Cellular and molecular events during oocyte maturation in mammals: molecules of cumulus-oocyte complex matrix and signalling pathways regulating meiotic progression.

Mammalian oocytes acquire their intrinsic ability in a stepwise manner through ovarian folliculogenesis, ultimately reaching the competence to undergo complete oocyte maturation at the final stage of Graafian follicle development. The fully-grown oocyte is tightly surrounded by compact layers of specialized granulosa cells (cumulus cells) to form a cumulus-oocyte complex (COC). After a preovulatory gonadotrophin surge, the COCs rapidly organize a special muco-elastic extracellular matrix (ECM) consisting of large amounts of hyaluronan (HA) and HA binding matrix glycoproteins. Simultaneously, the oocytes undergo meiotic resumption and cytoplasmic modification and attain the fertilizable metaphase II (MII) stage. These cellular events that immediately occur in COCs in the ovulatory phase are strictly regulated by pituitary hormones, steroids, growth factors and so on. Knowledge of the efficient mechanisms and the downstream cascades of the key molecules controlling oocyte maturation may gradually lead to improvement of the present oocyte/ embryo culture systems and gamete biotechnology. Recent studies by our group imply that i) the interaction of HA-CD44 identified in the porcine COC matrix is likely to participate in gap junctional communication and meiotic progression, and that ii) phosphatidylinositol 3-kinase (P13-K) and Akt contribute to the progress of follicle stimulating hormone (FSH)-induced meiosis in mice. Furthermore, this review focuses on the current understanding of biosynthetic regulation, the presumptive role of COC matrix molecules and the signalling pathways for meiotic modulators, such as the protein kinase A (PKA) pathway, the P13-K/Akt pathway and the mitogen activated protein kinase (MAPK) pathway.