Signalling pathways in oocyte meiotic maturation.

Immature oocytes become fertilisable eggs through a process called meiotic maturation, which is often induced by specific hormones. Oocyte maturation involves the activation of various signal transduction pathways that converge to activate maturation-promoting factor (MPF); this is a key activity that catalyses entry into M-phase of meiosis I and meiosis II. Whereas the function of MPF in promoting oocyte maturation is ubiquitous, there are species-dependent differences in the signalling pathways leading to MPF activation. The poster is based mainly on the signalling pathways elucidated in oocytes of the African clawed frog Xenopus laevis, which is probably the most intensively studied model system for meiotic maturation. Important differences between oocytes from Xenopus and other organisms are discussed below. Further details and primary references can be found in several recent reviews (Abrieu et al., 2001; Ferrell, 1999; Karaiskou et al., 2001; Maller et al., 2001; Nebreda and Ferby, 2000; Yamashita et al., 2000). Fully grown oocytes are arrested at the first meiotic prophase. The hallmarks of meiotic maturation in vertebrate oocytes are (1) resumption of meiosis I which includes germinal vesicle breakdown (GVBD), chromosome condensation and spindle formation, (2) the transition between meiosis I and meiosis II, including inhibition of S-phase and (3) arrest in metaphase II because of cytostatic factor (CSF) activity. Meiosis II is completed after fertilisation of the mature oocyte (or egg). By contrast, oocytes of many invertebrates proceed during maturation only to metaphase of meiosis I, which is when they are fertilised.