Mitosis and cytokinesis in androgonidia ofVolvox carteri f.weismannia

Summary Reproductive cells (androgonidia) ofVolvox carteri f.weismannia divide to form packets of 64 or 128 sperm cells. The androgonidium morphology, stages of mitosis, and cytokinesis were examined by electron microscopy. The biflagellate androgonidium loses its flagella before mitosis but the flagellar bases at the anterior end of the cell are retained. Two additional basal bodies are formed and the nucleus migrates from its central position to the area of the basal bodies before mitosis begins. A five-layered kinetochore is present on the chromosomes and remnant nucleolar material persists during mitosis. A furrow at the chloroplast end of the cell and the formation of phycoplast microtubules and vesicles signal the beginning of cytokinesis at early telophase. The cells maintain cytoplasmic connections until after the packet of sperm cells completes its development.     

MITOSIS, CELL WALL AND FLAGELLA SYNTHESIS IN SYNCHRONIZED CULTURES OF THE PRASINOPHYTE, SCHERFFELIA DUBIA

Cell division occurs within the parental cell wall, yielding two progeny cells. Since Scherffelia dubia sheds all four flagella prior to cell division, the maturing progeny cells must regenerate new cell walls and flagella during and/or after cytokinesis. To better understand these processes, we have synchronized cell division in cultures of S. dubia and observed all stages of mitosis, cytokinesis, and progeny cell maturation, including flagella and cell wall formation, via DAPI staining of fixed cells, DIC microscopy of live cells embedded in agarose and standard TEM. Microscopical observations revealed the following sequence of events: 1) Golgi stacks divide during late interphase and immediately begin producing theca scales; 2) deflagellation and release of the parental cell wall from the plasma membrane occurs during early prophase; 3) synthesis of theca and flagella scales within the Golgi and/or scale reticulum continues throughout mitosis; 4) during cytokinesis, a coalescence of vesicles containing theca scales at the posterior end of the cell results in a cleavage furrow slightly diagonal to the cells' longitudinal axis (40 min); 5) post-mitotic nascent basal body formation and flagella elongation at the inherited basal bodies (and later at the mature nascent basal bodies) occurs concurrently with continued cell wall synthesis; 6) the cleavage furrow rotates into a transverse position (35 min); 7) reorientation of the nuclei results in a "head to tail" orientation of the maturing progeny cells; and 8) matured progeny cells emerge from the posterior end of the parental theca not before 8 hrs after the onset of mitosis.     

Cytokinesis in the C. elegans embryo: regulating contractile forces and a late role for the central spindle

Genetic and molecular studies in the nematode Caenorhabditis elegans have identified multiple essential pathways that regulate and execute cytokinesis in early embryonic cells. These pathways influence both the microfilament cytoskeleton and the microtubule cytoskeleton. Microfilaments are enriched throughout the cell cortex at all times during the cell cycle in embryonic cells. Cortical microfilaments are required for multiple processes in embryonic cells, including polar body extrusion during meiosis, anterior-posterior axis specification by the sperm-donated microtubule-organizing center, and cytokinesis during mitosis. In addition to contractile apparatus proteins that are required positively for cleavage furrow ingression, the Nedd8 ubiquitin-like protein modification pathway negatively regulates contractile forces outside the cleavage furrow during cytokinesis. Another pathway that acts positively during cytokinesis involves the mitotic spindle. The central spindle, where anti-parallel non-kinetochore microtubules overlap and are cross-linked, is required for a late step in cytokinesis, and other pathway(s) involved in membrane addition during cytokinesis may also require the central spindle. The amenability of C. elegans to classical genetics, the ease of reducing gene function with RNA interference, the completion of the genome sequence, and the availability of transgenic GFP fusion proteins that render the cytoskeleton fluorescent, all serve to make the early worm embryo an especially promising system for further advances in the identification of cytokinesis pathways, and in defining their interactions.     

The association of calmodulin with central spindle regulates the initiation of cytokinesis in HeLa cells

Calmodulin is a major cytoplasmic calcium receptor that performs multiple functions in the cell including cytokinesis. Central spindle appears between separating chromatin masses after metaphase-anaphase transition. The interaction of microtubules from central spindle with cell cortex regulates the cleavage furrow formation. In this paper, we use green fluorescence protein (GFP)-tagged calmodulin as a living cell probe to examine the detailed dynamic redistribution and co-localization of calmodulin with central spindle during cytokinesis and the function of this distribution pattern in a tripolar HeLa cell model. We found that calmodulin is associated with spindle microtubules during mitosis and begins to aggregate with central spindle after anaphase initiation. The absence of either central spindle or central spindle-distributed calmodulin is correlated with the defect in the formation of cleavage furrow, where contractile ring-distributed CaM is also extinct. Further analysis found that both the assembly of central spindle and the formation of cleavage furrow are affected by the W7 treatment. The microtubule density of central spindle was decreased after the treatment. Only less than 10% of the synchronized cells enter cytokinesis when treated with 25 microM W7, and the completion time of furrow regression is also delayed from 10 min to at least 40 min. It is suggested that calmodulin plays a significant role in cytokinesis including furrow formation and regression, The understanding of the interaction between calmodulin and microtubules may give us insight into the mechanism through which calmodulin regulates cytokinesis.     

Dissociation of cytokinesis initiation from mitotic control in a eukaryote

Cytokinesis is initiated only after mitotic exit in eukaryotes. However, in the insect (procyclic) form of an ancient protist, Trypanosoma brucei, a blockade at the G2/M checkpoint results in an enrichment of anucleate cells (zoids), suggesting separated regulations between mitosis and cytokinesis (X. Tu and C. C. Wang, J. Biol. Chem. 279:20519-20528, 2004). Polo-like kinases (Plks) are known to play critical roles in controlling both mitosis and cytokinesis. A single Plk homologue in T. brucei, TbPLK, was found to be capable of complementing the Plk (Cdc5) functions in Saccharomyces cerevisiae, thus raising the question of how it may function in the trypanosome with cytokinesis dissociated from mitosis. Depletion of TbPLK in the procyclic form of T. brucei by RNA interference resulted in growth arrest with accumulation of multiple nuclei, kinetoplasts, basal bodies, and flagella in approximately equal numbers among individual cells. There were, however, few zoids detectable, indicating inhibited cytokinesis with unblocked mitosis and kinetoplast segregation. TbPLK is thus apparently involved only in initiating cytokinesis in T. brucei. Overexpression of TbPLK in the trypanosome did not affect cell growth, but 13% of the resulting population was in the zoid form, suggesting runaway cytokinesis. An immunofluorescence assay indicated that TbPLK was localized in a chain of likely flagellum attachment zones in the cytoskeleton. In a dividing cell, a new line of such zones appeared closely paralleling the existing one, which could constitute the cleavage furrow. An exposed region of TbPLK at the anterior tip of the cell may provide the trigger of cytokinesis. Taken together, our results revealed a novel mechanism of cytokinesis initiation in the trypanosome that may serve as a useful model for further in-depth investigations.     

Endocytosis resumes during late mitosis and is required for cytokinesis

Recent work has underscored the importance of membrane trafficking events during cytokinesis. For example, targeted membrane secretion occurs at the cleavage furrow in animal cells, and proteins that regulate endocytosis also influence the process of cytokinesis. Nonetheless, the prevailing dogma is that endosomal membrane trafficking ceases during mitosis and resumes after cell division is complete. In this study, we have characterized endocytic membrane trafficking events that occur during mammalian cell cytokinesis. We have found that, although endocytosis ceases during the early stages of mitosis, it resumes during late mitosis in a temporally and spatially regulated pattern as cells progress from anaphase to cytokinesis. Using fixed and live cell imaging, we have found that, during cleavage furrow ingression, vesicles are internalized from the polar region and subsequently trafficked to the midbody area during later stages of cytokinesis. In addition, we have demonstrated that cytokinesis is inhibited when clathrin-mediated endocytosis is blocked using a series of dominant negative mutants. In contrast to previous thought, we conclude that endocytosis resumes during the later stages of mitosis, before cytokinesis is completed. Furthermore, based on our findings, we propose that the proper regulation of endosomal membrane traffic is necessary for the successful completion of cytokinesis.     

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.     

Endocytic traffic in animal cell cytokinesis

Cytokinesis is the final step of mitosis whereby two daughter cells physically separate. It is initiated by the assembly of an actomyosin contractile ring at the mitotic cell equator, which constricts the cytoplasm between the two reforming nuclei resulting in the formation of a narrow intercellular bridge filled with central spindle microtubule bundles. Cytokinesis terminates with the cleavage of the intercellular bridge in a poorly understood process called abscission. Recent work has highlighted the importance of membrane trafficking events occurring from membrane compartments flanking the bridge to the central midbody region. In particular, polarized delivery of endocytic recycling membranes is essential for completion of animal cell cytokinesis. Why endocytic traffic occurs within the intercellular bridge remains largely mysterious and its significance for cytokinesis will be discussed.     

Zellteilung und Bildung von Innenschalen beiHantzschia amphioxys undAchnanthes coarctata

Before cytokinesis, the identically constructed chromatophores ofHantzschia amphioxys andAchnanthes coarctata are transformed into less effigurated bodies. In normal cytokinesis, the course of mitosis, chromatophore division, and cleavage furrowing are exactly synchronized. The division of the chromatophore appears as a passive process, i.e. intersection by the cleavage furrow. In inequal cell divisions before the formation of inner valves cytokinesis can take place without chromatophore division. Once chromatophore division without mitosis and cytokinesis was observed. InHantzschia there are three types of inner valve formation, inAchnanthes coarctata only two. The inner valves develop under unfavorable growth conditions, the cells possessing them, however, are not resting spores as in some other diatoms. InHantzschia, auxospore formation is suppressed under the cultural conditions used, the cells multiply intensely without diminution.

     

EVI5 protein associates with the INCENP-aurora B kinase-survivin chromosomal passenger complex and is involved in the completion of cytokinesis

EVI5 has been shown to be a novel centrosomal protein in interphase cells. In this report, we demonstrate using immunofluorescence microscopy that EVI5 has a dynamic distribution during mitosis, being associated with the mitotic spindle through anaphase and remaining within the midzone and midbody until completion of cytokinesis. Knockdown of EVI5 using siRNA results in a multinucleate phenotype, which is consistent with an essential role for this protein in the completion of cytokinesis. The EVI5 protein also undergoes posttranslational modifications during the cell cycle, which involve phosphorylation in early mitosis and proteolytic cleavage during late mitosis and cytokinesis. Since the subcellular distribution of the EVI5 protein was similar to that characteristic of chromosomal passenger proteins during the terminal stages of cytokinesis, we used immunoprecipitation and GST pull-down approaches to demonstrate that EVI5 is associated with the aurora B kinase protein complex (INCENP, aurora B kinase and survivin). Together, these data provide evidence that EVI5 is an essential component of the protein machinery facilitating the final stages of cell septation at the end of mitosis.     

Contractile ring-independent localization of DdINCENP, a protein important for spindle stability and cytokinesis

Dictyostelium DdINCENP is a chromosomal passenger protein associated with centromeres, the spindle midzone, and poles during mitosis and the cleavage furrow during cytokinesis. Disruption of the single DdINCENP gene revealed important roles for this protein in mitosis and cytokinesis. DdINCENP null cells lack a robust spindle midzone and are hypersensitive to microtubule-depolymerizing drugs, suggesting that their spindles may not be stable. Furthermore DdCP224, a protein homologous to the microtubule-stabilizing protein TOGp/XMAP215, was absent from the spindle midzone of DdINCENP null cells. Overexpression of DdCP224 rescued the weak spindle midzone defect of DdINCENP null cells. Although not required for the localization of the myosin II contractile ring and subsequent formation of a cleavage furrow, DdINCENP is important for the abscission of daughter cells at the end of cytokinesis. Finally, we show that the localization of DdINCENP at the cleavage furrow is modulated by myosin II but it occurs by a mechanism different from that controlling the formation of the contractile ring.     

FINE STRUCTURE OF CELL DIVISION IN CHLAMYDOMONAS REINHARDI : Basal Bodies and Microtubules

Cell division in log-phase cultures of the unicellular, biflagellate alga, Chlamydomonas reinhardi, has been studied with the electron microscope. The two basal bodies of the cell replicate prior to cytokinesis; stages in basal body formation are presented. At the time of cell division, the original basal bodies detach from the flagella, and the four basal bodies appear to be involved in the orientation of the plane of the cleavage furrow. Four sets of microtubules participate in cell division. Spindle microtubules are involved in a mitosis that is marked by the presence of an intact nuclear envelope. A band of microtubules arcs over the mitotic nucleus, indicating the future cleavage plane. A third set of microtubules appears between the daughter nuclei at telophase, and microtubules comprising the "cleavage apparatus" radiate from the basal bodies and extend along both sides of the cleavage furrow during cytokinesis. Features of cell division in C. reinhardi are discussed and related to cell division in other organisms. It is proposed that microtubules participate in the formation of the cleavage furrow in C. reinhardi.     

Colocalization of TD-60 and INCENP throughout G2 and mitosis: evidence for their possible interaction in signalling cytokinesis

TD-60 and INCENP are two members of the chromosome passenger protein family, and each has been suggested to play a role in the control of cytokinesis. Here we demonstrate by confocal immunofluorescence microscopy that TD-60 and INCENP distribute identically throughout the cell cycle. Both appear coordinately in G2-phase nuclei and become concentrated at centromeres during prophase. TD-60 and INCENP both then leave the chromosome together during anaphase and redistribute to the spindle midzone, as do other chromosome passenger proteins, and traverse the entire equatorial diameter from cortex to cortex. By image overlay and pixel count analysis we show that TD-60 and INCENP are distinct among known chromosome passenger proteins in extending beyond the spindle to the cortex. Further, we show that the cytokinesis-associated protein kinase AIM-1 also shares this distribution property. We suggest that this redistribution is required to signal cytokinesis. TD-60 and INCENP also show identical localization in cells that exit mitosis in the presence of dihydrocytochalasin B (DCB), an inhibitor of actin assembly. Such cells can resume cleavage upon removal of DCB and in a recovery subpopulation that cleaves only on one side, these proteins both colocalize to the cortex only where a cleavage furrow forms. Given the coincident distribution of TD-60 and INCENP during both interphase and mitosis, we suggest that these proteins may cooperate, perhaps within a protein complex, in signalling cytokinesis. Such a mechanism, using chromosome passenger proteins, may ensure that cytokinesis occurs only between the separated chromatids, and only after they have segregated. Received: 12 August 1998; in revised form: 1 September 1998 / Accepted: 2 September 1998     

Electron microscopic observations on the maintenance of the tight junction during cell division in the epithelium of the mouse small intestine.

Dividing epithelial cells in the mouse small intestine were examined by thin-section electron microscopy with special attention given to the mode of cytokinesis. As the columnar epithelial cells entered mitosis in the crypt, they became rounded, maintaining their junctional complexes with neighboring cells while detaching themselves from the basal lamina. In such rounded cells the mitotic apparatus was formed with its long axis parallel to the luminal surface. Replicated centrioles moved down from the apical region to locate themselves lateral to the nucleus, where they served as the poles of the mitotic spindle. During mitosis the cell retained microvilli on its luminal surface, though the terminal web became much thinner. At telophase the formation of a cleavage furrow proceeded asymmetrically from the basal side alone, and thus the contractile ring which was prominent at the base of the furrow, merged with the terminal web. Eventually, an intercellular bridge with a midbody was formed on the luminal surface. The space in the furrow was occupied by the flattened cytoplasmic processes of the neighboring cells. The tight junction was also seen on the basolateral surface of the intercellular bridge with the underlying neighboring cells. At very late telophase the intercellular bridge was disconnected from the neighboring cells and protruded into the lumen. These observations have led us to propose a mode by which the simple columnar epithelium maintain the tight junctional seal during cell division in the crypt of the small intestinal epithelium.     

Transitin, a nestin-like intermediate filament protein, mediates cortical localization and the lateral transport of Numb in mitotic avian neuroepithelial cells

Neuroepithelium is an apicobasally polarized tissue that contains neural stem cells and gives rise to neurons and glial cells of the central nervous system. The cleavage orientation of neural stem cells is thought to be important for asymmetric segregation of fate-determinants, such as Numb. Here, we show that an intermediate filament protein, transitin, colocalizes with Numb in the cell cortex of mitotic neuroepithelial cells, and that transitin anchors Numb via a physical interaction. Detailed immunohistological and time-lapse analyses reveal that basal Numb-transitin complexes shift laterally during mitosis, allowing asymmetric segregation of Numb-transitin to one of the daughter cells, even when the cell cleavage plane is perpendicular to the ventricular surface. In addition, RNA interference (RNAi) knockdown of the transitin gene reveals its involvement in neurogenesis. These results indicate that transitin has important roles in determining the intracellular localization of Numb, which regulates neurogenesis in the developing nervous system of avian embryos.     

Microtubule polarity and dynamics in the control of organelle positioning,segregation, and cytokinesis in the trypanosome cell cycle

Trypanosoma brucei has a precisely ordered microtubule cytoskeleton whose morphogenesis is central to cell cycle events such as organelle positioning, segregation, mitosis, and cytokinesis. We have defined microtubule polarity and show the + ends of the cortical microtubules to be at the posterior end of the cell. Measurements of organelle positions through the cell cycle reveal a high degree of coordinate movement and a relationship with overall cell extension. Quantitative analysis of the segregation of the replicated mitochondrial genome (the kinetoplast) by the flagellar basal bodies identifies a new G2 cell cycle event marker. The subsequent mitosis then positions one "daughter" nucleus into the gap between the segregated basal bodies/kinetoplasts. The anterior daughter nucleus maintains its position relative to the anterior of the cell, suggesting an effective yet cryptic nuclear positioning mechanism. Inhibition of microtubule dynamics by rhizoxin results in a phenomenon whereby cells, which have segregated their kinetoplasts yet are compromised in mitosis, cleave into a nucleated portion and a flagellated, anucleate, cytoplast. We term these cytoplasts "zoids" and show that they contain the posterior (new) flagellum and associated basal-body/kinetoplast complex. Examination of zoids suggests a role for the flagellum attachment zone (FAZ) in defining the position for the axis of cleavage in trypanosomes. Progression through cytokinesis, (zoid formation) while mitosis is compromised, suggests that the dependency relationships leading to the classical cell cycle check points may be altered in trypanosomes, to take account of the need to segregate two unit genomes (nuclear and mitochondrial) in this cell.     

Vesicle trafficking and membrane remodelling in cytokinesis

All cells complete cell division by the process of cytokinesis. At the end of mitosis, eukaryotic cells accurately mark the site of division between the replicated genetic material and assemble a contractile ring comprised of myosin II, actin filaments and other proteins, which is attached to the plasma membrane. The myosin-actin interaction drives constriction of the contractile ring, forming a cleavage furrow (the so-called 'purse-string' model of cytokinesis). After furrowing is completed, the cells remain attached by a thin cytoplasmic bridge, filled with two anti-parallel arrays of microtubules with their plus-ends interdigitating in the midbody region. The cell then assembles the abscission machinery required for cleavage of the intercellular bridge, and so forms two genetically identical daughter cells. We now know much of the molecular detail of cytokinesis, including a list of potential genes/proteins involved, analysis of the function of some of these proteins, and the temporal order of their arrival at the cleavage site. Such studies reveal that membrane trafficking and/or remodelling appears to play crucial roles in both furrowing and abscission. In the present review, we assess studies of vesicular trafficking during cytokinesis, discuss the role of the lipid components of the plasma membrane and endosomes and their role in cytokinesis, and describe some novel molecules implicated in cytokinesis. The present review covers experiments performed mainly on tissue culture cells. We will end by considering how this mechanistic insight may be related to cytokinesis in other systems, and how other forms of cytokinesis may utilize similar aspects of the same machinery.     

Centrosome/Spindle Pole–associated Protein Regulates Cytokinesis via Promoting the Recruitment of MyoGEF to the Central Spindle

Cooperative communications between the central spindle and the contractile ring are critical for the spatial and temporal regulation of cytokinesis. Here we report that MyoGEF, a guanine nucleotide exchange factor that localizes to the central spindle and cleavage furrow, interacts with centrosome/spindle pole-associated protein (CSPP), which is concentrated at the spindle pole and central spindle during mitosis and cytokinesis. Both in vitro and in vivo pulldown assays show that MyoGEF interacts with CSPP. The C-terminus of MyoGEF and N-terminus of CSPP are required for their interaction. Immunofluorescence analysis indicates that MyoGEF and CSPP colocalize at the central spindle. Depletion of CSPP or MyoGEF by RNA-interference (RNAi) not only causes defects in mitosis and cytokinesis, such as metaphase arrest and furrow regression, but also mislocalization of nonmuscle myosin II with a phosphorylated myosin regulatory light chain (p-MRLC). Importantly, CSPP depletion by RNAi interferes with MyoGEF localization at the central spindle. Finally, MyoGEF interacts with ECT2, and RNAi-mediated depletion of MyoGEF leads to mislocalization of ECT2 and RhoA during cytokinesis. Therefore, we propose that CSPP interacts with and recruits MyoGEF to the central spindle, where MyoGEF contributes to the spatiotemporal regulation of cytokinesis.     

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.     

Endomitotic Megakaryocytes that Form a Bipolar Spindle Exhibit Cleavage Furrow Ingression Followed by Furrow Regression

Megakaryocyte (MK) differentiation is marked by the development of progressive polyploidy, due to repeated incomplete cell cycles in which mitosis is aborted during anaphase, a process termed endomitosis. We have postulated that anaphase in endomitotic MKs diverges from diploid mitosis at a point distal to the assembly of the midzone, possibly involving impaired cleavage furrow progression. To define the extent of furrow initiation and ingression in endomitosis, we performed time-lapse imaging of MKs expressing yellow fluorescent protein (YFP)-tubulin and monitored shape change as they progressed through anaphase. We found that in early endomitotic cells that have a bipolar spindle, cleavage furrows form that can undergo significant ingression, but furrows regress to produce polyploid cells. Compared to cells that divide, cells that exhibit furrow regression have a slower rate of furrow ingression and do not furrow as deeply. More highly polyploid MKs undergoing additional endomitotic cycles also show measurable furrowing that is followed by regression, but the magnitude of the shape change is less than seen in the early MKs. This suggests that in the earliest endomitotic cycles when there is formation of a bipolar spindle, the failure of cytokinesis occurs late, following assembly and initial constriction of the actin/myosin ring, whereas in endomitotic MKs that are already polyploid there is secondary inhibition of furrow progression. This behavior of furrow ingression followed by regression may explain why midbody remnants are occasionally observed in polyploid MKs. This finding has important implications for the potential mechanisms for cytokinesis failure in endomitosis.