The molecular and cellular features of 2-cell-like cells: a reference guide

Chromatin structure in totipotent mouse early preimplantation embryos

Totipotency refers to the ability of a single cell to give rise to all the different cell types in the body. Terminally differentiated germ cells (sperm and oocytes) undergo reprogramming, which results in the acquisition of totipotency in zygotes. Since the 1990s, numerous studies have focused on the mechanisms of totipotency. With the emergence of the concept of epigenetic reprogramming, which is important for the undifferentiated and differentiated states of cells, the epigenomes of germ cells and fertilized oocytes have been thoroughly analyzed. However, in early immunostaining studies, detailed epigenomic information was difficult to obtain. In recent years, the explosive development of next-generation sequencing has made it possible to acquire genome-wide information and the rise of genome editing has facilitated the analysis of knockout mice, which was previously difficult. In addition, live imaging can effectively analyze zygotes and 2-cell embryos, for which the number of samples is limited, and provides biological insights that cannot be obtained by other methods. In this review, the progress of our research using these advanced techniques is traced back from the present to its earliest years.

Epigenetic reprogramming in the transition from pluripotency to totipotency

Mammalian development commences with the zygote, which can differentiate into both embryonic and extraembryonic tissues, a capability known as totipotency. Only the zygote and embryos around zygotic genome activation (ZGA) (two-cell embryo stage in mice and eight-cell embryo in humans) are totipotent cells. Epigenetic modifications undergo extremely extensive changes during the acquisition of totipotency and subsequent development of differentiation. However, the underlying molecular mechanisms remain elusive. Recently, the discovery of mouse two-cell embryo-like cells, human eight-cell embryo-like cells, extended pluripotent stem cells and totipotent-like stem cells with extra-embryonic developmental potential has greatly expanded our understanding of totipotency. Experiments with these in vitro models have led to insights into epigenetic changes in the reprogramming of pluri-to-totipotency, which have informed the exploration of preimplantation development. In this review, we highlight the recent findings in understanding the mechanisms of epigenetic remodeling during totipotency capture, including RNA splicing, DNA methylation, chromatin configuration, histone modifications, and nuclear organization.

Jump-starting life: balancing transposable element co-option and genome integrity in the developing mammalian embryo

Remnants of transposable elements (TEs) are widely expressed throughout mammalian embryo development. Originally infesting our genomes as selfish elements and acting as a source of genome instability, several of these elements have been co-opted as part of a complex system of genome regulation. Many TEs have lost transposition ability and their transcriptional potential has been tampered as a result of interactions with the host throughout evolutionary time. It has been proposed that TEs have been ultimately repurposed to function as gene regulatory hubs scattered throughout our genomes. In the early embryo in particular, TEs find a perfect environment of naïve chromatin to escape transcriptional repression by the host. As a consequence, it is thought that hosts found ways to co-opt TE sequences to regulate large-scale changes in chromatin and transcription state of their genomes. In this review, we discuss several examples of TEs expressed during embryo development, their potential for co-option in genome regulation and the evolutionary pressures on TEs and on our genomes.

Systematic evaluation of retroviral LTRs as cis-regulatory elements in mouse embryos

In mammals, many retrotransposons are de-repressed during zygotic genome activation (ZGA). However, their functions in early development remain elusive largely due to the challenge to simultaneously manipulate thousands of retrotransposon insertions in embryos. Here, we applied CRISPR interference (CRISPRi) to perturb the long terminal repeat (LTR) MT2_Mm, a well-known ZGA and totipotency marker that exists in ∼2,667 insertions throughout the mouse genome. CRISPRi robustly perturbed 2,485 (∼93%) MT2_Mm insertions and 1,090 (∼55%) insertions of the closely related MT2C_Mm in 2-cell embryos. Remarkably, such perturbation caused downregulation of hundreds of ZGA genes and embryonic arrest mostly at the morula stage. Mechanistically, MT2 LTRs are globally enriched for open chromatin and H3K27ac and function as promoters/enhancers downstream of OBOX/DUX proteins. Thus, we not only provide direct evidence to support the functional importance of MT2 activation in development but also systematically define cis-regulatory function of MT2 in embryos by integrating functional perturbation and multi-omic analyses.

A maternal-effect Padi6 variant causes nuclear and cytoplasmic abnormalities in oocytes, as well as failure of epigenetic reprogramming and zygotic genome activation in embryos

Maternal inactivation of genes encoding components of the subcortical maternal complex (SCMC) and its associated member, PADI6, generally results in early embryo lethality. In humans, SCMC gene variants were found in the healthy mothers of children affected by multilocus imprinting disturbances (MLID). However, how the SCMC controls the DNA methylation required to regulate imprinting remains poorly defined. We generated a mouse line carrying a Padi6 missense variant that was identified in a family with Beckwith-Wiedemann syndrome and MLID. If homozygous in female mice, this variant resulted in interruption of embryo development at the two-cell stage. Single-cell multiomic analyses demonstrated defective maturation of Padi6 mutant oocytes and incomplete DNA demethylation, down-regulation of zygotic genome activation (ZGA) genes, up-regulation of maternal decay genes, and developmental delay in two-cell embryos developing from Padi6 mutant oocytes but little effect on genomic imprinting. Western blotting and immunofluorescence analyses showed reduced levels of UHRF1 in oocytes and abnormal localization of DNMT1 and UHRF1 in both oocytes and zygotes. Treatment with 5-azacytidine reverted DNA hypermethylation but did not rescue the developmental arrest of mutant embryos. Taken together, this study demonstrates that PADI6 controls both nuclear and cytoplasmic oocyte processes that are necessary for preimplantation epigenetic reprogramming and ZGA.

Maternal KLF17 controls zygotic genome activation by acting as a messenger for RNA Pol II recruitment in mouse embryos

Initiation of timely and sufficient zygotic genome activation (ZGA) is crucial for the beginning of life, yet our knowledge of transcription factors (TFs) contributing to ZGA remains limited. Here, we screened the proteome of early mouse embryos after cycloheximide (CHX) treatment and identified maternally derived KLF17 as a potential TF for ZGA genes. Using a conditional knockout (cKO) mouse model, we further investigated the role of maternal KLF17 and found that it promotes embryonic development and full fertility. Mechanistically, KLF17 preferentially binds to promoters and recruits RNA polymerase II (RNA Pol II) in early 2-cell embryos, facilitating the expression of major ZGA genes. Maternal Klf17 knockout resulted in a downregulation of 9% of ZGA genes and aberrant RNA Pol II pre-configuration, which could be partially rescued by introducing exogenous KLF17. Overall, our study provides a strategy for screening essential ZGA factors and identifies KLF17 as a crucial TF in this process.

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.

Transition from totipotency to pluripotency in mice: insights into molecular mechanisms

Totipotency is the ability of a single cell to develop into a full organism and, in mammals, is strictly associated with the early stages of development following fertilization. This unlimited developmental potential becomes quickly restricted as embryonic cells transition into a pluripotent state. The loss of totipotency seems a consequence of the zygotic genome activation (ZGA), a process that determines the switch from maternal to embryonic transcription, which in mice takes place following the first cleavage. ZGA confers to the totipotent cell a transient transcriptional profile characterized by the expression of stage-specific genes and a set of transposable elements that prepares the embryo for subsequent development. The timely silencing of this transcriptional program during the exit from totipotency is required to ensure proper development. Importantly, the molecular mechanisms regulating the transition from totipotency to pluripotency have remained elusive due to the scarcity of embryonic material. However, the development of new in vitro totipotent-like models together with advances in low-input genome-wide technologies, are providing a better mechanistic understanding of how this important transition is achieved. This review summarizes the current knowledge on the molecular determinants that regulate the exit from totipotency.

Quiescence enables unrestricted cell fate in naive embryonic stem cells

Quiescence in stem cells is traditionally considered as a state of inactive dormancy or with poised potential. Naive mouse embryonic stem cells (ESCs) can enter quiescence spontaneously or upon inhibition of MYC or fatty acid oxidation, mimicking embryonic diapause in vivo. The molecular underpinning and developmental potential of quiescent ESCs (qESCs) are relatively unexplored. Here we show that qESCs possess an expanded or unrestricted cell fate, capable of generating both embryonic and extraembryonic cell types (e.g., trophoblast stem cells). These cells have a divergent metabolic landscape comparing to the cycling ESCs, with a notable decrease of the one-carbon metabolite S-adenosylmethionine. The metabolic changes are accompanied by a global reduction of H3K27me3, an increase of chromatin accessibility, as well as the de-repression of endogenous retrovirus MERVL and trophoblast master regulators. Depletion of methionine adenosyltransferase Mat2a or deletion of Eed in the polycomb repressive complex 2 results in removal of the developmental constraints towards the extraembryonic lineages. Our findings suggest that quiescent ESCs are not dormant but rather undergo an active transition towards an unrestricted cell fate.

Glutathione safeguards TET-dependent DNA demethylation and is critical for the acquisition of totipotency and pluripotency during preimplantation development

During early development, both genome-wide epigenetic reprogramming and metabolic remodeling are hallmark changes of normal embryogenesis. However, little is known about their relationship and developmental functions during the preimplantation window, which is essential for the acquisition of totipotency and pluripotency. Herein, we reported that glutathione (GSH), a ubiquitous intracellular protective antioxidant that maintains mitochondrial function and redox homeostasis, plays a critical role in safeguarding postfertilization DNA demethylation and is essential for establishing developmental potential in preimplantation embryos. By profiling mitochondria-related transcriptome that coupled with different pluripotency, we found GSH is a potential marker that is tightly correlated with full pluripotency, and its beneficial effect on prompting developmental potential was functionally conformed using in vitro fertilized mouse and bovine embryos as the model. Mechanistic study based on preimplantation embryos and embryonic stem cells further revealed that GSH prompts the acquisition of totipotency and pluripotency by facilitating ten-eleven-translocation (TET)-dependent DNA demethylation, and ascorbic acid (AsA)-GSH cycle is implicated in the process. In addition, we also reported that GSH serves as an oviductal paracrine factor that supports development potential of preimplantation embryos. Thus, our results not only advance the current knowledge of functional links between epigenetic reprogramming and metabolic remodeling during preimplantation development but also provided a promising approach for improving current in vitro culture system for assisted reproductive technology.

Retrotransposon renaissance in early embryos

Despite being the predominant genetic elements in mammalian genomes, retrotransposons were often dismissed as genomic parasites with ambiguous biological significance. However, recent studies reveal their functional involvement in early embryogenesis, encompassing crucial processes such as zygotic genome activation (ZGA) and cell fate decision. This review underscores the paradigm shift in our understanding of retrotransposon roles during early preimplantation development, as well as their rich functional reservoir that is exploited by the host to provide cis-regulatory elements, noncoding RNAs, and functional proteins. The rapid advancement in long-read sequencing, low input multiomics profiling, advanced in vitro systems, and precise gene editing techniques encourages further dissection of retrotransposon functions that were once obscured by the intricacies of their genomic footprints.

Nuclear RNA catabolism controls endogenous retroviruses, gene expression asymmetry, and dedifferentiation

Endogenous retroviruses (ERVs) are remnants of ancient parasitic infections and comprise sizable portions of most genomes. Although epigenetic mechanisms silence most ERVs by generating a repressive environment that prevents their expression (heterochromatin), little is known about mechanisms silencing ERVs residing in open regions of the genome (euchromatin). This is particularly important during embryonic development, where induction and repression of distinct classes of ERVs occur in short temporal windows. Here, we demonstrate that transcription-associated RNA degradation by the nuclear RNA exosome and Integrator is a regulatory mechanism that controls the productive transcription of most genes and many ERVs involved in preimplantation development. Disrupting nuclear RNA catabolism promotes dedifferentiation to a totipotent-like state characterized by defects in RNAPII elongation and decreased expression of long genes (gene-length asymmetry). Our results indicate that RNA catabolism is a core regulatory module of gene networks that safeguards RNAPII activity, ERV expression, cell identity, and developmental potency.

Molecular mechanisms underlying totipotency

Numerous efforts to understand pluripotency in mammals, using pluripotent stem cells in culture, have enabled the generation of artificially induced pluripotent stem cells, which serve as a valuable source for regenerative medicine and the creation of disease models. In contrast to these tremendous successes in the pluripotency field in the past few decades, our understanding of totipotency, which is highlighted by its broader plasticity than pluripotency, is still limited. This is largely attributable to the scarcity of available materials and the lack of in vitro models. However, recent technological advances have unveiled molecular features that characterize totipotent cells. Single-cell or low-input sequencing technologies allow the dissection of pre- and post-fertilization developmental processes at the molecular level with high resolution. In this review, we describe some of the key findings in understanding totipotency and discuss how totipotency is acquired at the beginning of life.

Systematic Perturbation of Thousands of Retroviral LTRs in Mouse Embryos

In mammals, many retrotransposons are de-repressed during zygotic genome activation (ZGA). However, their functions in early development remain elusive largely due to the challenge to simultaneously manipulate thousands of retrotransposon insertions in embryos. Here, we employed epigenome editing to perturb the long terminal repeat (LTR) MT2_Mm, a well-known ZGA and totipotency marker that exists in ~2667 insertions throughout the mouse genome. CRISPRi robustly repressed 2485 (~93%) MT2_Mm insertions and 1090 (~55%) insertions of the closely related MT2C_Mm in 2-cell embryos. Remarkably, such perturbation caused down-regulation of hundreds of ZGA genes at the 2-cell stage and embryonic arrest mostly at the morula stage. Mechanistically, MT2_Mm/MT2C_Mm primarily served as alternative ZGA promoters activated by OBOX proteins. Thus, through unprecedented large-scale epigenome editing, we addressed to what extent MT2_Mm/MT2C_Mm regulates ZGA and preimplantation development. Our approach could be adapted to systematically perturb retrotransposons in other mammalian embryos as it doesn't require transgenic animals.

Hyperosmotic stress induces 2-cell-like cells through ROS and ATR signaling

Mouse embryonic stem cells (ESCs) display pluripotency features characteristic of the inner cell mass of the blastocyst. Mouse embryonic stem cell cultures are highly heterogeneous and include a rare population of cells, which recapitulate characteristics of the 2-cell embryo, referred to as 2-cell-like cells (2CLCs). Whether and how ESC and 2CLC respond to environmental cues has not been fully elucidated. Here, we investigate the impact of mechanical stress on the reprogramming of ESC to 2CLC. We show that hyperosmotic stress induces 2CLC and that this induction can occur even after a recovery time from hyperosmotic stress, suggesting a memory response. Hyperosmotic stress in ESCs leads to accumulation of reactive-oxygen species (ROS) and ATR checkpoint activation. Importantly, preventing either elevated ROS levels or ATR activation impairs hyperosmotic-mediated 2CLC induction. We further show that ROS generation and the ATR checkpoint act within the same molecular pathway in response to hyperosmotic stress to induce 2CLCs. Altogether, these results shed light on the response of ESC to mechanical stress and on our understanding of 2CLC reprogramming.

In Vitro Embryogenesis and Gastrulation Using Stem Cells in Mice and Humans

During early mammalian embryonic development, fertilized one-cell embryos develop into pre-implantation blastocysts and subsequently establish three germ layers through gastrulation during post-implantation development. In recent years, stem cells have emerged as a powerful tool to study embryogenesis and gastrulation without the need for eggs, allowing for the generation of embryo-like structures known as synthetic embryos or embryoids. These in vitro models closely resemble early embryos in terms of morphology and gene expression and provide a faithful recapitulation of early pre- and post-implantation embryonic development. Synthetic embryos can be generated through a combinatorial culture of three blastocyst-derived stem cell types, such as embryonic stem cells, trophoblast stem cells, and extraembryonic endoderm cells, or totipotent-like stem cells alone. This review provides an overview of the progress and various approaches in studying in vitro embryogenesis and gastrulation in mice and humans using stem cells. Furthermore, recent findings and breakthroughs in synthetic embryos and gastruloids are outlined. Despite ethical considerations, synthetic embryo models hold promise for understanding mammalian (including humans) embryonic development and have potential implications for regenerative medicine and developmental research.

p16High senescence restricts cellular plasticity during somatic cell reprogramming

Despite advances in four-factor (4F)-induced reprogramming (4FR) in vitro and in vivo, how 4FR interconnects with senescence remains largely under investigated. Here, using genetic and chemical approaches to manipulate senescent cells, we show that removal of p16High cells resulted in the 4FR of somatic cells into totipotent-like stem cells. These cells expressed markers of both pluripotency and the two-cell embryonic state, readily formed implantation-competent blastoids and, following morula aggregation, contributed to embryonic and extraembryonic lineages. We identified senescence-dependent regulation of nicotinamide N-methyltransferase as a key mechanism controlling the S-adenosyl-L-methionine levels during 4FR that was required for expression of the two-cell genes and acquisition of an extraembryonic potential. Importantly, a partial 4F epigenetic reprogramming in old mice was able to reverse several markers of liver aging only in conjunction with the depletion of p16High cells. Our results show that the presence of p16High senescent cells limits cell plasticity, whereas their depletion can promote a totipotent-like state and histopathological tissue rejuvenation during 4F reprogramming.

Transcriptomic differences between human 8-cell-like cells reprogrammed with different methods

Embryonic genome activation (EGA) is a critical step in embryonic development. However, while EGA has been studied in mice using mouse 2-cell-like cells, human EGA remains incompletely elucidated due to the lack of an in vitro cell model recapitulating the early blastomere stage in humans. Recently, five groups independently reported human 8-cell-like cells (8CLCs, also called induced blastomere-like cells) developed from pluripotent stem cells and used single-cell RNA sequencing (scRNA-seq) to specify their cellular identities. Here we summarize the methods developed to produce the 8CLCs and compare their transcriptomic profiles by integrating them with the scRNA-seq datasets of human embryos. These observations will allow comparison and validation of the models, stimulate further in-depth research to characterize the genes involved in human EGA and pre-implantation development, and facilitate studies on human embryogenesis.

Regulation of mammalian totipotency: a molecular perspective from in vivo and in vitro studies

In mammals, cells acquire totipotency at fertilization. Embryonic genome activation (EGA), which occurs at the 2-cell stage in the mouse and 4- to 8-cell stage in humans, occurs during the time window at which embryonic cells are totipotent and thus it is thought that EGA is mechanistically linked to the foundations of totipotency. The molecular mechanisms that lead to the establishment of totipotency and EGA had been elusive for a long time, however, recent advances have been achieved with the establishment of new cell lines with greater developmental potential and the application of novel low-input high-throughput techniques in embryos. These have unveiled several principles of totipotency related to its epigenetic makeup but also to characteristic features of totipotent cells. In this review, we summarize and discuss current views exploring some of the key drivers of totipotency from both in vitro cell culture models and embryogenesis in vivo.

Human 8-cell embryo-like cells from pluripotent stem cells

The totipotent embryo initiates transcription during zygotic or embryonic genome activation (EGA, ZGA). ZGA occurs at the 8-cell stage in humans and its failure leads to developmental arrest. Understanding the molecular pathways underlying ZGA and totipotency is essential to comprehend human development. Recently, human 8-cell-like cells (8CLCs) have been discovered in vitro that resemble the 8-cell embryo. 8CLCs exist among naive pluripotent stem cells and can be induced genetically or chemically. Their ZGA-like transcriptome, transposable element activation, 8-cell embryo-specific protein expression, and developmental properties make them an exceptional model system to study early embryonic cell-state transitions and human totipotency programs in vitro.

A genome-wide screen reveals new regulators of the 2-cell-like cell state

In mammals, only the zygote and blastomeres of the early embryo are totipotent. This totipotency is mirrored in vitro by mouse '2-cell-like cells' (2CLCs), which appear at low frequency in cultures of embryonic stem cells (ESCs). Because totipotency is not completely understood, we carried out a genome-wide CRISPR knockout screen in mouse ESCs, searching for mutants that reactivate the expression of Dazl, a gene expressed in 2CLCs. Here we report the identification of four mutants that reactivate Dazl and a broader 2-cell-like signature: the E3 ubiquitin ligase adaptor SPOP, the Zinc-Finger transcription factor ZBTB14, MCM3AP, a component of the RNA processing complex TREX-2, and the lysine demethylase KDM5C. All four factors function upstream of DPPA2 and DUX, but not via p53. In addition, SPOP binds DPPA2, and KDM5C interacts with ncPRC1.6 and inhibits 2CLC gene expression in a catalytic-independent manner. These results extend our knowledge of totipotency, a key phase of organismal life.

RBBP4 is an epigenetic barrier for the induced transition of pluripotent stem cells into totipotent 2C-like cells

Cellular totipotency is critical for whole-organism generation, yet how totipotency is established remains poorly illustrated. Abundant transposable elements (TEs) are activated in totipotent cells, which is critical for embryonic totipotency. Here, we show that the histone chaperone RBBP4, but not its homolog RBBP7, is indispensable for maintaining the identity of mouse embryonic stem cells (mESCs). Auxin-induced degradation of RBBP4, but not RBBP7, reprograms mESCs to the totipotent 2C-like cells. Also, loss of RBBP4 enhances transition from mESCs to trophoblast cells. Mechanistically, RBBP4 binds to the endogenous retroviruses (ERVs) and functions as an upstream regulator by recruiting G9a to deposit H3K9me2 on ERVL elements, and recruiting KAP1 to deposit H3K9me3 on ERV1/ERVK elements, respectively. Moreover, RBBP4 facilitates the maintenance of nucleosome occupancy at the ERVK and ERVL sites within heterochromatin regions through the chromatin remodeler CHD4. RBBP4 depletion leads to the loss of the heterochromatin marks and activation of TEs and 2C genes. Together, our findings illustrate that RBBP4 is required for heterochromatin assembly and is a critical barrier for inducing cell fate transition from pluripotency to totipotency.

Multiple repeat regions within mouse DUX recruit chromatin regulators to facilitate an embryonic gene expression program

The embryonic transcription factor DUX regulates chromatin opening and gene expression in totipotent cleavage-stage mouse embryos, and its expression in embryonic stem cells promotes their conversion to 2-cell embryo-like cells (2CLCs) with extraembryonic potential. However, little is known regarding which domains within mouse DUX interact with particular chromatin and transcription regulators. Here, we reveal that the C-terminus of mouse DUX contains five uncharacterized ~100 amino acid (aa) repeats followed by an acidic 14 amino acid tail. Unexpectedly, structure-function approaches classify two repeats as 'active' and three as 'inactive' in cleavage/2CLC transcription program enhancement, with differences narrowed to a key 6 amino acid section. Our proximity dependent biotin ligation (BioID) approach identified factors selectively associated with active DUX repeat derivatives (including the 14aa 'tail'), including transcription and chromatin factors such as SWI/SNF (BAF) complex, as well as nucleolar factors that have been previously implicated in regulating the Dux locus. Finally, our mechanistic studies reveal cooperativity between DUX active repeats and the acidic tail in cofactor recruitment, DUX target opening, and transcription. Taken together, we provide several new insights into DUX structure-function, and mechanisms of chromatin and gene regulation.

Cross-activation of FGF, NODAL, and WNT pathways constrains BMP-signaling-mediated induction of the totipotent state in mouse embryonic stem cells

Embryonic stem cells (ESCs) are an attractive model to study the relationship between signaling and cell fates. Cultured mouse ESCs can exist in multiple states resembling distinct stages of early embryogenesis, such as totipotent, pluripotent, primed, and primitive endoderm. The signaling mechanisms regulating the totipotent state and coexistence of these states are poorly understood. Here we identify bone morphogenetic protein (BMP) signaling as an inducer of the totipotent state. However, we discover that BMP's role is constrained by the cross-activation of FGF, NODAL, and WNT pathways. We exploit this finding to enhance the proportion of totipotent cells by rationally inhibiting the cross-activated pathways. Single-cell mRNA sequencing reveals that induction of the totipotent state is accompanied by suppression of primed and primitive endoderm states. Furthermore, reprogrammed totipotent cells we generate in culture resemble totipotent cells of preimplantation embryo. Our findings reveal a BMP signaling mechanism regulating both the totipotent state and heterogeneity of ESCs.

Transcription of MERVL retrotransposons is required for preimplantation embryo development

Zygotic genome activation (ZGA) is a critical postfertilization step that promotes totipotency and allows different cell fates to emerge in the developing embryo. MERVL (murine endogenous retrovirus-L) is transiently upregulated at the two-cell stage during ZGA. Although MERVL expression is widely used as a marker of totipotency, the role of this retrotransposon in mouse embryogenesis remains elusive. Here, we show that full-length MERVL transcripts, but not encoded retroviral proteins, are essential for accurate regulation of the host transcriptome and chromatin state during preimplantation development. Both knockdown and CRISPRi-based repression of MERVL result in embryonic lethality due to defects in differentiation and genomic stability. Furthermore, transcriptome and epigenome analysis revealed that loss of MERVL transcripts led to retention of an accessible chromatin state at, and aberrant expression of, a subset of two-cell-specific genes. Taken together, our results suggest a model in which an endogenous retrovirus plays a key role in regulating host cell fate potential.

Systematic identification of factors involved in the silencing of germline genes in mouse embryonic stem cells

In mammals, many germline genes are epigenetically repressed to prevent their illegitimate expression in somatic cells. To advance our understanding of the mechanisms restricting the expression of germline genes, we analyzed their chromatin signature and performed a CRISPR-Cas9 knock-out screen for genes involved in germline gene repression using a Dazl-GFP reporter system in mouse embryonic stem cells (mESCs). We show that the repression of germline genes mainly depends on the polycomb complex PRC1.6 and DNA methylation, which function additively in mESCs. Furthermore, we validated novel genes involved in the repression of germline genes and characterized three of them: Usp7, Shfm1 (also known as Sem1) and Erh. Inactivation of Usp7, Shfm1 or Erh led to the upregulation of germline genes, as well as retrotransposons for Shfm1, in mESCs. Mechanistically, USP7 interacts with PRC1.6 components, promotes PRC1.6 stability and presence at germline genes, and facilitates DNA methylation deposition at germline gene promoters for long term repression. Our study provides a global view of the mechanisms and novel factors required for silencing germline genes in embryonic stem cells.

The regulation of transcription elongation in embryonic stem cells

Transcription elongation is a fundamental molecular process which is accurately regulated to ensure proper gene expression in cellular activities whereas its malfunction is associated with impaired cellular functions. Embryonic stem cells (ESCs) have significant value in regenerative medicine due to their self-renewal ability and their potential to differentiate to almost all types of cells. Therefore, dissection of the exact regulatory mechanism of transcription elongation in ESCs is crucial for both basic research and their clinical applications. In this review, we discuss the current understanding on the regulatory mechanisms of transcription elongation mediated by transcription factors and epigenetic modifications in ESCs.

Rif1 interacts with non-canonical polycomb repressive complex PRC1.6 to regulate mouse embryonic stem cells fate potential

Mouse embryonic stem cells (mESCs) cycle in and out of a transient 2-cell (2C)-like totipotent state, driven by a complex genetic circuit involves both the coding and repetitive sections of the genome. While a vast array of regulators, including the multi-functional protein Rif1, has been reported to influence the switch of fate potential, how they act in concert to achieve this cellular plasticity remains elusive. Here, by modularizing the known totipotency regulatory factors, we identify an unprecedented functional connection between Rif1 and the non-canonical polycomb repressive complex PRC1.6. Downregulation of the expression of either Rif1 or PRC1.6 subunits imposes similar impacts on the transcriptome of mESCs. The LacO-LacI induced ectopic colocalization assay detects a specific interaction between Rif1 and Pcgf6, bolstering the intactness of the PRC1.6 complex. Chromatin immunoprecipitation followed by sequencing (ChIP-seq) analysis further reveals that Rif1 is required for the accurate targeting of Pcgf6 to a group of genomic loci encompassing many genes involved in the regulation of the 2C-like state. Depletion of Rif1 or Pcgf6 not only activates 2C genes such as Zscan4 and Zfp352, but also derepresses a group of the endogenous retroviral element MERVL, a key marker for totipotency. Collectively, our findings discover that Rif1 can serve as a novel auxiliary component in the PRC1.6 complex to restrain the genetic circuit underlying totipotent fate potential, shedding new mechanistic insights into its function in regulating the cellular plasticity of embryonic stem cells.

Exploration of nuclear body-enhanced sumoylation reveals that PML represses 2-cell features of embryonic stem cells

Membrane-less organelles are condensates formed by phase separation whose functions often remain enigmatic. Upon oxidative stress, PML scaffolds Nuclear Bodies (NBs) to regulate senescence or metabolic adaptation. PML NBs recruit many partner proteins, but the actual biochemical mechanism underlying their pleiotropic functions remains elusive. Similarly, PML role in embryonic stem cell (ESC) and retro-element biology is unsettled. Here we demonstrate that PML is essential for oxidative stress-driven partner SUMO2/3 conjugation in mouse ESCs (mESCs) or leukemia, a process often followed by their poly-ubiquitination and degradation. Functionally, PML is required for stress responses in mESCs. Differential proteomics unravel the KAP1 complex as a PML NB-dependent SUMO2-target in arsenic-treated APL mice or mESCs. PML-driven KAP1 sumoylation enables activation of this key epigenetic repressor implicated in retro-element silencing. Accordingly, Pml^-/- mESCs re-express transposable elements and display 2-Cell-Like features, the latter enforced by PML-controlled SUMO2-conjugation of DPPA2. Thus, PML orchestrates mESC state by coordinating SUMO2-conjugation of different transcriptional regulators, raising new hypotheses about PML roles in cancer.

The Divergent Pluripotent States in Mouse and Human Cells

Pluripotent stem cells (PSCs), which can self-renew and give rise to all cell types in all three germ layers, have great potential in regenerative medicine. Recent studies have shown that PSCs can have three distinct but interrelated pluripotent states: naive, formative, and primed. The PSCs of each state are derived from different stages of the early developing embryo and can be maintained in culture by different molecular mechanisms. In this review, we summarize the current understanding on features of the three pluripotent states and review the underlying molecular mechanisms of maintaining their identities. Lastly, we discuss the interrelation and transition among these pluripotency states. We believe that comprehending the divergence of pluripotent states is essential to fully harness the great potential of stem cells in regenerative medicine.

Epigenetics as "conductor" in "orchestra" of pluripotent states

Pluripotent character is described as the potency of cells to differentiate into all three germ layers. The best example to reinstate the term lies in the context of embryonic stem cells (ESCs). Pluripotent ESC describes the in vitro status of those cells that originate during the complex process of embryogenesis. Pre-implantation to post-implantation development of embryo embrace cells with different levels of stemness. Currently, four states of pluripotency have been recognized, in the progressing order of "naïve," "poised," "formative," and "primed." Epigenetics act as the "conductor" in this "orchestra" of transition in pluripotent states. With a distinguishable gene expression profile, these four states associate with different epigenetic signatures, sometimes distinct while otherwise overlapping. The present review focuses on how epigenetic factors, including DNA methylation, bivalent chromatin, chromatin remodelers, chromatin/nuclear architecture, and microRNA, could dictate pluripotent states and their transition among themselves.

Transient DUX4 expression in human embryonic stem cells induces blastomere-like expression program that is marked by SLC34A2

Embryonic genome activation (EGA) is critical for embryonic development. However, our understanding of the regulatory mechanisms of human EGA is still incomplete. Human embryonic stem cells (hESCs) are an established model for studying developmental processes, but they resemble epiblast and are sub-optimal for modeling EGA. DUX4 regulates human EGA by inducing cleavage-stage-specific genes, while it also induces cell death. We report here that a short-pulsed expression of DUX4 in primed hESCs activates an EGA-like gene expression program in up to 17% of the cells, retaining cell viability. These DUX4-induced cells resembled eight-cell stage blastomeres and were named induced blastomere-like (iBM) cells. The iBM cells showed marked reduction of POU5F1 protein, as previously observed in mouse two-cell-like cells. Finally, the iBM cells were successfully enriched using an antibody against NaPi2b (SLC34A2), which is expressed in human blastomeres. The iBM cells provide an improved model system to study human EGA transcriptome.

A critical review on induced totipotent stem cells: Types and methods

Totipotent stem cells are cells with the capacity to form an entire embryo. Many attempts have been made to convert other types of cells to totipotent stem cells which we called induced totipotent stem cells. Various aspects of these cells such as transcriptional and epigenetics networks are unique. By taking advantage of these aspects, efficient methods have been provided to induce totipotent stem cells. Although this advancement is significant, many aspects of induction such as the underlying mechanism remain to be elucidated. On the other hand, embryonic stem cells usually are the source of induction which raise important questions regarding if these methods are induction or promotion of 2C intrinsic totipotent cells in ESC culture. Here, we review the latest mouse progress in underling mechanism of induction of totipotent stem cells. In addition, we follow up on the progress of Blastoids derived from totipotent stem cells.

Epigenetic Reprogramming in Early Animal Development

Dramatic nuclear reorganization occurs during early development to convert terminally differentiated gametes to a totipotent zygote, which then gives rise to an embryo. Aberrant epigenome resetting severely impairs embryo development and even leads to lethality. How the epigenomes are inherited, reprogrammed, and reestablished in this critical developmental period has gradually been unveiled through the rapid development of technologies including ultrasensitive chromatin analysis methods. In this review, we summarize the latest findings on epigenetic reprogramming in gametogenesis and embryogenesis, and how it contributes to gamete maturation and parental-to-zygotic transition. Finally, we highlight the key questions that remain to be answered to fully understand chromatin regulation and nuclear reprogramming in early development.

Maternal Factor Dppa3 Activates 2C-Like Genes and Depresses DNA Methylation in Mouse Embryonic Stem Cells

Mouse embryonic stem cells (ESCs) contain a rare cell population of “two-cell embryonic like” cells (2CLCs) that display similar features to those found in the two-cell (2C) embryo and thus represent an in vitro model for studying the progress of zygotic genome activation (ZGA). However, the positive regulator determinants of the 2CLCs’ conversion and ZGA have not been completely elucidated. Here, we identify a new regulator promoting 2CLCs and ZGA transcripts. Through a combination of overexpression (OE), knockdown (KD), together with transcriptional analysis and methylome analysis, we find that Dppa3 regulates the 2CLC-associated transcripts, DNA methylation, and 2CLC population in ESCs. The differentially methylated regions (DMRs) analysis identified 6,920 (98.2%) hypomethylated, whilst only 129 (1.8%) hypermethylated, regions in Dppa3 OE ESCs, suggesting that Dppa3 facilitates 2CLCs reprogramming. The conversion to 2CLCs by overexpression of Dppa3 is also associated with DNA damage response. Dppa3 knockdown manifest impairs transition into the 2C-like state. Global DNA methylome and chromatin state analysis of Dppa3 OE ESCs reveal that Dppa3 facilitates the chromatin configuration to 2CLCs reversion. Our finding for the first time elucidates a novel role of Dppa3 in mediating the 2CLC conversion, and suggests that Dppa3 is a new regulator for ZGA progress.

Genome architecture and totipotency: An intertwined relation during early embryonic development

Chromosomes are not randomly packed and positioned into the nucleus but folded in higher-order chromatin structures with defined functions. However, the genome of a fertilized embryo undergoes a dramatic epigenetic reprogramming characterized by extensive chromatin relaxation and the lack of a defined three-dimensional structure. This reprogramming is followed by a slow genome refolding that gradually strengthens the chromatin architecture during preimplantation development. Interestingly, genome refolding during early development coincides with a progressive loss of developmental potential suggesting a link between chromatin organization and cell plasticity. In agreement, loss of chromatin architecture upon depletion of the insulator transcription factor CTCF in embryonic stem cells led to the upregulation of the transcriptional program found in totipotent cells of the embryo, those with the highest developmental potential. This essay will discuss the impact of genome folding in controlling the expression of transcriptional programs involved in early development and their plastic-associated features.

Research Progress of Totipotent Stem Cells

Totipotent stem cells (TSCs), can develop into complete organisms, used in biological fields such as regenerative medicine, mammalian breeding, and conservation. However, cells from early-stage embryos cultured are hard to self-renew and maintain developmental totipotency, which becomes a key factor limiting the research of TSCs. Fortunately, a break-through in the study of induced pluripotent stem cells returning to their totipotent state has been made, resulting in the establishment of multiple TSCs and igniting a new wave of stem cell research. Furthermore, the blastocyst-like structures can be generated by the established TSCs, which lays a foundation for synthetic embryos in vitro. In this review, we summarize the totipotent stage of the early embryos, the establishment and cultivation of TSCs, and the developmental ability exploration of TSCs to promote further research of TSCs.

DNA replication fork speed underlies cell fate changes and promotes reprogramming

Totipotency emerges in early embryogenesis, but its molecular underpinnings remain poorly characterized. In the present study, we employed DNA fiber analysis to investigate how pluripotent stem cells are reprogrammed into totipotent-like 2-cell-like cells (2CLCs). We show that totipotent cells of the early mouse embryo have slow DNA replication fork speed and that 2CLCs recapitulate this feature, suggesting that fork speed underlies the transition to a totipotent-like state. 2CLCs emerge concomitant with DNA replication and display changes in replication timing (RT), particularly during the early S-phase. RT changes occur prior to 2CLC emergence, suggesting that RT may predispose to gene expression changes and consequent reprogramming of cell fate. Slowing down replication fork speed experimentally induces 2CLCs. In vivo, slowing fork speed improves the reprogramming efficiency of somatic cell nuclear transfer. Our data suggest that fork speed regulates cellular plasticity and that remodeling of replication features leads to changes in cell fate and reprogramming.

Totipotent cells in mouse embryos and 2-cell-like cells have slow DNA replication fork speed. Perturbations that slow replication fork speed promote 2-cell-like cell emergence and improve somatic cell nuclear transfer reprogramming and formation of induced pluripotent stem cell colonies.

Maternal Dppa2 and Dppa4 are dispensable for zygotic genome activation but important for offspring survival

Zygotic genome activation (ZGA) represents the initiation of transcription following fertilisation. Despite its importance, we know little of the molecular events that initiate mammalian ZGA in vivo. Recent in vitro studies in mouse embryonic stem cells have revealed developmental pluripotency associated 2 and 4 (Dppa2/4) as key regulators of ZGA-associated transcription. However, their roles in initiating ZGA in vivo remain unexplored. We reveal that Dppa2/4 proteins are present in the nucleus at all stages of preimplantation development and associate with mitotic chromatin. We generated conditional single and double maternal knockout mouse models to deplete maternal stores of Dppa2/4. Importantly, Dppa2/4 maternal knockout mice were fertile when mated with wild-type males. Immunofluorescence and transcriptome analyses of two-cell embryos revealed that, although ZGA took place, there were subtle defects in embryos that lacked maternal Dppa2/4. Strikingly, heterozygous offspring that inherited the null allele maternally had higher preweaning lethality than those that inherited the null allele paternally. Together, our results show that although Dppa2/4 are dispensable for ZGA transcription, maternal stores have an important role in offspring survival, potentially via epigenetic priming of developmental genes.

Summary: Conditional knockout mouse models reveal maternal stores of Dppa2 and Dppa4 are dispensable for zygotic genome activation, contrasting findings in embryonic stem cells. However, both maternal and zygotic Dppa2/4 are required for development.

DPPA2 and DPPA4 are dispensable for mouse zygotic genome activation and pre-implantation development

How maternal factors in oocytes initiate zygotic genome activation (ZGA) remains elusive in mammals, partly due to the challenge of de novo identification of key factors using scarce materials. Two-cell (2C)-like cells have been widely used as an in vitro model in order to understand mouse ZGA and totipotency because of their expression of a group of two-cell embryo-specific genes and their simplicity for genetic manipulation. Recent studies indicate that DPPA2 and DPPA4 are required for establishing the 2C-like state in mouse embryonic stem cells in a DUX-dependent manner. These results suggest that DPPA2 and DPPA4 are essential maternal factors that regulate Dux and ZGA in embryos. By analyzing maternal knockout and maternal-zygotic knockout embryos, we unexpectedly found that DPPA2 and DPPA4 are dispensable for Dux activation, ZGA and pre-implantation development. Our study suggests that 2C-like cells do not fully recapitulate two-cell embryos in terms of regulation of two-cell embryo-specific genes, and, therefore, caution should be taken when studying ZGA and totipotency using 2C-like cells as the model system.

Building the genome architecture during the maternal to zygotic transition

Proper higher-order chromatin folding can profoundly impact gene expression. In early animal development, chromatin undergoes dramatic reorganization to convert terminally differentiated gametes to early embryos. The recent rapid development of ultra-sensitive chromatin analysis technologies has revealed a drastic reconstruction of chromatin architecture, which includes a surprising relaxation followed by de novo and slow establishment of 3D genome organization in early embryos. Such progress adds another fascinating dimension to epigenetic reprogramming in early development that also features degradation of maternal RNA and activation of the zygotic genome during the maternal to zygotic transition (MZT). Nevertheless, the role of higher-order chromatin architecture in this critical developmental time window is yet to be understood. In this article, we review the latest findings from a variety of species (with a primary focus on mammals) on the establishment of 3D chromatin structure in gametogenesis and early development. These data shed light on how chromatin organization is regulated, and how it coordinates with MZT and other crucial events in early development. Finally, we discuss the crucial questions that remain to be answered in the future.

Foxd3 controls heterochromatin-mediated repression of repeat elements and 2-cell state transcription

Repeat element transcription plays a vital role in early embryonic development. The expression of repeats such as MERVL characterises mouse embryos at the 2‐cell stage and defines a 2‐cell‐like cell (2CLC) population in a mouse embryonic stem cell culture. Repeat element sequences contain binding sites for numerous transcription factors. We identify the forkhead domain transcription factor FOXD3 as a regulator of major satellite repeats and MERVL transcription in mouse embryonic stem cells. FOXD3 binds to and recruits the histone methyltransferase SUV39H1 to MERVL and major satellite repeats, consequentially repressing the transcription of these repeats by the establishment of the H3K9me3 heterochromatin modification. Notably, depletion of FOXD3 leads to the de‐repression of MERVL and major satellite repeats as well as a subset of genes expressed in the 2‐cell state, shifting the balance between the stem cell and 2‐cell‐like population in culture. Thus, FOXD3 acts as a negative regulator of repeat transcription, ascribing a novel function to this transcription factor.

Comparative Analyses of Single-Cell Transcriptomic Profiles between In Vitro Totipotent Blastomere-like Cells and In Vivo Early Mouse Embryonic Cells

The developmental potential within pluripotent cells in the canonical model is restricted to embryonic tissues, whereas totipotent cells can differentiate into both embryonic and extraembryonic tissues. Currently, the ability to culture in vitro totipotent cells possessing molecular and functional features like those of an early embryo in vivo has been a challenge. Recently, it was reported that treatment with a single spliceosome inhibitor, pladienolide B (plaB), can successfully reprogram mouse pluripotent stem cells into totipotent blastomere-like cells (TBLCs) in vitro. The TBLCs exhibited totipotency transcriptionally and acquired expanded developmental potential with the ability to yield various embryonic and extraembryonic tissues that may be employed as novel mouse developmental cell models. However, it is disputed whether TBLCs are ‘true’ totipotent stem cells equivalent to in vivo two-cell stage embryos. To address this question, single-cell RNA sequencing was applied to TBLCs and cells from early mouse embryonic developmental stages and the data were integrated using canonical correlation analyses. Differential expression analyses were performed between TBLCs and multi-embryonic cell stages to identify differentially expressed genes. Remarkably, a subpopulation within the TBLCs population expressed a high level of the totipotent-related genes Zscan4s and displayed transcriptomic features similar to mouse two-cell stage embryonic cells. This study underscores the subtle differences between in vitro derived TBLCs and in vivo mouse early developmental cell stages at the single-cell transcriptomic level. Our study has identified a new experimental model for stem cell biology, namely ‘cluster 3’, as a subpopulation of TBLCs that can be molecularly defined as near totipotent cells.

Transposable Element Dynamics and Regulation during Zygotic Genome Activation in Mammalian Embryos and Embryonic Stem Cell Model Systems

Transposable elements (TEs) are mobile genetic sequences capable of duplicating and reintegrating at new regions within the genome. A growing body of evidence has demonstrated that these elements play important roles in host genome evolution, despite being traditionally viewed as parasitic elements. To prevent ectopic activation of TE transposition and transcription, they are epigenetically silenced in most somatic tissues. Intriguingly, a specific class of TEs-retrotransposons-is transiently expressed at discrete phases during mammalian development and has been linked to the establishment of totipotency during zygotic genome activation (ZGA). While mechanisms controlling TE regulation in somatic tissues have been extensively studied, the significance underlying the unique transcriptional reactivation of retrotransposons during ZGA is only beginning to be uncovered. In this review, we summarize the expression dynamics of key retrotransposons during ZGA, focusing on findings from in vivo totipotent embryos and in vitro totipotent-like embryonic stem cells (ESCs). We then dissect the functions of retrotransposons and discuss how their transcriptional activities are finetuned during early stages of mammalian development.

Somatic Reprogramming-Above and Beyond Pluripotency

Pluripotent stem cells, having long been considered the fountain of youth, have caught the attention of many researchers from diverse backgrounds due to their capacity for unlimited self-renewal and potential to differentiate into all cell types. Over the past 15 years, the advanced development of induced pluripotent stem cells (iPSCs) has displayed an unparalleled potential for regenerative medicine, cell-based therapies, modeling human diseases in culture, and drug discovery. The transcription factor quartet (Oct4, Sox2, Klf4, and c-Myc) reprograms highly differentiated somatic cells back to a pluripotent state recapitulated embryonic stem cells (ESCs) in different aspects, including gene expression profile, epigenetic signature, and functional pluripotency. With the prior fruitful studies in SCNT and cell fusion experiments, iPSC finds its place and implicates that the differentiated somatic epigenome retains plasticity for re-gaining the pluripotency and further stretchability to reach a totipotency-like state. These achievements have revolutionized the concept and created a new avenue in biomedical sciences for clinical applications. With the advent of 15 years’ progress-making after iPSC discovery, this review is focused on how the current concept is established by revisiting those essential landmark studies and summarizing its current biomedical applications status to facilitate the new era entry of regenerative therapy.

CTCF is a barrier for 2C-like reprogramming

Totipotent cells have the ability to generate embryonic and extra-embryonic tissues. Interestingly, a rare population of cells with totipotent-like potential, known as 2 cell (2C)-like cells, has been identified within ESC cultures. They arise from ESC and display similar features to those found in the 2C embryo. However, the molecular determinants of 2C-like conversion have not been completely elucidated. Here, we show that the CCCTC-binding factor (CTCF) is a barrier for 2C-like reprogramming. Indeed, forced conversion to a 2C-like state by the transcription factor DUX is associated with DNA damage at a subset of CTCF binding sites. Depletion of CTCF in ESC efficiently promotes spontaneous and asynchronous conversion to a 2C-like state and is reversible upon restoration of CTCF levels. This phenotypic reprogramming is specific to pluripotent cells as neural progenitor cells do not show 2C-like conversion upon CTCF-depletion. Furthermore, we show that transcriptional activation of the ZSCAN4 cluster is necessary for successful 2C-like reprogramming. In summary, we reveal an unexpected relationship between CTCF and 2C-like reprogramming.

Keeping your options open: insights from Dppa2/4 into how epigenetic priming factors promote cell plasticity

The concept of cellular plasticity is particularly apt in early embryonic development, where there is a tug-of-war between the stability and flexibility of cell identity. This balance is controlled in part through epigenetic mechanisms. Epigenetic plasticity dictates how malleable cells are to change by adjusting the potential to initiate new transcriptional programmes. The higher the plasticity of a cell, the more readily it can adapt and change its identity in response to external stimuli such as differentiation cues. Epigenetic plasticity is regulated in part through the action of epigenetic priming factors which establish this permissive epigenetic landscape at genomic regulatory elements to enable future transcriptional changes. Recent studies on the DNA binding proteins Developmental Pluripotency Associated 2 and 4 (Dppa2/4) support their roles as epigenetic priming factors in facilitating cell fate transitions. Here, using Dppa2/4 as a case study, the concept of epigenetic plasticity and molecular mechanism of epigenetic priming factors will be explored. Understanding how epigenetic priming factors function is key not only to improve our understanding of the tight control of development, but also to give insights into how this goes awry in diseases of cell identity, such as cancer.

There is another: H3K27me3-mediated genomic imprinting

DNA methylation has long been considered the primary epigenetic mediator of genomic imprinting in mammals. Recent epigenetic profiling during early mouse development revealed the presence of domains of trimethylation of lysine 27 on histone H3 (H3K27me3) and chromatin compaction specifically at the maternally derived allele, independent of DNA methylation. Within these domains, genes are exclusively expressed from the paternally derived allele. This novel mechanism of noncanonical imprinting plays a key role in the development of mouse extraembryonic tissues and in the regulation of imprinted X-chromosome inactivation, highlighting the importance of parentally inherited epigenetic histone modifications. Here, we discuss the mechanisms underlying H3K27me3-mediated noncanonical imprinting in perspective of the dynamic chromatin landscape during early mouse development and explore evolutionary origins of noncanonical imprinting.

Autonomy in the Development of Stem Cell-Derived Embryoids: Sprouting Blastocyst-Like Cysts, and Ethical Implications

The experimental production of complex structures resembling mammalian embryos (e.g., blastoids, gastruloids) from pluripotent stem cells in vitro has become a booming research field. Since some of these embryoid models appear to reach a degree of complexity that may come close to viability, a broad discussion has set in with the aim to arrive at a consensus on the ethical implications with regard to acceptability of the use of this technology with human cells. The present text focuses on aspects of the gain of organismic wholeness of such stem cell-derived constructs, and of autonomy of self-organization, raised by recent reports on blastocyst-like cysts spontaneously budding in mouse stem cell cultures, and by previous reports on likewise spontaneous formation of gastrulating embryonic disc-like structures in primate models. Mechanisms of pattern (axis) formation in early embryogenesis are discussed in the context of self-organization of stem cell clusters. It is concluded that ethical aspects of development of organismic wholeness in the formation of embryoids need to receive more attention in the present discussions about new legal regulations in this field.