Effects of inhibin A and activin A during in vitro maturation of bovine oocytes in hormone- and serum-free medium.

The aim of this study was to investigate whether inhibin A and/or activin A play a role in the acquisition of oocyte competence during the final stages of oogenesis. The particular goal was to establish whether inhibin A and activin A exert development-enhancing effects during in vitro maturation in serum-free media and whether such effects are related to changes in the kinetics of meiotic resumption and/or fertilization rates. Cumulus-oocyte-complexes (COCs) were matured in two control media (Medium 199 [M199] with hormones and serum, hormone-serum control; M199 + 0.6% BSA, BSA-control) and nine treatment media (M199 with 0.6% BSA containing 100, 10, and 1 ng/ml of recombinant human inhibin A, recombinant human activin A, and the combination of the two). Oocytes were fertilized and cultured using standard procedures. Cleavage was assessed at 54 h and blastocyst development at 8 days after in vitro fertilization. Kinetics of oocyte maturation and the fertilization rates were evaluated after fixing and staining (Hoechst 33342) of oocytes at 8, 16, and 22 h after onset of in vitro maturation or of presumptive zygotes at 12 h after in vitro fertilization, respectively. Although there was no effect on cleavage rates, inhibin A and activin A significantly enhanced postcleavage development at concentrations of 10 ng/ml (57.7 +/- 7.5% and 56.6 +/- 11.7%, respectively) and 100 ng/ml (50.6 +/- 18.6% and 56.4 +/- 4.0%, respectively) compared to that in the BSA-control group (24.6 +/- 3.2%). Whereas inhibin A- and activin A-treated oocytes showed development-enhancing effects similar to those in the hormone-serum controls, these groups differed with regard to the kinetics of meiotic resumption. Likewise, the enhanced development of the hormone-serum control and the inhibin A/activin A-treated oocytes was not related to increased fertilization rates relative to the BSA-control. These results suggest that inhibin A and activin A may play important roles during the final stages of oogenesis and that recombinant inhibins and activins are useful compounds for the development of a serum-free culture system for in vitro maturation of oocytes from cattle and possibly other mammalian species.