The roles of FGF and MAP kinase signaling in the segregation of the epiblast and hypoblast cell lineages in bovine and human embryos

Pluripotent cell states and fates in human embryo models

Pluripotency, the capacity to generate all cells of the body, is a defining property of a transient population of epiblast cells found in pre-, peri- and post-implantation mammalian embryos. As development progresses, the epiblast cells undergo dynamic transitions in pluripotency states, concurrent with the specification of extra-embryonic and embryonic lineages. Recently, stem cell-based models of pre- and post-implantation human embryonic development have been developed using stem cells that capture key properties of the epiblast at different developmental stages. Here, we review early primate development, comparing pluripotency states of the epiblast in vivo with cultured pluripotent cells representative of these states. We consider how the pluripotency status of the starting cells influences the development of human embryo models and, in turn, what we can learn about the human pluripotent epiblast. Finally, we discuss the limitations of these models and questions arising from the pioneering studies in this emerging field.

Coupling of cell shape, matrix and tissue dynamics ensures embryonic patterning robustness

Tissue patterning coordinates morphogenesis, cell dynamics and fate specification. Understanding how precision in patterning is robustly achieved despite inherent developmental variability during mammalian embryogenesis remains a challenge. Here, based on cell dynamics quantification and simulation, we show how salt-and-pepper epiblast and primitive endoderm (PrE) cells pattern the inner cell mass of mouse blastocysts. Coupling cell fate and dynamics, PrE cells form apical polarity-dependent actin protrusions required for RAC1-dependent migration towards the surface of the fluid cavity, where PrE cells are trapped due to decreased tension. Concomitantly, PrE cells deposit an extracellular matrix gradient, presumably breaking the tissue-level symmetry and collectively guiding their own migration. Tissue size perturbations of mouse embryos and their comparison with monkey and human blastocysts further demonstrate that the fixed proportion of PrE/epiblast cells is optimal with respect to embryo size and tissue geometry and, despite variability, ensures patterning robustness during early mammalian development.

Divergent destinies: insights into the molecular mechanisms underlying EPI and PE fate determination

Mammalian pre-implantation development is entirely devoted to the specification of extra-embryonic lineages, which are fundamental for embryo morphogenesis and support. The second fate decision is taken just before implantation, as defined by the epiblast (EPI) and the primitive endoderm (PE) specification. Later, EPI forms the embryo proper and PE contributes to the formation of the yolk sac. The formation of EPI and PE as molecularly and morphologically distinct lineages is the final step of a multistage process, which begins when bipotent progenitor cells diverge into separate fates. Despite advances in uncovering the molecular mechanisms underlying the differential transcriptional patterns that dictate how apparently identical cells make fate decisions and how lineage integrity is maintained, a detailed overview of these mechanisms is still lacking. In this review, we dissect the EPI and PE formation process into four stages (initiation, specification, segregation, and maintenance) and we provide a comprehensive understanding of the molecular mechanisms involved in lineage establishment in the mouse. In addition, we discuss the conservation of key processes in humans, based on the most recent findings.

Establishment of bovine extraembryonic endoderm cells

Understanding the mechanisms of hypoblast development and its role in the implantation is critical for improving farm animal reproduction, but it is hampered by the lack of research models. Here we report that a chemical cocktail (FGF4, BMP4, IL-6, XAV939, and A83-01) enables de novo derivation and long-term culture of bovine extraembryonic endoderm cells (bXENs). Transcriptomic and epigenomic analyses confirmed the identity of bXENs and revealed that they are resemble hypoblast lineages of early bovine peri-implantation embryos. We showed that bXENs help maintain the stemness of bovine ESCs and prevent them from differentiation. In the presence of a signaling cocktail sustaining bXENs, the growth and progression of epiblasts are also facilitated in the developing pre-implantation embryo. Furthermore, through 3D assembly of bXENs with bovine ESCs and TSCs, we developed an improved bovine blastocyst like structure (bovine blastoid) that resembles blastocyst. The bovine XENs and blastoids established in this study represent accessible in vitro models for understanding hypoblast development and improving reproductive efficiency in livestock species.

Relationship of PSC to embryos: Extending and refining capture of PSC lines from mammalian embryos

Pluripotent stem cell lines derived from preimplantation mouse embryos have opened opportunities for the study of early mammalian development and generation of genetically uncompromised material for differentiation into specific cell types. Murine embryonic stem cells are highly versatile and can be engineered and introduced into host embryos, transferred to recipient females, and gestated to investigate gene function at multiple levels as well as developmental mechanisms, including lineage segregation and cell competition. In this review, we summarize the biomedical motivation driving the incremental modification to culture regimes and analyses that have advanced stem cell research to its current state. Ongoing investigation into divergent mechanisms of early developmental processes adopted by other species, such as agriculturally beneficial mammals and birds, will continue to enrich knowledge and inform strategies for future in vitro models.

mTOR activity paces human blastocyst stage developmental progression

Many mammals can temporally uncouple conception from parturition by pacing down their development around the blastocyst stage. In mice, this dormant state is achieved by decreasing the activity of the growth-regulating mTOR signaling pathway. It is unknown whether this ability is conserved in mammals in general and in humans in particular. Here, we show that decreasing the activity of the mTOR signaling pathway induces human pluripotent stem cells (hPSCs) and blastoids to enter a dormant state with limited proliferation, developmental progression, and capacity to attach to endometrial cells. These in vitro assays show that, similar to other species, the ability to enter dormancy is active in human cells around the blastocyst stage and is reversible at both functional and molecular levels. The pacing of human blastocyst development has potential implications for reproductive therapies.

Early human development and stem cell-based human embryo models

The use of stem cells to model the early human embryo promises to transform our understanding of developmental biology and human reproduction. In this review, we present our current knowledge of the first 2 weeks of human embryo development. We first focus on the distinct cell lineages of the embryo and the derivation of stem cell lines. We then discuss the intercellular crosstalk that guides early embryo development and how this crosstalk is recapitulated in vitro to generate stem cell-based embryo models. We highlight advances in this fast-developing field, discuss current limitations, and provide a vision for the future.

Early Cell Lineage Formation in Mammals: Complexity, Species Diversity, and Susceptibility to Disruptions Impacting Embryo Viability

The emergence of the earliest cell lineages in mammalian embryos is a complex process that utilizes an extensive network of chromatin regulators, transcription factors, cell polarity regulators, and cellular signaling pathways. These factors and pathways operate over a protracted period of time as embryos cleave, undergo compaction, and form blastocysts. The first cell fate specification event separates the pluripotent inner cell mass from the trophectoderm lineage. The second event separates pluripotent epiblast from hypoblast. This review summarizes over 50 years of study of these early lineage forming events, addressing the complexity of the network of interacting molecules, cellular functions and pathways that drive them, interspecies differences, and aspects of these mechanisms that likely underlie their high susceptibility to disruption by numerous environmental factors that can compromise embryo viability, such as maternal health and diet, environmental toxins, and other stressors.

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.

In vitro culture alters cell lineage composition and cellular metabolism of bovine blastocyst†

Profiling bovine blastocyst transcriptome at the single-cell level has enabled us to reveal the first cell lineage segregation, during which the inner cell mass (ICM), trophectoderm (TE), and an undefined population of transitional cells were identified. By comparing the transcriptome of blastocysts derived in vivo (IVV), in vitro from a conventional culture medium (IVC), and in vitro from an optimized reduced nutrient culture medium (IVR), we found a delay of the cell fate commitment to ICM in the IVC and IVR embryos. Developmental potential differences between IVV, IVC, and IVR embryos were mainly contributed by ICM and transitional cells. Pathway analysis of these non-TE cells between groups revealed highly active metabolic and biosynthetic processes, reduced cellular signaling, and reduced transmembrane transport activities in IVC embryos that may lead to reduced developmental potential. IVR embryos had lower activities in metabolic and biosynthetic processes but increased cellular signaling and transmembrane transport, suggesting these cellular mechanisms may contribute to improved blastocyst development compared to IVC embryos. However, the IVR embryos had compromised development compared to IVV embryos with notably over-active transmembrane transport activities that impaired ion homeostasis.

Naive pluripotent stem cell-based models capture FGF-dependent human hypoblast lineage specification

The hypoblast is an essential extraembryonic tissue set aside within the inner cell mass in the blastocyst. Research with human embryos is challenging. Thus, stem cell models that reproduce hypoblast differentiation provide valuable alternatives. We show here that human naive pluripotent stem cell (PSC) to hypoblast differentiation proceeds via reversion to a transitional ICM-like state from which the hypoblast emerges in concordance with the trajectory in human blastocysts. We identified a window when fibroblast growth factor (FGF) signaling is critical for hypoblast specification. Revisiting FGF signaling in human embryos revealed that inhibition in the early blastocyst suppresses hypoblast formation. In vitro, the induction of hypoblast is synergistically enhanced by limiting trophectoderm and epiblast fates. This finding revises previous reports and establishes a conservation in lineage specification between mice and humans. Overall, this study demonstrates the utility of human naive PSC-based models in elucidating the mechanistic features of early human embryogenesis.

Exposure of chimaeric embryos to exogenous FGF4 leads to the production of pure ESC-derived mice

Producing chimaeras constitutes the most reliable method of verifying the pluripotency of newly established cells. Moreover, forming chimaeras by injecting genetically modified embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) into the embryo is part of the procedure for generating transgenic mice, which are used for understanding gene function. Conventional methods for generating transgenic mice, including the breeding of chimaeras and tetraploid complementation, are time-consuming and cost-inefficient, with significant limitations that hinder their effectiveness and widespread applications. In the present study, we modified the traditional method of chimaera generation to significantly speed up this process by generating mice exclusively derived from ESCs. This study aimed to assess whether fully ESC-derived mice could be obtained by modulating fibroblast growth factor 4 (FGF4) levels in the culture medium and changing the direction of cell differentiation in the chimaeric embryo. We found that exogenous FGF4 directs all host blastomeres to the primitive endoderm fate, but does not affect the localisation of ESCs in the epiblast of the chimaeric embryos. Consequently, all FGF4-treated chimaeric embryos contained an epiblast composed exclusively of ESCs, and following transfer into recipient mice, these embryos developed into fully ESC-derived newborns. Collectively, this simple approach could accelerate the generation of ESC-derived animals and thus optimise ESC-mediated transgenesis and the verification of cell pluripotency. Compared to traditional methods, it could speed up functional studies by several weeks and significantly reduce costs related to maintaining and breeding chimaeras. Moreover, since the effect of stimulating the FGF signalling pathway is universal across different animal species, our approach can be applied not only to rodents but also to other animals, offering its utility beyond laboratory settings.

The first two blastomeres contribute unequally to the human embryo

Retrospective lineage reconstruction of humans predicts that dramatic clonal imbalances in the body can be traced to the 2-cell stage embryo. However, whether and how such clonal asymmetries arise in the embryo is unclear. Here, we performed prospective lineage tracing of human embryos using live imaging, non-invasive cell labeling, and computational predictions to determine the contribution of each 2-cell stage blastomere to the epiblast (body), hypoblast (yolk sac), and trophectoderm (placenta). We show that the majority of epiblast cells originate from only one blastomere of the 2-cell stage embryo. We observe that only one to three cells become internalized at the 8-to-16-cell stage transition. Moreover, these internalized cells are more frequently derived from the first cell to divide at the 2-cell stage. We propose that cell division dynamics and a cell internalization bottleneck in the early embryo establish asymmetry in the clonal composition of the future human body.

Why study human embryo development?

Understanding the processes and mechanisms underlying early human embryo development has become an increasingly active and important area of research. It has potential for insights into important clinical issues such as early pregnancy loss, origins of congenital anomalies and developmental origins of adult disease, as well as fundamental insights into human biology. Improved culture systems for preimplantation embryos, combined with the new tools of single cell genomics and live imaging, are providing new insights into the similarities and differences between human and mouse development. However, access to human embryo material is still restricted and extended culture of early embryos has regulatory and ethical concerns. Stem cell-derived models of different phases of human development can potentially overcome these limitations and provide a scalable source of material to explore the early postimplantation stages of human development. To date, such models are clearly incomplete replicas of normal development but future technological improvements can be envisaged. The ethical and regulatory environment for such studies remains to be fully resolved.

A comprehensive review: synergizing stem cell and embryonic development knowledge in mouse and human integrated stem cell-based embryo models

Mammalian stem cell-based embryo models have emerged as innovative tools for investigating early embryogenesis in both mice and primates. They not only reduce the need for sacrificing mice but also overcome ethical limitations associated with human embryo research. Furthermore, they provide a platform to address scientific questions that are otherwise challenging to explore in vivo. The usefulness of a stem cell-based embryo model depends on its fidelity in replicating development, efficiency and reproducibility; all essential for addressing biological queries in a quantitative manner, enabling statistical analysis. Achieving such fidelity and efficiency requires robust systems that demand extensive optimization efforts. A profound understanding of pre- and post-implantation development, cellular plasticity, lineage specification, and existing models is imperative for making informed decisions in constructing these models. This review aims to highlight essential differences in embryo development and stem cell biology between mice and humans, assess how these variances influence the formation of partially and fully integrated stem cell models, and identify critical challenges in the field.

Efficient derivation of embryonic stem cells and primordial germ cell-like cells in cattle

The induction of the germ cell lineage from pluripotent stem cells (in vitro gametogenesis) will help understand the mechanisms underlying germ cell differentiation and provide an alternative source of gametes for reproduction. This technology is especially important for cattle, which are among the most important livestock species for milk and meat production. Here, we developed a new method for robust induction of primordial germ cell-like cells (PGCLCs) from newly established bovine embryonic stem (bES) cells. First, we refined the pluripotent culture conditions for pre-implantation embryos and ES cells. Inhibition of RHO increased the number of epiblast cells in the pre-implantation embryos and dramatically improved the efficiency of ES cell establishment. We then determined suitable culture conditions for PGCLC differentiation using bES cells harboring BLIMP1-tdTomato and TFAP2C-mNeonGreen (BTTN) reporter constructs. After a 24-h culture with bone morphogenetic protein 4 (BMP4), followed by three-dimensional culture with BMP4 and a chemical agonist and WNT signaling chemical antagonist, bES cells became positive for the reporters. A set of primordial germ cells (PGC) marker genes, including PRDM1/BLIMP1, TFAP2C, SOX17, and NANOS3, were expressed in BTTN-positive cells. These bovine PGCLCs (bPGCLCs) were isolated as KIT/CD117-positive and CD44-negative cell populations. We anticipate that this method for the efficient establishment of bES cells and induction of PGCLCs will be useful for stem cell-based reproductive technologies in cattle.

Hallmarks of totipotent and pluripotent stem cell states

Though totipotency and pluripotency are transient during early embryogenesis, they establish the foundation for the development of all mammals. Studying these in vivo has been challenging due to limited access and ethical constraints, particularly in humans. Recent progress has led to diverse culture adaptations of epiblast cells in vitro in the form of totipotent and pluripotent stem cells, which not only deepen our understanding of embryonic development but also serve as invaluable resources for animal reproduction and regenerative medicine. This review delves into the hallmarks of totipotent and pluripotent stem cells, shedding light on their key molecular and functional features.

Molecular profiling of human blastocysts reveals primitive endoderm defects among embryos of decreased implantation potential

Human embryo implantation is remarkably inefficient, and implantation failure remains among the greatest obstacles in treating infertility. Gene expression data from human embryos have accumulated rapidly in recent years; however, identification of the subset of genes that determine successful implantation remains a challenge. We leverage clinical morphologic grading-known for decades to correlate with implantation potential-and transcriptome analyses of matched embryonic and abembryonic samples to identify factors and pathways enriched and depleted in human blastocysts of good and poor morphology. Unexpectedly, we discovered that the greatest difference was in the state of extraembryonic primitive endoderm (PrE) development, with relative deficiencies in poor morphology blastocysts. Our results suggest that implantation success is most strongly influenced by the embryonic compartment and that deficient PrE development is common among embryos with decreased implantation potential. Our study provides a valuable resource for those investigating the markers and mechanisms of human embryo implantation.

Using extended growth of cattle embryos in culture to gain insights into bovine developmental events on embryonic days 8 to 10

We have previously described an extended embryo culture system, based on uterine composition, growth factors and the cell culture additive B27, for growing cattle embryos in vitro beyond embryonic day 7. Here, extended in vitro embryos are compared to embryos developed in the uterus and are used to establish a developmental staging framework useful for understanding developmental events occurring until Day 10. Immunofluorescence or mRNA expression of the ICM/epiblast markers OCT4, SOX2 and NANOG, hypoblast markers GATA6, SOX17 and GATA4 and trophoblast genes CDX2, GATA3, ASCL2 and IFNT revealed the presence of four stages during this period that can be molecularly distinguished. These are expanded blastocyst, hatched blastocyst, hypoblast layering and early hypoblast migration. Interestingly NANOG and SOX17 show reciprocal expression at the expanded blastocyst stage, well before SOX2 and GATA6 expression refines to a similar so-called "salt and pepper" mutually exclusive expression in the ICM at the hatched blastocyst stage. GATA4 expression is only seen from stages when the hypoblast starts migrating around the blastocyst cavity. Intriguingly, trophoblast still expresses GATA6 and OCT4 in all cells during the expanded blastocyst phase, while SOX2 and SOX17 are seen in only some trophoblast cells. By the hypoblast-epiblast layering stage no trophoblast expression remains except for that of OCT4 protein, which starts waning in trophoblast once the hypoblast begins migrating. Lastly, it is shown that cultured embryos exhibit increased expression of the stress marker TP53 in the epiblast and hypoblast at late stages in comparison to embryos produced in the uterine environment.

Haploid androgenetic development of bovine embryos reveals imbalanced WNT signaling and impaired cell fate differentiation†

Haploid embryos have contributed significantly to our understanding of the role of parental genomes in development and can be applied to important biotechnology for human and animal species. However, development to the blastocyst stage is severely hindered in bovine haploid androgenetic embryos (hAE). To further our understanding of such developmental arrest, we performed a comprehensive comparison of the transcriptomic profile of morula-stage embryos, which were validated by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) of transcripts associated with differentiation in haploid and biparental embryos. Among numerous disturbances, results showed that pluripotency pathways, especially the wingless-related integration site (WNT) signaling, were particularly unbalanced in hAE. Moreover, transcript levels of KLF4, NANOG, POU5F1, SOX2, CDX2, CTNNBL1, AXIN2, and GSK3B were noticeably altered in hAE, suggesting disturbance of pluripotency and canonical WNT pathways. To evaluate the role of WNT on hAE competence, we exposed early Day-5 morula stage embryos to the GSK3B inhibitor CHIR99021. Although no alterations were observed in pluripotency and WNT-related transcripts, exposure to CHIR99021 improved their ability to reach the blastocysts stage, confirming the importance of the WNT pathway in the developmental outcome of bovine hAE.

In vitro models of human hypoblast and mouse primitive endoderm

The primitive endoderm (PrE, also named hypoblast), a predominantly extraembryonic epithelium that arises from the inner cell mass (ICM) of the mammalian pre-implantation blastocyst, plays a fundamental role in embryonic development, giving rise to the yolk sac, establishing the anterior-posterior axis and contributing to the gut. PrE is specified from the ICM at the same time as the epiblast (Epi) that will form the embryo proper. While in vitro cell lines resembling the pluripotent Epi have been derived from a variety of conditions, only one model system currently exists for the PrE, naïve extraembryonic endoderm (nEnd). As a result, considerably more is known about the gene regulatory networks and signalling requirements of pluripotent stem cells than nEnd. In this review, we describe the ontogeny and differentiation of the PrE or hypoblast in mouse and primate and then discuss in vitro cell culture models for different extraembryonic endodermal cell types.

Pluripotency and embryonic lineage genes expression in the presence of small molecule inhibitors of FGF, TGFβ and GSK3 during pre-implantation development of goat embryos

Generating stable livestock pluripotent stem cells (PSCs) can be used for complex genome editing, cellular agriculture, gamete generation, regenerative medicine and in vitro breeding schemes. Over the past decade, significant progress has been made in characterizing pluripotency markers for livestock species. In this study, we investigated embryo development and gene expression of the core pluripotency triad (OCT4, NANOG, SOX2) and cell lineage commitment markers (REX1, CDX2, GATA4) in the presence of three small molecules and their combination [PD0325901 (FGF inhibitor), SB431542 (TGFβ inhibitor), and CHIR99021 (GSK3B inhibitor)] from day 2-7 post-insemination in goat. Significant reduction in rate of blastocyst formation was observed when SB was used along with PD or CHIR and their three combinations had more sever effect. SB and CHIR decreased the expression of SOX2 while increasing the GATA4 expression. PD decrease the relative expression of NANOG, OCT4 and GATA4, while increased the expression of REX1. Among the combination of two molecules, only SB + CHIR combination significantly decreased the expression of GATA4, while the combination of the three molecules significantly decreases the expression of NANOG, SOX2 and CDX2. According to these results, the inhibition of the FGF signaling pathway, by PD may lead to blocking the hypoblast formation as observed by reduction of GATA4. OCT4 and NANOG expressions did not show signs of maintenance pluripotency. GATA4, NANOG and OCT4 in the PD group were downregulated and REX1 as EPI-marker was upregulated thus REX1 may be considered as a marker of EPI/ICM in goat.

Self-patterning of human stem cells into post-implantation lineages

Investigating human development is a substantial scientific challenge due to the technical and ethical limitations of working with embryonic samples. In the face of these difficulties, stem cells have provided an alternative to experimentally model inaccessible stages of human development in vitro1-13. Here we show that human pluripotent stem cells can be triggered to self-organize into three-dimensional structures that recapitulate some key spatiotemporal events of early human post-implantation embryonic development. Our system reproducibly captures spontaneous differentiation and co-development of embryonic epiblast-like and extra-embryonic hypoblast-like lineages, establishes key signalling hubs with secreted modulators and undergoes symmetry breaking-like events. Single-cell transcriptomics confirms differentiation into diverse cell states of the perigastrulating human embryo14,15 without establishing placental cell types, including signatures of post-implantation epiblast, amniotic ectoderm, primitive streak, mesoderm, early extra-embryonic endoderm, as well as initial yolk sac induction. Collectively, our system captures key features of human embryonic development spanning from Carnegie stage16 4-7, offering a reproducible, tractable and scalable experimental platform to understand the basic cellular and molecular mechanisms that underlie human development, including new opportunities to dissect congenital pathologies with high throughput.

Genome-wide identification and evolutionary analysis of the FGF gene family in buffalo

Fibroblast growth factors (FGFs) are important polypeptide growth factors that play a critical role in many developmental processes, including differentiation, cell proliferation, and migration in mammals. This study employs in silico analyses to characterize the FGF gene family in buffalo, investigating their genome-wide identification, physicochemical properties, and evolutionary patterns. For this purpose, genomic and proteomic sequences of buffalo, cattle, goat, and sheep were retrieved from NCBI database. We identified a total of 22 FGF genes in buffalo. Physicochemical properties observed through ProtParam tool showed notable features of these proteins including in-vitro instability, thermostability, hydrophilicity, and basic nature. Phylogenetic analysis grouped 22 identified genes into nine sub-families based on evolutionary relationships. Additionally, analysis of gene structure, motif patterns, and conserved domains using TBtools revealed the remarkable conservation of this gene family across selected species throughout the course of evolution. Comparative amino acid analysis performed through ClustalW demonstrated significant conservation between buffalo and cattle FGF proteins. Mutational analysis showed three non-synonymous mutations at positions R103 > G, P7 > L, and E98 > Q in FGF4, FGF6, and FGF19, respectively in buffalo. Duplication events revealed only one segmental duplication (FGF10/FGF22) in buffalo and two in cattle (FGF10/FGF22 and FGF13/FGF13-like) with Ka/Ks values <1 indicating purifying selection pressure for these duplications. Comparison of protein structures of buffalo, goat, and sheep exhibited more similarities in respective structures. In conclusion, our study highlights the conservation of the FGF gene family in buffalo during evolution. Furthermore, the identified non-synonymous mutations may have implications for the selection of animals with better performance.Communicated by Ramaswamy H. Sarma.

Pre-Implantation Bovine Embryo Evaluation-From Optics to Omics and Beyond

Approximately 80% of the ~1.5 million bovine embryos transferred in 2021 were in vitro produced. However, only ~27% of the transferred IVP embryos will result in live births. The ~73% pregnancy failures are partly due to transferring poor-quality embryos, a result of erroneous stereomicroscopy-based morphological evaluation, the current method of choice for pre-transfer embryo evaluation. Numerous microscopic (e.g., differential interference contrast, electron, fluorescent, time-lapse, and artificial-intelligence-based microscopy) and non-microscopic (e.g., genomics, transcriptomics, epigenomics, proteomics, metabolomics, and nuclear magnetic resonance) methodologies have been tested to find an embryo evaluation technique that is superior to morphologic evaluation. Many of these research tools can accurately determine embryo quality/viability; however, most are invasive, expensive, laborious, technically sophisticated, and/or time-consuming, making them futile in the context of in-field embryo evaluation. However accurate they may be, using complex methods, such as RNA sequencing, SNP chips, mass spectrometry, and multiphoton microscopy, at thousands of embryo production/collection facilities is impractical. Therefore, future research is warranted to innovate field-friendly, simple benchtop tests using findings already available, particularly from omics-based research methodologies. Time-lapse monitoring and artificial-intelligence-based automated image analysis also have the potential for accurate embryo evaluation; however, further research is warranted to innovate economically feasible options for in-field applications.

In Vitro Culture Alters Cell Lineage Composition and Cellular Metabolism of Bovine Blastocyst

Profiling transcriptome at single cell level of bovine blastocysts derived in vivo (IVV), in vitro from conventional culture medium (IVC), and reduced nutrient culture medium (IVR) has enabled us to reveal cell lineage segregation, during which forming inner cell mass (ICM), trophectoderm (TE), and an undefined population of transitional cells. Only IVV embryos had well-defined ICM, indicating in vitro culture may delay the first cell fate commitment to ICM. Differences between IVV, IVC and IVR embryos were mainly contributed by ICM and transitional cells. Pathway analysis by using the differentially expressed genes of these non-TE cells between groups pointed to highly active metabolic and biosynthetic processes, with reduced cellular signaling and membrane transport in IVC embryos, which may lead to reduced developmental potential. IVR embryos had lower activities in metabolic and biosynthetic processes, but increased cellular signaling and membrane transport, suggesting these cellular mechanisms may contribute to the improved blastocyst development compared to IVC embryos. However, the IVR embryos had compromised development when compared to IVV embryos with notably over-active membrane transport activities that led to impaired ion homeostasis.

Lineage segregation in human pre-implantation embryos is specified by YAP1 and TEAD1

STUDY QUESTION

Which processes and transcription factors specify the first and second lineage segregation events during human preimplantation development?

SUMMARY ANSWER

Differentiation into trophectoderm (TE) cells can be initiated independently of polarity; moreover, TEAD1 and YAP1 co-localize in (precursor) TE and primitive endoderm (PrE) cells, suggesting a role in both the first and the second lineage segregation events.

WHAT IS KNOWN ALREADY

We know that polarity, YAP1/GATA3 signalling and phospholipase C signalling play a key role in TE initiation in compacted human embryos, however, little is known about the TEAD family of transcription factors that become activated by YAP1 and, especially, whether they play a role during epiblast (EPI) and PrE formation. In mouse embryos, polarized outer cells show nuclear TEAD4/YAP1 activity that upregulates Cdx2 and Gata3 expression while inner cells exclude YAP1 which upregulates Sox2 expression. The second lineage segregation event in mouse embryos is orchestrated by FGF4/FGFR2 signalling which could not be confirmed in human embryos; TEAD1/YAP1 signalling also plays a role during the establishment of mouse EPI cells.

STUDY DESIGN, SIZE, DURATION

Based on morphology, we set up a development timeline of 188 human preimplantation embryos between Day 4 and 6 post-fertilization (dpf). The compaction process was divided into three subgroups: embryos at the start (C0), during (C1), and at the end (C2) of, compaction. Inner cells were identified as cells that were entirely separated from the perivitelline space and enclosed by cellular contacts on all sides. The blastulation process was divided into six subgroups, starting with early blastocysts with sickle-cell shaped outer cells (B0) and further on, blastocysts with a cavity (B1). Full blastocysts (B2) showed a visible ICM and outer cells referred to as TE. Further expanded blastocysts (B3) had accumulated fluid and started to expand due to TE cell proliferation and zona pellucida (ZP) thinning. The blastocysts then significantly expanded further (B4) and started to hatch out of the ZP (B5) until they were fully hatched (B6).

PARTICIPANTS/MATERIALS, SETTING, METHODS

After informed consent and the expiration of the 5-year cryopreservation duration, 188 vitrified high quality eight-cell stage human embryos (3 dpf) were warmed and cultured until the required stages were reached. We also cultured 14 embryos that were created for research until the four- and eight-cell stage. The embryos were scored according to their developmental stage (C0-B6) displaying morphological key differences, rather than defining them according to their chronological age. They were fixed and immunostained for different combinations of cytoskeleton (F-actin), polarization (p-ERM), TE (GATA3), EPI (NANOG), PrE (GATA4 and SOX17), and members of the Hippo signalling pathway (YAP1, TEAD1 and TEAD4). We choose these markers based on previous observations in mouse embryos and single cell RNA-sequencing data of human embryos. After confocal imaging (LSM800, Zeiss), we analysed cell numbers within each lineage, different co-localization patterns and nuclear enrichment.

MAIN RESULTS AND THE ROLE OF CHANCE

We found that in human preimplantation embryos compaction is a heterogeneous process that takes place between the eight-cell to the 16-cell stages. Inner and outer cells are established at the end of the compaction process (C2) when the embryos contain up to six inner cells. Full apical p-ERM polarity is present in all outer cells of compacted C2 embryos. Co-localization of p-ERM and F-actin increases steadily from 42.2% to 100% of the outer cells, between C2 and B1 stages, while p-ERM polarizes before F-actin (P < 0.00001). Next, we sought to determine which factors specify the first lineage segregation event. We found that 19.5% of the nuclei stain positive for YAP1 at the start of compaction (C0) which increases to 56.1% during compaction (C1). At the C2 stage, 84.6% of polarized outer cells display high levels of nuclear YAP1 while it is absent in 75% of non-polarized inner cells. In general, throughout the B0-B3 blastocyst stages, polarized outer/TE cells are mainly positive for YAP1 and non-polarized inner/ICM cells are negative for YAP1. From the C1 stage onwards, before polarity is established, the TE marker GATA3 is detectable in YAP1 positive cells (11.6%), indicating that differentiation into TE cells can be initiated independently of polarity. Co-localization of YAP1 and GATA3 increases steadily in outer/TE cells (21.8% in C2 up to 97.3% in B3). Transcription factor TEAD4 is ubiquitously present throughout preimplantation development from the compacted stage onwards (C2-B6). TEAD1 displays a distinct pattern that coincides with YAP1/GATA3 co-localization in the outer cells. Most outer/TE cells throughout the B0-B3 blastocyst stages are positive for TEAD1 and YAP1. However, TEAD1 proteins are also detected in most nuclei of the inner/ICM cells of the blastocysts from cavitation onwards, but at visibly lower levels as compared to that in TE cells. In the ICM of B3 blastocysts, we found one main population of cells with NANOG+/SOX17-/GATA4- nuclei (89.1%), but exceptionally we found NANOG+/SOX17+/GATA4+ cells (0.8%). In seven out of nine B3 blastocysts, nuclear NANOG was found in all the ICM cells, supporting the previously reported hypothesis that PrE cells arise from EPI cells. Finally, to determine which factors specify the second lineage segregation event, we co-stained for TEAD1, YAP1, and GATA4. We identified two main ICM cell populations in B4-6 blastocysts: the EPI (negative for the three markers, 46.5%) and the PrE (positive for the three markers, 28.1%) cells. We conclude that TEAD1 and YAP1 co-localise in (precursor) TE and PrE cells, indicating that TEAD1/YAP1 signalling plays a role in the first and the second lineage segregation events.

LIMITATIONS, REASONS FOR CAUTION

In this descriptive study, we did not perform functional studies to investigate the role of TEAD1/YAP1 signalling during the first and second lineage segregation events.

WIDER IMPLICATIONS OF THE FINDINGS

Our detailed roadmap on polarization, compaction, position and lineage segregation events during human preimplantation development paves the way for further functional studies. Understanding the gene regulatory networks and signalling pathways involved in early embryogenesis could ultimately provide insights into why embryonic development is sometimes impaired and facilitate the establishment of guidelines for good practice in the IVF lab.

STUDY FUNDING/COMPETING INTERESTS

This work was financially supported by Wetenschappelijk Fonds Willy Gepts (WFWG) of the University Hospital UZ Brussel (WFWG142) and the Fonds Wetenschappelijk Onderzoek-Vlaanderen (FWO, G034514N). M.R. is doctoral fellow at the FWO. The authors have no conflicts of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.

Developmental Hurdles That Can Compromise Pregnancy during the First Month of Gestation in Cattle

Several key developmental events are associated with early embryonic pregnancy losses in beef and dairy cows. These developmental problems are observed at a greater frequency in pregnancies generated from in-vitro-produced bovine embryos. This review describes critical problems that arise during oocyte maturation, fertilization, early embryonic development, compaction and blastulation, embryonic cell lineage specification, elongation, gastrulation, and placentation. Additionally, discussed are potential remediation strategies, but unfortunately, corrective actions are not available for several of the problems being discussed. Further research is needed to produce bovine embryos that have a greater likelihood of surviving to term.

Establishment of the fetal-maternal interface: developmental events in human implantation and placentation

Early pregnancy is a complex and well-orchestrated differentiation process that involves all the cellular elements of the fetal-maternal interface. Aberrant trophoblast-decidual interactions can lead to miscarriage and disorders that occur later in pregnancy, including preeclampsia, intrauterine fetal growth restriction, and preterm labor. A great deal of research on the regulation of implantation and placentation has been performed in a wide range of species. However, there is significant species variation regarding trophoblast differentiation as well as decidual-specific gene expression and regulation. Most of the relevant information has been obtained from studies using mouse models. A comprehensive understanding of the physiology and pathology of human implantation and placentation has only recently been obtained because of emerging advanced technologies. With the derivation of human trophoblast stem cells, 3D-organoid cultures, and single-cell analyses of differentiated cells, cell type-specific transcript profiles and functions were generated, and each exhibited a unique signature. Additionally, through integrative transcriptomic information, researchers can uncover the cellular dysfunction of embryonic and placental cells in peri-implantation embryos and the early pathological placenta. In fact, the clinical utility of fetal-maternal cellular trafficking has been applied for the noninvasive prenatal diagnosis of aneuploidies and the prediction of pregnancy complications. Furthermore, recent studies have proposed a viable path toward the development of therapeutic strategies targeting placenta-enriched molecules for placental dysfunction and diseases.

Evidence implicating sequential commitment of the founder lineages in the human blastocyst by order of hypoblast gene activation

Successful human pregnancy depends upon rapid establishment of three founder lineages: the trophectoderm, epiblast and hypoblast, which together form the blastocyst. Each plays an essential role in preparing the embryo for implantation and subsequent development. Several models have been proposed to define the lineage segregation. One suggests that all lineages specify simultaneously; another favours the differentiation of the trophectoderm before separation of the epiblast and hypoblast, either via differentiation of the hypoblast from the established epiblast, or production of both tissues from the inner cell mass precursor. To begin to resolve this discrepancy and thereby understand the sequential process for production of viable human embryos, we investigated the expression order of genes associated with emergence of hypoblast. Based upon published data and immunofluorescence analysis for candidate genes, we present a basic blueprint for human hypoblast differentiation, lending support to the proposed model of sequential segregation of the founder lineages of the human blastocyst. The first characterised marker, specific initially to the early inner cell mass, and subsequently identifying presumptive hypoblast, is PDGFRA, followed by SOX17, FOXA2 and GATA4 in sequence as the hypoblast becomes committed.

Pluripotent Stem Cells as a Model for Human Embryogenesis

Pluripotent stem cells (PSCs; embryonic stem cells and induced pluripotent stem cells) can recapitulate critical aspects of the early stages of embryonic development; therefore, they became a powerful tool for the in vitro study of molecular mechanisms that underlie blastocyst formation, implantation, the spectrum of pluripotency and the beginning of gastrulation, among other processes. Traditionally, PSCs were studied in 2D cultures or monolayers, without considering the spatial organization of a developing embryo. However, recent research demonstrated that PSCs can form 3D structures that simulate the blastocyst and gastrula stages and other events, such as amniotic cavity formation or somitogenesis. This breakthrough provides an unparalleled opportunity to study human embryogenesis by examining the interactions, cytoarchitecture and spatial organization among multiple cell lineages, which have long remained a mystery due to the limitations of studying in utero human embryos. In this review, we will provide an overview of how experimental embryology currently utilizes models such as blastoids, gastruloids and other 3D aggregates derived from PSCs to advance our understanding of the intricate processes involved in human embryo development.

Species-specific molecular differentiation of embryonic inner cell mass and trophectoderm: A systematic review

A wide-ranging review study regarding the molecular characterization of the first cell lineages of the developmental embryo is lacking, especially for the primary events during earliest differentiation which leads to the determination of cellular fate. Here, a systematic review and meta-analysis were conducted according to PRISMA guidelines. MEDLINE-PubMed was searched based on an established search strategy through April 2021. Thirty-six studies fulfilling the inclusion criteria were subjected to qualitative and quantitative analysis. Among the studies, 50 % (18/36) used mice as an animal model, 22.2 % (8/36) pigs, 16.7 % (6/36) cattle, 5.5 % (2/36) humans, and 2.8 % (1/36) goats as well as 2.8 % (1/36) equine. Our results demonstrated that each of the first cell lineages of embryos requires a certain pattern of expression to establish the cellular determination of fate. Moreover, these patterns are shared by many species, particularly for those molecules that have already been identified in the literature as biomarkers. In conclusion, the present study integrated carefully chosen studies regarding embryonic development and first cellular decisions in mammalian species and summarized the information about the differential characterization of the first cell lineages and their possible relationship with specific gene expression.

Inhibition of FGF receptor impairs primitive endoderm differentiation in bovine embryos

The first cellular differentiation event in the pre-implantation embryo results in the trophectoderm (TE) and the inner cell mass (ICM). A second event occurs in the latter, resulting in the epiblast and the primitive endoderm (PE). This second differentiation is still not fully characterized in bovine development, although it is likely to involve FGF signalling. Thus, in this study, we tested the hypothesis that stimulation or inhibition of the FGF pathway during bovine embryo in vitro culture would only interfere with PE differentiation if maintained until later blastocyst stages. At first, we characterized the expression of PE marker SOX17 at different blastocyst stages. Then, we treated in vitro produced embryos during different windows of time: days 5.0-7.0 (D5-D7), D7-D9, and D5-D9 with 1 μg/ml FGF4 and 1 μg/ml heparin or 1 mM FGFR inhibitor, AZD4547. We observed that the SOX17-positive cell number only increases in late-stage blastocysts compared to early stages. Treatment of embryos with FGF4 did not change the number of SOX17-positive cells, while inhibition of FGFR signalling reduced SOX17-positive cells from D5-D7 and completely ablated SOX17 expression when kept until D9. In conclusion, FGFR inhibition repressed PE differentiation in bovine embryos at all time points, although stimulation with FGF4 did not interfere with PE cell numbers.

Effects of fetal bovine serum on trophectoderm and primitive endoderm cell allocation of in vitro-produced bovine embryos

Supplementing embryonic culture medium with fetal bovine serum (FBS) renders this medium undefined. Glucose and growth factors present in FBS may affect the results of cell differentiation studies. This study tested the hypothesis that FBS supplementation during in vitro culture (IVC) alters cell differentiation in early bovine embryo development. Bovine embryos were produced in vitro and randomly distributed into three experimental groups at 90 h post insemination (90 hpi): the KSOM-FBS group, which consisted of a 5% (v/v) FBS supplementation; the KSOM33 group, with the renewal of 33% of medium volume; and the KSOM-Zero group, without FBS supplementation nor renewal of the culture medium. The results showed that the blastocyst rate (blastocyst/oocytes) at 210 hpi in the KSOM-FBS group was higher than in the KSOM-Zero group but not different from the KSOM33 group. There were no significant changes in metabolism-related aspects, such as fluorescence intensities of CellROX Green and MitoTracker Red or reduced nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD+). Immunofluorescence analysis of CDX2 revealed that the lack of FBS or medium supplementation reduced the number of trophectoderm (TE) cells and total cells. Immunofluorescence analysis revealed a reduction of SOX17-positive cell numbers after FBS supplementation compared with the KSOM33 group. Therefore, we concluded that FBS absence reduced blastocyst rates; however, no reduction occurred when there was a 33% volume renewal of the medium at 90 hpi. We also concluded that FBS supplementation altered TE and primitive endoderm cell allocation during early bovine embryo development.

Biophysical models of early mammalian embryogenesis

Embryo development is a critical and fascinating stage in the life cycle of many organisms. Despite decades of research, the earliest stages of mammalian embryogenesis are still poorly understood, caused by a scarcity of high-resolution spatial and temporal data, the use of only a few model organisms, and a paucity of truly multidisciplinary approaches that combine biological research with biophysical modeling and computational simulation. Here, we explain the theoretical frameworks and biophysical processes that are best suited to modeling the early mammalian embryo, review a comprehensive list of previous models, and discuss the most promising avenues for future work.

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.

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'.

Alternative mammalian strategies leading towards gastrulation: losing polar trophoblast (Rauber's layer) or gaining an epiblast cavity

Using embryological data from 14 mammalian orders, the hypothesis is presented that in placental mammals, epiblast cavitation and polar trophoblast loss are alternative developmental solutions to shield the central epiblast from extraembryonic signalling. It is argued that such reciprocal signalling between the edge of the epiblast and the adjoining polar trophoblast or edge of the mural trophoblast or with the amniotic ectoderm is necessary for the induction of gastrulation. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Truncation of IFT80 causes early embryonic loss in Holstein cattle associated with Holstein haplotype 2

Recessive alleles represent genetic risk in populations that have undergone bottleneck events. We present a comprehensive framework for identification and validation of these genetic defects, including haplotype-based detection, variant selection from sequence data, and validation using knockout embryos. Holstein haplotype 2 (HH2), which causes embryonic death, was used to demonstrate the approach. Holstein haplotype 2 was identified using a deficiency-of-homozygotes approach and confirmed to negatively affect conception rate and stillbirths. Five carriers were present in a group of 183 sequenced Holstein bulls selected to maximize the coverage of unique haplotypes. Three variants concordant with haplotype calls were found in HH2: a high-priority frameshift mutation resulting, and 2 low-priority variants (1 synonymous variant, 1 premature stop codon). The frameshift in intraflagellar 80 (IFT80) was confirmed in a separate group of Holsteins from the 1000 Bull Genomes Project that shared no animals with the discovery set. IFT80-null embryos were generated by truncating the IFT80 transcript at exon 2 or 11 using a CRISPR-Cas9 system. Abattoir-derived oocytes were fertilized in vitro, and zygotes were injected at the one-cell stage either with a guide RNA and CAS9 mRNA complex (n = 100) or Cas9 mRNA (control, n = 100) before return to culture, and replicated 3 times. IFT80 is activated at the 8-cell stage, and IFT80-null embryos arrested at this stage of development, which is consistent with data from mouse hypomorphs and HH2 carrier-to-carrier matings. This frameshift in IFT80 on chromosome 1 at 107,172,615 bp (p.Leu381fs) disrupts WNT and hedgehog signaling, and is responsible for the death of homozygous embryos.

The non-coding genome in early human development - Recent advancements

Not that long ago, the human genome was discovered to be mainly non-coding, that is comprised of DNA sequences that do not code for proteins. The initial paradigm that non-coding is also non-functional was soon overturned and today the work to uncover the functions of non-coding DNA and RNA in human early embryogenesis has commenced. Early human development is characterized by large-scale changes in genomic activity and the transcriptome that are partly driven by the coordinated activation and repression of repetitive DNA elements scattered across the genome. Here we provide examples of recent novel discoveries of non-coding DNA and RNA interactions and mechanisms that ensure accurate non-coding activity during human maternal-to-zygotic transition and lineage segregation. These include studies on small and long non-coding RNAs, transposable element regulation, and RNA tailing in human oocytes and early embryos. High-throughput approaches to dissect the non-coding regulatory networks governing early human development are a foundation for functional studies of specific genomic elements and molecules that has only begun and will provide a wider understanding of early human embryogenesis and causes of infertility.

Bioactive supplements influencing bovine in vitro embryo development

Ovum pickup and in vitro production (IVP) of bovine embryos are replacing traditional multiple ovulation embryo transfer (MOET) as the primary means for generating transferable embryos from genetically elite sires and dams. However, inefficiencies in the IVP process limit the opportunities to produce large numbers of transferable embryos. Also, the post-transfer competency of IVP embryos is inferior to embryos produced by artificial insemination or MOET. Numerous maternal, paternal, embryonic, and culture-related factors can have adverse effects on IVP success. This review will explore the various efforts made on describing how IVP embryo development and post-transfer competency may be improved by supplementing hormones, growth factors, cytokines, steroids and other bioactive factors found in the oviduct and uterus during early pregnancy. More than 40 of these factors, collectively termed as embryokines, are reviewed here. Several embryokines contain abilities to promote embryo development, including improving embryo survivability, improving blastomere cell numbers, and altering the distribution of blastomere cell types in blastocysts. A select few embryokines also can benefit pregnancy retention after IVP embryo transfer and improve neonatal calf health and performance, although very few embryokine-supplemented embryo transfer studies have been completed. Also, supplementing several embryokines at the same time holds promise for improving IVP embryo development and competency. However, more work is needed to explore the post-transfer consequences of adding these putative embryokines for any adverse outcomes, such as large offspring syndrome and poor postnatal health, and to specify the specific embryokine combinations that will best represent the ideal conditions found in the oviduct and uterus.

NANOG initiates epiblast fate through the coordination of pluripotency genes expression

The epiblast is the source of all mammalian embryonic tissues and of pluripotent embryonic stem cells. It differentiates alongside the primitive endoderm in a “salt and pepper” pattern from inner cell mass (ICM) progenitors during the preimplantation stages through the activity of NANOG, GATA6 and the FGF pathway. When and how epiblast lineage specification is initiated is still unclear. Here, we show that the coordinated expression of pluripotency markers defines epiblast identity. Conversely, ICM progenitor cells display random cell-to-cell variability in expression of various pluripotency markers, remarkably dissimilar from the epiblast signature and independently from NANOG, GATA6 and FGF activities. Coordination of pluripotency markers expression fails in Nanog and Gata6 double KO (DKO) embryos. Collectively, our data suggest that NANOG triggers epiblast specification by ensuring the coordinated expression of pluripotency markers in a subset of cells, implying a stochastic mechanism. These features are likely conserved, as suggested by analysis of human embryos.

Pluripotent epiblast cells segregate from primitive endoderm in the blastocyst inner cell mass (ICM). Here the authors show that mosaic epiblast differentiation during mouse and human preimplantation development initiates stochastically in ICM progenitors, independently of the FGF pathway, and requires NANOG activity

The functional diversity of the POUV-class proteins across vertebrates

POUV is a relatively newly emerged class of POU transcription factors present in jawed vertebrates (Gnathostomata). The function of POUV-class proteins is inextricably linked to zygotic genome activation (ZGA). A large body of evidence now extends the role of these proteins to subsequent developmental stages. While some functions resemble those of other POU-class proteins and are related to neuroectoderm development, others have emerged de novo. The most notable of the latter functions is pluripotency control by Oct4 in mammals. In this review, we focus on these de novo functions in the best-studied species harbouring POUV proteins-zebrafish, Xenopus (anamniotes) and mammals (amniotes). Despite the broad diversity of their biological functions in vertebrates, POUV proteins exert a common feature related to their role in safeguarding the undifferentiated state of cells. Here we summarize numerous pieces of evidence for these specific functions of the POUV-class proteins and recap available loss-of-function data.

OCT4/POU5F1 is indispensable for the lineage differentiation of the inner cell mass in bovine embryos

The mammalian blastocyst undergoes two lineage segregations, that is, formation of the trophectoderm and subsequently differentiation of the hypoblast (HB) from the inner cell mass, leaving the epiblast (EPI) as the remaining pluripotent lineage. To clarify the expression patterns of markers specific for these lineages in bovine embryos, we analyzed day 7, 9, and 12 blastocysts completely produced in vivo by staining for OCT4, NANOG, SOX2 (EPI), and GATA6, SOX17 (HB) and identified genes specific for these developmental stages in a global transcriptomics approach. To study the role of OCT4, we generated OCT4-deficient (OCT4 KO) embryos via somatic cell nuclear transfer or in vitro fertilization. OCT4 KO embryos reached the expanded blastocyst stage by day 8 but lost NANOG and SOX17 expression, while SOX2 and GATA6 were unaffected. Blastocysts transferred to recipient cows from day 6 to 9 expanded, but the OCT4 KO phenotype was not rescued by the uterine environment. Exposure of OCT4 KO embryos to exogenous FGF4 or chimeric complementation with OCT4 intact embryos did not restore NANOG or SOX17 in OCT4-deficient cells. Our data show that OCT4 is required cell autonomously for the maintenance of pluripotency of the EPI and differentiation of the HB in bovine embryos.

Pluripotent Core in Bovine Embryos: A Review

Early development in mammals is characterized by the ability of each cell to produce a complete organism plus the extraembryonic, or placental, cells, defined as pluripotency. During subsequent development, pluripotency is lost, and cells begin to differentiate to a particular cell fate. This review summarizes the current knowledge of pluripotency features of bovine embryos cultured in vitro, focusing on the core of pluripotency genes (OCT4, NANOG, SOX2, and CDX2), and main chemical strategies for controlling pluripotent networks during early development. Finally, we discuss the applicability of manipulating pluripotency during the morula to blastocyst transition in cattle species.

An eQTL variant of ALDH1A2 is associated with Kashin-Beck disease in Chinese population

INTRODUCTION

The aims of the study were to investigate the relationship between aldehyde dehydrogenase 1 family member A2 (ALDH1A2) and Kashin-Beck disease (KBD), explore the effects of the rs3204689 polymorphism and methylation status on the expression levels of ALDH1A2, and further clarify the pathogenesis of KBD.

MATERIALS AND METHODS

The genotype of ALDH1A2 rs3204689 was detected by PCR-RFLP in 103 KBD patients and 109 healthy controls in the whole blood. The mRNA level of ALDH1A2 was measured by qRT-PCR, and the protein expression was detected using IHC staining and Western blotting. The MSP-PCR was used to identify the ALDH1A2 methylation level.

RESULTS

There were significant differences in G/G, G/C, and C/C frequencies of ALDH1A2 rs3204689 between the KBD and control groups (χ² = 7.113, P = 0.029); the minor allele G of ALDH1A2 was associated with the risk of KBD (χ² = 5.984, P = 0.014). The mRNA and protein levels of ALDH1A2 were increased in the whole blood and cartilage of KBD patients compared with the controls (P = 0.049, P < 0.0001, P = 0.019). Meanwhile, a statistically significant difference was observed between G/G, G/C and C/C genotype on mRNA expression (P = 0.039). The methylation level of the ALDH1A2 gene promoter region showed no significant difference between the KBD and control groups (χ² = 0.317, P = 0.573).

CONCLUSION

Our case-control study indicates that the common variant rs3204689 near ALDH1A2 is associated with KBD in Chinese population. The risk allele G of rs3204689 is statistically linked to the high expression of ALDH1A2, which is up-regulated in the cartilage and whole blood of KBD patients. Our findings suggest a potential role of ALDH1A2 in the pathogenesis of KBD.