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In preovulatory follicles, oocytes are surrounded by numerous layers of cumulus cells in a known as the cumulus cell-oocyte complex (COC). After stimulation of ovulation by the LH surge, the morphology of COC is dramatically changed as a hyaluronan rich matrix accumulates within cumulus cell layer, and the oocyte resumes meiosis by progressing to the metaphase II. Although both changes induced by LH surge are essential for successful fertilization in vivo, the expression of LH receptors is not detected in the oocyte and is minimal (negligible) in cumulus cells compared with granulosa cells. However, cumulus cells express members of the EGF receptor family (ErbB family), prostaglandin receptors (EP2 and EP4) and cytokine family receptors that respond to specific ligands secreted by granulosa cells during the ovulation process. By these intermediary steps, the cumulus cells mediate LH signaling from granulosa cells to induce oocyte maturation. This minireview focuses on the role of cumulus cells in oocyte maturation at the physiological and molecular levels.
Prostaglandins (PGs) are implicated in various physiological and pathological functions because of their vasoactive, mitogenic, and differentiating properties. PGs have long been known to participate in various female reproductive functions. The cyclooxygenase (COX) isozymes, COX-1 and COX2, are the rate-limiting enzymes of PGs. Gene targeting studies have revealed that COX-2, but not COX-1, derived PGs are essential for ovulation, fertilization, implantation, and decidualization. However, the roles of PGs in oocyte maturation remain controversial. We have clarified that COX-2-deficient mice have defective oocyte meiotic and cytoplasmic maturation associated with defective cumulus expansion in vivo and in vitro. Lack of COX-2-derived PGE2 leads to impaired cumulus cell-oocyte interactions, which are critical for the production of fertilization-competent oocytes. This review summarizes the published data regarding the roles of PGs in oocyte maturation especially in gene targeting studies.
Mammalian oocytes acquire their intrinsic ability in a stepwise manner through ovarian folliculogenesis, ultimately becoming competent to undergo complete oocyte maturation at the final stage of the Graafian follicle. The fully-grown oocyte is tightly surrounded by compact layers of specialized granulosa cells (cumulus cells) known as the cumulus-oocyte complex. Oocyte maturation consists of the nuclear and the cytoplasmic maturation. Dynamic morphological changes such as cumulus expansion, chromosome alignment, and spindle formation are observed during the oocyte maturation. Mounting evidence that oocyte quality profoundly affects fertilization and subsequent embryo development drives the continued search for reliable predictors of oocyte developmental competence. It is necessary to understand the phenomenon and the molecular mechanisms active during oocyte maturation to obtain high-quality oocytes for in vitro maturation. In the present paper, we summarize the actions of the molecules that play key roles in meiotic progression and its control mechanism.
Maturation/M-phase promoting factor (MPF) activity is regulated by the Cyclin B concentration and phosphorylation states of Cdc2 kinase. In the present review, we summarize the factors regulating the synthesis and degradation of Cyclin B and the phosphorylation and dephosphorylation of Cdc2. We focuse on Aurora A and cytoplasmic polyadenylation element binding protein (CPEB), which are involved in cytoplasmic polyadenylation, for Cyclin B synthesis; on anaphase-promoting complex (APC) activating factors, FZR1 and CDC20, for Cyclin B degradation; on Cdk7 and Wee1B for Cdc2 phosphorylation, and on Cdc25 for Cdc2 dephosphorylation. We describe our recent results concerning these factors in porcine oocytes, and discusse overall regulation of MPF activity during porcine oocyte maturation.
To investigate the effect of media on mouse follicular development and oocyte maturation in vitro, we compared eight culture media. Early preantral follicles were mechanically dissected from the ovaries of 14-day-old mice and cultured for 10 days. The tested media were: α-MEM, D-MEM/F-12, D-MEM with high glucose (4.5 g/L) (D-MEM[H]) or low glucose (1 g/L) (D-MEM[L]), Weymouth, M199, IMDM, and RPMI1640. All of the media were supplemented with 5% FBS, ITS, 100 mlU/ ml rhFSH, 1 mlU/ml rhLH and, 0.5% gentamicin. Compared to the other media (P < 0.05), a higher percentage of follicles survived and antral-like cavity formation was seen after 10 days of culture when the follicles were cultured in α-MEM, Waymouth, D-MEM/F-12, or D-MEM(L). Among the four media with higher follicle survival, the oocyte diameter on day 10 of culture was largest in α-MEM and smallest in Waymouth. The highest percentages of MII oocytes were obtained when follicles were cultured in α-MEM, D-MEM(L), and D-MEM/F-12. In contrast, in Waymouth, no MII oocytes were obtained. The media used for mouse preantral follicle growth in vitro affected follicle survival and oocyte maturation. Of the media that we tested, α-MEM, D-MEM(L), and D-MEM/F-12 were superior for mouse preantral follicle cultures.
The aim of the present study was to reveal the effects of in vitro aging of mouse oocytes without cumulus cells on morphological alteration of their metaphase II (MII) spindles, ability to be fertilized in vitro and the developmental competence of the resulting embryos. To observe the MII spindles by immunofluorescence staining, oocytes without cumulus cells were divided into four groups: a non-aged control and three others aged for 10, 15 or 25 h. Although the incidence of morphologically normal spindles was significantly lower in the 25 h group, no significant increase of chromosome misalignment was observed in any of the aged groups (P < 0.05). For the in vitro fertilization (IVF) experiments, denuded oocytes were aged for 15, 16.5 or 18 h and the resulting normal zygotes with a second polar body were cultured in vitro for 120 h to assess their embryonic development. The fertilization rate was significantly lower only in the 18-h aged group. The rates of blastocyst formation were significantly lower in all aged oocyte groups, compared with non-aged controls, and blastocysts derived from aged oocytes had lower total cell numbers. Therefore, a significant decline of developmental competence appears in cumulus cell-removed oocytes aged for more than 15 h, even if they retain fertilizability.
It has been shown that epigenetic abnormalities are involved in both abnormal fetal development, as represented by large offspring syndrome, and the onset of diseases affecting vital prognosis, such as metabolic syndrome and malignancy. And if assisted reproductive technology (ART) causes epigenetic abnormalities, then this must be avoided. We compared the expression of imprinted genes (Igf2, Peg3, Snrpn and Kcnq1ot1), which are expressed in early embryos and are involved in fetal and placental development, using in vivo and in vitro mouse blastocysts. In order to compare the changes in gene expression under different culture conditions, in vitro embryos were incubated using two culture media (KSOM/AA and M16) and two gas phases (O2 5% and 20%). Between in vivo embryos and KSOM/AA embryos, no marked differences were seen in gene expression. However, between in vivo embryos and M16 embryos, changes in gene expression patterns were confirmed. By changing the incubator's O2 concentration, even when using the same culture medium, changes in imprinted gene expression were confirmed, clarifying that the expression of imprinted genes in early mouse embryos is related to culture medium components and oxygen concentration. It is important for ART to improve culture conditions.