Molecular mechanisms of the initiation of oocyte maturation: general and species-specific aspects.

Stimulated by maturation-inducing hormone secreted from follicle cells surrounding the oocytes, fully-grown oocytes mature and become fertilisable. During maturation, immature oocytes resume meiosis arrested at the first prophase and proceed to the first or second metaphase at which they are naturally inseminated. Paying special attention to general and species-specific aspects, we summarise the mechanisms regulating the initial phase of oocyte maturation, from the reception of hormonal signals on the oocyte surface to activation of the maturation-promoting factor in the cytoplasm, in amphibians, fishes, mammals and marine invertebrates.