Regulation of the nuclear maturation of preovulatory oocytes by KITL. (A) Lack of effects of KITL on GVBD of oocytes. Preovulatory follicles were cultured without (control, C) or with EGF (2 μg/ml) or KITL (50 and 100 ng/ml) for 8 h before evaluation of oocytes undergoing GVBD. (n = 3, 42–48 follicles per experimental group). (B and C) Effects of KITL treatment on first polar body extrusion by cultured cumulus oocyte complexes (B; COCs) and denuded oocytes (C; DOs). COCs or denuded oocytes isolated from preovulatory follicles were cultured without (control, C) or with KITL for 24 h. After culture, the percentage of oocytes showing first polar body extrusion was determined (n = 6, 49–61 oocytes per experimental group). (D) Neutralizing effects of the anti-KITL antibody on KITL stimulation of first polar body extrusion. COCs were cultured with KITL (5 ng/ml) with or without the anti-KITL antibody. (n = 3, 75–79 oocytes per experimental group). E) Effects of KITL treatment on the levels of cyclin B1 protein in MI oocytes. COCs were cultured without (control, C) or with KITL (5 ng/ml) for 7 h. After removal of cumulus cells, oocytes were subjected to immunoblotting. (Upper panel) A representative immunoblot showing the cyclin B1 protein at 60 kDa and β-actin protein at 42 kDa. (Lower panel) Densitometric analyses of relative expression levels of cyclin B1 protein. The cyclin B1 levels were represented as fold increases to the control group (mean ± SEM, n = 3). *, P < 0.05 vs. control group.