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Author(s): De Wu, Queenie Ching-Kwan Cheung, Lihua Wen, and Julang Li
Publication Date: June 28, 2006
Reference: BIOLOGY OF REPRODUCTION 75, 547–554 (2006)
Country: Canada

Summary:

In livestock, most of the follicles on the ovarian surface are small follicles. A procedure that supports the in vitro growth and maturation of these small follicle-derived oocytes may offer a new source of useable oocytes for both biotechnological and fundamental research purposes. The objective of the current study was to test the hypothesis that providing a more growth-supporting and less maturation-promoting environment during the first phase of small follicle-derived oocyte maturation may improve oocyte competence for meiosis and embryo development upon activation. In our small follicle-derived oocyte growth-maturation system (SGM group), cumulus-oocyte complexes (COCs) from small follicles (1–3 mm) were first cultured in oocyte growth medium for 24 h, then in oocyte maturation medium for 20 h. As controls, COCs from small (SM group) and large (LM group) follicles were cultured using a conventional in vitro maturation (IVM) approach in which they were directly cultured in oocytematurationmedium. At 24 h of culture, the percentage of small follicle-derived oocytes that underwent germinal vesicle breakdown (GVBD) in the SGM group was comparable to that of large follicle-derived oocytes (LM group) but was significantly higher than that of the SM group (P <0.05). At 44 h of culture, compared to 36% in the SM group, 55% of the SGM group oocytes reached metaphase II (MII; P <0.05). In addition, the level of cyclin B in oocytes of the SGMgroup was comparable to that of oocytes from LM group and was significantly higher than that of oocytes from the SM group (P <0.05).When activated and in vitro fertilized (IVF), 7.3 and 9.0 times more parthenogenetic and IVF embryos developed to blastocyst stage in the SGMgroup than in the SMgroup (P < 0.05). ThemRNA expression levels of three developmentally important genes—DNA-methyltransferase 1, Pou domain class 5 transcription factor 1, and Fibroblast growth factor receptor 2—in embryos of the SGM group were comparable to those of embryos developed from the LMgroup, whereas they were significantly lower in those of the SM group (P < 0.05). Our data suggest that the oocyte growthmaturation system facilitates the final stage of oocyte growth and thus resulted in better oocyte nuclear, cytoplasmic maturation, and developmental competency compared with the conventional direct oocyte maturation system.

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