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Estrogen metabolism by the equine embryo proper during the fourth week of pregnancy

Department of Biomedical Sciences, University of Guelph, Guelph, Ontario, Canada N1G2W1

Correspondence should be addressed to J I Raeside; Email: jraeside{at}uoguelph.ca

Estrogen production by the trophoblast is considered important in early equine pregnancy and leads to high concentrations in yolk-sac (Y-S) fluid. The embryo proper is a potential site for their action. We examined estrogen metabolism in the embryo proper because some actions of estrogens are derived from locally formed metabolites. The embryo proper, as well as separated extraembryonic tissues, of conceptuses collected about day 25 of pregnancy, were incubated with ³[H]-estrone (E₁) and ³[H]-estradiol (E₂). Steroids were recovered from media by solid-phase extraction and eluted separately as unconjugated and conjugated fractions. Profiles of free and sulfo-conjugated fractions were obtained by HPLC. Some differences and similarities were noted for the embryo proper as compared to the extraembryonic tissues. No reduction of E₁ to E₂ was noted for the embryo proper and allantois, but some was seen with the bilaminar Y-S wall. Less conversion of E₂ to E₁ occurred in the embryo proper than in the extraembryonic tissues. Profiles for hydrolyzed sulfates from incubation of the embryo proper were very similar for both substrates, mainly with E₁ present. Thus, low levels of reductase and high levels of oxido- activities were apparent for the 17β-hydroxysteroid dehydrogenase enzymes. Further evidence of an active role for the embryo proper was seen as minor, polar products, and an unknown compound eluting between E₂ and E₁. These findings show, for the first time, that the embryo proper can metabolize estrogens that are found in Y-S fluid – a function of potential significance at this stage in its development.