[Mg]/[Ca] determines Ca response at fertilization: tuning of adult phenotype?

Alteration of the postnatal phenotype has sparked great concern about the developmental impact of culture media used at fertilization. However, the mechanisms and compounds involved are yet to be determined. Here, we used the Ca responses from mouse eggs fertilized by ICSI as a dynamic and quantitative marker to understand the role of compounds in egg functioning and establish possible correlations with adult phenotypes. We computed 134 Ca responses from the first to the last oscillation in media with specific formulations. Analyses demonstrate that eggs generated two times as many Ca oscillations in KSOM as in M16 media (18.8 ± 7.0 vs 9.2 ± 2.5). Moreover, the time increment of the delay between two consecutive oscillations, named TIbO, is the most sensitive coefficient characterizing the mechanism that paces Ca oscillations once the egg has been fertilized. Neither doubling external free Ca nor dispermic fertilization increased significantly the total number of Ca oscillations. In contrast, removing Mg from the M16 boosted Ca oscillations to 54.0 ± 35.2. Hence, [Mg]/[Ca] appears to determine the number, duration and frequency of the Ca oscillations. These changes were correlated with long-term effects. The rate of female's growth was impacted with the 'KSOM' females having only half the fat deposit of 'M16' females. Moreover, adult animals issued from M16 had significantly smaller brain weight vs 'KSOM' and 'control' animals. TIbO is a new Ca coefficient that gauges the very early functional impact of culture media. It offers the possibility of establishing correlations with postnatal consequences according to IVF medium formulation.Free French abstract: A French translation of this abstract is freely available at http://www.reproduction-online.org/content/154/5/675/suppl/DC2.