Actin filaments: key players in the control of asymmetric divisions in mouse oocytes.

Meiotic maturation is characterized by the succession of two asymmetric divisions each giving rise to a small polar body and a large oocyte. These highly asymmetric divisions are characteristic of meiosis in higher organisms. They allow most of the maternal stores to be retained in the oocyte, a vital property for further embryo development. In mouse oocytes, the asymmetry is ensured by the migration and the anchoring of the division spindle to the cortex in meiosis I and by its anchoring to the cortex in meiosis II. In addition, and subsequent to this off-centre positioning of the spindle, a differentiation of the cortex overhanging the chromosomes takes place and is necessary for the extrusion of small polar bodies. In the present review, we will emphasize the role of the actin cytoskeleton in the control of spindle positioning, spindle anchoring to the cortex and cortical differentiation.