The Apical Domain Is Required and Sufficient for the First Lineage Segregation in the Mouse Embryo

The first lineage determination in mammals

This review describes in detail the morphological, cytoskeletal and gene expression events leading to the gene regulatory network bifurcation point of trophoblast and inner cell mass cells in a variety of mammalian preimplantation embryos. The interrelated processes of compaction and polarity establishment are discussed in terms of how they affect YAP/WWTR activity and the location and fate of cells. Comparisons between mouse, human, cattle, pig and rabbit embryos suggest a conserved role for YAP/WWTR signalling in trophoblast induction in eutherian animals though the mechanisms for, and timing of, YAP/WWTR activation differs among species. Downstream targets show further differences, with the trophoblast marker GATA3 being a direct target in all examined mammals, while CDX2-positive and SOX2-negative regulation varies.

The fusion of physics and biology in early mammalian embryogenesis

Biomechanics in embryogenesis is a dynamic field intertwining the physical forces and biological processes that shape the first days of a mammalian embryo. From the first cell fate bifurcation during blastulation to the complex symmetry breaking and tissue remodeling in gastrulation, mechanical cues appear critical in cell fate decisions and tissue patterning. Recent strides in mouse and human embryo culture, stem cell modeling of mammalian embryos, and biomaterial design have shed light on the role of cellular forces, cell polarization, and the extracellular matrix in influencing cell differentiation and morphogenesis. This chapter highlights the essential functions of biophysical mechanisms in blastocyst formation, embryo implantation, and early gastrulation where the interplay between the cytoskeleton and extracellular matrix stiffness orchestrates the intricacies of embryogenesis and placenta specification. The advancement of in vitro models like blastoids, gastruloids, and other types of embryoids, has begun to faithfully recapitulate human development stages, offering new avenues for exploring the biophysical underpinnings of early development. The integration of synthetic biology and advanced biomaterials is enhancing the precision with which we can mimic and study these processes. Looking ahead, we emphasize the potential of CRISPR-mediated genomic perturbations coupled with live imaging to uncover new mechanosensitive pathways and the application of engineered biomaterials to fine-tune the mechanical conditions conducive to embryonic development. This synthesis not only bridges the gap between experimental models and in vivo conditions to advancing fundamental developmental biology of mammalian embryogenesis, but also sets the stage for leveraging biomechanical insights to inform regenerative medicine.

Tead4 and Tfap2c generate bipotency and a bistable switch in totipotent embryos to promote robust lineage diversification

The mouse and human embryo gradually loses totipotency before diversifying into the inner cell mass (ICM, future organism) and trophectoderm (TE, future placenta). The transcription factors TFAP2C and TEAD4 with activated RHOA accelerate embryo polarization. Here we show that these factors also accelerate the loss of totipotency. TFAP2C and TEAD4 paradoxically promote and inhibit Hippo signaling before lineage diversification: they drive expression of multiple Hippo regulators while also promoting apical domain formation, which inactivates Hippo. Each factor activates TE specifiers in bipotent cells, while TFAP2C also activates specifiers of the ICM fate. Asymmetric segregation of the apical domain reconciles the opposing regulation of Hippo signaling into Hippo OFF and the TE fate, or Hippo ON and the ICM fate. We propose that the bistable switch established by TFAP2C and TEAD4 is exploited to trigger robust lineage diversification in the developing embryo.

Temporally distinct roles of Aurora A in polarization of the C. elegans zygote

During asymmetric cell division, cell polarity is coordinated with the cell cycle to allow proper inheritance of cell fate determinants and the generation of cellular diversity. In the Caenorhabditis elegans zygote, polarity is governed by evolutionarily conserved Partitioning-defective (PAR) proteins that segregate to opposing cortical domains to specify asymmetric cell fates. Timely establishment of PAR domains requires a cell cycle kinase, Aurora A (AIR-1 in C. elegans). Aurora A depletion by RNAi causes a spectrum of phenotypes including reversed polarity, excess posterior domains and no posterior domain. How depletion of a single kinase can cause seemingly opposite phenotypes remains obscure. Using an auxin-inducible degradation system and drug treatments, we found that AIR-1 regulates polarity differently at different times of the cell cycle. During meiosis I, AIR-1 acts to prevent later formation of bipolar domains, whereas in meiosis II, AIR-1 is necessary to recruit PAR-2 onto the membrane. Together, these data clarify the origin of multiple polarization phenotypes in RNAi experiments and reveal multiple roles of AIR-1 in coordinating PAR protein localization with cell cycle progression.

Mechanical strengthening of cell-cell adhesion during mouse embryo compaction

Compaction is the first morphogenetic movement of the eutherian mammals and involves a developmentally regulated adhesion process. Previous studies investigated cellular and mechanical aspects of compaction. During mouse and human compaction, cells spread onto each other as a result of a contractility-mediated increase in surface tension pulling at the edges of their cell-cell contacts. However, how compaction may affect the mechanical stability of cell-cell contacts remains unknown. Here, we used a dual pipette aspiration assay on cell doublets to quantitatively analyze the mechanical stability of compacting mouse embryos. We measured increased mechanical stability of contacts with rupture forces growing from 40 to 70 nN, which was highly correlated with cell-cell contact expansion. Analyzing the dynamic molecular reorganization of cell-cell contacts, we find minimal recruitment of the cell-cell adhesion molecule Cdh1 (also known as E-cadherin) to contacts but we observe its reorganization into a peripheral adhesive ring. However, this reorganization is not associated with increased effective bond density, contrary to previous reports in other adhesive systems. Using genetics, we reduce the levels of Cdh1 or replace it with a chimeric adhesion molecule composed of the extracellular domain of Cdh1 and the intracellular domain of Cdh2 (also known as N-cadherin). We find that reducing the levels of Cdh1 impairs the mechanical stability of cell-cell contacts due to reduced contact growth, which nevertheless show higher effective bond density than wild-type contacts of similar size. On the other hand, chimeric adhesion molecules cannot form large or strong contacts indicating that the intracellular domain of Cdh2 is unable to reorganize contacts and/or is mechanically weaker than the one of Cdh1 in mouse embryos. Together, we find that mouse embryo compaction mechanically strengthens cell-cell adhesion via the expansion of Cdh1 adhesive rings that maintain pre-compaction levels of effective bond density.

Perspectives on polarity - exploring biological asymmetry across scales

In this Perspective, Journal of Cell Science invited researchers working on cell and tissue polarity to share their thoughts on unique, emerging or open questions relating to their field. The goal of this article is to feature 'voices' from scientists around the world and at various career stages, to bring attention to innovative and thought-provoking topics of interest to the cell biology community. These voices discuss intriguing questions that consider polarity across scales, evolution, development and disease. What can yeast and protists tell us about the evolution of cell and tissue polarity in animals? How are cell fate and development influenced by emerging dynamics in cell polarity? What can we learn from atypical and extreme polarity systems? How can we arrive at a more unified biophysical understanding of polarity? Taken together, these pieces demonstrate the broad relevance of the fascinating phenomenon of cell polarization to diverse fundamental biological questions.

Supercontinuum intrinsic fluorescence imaging heralds free view of living systems

Optimal imaging strategies remain underdeveloped to maximize information for fluorescence microscopy while minimizing the harm to fragile living systems. Taking hint from the supercontinuum generation in ultrafast laser physics, we generated supercontinuum fluorescence from untreated unlabeled live samples before nonlinear photodamage onset. Our imaging achieved high-content cell phenotyping and tissue histology, identified bovine embryo polarization, quantified aging-related stress across cell types and species, demystified embryogenesis before and after implantation, sensed drug cytotoxicity in real-time, scanned brain area for targeted patching, optimized machine learning to track small moving organisms, induced two-photon phototropism of leaf chloroplasts under two-photon photosynthesis, unraveled microscopic origin of autumn colors, and interrogated intestinal microbiome. The results enable a facility-type microscope to freely explore vital molecular biology across life sciences.

Nonlinear topological symmetry protection in a dissipative system

We investigate experimentally and theoretically a system ruled by an intricate interplay between topology, nonlinearity, and spontaneous symmetry breaking. The experiment is based on a two-mode coherently-driven optical resonator where photons interact through the Kerr nonlinearity. In presence of a phase defect, the modal structure acquires a synthetic Möbius topology enabling the realization of spontaneous symmetry breaking in inherently bias-free conditions without fine tuning of parameters. Rigorous statistical tests confirm the robustness of the underlying symmetry protection, which manifests itself by a periodic alternation of the modes reminiscent of period-doubling. This dynamic also confers long term stability to various localized structures including domain walls, solitons, and breathers. Our findings are supported by an effective Hamiltonian model and have relevance to other systems of interacting bosons and to the Floquet engineering of quantum matter. They could also be beneficial to the implementation of coherent Ising machines.

Origin and development of primary animal epithelia

Epithelia are the first organized tissues that appear during development. In many animal embryos, early divisions give rise to a polarized monolayer, the primary epithelium, rather than a random aggregate of cells. Here, we review the mechanisms by which cells organize into primary epithelia in various developmental contexts. We discuss how cells acquire polarity while undergoing early divisions. We describe cases where oriented divisions constrain cell arrangement to monolayers including organization on top of yolk surfaces. We finally discuss how epithelia emerge in embryos from animals that branched early during evolution and provide examples of epithelia-like arrangements encountered in single-celled eukaryotes. Although divergent and context-dependent mechanisms give rise to primary epithelia, here we trace the unifying principles underlying their formation.

Developmental perturbation in human embryos: Clinical and biological significance learned from time-lapse images

Background

Time-lapse technology (TLT) has gained widespread adoption worldwide. In addition to facilitating the undisturbed culture of embryos, TLT offers the unique capability of continuously monitoring embryos to detect spatiotemporal changes. Although these observed phenomena play a role in optimal embryo selection/deselection, the clinical advantages of introducing TLT remain unclear. However, manual annotation of embryo perturbation could facilitate a comprehensive assessment of developmental competence. This process requires a thorough understanding of embryo observation and the biological significance associated with developmental dogma and variation. This review elucidates the typical behavior and variation of each phenomenon, exploring their clinical significance and research perspectives.

Methods

The MEDLINE database was searched using PubMed for peer-reviewed English-language original articles concerning human embryo development.

Main findings

TLT allows the observation of consecutive changes in embryo morphology, serving as potential biomarkers for embryo assessment. In assisted reproductive technology laboratories, several phenomena have not revealed their mechanism, posing difficulties such as fertilization deficiency and morula arrest.

Conclusion

A profound understanding of the biological mechanisms and significance of each phenomenon is crucial. Further collaborative efforts between the clinical and molecular fields following translational studies are required to advance embryonic outcomes and assessment.

Temporally distinct roles of Aurora A in polarization of the C. elegans zygote

During asymmetric cell division, coordination of cell polarity and the cell cycle is critical for proper inheritance of cell fate determinants and generation of cellular diversity. In Caenorhabditis elegans (C. elegans), polarity is established in the zygote and is governed by evolutionarily conserved Partitioning defective (PAR) proteins that localize to distinct cortical domains. At the time of polarity establishment, anterior and posterior PARs segregate to opposing cortical domains that specify asymmetric cell fates. Timely establishment of these PAR domains requires a cell cycle kinase, Aurora A (AIR-1 in C.elegans). Aurora A depletion by RNAi causes a spectrum of phenotypes including no posterior domain, reversed polarity, and excess posterior domains. How depletion of a single kinase can cause seemingly opposite phenotypes remains obscure. Using an auxin-inducible degradation system, drug treatments, and high-resolution microscopy, we found that AIR-1 regulates polarity via distinct mechanisms at different times of the cell cycle. During meiosis I, AIR-1 acts to prevent the formation of bipolar domains, while in meiosis II, AIR-1 is necessary to recruit PAR-2 onto the membrane. Together these data clarify the origin of the multiple polarization phenotypes observed in RNAi experiments and reveal multiple roles of AIR-1 in coordinating PAR protein localization with the progression of the cell cycle.

Apical PAR protein caps orient the mitotic spindle in C. elegans early embryos

During embryonic development, oriented cell divisions are important for patterned tissue growth and cell fate specification. Cell division orientation is controlled in part by asymmetrically localized polarity proteins, which establish functional domains of the cell membrane and interact with microtubule regulators to position the mitotic spindle. For example, in the 8-cell mouse embryo, apical polarity proteins form caps on the outside, contact-free surface of the embryo that position the mitotic spindle to execute asymmetric cell division. A similar radial or "inside-outside" polarity is established at an early stage in many other animal embryos, but in most cases, it remains unclear how inside-outside polarity is established and how it influences downstream cell behaviors. Here, we explore inside-outside polarity in C. elegans somatic blastomeres using spatiotemporally controlled protein degradation and live embryo imaging. We show that PAR polarity proteins, which form apical caps at the center of the contact-free membrane, localize dynamically during the cell cycle and contribute to spindle orientation and proper cell positioning. Surprisingly, isolated single blastomeres lacking cell contacts are able to break symmetry and form PAR-3/atypical protein kinase C (aPKC) caps. Polarity caps form independently of actomyosin flows and microtubules and can regulate spindle orientation in cooperation with the key polarity kinase aPKC. Together, our results reveal a role for apical polarity caps in regulating spindle orientation in symmetrically dividing cells and provide novel insights into how these structures are formed.

Proteomics reveals the underlying mechanism by which the first uneven division affects embryonic development in pig

One of the most typical abnormal cleavage patterns during early embryonic development is uneven division, but the first uneven division of pig zygote is common. Uneven division results in different daughter cell sizes and an uneven distribution of organelles such as lipid droplet, mitochondria, but the developmental capacity of daughter cells and proteomic changes of daughter cells are still unclear. Therefore, the developmental ability and proteomic quantification were investigated on blastomeres from even division (ED) or uneven division (UD) embryos at 2-cell stage in the present study. Firstly, the developmental ability was affected by the blastomeric size, when compared with medium blastomeres (MBs), the large blastomeres (LBs) with the higher cleavage rate but the small blastomeres (SBs) with the lower rate was observed. Subsequently, proteomic analysis was performed on blastomeres of LBs, MBs and SBs, a total of 109 DEPs were detected, which were involved in protein metabolism and processing, energy metabolism and ribosome. In particular, DEPs in LBs vs. SBs were focused on RNA binding and actin cytoskeletal tissue. Two protein-dense networks associated with RNA binding and cytoskeleton were revealed by further protein-protein interaction (PPI) analysis of DEPs in LBs vs. SBs, that DDX1 related to RNA binding and ACTB related to cytoskeleton were confirmed in UD embryos. Therefore, a briefly information of DEPs in blastomeres of 2-cell stage pig embryos was described in the present study, and it further confirmed that the formation of uneven division of the first cell cycle of pig embryos might be controlled by the cytoskeleton; the developmental capacity of daughter cells might be affected by the energy metabolism, RNA binding and ribosome, and further account for the developmental potential of the whole embryo.

Genetic reporter for live tracing fluid flow forces during cell fate segregation in mouse blastocyst development

Mechanical forces are known to be important in mammalian blastocyst formation; however, due to limited tools, specific force inputs and how they relay to first cell fate control of inner cell mass (ICM) and/or trophectoderm (TE) remain elusive. Combining in toto live imaging and various perturbation experiments, we demonstrate and measure fluid flow forces existing in the mouse blastocyst cavity and identify Klf2(Krüppel-like factor 2) as a fluid force reporter with force-responsive enhancers. Long-term live imaging and lineage reconstructions reveal that blastomeres subject to higher fluid flow forces adopt ICM cell fates. These are reinforced by internal ferrofluid-induced flow force assays. We also utilize ex vivo fluid flow force mimicking and pharmacological perturbations to confirm mechanosensing specificity. Together, we report a genetically encoded reporter for continuously monitoring fluid flow forces and cell fate decisions and provide a live imaging framework to infer force information enriched lineage landscape during development. VIDEO ABSTRACT.

Daughter cells inherit YAP localization from mother cells in early preimplantation embryos

The first stage of cell differentiation during mouse development is the differentiation into the trophectoderm and inner cell mass, which occurs during the 8-32-cell stages of preimplantation embryos. This differentiation is regulated by the Hippo signaling pathway. At the 32-cell stage, embryos establish a position-dependent distribution of the Hippo pathway coactivator, Yes-associated protein 1 (YAP, encoded by Yap1). The outer and inner cells showed nuclear and cytoplasmic localization of YAP, respectively. However, the process by which embryos establish position-dependent YAP localization remains elusive. Here, we established a YAP-reporter mouse line, Yap1mScarlet , and examined YAP-mScarlet protein dynamics during the 8-32-cell stages using live imaging. During mitosis, YAP-mScarlet diffused throughout the cells. YAP-mScarlet dynamics in daughter cells varied depending on the cell division patterns. YAP-mScarlet localization in daughter cells at the completion of cell division coincided with that in mother cells. Experimental manipulation of YAP-mScarlet localization in mother cells also altered its localization in daughter cells upon completion of cell division. In daughter cells, YAP-mScarlet localization gradually changed to the final pattern. In some divisions during the 8-16-cell stages, the cytoplasmic YAP-mScarlet localization preceded cell internalization. These results suggest that cell position is not a primary determinant of YAP localization and that the Hippo signaling status of the mother cell is inherited by the daughter cells, which likely contributes to the stabilization of the cell fate specification process beyond cell division.

Spatial positioning of preimplantation mouse embryo cells is regulated by mTORC1 and m7G-cap-dependent translation at the 8- to 16-cell transition

Preimplantation mouse embryo development involves temporal-spatial specification and segregation of three blastocyst cell lineages: trophectoderm, primitive endoderm and epiblast. Spatial separation of the outer-trophectoderm lineage from the two other inner-cell-mass (ICM) lineages starts with the 8- to 16-cell transition and concludes at the 32-cell stages. Accordingly, the ICM is derived from primary and secondary contributed cells; with debated relative EPI versus PrE potencies. We report generation of primary but not secondary ICM populations is highly dependent on temporal activation of mammalian target of Rapamycin (mTOR) during 8-cell stage M-phase entry, mediated via regulation of the 7-methylguanosine-cap (m7G-cap)-binding initiation complex (EIF4F) and linked to translation of mRNAs containing 5' UTR terminal oligopyrimidine (TOP-) sequence motifs, as knockdown of identified TOP-like motif transcripts impairs generation of primary ICM founders. However, mTOR inhibition-induced ICM cell number deficits in early blastocysts can be compensated by the late blastocyst stage, after inhibitor withdrawal; compensation likely initiated at the 32-cell stage when supernumerary outer cells exhibit molecular characteristics of inner cells. These data identify a novel mechanism specifically governing initial spatial segregation of mouse embryo blastomeres, that is distinct from those directing subsequent inner cell formation, contributing to germane segregation of late blastocyst lineages.

Human embryo live imaging reveals nuclear DNA shedding during blastocyst expansion and biopsy

Proper preimplantation development is essential to assemble a blastocyst capable of implantation. Live imaging has uncovered major events driving early development in mouse embryos; yet, studies in humans have been limited by restrictions on genetic manipulation and lack of imaging approaches. We have overcome this barrier by combining fluorescent dyes with live imaging to reveal the dynamics of chromosome segregation, compaction, polarization, blastocyst formation, and hatching in the human embryo. We also show that blastocyst expansion mechanically constrains trophectoderm cells, causing nuclear budding and DNA shedding into the cytoplasm. Furthermore, cells with lower perinuclear keratin levels are more prone to undergo DNA loss. Moreover, applying trophectoderm biopsy, a mechanical procedure performed clinically for genetic testing, increases DNA shedding. Thus, our work reveals distinct processes underlying human development compared with mouse and suggests that aneuploidies in human embryos may not only originate from chromosome segregation errors during mitosis but also from nuclear DNA shedding.

The nuclear lamina couples mechanical forces to cell fate in the preimplantation embryo via actin organization

During preimplantation development, contractile forces generated at the apical cortex segregate cells into inner and outer positions of the embryo, establishing the inner cell mass (ICM) and trophectoderm. To which extent these forces influence ICM-trophectoderm fate remains unresolved. Here, we found that the nuclear lamina is coupled to the cortex via an F-actin meshwork in mouse and human embryos. Actomyosin contractility increases during development, upregulating Lamin-A levels, but upon internalization cells lose their apical cortex and downregulate Lamin-A. Low Lamin-A shifts the localization of actin nucleators from nucleus to cytoplasm increasing cytoplasmic F-actin abundance. This results in stabilization of Amot, Yap phosphorylation and acquisition of ICM over trophectoderm fate. By contrast, in outer cells, Lamin-A levels increase with contractility. This prevents Yap phosphorylation enabling Cdx2 to specify the trophectoderm. Thus, forces transmitted to the nuclear lamina control actin organization to differentially regulate the factors specifying lineage identity.

Pre-hatching exposure to N2B27 medium improves post-hatching development of bovine embryos in vitro

Ungulate embryos undergo critical cell differentiation and proliferation events around and after blastocyst hatching. Failures in these processes lead to early pregnancy losses, which generate an important economic impact on farming. Conventional embryo culture media, such as SOF, are unable to support embryo development beyond hatching. In contrast, N2B27 medium supports early post-hatching development, evidencing a swift in embryonic nutritional requirements during this developmental window. Here, we investigate if earlier exposure to N2B27 could improve embryo development after hatching. Embryo culture in N2B27 from day (D) 5, 6 or 7 significantly enhanced complete hypoblast migration (>45 vs. ∼24%) and epiblast development into an embryonic disc (ED)-like structure at D12 (>40 vs. 23%), compared to embryos cultured in SOF up to D9. Culture in N2B27 from D5 significantly increased epiblast and hypoblast cell number in D8 blastocysts, but post-hatching embryos cultured in N2B27 from D5 or 6 frequently showed a disorganized distribution of epiblast cells. In conclusion, bovine embryo culture in N2B27 from D7 onwards improves subsequent post-hatching development. This improved fully in vitro system will be very useful to functionally explore cell differentiation mechanisms and the bases of early pregnancy failures without requiring animal experimentation.

Apical PAR-3 caps orient the mitotic spindle in C. elegans early embryos

During embryonic development, oriented cell divisions are important for patterned tissue growth and cell fate specification. Cell division orientation is controlled in part by asymmetrically localized polarity proteins, which establish functional domains of the cell membrane and interact with microtubule regulators to position the mitotic spindle. For example, in the 8-cell mouse embryo, apical polarity proteins form caps on the outside, contact-free surface of the embryo that position the mitotic spindle to execute asymmetric cell division. A similar radial or "inside-outside" polarity is established at an early stage in many other animal embryos, but in most cases it remains unclear how inside-outside polarity is established and how it influences downstream cell behaviors. Here, we explore inside-outside polarity in C. elegans somatic blastomeres using spatiotemporally controlled protein degradation and live embryo imaging. We show that PAR polarity proteins, which form apical caps at the center of the contact free membrane, localize dynamically during the cell cycle and contribute to spindle orientation and proper cell positioning. Surprisingly, apical PAR-3 can form polarity caps independently of actomyosin flows and the small GTPase CDC-42, and can regulate spindle orientation in cooperation with the key polarity kinase aPKC. Together, our results reveal a role for apical polarity caps in regulating spindle orientation in symmetrically dividing cells and provide novel insights into how these structures are formed.

Generating human blastoids modeling blastocyst-stage embryos and implantation

Human early development sets the stage for embryonic and adult life but remains difficult to investigate. A solution came from the ability of stem cells to organize into structures resembling preimplantation embryos-blastocysts-that we termed blastoids. This embryo model is available in unlimited numbers and could thus support scientific and medical advances. However, its predictive power depends on how faithfully it recapitulates the blastocyst. Here, we describe how we formed human blastoids that (1) efficiently achieve the morphology of the blastocyst and (2) form lineages according to the pace and sequence of blastocyst development, (3) ultimately forming cells that transcriptionally reflect the blastocyst (preimplantation stage). We employ three different commercially available 96- and 24-well microwell plates with results similar to our custom-made ones, and show that blastoids form in clinical in vitro fertilization medium and can be cryopreserved for shipping. Finally, we explain how blastoids replicate the directional process of implantation into endometrial organoids, specifically when these are hormonally stimulated. It takes 4 d for human blastoids to form and 10 d to prepare the endometrial implantation assay, and we have cultured blastoids up to 6 d (time-equivalent of day 13). On the basis of our experience, we anticipate that a person with ~1 year of human pluripotent stem cell culture experience and of organoid culture should be able to perform the protocol. Altogether, blastoids offer an opportunity to establish scientific and biomedical discovery programs for early pregnancy, and an ethical alternative to the use of embryos.

The role of embryonic stem cells, transcription and growth factors in mammals: A review

Mammals represent a relevant species in worldwide cultures with significant commercial value. These animals are considered an attractive large animal model for biomedical and biotechnology research. The development of large animal experimental models may open alternative strategies for investigating stem cells (SCs) physiology and potential application in the veterinary field. The embryonic stem cells (ESCs) are known to possess natural pluripotency that confers the ability to differentiate into various tissues in vivo and in vitro. These notable characteristics can be useful for research and innovative applications, including biomedicine, agriculture and industry. Transcription factors play a crucial role in preserving stem cell self-renewal, whereas growth factors are involved in both growth and differentiation. However, to date, many questions concerning pluripotency, cellular differentiation regulator genes, and other molecules such as growth factors and their interactions in many mammalian species remain unresolved. The purpose of this review is to provide an overall review regarding the study of ESCs in mammals and briefly discuss the role of transcription and growth factors.

Early Cell Specification in Mammalian Fertilized and Somatic Cell Nuclear Transfer Embryos

Early cell specification in mammalian preimplantation embryos is an intricate cellular process that leads to coordinated spatial and temporal expression of specific genes. Proper segregation into the first two cell lineages, the inner cell mass (ICM) and the trophectoderm (TE), is imperative for developing the embryo proper and the placenta, respectively. Somatic cell nuclear transfer (SCNT) allows the formation of a blastocyst containing both ICM and TE from a differentiated cell nucleus, which means that this differentiated genome must be reprogrammed to a totipotent state. Although blastocysts can be generated efficiently through SCNT, the full-term development of SCNT embryos is impaired mostly due to placental defects. In this review, we examine the early cell fate decisions in fertilized embryos and compare them to observations in SCNT-derived embryos, in order to understand if these processes are affected by SCNT and could be responsible for the low success of reproductive cloning.

The role of polarization and early heterogeneities in the mammalian first cell fate decision

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.

Journey of the mouse primitive endoderm: from specification to maturation

The blastocyst is a conserved stage and distinct milestone in the development of the mammalian embryo. Blastocyst stage embryos comprise three cell lineages which arise through two sequential binary cell fate specification steps. In the first, extra-embryonic trophectoderm (TE) cells segregate from inner cell mass (ICM) cells. Subsequently, ICM cells acquire a pluripotent epiblast (Epi) or extra-embryonic primitive endoderm (PrE, also referred to as hypoblast) identity. In the mouse, nascent Epi and PrE cells emerge in a salt-and-pepper distribution in the early blastocyst and are subsequently sorted into adjacent tissue layers by the late blastocyst stage. Epi cells cluster at the interior of the ICM, while PrE cells are positioned on its surface interfacing the blastocyst cavity, where they display apicobasal polarity. As the embryo implants into the maternal uterus, cells at the periphery of the PrE epithelium, at the intersection with the TE, break away and migrate along the TE as they mature into parietal endoderm (ParE). PrE cells remaining in association with the Epi mature into visceral endoderm. In this review, we discuss our current understanding of the PrE from its specification to its maturation. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Mechanisms of formation and functions of the early embryonic cavities

As the early mouse embryo develops, fundamental steps include the sequential formation of the first lumens in the murine conceptus. The first cavity established in the pre-implantation embryo is the blastocoel, followed by the emergence of the proamniotic cavity during the peri-implantation stages. The mouse embryo is a dynamic system which switches its modes of lumenogenesis before and after implantation. The blastocoel emerges in between the basolateral membranes, whereas the proamniotic cavity is formed on the apical interface. Defects in the sculpting of these luminal spaces are associated with developmental abnormalities and embryonic lethality. Here, we review the mechanisms by which these early embryonic cavities are formed and discuss the cavities in terms of their common and stage-specific principles of lumenogenesis and their functions.

Bisphenol Exposure Disrupts Cytoskeletal Organization and Development of Pre-Implantation Embryos

The endocrine disrupting activity of bisphenol compounds is well documented, but less is known regarding their impact on cell division and early embryo formation. Here, we tested the effects of acute in vitro exposure to bisphenol A (BPA) and its common substitute, bisphenol F (BPF), during critical stages of mouse pre-implantation embryo development, including the first mitotic division, cell polarization, as well as morula and blastocyst formation. Timing of initial cleavage was determined by live-cell imaging, while subsequent divisions, cytoskeletal organization and lineage marker labeling were assessed by high-resolution fluorescence microscopy. Our analysis reveals that brief culture with BPA or BPF impeded cell division and disrupted embryo development at all stages tested. Surprisingly, BPF was more detrimental to the early embryo than BPA. Notably, poor embryo development was associated with cytoskeletal disruptions of the actomyosin network, apical domain formation during cell polarization, actin ring zippering for embryo sealing and altered cell lineage marker profiles. These results underscore that bisphenols can disrupt cytoskeletal integrity and remodeling that is vital for early embryo development and raise concerns regarding the use of BPF as a 'safe' BPA substitute.

Trophectoderm formation: regulation of morphogenesis and gene expressions by RHO, ROCK, cell polarity, and HIPPO signaling

In brief

Trophectoderm is the first tissue to differentiate in the early mammalian embryo and is essential for hatching, implantation, and placentation. This review article discusses the roles of Ras homolog family members (RHO) and RHO-associated coiled-coil containing protein kinases (ROCK) in the molecular and cellular regulation of trophectoderm formation.

Abstract

The trophectoderm (TE) is the first tissue to differentiate during the preimplantation development of placental mammals. It constitutes the outer epithelial layer of the blastocyst and is responsible for hatching, uterine attachment, and placentation. Thus, its formation is the key initial step that enables the viviparity of mammals. Here, we first describe the general features of TE formation at the morphological and molecular levels. Prospective TE cells form an epithelial layer enclosing an expanding fluid-filled cavity by establishing the apical-basal cell polarity, intercellular junctions, microlumen, and osmotic gradient. A unique set of genes is expressed in TE that encode the transcription factors essential for the development of trophoblasts of the placenta upon implantation. TE-specific gene expressions are driven by the inhibition of HIPPO signaling, which is dependent on the prior establishment of the apical-basal polarity. We then discuss the specific roles of RHO and ROCK as essential regulators of TE formation. RHO and ROCK modulate the actomyosin cytoskeleton, apical-basal polarity, intercellular junctions, and HIPPO signaling, thereby orchestrating the epithelialization and gene expressions in TE. Knowledge of the molecular mechanisms underlying TE formation is crucial for assisted reproductive technologies in human and farm animals, as it provides foundation to help improve procedures for embryo handling and selection to achieve better reproductive outcomes.

Linker histone H1FOO is required for bovine preimplantation development by regulating lineage specification and chromatin structure

Linker histone H1 binds to the nucleosome and is implicated in the regulation of the chromatin structure and function. The H1 variant H1FOO is heavily expressed in oocytes and early embryos. However, given the poor homology of H1FOO among mammals, the functional role of H1FOO during preimplantation embryonic development remains largely unknown, especially in domestic animals. Here, we find that H1FOO is not only expressed in oocytes and preimplantation embryos but granulosa cells and spermatids in cattle. We then demonstrate that the interference of H1FOO results in preimplantation embryonic developmental arrest in cattle using either RNA editing or Trim-Away approach. H1FOO depletion leads to a compromised expression of critical lineage-specific genes at the morula stage and affects the establishment of cell polarity. Interestingly, H1FOO depletion causes a significant increase in the expression of genes encoding other linker H1 and core histones. Concurrently, there is an increase of H3K9me3 and H3K27me3, two markers of repressive chromatin and a decrease of H4K16ac, a marker of open chromatin. Importantly, overexpression of bovine H1FOO results in severe embryonic developmental defects. In sum, we propose that H1FOO controls the proper chromatin structure that is crucial for the fidelity of cell polarization and lineage specification during bovine preimplantation development.

Asynchronous division at 4-8-cell stage of preimplantation embryos affects live birth through ICM/TE differentiation

To improve the performance of assisted reproductive technology, it is necessary to find an indicator that can identify and select embryos that will be born or be aborted. We searched for indicators of embryo selection by comparing born/abort mouse embryos. We found that asynchronous embryos during the 4-8-cell stage were predisposed to be aborted. In asynchronous mouse embryos, the nuclear translocation of YAP1 in some blastomeres and compaction were delayed, and the number of ICMs was reduced. Hence, it is possible that asynchronous embryos have abnormal differentiation. When the synchrony of human embryos was observed, it was confirmed that embryos that did not reach clinical pregnancy had asynchrony as in mice. This could make synchrony a universal indicator common to all animal species.

Early human trophoblast development: from morphology to function

Human pregnancy depends on the proper development of the embryo prior to implantation and the implantation of the embryo into the uterine wall. During the pre-implantation phase, formation of the morula is followed by internalization of blastomeres that differentiate into the pluripotent inner cell mass lineage, while the cells on the surface undergo polarization and differentiate into the trophectoderm of the blastocyst. The trophectoderm mediates apposition and adhesion of the blastocyst to the uterine epithelium. These processes lead to a stable contact between embryonic and maternal tissues, resulting in the formation of a new organ, the placenta. During implantation, the trophectoderm cells start to differentiate and form the basis for multiple specialized trophoblast subpopulations, all of which fulfilling specific key functions in placentation. They either differentiate into polar cells serving typical epithelial functions, or into apolar invasive cells that adapt the uterine wall to progressing pregnancy. The composition of these trophoblast subpopulations is crucial for human placenta development and alterations are suggested to result in placenta-associated pregnancy pathologies. This review article focuses on what is known about very early processes in human reproduction and emphasizes on morphological and functional aspects of early trophoblast differentiation and subpopulations.

Emerging Role of Mechanical Forces in Cell Fate Acquisition

Mechanical forces are now recognized as key cellular effectors that together with genetic and cellular signals physically shape and pattern tissues and organs during development. Increasing efforts are aimed toward understanding the less explored role of mechanical forces in controlling cell fate decisions in embryonic development. Here we discuss recent examples of how differential forces feedback into cell fate specification and tissue patterning. In particular, we focus on the role of actomyosin-contractile force generation and transduction in affecting tissue morphogenesis and cell fate regulation in the embryo.

Mechanical Control of Cell Differentiation: Insights from the Early Embryo

Differentiation is the process by which a cell activates the expression of tissue-specific genes, downregulates the expression of potency markers, and acquires the phenotypic characteristics of its mature fate. The signals that regulate differentiation include biochemical and mechanical factors within the surrounding microenvironment. We describe recent breakthroughs in our understanding of the mechanical control mechanisms that regulate differentiation, with a specific emphasis on the differentiation events that build the early mouse embryo. Engineering approaches to reproducibly mimic the mechanical regulation of differentiation will permit new insights into early development and applications in regenerative medicine. Expected final online publication date for the Annual Review of Biomedical Engineering, Volume 24 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Cell size and polarization determine cytokinesis furrow ingression dynamics in mouse embryos

The final step of cell division, termed cytokinesis, comprises the constriction of a furrow that divides the cytoplasm to form two daughter cells. Although cytokinesis is well studied in traditional cell systems, how cytokinesis is regulated in complex multicellular settings and during cell-fate decisions is less well understood. Here, using live imaging and physical and molecular interventions, we find that the emergence of cell polarity during mouse embryo morphogenesis dramatically impacts cytokinesis mechanisms. Specifically, the assembly of the apical domain in outer cells locally inhibits the cytokinetic machinery, leading to an unexpected laterally biased cytokinesis.

Cytokinesis is the final step of cell division during which a contractile ring forms a furrow that partitions the cytoplasm in two. How furrow ingression is spatiotemporally regulated and how it is adapted to complex cellular environments and developmental transitions remain poorly understood. Here, we examine furrow ingression dynamics in the context of the early mouse embryo and find that cell size is a powerful determinant of furrow ingression speed during reductive cell divisions. In addition, the emergence of cell polarity and the assembly of the apical domain in outer cells locally inhibits the recruitment of cytokinesis components and thereby negatively regulates furrow ingression specifically on one side of the furrow. We show that this biasing of cytokinesis is not dependent upon cell–cell adhesion or shape but rather is cell intrinsic and is caused by a paucity of cytokinetic machinery in the apical domain. The results thus reveal that in the mouse embryo cell polarity directly regulates the recruitment of cytokinetic machinery in a cell-autonomous manner and that subcellular organization can instigate differential force generation and constriction speed in different zones of the cytokinetic furrow.

Mechanical regulation of early vertebrate embryogenesis

Embryonic cells grow in environments that provide a plethora of physical cues, including mechanical forces that shape the development of the entire embryo. Despite their prevalence, the role of these forces in embryonic development and their integration with chemical signals have been mostly neglected, and scrutiny in modern molecular embryology tilted, instead, towards the dissection of molecular pathways involved in cell fate determination and patterning. It is now possible to investigate how mechanical signals induce downstream genetic regulatory networks to regulate key developmental processes in the embryo. Here, we review the insights into mechanical control of early vertebrate development, including the role of forces in tissue patterning and embryonic axis formation. We also highlight recent in vitro approaches using individual embryonic stem cells and self-organizing multicellular models of human embryos, which have been instrumental in expanding our understanding of how mechanics tune cell fate and cellular rearrangements during human embryonic development.

Cortical softening elicits zygotic contractility during mouse preimplantation development

Actomyosin contractility is a major engine of preimplantation morphogenesis, which starts at the 8-cell stage during mouse embryonic development. Contractility becomes first visible with the appearance of periodic cortical waves of contraction (PeCoWaCo), which travel around blastomeres in an oscillatory fashion. How contractility of the mouse embryo becomes active remains unknown. We have taken advantage of PeCoWaCo to study the awakening of contractility during preimplantation development. We find that PeCoWaCo become detectable in most embryos only after the second cleavage and gradually increase their oscillation frequency with each successive cleavage. To test the influence of cell size reduction during cleavage divisions, we use cell fusion and fragmentation to manipulate cell size across a 20- to 60-μm range. We find that the stepwise reduction in cell size caused by cleavage divisions does not explain the presence of PeCoWaCo or their accelerating rhythm. Instead, we discover that blastomeres gradually decrease their surface tensions until the 8-cell stage and that artificially softening cells enhances PeCoWaCo prematurely. We further identify the programmed down-regulation of the formin Fmnl3 as a required event to soften the cortex and expose PeCoWaCo. Therefore, during cleavage stages, cortical softening, mediated by Fmnl3 down-regulation, awakens zygotic contractility before preimplantation morphogenesis.

During preimplantation morphogenesis, the mouse embryo relies on forces generated by the actomyosin cytoskeleton. This study uncovers how periodic actomyosin contractions increase in frequency during cleavage stages as blastomeres soften with each cleavage division.

Stain-free detection of embryo polarization using deep learning

Polarization of the mammalian embryo at the right developmental time is critical for its development to term and would be valuable in assessing the potential of human embryos. However, tracking polarization requires invasive fluorescence staining, impermissible in the in vitro fertilization clinic. Here, we report the use of artificial intelligence to detect polarization from unstained time-lapse movies of mouse embryos. We assembled a dataset of bright-field movie frames from 8-cell-stage embryos, side-by-side with corresponding images of fluorescent markers of cell polarization. We then used an ensemble learning model to detect whether any bright-field frame showed an embryo before or after onset of polarization. Our resulting model has an accuracy of 85% for detecting polarization, significantly outperforming human volunteers trained on the same data (61% accuracy). We discovered that our self-learning model focuses upon the angle between cells as one known cue for compaction, which precedes polarization, but it outperforms the use of this cue alone. By compressing three-dimensional time-lapsed image data into two-dimensions, we are able to reduce data to an easily manageable size for deep learning processing. In conclusion, we describe a method for detecting a key developmental feature of embryo development that avoids clinically impermissible fluorescence staining.

A monoastral mitotic spindle determines lineage fate and position in the mouse embryo

During mammalian development, the first asymmetric cell divisions segregate cells into inner and outer positions of the embryo to establish the pluripotent and trophectoderm lineages. Typically, polarity components differentially regulate the mitotic spindle via astral microtubule arrays to trigger asymmetric division patterns. However, early mouse embryos lack centrosomes, the microtubule-organizing centres (MTOCs) that usually generate microtubule asters. Thus, it remains unknown whether spindle organization regulates lineage segregation. Here we find that heterogeneities in cell polarity in the early 8-cell-stage mouse embryo trigger the assembly of a highly asymmetric spindle organization. This spindle arises in an unusual modular manner, forming a single microtubule aster from an apically localized, non-centrosomal MTOC, before joining it to the rest of the spindle apparatus. When fully assembled, this 'monoastral' spindle triggers spatially asymmetric division patterns to segregate cells into inner and outer positions. Moreover, the asymmetric inheritance of spindle components causes differential cell polarization to determine pluripotent versus trophectoderm lineage fate.

Epithelial dynamics during early mouse development

The first epithelia to arise in an organism face the challenge of maintaining the integrity of the newly formed tissue, while exhibiting the behavioral flexibility required for morphogenetic processes to occur effectively. Epithelial cells integrate biochemical and biomechanical cues, both intrinsic and extrinsic, in order to bring about the molecular changes which determine their morphology, behavior and fate. In this review we highlight recent advances in our understanding of the various dynamic processes that the emergent epithelial cells undergo during the first seven days of mouse development and speculate what the future holds in understanding the mechanistic bases for these processes through integrative approaches.

Live imaging YAP signalling in mouse embryo development

YAP protein is a critical regulator of mammalian embryonic development. By generating a near-infrared fusion YAP reporter mouse line, we have achieved high-resolution live imaging of YAP localization during mouse embryonic development. We have validated the reporter by demonstrating its predicted responses to blocking LATS kinase activity or blocking cell polarity. By time lapse imaging preimplantation embryos, we revealed a mitotic reset behaviour of YAP nuclear localization. We also demonstrated deep tissue live imaging in post-implantation embryos and revealed an intriguing nuclear YAP pattern in migrating cells. The YAP fusion reporter mice and imaging methods will open new opportunities for understanding dynamic YAP signalling in vivo in many different situations.

The second polar body contributes to the fate asymmetry in the mouse embryo

The polar bodies (PBs) are extruded microcells during oocyte meiosis and generally regarded as inessentials for embryonic development. Therefore, PBs have been widely used as important materials for pre-implantation genetic diagnosis in human. Here we report that the second PB (PB2) in the mouse zygote may play roles in cell-fate specification and post-implantation development. A subset of mRNAs encoding pluripotency-related factors are enriched in PB2. Nascent proteins may be synthesized in PB2 after fertilization and transport from PB2 to the zygote before the two-cell stage. The PB2-attached blastomere (pbB) at the two-cell stage, compared to the other blastomere (npbB), likely contributes more descendants to the inner cell mass (ICM) lineage in the blastocyst. Removal of PB2 from the zygote or transient blockage of material exchange between PB2 and the zygote by nocodazole treatment appears to cause a loss of the ICM fate bias of pbB. PB2 removal or nocodazole treatment also results in abnormal post-implantation development. Injection of PB2 lysate into pbB of PB2-removed two-cell-stage embryos may reset the cell-fate preference and rescue post-implantation development. Our data collectively suggest that PB2 would demarcate the earliest cell-fate asymmetry of the mouse zygote and be required for post-implantation development.

CDX2 downregulation in mouse mural trophectoderm during peri-implantation is heteronomous, dependent on the YAP-TEAD pathway and controlled by estrogen-induced factors

Purpose

To investigate the transition of CDX2 expression patterns in mouse trophectoderm (TE) and its regulatory mechanisms during implantation.

Methods

Mouse E3.5-4.5 blastocysts were used to immunostain CDX2, YAP, TEAD4, and ESRRB. Endogenous estrogen signaling was perturbed by administrating estrogen receptor antagonist ICI 182,780 or ovariectomy followed by administration of progesterone and β-estradiol to elucidate the relationship between the transition of CDX2 expression patterns and ovarian estrogen-dependent change in the uterine environment.

Results

CDX2 expression was gradually downregulated in the mural TE from E4.0 in vivo, whereas CDX2 downregulation was not observed in blastocysts cultured in KSOM. Fetal bovine serum (FBS) supplementation in KSOM induced CDX2 downregulation independently of blastocyst attachment to dishes. CDX2 downregulation in the mural TE was repressed by administration of ICI 182,780 or by ovariectomy, and administration of β-estradiol into ovariectomized mice retriggered CDX2 downregulation. Furthermore, Cdx2 expression in the mural TE might be controlled by the YAP-TEAD pathway.

Conclusions

CDX2 downregulation was induced heteronomously in the mural TE from E4.0 by uterus-derived factors, the secretion of which was stimulated by ovarian estrogen.

ECM-integrin signalling instructs cellular position-sensing to pattern the early mouse embryo

Development entails patterned emergence of diverse cell types within the embryo. In mammals, cells positioned inside the embryo give rise to the inner cell mass (ICM), which eventually forms the embryo itself. Yet, the molecular basis of how these cells recognise their ‘inside’ position to instruct their fate is unknown. Here, we show that provision of extracellular matrix (ECM) to isolated embryonic cells induces ICM specification and alters the subsequent spatial arrangement between epiblast (EPI) and primitive endoderm (PrE) cells that emerge within the ICM. Notably, this effect is dependent on integrin β1 activity and involves apical-to-basal conversion of cell polarity. We demonstrate that ECM-integrin activity is sufficient for ‘inside’ positional signalling and is required for correct EPI/PrE patterning. Thus, our findings highlight the significance of ECM-integrin adhesion in enabling position sensing by cells to achieve tissue patterning.

Summary: The importance of patterned cell-extracellular matrix (ECM) interactions in early mouse development: ECM signals can modulate both cell fate and the relative spatial arrangement between cells.

Combined effect of cell geometry and polarity domains determines the orientation of unequal division

Cell division orientation is thought to result from a competition between cell geometry and polarity domains controlling the position of the mitotic spindle during mitosis. Depending on the level of cell shape anisotropy or the strength of the polarity domain, one dominates the other and determines the orientation of the spindle. Whether and how such competition is also at work to determine unequal cell division (UCD), producing daughter cells of different size, remains unclear. Here, we show that cell geometry and polarity domains cooperate, rather than compete, in positioning the cleavage plane during UCDs in early ascidian embryos. We found that the UCDs and their orientation at the ascidian third cleavage rely on the spindle tilting in an anisotropic cell shape, and cortical polarity domains exerting different effects on spindle astral microtubules. By systematically varying mitotic cell shape, we could modulate the effect of attractive and repulsive polarity domains and consequently generate predicted daughter cell size asymmetries and position. We therefore propose that the spindle position during UCD is set by the combined activities of cell geometry and polarity domains, where cell geometry modulates the effect of cortical polarity domain(s).

Klf5 establishes bi-potential cell fate by dual regulation of ICM and TE specification genes

Early blastomeres of mouse preimplantation embryos exhibit bi-potential cell fate, capable of generating both embryonic and extra-embryonic lineages in blastocysts. Here we identify three major two-cell-stage (2C)-specific endogenous retroviruses (ERVs) as the molecular hallmark of this bi-potential plasticity. Using the long terminal repeats (LTRs) of all three 2C-specific ERVs, we identify Krüppel-like factor 5 (Klf5) as their major upstream regulator. Klf5 is essential for bi-potential cell fate; a single Klf5-overexpressing embryonic stem cell (ESC) generates terminally differentiated embryonic and extra-embryonic lineages in chimeric embryos, and Klf5 directly induces inner cell mass (ICM) and trophectoderm (TE) specification genes. Intriguingly, Klf5 and Klf4 act redundantly during ICM specification, whereas Klf5 deficiency alone impairs TE specification. Klf5 is regulated by multiple 2C-specific transcription factors, particularly Dux, and the Dux/Klf5 axis is evolutionarily conserved. The 2C-specific transcription program converges on Klf5 to establish bi-potential cell fate, enabling a cell state with dual activation of ICM and TE genes.

Using multiple 2C-specific ERV cell fate markers, Kinisu et al. identify Klf5 as a key transcription factor that confers a 2C-like developmental potential and activates ICM and TE specification genes. Klf5 and Klf4 act redundantly for ICM and TE specification in mouse preimplantation embryos.

The human sperm basal body is a complex centrosome important for embryo preimplantation development

The mechanism of conversion of the human sperm basal body to a centrosome after fertilization, and its role in supporting human early embryogenesis, has not been directly addressed so far. Using proteomics and immunofluorescence studies, we show here that the human zygote inherits a basal body enriched with centrosomal proteins from the sperm, establishing the first functional centrosome of the new organism. Injection of human sperm tails containing the basal body into human oocytes followed by parthenogenetic activation, showed that the centrosome contributes to the robustness of the early cell divisions, increasing the probability of parthenotes reaching the compaction stage. In the absence of the sperm-derived centrosome, pericentriolar material (PCM) components stored in the oocyte can form de novo structures after genome activation, suggesting a tight PCM expression control in zygotes. Our results reveal that the sperm basal body is a complex organelle which converts to a centrosome after fertilization, ensuring the early steps of embryogenesis and successful compaction. However, more experiments are needed to elucidate the exact molecular mechanisms of centrosome inheritance in humans.

Transposable elements shape the evolution of mammalian development

Transposable elements (TEs) promote genetic innovation but also threaten genome stability. Despite multiple layers of host defence, TEs actively shape mammalian-specific developmental processes, particularly during pre-implantation and extra-embryonic development and at the maternal-fetal interface. Here, we review how TEs influence mammalian genomes both directly by providing the raw material for genetic change and indirectly via co-evolving TE-binding Krüppel-associated box zinc finger proteins (KRAB-ZFPs). Throughout mammalian evolution, individual activities of ancient TEs were co-opted to enable invasive placentation that characterizes live-born mammals. By contrast, the widespread activity of evolutionarily young TEs may reflect an ongoing co-evolution that continues to impact mammalian development.

A theoretical understanding of mammalian preimplantation development

The blastocyst has long been a hallmark system of study in developmental biology due to its importance in mammalian development and clinical relevance for assisted reproductive technologies. In recent years, the blastocyst is emerging as a system of study for mathematical modelling. In this review, we compile, to our knowledge, all models describing preimplantation development. Coupled with experiments, these models have provided insight regarding the morphogenesis and cell-fate specification throughout preimplantation development. In the case of cell-fate specification, theoretical models have provided mechanisms explaining how proportion of cell types are established and maintained when confronted to perturbations. For cell-shape based models, they have described quantitatively how mechanical forces sculpt the blastocyst and even predicted how morphogenesis could be manipulated. As theoretical biology develops, we believe the next critical stage in modelling involves an integration of cell fate and mechanics to provide integrative models of development at distinct spatiotemporal scales. We discuss how, building on a balanced base of mechanical and chemical models, the preimplantation embryo will play a key role in integrating these two faces of the same coin.

Mechanobiology of the female reproductive system

BACKGROUND

Mechanobiology in the field of human female reproduction has been extremely challenging technically and ethically.

METHODS

The present review provides the current knowledge on mechanobiology of the female reproductive system. This review focuses on the early phases of reproduction from oocyte development to early embryonic development, with an emphasis on current progress.

MAIN FINDINGS RESULTS

Optimal, well-controlled mechanical cues are required for female reproductive system physiology. Many important questions remain unanswered; whether and how mechanical imbalances among the embryo, decidua, and uterine muscle contractions affect early human embryonic development, whether the biomechanical properties of oocytes/embryos are potential biomarkers for selecting high-quality oocytes/embryos, whether mechanical properties differ between the two major compartments of the ovary (cortex and medulla) in normally ovulating human ovaries, whether durotaxis is involved in several processes in addition to embryonic development. Progress in mechanobiology is dependent on development of technologies that enable precise physical measurements.

CONCLUSION

More studies are needed to understand the roles of forces and changes in the mechanical properties of female reproductive system physiology. Recent and future technological advancements in mechanobiology research will help us understand the role of mechanical forces in female reproductive system disorders/diseases.

Cell fate determination and Hippo signaling pathway in preimplantation mouse embryo

First cell fate determination plays crucial roles in cell specification during early phases of embryonic development. Three classical concepts have been proposed to explain the lineage specification mechanism of the preimplantation embryo: inside-outside, pre-patterning, and polarity models. Transcriptional effectors of the Hippo signal pathway are YAP and TAZ activators that can create a shuttle between the cytoplasm and the nucleus. Despite different localizations of YAP in the cell, it determines the fate of ICM and TE. How the decisive cue driving factors that determine YAP localization are coordinated remains a central unanswered question. How can an embryonic cell find its position? The objective of this review is to summarize the molecular and mechanical aspects in cell fate decision during mouse preimplantation embryonic development. The findings will reveal the relationship between cell-cell adhesion, cell polarity, and determination of cell fate during early embryonic development in mice and elucidate the inducing/inhibiting mechanisms that are involved in cell specification following zygotic genome activation and compaction processes. With future studies, new biophysical and chemical cues in the cell fate determination will impart significant spatiotemporal effects on early embryonic development. The achieved knowledge will provide important information to the development of new approaches to be used in infertility treatment and increase the success of pregnancy.