We investigate the mechanisms that direct ‘cell fate’ in human embryos and stem cells. This means studying the different factors that tell embryonic cells which type of cell to become.
After a human egg is fertilised, the cells multiply as the embryo grows. After five days there are around 100 cells, under 10 of which are embryonic epiblast cells that go on to form the foetus – these are pluripotent cells, as they are capable of becoming any type of cell in the body. The remaining 90 or so cells will go on to form either the placenta or the yolk sac.
We’re trying to understand how these early human pluripotent embryonic cells are established, how they remain pluripotent and how this process if turned off when the cells specialise. We’re trying to map out the complex hierarchy of different genes that control cell activity in early development, determine the influence of factors outside of the cells and understand the similarities and differences between human and mouse development.
The processes that underpin early development and stem cell pluripotency are fundamental to human biology. If we knew how these processes worked, this knowledge could inform the understanding and treatment of infertility and developmental disorders. We could also use this knowledge to improve our use of stem cells in both science and medicine.
The allocation of cells to a specific lineage is regulated by the activities of key signalling pathways and developmentally regulated transcription factors. The focus of our research is to understand the influence of signalling and transcription factors on differentiation during early human development. During preimplantation development, totipotent human zygotes undergo subsequent rounds of mitotic cell divisions leading to the divergence of pluripotent embryonic cells, which form the foetus, and extra-embryonic cells, which contribute to the placenta and yolk sac.
The central question we are addressing is what are the molecular mechanisms that regulate embryonic pluripotency and how is it disengaged during cellular differentiation? We seek to define the genetic hierarchy acting during differentiation, the influence of extracellular signalling and the extent to which these mechanisms are conserved between humans and mice.
Implantation of a human embryo into the endometrium is a crucial event in gestation, as it marks the initiation of a pregnancy and is prone to high failure rates. We have limited understanding of these stages because of the inaccessibility of implanting embryos and the lack of suitable model systems. Here, we establish an in vitro model that recapitulates the luminal, glandular, and stromal compartments of the superficial layer of receptive human endometrium. Human embryos and blastoids implant into the endometrial model, achieving post-implantation hallmarks including advanced trophoblast structures that underlie early events in placental development. Single-cell RNA sequencing of the embryo-endometrial interface at day 14 uncovers predicted molecular interactions between conceptus and endometrium. Disrupting signaling interactions between extravillous trophoblast and endometrial stromal cells caused defects in trophoblast outgrowth, demonstrating the importance of crosstalk processes to sustain embryogenesis. This platform opens the opportunity to investigate early stages of human embryo implantation.
The placental DNA methylation landscape is unique, with widespread partially methylated domains (PMDs). The placental "methylome" is conserved across mammals, a shared feature of many cancers, and extensively studied for links with pregnancy complications. Human trophoblast stem cells (hTSCs) offer exciting potential for functional studies to better understand this epigenetic feature; however, whether the hTSC epigenome recapitulates primary trophoblast remains unclear. We find that hTSCs exhibit an atypical methylome compared with trophectoderm and 1 trimester cytotrophoblast. Regardless of cell origin, oxygen levels, or culture conditions, hTSCs show localized DNA methylation within transcribed gene bodies and a complete loss of PMDs. Unlike early human trophoblasts, hTSCs display a notable absence of DNMT3L expression, which is necessary for PMD establishment in mouse trophoblasts. Remarkably, we demonstrate that ectopic expression of DNMT3L in hTSCs restores placental PMDs, supporting a conserved role for DNMT3L in de novo methylation in trophoblast development in human embryogenesis.
The human blastocyst contains the pluripotent epiblast from which human embryonic stem cells (hESCs) can be derived. ACTIVIN/NODAL signaling maintains expression of the transcription factor NANOG and in vitro propagation of hESCs. It is unknown whether this reflects a functional requirement for epiblast development in human embryos. Here, we characterized NODAL signaling activity during pre-implantation human development. We showed that NANOG is an early molecular marker restricted to the nascent human pluripotent epiblast and was initiated prior to the onset of NODAL signaling. We further demonstrated that expression of pluripotency-associated transcription factors NANOG, SOX2, OCT4, and KLF17 were maintained in the epiblast in the absence of NODAL signaling activity. Genome-wide transcriptional analysis showed that NODAL signaling inhibition did not decrease NANOG transcription or impact the wider pluripotency-associated gene regulatory network. These data suggest differences in the signaling requirements regulating pluripotency in the pre-implantation human epiblast compared with existing hESC culture.