title: Positioning to get out of meiosis: the asymmetry of division.
abstract: BACKGROUND
  During meiosis, mammalian oocytes undergo two successive cell divisions without an intermediate replicative phase. This brief period, called 'meiotic maturation', is crucial for the formation of an egg capable of being fertilized and of generating viable and euploid offspring.
  
  
  METHODS
  We review our current knowledge of the cellular and molecular mechanisms that control asymmetry and appear to be shared between mammalian species, as well as the associated misfunctions that impair the formation of functional female gametes.
  
  
  CONCLUSIONS
  The two successive divisions that comprise mammalian oogenesis are asymmetric. They lead to the formation of small polar bodies and the large and polarized egg. This asymmetry depends upon the dynamic organization of the oocyte cytoskeleton during both divisions. During meiosis I, microfilaments and associated molecules ensure the targeting of the microtubule spindle at the oocyte periphery. During meiosis II, they anchor the spindle under the plasma membrane. In parallel, the cortex overhanging the spindle is dramatically reorganized. Establishment and maintenance of this cortical domain are crucial for the completion of fertilization. Loss of this differentiated area is characteristic of ageing or low-quality gametes and associated with increased maternal age or post-ovulatory ageing.
authors.affiliation: Developmental Biology, CNRS UMR7622, Université Pierre et Marie Curie, Paris, France. sbrunet@snv.jussieu.fr
authors.forename: Stéphane
authors.initials: S
authors.lastname: Brunet
authors.forename: Marie Hélène
authors.initials: MH
authors.lastname: Verlhac
mesh_terms: D008841:Actin Cytoskeleton / Q000502:physiology / Q000648:ultrastructure; D000818:Animals; D016764:Cell Polarity; D005260:Female; D005306:Fertilization / Q000502:physiology; D006801:Humans; D008322:Mammals; D008540:Meiosis / Q000502:physiology*; D008870:Microtubules / Q000502:physiology / Q000648:ultrastructure; D009865:Oocytes / Q000166:cytology* / Q000648:ultrastructure; D008941:Spindle Apparatus / Q000502:physiology / Q000648:ultrastructure