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Self-Organising Capacity of Stem Cells during Implantation and Early Post-implantation Development: Implications for Human Development

Periodic Reporting for period 3 - EPIROSE (Self-Organising Capacity of Stem Cells during Implantation and Early Post-implantation Development: Implications for Human Development)

Reporting period: 2019-01-01 to 2020-06-30

 Implantation is a defining event in mammalian pregnancy when the body axes are established and the embryonic germ layers are created. Despite its importance, this is the most enigmatic part of development, because upon implantation, the very small embryo becomes inaccessible within the body of the mother. We aim to understand how architectural features and signalling events integrate to regulate successive cell fate decisions and tissue morphogenesis during implantation and early post-implantation development.

By developing techniques to use stem cells to recapitulate embryo development in vitro, we have begun to fill the knowledge gap between pre and post- implantation development. We are also discovering the self-organising capabilities of stem cells and their ability to recapitulate embryonic development, with untold potential for regenerative medicine. An understanding of mechanisms that intertwine lineage specification, developmental plasticity and tissue morphogenesis at this critical developmental transition in the mouse model will give insight into the pathological embryo lethality and congenital malformations.

Our specific overall objectives are: 1. To characterize the spatio-temporal sequence of cellular mechanisms of peri-implantation morphogenesis in the mouse; 2. To determine cell autonomous molecular mechanisms of rosette formation in post-implantation development; 3. To determine non cell autonomous molecular mechanisms underlying the communication between embryonic and extra-embryonic compartments; 4. To identify the mechanisms behind the developmental plasticity of the peri-implantation embryo; 5: To establish the principles of self-organisation of mouse embryonic stem (ES) cells and to which extent they can mimic peri-implantation development.
1. We have characterised the complex process whereby the major cavity of the embryo (pro-amniotic cavity) is formed. We have found that an extra-embryonic cavity forms via tissue folding in contrast to the cavity of the pluripotent embryonic cells, which forms through characteristic hollowing of a rosette-shaped intermediate. Live imaging has helped elucidate how the polarised resolution of such rosettes orchestrates tissue remodelling revealing a new model for cavity extension and fusion.

2. We have found that the formation of the first rosette within the pluripotent epiblast cells, which will ultimately form the embryo proper, requires a defined series of cellular events that we have characterised molecularly. These cellular transitions are accompanied by changes in the pattern of gene expression from a state of naive pluripotency to one in which the cells become primed to begin to differentiate.

3. To gain insight into the molecular mechanisms driving post-implantation morphogenesis, we generated spatial transcriptome maps of implanting embryos along the proximo-distal axis of embryo and identified differentially expressed genes. We confirmed that the naïve pluripotency network has been dismantled by embryonic day E5.0 in agreement with our work in aim 2.

4. We have found that the pluripotent epiblast of artificially enlarged embryos can be down-sized by apoptosis. Moreover, small clumps of ES cells undergo lumenogenesis without cell death, whereas bigger structures require apoptosis to open the cavity. We found that cells with abnormal chromosome numbers can be eliminated both before and after implantation by apoptosis and are compensated by remaining cells with normal ploidy. This deepens our understanding of mechanisms underlying subfertility, developmental defects and failed pregnancies.

5. We have been able to generate synthetic mouse embryo-like structures from embryonic (ES) and trophoblast (TS) stem cells cultured in a gel of extracellular matrix. These can mimic development of embryonic and extra-embryonic tissues of natural embryos and generate mesoderm and primordial germ cells. By incorporating extra-embryonic endoderm stem cells (XEN cells), we generated synthetic embryo-like that achieve axis establishment, gastrulation, and definitive endoderm formation.
The findings from our laboratory are generating huge social interest over the last couple of years. We have become increasingly active in public engagement, reflecting the important implications of our research in mouse embryogenesis towards early human development, which is of natural interest to the general public. For example, during 2016 I was invited to give a TED-style presentation “Conception and Early Development” at the Stanford ChildX Symposium, aimed at the public and designed to inspire innovation that improves paediatric and maternal health. One theme of the presentation was our finding that aneuploid cells are eliminated from the tissue that forms the foetus by apoptosis and yet tolerated by extra-embryonic tissues that build placenta. This has important ramifications for prenatal diagnosis by the sampling of single cells from pre-implantation embryos because aneuploidy detected in this way would not necessary mean that the pregnancy is destined to a failure.

Our recent Science publication (Harrison et al., Science (2017) 356, 153) showing that embryonic and extra-embryonic stem cells can be grown in culture to generate embryo-like-structures that mimic the natural embryogenesis has attracted extensive media coverage due to potential implications for wider-society. For example, the use of these ‘artificial embryos’ to improve the understanding the very early stages of embryo development is of interest as this knowledge may help explain why a significant number of human pregnancies fail at this time.

These findings have given us opportunities to interact with journalists from the popular press and to engage with the general public.