a proliferation of microtubules outside the nuclear envelope. Gaps then appear in the nuclear envelope, admitting the microtubules into the nucleus, where they form a spindle. A unique feature of mitosis in O. danica is that the 2 rhizoplasts form the poles of the spindle, spindle microtubules inserting directly onto the rhizoplasts. Some of the spindle microtubules extend from pole to pole; others appear to attach to the chromosomes. Kinetochores, however, are not present. The nuclear envelope breaks down, except, in the regions adjacent, to the chloroplasts; chloroplast ER remains intact throughout mitosis. At late anaphase the chromosomes come to lie against part of the chloroplast ER. This segment of the chloroplast ER appears to be incorporated as part of the reforming nuclear envelope, thus reestablishing the characteristic nuclear envelope—chloroplast ER association of the interphase cell.     

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.     

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.     

Ultrastructure of Mitosis and Cytokinesis In the Colorless Flagellate Katablepharis Ovalis Skuja

ABSTRACT. Mitosis and cytokinesis in Katablepharis ovalis , a colorless flagellate, was investigated. Two new flagella are produced prior to prophase, resulting in a motile quadriflagellate cell during mitosis. the inner array of microtubules of the feeding apparatus disappears before prophase begins. the nuclear envelope disperses during prophase, apparently being converted into rough endoplasmic reticulum. the chromatin condenses and the nucleolus disperses with spindle microtubules appearing oriented perpendicular to the longitudinal axis of the cell. At metaphase, the chromatin is condensed as a single disc-shaped mass and rough endoplasmic reticulum flanks the chromatin mass on each side. Groups of spindle microtubules pass through tunnels in the rough endoplasmic reticulum and through electron-translucent areas of the chromatin. the spindle microtubules end at a number of minipoles in the cytoplasm. Vesicles, ribosomes, mitochondria and endoplasmic reticulum migrate among the spindle microtubules. There is no polar body or any electrondense area associated with the spindle poles. the basal bodies of the flagella remain attached to the axonemes and do not participate in mitosis. In anaphase, the chromatin separates and migrates to the poles. During telophase, the nuclear envelope reforms from the rough endoplasmic reticulum and the nucleoli reappear. the spindle microtubules are persistent during telophase. Cytokinesis occurs by longitudinal fission, starting at the anterior end and progressing posteriorly. Cytokinesis may be driven by elongation of the spindle microtubules since there is no visible structure associated with the furrowing.     

Cytotoxic effects exerted by Tritrichomonas foetus pseudocysts

The protozoan parasite Tritrichomonas foetus displays a pear-shaped form and a pseudocyst stage. However, little is known about the biology of the pseudocyst. The aim of this work was to assess whether pseudocysts exert cytotoxic effects during their interaction with MDCK cells (an epithelial kidney canine cell line) and compare their behavior to that of the pear-shaped parasites. Pseudocysts and pear-shaped parasites from both cultured and freshly isolated T. foetus were used. Electron microscopy revealed that the epithelial cells exhibited more signs of injury, such as depletion of microvilli, retraction from neighboring cells and swollen mitochondria with loss of electron density in the matrix, when the pseudocysts were used in interaction experiments. In addition, during the co-incubation with MDCK cells, pseudocysts exhibited a more intense amoeboid transformation than that found in pear-shaped parasites. The MTT viability assay demonstrated that the pseudocysts were more cytotoxic when in contact with host cells as compared to the flagellated pear-shaped parasites. The JC-1 viability assay revealed that pseudocysts induced a higher loss of mitochondrial membrane potential compared to pear-shaped parasites. Pseudocysts undergoing a budding process were observed after 2.5h of co-incubation with MDCK cells. Our results suggest that the T. foetus pseudocyst might be a more aggressive form.     

Mitotic spindle form and function

The Saccharomyces cerevisiae mitotic spindle in budding yeast is exemplified by its simplicity and elegance. Microtubules are nucleated from a crystalline array of proteins organized in the nuclear envelope, known as the spindle pole body in yeast (analogous to the centrosome in larger eukaryotes). The spindle has two classes of nuclear microtubules: kinetochore microtubules and interpolar microtubules. One kinetochore microtubule attaches to a single centromere on each chromosome, while approximately four interpolar microtubules emanate from each pole and interdigitate with interpolar microtubules from the opposite spindle to provide stability to the bipolar spindle. On the cytoplasmic face, two to three microtubules extend from the spindle pole toward the cell cortex. Processes requiring microtubule function are limited to spindles in mitosis and to spindle orientation and nuclear positioning in the cytoplasm. Microtubule function is regulated in large part via products of the 6 kinesin gene family and the 1 cytoplasmic dynein gene. A single bipolar kinesin (Cin8, class Kin-5), together with a depolymerase (Kip3, class Kin-8) or minus-end-directed kinesin (Kar3, class Kin-14), can support spindle function and cell viability. The remarkable feature of yeast cells is that they can survive with microtubules and genes for just two motor proteins, thus providing an unparalleled system to dissect microtubule and motor function within the spindle machine.     

The spindle pole body duplicates in early G1 phase in the pathogenic yeast Exophiala dermatitidis: an ultrastructural study

The spindle pole body of the pathogenic yeast Exophiala dermatitidis was observed during the cell cycle using freeze-substitution and serial ultrathin sectioning electron microscopy. The spindle pole body was located on the outer membrane of the nuclear envelope and consisted of two disk elements connected by an intervening midpiece in G1 through G2 phases. Each disk element was composed of filamentous materials and measured 150 nm in diameter and 100 nm in thickness. The midpiece had higher electron density and measured 60 nm in length and 40 nm in thickness. At the beginning of prophase, each disk element of the spindle pole body enlarged to more than double in size. They were separated on the nuclear envelope, and associated with numerous cytoplasmic microtubules. At mitosis, the spindle pole body entered the nuclear envelope, associated with numerous nuclear microtubules, and was located at the spindle poles. At the end of telophase, it was extruded back into the cytoplasm from the nuclear envelope. Three-dimensional analysis of cells in different cell cycles suggested that duplication of the spindle pole body took place in early G1 phase. Thus, the location, structure, and duplication cycle of the E. dermatitidis spindle pole body were different from those of Saccharomyces cerevisiae.     

Dynamics of the nuclear envelope and of nuclear pore complexes during mitosis in the Drosophila embryo

Early embryonic development in Drosophila melanogaster is marked by a series of thirteen very rapid (10-15 min) and highly synchronous nuclear divisions, the last four of which occur just beneath the embryo surface. A total of some 6000 blastoderm nuclei result, which are subsequently enclosed by furrow membranes to form the cellular blastoderm. We have examined the fine structure of nuclear division in late syncytial embryos. The mitotic spindle forms adjacent to the nuclear envelope on the side facing the embryo surface. During prophase, astral microtubules deform the nuclear envelope which then ruptures at the poles at the onset of prometaphase. The nuclear envelope remains essentially intact elsewhere throughout mitosis. A second envelope begins to form around the nuclear envelope in prometaphase and is completed by metaphase; the entire double layered structure, referred to as the spindle envelope, persists through early in the ensuing interphase. Pole cell spindles are enclosed by identical spindle envelopes. Interphase and prophase nuclei contain nuclear pore complexes (PCs) of standard dimensions and morphology. In prometaphase PCs become much less electron-dense, although they retain their former size and shape. By metaphase, no semblance of PC structure remains, and instead, both layers of the spindle envelope are interrupted by numerous irregular fenestrae. PCs are presumably disassembled into their component parts during mitosis, and reassembled subsequently. Yolk nuclei remain among the central yolk mass when most nuclei migrate to the surface, cease to divide, yet become polyploid. These nuclei nonetheless lose and regain PCs in synchrony with the dividing blastoderm nuclei. In addition, they gain and lose a second fenestrated membrane layer with the same timing. Cytoplasmic membranes containing PCs (annulate lamellae) also lose and regain pores in synchrony with the two classes of nuclear envelopes. The factors that affect the integrity of PCs in dividing blastoderm nuclei appear to affect those in other membrane systems to an equivalent degree and with identical timing.     

Plant TPX2 and related proteins

At the onset of mitosis, microtubules form a bipolar spindle around the prophase nucleus. TPX2 is phosphorylated during mitosis and acts as a spindle assembly factor that nucleates microtubules in the close vicinity of chromosomes, independent of the centrosomes. Furthermore, it activates the kinase Aurora A and targets the Xenopus kinesin-like protein 2 to spindle poles. We have characterized the plant orthologue of TPX2 that possesses all identified functional domains of its animal counterpart. Moreover, we have demonstrated that it is exported before nuclear envelope breakdown and that its activity around the nuclear envelope is essential for prospindle assembly. Here, we compare the sequences of several characterized TPX2 domains, allowing us to define TPX2. We propose that true TPX2 orthologues share simultaneously all these conserved domains and that other proteins possessing only some of these functional blocks may be considered as TPX2-related proteins.Key words: mitosis, microtubules, spindle assembly, TPX2 signature, targeting domains, Prosite motifs, evolution     

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.     

Mitosis of the free-living flagellate Bodo saltans strain Ps+ (Kinetoplastidea, Bodonida)

Mitosis of the free-living flagellate Bodo saltans of the Ps+ strain characterized by the presence of prokaryotic cytobionts in the perinuclear space was studied. Division of B. saltans Ps+ nuclei occurs by the closed intranuclear type of mitosis without condensation of chromosomes. At the initial stages of nuclear division, consecutive anlage of two spatially separated microtubular spindles begins. The spindle containing about 20 microtubules appears first, then, at an angle of 30–40° to it, the second spindle containing half as many microtubules is formed. The microtubules of the first spindle are associated with 4 pairs of kinetochores, the microtubules of the second one—with 2 pairs. The kinetochores of B. saltans Ps+ have a pronounced laminar structure. Both spindles rest with their ends directly on the internal membrane of the nuclear envelope and form 4 well-pronounced poles. The equatorial phase of mitosis in B. saltans Ps+ is not revealed. The divergence of sister kinetochores towards the poles occurs independently in each spindle. At the elongation phase of mitosis, the poles of both spindles are united in pairs to form a single bipolar structure composed of two loose bundles of microtubules. At this stage of nuclear division, the kinetochores reach the poles of the subspindles and cease to be visible. At subsequent nuclear division stages the nucleus acquires a dumbbell shape. During the reorganization phase the sister nuclei are separated. In the perinuclear space of the interphase nuclei of B. saltans Ps+, 1–2 prokaryotic cytobionts are present. In the course of mitosis, these organisms divide intensively, such that their number can reach 20 and more per nucleus. During separation of sister nuclei, the “excessive” cytobionts are released into the cytoplasmic vacuoles formed by external membranes of the nuclear envelope.     

MITOSIS IN THE OCTAFLAGELLATE PRASINOPHYTE,PYRAMIMONAS AMYLIFERA (CHLOROPHYTA)1

Cell division is described in the octaflagellate prasinophyte Pyramimonas amylifera Conrad and is compared in related genera. Basal bodies replicate at preprophase and move toward the poles. Cells remain motile throughout division. The nuclear envelope disperses and chromosomes begin to condense at prophase. Pairs of multilayered kinetochores are evident on the chromosomes of the metaphase plate. Spindle microtubules extending from the region of the basal bodies and rhizoplasts attach to the kinetochores or extend from pole to pole. Numerous vesicles and ribosomes have entered the nuclear region and the incipient cleavage furrow invaginates. The chromosomes move toward the poles at anaphase leaving a broad interzonal spindle between the two chromosomal plates. The nuclear envelope reforms first around the chromatin on the side adjacent to the spindle poles and later on the interzonal side. The cleavage furrow progresses into the interzonal spindle at telophase. By late telophase the nucleoli have reformed and the chromosomes have decondensed. The interzonal spindle has not been observed late in telophase. As the cleavage furrow nears completion the cells begin to twist and contort, ultimately separating the two cells.     

A tense time for the nuclear envelope

When many cells divide, the nuclear envelope poses a problem: the spindle microtubules can't access the chromosomes. Two recent papers in Cell describe how the spindle solves this problem by literally pulling open the nucleus at the beginning of mitosis.     

Electron microscopy of dividing cells

During intranuclear mitosis in plasmodia of Physarum polycephalum the primordium of spindle microtubules which is a somewhat electron opaque, amorphous structure with fibrous or granular elements, occurs in the center of early prophase, 20 to 30 minutes before metaphase. Then, the primordium seems to divide into two parts. Spindle microtubules develop radially from the primordia of spindle microtubules. These spindle microtubules increase in number and length during prophase. Spindle microtubules are completed in about five minutes before metaphase. The nuclear envelope remains intact during prophase, but after metaphase it breaks at the polar regions. The nuclear envelope of the daughter nucleus is re-formed from the original nuclear envelope.The authors wish to express their thanks Professor K. Ueda for valuable advice. They would also like to thank Professor N. Kamiya of Osaka University who kindly supplied the material (Physarum polycephalum) used in the present study.     

Contributions of the axostyle and flagella to closed mitosis in the protists Tritrichomonas foetus and Trichomonas vaginalis

Tritrichomonas foetus and Trichomonas vaginalis are protists that undergo closed mitosis: the nuclear envelope remains intact and the spindle remains extranuclear. Here we show, in disagreement with previous studies, that the axostyle does not disappear during mitosis but rather actively participates in it. We document the main structural modifications of the cell during its cell cycle using video enhanced microscopy and computer animation, bright field light microscopy, confocal laser scanning microscopy, and scanning and transmission electron microscopy. We propose six phases in the trichomonad's cell cycle: an orthodox interphase, a pre-mitotic phase, and four stages during the cell division process. We report that in T. foetus and T. vaginalis: a) all skeletal structures such as the costa, pelta-axostyle system, basal bodies, flagella, and associated filaments of the mastigont system are duplicated in a pre-mitotic phase; b) the axostyle does not disappear during mitosis, otherwise playing a fundamental role in this process; c) axostyles participate in the changes in the cell shape, contortion of the anterior region of the cell, and karyokinesis; d) flagella are not under assembly during mitosis, as previously stated by others, but completely formed before it; and e) cytokinesis is powered in part by cell locomotion.     

FINE STRUCTURAL STUDIES ON THE INTERPHASE AND DIVIDING APICAL CELLS OF SPHACELARIA TRIBULOIDES (PHAEOPHYTA)1

The apical cells of Sphacelaria tribuloides Menegh. are larger than other thallus cells, contain more organelles and appear polarized. Their tip portion, where they grow, contains a well developed Golgi apparatus, abundant endoplasmic reticulum (ER) membranes, mitochondria, chloroplasts and a large number of small vacuoles. It seems likely that a continuous flow of membranous material from the ER membranes to the dictyosomes and from the latter to the plasmalemma of the extending tip portion takes place. In contrast, the basal pole possesses fewer organelles and is occupied mainly by large-sized, sometimes central vacuoles. The apical cells undergo two distinct types of highly asymmetrical differential divisions giving rise to cells of the thallus and hair initials. During the early stages of mitosis the nuclear envelope remains intact, except for fenestrated poles. Microtubules pass through the fenestrae into the nucleoplasm. During meta-phase, a typical chromosome plate is organized. The sites of attachment of spindle microtubules to the chromosomes are structurally different from the rest of the chromosomes. At late anaphase, the nuclear envelope breaks down completely. During telophase, a new membrane encloses the chromosomes which are decondensed and the nucleoli are reorganized. Cytokinesis proceeds long after mitosis at a stage in which the nuclei have increased in size and have moved farther apart. A membranous furrow develops centripetally, without the participation of microtubules. However, microtubules traverse the thin cytoplasmic strands which, in both interphase and cytokinetic cells, meander among the vacuoles of the basal pole of the cell and the internuclear space. Dictyosomes appear to be involved in the subsequent wall deposition.     

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.     

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.     

Dynamics of the endoplasmic reticulum during early development of Drosophila melanogaster

In this study, we analyze for the first time endoplasmic reticulum (ER) dynamics and organization during oogenesis and embryonic divisions of Drosophila melanogaster using a Protein Disulfide Isomerase (PDI) GFP chimera protein. An accumulation of ER material into the oocyte takes place during the early steps of oogenesis. The compact organization of ER structures undergoes a transition to an expanded reticular network at fertilization. At the syncytial stage, this network connects to the nuclear envelope as each nucleus divides. Time-lapse confocal microscopy on PDI transgenic embryos allowed us to characterize a rapid redistribution of the ER during the mitotic phases. The ER network is massively recruited to the spindle poles in prophase. During metaphase most of the ER remains concentrated at the spindle poles and shortly thereafter forms several layers of membranes along the ruptured nuclear envelope. Later, during telophase an accumulation of ER material occurs at the spindle equator. We also analyzed the subcellular organization of the ER network at the ultrastructural level, allowing us to corroborate the results from confocal microscopy studies. This dynamic redistribution of ER suggests an unexpected regulatory function for this organelle during mitosis.     

The plant TPX2 protein regulates prospindle assembly before nuclear envelope breakdown

The Targeting Protein for Xklp2 (TPX2) is a central regulator of spindle assembly in vertebrate cells. The absence or excess of TPX2 inhibits spindle formation. We have defined a TPX2 signature motif that is present once in vertebrate sequences but twice in plants. Plant TPX2 is predominantly nuclear during interphase and is actively exported before nuclear envelope breakdown to initiate prospindle assembly. It localizes to the spindle microtubules but not to the interdigitating polar microtubules during anaphase or to the phragmoplast as it is rapidly degraded during telophase. We characterized the Arabidopsis thaliana TPX2-targeting domains and show that the protein is able to rescue microtubule assembly in TPX2-depleted Xenopus laevis egg extracts. Injection of antibodies to TPX2 into living plant cells inhibits the onset of mitosis. These results demonstrate that plant TPX2 already functions before nuclear envelope breakdown. Thus, plants have adapted nuclear-cytoplasmic shuttling of TPX2 to maintain proper spindle assembly without centrosomes.