Investigating the functional crosstalk between human embryo and endometrium within the implantation niche

The implantation of a human embryo into a receptive endometrium is a complex and necessary step in the initiation of pregnancy. This process represents the point of failure in most unsuccessful pregnancies and is a barrier to successful infertility treatments. Our understanding of how embryos implant and how they establish a functional crosstalk with the endometrium is limited because of the inaccessibility of embryos in the days immediately following implantation.

To address this important gap, we have developed a novel in vitro three-dimensional, cell-engineered system that recapitulates the spatial architecture and physiology of the receptive human endometrium, which enables us for the first time to investigate the properties of human embryo implantation and early post-implantation development. Human blastocysts and stem cell-based blastoid embryo models attach with high efficiency to the endometrial surface, invade through the epithelial layer, and undergo deep, interstitial implantation into the stromal compartment.

In these conditions, developing embryos replicate the morphology of in vivo embryos with faithful tissue organisation and developmental landmarks. Trophoblast cells undergo substantial proliferation, lineage specification and integration within the endometrium. Profiling single-cell transcriptomes in a small number of embryos implanted within the in vitro endometrium at day 14 of development (the UK legal limit) revealed the diversity of cell types already formed at this stage.

In this proposal, we will build on these exciting advances by decoding the communication signals between invading trophoblast and maternal endometrium that are required for early post-implantation human embryo development. By identifying and functionally testing the ligand-receptor interactions in an in vitro setting, our work will transform our understanding of the initial molecular crosstalk between embryo and mother.

This new knowledge could lead to direct health benefits by informing the design of new therapies to improve the success rates of infertility treatments and by uncovering potential causes of early miscarriage. The specific objectives of the proposal are: 1) Characterise the embryo implantation niche and predict regulators of embryo-maternal interactions.

We will use our in vitro endometrial model together with human embryos to characterise the implantation niche at Days 8, 11 and 14 of development by single cell profiling and spatial transcriptomics. We will use these integrated data sets to predict cell-type specific ligand-receptor interactions. The outcome will be to obtain a comprehensive data set of factors expressed by trophoblast sub-types and endometrial sub-types and predictions of their functional interactions.

2) Determine functional molecular interactions within the implantation niche.

Here, we will functionally disrupt candidate ligand-receptor pairings in the trophoblast lineage of stem cell-based blastoid models or in the endometrial stromal cells within the implantation model. We will assess impact on implantation and growth. The outcomes will be to determine the functional mediators of embryo-maternal crosstalk in early postimplantation development.

3) Establish the role of specific ligand­-receptor interactions for trophoblast invasion.

To investigate the dynamics of embryo implantation and effect on trophoblast invasion following the disruption of targeted ligand-receptors, we will live-image blastoid and embryo implantation and early postimplantation development and quantify key parameters. The outcome will be to determine the requirement of specific embryo-maternal interactions on the integration and invasion of trophoblast within endometrium.

The successful completion of this research proposal will uncover vital new insights into the molecular crosstalk between embryo and endometrium during implantation and early development. Translational leads will be pursued with our project partners.