O-093 Endometrium-blastocyst cross talk: how far are we from understanding?
Authors: Campagnolo L 
Published: June 2023

Abstract
The early events characterising implantation of the human embryo represent a critical step in which proper activation of orchestrated molecular pathways regulate the success of pregnancy. Understanding this complex network is considered by many the last frontier of human reproduction. Although outstanding research advancements have provided clinicians tools to select the best quality embryo, identification of the optimal timing in which the endometrium is ready to accommodate it remains a challenge. Over the last decade several studies have identified many key players involved in the preparation of the endometrium to implantation and several diffusible signals guiding the embryo in its journey toward the maternal endometrial tissue have been uncovered (Massimiani et al. 2020). However, pieces of the puzzle are still missing.
For convenience, the process of implantation is subdivided into several sequential processes, the completion of one being propaedeutic to the following. Once the blastocyst has arrived in close proximity to the uterine epithelium, the first event occurring is its escape from the zona pellucida. The molecular basis of blastocyst hatching remains poorly understood, and species-specific differences have been reported. Using the mouse model, we recently demonstrated that thyroid hormone up-regulates the expression of specific lytic enzymes (Isp1 and Isp2) known mediators of mouse embryo zona lysis. Interestingly, we observed that TH-mediated upregulation of Isp1 and Isp2 is strictly dependent on the presence of endometrial stromal cell feeder layer, clearly indicating that blastocyst–endometrium interaction is indispensable for TH-mediated increased expression of these proteases. So far, no molecular signals regulating hatching in humans have been identified, and only mechanical forces have been proposed to induce zona opening; however very recently the possibility that proteases may be involved also in humans has been suggested (Almagor et al., 2020), although further studies are needed to support this hypothesis.
Once free from the zona, the embryo contacts the endometrial epithelium and starts its journey to further proceed in the consolidation of pregnancy. Hormones, pro- and anti-inflammatory mediators, growth factors and their receptors, adhesion molecules, and proteases and protease inhibitors have been reported in the regulation of the process of embryo apposition, adhesion, and invasion (Massimiani et al., 2020). Among the many molecular signals, the Notch pathway has been recognized key in embryo-endometrium cross talk (Cuman et al., 2014). Transient activation of the Notch1 receptor has been demonstrated in the process of decidualization (Afshar et al., 2012), a limiting step to implantation, and blastocyst-conditioned medium has been shown to induce the expression of Notch family members in decidual cells (Hess et al., 2007). Expression of notch receptors and ligands has been reported in the blastocyst trophectoderm as well (Cuman et al., 2014). We previously demonstrated that the secreted factor epidermal growth factor-like domain 7 (EGFL7) is a novel modulator of the Notch pathway, it is expressed by trophoblast cells of the mouse blastocyst (Fitch et al., 2004; Lacko et al., 2014) and it regulates human trophoblast migration and invasion ability by activating the Notch pathway (Massimiani et al., 2015). Data will be presented demonstrating that EGFL7 is expressed in the human endometrium and its expression is mainly localized in the glandular epithelium throughout the menstrual cycle. EGFL7 is significantly upregulated in whole endometrial tissue during the secretory phase, particularly in the stromal compartment; this observation is further supported by in vitro studies showing its upregulation in decidualized endometrial stromal cells, suggesting a role for EGFL7 in implantation. Further support is provided by the evidence that its levels are strongly downregulated in the endometrium of women with impaired fertility. We propose EGFL7 as a novel player involved in the embryo-endometrium cross talk.