Hematoxylin staining reveals a decrease in nucleolar diameter of pig oocytes before germinal vesicle breakdown.

During oocyte growth, the morphology of the nucleolus changes into a compact and homogenous structure. The compact nucleoli in full-grown oocytes are not stained by aceto-orcein staining or immunofluorescence staining. In this study, we developed a hematoxylin staining method for pig oocytes in whole-mount preparations to visualize the nucleoli. Nucleoli of growing and full-grown oocytes were stained blue with hematoxylin. Using this staining method, the changes in the oocyte nucleolus during maturation were examined. The nucleolar diameter gradually decreased in maturing oocytes (10.7 ± 0.1 μm to 9.0 ± 0.7 μm, P<0.05) before germinal vesicle breakdown (GVBD). The results suggest that the nucleolar volume of oocytes decreases before GVBD.