Production of free estrogens and estrogen conjugates by the preimplantation equine embryo

In vitro production of free estrogens and estrogen conjugates by intact Day 12.5, 13.5 and 14.5 equine embryos was measured at 2-h intervals over a 24-h culture period. Production of free estrogens was higher for Day 14.5 than Day 12.5 embryos. Differences in production of conjugated estrogens were not significant, but a trend toward increased production with increased age of embryo was apparent. No trend toward increased free and conjugated estrogen production per cell was observed with age. Embryo diameter and number of cells increased with age but varied considerably within groups. The amount of free and conjugated estrogens measured in blastocoelic fluid did not decrease over the 24-h culture period, suggesting that estrogens detected in culture medium were produced by the embryo and not the result of leakage of maternal estrogen from the blastocoele. The results of this study support previous results that estrogen production increases with development of equine embryos. This increase in estrogen production appears to be more closely associated with the diameter of the embryo, and hence its number of cells, than with increased intracellular steroidogenic activity.