Formation of cortical granule free domains in mouse oocytes. Redistribution and pre-fertilization release of cortical granules prior to gamete fusion are shown in this schematic diagram. Unfertilized zona intact oocytes in different developmental stages and fertilized zona intact oocytes are shown chronologically. (A - D) Germinal vesicle intact oocytes (in prophase I) prior to ovulation; (E - G) Metaphase II unfertilized oocytes following ovulation; (H) Fertilized oocyte. Physical localization of cortical granules in the cortex is shown with areas in grey shade. Cortical granules are evenly distributed in the cortex of germinal vesicle intact oocytes prior to ovulation (A). A small population of granules undergoes redistribution (B), and an area devoid of cortical granule (i.e. first CGFD) is formed (C). During polar body extrusion, pre-fertilization release of cortical granules is evident in the cleavage furrow (D). Following ovulation, cortical granules undergoes redistribution (F), and an area devoid of cortical granules (i.e. second CGFD) is formed again (G). Following sperm penetration, the remaining cortical granules undergo exocytosis and release their contents into the extracellular space (H). Arrow with dotted line = cortical granule migration; arrow with solid line = the site of cleavage furrow; mosaic grey shade = cortical granule pre-fertilization; PB = polar body.