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The first cell fate decision is the process by which cells of an embryo take on distinct lineage identities for the first time, representing the beginning of patterning during development. In mammals, this process separates an embryonic inner cell mass lineage (future new organism) from an extra-embryonic trophectoderm lineage (future placenta), and in the mouse, this is classically attributed to the consequences of apical-basal polarity. The mouse embryo acquires this polarity at the 8-cell stage, indicated by cap-like protein domains on the apical surface of each cell; those cells which retain polarity over subsequent divisions are specified as trophectoderm, and the rest as inner cell mass. Recent research has advanced our knowledge of this process – this review will discuss mechanisms behind the establishment of polarity and distribution of the apical domain, different factors affecting the first cell fate decision including heterogeneities between cells of the very early embryo, and the conservation of developmental mechanisms across species, including human.

Individual cells within de novo polarising tubes and cavities must integrate their forming apical domains into a centralised apical membrane initiation site (AMIS). This is necessary to enable organised lumen formation within multi-cellular tissue. Despite the well-documented importance of cell division in localising the AMIS, we have found a division-independent mechanism of AMIS localisation that relies instead on Cadherin-mediated cell-cell adhesion. Our study of de novo polarising mouse embryonic stem cells (mESCs) cultured in 3D suggests that cell-cell adhesion localises apical proteins such as PAR-6 to a centralised AMIS. Unexpectedly, we also found that mESC clusters lacking functional E-cadherin still formed a lumen-like cavity in the absence of AMIS localisation but did so at a later stage of development via a "closure" mechanism, instead of via hollowing. This work suggests that there are two, interrelated mechanisms of apical polarity localisation: cell adhesion and cell division. Alignment of these mechanisms in space allows for redundancy in the system and ensures the development of a coherent epithelial structure within a growing organ.

Mouse embryos are an invaluable tool to study mammalian development but establishment of novel transgenic mouse lines is a costly and time-consuming endeavour. Stem cell models, on the other hand, lend themselves to quick genetic manipulation and are easily scalable, yet they do not successfully capture morphogenesis.Here, we report stem cell-based structures called ETiX-embryoids that can recapitulate mouse post-implantation morphogenesis. ETiX-embryoids form by aggregating embryonic and trophoblast stem cells to embryonic stem cells transiently expressing Gata4. Over the course of 4 days, these structures develop like mouse post-implantation egg cylinders; they then undertake and complete gastrulation on day 5 and 6, respectively. On day 7, ETiX-embryoids undergo neural induction and form the mouse anterior-posterior axis from neural folds to tail bud by movements of convergent extension. On day 8 they form a brain, establish a beating heart-like structure and a gut tube, as they develop inside extraembryonic membranes equivalent to amnion and yolk sac. We detail here the experimental set up and procedure required to generate ETiX-embryoids, and anticipate that they will establish themselves as a novel, powerful tool to study mammalian development without generating a novel mouse lines.

Several in vitro models have been developed to recapitulate mouse embryogenesis solely from embryonic stem cells (ESCs). Despite mimicking many aspects of early development, they fail to capture the interactions between embryonic and extraembryonic tissues. To overcome this difficulty, we have developed a mouse ESC-based in vitro model that reconstitutes the pluripotent ESC lineage and the two extraembryonic lineages of the post-implantation embryo by transcription-factor-mediated induction. This unified model recapitulates developmental events from embryonic day 5.5 to 8.5, including gastrulation; formation of the anterior-posterior axis, brain, and a beating heart structure; and the development of extraembryonic tissues, including yolk sac and chorion. Comparing single-cell RNA sequencing from individual structures with time-matched natural embryos identified remarkably similar transcriptional programs across lineages but also showed when and where the model diverges from the natural program. Our findings demonstrate an extraordinary plasticity of ESCs to self-organize and generate a whole-embryo-like structure.

Embryonic stem (ES) cells can undergo many aspects of mammalian embryogenesis in vitro1–5, but their developmental potential is substantially extended by interactions with extraembryonic stem cells, including trophoblast stem (TS) cells, extraembryonic endoderm stem (XEN) cells and inducible XEN (iXEN) cells6–11. Here we assembled stem cell-derived embryos in vitro from mouse ES cells, TS cells and iXEN cells and showed that they recapitulate the development of whole natural mouse embryo in utero up to day 8.5 post-fertilization. Our embryo model displays headfolds with defined forebrain and midbrain regions and develops a beating heart-like structure, a trunk comprising a neural tube and somites, a tail bud containing neuromesodermal progenitors, a gut tube, and primordial germ cells. This complete embryo model develops within an extraembryonic yolk sac that initiates blood island development. Notably, we demonstrate that the neurulating embryo model assembled from Pax6-knockout ES cells aggregated with wild-type TS cells and iXEN cells recapitulates the ventral domain expansion of the neural tube that occurs in natural, ubiquitous Pax6-knockout embryos. Thus, these complete embryoids are a powerful in vitro model for dissecting the roles of diverse cell lineages and genes in development. Our results demonstrate the self-organization ability of ES cells and two types of extraembryonic stem cells to reconstitute mammalian development through and beyond gastrulation to neurulation and early organogenesis.

Mammalian embryos sequentially differentiate into trophectoderm and an inner cell mass, the latter of which differentiates into primitive endoderm and epiblast. Trophoblast stem (TS), extraembryonic endoderm (XEN) and embryonic stem (ES) cells derived from these three lineages can self-assemble into synthetic embryos, but the mechanisms remain unknown. Here, we show that a stem cell-specific cadherin code drives synthetic embryogenesis. The XEN cell cadherin code enables XEN cell sorting into a layer below ES cells, recapitulating the sorting of epiblast and primitive endoderm before implantation. The TS cell cadherin code enables TS cell sorting above ES cells, resembling extraembryonic ectoderm clustering above epiblast following implantation. Whereas differential cadherin expression drives initial cell sorting, cortical tension consolidates tissue organization. By optimizing cadherin code expression in different stem cell lines, we tripled the frequency of correctly formed synthetic embryos. Thus, by exploiting cadherin codes from different stages of development, lineage-specific stem cells bypass the preimplantation structure to directly assemble a postimplantation embryo.

Embryo-like structures generated from stem cells can achieve varying developmental milestones, but none have been shown to progress through gastrulation, neurulation, and organogenesis.1–7 Here, we show that “ETiX” mouse embryos, established from embryonic stem cells aggregated with trophoblast stem cells and inducible extraembryonic endoderm stem cells, can develop through gastrulation and beyond to undertake neural induction and generate the progenitors needed to create the entire organism. The head-folds of ETiX embryos show anterior expression of Otx2, defining forebrain and midbrain regions that resemble those of the natural mouse embryo. ETiX embryos also develop beating hearts, trunk structures comprising a neural tube and somites, tail buds containing neuromesodermal progenitors and primordial germ cells, and gut tubes derived from definitive endoderm. A fraction of ETiX embryos show neural tube abnormalities, which can be partially rescued by treatment with the metabolically active form of folic acid, reminiscent of common birth defects and therapies in humans. Notably, ETiX embryos also develop a yolk sac with blood islands. Overall, ETiX embryos uniquely recapitulate natural embryos, developing further than any other stem-cell derived model, through multiple post-implantation stages and within extra-embryonic membranes.

Summary Stem cell-based embryo models by cultured pluripotent and extra-embryonic lineage stem cells are novel platforms to model early postimplantation development. We showed that induced pluripotent stem cells (iPSCs) could form ITS (iPSCs and trophectoderm stem cells) and ITX (iPSCs, trophectoderm stem cells, and XEN cells) embryos, resembling the early gastrula embryo developed in vivo. To facilitate the efficient and unbiased analysis of the stem cell-based embryo model, we set up a machine learning workflow to extract multi-dimensional features and perform quantification of ITS embryos using 3D images collected from a high-content screening system. We found that different PSC lines differ in their ability to form embryo-like structures. Through high-content screening of small molecules and cytokines, we identified that BMP4 best promoted the morphogenesis of the ITS embryo. Our study established an innovative strategy to analyze stem cell-based embryo models and uncovered new roles of BMP4 in stem cell-based embryo models., Graphical abstract, Highlights • Set up machine learning-assisted image analysis to study stem cell-based embryos • Mouse iPSCs can form ITS or ITX embryos with TSCs and XEN cells • Different PSC lines differ in their abilities to form embryo-like structures • High-content screen identified BMP4 best promoted the morphogenesis of ITS embryos, In this article, Dr. J. Na and colleagues established a machine learning-assisted image analysis method to perform unbiased and automated analysis of stem cell-based embryos. With this powerful technology, the authors compared different pluripotent stem cell lines, optimized culture conditions, and conducted a small-molecule and cytokine screen to identify factors that can promote the development of stem cell-based embryos.

Although high-throughput RNA sequencing (RNA-seq) has greatly advanced small non-coding RNA (sncRNA) discovery, the currently widely used complementary DNA library construction protocol generates biased sequencing results. This is partially due to RNA modifications that interfere with adapter ligation and reverse transcription processes, which prevent the detection of sncRNAs bearing these modifications. Here, we present PANDORA-seq (panoramic RNA display by overcoming RNA modification aborted sequencing), employing a combinatorial enzymatic treatment to remove key RNA modifications that block adapter ligation and reverse transcription. PANDORA-seq identified abundant modified sncRNAs—mostly transfer RNA-derived small RNAs (tsRNAs) and ribosomal RNA-derived small RNAs (rsRNAs)—that were previously undetected, exhibiting tissue-specific expression across mouse brain, liver, spleen and sperm, as well as cell-specific expression across embryonic stem cells (ESCs) and HeLa cells. Using PANDORA-seq, we revealed unprecedented landscapes of microRNA, tsRNA and rsRNA dynamics during the generation of induced pluripotent stem cells. Importantly, tsRNAs and rsRNAs that are downregulated during somatic cell reprogramming impact cellular translation in ESCs, suggesting a role in lineage differentiation.

At implantation, the mouse embryo undergoes a critical transformation which requires the precise spatiotemporal control of signalling pathways necessary for morphogenesis and developmental progression. The role played by such signalling pathways during this transition are largely unexplored, due to the inaccessibility of the embryo during the implantation when it becomes engulfed by uterine tissues. Genetic studies demonstrate that mutant embryos for BMPs die around gastrulation. Here we have aimed to dissect the role of BMPs during pre-to post-implantation transition by using a protocol permitting the development of the embryo beyond implantation stages in vitro and using stem cells to mimic post-implantation tissue organisation. By assessing both the canonical and non-canonical mechanisms of BMP, we show that the loss of canonical BMP activity compromises the extra-embryonic ectoderm development. Our analyses demonstrate that BMP signalling maintains stem cell populations within both embryonic/extra-embryonic tissues during pre-to post-implantation development. These results may provide insight into the role played by BMP signalling in controlling early embryogenesis.

Early embryogenesis is a conserved and self-organized process. In the mammalian embryo, the potential for self-organization is manifested in its extraordinary developmental plasticity, allowing a correctly patterned embryo to arise despite experimental perturbation. The underlying mechanisms enabling such regulative development have long been a topic of study. In this Review, we summarize our current understanding of the self-organizing principles behind the regulative nature of the early mammalian embryo. We argue that geometrical constraints, feedback between mechanical and biochemical factors, and cellular heterogeneity are all required to ensure the developmental plasticity of mammalian embryo development.

Synthetic embryology aims to develop embryo-like structures from stem cells to provide new insight into early stages of mammalian development. Recent advances in synthetic embryology have highlighted the remarkable capacity of stem cells to self-organize under certain biochemical or biophysical stimulations, generating structures that recapitulate the fate and form of early mouse/human embryos, in which symmetry breaking, pattern formation, or proper morphogenesis can be observed spontaneously. Here we review recent progress on the design principles for different types of embryoids and discuss the impact of different biochemical and biophysical factors on the process of stem-cell self-organization. We also offer our thoughts about the principal future challenges.

Funder: Weston Havens Foundation; doi: https://doi.org/10.13039/100011223, Aneuploidy, the presence of an abnormal number of chromosomes, is a major cause of early pregnancy loss in humans. Yet, the developmental consequences of specific aneuploidies remain unexplored. Here, we determine the extent of post-implantation development of human embryos bearing common aneuploidies using a recently established culture platform. We show that while trisomy 15 and trisomy 21 embryos develop similarly to euploid embryos, monosomy 21 embryos exhibit high rates of developmental arrest, and trisomy 16 embryos display a hypo-proliferation of the trophoblast, the tissue that forms the placenta. Using human trophoblast stem cells, we show that this phenotype can be mechanistically ascribed to increased levels of the cell adhesion protein E-CADHERIN, which lead to premature differentiation and cell cycle arrest. We identify three cases of mosaicism in embryos diagnosed as full aneuploid by pre-implantation genetic testing. Our results present the first detailed analysis of post-implantation development of aneuploid human embryos.

Decidual remodelling of midluteal endometrium leads to a short implantation window after which the uterine mucosa either breaks down or is transformed into a robust matrix that accommodates the placenta throughout pregnancy. To gain insights into the underlying mechanisms, we established and characterised endometrial assembloids, consisting of gland organoids and primary stromal cells. Single-cell transcriptomics revealed that decidualized assembloids closely resemble midluteal endometrium, harbouring differentiated and senescent subpopulations in both glands and stroma. We show that acute senescence in glandular epithelium drives secretion of multiple canonical implantation factors, whereas in the stroma it calibrates the emergence of anti-inflammatory decidual cells and pro-inflammatory senescent decidual cells. Pharmacological inhibition of stress responses in pre-decidual cells accelerated decidualization by inhibiting senescence and mesenchymal-epithelial transition, processes involved in endometrial breakdown and regeneration, respectively. Accelerated decidualization resulted in entrapment of co-cultured human blastocysts in a largely static decidual matrix. By contrast, the presence of senescent decidual cells created a dynamic implantation environment, enabling embryo expansion and attachment, although their persistence led to gradual disintegration of assembloids. Our findings demonstrate that senescence controls endometrial fate decisions at implantation and highlight how endometrial assembloids may accelerate the discovery of new treatments to prevent reproductive failure.

Following implantation, the human embryo undergoes major morphogenetic transformations that establish the future body plan. While the molecular events underpinning this process are established in mice, they remain unknown in humans. Here we characterise key events of human embryo morphogenesis, in the period between implantation and gastrulation, using single-cell analyses and functional studies. First, the embryonic epiblast cells transition through different pluripotent states and act as a source of FGF signals that ensure proliferation of both embryonic and extra-embryonic tissues. In a subset of embryos, we identify a group of asymmetrically positioned extra-embryonic hypoblast cells expressing inhibitors of BMP, NODAL and WNT signalling pathways. We suggest that this group of cells can act as the anterior singalling centre to pattern the epiblast. These results provide insights into pluripotency state transitions, the role of FGF signalling and the specification of anterior-posterior axis during human embryo development., Single cell analysis of early human embryos identifies key changes in pluripotency, the requirement of FGF signalling for embryo survival, and defines a putative anterior-like region of hypoblast cells, providing insights into how early human development is regulated.

During embryonic development, a fertilized egg transitions into a complex organism, whereby diverse cell types are spatially organized into functional tissues. This sequential unfurling of complexity is exemplified perhaps best at gastrulation, where the major axes of the adult are laid down and the principal germ layers are defined in tandem with a dramatic change in morphology (Tam and Behringer, 1997; Arnold and Robertson, 2009). Understanding the mechanisms by which this highly choreographed process plays out demands investigation at different spatial scales (Shahbazi et al., 2019): from the cell-intrinsic gene-regulatory networks that govern cell fate decisions; to the signaling interactions that coordinate divergent fate trajectories across tissues. It requires understanding of how these regulatory processes are coupled to global characteristics including embryonic geometry, patterning, and inter-tissue interactions. Dissecting mechanisms across such scales — and crucially understanding their interactions — requires the capacity to manipulate genes, signals and morphology, a task that has proved challenging particularly for mammalian experimental embryology given the inaccessibility of the conceptus at implantation, a time when many of these important events occur (Hadjantonakis et al., 2020; Shahbazi and Zernicka-Goetz, 2018). Synthetic embryology has risen to this challenge, engineering embryo-like structures — “stembryos” — using stem cells derived from embryos situating cells in both near-native and more foreign contexts, and probing the consequences for patterning and morphogenesis. Given the recent explosion of diversity in stembryo models (Harrison et al., 2017; Rivron et al., 2018; Shao et al., 2017a; Sozen et al., 2018; Van den Brink et al., 2014; Veenvliet et al., 2020; Warmflash et al., 2014; Amadei et al., 2020), this Special Issue reflects on the successes made and the challenges that remain in using in vitro culture platforms to study early mammalian embryogenesis. Here, we offer a perspective on the purpose and utility of synthetic embryology, proposing that focusing efforts on probing fundamental mechanisms of self-organization, spurred via novel bioengineering strategies, can help unify the field towards its shared motivations of understanding natural development and building reliable, translatable experimental models.

Understanding human development is of fundamental biological and clinical importance. Yet despite its significance, insights into early developmental events in humans still remain largely unknown. While recent advances show that stem cells can mimic embryogenesis1–9to unravel hidden developmental mechanisms, a stem cell-based model of early human embryogenesis is lacking. Here, we use human extended pluripotent stem cells10to reconstitute early human development in 3-dimensions and recapitulate early embryo-like events. We first perform a systematic characterisation to reveal unique signalling requirements for building the human pre-implantation blastocyst. Further, we show that thesein vitrostem cell-derived blastocyst-like structures are able to undertake spatiotemporal self-organisation to mimic peri-implantation remodelling in which a polarised rosette opens up the amniotic cavity within a developing disc. The hallmarks of human early development displayed by this stem cell-basedin vitromodel mimics features of embryonic day 3 to day 9/10 of natural development. Thus, this platform represents a tractable model system to contribute to the basic understanding of cellular and molecular mechanisms governing early embryonic events in humans and to provide valuable insights into the design of differentiation protocols for human stem cells in clinical applications.

Summary The development of mouse embryos can be partially recapitulated by combining embryonic stem cells (ESCs), trophoblast stem cells (TS), and extra-embryonic endoderm (XEN) stem cells to generate embryo-like structures called ETX embryos. Although ETX embryos transcriptionally capture the mouse gastrula, their ability to recapitulate complex morphogenic events such as gastrulation is limited, possibly due to the limited potential of XEN cells. To address this, we generated ESCs transiently expressing transcription factor Gata4, which drives the extra-embryonic endoderm fate, and combined them with ESCs and TS cells to generate induced ETX embryos (iETX embryos). We show that iETX embryos establish a robust anterior signaling center that migrates unilaterally to break embryo symmetry. Furthermore, iETX embryos gastrulate generating embryonic and extra-embryonic mesoderm and definitive endoderm. Our findings reveal that replacement of XEN cells with ESCs transiently expressing Gata4 endows iETX embryos with greater developmental potential, thus enabling the study of the establishment of anterior-posterior patterning and gastrulation in an in vitro system., Graphical Abstract, Highlights • Stem cells generate mouse-embryo-like structures with improved potential • These structures undertake anterior visceral endoderm formation and gastrulation • Single-cell sequencing shows improved resemblance to mouse embryo, Amadei et al. have generated stem-cell-based structures that resemble mouse post-implantation embryos and have the potential to form the anterior-posterior axis and undergo gastrulation in vitro. Single-cell sequencing shows gene-expression patterns similar to those of the natural embryo at a comparable stage of development.

Early human post-implantation development involves extensive growth combined with a series of complex morphogenetic events. The lack of precise spatial and temporal control over these processes leads to pregnancy loss. Given the ethical and technical limitations in studying the natural human embryo, alternative approaches are needed to investigate mechanisms underlying this critical stage of human development. Here, we present an overview of the different stem cells and stem cell-derived models which serve as useful, albeit imperfect, tools in understanding human embryogenesis. Current models include stem cells that represent each of the three earliest lineages: human embryonic stem cells corresponding to the epiblast, hypoblast-like stem cells and trophoblast stem cells. We also review the use of human embryonic stem cells to model complex aspects of epiblast morphogenesis and differentiation. Additionally, we propose that the combination of both embryonic and extra-embryonic stem cells to form three-dimensional embryo models will provide valuable insights into cell-cell chemical and mechanical interactions that are essential for natural embryogenesis.

Timing and trigger of cell polarization During mammalian embryo development, the first cell fate decision separates the progenitors of the trophectoderm (destined to form the placenta) from the inner cell mass (which forms all tissues of the embryonic and yolk sac). A key event for this first lineage segregation is the establishment of apicobasal cell polarity. This event is set to occur at a fixed developmental stage. The factors that trigger the establishment of cell polarity as well as its temporal regulation have remained unknown. Zhu et al. show in mouse embryos that three molecular regulators—Tfap2c, Tead4, and RhoA—are sufficient to advance the timing of cell polarization with subsequent cell fate specification and morphogenesis. Science , this issue p. eabd2703

Pluripotent stem cells (PSCs) transition between cell states in vitro, reflecting developmental changes in the early embryo. PSCs can be stabilized in the naive state by blocking extracellular differentiation stimuli, particularly FGF–MEK signalling. Here, we report that multiple features of the naive state in human and mouse PSCs can be recapitulated without affecting FGF–MEK signalling or global DNA methylation. Mechanistically, chemical inhibition of CDK8 and CDK19 (hereafter CDK8/19) kinases removes their ability to repress the Mediator complex at enhancers. CDK8/19 inhibition therefore increases Mediator-driven recruitment of RNA polymerase II (RNA Pol II) to promoters and enhancers. This efficiently stabilizes the naive transcriptional program and confers resistance to enhancer perturbation by BRD4 inhibition. Moreover, naive pluripotency during embryonic development coincides with a reduction in CDK8/19. We conclude that global hyperactivation of enhancers drives naive pluripotency, and this can be achieved in vitro by inhibiting CDK8/19 kinase activity. These principles may apply to other contexts of cellular plasticity., M.N.S. was funded by a Leverhulme Trust early career fellowship. Work in the laboratory of M.Z.-G. was funded by the Wellcome Trust (098287/Z/12/Z) and the European Research Council (ERC) (669198). I.C. was funded by the Secretaria d’Universitats i Recerca de la Generalitat de Catalunya and European Social Fund. I.A. and P.S. were supported by the Fondation pour la Recherche Medicale (DEQ20170336757), Infrastructure Nationale en Biologie et Santé INGESTEM (ANR-11-INBS-0009), IHU-B CESAME (ANR-10-IBHU-003), LabEx REVIVE (ANR-10-LABX-73), LabEx DEVweCAN (ANR-10-LABX-0061) and LabEx CORTEX (ANR-11-LABX-0042) of University of Lyon within the programme ‘Investissements d’Avenir’ (ANR-11-IDEX-0007). Research by J.P., S.M. and C.B.-A. was supported in part by a grant from the Spanish Ministry of Economy and Competitiveness (SAF2013-44267-R) and by the CNIO. Work in the laboratory of D.F. was funded by the Institut National du Cancer (PLBIO10-068 and PLBIO15-005) and the Ligue National Contre le Cancer (EL2018.LNCC/DF). Work in the laboratory of N.M. was funded by the ERC, under the European Union Horizon 2020 research and innovation programme (StG-2014–640525_REGMAMKID), the Spanish Association Against Cancer (AECC/LABAE16006), Carlos III Health Institute (Red TerCel, CardioCel, RD16/0011/0027), Ministry of Economy and Competitiveness (MINECO) projects SAF2017–89782-R, SAF2015–72617-EXP and RYC-2014–16242, and the CERCA/Government of Catalonia (2017 SGR 1306). Work in the laboratory of S.O. was funded by SAF2013–44866-R from MINECO Spain. Work in the laboratory of M.F.F. was funded by Plan Nacional de I+D+I 2013–2016/FEDER (PI15/00892, to M.F.F. and A.F.F.); the ISCIII-Subdireccion General de Evaluación y Fomento de la Investigación and Plan Nacional de I+D+I 2008–2011/FEDER (CP11/00131, to A.F.F.); IUOPA (to M.I.S.); and the Asturias Regional Government (GRUPIN14–052, to M.F.F.). The IUOPA is supported by the Obra Social Liberbank-Cajastur, Spain. Work in the laboratory of M.S. was funded by the CNIO, the IRB and by grants from Spanish Ministry of Economy co-funded by the European Regional Development Fund (SAF2017-82613-R), ERC (ERC-2014-AdG/669622), Botin Foundation, Banco Santander (Santander Universities Global Division), laCaixa Foundation and Secretaria d’Universitats i Recerca del Departament d’Empresa i Coneixement of Catalonia (Grup de Recerca consolidat 2017 SGR 282).

The formation of differential cell lineages in the mammalian blastocyst from the totipotent zygote is crucial for implantation and the success of the whole pregnancy. The first lineage segregation generates the polarised trophectoderm (TE) tissue, which forms the placenta, and the apolar inner cell mass (ICM), which mainly gives rise to all foetal tissues and also the yolk sac1–3. The mechanism underlying this cell fate segregation has been extensively studied in the mouse embryo4,5. However, when and how it takes place in the human embryo remains unclear. Here, using time-lapse imaging and 325 surplus human embryos, we provide a detailed characterisation of morphological events and transcription factor expression and localisation to understand how they lead to the first lineage segregation in human embryogenesis. We show that the first lineage segregation of the human embryo is triggered by cell polarisation that occurs at the 8-cell stage in two sequential steps. In the first step, F-actin becomes apically polarised concomitantly with embryo compaction. In the second step, the Par complex becomes polarised to form the apical cellular domain. Mechanistically, we show that activation of Phospholipase C (PLC) triggers actin polarisation and is therefore essential for apical domain formation, as is the case in mouse embryos6. Finally, we show that, in contrast to the mouse embryo, the key extra-embryonic determinant GATA37,8 is expressed not only in extra-embryonic lineage precursors upon blastocyst formation. However, the cell polarity machinery enhances the expression and nuclear accumulation of GATA3. In summary, our results demonstrate for the first time that cell polarisation reinforces the first lineage segregation in the human embryo.