The art of choreographing asymmetric cell division

Contributions of the actin cytoskeleton to the emergence of polarity during maturation in human oocytes

In mature mammalian oocytes, cortical f-actin distribution is polarized, as evidenced by a prominent cap subtended by the metaphase II (MII) spindle. Formation of a polarized actin cap is a consequence of a complex actomyosin-driven contractile process that directs polar body extrusion. Human mature oocytes also display a network of suboolemmal actin, but so far there has been no suggestion of an actin-rich domain in the vicinity of the spindle. By high-resolution confocal microscopy, we generated semi-quantitative data of the actin cytoskeleton in human mature and immature oocytes, with the aim to better understand the characteristics and remodelling of this cytoskeletal component in the female gamete. In mature MII oocytes, the cortical domain near the spindle showed a more intense actin signal in comparison to the opposite cortical domain (177.2±59.0 versus 126.8±61.0, P<0.0001; data expressed in arbitrary units). The extent of cortical f-actin polarity was comparable between in vivo and in vitro matured oocytes. However, both the degree of polarity and relative abundance of signal were diminished with increasing maternal age. Mean intensity of cytoplasmic actin was significantly higher in oocytes matured in vitro derived from in vitro maturation (IVM) cycle, in comparison to oocytes matured in vivo or in vitro obtained from controlled ovarian stimulation cycles (35.0±8.0, 21.1±12.4 and 25.9±8.6, respectively; P=0.025). In germinal vesicle (GV)-stage oocytes obtained from both IVM and controlled ovarian stimulation cycles, cortical actin did not appear polarized, irrespective of whether the GV was located centrally or asymmetrically. These data indicate that, during maturation, cortical actin acquires a polarized distribution involving an accumulation in the domain adjacent the spindle. They also propose new questions concerning the existence of cytoplasmic actin in mature oocytes. Finally, they are suggestive of an influence of maternal age on the actin cytoskeleton.