The second week after mating is recognised as critical to the successful maintenance of pregnancy in the pig, and is believed to be when Meishan females gain much of their reproductive advantage. In this experiment, the composition of uterine fluids, and conceptus and endometrial culture medium was examined in 14 Meishan gilts on Days 11-15 of pregnancy. Positive relationships were detected between blastocyst size and both oestradiol secretion (P < 0.05) and the incorporation of radiolabelled leucine into conceptus secretory proteins (P < 0.05), The incorporation of radiolabel was also related to oestradiol secretion (P < 0.01) in individual littermate spherical and ovoid conceptuses. Although there were no consistent temporal changes in the endometrial secretory protein profile, two proteins believed to be retinol binding proteins were absent from uterine flushes on Day 11, but present on all subsequent days. In conceptus culture medium, acidic proteins were predominant on Days 12 and 13, while basic proteins predominated on Days 14 and 15. Conceptus secretion of proteins with antiviral activity, detected on Days 14 and 15, was reduced by almost 50% when conceptuses were cultured in the presence of endometrial tissue. These data provide valuable new information on the biochemical dialogue occurring between Meishan conceptuses and the endometrium during a critical stage of pregnancy, and can be considered to reveal interactions characteristic of prolific pigs.
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