title: Mechanism of the chromosome-induced polar body extrusion in mouse eggs.
abstract: BACKGROUND
  An oocyte undergoes two rounds of asymmetric division to generate a haploid gamete and two small polar bodies designed for apoptosis. Chromosomes play important roles in specifying the asymmetric meiotic divisions in the oocytes but the underlying mechanism is poorly understood.
  
  
  RESULTS
  Chromosomes independently induce spindle formation and cortical actomyosin assembly into special cap and ring structures in the cortex of the oocyte. The spindle and the cortical cap/ring interact to generate mechanical forces, leading to polar body extrusion. Two distinct force-driven membrane changes were observed during 2nd polar body extrusion: a protrusion of the cortical cap and a membrane invagination induced by an anaphase spindle midzone. The cortical cap protrusion and invagination help rotate the spindle perpendicularly so that the spindle midzone can induce bilateral furrows at the shoulder of the protruding cap, leading to an abscission of the polar body. It is interesting to note that while the mitotic spindle midzone induces bilateral furrowing, leading to efficient symmetric division in the zygote, the meiotic spindle midzone induced cytokinetic furrowing only locally.
  
  
  CONCLUSIONS
  Distinct forces driving cortical cap protrusion and membrane invagination are involved in spindle rotation and polar body extrusion during meiosis II in mouse oocytes.
authors.affiliation: Department of Obstetrics and Gynecology and Reproductive Biology, 75 Francis Street, Brigham and Women's Hospital, Harvard Medical School, Boston, MA 02115, USA. mdeng3@partners.org.
authors.forename: Qiong
authors.initials: Q
authors.lastname: Wang
authors.forename: Catherine
authors.initials: C
authors.lastname: Racowsky
authors.forename: Manqi
authors.initials: M
authors.lastname: Deng