Bioscientifica Proceedings

This review attempts to integrate available data on embryo-maternal communication during maternal recognition of pregnancy and implantation in the pig. Progesterone (P₄) is an essential hormone that makes the uterus receptive to accept conceptuses for implantation and subsequent placentation. As well as P₄, the receptive stage of the endometrium is further primed by paracrine factors secreted by the conceptus prior to and during implantation. Oestrogen secretion by elongating conceptuses on days 11 to 12 is necessary for the maternal recognition of pregnancy to protect the corpus luteum from regression. Oestrogens increase the expression of various uterine secretory proteins that are components of the histotroph, which in turn support conceptus growth and implantation. The PGE₂ positive feedback loop in the endometrium contributes to an increased PGE₂/PGF_2α ratio during the peri-implantation period. Greater synthesis of prostacyclin by the uterus, along with activation of the VEGF-receptor system, may be involved in regulating the uterine vascular bed and blood flow, both being prerequisite for sufficient embryo nutrition. HOXA10 expressed by the endometrium may be involved in tissue remodelling and proliferation during conceptus implantation. LIF, IL6 and their receptors are present at the maternal-conceptus interface, and both cytokines increase the attachment and proliferation of trophoblast cells. TGFβ1 stimulates the synthesis of fibronectin, which is necessary for firm adhesion between trophoblast and maternal tissue. Moreover, cytokines regulate a maternal immune response that is permissive for successful pregnancy establishment.

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