1
|
Yoshida N, Arai A, Aoki M, Moriya M, Sekiguchi K, Shimizu T. The cleavage program in the 2d cell lineage of Tubifex embryos. J Morphol 2019; 280:568-586. [PMID: 30762252 DOI: 10.1002/jmor.20966] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2018] [Revised: 01/27/2019] [Accepted: 01/30/2019] [Indexed: 11/07/2022]
Abstract
Early development in clitellate annelids is characterized by a highly stereotyped sequence of unequal, spiral cleavages. Cell 2d (i.e., the second micromere of the D quadrant) in the oligochaete Tubifex tubifex also undergoes an evolutionarily conserved sequence of cell division to produce four bilateral pairs of ectodermal teloblasts that act as embryonic stem cells. This study was conducted to characterize each of the 15 rounds of cell division that occur in the 2d cell lineage in this clitellate. After its occurrence, cell 2d undergoes three rounds of highly unequal divisions, giving off the first smaller daughter cell toward the posterior right of the larger daughter cell, the second cell toward the posterior left, and the third cell toward the anterior side of the cell; the larger daughter cell that results from the third division (i.e., the great-granddaughter cell of 2d) then divides equally into a bilateral pair of NOPQ proteloblasts. Cell NOPQ on either side of the embryo undergoes 11 rounds of cell division, during which ectoteloblasts N, Q, and O/P are produced in this order. After its appearance, NOPQ undergoes highly unequal divisions twice cutting off the smaller cells toward the anterior end of the embryo and then divides almost equally into ectoteloblast N and proteloblast OPQ. After its appearance, OPQ undergoes highly unequal divisions twice giving off the first smaller cell toward the anterior and the second smaller cell toward the posterior of the embryo and then divides almost equally into ectoteloblast Q and proteloblast OP. Finally, OP undergoes highly unequal division four times after its birth budding off the smaller cells toward the anterior and then cleaves equally into ectoteloblasts O and P. In the unequally dividing cells of the 2d cell lineage, the mitotic apparatus (MA), which forms at the cell's center, moves eccentrically toward the cortical site where the smaller cell will be given off. The moving MA is oriented perpendicular to the surface it approaches, and its peripheral pole becomes closely associated with the cell cortex. In contrast, the MA involved in the equal divisions remains in the cell center throughout mitosis. The key features of the cleavage program in the 2d cell lineage are discussed in light of the present observations. The mechanical aspects of unequal cleavage in the 2d cell lineage and the modes of specification of MA orientation are discussed. A comparison of the cleavage mode in the 2d cell lineage is also performed among six selected clitellate annelid species.
Collapse
Affiliation(s)
- Noriyuki Yoshida
- Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan
| | - Asuna Arai
- Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan
| | - Momoe Aoki
- Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan
| | - Miho Moriya
- Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan
| | - Kaho Sekiguchi
- Division of Life Sciences, Graduate School of Life Science, Hokkaido University, Sapporo, Japan
| | - Takashi Shimizu
- Department of Biological Sciences, Faculty of Science, Hokkaido University, Sapporo, Japan
| |
Collapse
|
2
|
Nakama AB, Chou HC, Schneider SQ. The asymmetric cell division machinery in the spiral-cleaving egg and embryo of the marine annelid Platynereis dumerilii. BMC DEVELOPMENTAL BIOLOGY 2017; 17:16. [PMID: 29228898 PMCID: PMC5725810 DOI: 10.1186/s12861-017-0158-9] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 05/10/2017] [Accepted: 11/23/2017] [Indexed: 12/29/2022]
Abstract
BACKGROUND Over one third of all animal phyla utilize a mode of early embryogenesis called 'spiral cleavage' to divide the fertilized egg into embryonic cells with different cell fates. This mode is characterized by a series of invariant, stereotypic, asymmetric cell divisions (ACDs) that generates cells of different size and defined position within the early embryo. Astonishingly, very little is known about the underlying molecular machinery to orchestrate these ACDs in spiral-cleaving embryos. Here we identify, for the first time, cohorts of factors that may contribute to early embryonic ACDs in a spiralian embryo. RESULTS To do so we analyzed stage-specific transcriptome data in eggs and early embryos of the spiralian annelid Platynereis dumerilii for the expression of over 50 candidate genes that are involved in (1) establishing cortical domains such as the partitioning defective (par) genes, (2) directing spindle orientation, (3) conveying polarity cues including crumbs and scribble, and (4) maintaining cell-cell adhesion between embryonic cells. In general, each of these cohorts of genes are co-expressed exhibiting high levels of transcripts in the oocyte and fertilized single-celled embryo, with progressively lower levels at later stages. Interestingly, a small number of key factors within each ACD module show different expression profiles with increased early zygotic expression suggesting distinct regulatory functions. In addition, our analysis discovered several highly co-expressed genes that have been associated with specialized neural cell-cell recognition functions in the nervous system. The high maternal contribution of these 'neural' adhesion complexes indicates novel general adhesion functions during early embryogenesis. CONCLUSIONS Spiralian embryos are champions of ACD generating embryonic cells of different size with astonishing accuracy. Our results suggest that the molecular machinery for ACD is already stored as maternal transcripts in the oocyte. Thus, the spiralian egg can be viewed as a totipotent yet highly specialized cell that evolved to execute fast and precise ACDs during spiral cleaving stages. Our survey identifies cohorts of factors in P. dumerilii that are candidates for these molecular mechanisms and their regulation, and sets the stage for a functional dissection of ACD in a spiral-cleaving embryo.
Collapse
Affiliation(s)
- Aron B. Nakama
- Department of Genetics, Development and Cell Biology, Iowa State University, 503 Science Hall II, Ames, IA 50011 USA
| | - Hsien-Chao Chou
- Department of Genetics, Development and Cell Biology, Iowa State University, 503 Science Hall II, Ames, IA 50011 USA
- current address: Center for Cancer Research, National Institutes of Health, Bethesda, MD 20894 USA
| | - Stephan Q. Schneider
- Department of Genetics, Development and Cell Biology, Iowa State University, 503 Science Hall II, Ames, IA 50011 USA
| |
Collapse
|
3
|
Abstract
Early embryos of the clitellate annelid Tubifex (oligochaete) undergo a series of unequal spiral cell divisions before the descendants of the D quadrant micromeres (cells 2d and 4d) divide bilaterally. Here, we show that inhibition of zygotic transcription by microinjection of α-amanitin (transcription inhibitor) exclusively converts unequal cleavage in cell 2d11 (granddaughter of 2d) into equal cleavage while other unequal cleavages and ensuing bilateral cleavages in cells 4d and 2d111 (great-granddaughter of 2d) all proceed in a normal fashion in the presence of this inhibitor. These results differ significantly from those reported for embryos of another clitellate annelid Helobdella (leech), in which inhibition of transcription converts bilateral (symmetric) cleavages in cells DNOPQ"' and DM" (equivalent to 2d111 and 4d) into unequal (asymmetric) cleavages while having no apparent effect on unequal cleavage in DNOPQ" (equivalent to 2d11). These differences imply distinct mechanisms for the control of the unequal-to-bilateral transition in the two clitellate annelids.
Collapse
Affiliation(s)
- Momoe Aoki
- Division of Biological Sciences, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan
| | - Takashi Shimizu
- Division of Biological Sciences, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan.
| |
Collapse
|
4
|
Powers KG, Blackburn DG. Morphological specializations of the yolk sac for yolk processing in embryonic corn snakes (Pantherophis guttatus: Colubridae). J Morphol 2017; 278:768-779. [DOI: 10.1002/jmor.20671] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2017] [Revised: 02/11/2017] [Accepted: 02/20/2017] [Indexed: 11/12/2022]
Affiliation(s)
- Kathryn G. Powers
- Department of Biology, and Electron Microscopy Center; Trinity College; Hartford Connecticut 06106
| | - Daniel G. Blackburn
- Department of Biology, and Electron Microscopy Center; Trinity College; Hartford Connecticut 06106
| |
Collapse
|
5
|
Lyons DC, Weisblat DA. D quadrant specification in the leech Helobdella: actomyosin contractility controls the unequal cleavage of the CD blastomere. Dev Biol 2009; 334:46-58. [PMID: 19607823 PMCID: PMC3077801 DOI: 10.1016/j.ydbio.2009.07.007] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/28/2009] [Revised: 06/27/2009] [Accepted: 07/04/2009] [Indexed: 10/20/2022]
Abstract
The unequal division of the CD blastomere at second cleavage is critical in establishing the second embryonic axis in the leech Helobdella, as in other unequally cleaving spiralians. When CD divides, the larger D and smaller C blastomeres arise invariantly on the left and right sides of the embryo, respectively. Here we show that stereotyped cellular dynamics, including the formation of an intercellular blastocoel, culminate in a morphological left-right asymmetry in the 2-cell embryo, which precedes cytokinesis and predicts the chirality of the second cleavage. In contrast to the unequal first cleavage, the unequal second cleavage does not result from down-regulation of one centrosome, nor from an asymmetry within the spindle itself. Instead, the unequal cleavage of the CD cell entails a symmetric mitotic apparatus moving and anisotropically growing rightward in an actomyosin-dependent process. Our data reveal that mechanisms controlling the establishment of the D quadrant differ fundamentally even among the monophyletic clitellate annelids. Thus, while the homologous spiral cleavage pattern is highly conserved in this clade, it has diverged significantly at the level of cell biological mechanisms. This combination of operational conservation and mechanistic divergence begins to explain how the spiral cleavage program has remained so refractory to change while, paradoxically, accommodating numerous modifications throughout evolution.
Collapse
Affiliation(s)
- Deirdre C. Lyons
- 385 Life Sciences Addition, Department of Molecular & Cell Biology, U.C. Berkeley, Berkeley CA 94720, 510 642 2697
| | - David A. Weisblat
- 385 Life Sciences Addition, Department of Molecular & Cell Biology, U.C. Berkeley, Berkeley CA 94720, 510 642 8309
| |
Collapse
|
6
|
Ren X, Weisblat DA. Asymmetrization of first cleavage by transient disassembly of one spindle pole aster in the leech Helobdella robusta. Dev Biol 2006; 292:103-15. [PMID: 16458880 DOI: 10.1016/j.ydbio.2005.12.049] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2005] [Revised: 11/15/2005] [Accepted: 12/21/2005] [Indexed: 01/10/2023]
Abstract
Unequal first cleavage is characteristic of a diverse group of protostome animals. In the nematode Caenorhabditis elegans, unequal first cleavage is achieved through the interaction of an apparently symmetric mitotic spindle apparatus with a clearly polarized cell cortex. In the clitellate annelid Tubifex tubifex, by contrast, the spindle is monastral and contains only one gamma-tubulin-reactive centrosome; this monastral spindle is inherently asymmetric throughout mitosis. Here, we have used immunostaining for beta- and gamma-tubulin to follow spindle dynamics during the unequal first cleavage in another clitellate annelid, the leech Helobdella robusta. We find that the mitotic spindle is diastral and symmetric through early metaphase, then becomes asymmetric following the transient down-regulation of one centrosome, as judged by gamma-tubulin immunofluorescence. Low levels of drugs that affect microtubule dynamics can symmetrize the first cleavage without affecting the gamma-tubulin dynamics. Our results provide a striking example of the evolvability of cellular mechanisms underlying an unambiguously homologous developmental process.
Collapse
Affiliation(s)
- Xiaoyun Ren
- Department of Molecular and Cell Biology, 385 LSA, University of California, Berkeley, CA 94720-3200, USA
| | | |
Collapse
|
7
|
McCarthy EK, Goldstein B. Asymmetric spindle positioning. Curr Opin Cell Biol 2006; 18:79-85. [PMID: 16361093 PMCID: PMC2186777 DOI: 10.1016/j.ceb.2005.12.006] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/09/2005] [Accepted: 12/01/2005] [Indexed: 12/16/2022]
Abstract
When a spindle is positioned asymmetrically in a dividing cell, the resulting daughter cells are unequal in size. Asymmetric spindle positioning is driven by regulated forces that can pull or push a spindle. The physical and molecular mechanisms that can position spindles asymmetrically have been studied in several systems, and some themes have begun to emerge from recent research. Recent work in budding yeast has presented a model for how cytoskeletal motors and cortical capture molecules can function in orienting and positioning a spindle. The temporal regulation of microtubule-based pulling forces that move a spindle has been examined in one animal system. Although the spindle positioning force generators have not been identified in most animal systems, the forces have been found to be regulated by both PAR polarity proteins and G-protein signaling pathways in more than one animal system.
Collapse
Affiliation(s)
- Erin K McCarthy
- Biology Department, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3280, USA
| | | |
Collapse
|
8
|
Yoshigaki T. Simulation of the mechanism of determining the position of the cleavage furrow in cytokinesis of sea urchin eggs. Math Biosci 2001; 170:17-58. [PMID: 11259802 DOI: 10.1016/s0025-5564(00)00066-3] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
In cytokinesis of sea urchin eggs, the numerical density of astral microtubules extending close to the cell surface has been thought to determine the position of the cleavage furrow. In the present study, a new model was constructed to simulate the relationship between the microtubule density and the furrow formation. In the model, gradients of the microtubule density drive fluid membrane proteins whose accumulation triggers the formation of contractile-ring microfilaments. The model could explain the behavior of the cleavage furrow under various experimental conditions. These simulations revealed two aspects of furrow formation. One is that in some cases, the cleavage furrow appears in a surface region where the microtubule density has neither a minimum nor a maximum. In all furrow regions, however, the second derivative of the microtubule-density function has large positive values. Membrane proteins greatly slow down to accumulate in such a region. The other is that the cleavage furrow is mobile, not fixed in one position, because of the fluidity of membrane proteins. These results strongly suggested that the mitotic apparatus determines the position of the cleavage furrow by redistributing membrane proteins through gradients of the microtubule density at the cell surface.
Collapse
Affiliation(s)
- T Yoshigaki
- 4-4-16-305 Izumi-chou, Nishi Tokyo, 202-0011, Tokyo, Japan.
| |
Collapse
|
9
|
Abstract
Taxol enhanced assembly and stability of microtubules in the mitotic apparatus and subsequently inhibited chromosome movement and cleavage when injected in the sea urchin egg as reported previously [Y. Hamaguchi et al., 1987: Cell Struct. Funct. 12:43-52]. In this study, to examine the local effect of taxol on cleavage, taxol was injected in small doses. When taxol was injected into the cortical region of the equatorial plane, the birefringence (BR) of the mitotic apparatus near the injection site increased, chromosome movement became slow near this site, and then cleavage furrow formation was inhibited in the cortex near the site, although chromosome movement and cleavage furrow formation were apparently normal in the other side of the cell. When taxol was injected in the polar cortex, BR of the mitotic apparatus near the injection site was enhanced and then the cleavage furrow was displaced toward this site from the equator. Accordingly, resultant blastomeres were unequal in size. The enhancement of microtubule assembly and stabilization of microtubule dynamics, which were detected as the increase in BR were confirmed by immunofluorescence microscopy with anti-tubulin antibody. Consequently, it was found that taxol injection caused local suppression of dynamics of microtubules in the cell, thereby modifying cleavage furrow formation.
Collapse
Affiliation(s)
- Y Hamaguchi
- Biological Laboratory, Faculty of Bioscience and Biotechnology, Tokyo Institute of Technology, Japan.
| |
Collapse
|
10
|
Weisblat DA, Huang FZ, Isaksen DE, Liu NJ, Chang P. The other side of the embryo: an appreciation of the non-D quadrants in leech embryos. Curr Top Dev Biol 1999; 46:105-32. [PMID: 10417878 DOI: 10.1016/s0070-2153(08)60327-0] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/13/2023]
Affiliation(s)
- D A Weisblat
- Department of Molecular and Cell Biology, University of California, Berkeley 94720, USA
| | | | | | | | | |
Collapse
|
11
|
Nishida H, Morokuma J, Nishikata T. Maternal cytoplasmic factors for generation of unique cleavage patterns in animal embryos. Curr Top Dev Biol 1999; 46:1-37. [PMID: 10417875 DOI: 10.1016/s0070-2153(08)60324-5] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Affiliation(s)
- H Nishida
- Department of Life Science, Tokyo Institute of Technology, Yokohama, Japan
| | | | | |
Collapse
|
12
|
Nishikata T, Hibino T, Nishida H. The centrosome-attracting body, microtubule system, and posterior egg cytoplasm are involved in positioning of cleavage planes in the ascidian embryo. Dev Biol 1999; 209:72-85. [PMID: 10208744 DOI: 10.1006/dbio.1999.9244] [Citation(s) in RCA: 90] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Many kinds of animal embryos exhibit stereotyped cleavage patterns during early embryogenesis. In the ascidian Halocynthia roretzi, cleavage patterns are invariant but they are complicated by successive unequal cleavages that occur in the posterior region. Here we report the essential roles of a novel structure, called the centrosome-attracting body (CAB), which exists in the posterior pole cortex of cleaving embryos, in generating unequal cleavages. By removing and transplanting posterior egg cytoplasm and by treatment with sodium dodecyl sulfate, we demonstrated that loss of the CAB resulted in abolishment of unequal cleavage, while ectopic formation of the CAB caused ectopic unequal cleavages to occur. Experiments with a microtubule inhibitor demonstrated that the centrosome and nucleus were attracted toward the posterior cortex, where the CAB is located, by shortening of microtubule bundles formed between the centrosome and the CAB. Consequently, the mitotic apparatus was positioned asymmetrically, resulting in unequal cleavage. Immunohistochemistry provided evidence that a microtubule motor protein, a kinesin or kinesin-like molecule, may be associated with the CAB. Formation of the CAB during the early cleavage stage was resistant to treatment with the microtubule inhibitor. In contrast, the integrity of the CAB was lost upon treatment with a microfilament inhibitor. We propose that the CAB plays key roles in the orientation and positioning of cleavage planes during unequal cell division.
Collapse
Affiliation(s)
- T Nishikata
- Faculty of Science, Konan University, Kobe, 658-8501, Japan
| | | | | |
Collapse
|
13
|
Abstract
Unequal cleavage that produces two blastomeres of different size is a cleavage pattern that many animals in a variety of phyla, particularly in Spiralia, adopt during early development. This cleavage pattern is apparently instrumental for asymmetric segregation of developmental potential, but it is also indispensable for normal embryogenesis in many animals. Mechanically, unequal cleavage is achieved by either simple unequal cytokinesis or by forming a polar lobe at the egg's vegetal pole. In the present paper, the mechanisms for unequal cytokinesis involved in the first three cleavages in the oligochaete annelid Tubifex are reviewed. The three unequal cleavages are all brought about by an asymmetrically organized mitotic apparatus (MA). The MA of the first cleavage is monastral in that an aster is present at one pole of a bipolar spindle but not at the other. This monastra form, which arises as a result of the involvement of a single centrosome in the MA assembly, is both necessary and sufficient for unequal first cleavage. The egg cortex during the first mitosis is devoid of the ability to remodel spindle poles. In contrast to the non-cortical mechanisms for the first cleavage, asymmetry in the MA organization at the second and third cleavages depends solely on specialized properties of the cell cortex, to which one spindle pole is physically connected. A cortical attachment site for the second cleavage spindle is generated de novo at the cleavage membrane resulting from the first cleavage; it is an actin-based, cell contact-dependent structure. The cortical microtubule attachment site for the third cleavage, which functions independently of contact with other cells, is not generated at the cleavage membrane resulting from the second cleavage, but is located at the animal pole; it may originate from the second polar body formation and become functional at the 4-cell stage.
Collapse
Affiliation(s)
- T Shimizu
- Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan
| | | | | |
Collapse
|
14
|
Shimizu T. Separation of microvilli from Tubifex eggs upon activation: its inhibition by concanavalin A hinders ooplasmic segregation and cleavage. Dev Growth Differ 1997; 39:787-94. [PMID: 9493838 DOI: 10.1046/j.1440-169x.1997.t01-3-00014.x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
The surface of mature eggs of the freshwater oligochaete Tubifex exhibits numerous microvilli. Upon activation, microvilli become narrower at their base and separated from the ooplasmic surface. Here it is shown that concanavalin A (Con A) reversibly inhibits the separation of microvilli from activated Tubifex eggs. The Con A-treated eggs undergo meioses and mitoses at a normal rate. Microvilli on these eggs change their length in a meiotic cycle-dependent manner; their core bundles of microfilaments elongate significantly during the second meiosis. The Con A-treated eggs fail to complete polar body formation, ooplasmic segregation and cleavages. Treatment with Con A of eggs that have accomplished microvillar separation does not exert any inhibitory effect on their development. Succinyl-Con A, a dimeric derivative of Con A, does not prevent microvillar separation, suggesting that the tetravalent form of Con A is essential for Con A to exert its inhibitory effect on microvillar separation.
Collapse
Affiliation(s)
- T Shimizu
- Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan
| |
Collapse
|
15
|
Ishii R, Shimizu T. Reproductive Capability of Maternal Centrosomes in the Tubifex Egg. Zoolog Sci 1997. [DOI: 10.2108/zsj.14.961] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
|
16
|
Ishii R, Shimizu T. Equalization of unequal first cleavage in the Tubifex egg by introduction of an additional centrosome: implications for the absence of cortical mechanisms for mitotic spindle asymmetry. Dev Biol 1997; 189:49-56. [PMID: 9281336 DOI: 10.1006/dbio.1997.8653] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
The first cleavage in the Tubifex egg is unequal and involves a monastral mitotic apparatus (MA), which remains at the egg's center from metaphase through telophase. The monastral form of the MA is thought to arise due to the involvement of a single maternal centrosome in the MA assembly (R. Ishii and T. Shimizu, Dev. Growth Differ. 37, 687-701, 1995). To investigate the mechanisms that generate asymmetry in this division, we have examined the cleavage patterns in eggs that are manipulated to inherit two centrosomes during the first mitosis. When eggs are prevented from extruding polar bodies, centrosomes located at both poles of the first meiotic spindle persist into the first mitosis without showing any sign of duplication and generate astral spindle poles. Eggs that inherit these centrosomes exhibit two types of MA configurations, viz., an amphiastral MA and twin monastral MAs. We found that eggs with amphiastral MAs undergo bipolar equal divisions, while those with twin MAs divide into three cells simultaneously (tripolar divisions). The amphiastral MAs are located at the egg's center and their astral poles are organized symmetrically, suggesting that Tubifex eggs are unable to generate asymmetry in the amphiastral MAs during the first mitosis. These results suggest that inheritance of a single centrosome during the first mitosis is critical for Tubifex eggs to undergo unequal cleavage. We propose that the cortical mechanisms for MA asymmetry are lacking in the Tubifex egg during the first mitosis.
Collapse
Affiliation(s)
- R Ishii
- Graduate School of Science, Hokkaido University, Sapporo, 060, Japan
| | | |
Collapse
|
17
|
Shimizu T. Reorganization of the cortical actin cytoskeleton during maturation division in the Tubifex egg: possible involvement of protein kinase C. Dev Biol 1997; 188:110-21. [PMID: 9245516 DOI: 10.1006/dbio.1997.8606] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Tubifex eggs undergo a drastic reorganization of the cortical actin cytoskeleton during metaphase of the second meiosis. At the end of the first meiosis, the egg cortex displays only scattered actin filaments and tiny dots of F-actin; during the following 90 min, cortical F-actin gradually increases in amount, becomes organized into foci that are interlinked by actin bundles, and generates a geodesic dome-like organization. In this study, we have characterized this reorganization of the cortical actin cytoskeleton. In living eggs injected with rhodamine-phalloidin at the beginning of the second meiosis, cortical actin assembly (i.e., formation of actin foci and bundles) proceeds normally, but labeled F-actin is not found to be included significantly in the formed cortical actin network, suggesting that the increase in cortical F-actin is not simply ascribable to the recruitment of preexisting actin filaments. Cortical actin assembly can be induced precociously not only by calcium ionophore A23187 but also by a phorbol ester PMA, an agonist of protein kinase C (PKC). Conversely, the formation of actin foci and bundles is inhibited by PKC antagonists, although cortical F-actin increases to some extent in the presence of these inhibitors. Similar inhibition of the cortical reorganization is elicited in eggs whose intracellular free calcium level ([Ca2+]i) has been clamped low by microinjection of a calcium chelator BAPTA. The treatment of BAPTA-injected eggs with PMA results in the formation of actin foci and bundles. An experiment with eggs injected with fluo-3 shows that [Ca2+]i increases during metaphase of the second meiosis. These results suggest that the reorganization of cortical actin during metaphase of the second meiosis requires activation of PKC, which depends on increases in [Ca2+]i.
Collapse
Affiliation(s)
- T Shimizu
- Graduate School of Science, Hokkaido University, Sapporo, 060, Japan
| |
Collapse
|
18
|
Takahashi H, Shimizu T. Role of intercellular contacts in generating an asymmetric mitotic apparatus in the Tubifex embryo. Dev Growth Differ 1997; 39:351-62. [PMID: 9227902 DOI: 10.1046/j.1440-169x.1997.t01-2-00011.x] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
The 2-cell stage embryo of Tubifex is composed of a smaller cell, AB, and a larger cell, CD. At the second cleavage, the CD-cell divides unequally. The mitotic apparatus (MA) involved in this division is organized asymmetrically: the MA pole to be segregated to a smaller cell is flattened and truncated, and associated with the anterior cortex facing the AB-cell, while the other pole is symmetric and located more centrally. The present study was undertaken to elucidate the mechanism that generates asymmetry in the MA organization in CD-cells. When CD-cell nuclei, which are normally located near the anterior cortex, were displaced toward the posterior end of the cell (i.e. opposite AB-cells) by centrifugation, MA assembled ectopically there, and were bilaterally symmetric in organization. Similar symmetric MA were formed in isolated CD-cells, which divided more equally than intact cells. This equality of cell division was dramatically reduced if the anterior surface of isolated CD-cells formed contact with other cells, such as AB-, C- and 4D-cells. The MA that formed in these reconstituted embryos were asymmetric in organization; one MA pole was always found to be truncated and apposed to the cortical site at the cell contact. Symmetric MA were also observed in cytochalasin-treated embryos. Together with the finding that one of the MA poles is physically attached to the anterior cortex of the intact CD-cell, these results suggest that factors generating asymmetry in the spatial organization of MA poles reside at the anterior cortex of the CD-cell and that this cortical mechanism is dependent upon cell contacts.
Collapse
Affiliation(s)
- H Takahashi
- Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan
| | | |
Collapse
|
19
|
Behaviour of centrosomes in early Tubifex embryos: asymmetric segregation and mitotic cycle-dependent duplication. ACTA ACUST UNITED AC 1996; 205:290-299. [PMID: 28306032 DOI: 10.1007/bf00365807] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/17/1995] [Accepted: 09/21/1995] [Indexed: 10/26/2022]
Abstract
An antibody raised against a highly conserved peptide of γ-tubulin (Joshi et al. 1992) recognized a 50 kDa polypeptide in centrosomes in Tubifex embryos. Centrosomes labelled with this antibody are found at both poles of the first meiotic spindle and at the inner pole of the second meiotic spindle. At the transition to the second meiosis, there is no change in morphology of the centrosomes which are retained in the egg proper. In contrast, as the second meiosis proceeds from anaphase to telophase, centrosomes labelled with the antibody gradually become smaller, but are still recognized as tiny dots; each egg exhibits no more than one tiny dot. The first cleavage spindles exhibit a centrosome at one pole but not at the other. The spindle pole with a centrosome forms an aster which is inherited by the larger cell, CD, of the two-cell embryo; the centrosome-free spindle pole then becomes anastral and is segregated to a smaller cell AB. Centrosomes are present in the C and D cell lineages but not in the A and B lineages, at least up to the eighth cleavage cycle. During cleavage stages, centrosomes duplicate early in telophase of each mitosis, and their size changes in a cell cycle-specific fashion. Centrosomes which otherwise duplicate asynchronously in separate cells do so synchronously in a common cytoplasm. Centrosome duplication is inhibited by nocodazole but not by cytochalasin D. An examination of embryos treated with cycloheximide or aphidicolin also suggests that centrosome duplication during cleavages requires protein synthesis but no DNA replication per se. These results suggest that the centrosome cycle in Tubifex blastomeres is linked to the mitotic cycle more closely than is that in other animals.
Collapse
|
20
|
Shimizu T. Role of the cytoskeleton in the generation of spatial patterns in Tubifex eggs. Curr Top Dev Biol 1996; 31:197-235. [PMID: 8746666 DOI: 10.1016/s0070-2153(08)60228-8] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Affiliation(s)
- T Shimizu
- Division of Biological Sciences, Hokkaido University, Sapporo, Japan
| |
Collapse
|