Figure 2.

Final oocyte maturation. The midcycle gonadotropin surge causes a decrease in intraoocyte cAMP. The oocyte is removed from meiotic arrest and undergoes a series of coordinated changes affecting both the nucleus and cytoplasm. During nuclear maturation, the haploid metaphase I oocyte extrudes half of its genetic material in a polar body and transitions toward a haploid metaphase II gamete. To achieve this, the germinal vesicle breaks down (GVBD) and chromosomes align along the spindle before separation of genetic material occurs and the polar body is extruded. During oocyte maturation, cytoplasmic and nuclear maturation both occur in related but independent processes. Cytoplasmic maturation prepares the oocyte to meet the metabolic demands of fertilization and embryo growth through changes in organelles. Prior to germinal vesicle breakdown, mitochondria surround the germinal vesicle (GV), and the Golgi apparatus remains intact. By the end of oocyte maturation, mitochondria are associated with smooth endoplasmic reticulum, the Golgi body has been fragmented, and the polar body is extruded. Adapted from Mao et al. 2014 (69).