Mitotic Asters Research Articles

Reduction of cortical pulling at mitotic entry facilitates aster centration.

Equal cell division relies upon astral microtubule-based centering mechanisms, yet how the interplay between mitotic entry, cortical force generation and long astral microtubules leads to symmetric cell division is not resolved. We report that a cortically located sperm aster displaying long astral microtubules that penetrate the whole zygote does not undergo centration until mitotic entry. At mitotic entry, we find that microtubule-based cortical pulling is lost. Quantitative measurements of cortical pulling and cytoplasmic pulling together with physical simulations suggested that a wavelike loss of cortical pulling at mitotic entry leads to aster centration based on cytoplasmic pulling. Cortical actin is lost from the cortex at mitotic entry coincident with a fall in cortical tension from ∼300pN/µm to ∼100pN/µm. Following the loss of cortical force generators at mitotic entry, long microtubule-based cytoplasmic pulling is sufficient to displace the aster towards the cell center. These data reveal how mitotic aster centration is coordinated with mitotic entry in chordate zygotes.

Kif2 localizes to a subdomain of cortical endoplasmic reticulum that drives asymmetric spindle position

Asymmetric positioning of the mitotic spindle is a fundamental process responsible for creating sibling cell size asymmetry; however, how the cortex causes the depolymerization of astral microtubules during asymmetric spindle positioning has remained elusive. Early ascidian embryos possess a large cortical subdomain of endoplasmic reticulum (ER) that causes asymmetric spindle positioning driving unequal cell division. Here we show that the microtubule depolymerase Kif2 localizes to this subdomain of cortical ER. Rapid live-cell imaging reveals that microtubules are less abundant in the subdomain of cortical ER. Inhibition of Kif2 function prevents the development of mitotic aster asymmetry and spindle pole movement towards the subdomain of cortical ER, whereas locally increasing microtubule depolymerization causes exaggerated asymmetric spindle positioning. This study shows that the microtubule depolymerase Kif2 is localized to a cortical subdomain of endoplasmic reticulum that is involved in asymmetric spindle positioning during unequal cell division.

Size Scaling of Microtubule Assemblies in Early Xenopus Embryos.

The first 12 cleavage divisions in Xenopus embryos provide a natural experiment in size scaling, as cell radius decreases ∼16-fold with little change in biochemistry. Analyzing both natural cleavage and egg extract partitioned into droplets revealed that mitotic spindle size scales with cell size, with an upper limit in very large cells. We discuss spindle-size scaling in the small- and large-cell regimes with a focus on the "limiting-component" hypotheses. Zygotes and early blastomeres show a scaling mismatch between spindle and cell size. This problem is solved, we argue, by interphase asters that act to position the spindle and transport chromosomes to the center of daughter cells. These tasks are executed by the spindle in smaller cells. We end by discussing possible mechanisms that limit mitotic aster size and promote interphase aster growth to cell-spanning dimensions.

The taccalonolides and paclitaxel cause distinct effects on microtubule dynamics and aster formation

BackgroundMicrotubule stabilizers suppress microtubule dynamics and, at the lowest antiproliferative concentrations, disrupt the function of mitotic spindles, leading to mitotic arrest and apoptosis. At slightly higher concentrations, these agents cause the formation of multiple mitotic asters with distinct morphologies elicited by different microtubule stabilizers.ResultsWe tested the hypothesis that two classes of microtubule stabilizing drugs, the taxanes and the taccalonolides, cause the formation of distinct aster structures due, in part, to differential effects on microtubule dynamics. Paclitaxel and the taccalonolides suppressed the dynamics of microtubules formed from purified tubulin as well as in live cells. Both agents suppressed microtubule dynamic instability, with the taccalonolides having a more pronounced inhibition of microtubule catastrophe, suggesting that they stabilize the plus ends of microtubules more effectively than paclitaxel. Live cell microscopy was also used to evaluate the formation and resolution of asters after drug treatment. While each drug had similar effects on initial formation, substantial differences were observed in aster resolution. Paclitaxel-induced asters often coalesced over time resulting in fewer, larger asters whereas numerous compact asters persisted once they were formed in the presence of the taccalonolides.ConclusionsWe conclude that the increased resistance of microtubule plus ends to catastrophe may play a role in the observed inability of taccalonolide-induced asters to coalesce during mitosis, giving rise to the distinct morphologies observed after exposure to these agents.

Morphofunctional transformations of the yolk syncytial layer during zebrafish development

The yolk syncytial layer (YSL) is a provisory extraembryonic structure of teleost fishes and representatives of some other taxa with meroblastic cleavage. The YSL of teleosts is a symplast with polymorphous polyploid nuclei. It is known to perform nutritional, morphogenetic, immune, and, probably, other functions. Data about the YSL organization, functioning and regulation is fragmentary. Although gene expression patterns and other aspects of YSL functioning have been studied in Danio rerio, the morphology of its YSL has not been described in detail. The study of zebrafish YSL structure on sequential developmental stages is necessary to recognize specific features of this important polyfunctional system in this model organism and to extend our knowledge about provisory systems. The thickness of the YSL and the distribution of its nuclei are not uniform on each stage and change during development. During oblong and sphere stages the internal YSL (I-YSL) is filled with yolk inclusions; interphase yolk syncytial nuclei (YSN) and mitotic asters can be seen. During doming and epiboly the external YSL (E-YSL) is thicker than I-YSL. On the subsequent stages the YSL is thickened caudally. The dorsal YSL part is thickened during early segmentation stages and becomes the thinnest YSL region later. The anterior part of the YSL is thin, but enlarges during larval period. The YSN of different size and diverse forms, from regular to lobed, are present and form clusters. The number of irregular-shaped nuclei increases during development. The YSL thickens in the end of endotrophic and in the course of endo-exotrophic period, and its cytoplasm contains numerous yolk inclusions. After yolk exhaustion the YSL is flat. As the YSL degrades, the YSN become pycnotic, and the YSL remnant probably is cleared by phagocytes.

Gene silencing and antitumoral effects of Eg5 or Ran siRNA oligoaminoamide polyplexes.

Two antitumoral siRNAs (directed against target genes Eg5 and Ran) complexed with one of three sequence-defined cationic oligomers were compared in gene silencing in vitro and antitumoral in vivo efficacy upon intratumoral injection. Two lipo-oligomers (T-shape 49, i-shape 229) and the three-arm oligomer 386 were chosen because of their high efficiency in previous marker gene silencing screens. The oligomers showed very similar target-specific gene knockdown in murine neuroblastoma cells. Silencing persisted only for a short period (maximum on day 1 at mRNA and day 2 at protein level) triggering siRNA specific in vitro tumor cell killing. The fastest onset of protein knockdown and strongest antitumoral effect was mediated by oligomer 386. Tumor growth reduction in vivo was evaluated in the subcutaneous Neuro2A mouse model. Intratumoral injections of either Eg5 or Ran siRNA/oligomer 49 polyplexes led to reduced tumor growth and prolonged survival of mice compared to control siRNA and buffer treatment. Target knockdown was evidenced in tumors by mitotic Aster formation for Eg5 knockdown and apoptotic TUNEL stain for Ran knockdown. Ran siRNA displayed better antitumoral efficacy and was chosen for in vivo comparison of the oligomers. A very clear order of antitumoral activity (oligomer 386 > 49 > 229) was observed. In summary, the similar in vitro gene silencing efficiencies on mRNA level by the tested oligomers did not correlate with the observed therapeutic effects in vivo. Oligomer 386 with the fastest onset of protein knockdown and best in vitro cell killing mediated the best in vivo antitumoral efficacy.

Spindle Pole Mechanics Studied in Mitotic Asters: Dynamic Distribution of Spindle Forces through Compliant Linkages

During cell division, chromosomes must faithfully segregate to maintain genome integrity, and this dynamic mechanical process is driven by the macromolecular machinery of the mitotic spindle. However, little is known about spindle mechanics. For example, spindle microtubules are organized by numerous cross-linking proteins yet the mechanical properties of those cross-links remain unexplored. To examine the mechanical properties of microtubule cross-links we applied optical trapping to mitotic asters that form in mammalian mitotic extracts. These asters are foci of microtubules, motors, and microtubule-associated proteins that reflect many of the functional properties of spindle poles and represent centrosome-independent spindle-pole analogs. We observed bidirectional motor-driven microtubule movements, showing that microtubule linkages within asters are remarkably compliant (mean stiffness 0.025 pN/nm) and mediated by only a handful of cross-links. Depleting the motor Eg5 reduced this stiffness, indicating that Eg5 contributes to the mechanical properties of microtubule asters in a manner consistent with its localization to spindle poles in cells. We propose that compliant linkages among microtubules provide a mechanical architecture capable of accommodating microtubule movements and distributing force among microtubules without loss of pole integrity—a mechanical paradigm that may be important throughout the spindle.

Functionally Unequal Centrosomes Drive Spindle Orientation in Asymmetrically Dividing Drosophila Neural Stem Cells

Stem cell asymmetric division requires tight control of spindle orientation. To study this key process, we have recorded Drosophila larval neural stem cells (NBs) engineered to express fluorescent reporters for microtubules, pericentriolar material (PCM), and centrioles. We have found that early in the cell cycle, the two centrosomes become unequal: one organizes an aster that stays near the apical cortex for most of the cell cycle, while the other loses PCM and microtubule-organizing activity, and moves extensively throughout the cell until shortly before mitosis when, located near the basal cortex, it recruits PCM and organizes the second mitotic aster. Upon division, the apical centrosome remains in the stem cell, while the other goes into the differentiating daughter. Apical aster maintenance requires the function of Pins. These results reveal that spindle orientation in Drosophila larval NBs is determined very early in the cell cycle, and is mediated by asymmetric centrosome function.

Mechanisms of Spindle-Pole Organization Are Influenced by Kinetochore Activity in Mammalian Cells

The spindle is a fusiform bipolar-microtubule array that is responsible for chromosome segregation during mitosis. Focused poles are an essential feature of spindles in vertebrate somatic cells, and pole focusing has been shown to occur through a centrosome-independent self-organization mechanism where microtubule motors cross-link and focus microtubule minus ends. Most of our understanding of this mechanism for pole focusing derives from studies performed in cell-free extracts devoid of centrosomes and kinetochores. Here, we examine how sustained force from kinetochores influences the mechanism of pole focusing in cultured cells. We show that the motor-driven self-organization activities associated with NuMA (i.e., cytoplasmic dynein) and HSET are not necessary for pole focusing if sustained force from kinetochores is inhibited in Nuf2- or Mis12-deficient cells. Instead, pole organization relies on TPX2 as it cross-links spindle microtubules to centrosome-associated mitotic asters. Thus, both motor-driven and static-cross-linking mechanisms contribute to spindle-pole organization, and kinetochore activity influences the mechanism of spindle-pole organization. The motor-driven self-organization of microtubule minus ends at spindle poles is needed to organize spindle poles in vertebrate somatic cells when kinetochores actively exert force on spindle microtubules.

An Essential Function of the C. elegans Ortholog of TPX2 Is to Localize Activated Aurora A Kinase to Mitotic Spindles

In vertebrates, the microtubule binding protein TPX2 is required for meiotic and mitotic spindle assembly. TPX2 is also known to bind to and activate Aurora A kinase and target it to the spindle. However, the relationship between the TPX2-Aurora A interaction and the role of TPX2 in spindle assembly is unclear. Here, we identify TPXL-1, a C. elegans protein that is the first characterized invertebrate ortholog of TPX2. We demonstrate that an essential role of TPXL-1 during mitosis is to activate and target Aurora A to microtubules. Our data suggest that this targeting stabilizes microtubules connecting kinetochores to the spindle poles. Thus, activation and targeting of Aurora A appears to be an ancient and conserved function of TPX2 that plays a central role in mitotic spindle assembly.

Overexpression of truncated γ-tubulins disrupts mitotic aster formation in Xenopus oocyte extracts

Mechanisms of spindle pole formation rely on minus-end-directed motor proteins. gamma-Tubulin is present at the centre of poles, but its function during pole formation is completely unknown. To address the role of gamma-tubulin in spindle pole formation, we overexpressed GFP (green fluorescent protein)-fused gamma-tubulin (gamma-Tu-GFP) in Xenopus oocytes and produced self-assembled mitotic asters in the oocyte extracts. gamma-Tu-GFP associated with endogenous alpha-, beta- and gamma-tubulin, suggesting that it acts in the same manner as that of endogenous gamma-tubulin. During the process of aster formation, gamma-Tu-GFP aggregated as dots on microtubules, and then the dots were translocated to the centre of the aster along microtubules in a manner dependent on cytoplasmic dynein activity. Inhibition of the function of gamma-tubulin by an anti-gamma-tubulin antibody resulted in failure of microtubule organization into asters. This defect was restored by overexpression of gamma-Tu-GFP, confirming the necessity of gamma-tubulin in microtubule recruitment for aster formation. We also examined the effects of truncated gamma-tubulin mutants, which are difficult to solubly express in other systems, on aster formation. The middle part of gamma-tubulin caused abnormal organization of microtubules in which minus ends of microtubules were not tethered, but dispersed. An N-terminus-deleted mutant prevented recruitment of microtubules into asters, similar to the effect of the anti-gamma-tubulin antibody. The results indicate possible roles of gamma-tubulin in spindle pole formation and show that the system developed in the present study could be useful for analysing roles of many proteins that are difficult to solubly express.

RGS14 is a Microtubule-Associated Protein

Heterotrimeric G-proteins and their regulators are emerging as important players in modulating microtubule polymerization dynamics and in spindle force generation during cell division in C. elegans, D. melanogaster, and mammals. We recently demonstrated that RGS14 is required for completion of the first mitotic division of the mouse embryo, and that it regulates microtubule organization in vivo. Here, we demonstrate that RGS14 is a microtubule associated protein and a component of the mitotic spindle that may regulate microtubule polymerization and spindle organization. Taxol-stabilized tubulin, but not depolymerized tubulin co-immunoprecipitates with RGS14 from cell extracts. Furthermore, RGS14 co-purifies with tubulin from porcine brain following multiple rounds of microtubule polymerization/depolymerization and binds directly to microtubules formed in vitro from pure tubulin (KD=1.3 +/- 0.3 ?M). Both RGS14 and G?i1 in the presence of exogenous GTP promote tubulin polymerization, which is dependent on additional microtubule associated proteins. However, preincubation of RGS14 with G?i1-GDP precludes either from promoting microtubule polymerization, suggesting that a functional GTP/GDP cycle is necessary. Finally, we show that RGS14 is a component of mitotic asters formed in vitro from HeLa cell extracts and that depletion of RGS14 from cell extracts blocks aster formation. Collectively, these results show that RGS14 is a microtubule associated protein that may modulate microtubule dynamics and spindle formation.

Mitotic Regulation of Protein 4.1R Involves Phosphorylation by cdc2 Kinase

The nonerythrocyte isoform of the cytoskeletal protein 4.1R (4.1R) is associated with morphologically dynamic structures during cell division and has been implicated in mitotic spindle function. In this study, we define important 4.1R isoforms expressed in interphase and mitotic cells by RT-PCR and mini-cDNA library construction. Moreover, we show that 4.1R is phosphorylated by p34cdc2 kinase on residues Thr60 and Ser679 in a mitosis-specific manner. Phosphorylated 4.1R135 isoform(s) associate with tubulin and Nuclear Mitotic Apparatus protein (NuMA) in intact HeLa cells in vivo as well as with the microtubule-associated proteins in mitotic asters assembled in vitro. Recombinant 4.1R135 is readily phosphorylated in mitotic extracts and reconstitutes mitotic aster assemblies in 4.1R-immunodepleted extracts in vitro. Furthermore, phosphorylation of these residues appears to be essential for the targeting of 4.1R to the spindle poles and for mitotic microtubule aster assembly in vitro. Phosphorylation of 4.1R also enhances its association with NuMA and tubulin. Finally, we used siRNA inhibition to deplete 4.1R from HeLa cells and provide the first direct genetic evidence that 4.1R is required to efficiently focus mitotic spindle poles. Thus, we suggest that 4.1R is a member of the suite of direct cdc2 substrates that are required for the establishment of a bipolar spindle.

Protein 4.1R, a Microtubule-associated Protein Involved in Microtubule Aster Assembly in Mammalian Mitotic Extract

Non-erythroid protein 4.1R (4.1R) consists of a complex family of isoforms. We have shown that 4.1R isoforms localize at the mitotic spindle/spindle poles and associate in a complex with the mitotic-spindle organization proteins Nuclear Mitotic Apparatus protein (NuMA), dynein, and dynactin. We addressed the mitotic function of 4.1R by investigating its association with microtubules, the main component of the mitotic spindles, and its role in mitotic aster assembly in vitro. 4.1R appears to partially co-localize with microtubules throughout the mitotic stages of the cell cycle. In vitro sedimentation assays showed that 4.1R isoforms directly interact with microtubules. Glutathione S-transferase (GST) pull-down assays using GST-4.1R fusions and mitotic cell extracts further showed that the association of 4.1R with tubulin results from both the membrane-binding domain and C-terminal domain of 4.1R. Moreover, 4.1R, but not actin, is a mitotic microtubule-associated protein; 4.1R associates with microtubules in the microtubule pellet of the mitotic asters assembled in mammalian cell-free mitotic extract. The organization of microtubules into asters depends on 4.1R in that immunodepletion of 4.1R from the extract resulted in randomly dispersed microtubules. Furthermore, adding a 135-kDa recombinant 4.1R reconstituted the mitotic asters. Finally, we demonstrated that a mitotic 4.1R isoform appears to form a complex in vivo with tubulin and NuMA in highly synchronized mitotic HeLa extracts. Our results suggest that a 135-kDa non-erythroid 4.1R is important to cell division, because it participates in the formation of mitotic spindles and spindle poles through its interaction with mitotic microtubules.

Ray Rappaport chronology: Twenty-five years of seminal papers on cytokinesis in the Journal of Experimental Zoology.

Between 1960 and 1993 Ray Rappaport (Fig. 1) published a series of seminal articles in the Journal of Experimental Zoology that defined the fundamental rules for how the mitotic apparatus determines the position of the cleavage furrow. This lasting achievement is recognized in every cell biology textbook and his story is told in every cell biology class. His legendary experiments remain fresh and his papers still make exciting reading. This appeal comes in part from Ray’s gift as a storyteller, but it’s mainly the events in these riveting stories that hold one’s attention. The work is so important that his strategies and ideas strongly influence current research on cell division. We still debate the meaning of his experiments. This remarkable accomplishment was achieved in a manner that has been all but lost in contemporary biological research. Ray did virtually all of his experiments with his own two hands during summers at the Mt. Desert Island Biological Laboratory in Salisbury Cove Maine. During the rest of the year he taught at Union College in Schenectady New York. The Mt. Desert Island Biological Laboratory is a rustic collection of wooden buildings, a world apart from the pretentious palaces designed by signature architects that house many biological research labs today. Ray’s equipment was as modest as the lab in Maine. It consisted of a Unitron inverted microscope, a deFonbrune microforge, a dissecting microscope, micromanipulators, a simple centrifuge, and various observation chambers that he fabricated himself from materials available at a hardware store. Ray used a rowboat to collect sand dollars from the local waters for his experiments. He never used an electron microscope or a fluorescence microscope. The work was done without large research grants, graduate students, postdoctoral fellows, or research assistants. A couple of undergraduates helped with two early papers. Ray’s wife Barbara was a constant source of support, helped with writing, and co-authored some later papers. Although Ray never worked with a gene, protein, or antibody, he defined the rules for how the mitotic apparatus stimulates the cortex to form a furrow. He showed that the asters of the mitotic spindle are the source of this positive signal (at least in echinoderm eggs), measured the rate that the stimulus moves from the mitotic asters to the cortex, identified the time required for the stimulus to make a lasting impression on the cortex, and discovered a latent period between receipt of the signal and the formation of a visible

The kinetically dominant assembly pathway for centrosomal asters in Caenorhabditis elegans is gamma-tubulin dependent.

γ-Tubulin–containing complexes are thought to nucleate and anchor centrosomal microtubules (MTs). Surprisingly, a recent study (Strome, S., J. Powers, M. Dunn, K. Reese, C.J. Malone, J. White, G. Seydoux, and W. Saxton. Mol. Biol. Cell. 12:1751–1764) showed that centrosomal asters form in Caenorhabditis elegans embryos depleted of γ-tubulin by RNA-mediated interference (RNAi). Here, we investigate the nucleation and organization of centrosomal MT asters in C. elegans embryos severely compromised for γ-tubulin function. We characterize embryos depleted of ∼98% centrosomal γ-tubulin by RNAi, embryos expressing a mutant form of γ-tubulin, and embryos depleted of a γ-tubulin–associated protein, CeGrip-1. In all cases, centrosomal asters fail to form during interphase but assemble as embryos enter mitosis. The formation of these mitotic asters does not require ZYG-9, a centrosomal MT-associated protein, or cytoplasmic dynein, a minus end–directed motor that contributes to self-organization of mitotic asters in other organisms. By kinetically monitoring MT regrowth from cold-treated mitotic centrosomes in vivo, we show that centrosomal nucleating activity is severely compromised by γ-tubulin depletion. Thus, although unknown mechanisms can support partial assembly of mitotic centrosomal asters, γ-tubulin is the kinetically dominant centrosomal MT nucleator.

Fumarylacetoacetate, the metabolite accumulating in hereditary tyrosinemia, activates the ERK pathway and induces mitotic abnormalities and genomic instability.

Patients suffering from the metabolic disease hereditary tyrosinemia type I (HT1), caused by fumarylacetoacetate hydrolase deficiency, have a high risk of developing liver cancer. We report that a sub-apoptogenic dose of fumarylacetoacetate (FAA), the mutagenic metabolite accumulating in HT1, induces spindle disturbances and segregational defects in both rodent and human cells. Mitotic abnormalities, such as distorted spindles, lagging chromosomes, anaphase/telophase chromatin bridges, aberrant karyokinesis/cytokinesis and multinucleation were observed. Some mitotic asters displayed a large pericentriolar material cloud and/or altered distribution of the spindle pole-associated protein NuMA. FAA-treated cells developed micronuclei which were predominantly CREST-positive, suggesting chromosomal instability. The Golgi complex was rapidly disrupted by FAA, without evident microtubules/tubulin alterations, and a sustained activation of the extracellular signal-regulated protein kinase (ERK) was also observed. Primary skin fibroblasts derived from HT1 patients, not exogenously treated with FAA, showed similar mitotic-derived alterations and ERK activation. Biochemical data suggest that FAA causes ERK activation through a thiol-regulated and tyrosine kinase-dependent, but growth factor receptor- and protein kinase C-independent pathway. Pre-treatment with the MEK inhibitor PD98059 and the Ras farnesylation inhibitor B581 decreased the formation of CREST-positive micronuclei by approximately 75%, confirming the partial contribution of the Ras/ERK effector pathway to the induction of chromosomal instability by FAA. Replenishment of intracellular glutathione (GSH) with GSH monoethylester abolished ERK activation and reduced the chromosomal instability induced by FAA by 80%. Together these results confirm and extend the previously reported genetic instability occurring in cells from HT1 patients and allow us to speculate that this tumorigenic-related phenomenon may rely on the biochemical/cellular effects of FAA as a thiol-reacting and organelle/mitotic spindle-disturbing agent.

Methods for the study of centrosome-independent spindle assembly in Xenopus extracts

Publisher Summary This chapter discusses methods for the study of both centrosome-mediated and centrosome-independent spindle assembly in Xenopus egg extracts. The chapter describes simple assays for microtubule polymerization and organization, through the addition of dimethyl sulfoxide (DMSO) or purified centrosomes to form mitotic asters. The protocols described include microtubule aster reactions, bead spindle reactions, and inhibition of extract proteins. This system is ideal for studying mechanisms of self-organization, including microtubule nucleation around chromatin and motor-dependent function. The value of these methods extends beyond the study of microtubule self-organization, however. Both centrosome-mediated and centrosome-independent spindle assembly share mechanisms in common. Although centrosomes constitute visible and dominant organizing centers when they are present, motor functions are superimposed and are crucial for spindle organization, as they are when centrosomes are absent. Therefore, the study of both types of spindle assembly is important to understand the principles of microtubule organization in mitosis and meiosis.

Cytoskeletal mechanisms of ooplasmic segregation in annelid eggs.

Annelid embryos are comprised of yolk-deficient animal and yolk-filled vegetal blastomeres. This "unipolar" organization along the animal-vegetal axis (in terms of ooplasmic distribution) is generated via selective segregation of yolk-free, clear cytoplasm to the animal blastomeres. The pathway that leads to the unipolar organization is different between polychaetes and clitellates (i.e., oligochaetes and hirudinidans). In polychaetes, the clear cytoplasm domain, which is established through ooplasmic segregation at the animal side of the egg, is simply cut up by unequal equatorial cleavage. In clitellates, localization of clear cytoplasm to animal blastomeres is preceded by unification of the initially separated polar domains of clear cytoplasm, which result from bipolar ooplasmic segregation. In this article, I have reviewed recent studies on cytoskeletal mechanisms for ooplasmic localization during early annelid development. Annelid eggs accomplish ooplasmic rearrangements through various combinations of three cytoskeletal mechanisms, which are mediated by actin microfilaments, microtubules and mitotic asters, respectively. One of the unique features of annelid eggs isthat a homologous process is driven by distinct cytoskeletal elements. Annelid eggs may provide an intriguing system to investigate not only mechanical aspects of ooplasmic segregation but also evolutionary divergence of cytoskeletal mechanisms that operate in a homologous process.

Basal bodies and centrioles: Their function and structure

Publisher Summary The centriole is a darkly staining dot at the center of each mitotic aster and near the nucleus in interphase cells. The staining is observed at the center of the differentiated cytoplasm termed the “centrosome.” This chapter discusses the structure and function of centrioles and basal bodies. Centrioles can interconvert to basal bodies, which are needed for the assembly of cilia or flagella. Several examples demonstrate that basal bodies and centrioles are interchangeable. Centrioles play an essential role in mitosis. The structure of centrioles in various cell types and organelles is discussed. The structure of basal bodies is similar to that of centrioles. Both structures are highly conserved in a wide range of organisms. Basal bodies and centrioles have several appendages that are associated with the microtubules. The basal body undergoes a transition to double microtubules. Several microtubule structures are associated with basal bodies. The structure of Chlamydomonas is described, along with the role of basal bodies in flagellar assembly and the organization of interphase Chlamydomonas cells. Some other associated structures discussed are centrin, assemblin, Bap 95, and Bap90. The function of basal bodies and centrioles revealed by mutational analysis is also described.