Published online by Cambridge University Press: 24 December 2019
Mature human gametes ready for fertilization differ in their state of nuclear maturation: the spermatozoon has completed meiosis and the oocyte is arrested at metaphase II. However, both gametes must also undergo a process of cytoplasmic maturation before they are capable of fertilization. This involves a complex series of biochemical, physiological and structural events that occur in a carefully orchestrated temporal and spatial pattern in parallel with, but independent from, nuclear maturation. Cytoplasmic and nuclear maturation are often asynchronous (Dale, 2018a): therefore, a cohort of human metaphase II oocytes collected after controlled ovarian hyperstimulation in an IVF program may appear to be similar with regards to the nuclear apparatus, but they are in fact at various stages of cytoplasmic maturation. This may partly explain the different developmental capabilities of embryos generated from a single cohort of oocytes.