Functional characterization of the ability of FGF8 to enhance porcine oocyte maturation

Fibroblast growth factor 8 (FGF8) is known to act as an oocyte-secreted factor (OSF) of oocyte maturation in several mammalian species; however, its expression and function in the porcine ovary remain largely unexplored. This study aimed to investigate the localization patterns of FGF8 and its receptors, fibroblast growth factor receptor 3c (FGFR3c) and fibroblast growth factor receptor 4 (FGFR4), during follicular development, and to evaluate the effects of FGF8 supplementation during in vitro maturation (IVM) of porcine cumulus-oocyte complexes (COCs). Immunohistochemistry (IHC) revealed that FGF8 was present in both oocytes and surrounding somatic cells, FGFR3c was primarily localized to somatic cells, and FGFR4 was detected mainly in oocytes. Supplementation with FGF8 during IVM enhanced nuclear maturation, cytoplasmic maturation, cumulus expansion, and subsequent embryonic development, as compared to the controls. In addition, blastocysts derived from FGF8-treated oocytes exhibited a significantly reduced proportion of apoptotic cells, indicating improved blastocyst quality. At the molecular level, treatment with 100 ng/mL FGF8 significantly increased the expression of mRNA associated with glycolysis, antioxidative response, and anti-apoptotic pathways in cumulus cells, and upregulated the expression of OSFs and maternal-effect genes (MEGs) in oocytes. These molecular changes support the notion that FGF8 facilitates oocyte developmental competence via both metabolic and protective mechanisms. Collectively, our findings demonstrate, for the first time, the spatial expression of FGF8 and its receptors in porcine ovaries and confirm that FGF8 positively regulates porcine oocyte maturation through transcriptional changes in both cumulus cells and oocytes.