Developmental plasticity is bound by pluripotency and the Fgf and Wnt signaling pathways

Cellular mechanisms of monozygotic twinning: clues from assisted reproduction

BACKGROUND

Monozygotic (MZ) twins are believed to arise from the fission of a single fertilized embryo at different stages. Monochorionic MZ twins, who share one chorion, originate from the splitting of the inner cell mass (ICM) within a single blastocyst. In the classic model for dichorionic MZ twins, the embryo splits before compaction, developing into two blastocysts. However, there are a growing number of ART cases where a single blastocyst transfer results in dichorionic MZ twins, indicating that embryo splitting may occur even after blastocyst formation.

OBJECTIVE AND RATIONALE

For monochorionic MZ twins, we conducted a comprehensive analysis of the cellular mechanisms involved in ICM splitting, drawing from both ART cases and animal experiments. In addition, we critically re-examine the classic early splitting model for dichorionic MZ twins. We explore cellular mechanisms leading to two separated blastocysts in ART, potentially causing dichorionic MZ twins.

SEARCH METHODS

Relevant studies including research articles, reviews, and conference papers were searched in the PubMed database. Cases of MZ twins from IVF clinics were found by using combinations of terms including 'monozygotic twins' with 'IVF case report', 'ART', 'single embryo transfer', or 'dichorionic'. The papers retrieved were categorized based on the implicated mechanisms or as those with unexplained mechanisms. Animal experiments relating to MZ twins were found using 'mouse embryo monozygotic twins', 'mouse 8-shaped hatching', 'zebrafish janus mutant', and 'nine-banded armadillo embryo', along with literature collected through day-to-day reading. The search was limited to articles in English, with no restrictions on publication date or species.

OUTCOMES

For monochorionic MZ twins, ART cases and mouse experiments demonstrate evidence that a looser ICM in blastocysts has an increased chance of ICM separation. Physical forces facilitated by blastocoel formation or 8-shaped hatching are exerted on the ICM, resulting in monochorionic MZ twins. For dichorionic MZ twins, the classic model resembles artificial cloning of mouse embryos in vitro, requiring strictly controlled splitting forces, re-joining prevention, and proper aggregation, which allows the formation of two separate human blastocysts under physiological circumstances. In contrast, ART procedures involving the transfer of a single blastocysts after atypical hatching or vitrified-warmed cycles might lead to blastocyst separation. Differences in morphology, molecular mechanisms, and timing across various animal model systems for MZ twinning can impede this research field. As discussed in future directions, recent developments of innovative in vitro models of human embryos may offer promising avenues for providing fundamental novel insights into the cellular mechanisms of MZ twinning during human embryogenesis.

WIDER IMPLICATIONS

Twin pregnancies pose high risks to both the fetuses and the mother. While single embryo transfer is commonly employed to prevent dizygotic twin pregnancies in ART, it cannot prevent the occurrence of MZ twins. Drawing from our understanding of the cellular mechanisms underlying monochorionic and dichorionic MZ twinning, along with insights into the genetic mechanisms, could enable improved prediction, prevention, and even intervention strategies during ART procedures.

REGISTRAITON NUMBER

N/A.

NCOA3 knockdown delays human embryo development

Embryonic development is a precisely controlled sequential process influenced by complex external and internal factors; therefore, this process holds paramount significance in the context of in vitro fertilization and embryo transfer (IVF-ET), with internal oocyte and embryo quality being pivotal in determining its success. Nuclear receptor coactivator 3 (NCOA3), a member of the p160 nuclear receptor coactivators family, has been extensively studied in tumorigenesis and reportedly plays a crucial role in maintaining pluripotency in mouse embryonic stem cells (ESCs). However, its functions in human embryo development remain largely unexplored. In this study, we collected human samples, including oocytes, zygotes, and embryos, from patients at the First Affiliated Hospital of Zhengzhou University to investigate whether NCOA3 regulates human embryonic development. To this end, we employed various assays, including immunofluorescence, quantitative real-time PCR (qPCR), microinjection, and RNA sequencing. Our findings suggested that NCOA3 expression level was low in inferior embryos (with >50 % fragmentation), and its presence is closely related to the expression of the pluripotency factor NANOG. Deletion of NCOA3 delays human embryonic development. Single-oocyte RNA sequencing revealed that NCOA3 primarily participates in metabolic alterations in oocytes. In sum, these findings elucidate the pivotal roles of NCOA3 in human embryonic development-NCOA3 deletion compromise the developmental potential of embryos. These mechanistic insights into the role of NCOA3 in human embryonic development not only advances our understanding of the intricate molecular mechanisms involved but also holds potential implications for improving assisted reproductive technologies (ART) and addressing developmental disorders in human embryos.

Overlapping peri-implantation phenotypes of ZNHIT1 and ZNHIT2 despite distinct functions during early mouse development

Mammalian preimplantation development culminates in the formation of a blastocyst which undergoes extensive gene expression regulation to successfully implant into the maternal endometrium. Zinc-finger HIT domain-containing (ZNHIT) 1 and 2 are members of a highly conserved family, yet they have been identified as subunits of distinct complexes. Here we report that knockout of either Znhit1 or Znhit2 results in embryonic lethality during peri-implantation stages. Znhit1 and Znhit2 mutant embryos have overlapping phenotypes, including reduced proportion of SOX2-positive ICM cells, a lack of Fgf4 expression and aberrant expression of NANOG and SOX17. Furthermore, we find that the similar phenotypes are caused by distinct mechanisms. Specifically, embryos lacking ZNHIT1 likely fail to incorporate sufficient H2A.Z at the promoter region of Fgf4 and other genes involved in cell projection organization resulting in impaired invasion of trophoblast cells during implantation. In contrast, Znhit2 mutant embryos display a complete lack of nuclear EFTUD2, a key component of U5 spliceosome, indicating a global splicing deficiency. Our findings unveil the indispensable yet distinct roles of ZNHIT1 and ZNHIT2 in early mammalian embryonic development.

Proteome asymmetry in mouse and human embryos before fate specification

Pre-patterning of the embryo, driven by spatially localized factors, is a common feature across several non-mammalian species 1-4 . However, mammals display regulative development and thus it was thought that blastomeres of the embryo do not show such pre-patterning, contributing randomly to the three lineages of the blastocyst: the epiblast, primitive endoderm and trophectoderm that will generate the new organism, the yolk sac and placenta respectively 4-6 . Unexpectedly, early blastomeres of mouse and human embryos have been reported to have distinct developmental fates, potential and heterogeneous abundance of certain transcripts 7-12 . Nevertheless, the extent of the earliest intra-embryo differences remains unclear and controversial. Here, by utilizing multiplexed and label-free single-cell proteomics by mass-spectrometry 13 , we show that 2-cell mouse and human embryos contain an alpha and a beta blastomere as defined by differential abundance of hundreds of proteins exhibiting strong functional enrichment for protein synthesis, transport, and degradation. Such asymmetrically distributed proteins include Gps1 and Nedd8, depletion or overexpression of which in one blastomere of the 2-cell embryo impacts lineage segregation. These protein asymmetries increase at 4-cell stage. Intriguingly, halved mouse zygotes display asymmetric protein abundance that resembles alpha and beta blastomeres, suggesting differential proteome localization already within zygotes. We find that beta blastomeres give rise to a blastocyst with a higher proportion of epiblast cells than alpha blastomeres and that vegetal blastomeres, which are known to have a reduced developmental potential, are more likely to be alpha. Human 2-cell blastomeres also partition into two clusters sharing strong concordance with clusters found in mouse, in terms of differentially abundant proteins and functional enrichment. To our knowledge, this is the first demonstration of intra-zygotic and inter-blastomere proteomic asymmetry in mammals that has a role in lineage segregation.

The primitive endoderm supports lineage plasticity to enable regulative development

Mammalian blastocyst formation involves the specification of the trophectoderm followed by the differentiation of the inner cell mass into embryonic epiblast and extra-embryonic primitive endoderm (PrE). During this time, the embryo maintains a window of plasticity and can redirect its cellular fate when challenged experimentally. In this context, we found that the PrE alone was sufficient to regenerate a complete blastocyst and continue post-implantation development. We identify an in vitro population similar to the early PrE in vivo that exhibits the same embryonic and extra-embryonic potency and can form complete stem cell-based embryo models, termed blastoids. Commitment in the PrE is suppressed by JAK/STAT signaling, collaborating with OCT4 and the sustained expression of a subset of pluripotency-related transcription factors that safeguard an enhancer landscape permissive for multi-lineage differentiation. Our observations support the notion that transcription factor persistence underlies plasticity in regulative development and highlight the importance of the PrE in perturbed development.

In vitro developmental competence of bovine demi-embryos generated by blastomere separation and blastocyst bisection

The efficiency of bovine in vitro embryo production can be significantly improved by splitting embryos at different stages. However, the blastocyst quality of in vitro-produced demi-embryos remains unexplored. The objective of this research was to compare embryo developmental rates and quality of bovine demi-embryos produced by two different strategies: (a) embryo bisection (BSEC) and (b) 2-cell blastomere separation (BSEP). To determine demi-embryos quality, we evaluated total blastocyst cell number and proportion of SOX2+ cells. Additionally, the expression of SOX2, NANOG, OCT4, CDX2, IFNT, BAX and BCL genes and let-7a and miRNA-30c Micro RNAs was analysed. BSEP resulted in improved blastocyst development, higher ICM cells and a significantly higher expression of IFNΤ than demi-embryos produced by BSEC. Let-7a, which is associated with low pregnancy establishment was detected in BSEC, while miRNA-30c expression was observed in all treatments. In conclusion, BSEP of 2-cell embryos is more efficient to improve in vitro bovine embryo development and to produce good quality demi-embryos based on ICM cell number and the expression pattern of the genes explored compared to BSEC.

Development and growth of bovine calves demi-embryos

The aim of this study was to investigate the growth and development of animals produced from demi-embryos and compare them with whole embryos from fetus to adult life. To achieve this, calves produced from fresh demi-embryos and whole embryos were individually transferred and monitored from 60 days of pregnancy until slaughter at 550 days. Ultrasound scans were conducted on fetuses at 60 and 90 days to evaluate the biparietal, abdominal, umbilical cord, orbital, and aorta diameters. Subsequently, morphological traits of newborn calves were measured at 0, 7, and 21 days (N = 18). Live weight was recorded at birth, weaning, and every 30 days thereafter until slaughter at 550 days. The growth curve of each group was modeled using logistic regression, and the factors of the respective functions were compared. As early as 60 days of pregnancy, ultrasound evaluations revealed no morphometric differences between fetuses produced from demi-embryos and those from whole embryos. This lack of differentiation persisted in the morphometric evaluations of newborns up to 21 days of age, as well as in live weight and the growth curve from birth to slaughter. Moreover, there were no significant differences between the groups in terms of rib eye area and fat thickness evolution. Consequently, individuals from demi-embryos exhibited no discernible disparities to those whole embryos in growth and development from 60 days of gestation, through birth, and into adulthood.

Endogenous retroviruses shape pluripotency specification in mouse embryos

The smooth and precise transition from totipotency to pluripotency is a key process in embryonic development, generating pluripotent stem cells capable of forming all cell types. While endogenous retroviruses (ERVs) are essential for early development, their precise roles in this transition remains mysterious. Using cutting-edge genetic and biochemical techniques in mice, we identify MERVL-gag, a retroviral protein, as a crucial modulator of pluripotent factors OCT4 and SOX2 during lineage specification. MERVL-gag tightly operates with URI, a prefoldin protein that concurs with pluripotency bias in mouse blastomeres, and which is indeed required for totipotency-to-pluripotency transition. Accordingly, URI loss promotes a stable totipotent-like state and embryo arrest at 2C stage. Mechanistically, URI binds and shields OCT4 and SOX2 from proteasome degradation, while MERVL-gag displaces URI from pluripotent factor interaction, causing their degradation. Our findings reveal the symbiotic coevolution of ERVs with their host cells to ensure the smooth and timely progression of early embryo development.

Blastomeres of 8-cell mouse embryos differ in their ability to generate embryonic stem cells and produce lines with different transcriptional signatures

Embryonic stem cell (ESC) derivation from single blastomeres of 8-cell mouse embryos results in lower derivation rates than that from whole blastocysts, raising a biological question about the developmental potential of sister blastomeres. We aimed to assess the ability of 8-cell blastomeres to produce epiblast cells and ESC lines after isolation, and the properties of the resulting lines. Our results revealed unequal competence among sister blastomeres to produce ESC lines. At least half of the blastomeres possess a lower potential to generate ESCs, although culture conditions and blastomeres plasticity can redirect their non-pluripotent fate towards the epiblast lineage, allowing us to generate up to seven lines from the same embryo. Lines originated from the same embryo segregated into two groups according to their transcriptional signatures. While the expression of genes related to pluripotency and development was higher in one group, no differences were found in their trilineage differentiation ability. These results may help to improve our understanding of the ESC derivation process from single blastomeres and cell fate determination in the preimplantation mouse embryos.

Progression of the pluripotent epiblast depends upon the NMD factor UPF2

Nonsense-mediated RNA decay (NMD) is a highly conserved RNA turnover pathway that degrades RNAs harboring in-frame stop codons in specific contexts. Loss of NMD factors leads to embryonic lethality in organisms spanning the phylogenetic scale, but the mechanism remains unknown. Here, we report that the core NMD factor, UPF2, is required for expansion of epiblast cells within the inner cell mass of mice in vivo. We identify NMD target mRNAs in mouse blastocysts - both canonical and alternatively processed mRNAs - including those encoding cell cycle arrest and apoptosis factors, raising the possibility that NMD is essential for embryonic cell proliferation and survival. In support, the inner cell mass of Upf2-null blastocysts rapidly regresses with outgrowth and is incompetent for embryonic stem cell derivation in vitro. In addition, we uncovered concordant temporal- and lineage-specific regulation of NMD factors and mRNA targets, indicative of a shift in NMD magnitude during peri-implantation development. Together, our results reveal developmental and molecular functions of the NMD pathway in the early embryo.

New insights into the epitranscriptomic control of pluripotent stem cell fate

Each cell in the human body has a distinguishable fate. Pluripotent stem cells are challenged with a myriad of lineage differentiation options. Defects are more likely to be fatal to stem cells than to somatic cells due to the broad impact of the former on early development. Hence, a detailed understanding of the mechanisms that determine the fate of stem cells is needed. The mechanisms by which human pluripotent stem cells, although not fully equipped with complex chromatin structures or epigenetic regulatory mechanisms, accurately control gene expression and are important to the stem cell field. In this review, we examine the events driving pluripotent stem cell fate and the underlying changes in gene expression during early development. In addition, we highlight the role played by the epitranscriptome in the regulation of gene expression that is necessary for each fate-related event.

The Impact of Two Embryo Culture Media, Synthetic Oviduct Fluid and Commercial BO, on pre-and post-Implantation Development of Cloned SAANEN Goat Embryos

BACKGROUND

Somatic cell nuclear transfer (SCNT) is an approach for the propagation of elite animals. In vitro condition, especially the composition of culture media has a profound effect on the developmental competency of in vitro derived e mbryos. There are limited studies evaluating the effect of culture media on SCNT outcomes. To address this gap, we compare the effect of two culture media synthetic oviduct fluid (SOF) vs. commercial bracket-oliphant (BO) on developmental comptenecy.

MATERIALS AND METHODS

In this experimental study, embryos derived from in vitro fertilized (IVF) and SCNT were cultured in both BO and SOF media for 7 days. In addition to the assessment of cleavage and blastocyst on day 3, and 7, the quantitative expression of 16 genes in theresultant blastocysts were assessed. The resultant SCNT blastocysts from SOF and BO groups were also transferred to the synchronized recipient for developmental competency to term.

RESULTS

The blastocyst rate in the BO medium was significantly higher than that of the SOF medium in the SCNT group (P<0.05). All of the examined genes showed increased expression levels in SCNT blastocyst in both media compared to IVF Blastocyst. In the IVF group, Oct4, Bmpr1, and Gcn5 showed significantly higher expression in the SOF medium compared to the BO medium while Akt, Fgfr4, Sox2 showed significantly lower expression in the SOF medium compared to the BO medium. In the SCNT group, Fgfr4, Gcn5, Fzd, Ctnnb, Bmpr1, and Fgfr4 showed significantly higher expression in SOF compared to BO derived blastocyst.

CONCLUSION

It appears that in SCNT blastocysts, gene regulation is less controlled compared to IVF ones, irrespective of the type of medium. In addition, there are differences regarding certain genes expressions between IVF and SCNT derived blastocysts between SOF and BO, reiterating that culture composition affects developmental competency and gene expression.

The proteome, not the transcriptome, predicts that oocyte superovulation affects embryonic phenotypes in mice

Superovulation is the epitome for generating oocytes for molecular embryology in mice, and it is used to model medically assisted reproduction in humans. However, whether a superovulated oocyte is normal, is an open question. This study establishes for the first time that superovulation is associated with proteome changes that affect phenotypic traits in mice, whereas the transcriptome is far less predictive. The proteins that were differentially expressed in superovulated mouse oocytes and embryos compared to their naturally ovulated counterparts were enriched in ontology terms describing abnormal mammalian phenotypes: a thinner zona pellucida, a smaller oocyte diameter, increased frequency of cleavage arrest, and defective blastocyst formation, which could all be verified functionally. Moreover, our findings indicate that embryos with such abnormalities are negatively selected during preimplantation, and ascribe these abnormalities to incomplete ovarian maturation during the time of the conventional superovulation, since they could be corrected upon postponement of the ovulatory stimulus by 24 h. Our data place constraints on the common view that superovulated oocytes are suitable for drawing general conclusions about developmental processes, and underscore the importance of including the proteins in a modern molecular definition of oocyte quality.

Developmental capacity is unevenly distributed among single blastomeres of 2-cell and 4-cell stage mouse embryos

During preimplantation development, mammalian embryo cells (blastomeres) cleave, gradually losing their potencies and differentiating into three primary cell lineages: epiblast (EPI), trophectoderm (TE), and primitive endoderm (PE). The exact moment at which cells begin to vary in their potency for multilineage differentiation still remains unknown. We sought to answer the question of whether single cells isolated from 2- and 4-cell embryos differ in their ability to generate the progenitors and cells of blastocyst lineages. We revealed that twins were often able to develop into blastocysts containing inner cell masses (ICMs) with PE and EPI cells. Despite their capacity to create a blastocyst, the twins differed in their ability to produce EPI, PE, and TE cell lineages. In contrast, quadruplets rarely formed normal blastocysts, but instead developed into blastocysts with ICMs composed of only one cell lineage or completely devoid of an ICM altogether. We also showed that quadruplets have unequal capacities to differentiate into TE, PE, and EPI lineages. These findings could explain the difficulty of creating monozygotic twins and quadruplets from 2- and 4-cell stage mouse embryos.

Asymmetric expression of maternal mRNA governs first cell-fate decision

The goal of preimplantation development is to establish the fates of the embryonic and extra-embryonic cells. However, when and how cell fates are determined during early mammalian embryonic development remains unclear. We report that the high mobility group (HMG) protein family member HMGA1 was distributed differentially in mouse two-cell blastomeres. Knockdown of Hmga1 expression in one of the two cells reduced the number of cells contributing to the inner cell mass (ICM), suggesting that differential distribution of HMGA1 in the blastomeres in two-cell mouse embryos affected the selection of embryonic cell lineages. Mechanistically, HMGA1 promotes the expression of the ICM-specific gene Sox2. The results of this study show that mouse embryos demonstrate heterogeneity as early as the two-cell stage, and that these differences are related to cell-fate differentiation in early mouse embryos.

Cell fate determination and Hippo signaling pathway in preimplantation mouse embryo

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

The COP9 signalosome subunit 3 is necessary for early embryo survival by way of a stable protein deposit in mouse oocytes

Investigations of genes required in early mammalian development are complicated by protein deposits of maternal products, which continue to operate after the gene locus has been disrupted. This leads to delayed phenotypic manifestations and underestimation of the number of genes known to be needed during the embryonic phase of cellular totipotency. Here we expose a critical role of the gene Cops3 by showing that it protects genome integrity during the 2-cell stage of mouse development, in contrast to the previous functional assignment at postimplantation. This new role is mediated by a substantial deposit of protein (94th percentile of the proteome), divided between an exceptionally stable cortical rim, which is prevalent in oocytes, and an ancillary deposit in the embryonic nuclei. Since protein abundance and stability defeat prospects of DNA- or RNA-based gene inactivation in oocytes, we harnessed a classical method next to an emerging method for protein inactivation: antigen masking (for functional inhibition) versus TRIM21-mediated proteasomal degradation, also known as 'Trim away' (for physical removal). Both resulted in 2-cell embryo lethality, unlike the embryos receiving anti-green fluorescent protein. Comparisons between COPS3 protein-targeted and non-targeted embryos revealed large-scale transcriptome differences, which were most evident for genes associated with biological functions critical for RNA metabolism and for the preservation of genome integrity. The gene expression abnormalities associated with COPS3 inactivation were confirmed in situ by the occurrence of DNA endoreduplication and DNA strand breaks in 2-cell embryos. These results recruit Cops3 to the small family of genes that are necessary for early embryo survival. Overall, assigning genes with roles in embryogenesis may be less safe than assumed, if the protein products of these genes accumulate in oocytes: the inactivation of a gene at the protein level can expose an earlier phenotype than that identified by genetic techniques such as conventional gene silencing.

Totipotency of mouse zygotes extends to single blastomeres of embryos at the four-cell stage

In multicellular organisms, oocytes and sperm undergo fusion during fertilization and the resulting zygote gives rise to a new individual. The ability of zygotes to produce a fully formed individual from a single cell when placed in a supportive environment is known as totipotency. Given that totipotent cells are the source of all multicellular organisms, a better understanding of totipotency may have a wide-ranging impact on biology. The precise delineation of totipotent cells in mammals has remained elusive, however, although zygotes and single blastomeres of embryos at the two-cell stage have been thought to be the only totipotent cells in mice. We now show that a single blastomere of two- or four-cell mouse embryos can give rise to a fertile adult when placed in a uterus, even though blastomere isolation disturbs the transcriptome of derived embryos. Single blastomeres isolated from embryos at the eight-cell or morula stages and cultured in vitro manifested pronounced defects in the formation of epiblast and primitive endoderm by the inner cell mass and in the development of blastocysts, respectively. Our results thus indicate that totipotency of mouse zygotes extends to single blastomeres of embryos at the four-cell stage.

Principles of Self-Organization of the Mammalian Embryo

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

Developmental clock and mechanism of de novo polarization of the mouse embryo

Embryo polarization is critical for mouse development; however, neither the regulatory clock nor the molecular trigger that it activates is known. Here, we show that the embryo polarization clock reflects the onset of zygotic genome activation, and we identify three factors required to trigger polarization. Advancing the timing of transcription factor AP-2 gamma (Tfap2c) and TEA domain transcription factor 4 (Tead4) expression in the presence of activated Ras homolog family member A (RhoA) induces precocious polarization as well as subsequent cell fate specification and morphogenesis. Tfap2c and Tead4 induce expression of actin regulators that control the recruitment of apical proteins on the membrane, whereas RhoA regulates their lateral mobility, allowing the emergence of the apical domain. Thus, Tfap2c, Tead4, and RhoA are regulators for the onset of polarization and cell fate segregation in the mouse.

Formation of Osteochondral Organoids from Murine Induced Pluripotent Stem Cells

Osteoarthritis is a debilitating joint disease that is characterized by pathologic changes in both cartilage and bone, potentially involving cross talk between these tissues that is complicated by extraneous factors that are difficult to study in vivo. To create a model system of these cartilage-bone interactions, we developed an osteochondral organoid from murine induced pluripotent stem cells (iPSCs). Using this approach, we grew organoids from a single cell type through time-dependent sequential exposure of growth factors, namely transforming growth factor β-3 and bone morphogenic protein 2, to mirror bone development through endochondral ossification. The result is a cartilaginous region and a calcified bony region comprising an organoid with the potential for joint disease drug screening and investigation of genetic risk in a patient or disease-specific manner. Furthermore, we also investigated the possibility of the differentiated cells within the organoid to revert to a pluripotent state. It was found that while the cells themselves maintain the capacity for reinduction of pluripotency, encapsulation in the newly formed 3D matrix prevents this process from occurring, which could have implications for future clinical use of iPSCs.

ZC3H4-a novel Cys-Cys-Cys-His-type zinc finger protein-is essential for early embryogenesis in mice†

Zinc finger domains of the Cys-Cys-Cys-His (CCCH) class are evolutionarily conserved proteins that bind nucleic acids and are involved in various biological processes. Nearly 60 CCCH-type zinc finger proteins have been identified in humans and mice, most have not been functionally characterized. Here, we provide the first in vivo functional characterization of ZC3H4-a novel CCCH-type zinc finger protein. Our results show that although Zc3h4 mutant embryos exhibit normal morphology at E3.5 blastocyst stage, they cannot be recovered at E7.5 early post-gastrulation stage, suggesting implantation failure. Outgrowth assays reveal that mutant blastocysts either fail to hatch from the zona pellucida, or can hatch but do not form a typical inner cell mass colony, the source of embryonic stem cells (ESCs). Although there is no change in levels of reactive oxygen species, Zc3h4 mutants display severe DNA breaks and reduced cell proliferation. Analysis of lineage specification reveals that both epiblast and primitive endoderm lineages are compromised with severe reductions in cell number and/or specification in the mutant blastocysts. In summary, these findings demonstrate the essential role of ZC3H4 during early mammalian embryogenesis.

MCRS1 is essential for epiblast development during early mouse embryogenesis

Microspherule protein 1 (MCRS1, also known as MSP58) is an evolutionarily conserved protein that has been implicated in various biological processes. Although a variety of functions have been attributed to MCRS1 in vitro, mammalian MCRS1 has not been studied in vivo. Here we report that MCRS1 is essential during early murine development. Mcrs1 mutant embryos exhibit normal morphology at the blastocyst stage but cannot be recovered at gastrulation, suggesting an implantation failure. Outgrowth (OG) assays reveal that mutant blastocysts do not form a typical inner cell mass (ICM) colony, the source of embryonic stem cells (ESCs). Surprisingly, cell death and histone H4 acetylation analysis reveal that apoptosis and global H4 acetylation are normal in mutant blastocysts. However, analysis of lineage specification reveals that while the trophoblast and primitive endoderm are properly specified, the epiblast lineage is compromised and exhibits a severe reduction in cell number. In summary, our study demonstrates the indispensable role of MCRS1 in epiblast development during early mammalian embryogenesis.

Coordination between patterning and morphogenesis ensures robustness during mouse development

The mammalian preimplantation embryo is a highly tractable, self-organizing developmental system in which three cell types are consistently specified without the need for maternal factors or external signals. Studies in the mouse over the past decades have greatly improved our understanding of the cues that trigger symmetry breaking in the embryo, the transcription factors that control lineage specification and commitment, and the mechanical forces that drive morphogenesis and inform cell fate decisions. These studies have also uncovered how these multiple inputs are integrated to allocate the right number of cells to each lineage despite inherent biological noise, and as a response to perturbations. In this review, we summarize our current understanding of how these processes are coordinated to ensure a robust and precise developmental outcome during early mouse development. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

Aneuploid rescue precedes X-chromosome inactivation and increases the incidence of its skewness by reducing the size of the embryonic progenitor cell pool

STUDY QUESTION

Do monosomy rescue (MR) and trisomy rescue (TR) in preimplantation human embryos affect other developmental processes, such as X-chromosome inactivation (XCI)?

SUMMARY ANSWER

Aneuploid rescue precedes XCI and increases the incidence of XCI skewness by reducing the size of the embryonic progenitor cell pools.

WHAT IS KNOWN ALREADY

More than half of preimplantation human embryos harbor aneuploid cells, some of which can be spontaneously corrected through MR or TR. XCI in females is an indispensable process, which is predicted to start at the early-blastocyst phase.

STUDY DESIGN, SIZE, DURATION

We examined the frequency of XCI skewness in young females who carried full uniparental disomy (UPD) resulting from MR or TR/gamete complementation (GC). The results were statistically analyzed using a theoretical model in which XCI involves various numbers of embryonic progenitor cells.

PARTICIPANTS/MATERIALS, SETTING, METHODS

We studied 39 children and young adults ascertained by imprinting disorders. XCI ratios were determined by DNA methylation analysis of a polymorphic locus in the androgen receptor gene. We used Bayesian approach to assess the probability of the occurrence of extreme XCI skewness in the MR and TR/GC groups using a theoretical model of 1-12 cell pools.

MAIN RESULTS AND THE ROLE OF CHANCE

A total of 12 of 39 individuals (31%) showed skewed XCI. Extreme skewness was observed in 3 of 15 MR cases (20%) and 1 of 24 TR/GC cases (4.2%). Statistical analysis indicated that XCI in the MR group was likely to have occurred when the blastocyst contained three or four euploid embryonic progenitor cells. The estimated size of the embryonic progenitor cell pools was approximately one-third or one-fourth of the predicted size of normal embryos. The TR/GC group likely had a larger pool size at the onset of XCI, although the results remained inconclusive.

LIMITATIONS, REASONS FOR CAUTION

This is an observational study and needs to be validated by experimental analyses.

WIDER IMPLICATIONS OF THE FINDINGS

This study provides evidence that the onset of XCI is determined by an intrinsic clock, irrespectively of the number of embryonic progenitor cells. Our findings can also be applied to individuals without UPD or imprinting disorders. This study provides a clue to understand chromosomal and cellular dynamics in the first few days of human development, their effects on XCI skewing and the possible implications for the expression of X-linked diseases in females.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by the Grants-in-aid for Scientific Research on Innovative Areas (17H06428) and for Scientific Research (B) (17H03616) from Japan Society for the Promotion of Science (JSPS), and grants from Japan Agency for Medical Research and Development (AMED) (18ek0109266h0002 and 18ek0109278h0002), National Center for Child Health and Development and Takeda Science Foundation. The authors declare no conflict of interest.

TRIAL REGISTRATION NUMBER

Not applicable.

Differences in blastomere totipotency in 2-cell mouse embryos are a maternal trait mediated by asymmetric mRNA distribution

It is widely held that the first two blastomeres of mammalian embryos are equally totipotent and that this totipotency belongs to the group of regulative properties. However, this interpretation neglects an important aspect: evidence only came from successful monozygotic twins which can speak only for those pairs of half-embryos that are able to regulate in the first place. Are the frequently occurring incomplete pairs simply an artefact, or do they represent a real difference, be it in the imperfect blastomere's ability to regulate growth or in the distribution of any compound X that constrains regulation? Using the model system of mouse embryos bisected at the 2-cell stage after fertilization, we present evidence that the interblastomere differences evade regulation by external factors and are already latent in oocytes. Specifically, an interblastomere imbalance of epiblast production persists under the most diverse culture conditions and applies to the same extent in parthenogenetic counterparts. As a result, cases in which twin blastocysts continued to develop in only one member account for 65 and 57% of zygotic and parthenogenetic pairs, respectively. The interblastomere imbalance is related to the subcellular distribution of gene products, as documented for the epiblast-related gene Cops3, using mRNA FISH in super-resolution mode confocal microscopy. Blastomere patterns of Cops3 mRNA distribution are α-amanitin-resistant. Thus, the imbalance originates not from de novo transcription, but from influences which are effective before fertilisation. These data expose previously unrecognized limits of regulative capacities of 2-cell stage blastomeres and point to aspects of cytoplasmic organization of the mouse oocyte that segregate unequally to blastomeres during cleavage.

Autophagy-mediated apoptosis eliminates aneuploid cells in a mouse model of chromosome mosaicism

The high incidence of aneuploidy in the embryo is considered the principal cause for low human fecundity. However, the prevalence of aneuploidy dramatically declines as pregnancy progresses, with the steepest drop occurring as the embryo completes implantation. Despite the fact that the plasticity of the embryo in dealing with aneuploidy is fundamental to normal development, the mechanisms responsible for eliminating aneuploid cells are unclear. Here, using a mouse model of chromosome mosaicism, we show that aneuploid cells are preferentially eliminated from the embryonic lineage in a p53-dependent process involving both autophagy and apoptosis before, during and after implantation. Moreover, we show that diploid cells in mosaic embryos undertake compensatory proliferation during the implantation stages to confer embryonic viability. Together, our results indicate a close link between aneuploidy, autophagy, and apoptosis to refine the embryonic cell population and ensure only chromosomally fit cells proceed through development of the fetus.

The mechanisms behind the plasticity of embryos and how they deal with aneuploid cells are unclear. Here, the authors show that aneuploid cells in a mouse embryo are preferentially eliminated during pre- and peri-implantation development in a p53-dependent process involving both autophagy and apoptosis.

Regulation of size and scale in vertebrate spinal cord development

All vertebrates have a spinal cord with dimensions and shape specific to their species. Yet how species‐specific organ size and shape are achieved is a fundamental unresolved question in biology. The formation and sculpting of organs begins during embryonic development. As it develops, the spinal cord extends in anterior–posterior direction in synchrony with the overall growth of the body. The dorsoventral (DV) and apicobasal lengths of the spinal cord neuroepithelium also change, while at the same time a characteristic pattern of neural progenitor subtypes along the DV axis is established and elaborated. At the basis of these changes in tissue size and shape are biophysical determinants, such as the change in cell number, cell size and shape, and anisotropic tissue growth. These processes are controlled by global tissue‐scale regulators, such as morphogen signaling gradients as well as mechanical forces. Current challenges in the field are to uncover how these tissue‐scale regulatory mechanisms are translated to the cellular and molecular level, and how regulation of distinct cellular processes gives rise to an overall defined size. Addressing these questions will help not only to achieve a better understanding of how size is controlled, but also of how tissue size is coordinated with the specification of pattern.

This article is categorized under:

Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing

Signaling Pathways > Global Signaling Mechanisms

Nervous System Development > Vertebrates: General Principles

Assessing heterogeneity among single embryos and single blastomeres using open microfluidic design

The process by which a zygote develops from a single cell into a multicellular organism is poorly understood. Advances are hindered by detection specificity and sensitivity limitations of single-cell protein tools and by challenges in integrating multimodal data. We introduce an open microfluidic tool expressly designed for same-cell phenotypic, protein, and mRNA profiling. We examine difficult-to-study-yet critically important-murine preimplantation embryo stages. In blastomeres dissociated from less well-studied two-cell embryos, we observe no significant GADD45a protein expression heterogeneity, apparent at the four-cell stage. In oocytes, we detect differences in full-length versus truncated DICER-1 mRNA and protein, which are insignificant by the two-cell stage. Single-embryo analyses reveal intraembryonic heterogeneity, differences between embryos of the same fertilization event and between donors, and reductions in the burden of animal sacrifice. Open microfluidic design integrates with existing workflows and opens new avenues for assessing the cellular-to-molecular heterogeneity inherent to preimplantation embryo development.

Retrospective analysis: reproducibility of interblastomere differences of mRNA expression in 2-cell stage mouse embryos is remarkably poor due to combinatorial mechanisms of blastomere diversification

STUDY QUESTION

What is the prevalence, reproducibility and biological significance of transcriptomic differences between sister blastomeres of the mouse 2-cell embryo?

SUMMARY ANSWER

Sister 2-cell stage blastomeres are distinguishable from each other by mRNA analysis, attesting to the fact that differentiation starts mostly early in the mouse embryo; however, the interblastomere differences are poorly reproducible and invoke the combinatorial effects of known and new mechanisms of blastomere diversification.

WHAT IS KNOWN ALREADY

Transcriptomic datasets for single blastomeres in mice have been available for years but have never been systematically analysed together, although such an analysis may shed light onto some unclarified topics of early mammalian development. Two unknowns that remain are at which stage embryonic blastomeres start to diversify from each other and what is the molecular origin of that difference. At the earliest postzygotic stage, the 2-cell stage, opinions differ regarding the answer to these questions; one group claims that the first zygotic division yields two equal blastomeres capable of forming a full organism (totipotency) and another group claims evidence for interblastomere differences reminiscent of the prepatterning found in embryos of lower taxa. Regarding the molecular origin of interblastomere differences, there are four prevalent models which invoke (1) oocyte anisotropy, (2) sperm entry point, (3) partition errors of the transcript pool and (4) asynchronous embryonic genome activation in the two blastomeres.

STUDY DESIGN, SIZE, DURATION

Seven transcriptomic studies published between 2011 and 2017 were eligible for retrospective analysis, since both blastomeres of the mouse 2-cell embryo had been analysed individually regarding the original pair associations and since the datasets were made available in public repositories. Five of these studies, encompassing a total of 43 pairs of sister blastomeres, were selected for further analyses based on high interblastomere correlations of mRNA levels. A double cut-off was used to select mRNAs that had robust interblastomere differences both within and between embryos (hits). The hits of each study were compared and contrasted with the hits of the other studies using Venn diagrams. The hits shared by at least four of five studies were analysed further by bioinformatics.

PARTICIPANTS/MATERIALS, SETTING, METHODS

PubMed was systematically examined for mRNA expression profiles of single 2-cell stage blastomeres in addition to publicly available microarray datasets (GEO, ArrayExpress). Based on the original normalizations, data from seven studies were screened for pairwise sample correlation at the gene level (Spearman), and the top five datasets with the highest correlation were subjected to hierarchical cluster analysis. Interblastomere differences of gene expression were expressed as a ratio of the higher to the lower mRNA level for each pair of blastomeres. A double cut-off was used to make the call of interblastomere difference, accepting genes with mRNA ratios above 2 when observed in at least 50% of the pairs, and discarding the other genes. The proportion of interblastomere differences common to at least four of the five datasets was calculated. Finally, the corresponding gene, pathway and enrichment analyses were performed utilizing PANTHER and GORILLA platforms.

MAIN RESULTS AND THE ROLE OF CHANCE

An average of 17% of genes within the datasets are differently expressed between sister blastomeres, a proportion which falls to 1% when considering the differences that are common to at least four of the five studies. Housekeeping mRNAs were not included in the 17% and 1% gene lists, suggesting that the interblastomere differences do not occur simply by chance. The 1% of shared interblastomere differences comprise 100 genes, of which 35 are consistent with at least one of the four prevalent models of sister blastomere diversification. Bioinformatics analysis of the remaining 65 genes that are not consistent with the four models suggests that at least one more mechanism is at play, potentially related to the endomembrane system. Although there are many dimensions to the issue of reproducibility (biological, experimental, analytical), we consider that the sister blastomeres are poised to escape high interblastomere correlations of mRNA levels, because at least five sources of diversity superimpose on each other, accounting for at least 25 = 32 different states. As a result, interblastomere mRNA differences of a given 2-cell embryo are necessarily difficult to reproduce in another 2-cell embryo.

LARGE SCALE DATA

Data were as provided by the original studies (GSE21688, GSE22182, GSE27396, GSE45719, GSE57249, E-MTAB-3321, GSE94050).

LIMITATIONS, REASONS FOR CAUTION

The original studies present similarities (e.g. fertilization in vivo after ovarian stimulation) as well as differences (e.g. mouse strains, method and timing of blastomere separation). We identified robust mRNA differences between the sister blastomeres, but these differences are underestimated because our double cut-off method works with thresholds and affords more protection against false positives than false negatives. Regarding the false negatives, transcriptome analysis may have captured only part of the interblastomere differences due to: (1) the 2-fold cut-off not being sensitive enough to detect the remaining part of the interblastomere differences, (2) the detection limit of the transcriptomic methods not being sufficient, or (3) interblastomere differences being oblivious to transcriptomic identification because transcriptional changes are oscillatory or because differences are mediated non-transcriptionally or post-transcriptionally. Regarding the false positives, it seems unlikely that a difference was found just by chance for the same group of transcripts due to the same technical error, given that different laboratories produced the data.

WIDER IMPLICATIONS OF THE FINDINGS

It is clear that the sister blastomeres are distinguishable from each other by mRNA analysis even at the 2-cell stage; however, efforts to identify large stable patterns may be in vain. This elicits thoughts about the wisdom of adding new transcriptomic datasets to the ones that already exist; if all transcriptomic datasets produced so far show a reproducibility of 1%, then any future study would probably face the same issue again. Possibly, a solid identification of the 'large stable pattern that should be there but was not found' requires an even larger dataset than the sum of the seven datasets considered here. Conversely, small stable patterns may be easier to identify, but their biological relevance is less obvious. Alternatively, interblastomere differences may not be mediated by nucleic acids but by other cellular components.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by the Deutsche Forschungsgemeinschaft (grant DFG BO 2540-4-3 to M.B. and grant NO 413/3-3 to V.N.). The authors declare that they have no competing financial interests.

Mysteries in embryonic development: How can errors arise so frequently at the beginning of mammalian life?

Chromosome segregation errors occur frequently during female meiosis but also in the first mitoses of mammalian preimplantation development. Such errors can lead to aneuploidy, spontaneous abortions, and birth defects. Some of the mechanisms underlying these errors in meiosis have been deciphered but which mechanisms could cause chromosome missegregation in the first embryonic cleavage divisions is mostly a “mystery”. In this article, we describe the starting conditions and challenges of these preimplantation divisions, which might impair faithful chromosome segregation. We also highlight the pending research to provide detailed insight into the mechanisms and regulation of preimplantation mitoses.

Starting a new life is a challenging business. This Essay explores the changes at the oocyte-to-embryo transition to highlight the circumstances under which the very first and decisive — but ‘mysteriously’ error-prone — mitotic divisions occur.

The first cell-fate decision of mouse preimplantation embryo development: integrating cell position and polarity

During the first cell-fate decision of mouse preimplantation embryo development, a population of outer-residing polar cells is segregated from a second population of inner apolar cells to form two distinct cell lineages: the trophectoderm and the inner cell mass (ICM), respectively. Historically, two models have been proposed to explain how the initial differences between these two cell populations originate and ultimately define them as the two stated early blastocyst stage cell lineages. The 'positional' model proposes that cells acquire distinct fates based on differences in their relative position within the developing embryo, while the 'polarity' model proposes that the differences driving the lineage segregation arise as a consequence of the differential inheritance of factors, which exhibit polarized subcellular localizations, upon asymmetric cell divisions. Although these two models have traditionally been considered separately, a growing body of evidence, collected over recent years, suggests the existence of a large degree of compatibility. Accordingly, the main aim of this review is to summarize the major historical and more contemporarily identified events that define the first cell-fate decision and to place them in the context of both the originally proposed positional and polarity models, thus highlighting their functional complementarity in describing distinct aspects of the developmental programme underpinning the first cell-fate decision in mouse embryogenesis.

Tracing the origin of heterogeneity and symmetry breaking in the early mammalian embryo

A fundamental question in developmental and stem cell biology concerns the origin and nature of signals that initiate asymmetry leading to pattern formation and self-organization. Instead of having prominent pre-patterning determinants as present in model organisms (worms, sea urchin, frog), we propose that the mammalian embryo takes advantage of more subtle cues such as compartmentalized intracellular reactions that generate micro-scale inhomogeneity, which is gradually amplified over several cellular generations to drive pattern formation while keeping developmental plasticity. It is therefore possible that by making use of compartmentalized information followed by its amplification, mammalian embryos would follow general principle of development found in other organisms in which the spatial cue is more robustly presented.

Control of the proportion of inner cells by asymmetric divisions and the ensuing resilience of cloned rabbit embryos

Mammalian embryo cloning by nuclear transfer has a low success rate. This is hypothesized to correlate with a high variability of early developmental steps that segregate outer cells, which are fated to extra-embryonic tissues, from inner cells, which give rise to the embryo proper. Exploring the cell lineage of wild-type embryos and clones, imaged in toto until hatching, highlights the respective contributions of cell proliferation, death and asymmetric divisions to phenotypic variability. Preferential cell death of inner cells in clones, probably pertaining to the epigenetic plasticity of the transferred nucleus, is identified as a major difference with effects on the proportion of inner cell. In wild type and clones, similar patterns of outer cell asymmetric divisions are shown to be essential to the robust proportion of inner cells observed in wild type. Asymmetric inner cell division, which is not described in mice, is identified as a regulator of the proportion of inner cells and likely gives rise to resilient clones.

Summary: A unique quantitative approach based on complete reconstruction of the cell lineage that unveils an unknown mechanism of size control in cell populations of rabbit blastocysts, wild types or clones.

Cell Fate Decisions During Preimplantation Mammalian Development

The early mouse embryo offers a phenomenal system to dissect how changes in the mechanisms controlling cell fate are integrated with morphogenetic events at the single-cell level. New technologies based on live imaging have enabled the discovery of dynamic changes in the regulation of single genes, transcription factors, and epigenetic mechanisms directing early cell fate decision in the early embryo. Here, we review recent progress in linking molecular dynamic events occurring at the level of the single cell in vivo, to some of the key morphogenetic changes regulating early mouse development.

Dynamics of cytoplasm and cleavage divisions correlates with preimplantation embryo development

In vitro fertilization has become increasingly popular as an infertility treatment. In order to improve efficiency of this procedure, there is a strong need for a refinement of existing embryo assessment methods and development of novel, robust and non-invasive selection protocols. Studies conducted on animal models can be extremely helpful here, as they allow for more extensive research on the potential biomarkers of embryo quality. In the present paper, we subjected mouse embryos to non-invasive time-lapse imaging and combined the Particle Image Velocimetry analysis of cytoplasmic dynamics in freshly fertilized oocytes with the morphokinetic analysis of recordings covering 5 days of preimplantation development. Our results indicate that parameters describing cytoplasmic dynamics and cleavage divisions independently correspond to mouse embryo's capacity to form a high-quality blastocyst. We also showed for the first time that these parameters are associated with the percentage of abnormal embryonic cells with fragmented nuclei and with embryo's ability to form primitive endoderm, one of the cell lineages differentiated during preimplantation development. Finally, we present a model that links selected cytoplasmic and morphokinetic parameters reflecting frequency of fertilization-induced Ca²⁺-oscillations and timing of 4-cell stage and compaction with viability of the embryo assessed as the total number of cells at the end of its preimplantation development. Our results indicate that a combined analysis of cytoplasmic dynamics and morphokinetics may facilitate the assessment of embryo's ability to form high-quality blastocysts.

Totipotency segregates between the sister blastomeres of two-cell stage mouse embryos

Following fertilization in mammals, it is generally accepted that totipotent cells are exclusive to the zygote and to each of the two blastomeres originating from the first mitotic division. This model of totipotency was inferred from a minority of cases in which blastomeres produced monozygotic twins in mice. Was this due to experimental limitation or biological constraint? Here we removed experimental obstacles and achieved reliable quantification of the prevalence of dual totipotency among mouse two-cell stage blastomeres. We separated the blastomeres of 1,252 two-cell embryos, preserving 1,210 of the pairs. Two classes of monozygotic twins became apparent at the blastocyst stage: 27% formed a functional epiblast in both members (concordant), and 73% did so in only one member of the pair (discordant) - a partition that proved insensitive to oocyte quality, sperm-entry point, culture environment and pattern of cleavage. In intact two-cell embryos, the ability of sister blastomeres to generate epiblast was also skewed. Class discovery clustering of the individual blastomeres' and blastocysts' transcriptomes points to an innate origin of concordance and discordance rather than developmental acquisition. Our data place constraints on the commonly accepted idea that totipotency is allocated equally between the two-cell stage blastomeres in mice.

Potential of human twin embryos generated by embryo splitting in assisted reproduction and research

BACKGROUND

Embryo splitting or twinning has been widely used in veterinary medicine over 20 years to generate monozygotic twins with desirable genetic characteristics. The first human embryo splitting, reported in 1993, triggered fierce ethical debate on human embryo cloning. Since Dolly the sheep was born in 1997, the international community has acknowledged the complexity of the moral arguments related to this research and has expressed concerns about the potential for reproductive cloning in humans. A number of countries have formulated bans either through laws, decrees or official statements. However, in general, these laws specifically define cloning as an embryo that is generated via nuclear transfer (NT) and do not mention embryo splitting. Only the UK includes under cloning both embryo splitting and NT in the same legislation. On the contrary, the Ethics Committee of the American Society for Reproductive Medicine does not have a major ethical objection to transferring two or more artificially created embryos with the same genome with the aim of producing a single pregnancy, stating that 'since embryo splitting has the potential to improve the efficacy of IVF treatments for infertility, research to investigate the technique is ethically acceptable'.

OBJECTIVE AND RATIONALE

Embryo splitting has been introduced successfully to the veterinary medicine several decades ago and today is a part of standard practice. We present here an overview of embryo splitting experiments in humans and non-human primates and discuss the potential of this technology in assisted reproduction and research.

SEARCH METHODS

A comprehensive literature search was carried out using PUBMED and Google Scholar databases to identify studies on embryo splitting in humans and non-human primates. 'Embryo splitting' and 'embryo twinning' were used as the keywords, alone or in combination with other search phrases relevant to the topics of biology of preimplantation embryos.

OUTCOMES

A very limited number of studies have been conducted in humans and non-human primates. The published material, especially the studies with human embryos, is controversial. Some reports suggest that twinning technology will find clinical use in reproductive medicine in the future, whereas others conclude the opposite that human twin embryos created in vitro are unsuitable not only for clinical, but also for research, purposes.

WIDER IMPLICATIONS

The blastomere biopsy technique of embryo splitting seems to be unsuitable for either clinical or research purposes; however, embryo bisection, a preferable method of cloning in veterinary medicine, has not yet been tested on human embryos.

Reassembling embryos in vitro from component stem cells

Researchers at the University of Cambridge, UK have succeeded in reconstructing mouse embryos by combining pluripotent embryonic and multipotent trophoblast stem cells in a 3D scaffold; the study from the laboratory of Professor Zernicka-Goetz, recently published in Science, provides a break-through tool to probe early mammalian development outside the uterus. Achieving a similar feat with human cells might necessitate reconsideration of the 14-day rule as a limitation of such research.

Four simple rules that are sufficient to generate the mammalian blastocyst

Early mammalian development is both highly regulative and self-organizing. It involves the interplay of cell position, predetermined gene regulatory networks, and environmental interactions to generate the physical arrangement of the blastocyst with precise timing. However, this process occurs in the absence of maternal information and in the presence of transcriptional stochasticity. How does the preimplantation embryo ensure robust, reproducible development in this context? It utilizes a versatile toolbox that includes complex intracellular networks coupled to cell-cell communication, segregation by differential adhesion, and apoptosis. Here, we ask whether a minimal set of developmental rules based on this toolbox is sufficient for successful blastocyst development, and to what extent these rules can explain mutant and experimental phenotypes. We implemented experimentally reported mechanisms for polarity, cell-cell signaling, adhesion, and apoptosis as a set of developmental rules in an agent-based in silico model of physically interacting cells. We find that this model quantitatively reproduces specific mutant phenotypes and provides an explanation for the emergence of heterogeneity without requiring any initial transcriptional variation. It also suggests that a fixed time point for the cells' competence of fibroblast growth factor (FGF)/extracellular signal-regulated kinase (ERK) sets an embryonic clock that enables certain scaling phenomena, a concept that we evaluate quantitatively by manipulating embryos in vitro. Based on these observations, we conclude that the minimal set of rules enables the embryo to experiment with stochastic gene expression and could provide the robustness necessary for the evolutionary diversification of the preimplantation gene regulatory network.

The chromatin modifier Satb1 regulates cell fate through Fgf signalling in the early mouse embryo

The separation of embryonic from extra-embryonic tissues within the inner cell mass to generate the epiblast (EPI), which will form the new organism, from the primitive endoderm (PE), which will form the yolk sac, is a crucial developmental decision. Here, we identify a chromatin modifier, Satb1, with a distinct role in this decision. Satb1 is differentially expressed within 16-cell-stage embryos, with higher expression levels in the inner cell mass progenitor cells. Depleting Satb1 increases the number of EPI cells at the expense of PE. This phenotype can be rescued by simultaneous depletion of both Satb1 and Satb2, owing to their antagonistic effect on the pluripotency regulator Nanog. Consequently, increasing Satb1 expression leads to differentiation into PE and a decrease in EPI, as a result of the modulation of expression of several pluripotency- and differentiation-related genes by Satb1. Finally, we show that Satb1 is a downstream target of the Fgf signalling pathway, linking chromatin modification and Fgf signalling. Together, these results identify a role for Satb1 in the lineage choice between pluripotency and differentiation and further our understanding of early embryonic lineage segregation.

Somatic mutations reveal asymmetric cellular dynamics in the early human embryo

Somatic cells acquire mutations throughout the course of an individual's life. Mutations occurring early in embryogenesis are often present in a substantial proportion of, but not all, cells in postnatal humans and thus have particular characteristics and effects. Depending on their location in the genome and the proportion of cells they are present in, these mosaic mutations can cause a wide range of genetic disease syndromes and predispose carriers to cancer. They have a high chance of being transmitted to offspring as de novo germline mutations and, in principle, can provide insights into early human embryonic cell lineages and their contributions to adult tissues. Although it is known that gross chromosomal abnormalities are remarkably common in early human embryos, our understanding of early embryonic somatic mutations is very limited. Here we use whole-genome sequences of normal blood from 241 adults to identify 163 early embryonic mutations. We estimate that approximately three base substitution mutations occur per cell per cell-doubling event in early human embryogenesis and these are mainly attributable to two known mutational signatures. We used the mutations to reconstruct developmental lineages of adult cells and demonstrate that the two daughter cells of many early embryonic cell-doubling events contribute asymmetrically to adult blood at an approximately 2:1 ratio. This study therefore provides insights into the mutation rates, mutational processes and developmental outcomes of cell dynamics that operate during early human embryogenesis.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

Asynchronous fate decisions by single cells collectively ensure consistent lineage composition in the mouse blastocyst

Intercellular communication is essential to coordinate the behaviour of individual cells during organismal development. The preimplantation mammalian embryo is a paradigm of tissue self-organization and regulative development; however, the cellular basis of these regulative abilities has not been established. Here we use a quantitative image analysis pipeline to undertake a high-resolution, single-cell level analysis of lineage specification in the inner cell mass (ICM) of the mouse blastocyst. We show that a consistent ratio of epiblast and primitive endoderm lineages is achieved through incremental allocation of cells from a common progenitor pool, and that the lineage composition of the ICM is conserved regardless of its size. Furthermore, timed modulation of the FGF-MAPK pathway shows that individual progenitors commit to either fate asynchronously during blastocyst development. These data indicate that such incremental lineage allocation provides the basis for a tissue size control mechanism that ensures the generation of lineages of appropriate size.

Early embryonic cell fate and lineage specification is tightly regulated in the preimplantation mammalian embryo. Here, the authors quantitatively examine the ratio of epiblast to primitive endoderm lineages in the blastocyst and show composition of the inner cell mass is conserved, independent of its size.

Polarity in Cell-Fate Acquisition in the Early Mouse Embryo

Establishing polarity is a fundamental part of embryogenesis and can be traced back to the earliest developmental stages. It can be achieved in one of two ways: through the preexisting polarization of germ cells before fertilization or via symmetry breaking after fertilization. In mammals, it seems to be the latter, and we will discuss the various cytological and molecular events that lead up to this event, its mechanisms and the consequences. In mammals, the first polarization event occurs in the preimplantation period, when the embryo is but a cluster of cells, free-floating in the oviduct. This provides a unique, autonomous system to study the de novo polarization that is essential to life. In this review, we will cover modern and past studies on the polarization of the early embryo, using the mouse as a model system, as well as hypothesizing the potential implications and functions of the biological events involved.