Mechanisms of formation and functions of the early embryonic cavities

Unlocking the potential of stem-cell-derived 'synthetic' embryo models

Stem-cell-derived 'synthetic' embryo models represent a revolutionary avenue in developmental biology, offering unprecedented insights into embryogenesis and tissue formation. However, the majority of current research on embryo models resides predominantly in the engineering construction phase, with limited substantive applications. This review explores the utilization of these embryo models and their applications in deciphering fundamental developmental processes. We delve into the methodologies employed in generating these models, emphasizing their potential to advance our understanding of embryonic development and disease. By evaluating current advancements and challenges, this review provides a comprehensive overview of the opportunities and implications of employing stem-cell-derived embryo models.

Trans-scale live-imaging of an E5.5 mouse embryo using incubator-type biaxial light-sheet microscopy

During mouse embryonic development, the embryonic day (E) 5.5 stage represents a crucial period for the formation of the primitive body axis, where the symmetry breaking of cellular states influences the multicellular system. Elucidating the detailed mechanisms of this process necessitates a trans-layered dynamic observation of the embryo and all internal cells. In this report, we present our success in achieving in-toto single-cell observation in a whole hemisphere of an E5.5 embryo for 12 h, using a newly developed incubator-type biaxial light-sheet microscope. To achieve the success, we optimized our microscope system, including an incubator for culture stability, and refining the observation protocol to reduce phototoxicity. Our key discovery is that the scan speed during light-sheet formation plays a critical role in reducing phototoxicity, rather than the irradiation intensity or the interval time between frames. This innovative system not only enabled in-toto single-cell tracking but also led to the discovery of the abrupt shrinking of embryos whose contractile center was located at the extraembryonic ectoderm during monotonous growth up to the E6.5 stage.

Highly efficient construction of monkey blastoid capsules from aged somatic cells

Blastoids-blastocyst-like structures created in vitro-emerge as a valuable model for early embryonic development research. Non-human primates stem cell-derived blastoids are an ethically viable alternative to human counterparts, yet the low formation efficiency of monkey blastoid cavities, typically below 30%, has limited their utility. Prior research has predominantly utilized embryonic stem cells. In this work, we demonstrate the efficient generation of blastoids from induced pluripotent stem cells and somatic cell nuclear transfer embryonic stem cells derived from aged monkeys, achieving an 80% formation efficiency. We also introduce a hydrogel-based microfluidics platform for the scalable and reproducible production of size-adjustable, biodegradable blastoid capsules, providing a stable 3D structure and mechanical protection. This advancement in the high-efficiency, scalable production of monkey blastoid capsules from reprogrammed aged somatic cells significantly enhances the study of embryonic development and holds promise for regenerative medicine.

Regulation of trophectoderm morphogenesis by small GTPase RHOA through HIPPO signaling-dependent and -independent mechanisms in mouse preimplantation development

The trophectoderm (TE) is the first tissue to differentiate during the preimplantation development of the mammalian embryo. It forms the outer layer of the blastocyst and is responsible for generating the blastocoel, a fluid-filled cavity whose expansion is essential for successful hatching and implantation. Here, we investigated the role of the small GTPase RHOA in the morphogenesis of the TE, particularly its relationship with HIPPO signaling, using mouse embryos as a model. Inhibition of RHOA resulted in the failure to form a blastocoel and significantly altered the expression of numerous genes. Transcriptomic analysis revealed that 330 genes were down-regulated and 168 genes were up-regulated by more than two-fold. Notably, 98.4% of these transcriptional changes were reversed by simultaneous inhibition of LATS kinases, indicating that the transcriptional influence of RHOA is primarily mediated through HIPPO signaling. Many of the down-regulated genes are involved in critical processes of TE morphogenesis, such as apical-basal cell polarization, tight junction formation, and sodium and water transport, suggesting that RHOA supports TE development by enhancing the expression of morphogenesis-related genes through HIPPO signaling, specifically via TEAD transcription factors. However, RHOA inhibition also disrupted apical-basal polarity and tight junctions, effects that were not restored by LATS inhibition, pointing to additional HIPPO signaling-independent mechanisms by which RHOA controls TE morphogenesis. Furthermore, RHOA inhibition impaired cell viability at the late blastocyst stage, with partial rescue observed upon LATS inhibition, suggesting that RHOA maintains cell survival through both HIPPO signaling-dependent and -independent pathways. A deeper knowledge of the molecular mechanisms governing TE morphogenesis, including blastocoel expansion and cell viability, could significantly advance assisted reproductive technologies aimed at producing healthy blastocysts.

The Interplay of Molecular Factors and Morphology in Human Placental Development and Implantation

The placenta is a vital organ that supports fetal development by mediating nutrient and gas exchange, regulating immune tolerance, and maintaining hormonal balance. Its formation and function are tightly linked to the processes of embryo implantation and the establishment of a robust placental-uterine interface. Recent advances in molecular biology and histopathology have shed light on the key regulatory factors governing these processes, including trophoblast invasion, spiral artery remodeling, and the development of chorionic villi. This review integrates morphological and molecular perspectives on placental development, emphasizing the roles of cytokines, growth factors, and signaling pathways, such as VEGF and Notch signaling, in orchestrating implantation and placental formation. The intricate interplay between molecular regulation and morphological adaptations highlights the placenta's critical role as a dynamic interface in pregnancy. This review synthesizes current findings to offer clinicians and researchers a comprehensive understanding of the placenta's role in implantation, emphasizing its importance in maternal-fetal medicine. By integrating these insights, the review lays the groundwork for advancing diagnostic and therapeutic approaches that can enhance pregnancy outcomes and address related complications effectively.

The role of TEAD4 in trophectoderm commitment and development is not conserved in non-rodent mammals

The first lineage differentiation in mammals gives rise to the inner cell mass and the trophectoderm (TE). In mice, TEAD4 is a master regulator of TE commitment, as it regulates the expression of other TE-specific genes and its ablation prevents blastocyst formation, but its role in other mammals remains unclear. Herein, we have observed that TEAD4 ablation in two phylogenetically distant species (bovine and rabbit) does not impede TE differentiation, blastocyst formation and the expression of TE markers, such as GATA3 and CDX2, although a reduced number of cells in the inner cell mass was observed in bovine TEAD4 knockout (KO) blastocysts. Transcriptional analysis in bovine blastocysts revealed no major transcriptional effect of the ablation, although the expression of hypoblast and Hippo signalling-related genes tended to be decreased in KO embryos. Experiments were conducted in the bovine model to determine whether TEAD4 was required for post-hatching development. TEAD4 KO spherical conceptuses showed normal development of the embryonic disc and TE, but hypoblast migration rate was reduced. At later stages of development (tubular conceptuses), no differences were observed between KO and wild-type conceptuses.

Soft Polyethylene Glycol Hydrogels Support Human PSC Pluripotency and Morphogenesis

Lumenogenesis within the epiblast represents a critical step in early human development, priming the embryo for future specification and patterning events. However, little is known about the specific mechanisms that drive this process due to the inability to study the early embryo in vivo. While human pluripotent stem cell (hPSC)-based models recapitulate many aspects of the human epiblast, most approaches for generating these 3D structures rely on ill-defined, reconstituted basement membrane matrices. Here, we designed synthetic, nonadhesive polyethylene glycol (PEG) hydrogel matrices to better understand the role of matrix mechanical cues in iPSC morphogenesis, specifically elastic modulus. First, we identified a narrow range of hydrogel moduli that were conducive to the hPSC viability, pluripotency, and differentiation. We then used this platform to investigate the effects of the hydrogel modulus on lumenogenesis, finding that matrices of intermediate stiffness yielded the most epiblast-like aggregates. Conversely, stiffer matrices impeded lumen formation and apico-basal polarization, while the softest matrices yielded polarized but aberrant structures. Our approach offers a simple, modular platform for modeling the human epiblast and investigating the role of matrix cues in its morphogenesis.

Lumen expansion is initially driven by apical actin polymerization followed by osmotic pressure in a human epiblast model

Post-implantation, the pluripotent epiblast in a human embryo forms a central lumen, paving the way for gastrulation. Osmotic pressure gradients are considered the drivers of lumen expansion across development, but their role in human epiblasts is unknown. Here, we study lumenogenesis in a pluripotent-stem-cell-based epiblast model using engineered hydrogels. We find that leaky junctions prevent osmotic pressure gradients in early epiblasts and, instead, forces from apical actin polymerization drive lumen expansion. Once the lumen reaches a radius of ∼12 μm, tight junctions mature, and osmotic pressure gradients develop to drive further growth. Computational modeling indicates that apical actin polymerization into a stiff network mediates initial lumen expansion and predicts a transition to pressure-driven growth in larger epiblasts to avoid buckling. Human epiblasts show transcriptional signatures consistent with these mechanisms. Thus, actin polymerization drives lumen expansion in the human epiblast and may serve as a general mechanism of early lumenogenesis.

Effect of blastocyst development on hatching and embryo implantation

The mammalian zygote, formed after a sperm fertilizes an egg, undergoes several rounds of mitosis and morphogenesis to form the blastocyst. During the peri-implantation period, the blastocyst hatches out of the zona pellucida (ZP) and invades the receptive uterine endometrium. This process promotes maternal-fetal dialogue at the physiological and molecular level, thereby initiating the implantation process. Blastocyst hatching is a consequence of elevated osmotic pressure due to active Na+/K+ ion transporter in the blastocyst cavity, as well as proteases produced by trophectoderm (TE) that hydrolyze the ZP. This review summarizes the process underpinning blastocyst hatching, such as the hatching schedule, the location of TEs during initial hatching out of the ZP, the molecules involved in blastocyst hatching, and how these processes affect implantation events. Additionally, we focus on identifying crucial molecules that may influence the quality of implantation and predict the outcome of embryo implantation. Further understanding the mechanism of these molecules may help us to improve the efficiency of Assisted reproductive technology (ART) in livestock breeding. This review provides insight into embryonic development, specifically during the short-term process of blastocyst hatching and its effects on the following implantation.

Substrate nesting guides cyst morphogenesis of human pluripotent stem cells without 3D extracellular matrix overlay

Understanding the principles underlying the self-organization of stem cells into tissues is fundamental for deciphering human embryo development. Here, we report that, without three-dimensional (3D) extracellular matrix (ECM) overlay, human pluripotent stem cells (hPSCs) cultured on two-dimensional soft elastic substrates can self-organize into 3D cysts resembling the human epiblast sac in a stiffness-dependent manner. Our theoretical modeling predicts that this cyst organization is facilitated and guided by the spontaneous nesting of the soft substrate, which results from the adhesion-dependent mechanical interaction between cells and substrate. Such substrate nesting is sufficient for the 3D assembly and polarization of hPSCs required for cyst organization, even without 3D ECM overlay. Furthermore, we identify that the reversible substrate nesting and cyst morphogenesis also require appropriate activation of ROCK-Myosin II pathway. This indicates a unique set of tissue morphomechanical signaling mechanisms that clearly differ from the canonical cystogenic mechanism previously reported in 3D ECM. Our findings highlight an unanticipated synergy between mechanical microenvironment and mechanotransduction in controlling tissue morphogenesis and suggest a mechanics-based strategy for generation of hPSCs-derived models for early human embryogenesis. STATEMENT OF SIGNIFICANCE: Soft substrates can induce the self-organization of human pluripotent stem cells (hPSCs) into cysts without three-dimensional (3D) extracellular matrix (ECM) overlay. However, the underlying mechanisms by which soft substrate guides cystogenesis are largely unknown. This study shows that substrate nesting, resulting from cell-substrate interaction, plays an important role in cyst organization, including 3D assembly and apical-basal polarization. Additionally, actomyosin contractility mediated by the ROCK-Myosin II pathway also contributes to the substrate deformation and cyst morphology. These findings demonstrate the interplay between the mechanical microenvironment and cells in tissue morphogenesis, suggesting a mechanics-based strategy in building hPSC-derived models for early human embryo development.

Polarity inversion reorganizes the stem cell compartment of the trophoblast lineage

The extra-embryonic tissues that form the placenta originate from a small population of trophectoderm cells with stem cell properties, positioned at the embryonic pole of the mouse blastocyst. During the implantation stages, the polar trophectoderm rapidly proliferates and transforms into extra-embryonic ectoderm. The current model of trophoblast morphogenesis suggests that tissue folding reshapes the trophoblast during the blastocyst to egg cylinder transition. Instead of through folding, here we found that the tissue scale architecture of the stem cell compartment of the trophoblast lineage is reorganized via inversion of the epithelial polarity axis. Our findings show the developmental significance of polarity inversion and provide a framework for the morphogenetic transitions in the peri-implantation trophoblast.

Cloning by SCNT: Integrating Technical and Biology-Driven Advances

Somatic cell nuclear transfer (SCNT) into enucleated oocytes initiates nuclear reprogramming of lineage-committed cells to totipotency. Pioneer SCNT work culminated with cloned amphibians from tadpoles, while technical and biology-driven advances led to cloned mammals from adult animals. Cloning technology has been addressing fundamental questions in biology, propagating desired genomes, and contributing to the generation of transgenic animals or patient-specific stem cells. Nonetheless, SCNT remains technically complex and cloning efficiency relatively low. Genome-wide technologies revealed barriers to nuclear reprogramming, such as persistent epigenetic marks of somatic origin and reprogramming resistant regions of the genome. To decipher the rare reprogramming events that are compatible with full-term cloned development, it will likely require technical advances for large-scale production of SCNT embryos alongside extensive profiling by single-cell multi-omics. Altogether, cloning by SCNT remains a versatile technology, while further advances should continuously refresh the excitement of its applications.

Cadherin-mediated adhesion takes control

The formation of a centralised apical membrane initiation site (AMIS) is a key event in epithelial cell polarisation. A recent study by Liang et al demonstrates that AMIS localisation relies on cadherin-mediated cell adhesion.