Expression of progesterone and oestrogen receptors by early intrauterine equine conceptuses

Progesterone and oestrogen play essential roles in the maintenance of pregnancy in eutherian mammals and are thought to exert their effects on the developing conceptus indirectly, via the endometrium. In some species, early embryos have themselves been shown to express steroid receptors, thereby suggesting that reproductive steroids may also influence embryonic development directly. The aim of this study was to determine whether early intrauterine equine conceptuses express either the classical intracellular progesterone (PR) and oestrogen receptors (ERα and ERβ) or the more recently characterised membrane-bound progesterone receptors (PGRMC1 and mPR). Horse conceptuses recovered on days 7, 10 and 14 after ovulation (n = 8 at each stage) were examined for steroid receptor mRNA expression using quantitative rtPCR. Where commercial antibodies were available (PR, ERβ), receptor localisation was examined immunohistochemically in day 10, 12, 14, 15 and 16 conceptuses (n = 2 at each stage). mRNA for PR, PGRMC1 and mPR was detected at all stages examined, but while PGRMC1 and mPR expression increased during the day 7-14 period, PR expression decreased. ERα mRNA was not detected at any stage examined, whereas ERβ mRNA was detected in all day 14, some day 10 and no day 7 conceptuses. Immunoreactive ERβ receptors were localised to the trophectoderm of day 14-16 conceptuses; PR were not detected immunohistochemically in conceptus tissue. In summary, this study demonstrates that equine conceptuses express mRNA and, in the case of ERβ, protein for steroid hormone receptors during the period encompassing rapid conceptus growth, differentiation and maternal pregnancy recognition.