Osmolarity- and Stage-Dependent Effects of Glycine on Parthenogenetic Development of Pig Oocytes

The osmolarities of media that are most effective for in vitro culture of mammalian oocytes and embryos are lower than that of oviductal fluid. Oocytes and embryos can survive the high physiological osmolarity in vivo perhaps owing to the presence of amino acids such as glycine, which serve as organic osmolytes in the female reproductive tract. In the present study, the effects of glycine on the parthenogenetic development of pig oocytes were examined in hypotonic or isotonic media. The results showed that culturing oocytes in isotonic media improved the cleavage rates (P<0.01) at 2 days in culture but inhibited any further development beyond cleavage when compared with the hypotonic media. However, addition of 4 mM glycine to the isotonic media resulted in improved blastocyst formation rates compared with that observed in the hypotonic media (P<0.01), and there was no inhibition of development beyond the cleavage stages in oocytes. The beneficial effects of glycine were observed only when oocytes were cultured in isotonic media and glycine was added at day 2 or 3 in culture. The results from the present study indicate that an isotonic medium with glycine is useful for in vitro culture of pig oocytes and that glycine may protect pig oocytes against the detrimental effects of increased osmolarity.