Reproduction Abstracts

Implantation failure remains a bottleneck in assisted reproduction treatments (ARTs), as only ~25% of treatment cycles result in a live birth. During the early stages of implantation, endocrine and embryonic paracrine signals prime the receptivity of the uterine luminal epithelium (LE) before the trophectoderm (TE) of the blastocyst-stage embryo can mediate attachment and subsequently invade the uterine epithelium and stroma. We have developed an in vitro model using the human endometrial adenocarcinoma Ishikawa cell line with mouse blastocysts. Weak and reversible adhesive interactions were observed during co-culture of embryonic day (E) 4.5 blastocysts with Ishikawa cells over 28 h before embryos attained stable attachment over the following 20 h, progressing to breach the Ishikawa cells by 48 h. This process was steroid hormone-independent. Embryos that were initially cultured in the absence of Ishikawa cells (from E4.5-5.5) attached with the same kinetics as co-cultured embryos, but their ability to breach the epithelial layer was strongly impaired this effect was dependent on contact between blastocysts and Ishikawa cells as revealed by transwell co-culture experiments. Hyperosmolarity is a clinically relevant and experimentally well-defined embryonic stressor. Remarkably, repeated hyperosmolar stress (400mOsm osmolarity increase) at E5.5 in the absence of prior co-culture promoted embryonic breaching of the Ishikawa cells to levels seen with co-culture from E4.5. Furthermore, pharmacological inhibition revealed this effect to be dependent on signalling through the stress-activated protein kinase JNK. Stress is known to induce differentiation of embryonic cells, leading us to hypothesise that TE differentiation is required for breaching of the LE, and that both apposition with the LE and stress can promote implantation. These findings have clinical implications for ART as well as biological importance in understanding early pregnancy. Moreover, demonstrating that stress impacts this epigenetically critical developmental period has epidemiological relevance in line with the developmental origins of health and disease hypothesis.