Polar bodies – More a lack of understanding than a lack of respect

Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island, USA.
Molecular Reproduction and Development (Impact Factor: 2.53). 01/2011; 78(1):3-8. DOI: 10.1002/mrd.21266
Source: PubMed


Polar bodies are as diverse as the organisms that produce them. Although in many animals these cells often die following meiotic maturation of the oocyte, in other organisms they are an essential and diverse part of embryonic development. Here we highlight some of this diversity and summarize the evolutionary basis for their utility.

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    • "Both types of study are limited by the need to identify cell types by physical size when the cells involved are actively dividing and provide only snapshots of dynamic processes. Studies across animal species indicate a diversity of roles and fates for polar bodies (Schmerler and Wessel 2011). By using polar body counts to infer meiotic processes for the Myxozoa, assumptions have been made regarding their degradation rates, possible reassimilation , fusion, generation and function that may not be warranted. "

    Myxozoan Evolution, Ecology and Development, Edited by B. Okamura, A. Gruhl, J. Bartholomew, 01/2015: chapter Cellular Processes in Myxozoans: pages 139-154; Springer International Publishing Switzerland 2015., ISBN: 978-3-319-14752-9
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    • "The larger ovum contributes a haploid genome and most cytoplasmic components to the fertilized egg and developing organism. Despite their small size and minimal cytoplasmic content, polar bodies in some organisms are essential and contribute to embryonic development [Schmerler and Wessel, 2011]. However, in most species, they do not go on to serve any particular function and in fact degenerate, but their creation is critical for reduction of ploidy of the ovum. "
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    ABSTRACT: Polar body cytokinesis is the physical separation of a small polar body from a larger oocyte or ovum. This maternal meiotic division shares many similarities with mitotic and spermatogenic cytokinesis, but there are several distinctions, which will be discussed in this review. We synthesize results from many different model species, including those popular for their genetics and several that are more obscure in modern cell biology. The site of polar body division is determined before anaphase, by the eccentric, cortically associated meiotic spindle. Depending on the species, either the actin or microtubule cytoskeleton is required for spindle anchoring. Chromatin is necessary and sufficient to elicit differentiation of the associated cortex, via Ran-based signaling. The midzone of the anaphase spindle serves as a hub for regulatory complexes that elicit Rho activation, and ultimately actomyosin contractile ring assembly and contraction. Polar body cytokinesis uniquely requires another Rho family GTPase, Cdc42, for dynamic reorganization of the polar cortex. This is perhaps due to the considerable asymmetry of this division, wherein the polar body and the oocyte/ovum have distinct fates and very different sizes. Thus, maternal meiotic cytokinesis appears to occur via simultaneous polar relaxation and equatorial contraction, since the polar body is extruded from the spherical oocyte through the nascent contractile ring. As such, polar body cytokinesis is an interesting and important variation on the theme of cell division. © 2012 Wiley Periodicals, Inc.
    Cytoskeleton 11/2012; 69(11). DOI:10.1002/cm.21064 · 3.12 Impact Factor
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    • "This also makes the oocyte's cell divisions an interesting model for asymmetric cell divisions in general, relevant to numerous developmental and differentiation events (Grill 2010; Gönczy 2008). While the fates of the polar bodies differ among species (Schmerler and Wessel 2011), a conserved aspect of polar body function is the elimination of one set of chromosomes, leaving the haploid maternal genome component in the oocyte to merge with the haploid paternal contribution. These phenomena are crucial for reproductive success, as defects in processes during meiosis I or II (e.g., polar body emission, organization and stability of the metaphase I or II spindles) can compromise egg quality and egg competence to form a healthy embryo. "
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    ABSTRACT: Recent work shows that cytokinesis and other cellular morphogenesis events are tuned by an interplay among biochemical signals, cell shape, and cellular mechanics. In cytokinesis, this includes cross-talk between the cortical cytoskeleton and the mitotic spindle in coordination with cell cycle control, resulting in characteristic changes in cellular morphology and mechanics through metaphase and cytokinesis. The changes in cellular mechanics affect not just overall cell shape, but also mitotic spindle morphology and function. This review will address how these principles apply to oocytes undergoing the asymmetric cell divisions of meiosis I and II. The biochemical signals that regulate cell cycle timing during meiotic maturation and egg activation are crucial for temporal control of meiosis. Spatial control of the meiotic divisions is also important, ensuring that the chromosomes are segregated evenly and that meiotic division is clearly asymmetric, yielding two daughter cells - oocyte and polar body - with enormous volume differences. In contrast to mitotic cells, the oocyte does not undergo overt changes in cell shape with its progression through meiosis, but instead maintains a relatively round morphology with the exception of very localized changes at the time of polar body emission. Placement of the metaphase-I and -II spindles at the oocyte periphery is clearly important for normal polar body emission, although this is likely not the only control element. Here, consideration is given to how cellular mechanics could contribute to successful mammalian female meiosis, ultimately affecting egg quality and competence to form a healthy embryo.
    Molecular Reproduction and Development 10/2011; 78(10-11):769-77. DOI:10.1002/mrd.21358 · 2.53 Impact Factor
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