VN June 2023

Vetnews | Junie 2023 13 the endometrium requires the removal of surface glycocalyx components that previously inhibited this process and, in particular, mucins that coat the endometrial luminal epithelium (e.g. MUC1). In the mare, there does not appear to be a reduction in endometrial MUCI expression in preparation for attachment, but it may be significant that the blastocyst capsule is itself rich in mucin-like glycoproteins. As discussed previously, these capsular glycoproteins are desialylated at the time of conceptus fixation by a process that must be progesterone dependent since it fails if luteolysis is induced prior to fixation. In short, desialylation of the capsular glycoproteins may be an equine-specific equivalent to the progesterone-primed dismantling Of endometrial surface mucins in the preparation for trophectoderm-endometnum attachment. Whether degradation of the capsule alone IS sufficient to permit attachment is unknown, and it seems more likely that luteal progesterone and conceptus-derived signals combine to further promote and stabilize trophectoderm- endometrium attachment. The need for coordinated interaction between the progesterone- primed endometrium and the developing conceptus if pregnancy is toproceednormally is clearly Illustratedby thedelayeddevelopment of equine embryos transferred into the uterus of a recipientmare that ovulated five days after the donor. Indeed, it is likely that a species- specific minimum period of progesterone priming is required for the endometrium to be able to both provide adequate histotrophic nutrition and become receptive for implantation. Moreover, recent transcriptomic studies identified a range of cytokines growth factors and corresponding receptors that are upregulated either In the trophectoderm, the endometrium or both, in response to luteal progesterone and/or local trophectodermal hormones, including estrogens and PGE 2 , in the third and fourth weeks of pregnancy. There also appear to be roles formolecules known to be instrumental to implantation in other species, such as leukaemia inhibitory factor, osteopontin, integrins and various members, receptors and binding factors from the insulin-like and fibroblast growth factor families. However, it is not known how the endometrial surface is modified to play its contrasting roles in the two distinct implantation events seen during equine pregnancy, namely invasion of the highly proliferative chorionic girdle cells versus interdigitation of the remaining, non-invasive chorionic cells. Nevertheless, deficiencies in endometrial receptivity and/or aberrations in the embryo-maternal signalling processes required for implantation almost certainly contribute to the high incidence of pregnancy loss between the end of the conceptus mobile phase (day 16) and the onset of definitive placental formation (day 40) Conclusion During the first six weeks of pregnancy, the equine conceptus develops from a single cell ‘zygote’ to a fetus with a functional circulation and precursors of all of the organs required for postnatal life. To ensure the successful establishment and maintenance of pregnancy, embryo-maternal signalling plays critical roles in ensuring CL maintenance and, together with luteal progesterone, in preparing the endometrium for its role in implantation and establishment of a more stable platform for nutrient and gaseous exchange. For the full article and references – contact author v Article