Summary. The role of the embryo in promoting increased plasma concentrations of immunoreactive inhibin after conception in the marmoset monkey was determined by flushing embryos from the uterus between days 5 and 9 after ovulation (implantation commences on days 11–12). Blood samples were taken from each animal (three times a week) after ovulation until the end of the luteal phase. Plasma inhibin concentrations were measured using a radioimmunoassay based on antisera against a synthetic fragment of the α-subunit of human inhibin. When embryos were flushed on days 5 and 6 (n = 6) after ovulation inhibin concentrations did not exceed 250 ng ml−1 for the duration of the luteal phase. In contrast when embryos were flushed on days 7 (n = 4), 8 (n = 4) and 9 (n = 3) maximum concentrations of inhibin always exceeded 250 ng ml−1, reaching > 400 ng ml−1 when embryos were flushed on days 8 and 9. Inhibin concentrations remained high for the duration of the luteal phase, which varied in length between 20 and 32 days. Significantly (P < 0·01) higher mean plasma concentrations of immunoreactive inhibin were first recorded on days 7–8 after ovulation in animals that had embryos flushed on days 7, 8 and 9 compared with concentrations in animals that had embryos flushed on days 5 and 6. Inhibin could not be detected in the medium of embryos cultured for up to 2 weeks. In two control animals, inhibin concentrations exceeded 250 ng ml−1 after sham operation on day 6, whereas inhibin did not exceed 250 ng ml−1 when unfertilized eggs were flushed from the uterus of an animal on day 8 after ovulation. Progesterone concentrations did not differ significantly among animals irrespective of the day of embryo flushing. The results suggest that the marmoset pre-implantation embryo provides a signal on days 7–8 which triggers an increase in luteal inhibin production. The continued presence of this putative signal was not necessary for maintaining increased inhibin production.
Keywords: embryo; inhibin; pregnancy; marmoset monkey