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Neither gonadotropin nor cumulus cell expansion is needed for the maturation of competent porcine oocytes in vitro

In vitro maturation of oocytes from immature females is widely used in assisted reproductive technologies. Here we illustrate that cumulus cell (CC) expansion, once considered a key indicator of oocyte quality, is not needed for oocytes to mature to the metaphase II (MII) stage and to gain nuclear and cytoplasmic competence to produce offspring. Juvenile pig oocytes were matured in four different media: 1) Basal (-gonadotropins (GN)-FLI); 2) -GN+FLI (supplement of FGF2, LIF, and IGF1); 3) +GN-FLI; 4) +GN+FLI. There was no difference in maturation to MII or progression to the blastocyst stage after fertilization of oocytes that had been matured in -GN+FLI medium and oocytes matured in +GN+FLI medium. Only slight CC expansion occurred in the two media lacking GN compared to the two where GN was present. The cumulus-oocytes-complexes (COC) matured in +GN+FLI exhibited the greatest expansion. We conclude that FLI has a dual role. It is directly responsible for oocyte competence, a process where GN are not required, and, when GN are present, it has a downstream role in enhancing CC expansion. Our study also shows that elevated phosphorylated MAPK may not be a necessary correlate of oocyte maturation and that the greater utilization of glucose by COC observed in +GN+FLI medium probably plays a more significant role to meet the biosynthetic needs of the CC to expand than to attain oocyte developmental competence. Gene expression analyses have not been informative in providing a mechanism to explain how FLI medium enhances oocyte competence without promoting CC expansion.
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