VN June 2023

Vetnuus | June 2023 12 Although the functions of the capsule are largely speculative, that it is interposed between the trophectoderm and endometrium means it almost certainly plays some role in mediating embryo-maternal interaction and communication. Moreover, the capsule is essential for conceptus survival in utero since removing it from early day seven embryos by micromanipulation prior to embryo transfer prevents those embryos from developing into viable pregnancies. Conversely, although IVP embryos have no capsule when they reach the blastocyst stage, they have accumulated capsular glycoproteins within the perivitelline space. Subsequently, they develop an apparently normal capsule following transfer to the uterus of a suitable recipient mare. Physically, the capsule is both tough and elastic, and it has been proposed to help the blastocyst maintain its spherical shape and to provide mechanical protection during the ‘mobile phase’ when the delicate conceptus vesicle is ‘squeezed’ around the uterine lumen by myometrial contractions. In addition, the capsular glycoproteins are rich in negatively charged sialic acid residues that have been postulated to confer anti-adhesive properties and thereby facilitate conceptus migration, indeed, the end of the conceptus mobile phase is associated with extensive desialylation of the capsule. In this way, the capsule plays at least an indirect role in embryonic maternal recognition of pregnancy signalling (i.e. by promoting migration). More intriguingly, the capsule has been proposed to play an active role in embryo-maternal dialogue by acting as a ‘mail-box’ to store, modify or transfer endometrial proteins involved in nutrient transport (e.g. uterocalin) or in stimulating trophectoderm cell proliferation. Similarly, the capsule may act as a repository for trophectodermal proteins that modulate endometrial function or act in a paracrine fashion to regulate trophectoderm cell migration or proliferation (e.g. IGFBP3). Maternal recognition of pregnancy signalling The archetypal embryo-maternal signalling process during early intrauterine development is MRP, during which the developing embryo biochemically signals its presence to its dam to ensure that she modified to allow conceptus adhesion. An important aspect of endometrial preparation is a period of exposure to progesterone that results in a downregulation of progesterone receptors (PRs) in the luminal and glandular epithelial cells but not in the stromal cells. These stromal cells are proposed to produce‘progestamedins’that allow progesterone to fine-tune the preparations of the endometrial epithelium for its role in implantation, despite the ‘switching off’ of their own PRs. One of the first steps in implantation is the intimate apposition of the trophectoderm to the endometrium, an event that can only occur once the capsule starts to disintegrate at around day 23 of pregnancy. In most species, attachment of the trophectoderm to Article