Figure 1

A schematic illustration of the resumption of meiosis in different animal models. The immature oocyte is arrested in prophase I (PI) marked by the germinal vesicle (GV). Depending on the species, oocytes may be fertilized in PI, undergoes a second meiotic block at the metaphase I (MI), metaphase II (MII) or may complete meiosis before fertilization. MII is marked by one polar body (yellow). Resumption from the second meiotic block occurs upon sperm penetration leading to germinal vesicle breakdown (GVBD), meiosis completion and zygote formation marked by the two inner pronuclei and two polar bodies.