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

Determinant molecular markers for peri-gastrulating bovine embryo development

Peri-gastrulation defines the time frame between blastocyst formation and implantation that also corresponds in cattle to elongation, pregnancy recognition and uterine secretion. Optimally, this developmental window prepares the conceptus for implantation, placenta formation and fetal development. However, this is a highly sensitive period, as evidenced by the incidence of embryo loss or early post-implantation mortality after AI, embryo transfer or somatic cell nuclear transfer. Elongation markers have often been used within this time frame to assess developmental defects or delays, originating either from the embryo, the uterus or the dam. Comparatively, gastrulation markers have not received great attention, although elongation and gastrulation are linked by reciprocal interactions at the molecular and cellular levels. To make this clearer, this peri-gastrulating period is described herein with a focus on its main developmental landmarks, and the resilience of the landmarks in the face of biotechnologies is questioned.

Transcriptome profiling of bovine inner cell mass and trophectoderm derived from in vivo generated blastocysts

Background

This study describes the generation and analysis of the transcriptional profile of bovine inner cell mass (ICM) and trophectoderm (TE), obtained from in vivo developed embryos by using a bovine-embryo specific array (EmbryoGENE) containing 37,238 probes.

Results

A total of 4,689 probes were differentially expressed between ICM and TE, among these, 2,380 and 2,309 probes were upregulated in ICM and TE tissues, respectively (P ≤ 0.01, FC ≥ 2.0, FDR: 2.0). Ontological classification of the genes predominantly expressed in ICM emerged a range of functional categories with a preponderance of genes involved in basal and developmental signaling pathways including P53, TGFβ, IL8, mTOR, integrin, ILK, and ELF2 signalings. Cross-referencing of microarray data with two available in vitro studies indicated a marked reduction in ICM vs. TE transcriptional difference following in vitro culture of bovine embryos. Moreover, a great majority of genes that were found to be misregulated following in vitro culture of bovine embryos were known genes involved in epigenetic regulation of pluripotency and cell differentiation including DNMT1, GADD45, CARM1, ELF5 HDAC8, CCNB1, KDM6A, PRDM9, CDX2, ARID3A, IL6, GADD45A, FGFR2, PPP2R2B, and SMARCA2. Cross-species referencing of microarray data revealed substantial divergence between bovine and mouse and human in signaling pathways involved in early lineage specification.

Conclusions

The transcriptional changes occur during ICM and TE lineages specification in bovine is greater than previously understood. Therefore, this array data establishes a standard to evaluate the in vitro imprint on the transcriptome and to hypothesize the cross-species differences that allow in vitro acquisition of pluripotent ICM in human and mice but hinder that process in bovine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0096-3) contains supplementary material, which is available to authorized users.

Lineage-Specific Profiling Delineates the Emergence and Progression of Naive Pluripotency in Mammalian Embryogenesis

Naive pluripotency is manifest in the preimplantation mammalian embryo. Here we determine transcriptome dynamics of mouse development from the eight-cell stage to postimplantation using lineage-specific RNA sequencing. This method combines high sensitivity and reporter-based fate assignment to acquire the full spectrum of gene expression from discrete embryonic cell types. We define expression modules indicative of developmental state and temporal regulatory patterns marking the establishment and dissolution of naive pluripotency in vivo. Analysis of embryonic stem cells and diapaused embryos reveals near-complete conservation of the core transcriptional circuitry operative in the preimplantation epiblast. Comparison to inner cell masses of marmoset primate blastocysts identifies a similar complement of pluripotency factors but use of alternative signaling pathways. Embryo culture experiments further indicate that marmoset embryos utilize WNT signaling during early lineage segregation, unlike rodents. These findings support a conserved transcription factor foundation for naive pluripotency while revealing species-specific regulatory features of lineage segregation.

Signal Inhibition Reveals JAK/STAT3 Pathway as Critical for Bovine Inner Cell Mass Development

The inner cell mass (ICM) of mammalian blastocysts consists of pluripotent epiblast and hypoblast lineages, which develop into embryonic and extraembryonic tissues, respectively. We conducted a chemical screen for regulators of epiblast identity in bovine Day 8 blastocysts. From the morula stage onward, in vitro fertilized embryos were cultured in the presence of cell-permeable small molecules targeting nine principal signaling pathway components, including TGFbeta1, BMP, EGF, VEGF, PDGF, FGF, cAMP, PI3K, and JAK signals. Using 1) blastocyst quality (by morphological grading), 2) cell numbers (by differential stain), and 3) epiblast (FGF4, NANOG) and hypoblast (PDGFRa, SOX17) marker gene expression (by quantitative PCR), we identified positive and negative regulators of ICM development and pluripotency. TGFbeta1, BMP, and cAMP and combined VEGF/PDGF/FGF signals did not affect blastocyst development while PI3K was important for ICM growth but did not alter lineage-specific gene expression. Stimulating cAMP specifically increased NANOG expression, while combined VEGF/PDGF/FGF inhibition up-regulated epiblast and hypoblast markers. The strongest effects were observed by suppressing JAK1/2 signaling with AZD1480. This treatment interfered with ICM formation, but trophectoderm cell numbers and markers (CDX2, KTR8) were not altered. JAK inhibition repressed both epiblast and hypoblast transcripts as well as naive pluripotency-related genes (KLF4, TFCP2L1) and the JAK substrate STAT3. We found that tyrosine (Y) 705-phosphorylated STAT3 (pSTAT3(Y705)) was restricted to ICM nuclei, where it colocalized with SOX2 and NANOG. JAK inhibition abolished this ICM-exclusive pSTAT3(Y705) signal and strongly reduced the number of SOX2-positive nuclei. In conclusion, JAK/STAT3 activation is required for bovine ICM formation and acquisition of naive pluripotency markers.

Cyclin E1 plays a key role in balancing between totipotency and differentiation in human embryonic cells

STUDY HYPOTHESIS

We aimed to investigate if Cyclin E1 (CCNE1) plays a role in human embryogenesis, in particular during the early developmental stages characterized by a short cell cycle.

STUDY FINDING

CCNE1 is expressed in plenipotent human embryonic cells and plays a critical role during hESC derivation via the naïve state and, potentially, normal embryo development.

WHAT IS KNOWN ALREADY

A short cell cycle due to a truncated G1 phase has been associated with the high developmental capacity of embryonic cells. CCNE1 is a critical G1/S transition regulator. CCNE1 overexpression can cause shortening of the cell cycle and it is constitutively expressed in mouse embryonic stem cells and cancer cells.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

We investigated expression of CCNE1 in human preimplantation embryo development and embryonic stem cells (hESC). Functional studies included CCNE1 overexpression in hESC and CCNE1 downregulation in the outgrowths formed by plated human blastocysts. Analysis was performed by immunocytochemistry and quantitative real-time PCR. Mann-Whitney statistical test was applied.

MAIN RESULTS AND THE ROLE OF CHANCE

The CCNE1 protein was ubiquitously and constitutively expressed in the plenipotent cells of the embryo from the 4-cell stage up to and including the full blastocyst. During blastocyst expansion, CCNE1 was downregulated in the trophectoderm (TE) cells. CCNE1 shortly co-localized with NANOG in the inner cell mass (ICM) of expanding blastocysts, mimicking the situation in naïve hESC. In the ICM of expanded blastocysts, which corresponds with primed hESC, CCNE1 defined a subpopulation of cells different from NANOG/POU5F1-expressing pluripotent epiblast (EPI) cells and GATA4/SOX17-expressing primitive endoderm (PrE) cells. This CCNE1-positive cell population was associated with visceral endoderm based on transthyretin expression and marked the third cell lineage within the ICM, besides EPI and PrE, which had never been described before. We also investigated the role of CCNE1 by plating expanded blastocysts for hESC derivation. As a result, all the cells including TE cells re-gained CCNE1 and, consequently, NANOG expression, resembling the phenotype of naïve hESC. The inhibition of CCNE1 expression with siRNA blocked proliferation and caused degeneration of those plated cells.

LIMITATIONS, REASONS FOR CAUTION

The study is based on a limited number of good-quality human embryos donated to research.

WIDER IMPLICATIONS OF THE FINDINGS

Our study sheds light on the processes underlying the high developmental potential of early human embryonic cells. The CCNE1-positive plenipotent cell type corresponds with a phenotype that enables early human embryos to recover after fragmentation, cryodamage or (single cell) biopsy on day 3 for preimplantation genetic diagnosis. Knowledge on the expression and function of genes responsible for this flexibility will help us to better understand the undifferentiated state in stem cell biology and might enable us to improve technologies in assisted reproduction.

LARGE SCALE DATA

NA STUDY FUNDING AND COMPETING INTERESTS: This research is supported by grants from the Fund for Scientific Research - Flanders (FWO-Vlaanderen), the Methusalem (METH) of the VUB and Scientific Research Fond Willy Gepts of UZ Brussel. There are no competing interests.

Defining the three cell lineages of the human blastocyst by single-cell RNA-seq

Here, we provide fundamental insights into early human development by single-cell RNA-sequencing of human and mouse preimplantation embryos. We elucidate conserved transcriptional programs along with those that are human specific. Importantly, we validate our RNA-sequencing findings at the protein level, which further reveals differences in human and mouse embryo gene expression. For example, we identify several genes exclusively expressed in the human pluripotent epiblast, including the transcription factor KLF17. Key components of the TGF-β signalling pathway, including NODAL, GDF3, TGFBR1/ALK5, LEFTY1, SMAD2, SMAD4 and TDGF1, are also enriched in the human epiblast. Intriguingly, inhibition of TGF-β signalling abrogates NANOG expression in human epiblast cells, consistent with a requirement for this pathway in pluripotency. Although the key trophectoderm factors Id2, Elf5 and Eomes are exclusively localized to this lineage in the mouse, the human orthologues are either absent or expressed in alternative lineages. Importantly, we also identify genes with conserved expression dynamics, including Foxa2/FOXA2, which we show is restricted to the primitive endoderm in both human and mouse embryos. Comparison of the human epiblast to existing embryonic stem cells (hESCs) reveals conservation of pluripotency but also additional pathways more enriched in hESCs. Our analysis highlights significant differences in human preimplantation development compared with mouse and provides a molecular blueprint to understand human embryogenesis and its relationship to stem cells.

Summary: Single-cell RNA-sequencing of human and mouse embryos reveals conserved and human-specific transcriptional programmes as well as a functional requirement for TGFβ signalling in human embryos.

Influence of Sex on Basal and Dickkopf-1 Regulated Gene Expression in the Bovine Morula

Sex affects function of the developing mammalian embryo as early as the preimplantation period. There were two goals of the current objective. The first was to determine the degree and nature of differences in gene expression between female and male embryos in the cow at the morula stage of development. The second objective was to determine whether DKK1, a molecule known to alter differentiation of the blastocyst, would affect gene expression differently for female and male morulae. In Experiment 1, female and male embryos were treated with DKK1 at Day 5 after insemination. Morulae were harvested 24 h after treatment, pooled in groups of 20 for microarray analysis and RNA subjected to analysis of gene expression by microarray hybridization. There were 662 differentially expressed genes between females and males and 128 of these genes had a fold change ≥ 1.5 between the two sexes. Of the genes upregulated in females, 49.5% were located in the X chromosome. Functional analysis predicted that cell survival was greater in female embryos. Experiment 2 involved a similar design except that transcripts for 12 genes previously reported to be affected by sex, DKK1 or the interaction were quantified by quantitative polymerase chain reaction. Expression of all genes tested that were affected by sex in experiment 1 was affected in a similar manner in Experiment 2. In contrast, effects of DKK1 on gene expression were largely not repeatable in Experiment 2. The exception was for the Hippo signaling gene AMOT, which was inhibited by DKK1. In Experiment 3, embryos produced by fertilization with unsorted sperm were treated with DKK1 at Day 5 and abundance of transcripts for CDX2, GATA6, and NANOG determined at Days 5, 6 and 7 after insemination. There was no effect of DKK1 on expression of any of the three genes. In conclusion, female and male bovine embryos have a different pattern of gene expression as early as the morula stage, and this is due to a large extent to expression of genes in the X chromosomes in females. Differential gene expression between female and male embryos is likely the basis for increased resistance to cell death signals in female embryos and disparity in responses of female and male embryos to changes in the maternal environment.

Cell lineage allocation in equine blastocysts produced in vitro under varying glucose concentrations

Equine embryos developin vitroin the presence of high glucose concentrations, but little is known about their requirements for development. We evaluated the effect of glucose concentrations in medium on blastocyst development after ICSI. In experiment 1, there were no significant differences in rates of blastocyst formation among embryos cultured in our standard medium (DMEM/F-12), which contained >16 mM glucose, and those cultured in a minimal-glucose embryo culture medium (<1 mM; Global medium, GB), with either 0 added glucose for the first 5 days, then 20 mM (0-20) or 20 mM for the entire culture period (20-20). In experiment 2, there were no significant differences in the rates of blastocyst development (31–46%) for embryos cultured in four glucose treatments in GB (0-10, 0-20, 5-10, or 5-20). Blastocysts were evaluated by immunofluorescence for lineage-specific markers. All cells stained positively forPOU5F1. An inner cluster of cells was identified that included presumptive primitive endoderm cells (GATA6-positive) and presumptive epiblast (EPI) cells. The 5-20 treatment resulted in a significantly lower number of presumptive EPI-lineage cells than the 0-20 treatment did.GATA6-positive cells appeared to be allocated to the primitive endoderm independent of the formation of an inner cell mass, as was previously hypothesized for equine embryos. These data demonstrate that equine blastocyst development is not dependent on high glucose concentrations during early culture; rather, environmental glucose may affect cell allocation. They also present the first analysis of cell lineage allocation inin vitro-fertilized equine blastocysts. These findings expand our understanding of the factors that affect embryo development in the horse.

Primitive endoderm differentiation: from specification to epithelium formation

In amniotes, primitive endoderm (PrE) plays important roles not only for nutrient support but also as an inductive tissue required for embryo patterning. PrE is an epithelial monolayer that is visible shortly before embryo implantation and is one of the first three cell lineages produced by the embryo. We review here the molecular mechanisms that have been uncovered during the past 10 years on PrE and epiblast cell lineage specification within the inner cell mass of the blastocyst and on their subsequent steps of differentiation.

The post-inner cell mass intermediate: implications for stem cell biology and assisted reproductive technology

BACKGROUND

Until recently, the temporal events that precede the generation of pluripotent embryonic stem cells (ESCs) and their equivalence with specific developmental stages in vivo was poorly understood. Our group has discovered the existence of a transient epiblast-like structure, coined the post-inner cell mass (ICM) intermediate or PICMI, that emerges before human ESC (hESCs) are established, which supports their primed nature (i.e. already showing some predispositions towards certain cell types) of pluripotency.

METHODS

The PICMI results from the progressive epithelialization of the ICM and it expresses a mixture of early and late epiblast markers, as well as some primordial germ cell markers. The PICMI is a closer progenitor of hESCs than the ICM and it can be seen as the first proof of why all existing hESCs, until recently, display a primed state of pluripotency.

RESULTS

Even though the pluripotent characteristics of ESCs differ from mouse (naïve) to human (primed), it has recently been shown in mice that a similar process of self-organization at the transition from ICM to (naïve) mouse ESCs (mESCs) transforms the amorphous ICM into a rosette of polarized epiblast cells, a mouse PICMI. The transient PICMI stage is therefore at the origin of both mESCs and hESCs. In addition, several groups have now reported the conversion from primed to the naïve (mESCs-like) hESCs, broadening the pluripotency spectrum and opening new opportunities for the use of pluripotent stem cells.

CONCLUSIONS

In this review, we discuss the recent discoveries of mouse and human transient states from ICM to ESCs and their relation towards the state of pluripotency in the eventual stem cells, being naïve or primed. We will now further investigate how these intermediate and/or different pluripotent stages may impact the use of human stem cells in regenerative medicine and assisted reproductive technology.

Actin Disorganization Plays a Vital Role in Impaired Embryonic Development of In Vitro-Produced Mouse Preimplantation Embryos

Assisted reproductive technology (ART) is being increasingly applied to overcome infertility. However, the in vitro production process, the main procedure of ART, can lead to aberrant embryonic development and health-related problems in offspring. Understanding the mechanisms underlying the ART-induced side effects is important to improve the ART process. In this study, we carried out comparative transcriptome profiling between in vivo- (IVO) and in vitro- produced (IVP) mouse blastocysts. Our results suggested that aberrant actin organization might be a major factor contributing to the impaired development of IVP embryos. To test this, we examined the effect of actin disorganization on the development of IVP preimplantation embryos. Specific disruption of actin organization by cytochalasin B (CB) indicated that well-organized actin is essential for in vitro embryonic development. Supplementing the culture medium with 10^–9 M melatonin, a cytoskeletal modulator in adult somatic cells, significantly reversed the disrupted expression patterns of genes related to actin organization, including Arhgef2, Bcl2, Coro2b, Flnc, and Palld. Immunofluorescence analysis showed that melatonin treatment of IVP embryos significantly improved the distribution and organization of actin filaments (F-actin) from the 8-cell stage onwards. More importantly, we found that melatonin alleviated the CB-mediated aberrant F-actin distribution and organization and rescued CB-induced impaired embryonic development. This is the first study to indicate that actin disorganization is implicated in impaired development of IVP embryos during the preimplantation stage. We also demonstrated that improving actin organization is a promising strategy to optimize existing IVP systems.

In vitro culture of stem-like cells derived from somatic cell nuclear transfer bovine embryos of the Korean beef cattle species, HanWoo

We established and maintained somatic cell nuclear transfer embryo-derived stem-like cells (SCNT-eSLCs) from the traditional Korean beef cattle species, HanWoo (Bos taurus coreanae). Each SCNT blastocyst was placed individually on a feeder layer with culture medium containing three inhibitors of differentiation (3i). Primary colonies formed after 2-3 days of culture and the intact colonies were passaged every 5-6 days. The cells in each colony showed embryonic stem cell-like morphologies with a distinct boundary and were positive to alkaline phosphatase staining. Immunofluorescence and reverse transcription-polymerase chain reaction analyses also confirmed that these colonies expressed pluripotent markers. The colonies were maintained over 50 passages for more than 270 days. The cells showed normal karyotypes consisting of 60 chromosomes at Passage 50. Embryoid bodies were formed by suspension culture to analyse in vitro differentiation capability. Marker genes representing the differentiation into three germ layers were expressed. Typical embryonal carcinoma was generated after injecting cells under the testis capsule of nude mice, suggesting that the cultured cells may also have the potential of in vivo differentiation. In conclusion, we generated eSLCs from SCNT bovine embryos, using a 3i system that sustained stemness, normal karyotype and pluripotency, which was confirmed by in vitro and in vivo differentiation.

A mRNA landscape of bovine embryos after standard and MAPK-inhibited culture conditions: a comparative analysis

Background

Genes and signalling pathways involved in pluripotency have been studied extensively in mouse and human pre-implantation embryos and embryonic stem (ES) cells. The unsuccessful attempts to generate ES cell lines from other species including cattle suggests that other genes and pathways are involved in maintaining pluripotency in these species. To investigate which genes are involved in bovine pluripotency, expression profiles were generated from morula, blastocyst, trophectoderm and inner cell mass (ICM) samples using microarray analysis. As MAPK inhibition can increase the NANOG/GATA6 ratio in the inner cell mass, additionally blastocysts were cultured in the presence of a MAPK inhibitor and changes in gene expression in the inner cell mass were analysed.

Results

Between morula and blastocyst 3,774 genes were differentially expressed and the largest differences were found in blastocyst up-regulated genes. Gene ontology (GO) analysis shows lipid metabolic process as the term most enriched with genes expressed at higher levels in blastocysts. Genes with higher expression levels in morulae were enriched in the RNA processing GO term. Of the 497 differentially expressed genes comparing ICM and TE, the expression of NANOG, SOX2 and POU5F1 was increased in the ICM confirming their evolutionary preserved role in pluripotency. Several genes implicated to be involved in differentiation or fate determination were also expressed at higher levels in the ICM. Genes expressed at higher levels in the ICM were enriched in the RNA splicing and regulation of gene expression GO term. Although NANOG expression was elevated upon MAPK inhibition, SOX2 and POU5F1 expression showed little increase. Expression of other genes in the MAPK pathway including DUSP4 and SPRY4, or influenced by MAPK inhibition such as IFNT, was down-regulated.

Conclusion

The data obtained from the microarray studies provide further insight in gene expression during bovine embryonic development. They show an expression profile in pluripotent cells that indicates a pluripotent, epiblast-like state. The inability to culture ICM cells as stem cells in the presence of an inhibitor of MAPK activity together with the reported data indicates that MAPK inhibition alone is not sufficient to maintain a pluripotent character in bovine cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1448-x) contains supplementary material, which is available to authorized users.

Mouse and human blastocyst-derived stem cells: vive les differences

Lessons learned from conserved vertebrate developmental pathways have catalyzed rapid advances in pluripotent stem cell differentiation towards therapeutically relevant cell types. The most highly conserved phases of development are associated with the early patterning of the body plan - the so-called phylotypic stage. Both prior to and after this stage there is much more divergence across species. Developmental differences between human and mouse at the blastocyst and early post-implantation stages might help explain the differences among the different stem cell lines derived from these embryos. A better understanding of these early stages of human development will aid our ability to generate and manipulate human stem cells and their derivatives.

Inhibition of transforming growth factor β signaling promotes epiblast formation in mouse embryos

Early lineage segregation in preimplantation embryos and maintenance of pluripotency in embryonic stem cells (ESCs) are both regulated by specific signaling pathways. Small molecules have been shown to modulate these signaling pathways. We examined the influence of several small molecules and growth factors on second-lineage segregation of the inner cell mass toward hypoblast and epiblast lineage during mouse embryonic preimplantation development. We found that the second-lineage segregation is influenced by activation or inhibition of the transforming growth factor (TGF)β pathway. Inhibition of the TGFβ pathway from the two-cell, four-cell, and morula stages onward up to the blastocyst stage significantly increased the epiblast cell proliferation. The epiblast formed in the embryos in which TGFβ signaling was inhibited was fully functional as demonstrated by the potential of these epiblast cells to give rise to pluripotent ESCs. Conversely, activating the TGFβ pathway reduced epiblast formation. Inhibition of the glycogen synthase kinase (GSK)3 pathway and activation of bone morphogenetic protein 4 signaling reduced the formation of both epiblast and hypoblast cells. Activation of the protein kinase A pathway and of the Janus kinase/signal transducer and activator of transcription 3 pathway did not influence the second-lineage segregation in mouse embryos. The simultaneous inhibition of three pathways--TGFβ, GSK3β, and the fibroblast growth factor (FGF)/extracellular signal-regulated kinases (Erk)--significantly enhanced the proliferation of epiblast cells than that caused by inhibition of either TGFβ pathway alone or by combined inhibition of the GSK3β and FGF/Erk pathways only.

Expression profile of FGF receptors in preimplantation ovine embryos and the effect of FGF2 and PD173074

Fibroblast growth factors (FGFs) and their receptors (FGFRs) are increasingly recognized as important regulators of embryo development in mammals. This study investigated the importance of FGF signaling during in vitro development of ovine embryo. The mRNAs of four FGFR subtypes were detected throughout preimplantation development of in vitro fertilized (IVF) embryos, peaked in abundance at the morula stage, and decreased significantly at the blastocyst stage. To gain insight into the role of these mRNAs in embryo development, IVF embryos were cultured in the presence of FGF2 (100 or 500 ng/ml: beginning from days 1 or 4 to 7) or PD173074 (1 µM: beginning from days 1 to 7) as usual treatments for activation or inhibition of FGFRs, respectively. FGF2-supplementation did not affect the percentage of embryos that developed to the blastocyst, blastocyst cell count and the proportion of cells allocated in inner cell mass (ICM) and trophectoderm (TE) compared to control (p > 0.05). Also, increasing the dosage or duration of FGF2 treatment did not significantly alter blastocyst yield or differential cell count (p > 0.05). PD173074-mediated inhibition of FGFRs did not significantly affect blastocyst yield (p > 0.05). Assessment of expression profiles of lineage-associated markers revealed that FGF2 (500 ng/ml) supplementation: (i) significantly increased expression of putative hypoblast marker (GATA4), (ii) significantly decreased expression of putative epiblast (EPI) marker (NANOG) and (iii) did not change TE markers (CDX2 and IFNT) and pluripotency makers (OCT4, SOX2 and REX1). In summary, FGF2-mediated activation of FGFRs may promote a switch in transcriptional profile of ovine ICM from EPI- to hypoblast-associated gene expression.

A close look at the mammalian blastocyst: epiblast and primitive endoderm formation

During early development, the mammalian embryo undergoes a series of profound changes that lead to the formation of two extraembryonic tissues--the trophectoderm and the primitive endoderm. These tissues encapsulate the pluripotent epiblast at the time of implantation. The current model proposes that the formation of these lineages results from two consecutive binary cell fate decisions. The first controls the formation of the trophectoderm and the inner cell mass, and the second controls the formation of the primitive endoderm and the epiblast within the inner cell mass. While early mammalian embryos develop with extensive plasticity, the embryonic pattern prior to implantation is remarkably reproducible. Here, we review the molecular mechanisms driving the cell fate decision between primitive endoderm and epiblast in the mouse embryo and integrate data from recent studies into the current model of the molecular network regulating the segregation between these lineages and their subsequent differentiation.

Induction of a human pluripotent state with distinct regulatory circuitry that resembles preimplantation epiblast

Human embryonic stem cells (hESCs) are derived from the inner cell mass of the blastocyst. Despite sharing the common property of pluripotency, hESCs are notably distinct from epiblast cells of the preimplantation blastocyst. Here we use a combination of three small-molecule inhibitors to sustain hESCs in a LIF signaling-dependent hESC state (3iL hESCs) with elevated expression of NANOG and epiblast-enriched genes such as KLF4, DPPA3, and TBX3. Genome-wide transcriptome analysis confirms that the expression signature of 3iL hESCs shares similarities with native preimplantation epiblast cells. We also show that 3iL hESCs have a distinct epigenetic landscape, characterized by derepression of preimplantation epiblast genes. Using genome-wide binding profiles of NANOG and OCT4, we identify enhancers that contribute to rewiring of the regulatory circuitry. In summary, our study identifies a distinct hESC state with defined regulatory circuitry that will facilitate future analysis of human preimplantation embryogenesis and pluripotency.

Increased MAP kinase inhibition enhances epiblast-specific gene expression in bovine blastocysts

Mammalian blastocysts comprise three distinct lineages, namely, trophoblast, hypoblast, and epiblast, which develop into fetal placenta, extraembryonic yolk sac, and embryo proper, respectively. Pluripotent embryonic stem cells, capable of forming all adult cell types, can only be derived from the epiblast. In mouse and rat, this process is promoted by the double inhibition (2i) of mitogen-activated protein kinase kinase (MAP2K), which antagonizes FGF signaling, and glycogen synthase kinase 3 (GSK3), which stimulates the WNT pathway. We investigated variations of the 2i treatment on lineage segregation and pluripotency-related gene expression in bovine blastocysts. In vitro-fertilized embryos were cultured either in the presence of inhibitors of GSK3 (3 μM CHIR) and MAP2K (0.4 vs. 10 μM PD0325901, designated 2i and 2i+, respectively) or in 2i/2i+ with FGFR inhibitor (0.1 μM PD173074, designated 3i [2i and PD173074] and 3i+ [2i+ and PD173074]). Compared with 2i, both 2i+ and 3i+ potentiated the improvement in blastocyst morphology. Using an automated platform for multiplexed digital mRNA profiling, we simultaneously counted transcripts of 76 candidate genes in bovine blastocysts treated with multiple kinase inhibitors. We show that 2i+ medium specifically increased FGF4 and NANOG while reducing PDGFRalpha and SOX17 levels. The shift from a hypoblast to an epiblast gene expression signature was confirmed by quantitative PCR. A wide range of functionally related genes, including candidates involved in DNA methylation, were not significantly changed. This well-defined 2i+ effect was not observed after pharmacologically inhibiting FGF receptor or related MAP kinases (p38, JNK, and ERK5). In summary, our data suggest that increased MAP2K inhibition exerts its pluripotency-promoting effects through as yet unidentified signals.

The WNT signaling antagonist Dickkopf-1 directs lineage commitment and promotes survival of the preimplantation embryo

Successful embryonic development is dependent on factors secreted by the reproductive tract. Dickkopf-1 (DKK1), an antagonist of the wingless-related mouse mammary tumor virus (WNT) signaling pathway, is one endometrial secretory protein potentially involved in maternal-embryo communication. The purpose of this study was to investigate the roles of DKK1 in embryo cell fate decisions and competence to establish pregnancy. Using in vitro-produced bovine embryos, we demonstrate that exposure of embryos to DKK1 during the period of morula to blastocyst transition (between d 5 and 8 of development) promotes the first 2 cell fate decisions leading to increased differentiation of cells toward the trophectoderm and hypoblast lineages compared with that for control embryos treated with vehicle. Moreover, treatment of embryos with DKK1 or colony-stimulating factor 2 (CSF2; an endometrial cytokine known to improve embryo development and pregnancy establishment) between d 5 and 7 of development improves embryo survival after transfer to recipients. Pregnancy success at d 32 of gestation was 27% for cows receiving control embryos treated with vehicle, 41% for cows receiving embryos treated with DKK1, and 39% for cows receiving embryos treated with CSF2. These novel findings represent the first evidence of a role for maternally derived WNT regulators during this period and could lead to improvements in assisted reproductive technologies.

GATA6 levels modulate primitive endoderm cell fate choice and timing in the mouse blastocyst

Cells of the inner cell mass (ICM) of the mouse blastocyst differentiate into the pluripotent epiblast or the primitive endoderm (PrE), marked by the transcription factors NANOG and GATA6, respectively. To investigate the mechanistic regulation of this process, we applied an unbiased, quantitative, single-cell-resolution image analysis pipeline to analyze embryos lacking or exhibiting reduced levels of GATA6. We find that Gata6 mutants exhibit a complete absence of PrE and demonstrate that GATA6 levels regulate the timing and speed of lineage commitment within the ICM. Furthermore, we show that GATA6 is necessary for PrE specification by FGF signaling and propose a model where interactions between NANOG, GATA6, and the FGF/ERK pathway determine ICM cell fate. This study provides a framework for quantitative analyses of mammalian embryos and establishes GATA6 as a nodal point in the gene regulatory network driving ICM lineage specification.

Specific epiblast loss and hypoblast impairment in cattle embryos sensitized to survival signalling by ubiquitous overexpression of the proapoptotic gene BAD

Early embryonic lethality is common, particularly in dairy cattle. We made cattle embryos more sensitive to environmental stressors by raising the threshold of embryo survival signaling required to overcome the deleterious effects of overexpressing the proapoptotic protein BAD. Two primary fibroblast cell lines expressing BAD and exhibiting increased sensitivity to stress-induced apoptosis were used to generate transgenic Day13/14 BAD embryos. Transgenic embryos were normal in terms of retrieval rates, average embryo length or expression levels of the trophectoderm marker ASCL2. However both lines of BAD-tg embryos lost the embryonic disc and thus the entire epiblast lineage at significantly greater frequencies than either co-transferrred IVP controls or LacZ-tg embryos. Embryos without epiblast still contained the second ICM-derived lineage, the hypopblast, albeit frequently in an impaired state, as shown by reduced expression of the hypoblast markers GATA4 and FIBRONECTIN. This indicates a gradient of sensitivity (epiblast > hypoblast > TE) to BAD overexpression. We postulate that the greater sensitivity of specifically the epiblast lineage that we have seen in our transgenic model, reflects an inherent greater susceptibility of this lineage to environmental stress and may underlie the epiblast-specific death seen in phantom pregnancies.

Establishment of bovine trophoblast stem-like cells from in vitro-produced blastocyst-stage embryos using two inhibitors

The trophoblast (TR) is the first to differentiate during mammalian embryogenesis and play a pivotal role in the development of the placenta. We used a dual inhibitor system (PD0325901 and CHIR99021) with mixed feeders to successfully obtain bovine trophoblast stem-like (bTS) cells, which were similar in phenotype to mouse trophoblast stem cells (TSCs). The bTS cells that were generated using this system continually proliferated, displayed a normal diploid karyotype, and had no signs of altered morphology or differentiation even after 150 passages. These cells exhibited alkaline phosphatase (AP) activity and expressed pluripotency markers, such as OCT4, NANOG, SOX2, SSEA-1, SSEA-4, TRA-1-60, and TRA-1-81, and TR lineage markers such as CDX2, as determined by both immunofluorescence and reverse transcription-polymerase chain reaction (RT-PCR). Additionally, these cells generated dome-like structures, formed teratomas when injected into NOD-SCID mice, and differentiated into placenta TR cells in vitro. The microarray analysis of bTS cells showed high expression levels of many TR markers, such as TEAD4, EOMES, GATA3, ETS2, TFAP2A, ELF5, SMARCA4 (BRG1), CDH3, MASH2, HSD17B1, CYP11A1, PPARG, ID2, GCM1, HAND1, TDK, PAG, IFN-τ, and THAP11. The expression of many pluripotency markers, such as OCT4, SOX2, NANOG, and GDF3, was lower in bTS cells compared with in vitro-produced blastocysts; however, compared with bovine fetal fibroblasts, the expression of these pluripotency markers was elevated in bTS cells. The DNA methylation status of the promoter regions of OCT4, NANOG, and SOX2 was investigated, which were significantly higher in bTS cells (OCT4 23.90%, NANOG 74.40%, and SOX2 8.50%) compared with blastocysts (OCT4 8.90%, NANOG 34.4%, and SOX2 3.80%). In contrast, two promoter regions of CDX2 were hypomethylated in bTS cells (13.80% and 3.90%) compared with blastocysts (18.80% and 9.10%). The TSC lines that were established in this study may be used either for basic research that is focused on peri-implantation and placenta development or as donor cells for transgenic animal production.

Cell cycle-dependent expression of Dub3, Nanog and the p160 family of nuclear receptor coactivators (NCoAs) in mouse embryonic stem cells

Pluripotency of embryonic stem cells (ESC) is tightly regulated by a network of transcription factors among which the estrogen-related receptor β (Esrrb). Esrrb contributes to the relaxation of the G1 to S-phase (G1/S) checkpoint in mouse ESCs by transcriptional control of the deubiquitylase Dub3 gene, contributing to Cdc25A persistence after DNA damage. We show that in mESCs, Dub3 gene expression is cell cycle regulated and is maximal prior G1/S transition. In addition, following UV-induced DNA damage in G1, Dub3 expression markedly increases in S-phase also suggesting a role in checkpoint recovery. Unexpectedly, we also observed cell cycle-regulation of Nanog expression, and not Oct4, reaching high levels prior to G1/S transition, finely mirroring Cyclin E1 fluctuations. Curiously, while Esrrb showed only limited cell-cycle oscillations, transcript levels of the p160 family of nuclear receptor coactivators (NCoAs) displayed strong cell cycle-dependent fluctuations. Since NCoAs function in concert with Esrrb in transcriptional activation, we focussed on NCoA1 whose levels specifically increase prior onset of Dub3 transcription. Using a reporter assay, we show that NCoA1 potentiates Esrrb-mediated transcription of Dub3 and we present evidence of protein interaction between the SRC1 splice variant NCoA1 and Esrrb. Finally, we show a differential developmental regulation of all members of the p160 family during neural conversion of mESCs. These findings suggest that in mouse ESCs, changes in the relative concentration of a coactivator at a given cell cycle phase, may contribute to modulation of the transcriptional activity of the core transcription factors of the pluripotent network and be implicated in cell fate decisions upon onset of differentiation.

A rapid and efficient 2D/3D nuclear segmentation method for analysis of early mouse embryo and stem cell image data

Segmentation is a fundamental problem that dominates the success of microscopic image analysis. In almost 25 years of cell detection software development, there is still no single piece of commercial software that works well in practice when applied to early mouse embryo or stem cell image data. To address this need, we developed MINS (modular interactive nuclear segmentation) as a MATLAB/C++-based segmentation tool tailored for counting cells and fluorescent intensity measurements of 2D and 3D image data. Our aim was to develop a tool that is accurate and efficient yet straightforward and user friendly. The MINS pipeline comprises three major cascaded modules: detection, segmentation, and cell position classification. An extensive evaluation of MINS on both 2D and 3D images, and comparison to related tools, reveals improvements in segmentation accuracy and usability. Thus, its accuracy and ease of use will allow MINS to be implemented for routine single-cell-level image analyses.

Current state of the opportunities for derivation of germ-like cells from pluripotent stem cells: are you a man, or a mouse?

The concept of pluripotency as a prerogative of cells of early mammal embryos and cultured embryonic stem cells (ESC) has been invalidated with the advent of induced pluripotent stem cells. Later, it became clear that the ability to generate all cell types of the adult organism is also a questionable aspect of pluripotency, as there are cell types, such as germ cells, which are difficult to produce from pluripotent stem cells. Recently it has been proposed that there are at least two different states of pluripotency; namely, the naïve, or ground state, and the primed state, which may differ radically in terms of timeline of existence, signalling mechanisms, cell properties, capacity for differentiation into different cell types, etc. Germ-like male and female rodent cells have been successfully produced in vitro from ESC and induced pluripotent stem cells. The attempts to derive primate primordial germ cells (PGC) and germ cells in vitro from pluripotent stem cells, however, still have a low success rate, especially with the female germline. The paper reviews the properties of rodent and primate ESC with regard to their capacity for differentiation in vitro to germ-like cells, outlining the possible caveats to derivation of PGC and germ cells from primate and human pluripotent cells.

Regulation of pluripotency of inner cell mass and growth and differentiation of trophectoderm of the bovine embryo by colony stimulating factor 2

Colony-stimulating factor 2 (CSF2) enhances competence of the bovine embryo to establish and maintain pregnancy after the embryo is transferred into a recipient. Mechanisms involved could include regulation of lineage commitment, growth, or differentiation of the inner cell mass (ICM) and trophectoderm (TE). Experiments were conducted to evaluate regulation by CSF2 of pluripotency of the ICM and differentiation and growth of the TE. Embryos were cultured with 10 ng/ml recombinant bovine CSF2 or a vehicle control from Days 5 to 7 or 6 to 8 postinsemination. CSF2 increased the number of putative zygotes that developed to blastocysts when the percent of embryos becoming blastocysts in the control group was low but decreased blastocyst yield when blastocyst development in controls was high. ICM isolated from blastocysts by lysing the trophectoderm using antibody and complement via immunosurgery were more likely to survive passage when cultured on mitomycin C-treated fetal fibroblasts if derived from blastocysts treated with CSF2 than if from control blastocysts. There was little effect of CSF2 on characteristics of TE outgrowths from blastocysts. The exception was a decrease in outgrowth size for embryos treated with CSF2 from Days 5 to 7 and an increase in expression of CDX2 when treatment was from Days 6 to 8. Expression of the receptor subunit gene CSF2RA increased from the zygote stage to the 9-16 cell stage before decreasing to the blastocyst stage. In contrast, CSF2RB was undetectable at all stages. In conclusion, CSF2 improves competence of the ICM to survive in a pluripotent state and alters TE outgrowths. Actions of CSF2 occur through a signaling pathway that is likely to be independent of CSF2RB.

The cell-cycle state of stem cells determines cell fate propensity

Self-renewal and differentiation of stem cells are fundamentally associated with cell-cycle progression to enable tissue specification, organ homeostasis, and potentially tumorigenesis. However, technical challenges have impaired the study of the molecular interactions coordinating cell fate choice and cell-cycle progression. Here, we bypass these limitations by using the FUCCI reporter system in human pluripotent stem cells and show that their capacity of differentiation varies during the progression of their cell cycle. These mechanisms are governed by the cell-cycle regulators cyclin D1–3 that control differentiation signals such as the TGF-β-Smad2/3 pathway. Conversely, cell-cycle manipulation using a small molecule directs differentiation of hPSCs and provides an approach to generate cell types with a clinical interest. Our results demonstrate that cell fate decisions are tightly associated with the cell-cycle machinery and reveal insights in the mechanisms synchronizing differentiation and proliferation in developing tissues.

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Cell fate decisions are cell-cycle dependent

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Control of endoderm versus neuroectoderm differentiation by cyclin D-CDK4/6

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Cyclin Ds control TGF-β-Smad2/3 transcriptional activity

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Differentiation of hPSCs can be directed by manipulation of the cell cycle

Temporal reduction of LATS kinases in the early preimplantation embryo prevents ICM lineage differentiation

Cellular localization of the Yes-associated protein (YAP) is dependent on large tumor suppressor (LATS) kinase activity and initiates lineage specification in the preimplantation embryo. We temporally reduced LATS activity to disrupt this early event, allowing its reactivation at later stages. This interference resulted in an irreversible lineage misspecification and aberrant polarization of the inner cell mass (ICM). Complementation experiments revealed that neither epiblast nor primitive endoderm can be established from these ICMs. We therefore conclude that precisely timed YAP localization in early morulae is essential to prevent trophectoderm marker expression in, and lineage specification of, the ICM.

Changes in sub-cellular localisation of trophoblast and inner cell mass specific transcription factors during bovine preimplantation development

Background

Preimplantation bovine development is emerging as an attractive experimental model, yet little is known about the mechanisms underlying trophoblast (TE)/inner cell mass (ICM) segregation in cattle. To gain an insight into these processes we have studied protein and mRNA distribution during the crucial stages of bovine development. Protein distribution of lineage specific markers OCT4, NANOG, CDX2 were analysed in 5-cell, 8–16 cell, morula and blastocyst stage embryos. ICM/TE mRNA levels were compared in hatched blastocysts and included: OCT4, NANOG, FN-1, KLF4, c-MYC, REX1, CDX2, KRT-18 and GATA6.

Results

At the mRNA level the observed distribution patterns agree with the mouse model. CDX2 and OCT4 proteins were first detected in 5-cell stage embryos. NANOG appeared at the morula stage and was located in the cytoplasm forming characteristic rings around the nuclei. Changes in sub-cellular localisation of OCT4, NANOG and CDX2 were noted from the 8–16 cell onwards. CDX2 initially co-localised with OCT4, but at the blastocyst stage a clear lineage segregation could be observed. Interestingly, we have observed in a small proportion of embryos (2%) that CDX2 immunolabelling overlapped with mitotic chromosomes.

Conclusions

Cell fate specification in cattle become evident earlier than presently anticipated – around the time of bovine embryonic genome activation. There is an intriguing possibility that for proper lineage determination certain transcription factors (such as CDX2) may need to occupy specific regions of chromatin prior to its activation in the interphase nucleus. Our observation suggests a possible role of CDX2 in the process of epigenetic regulation of embryonic cell fate.

Would the real human embryonic stem cell please stand up?

Embryonic stem cells (ESCs) are now classified into two types of pluripotency: "naïve" and "primed" based upon their differing characteristics. Conventional human ESCs have much more in common with mouse epiblast stem cells and are now deemed to be primed. Naïve human ESCs that resemble mouse ESCs have recently been generated from their primed counterpart by cellular reprogramming. Isolation of naïve hESCs from human embryos has proven to be difficult. Is the inability to capture naïve hESCs the result of suboptimal derivation conditions or because they are so transient they cannot be "captured" in vitro? Prevailing evidence surrounding this issue are inconclusive and require additional human embryo research. However, negative public opinion regarding human embryo research, may make this an uphill battle. The solution may come from cellular reprogramming.

Epigenetics of reprogramming to induced pluripotency

Reprogramming to induced pluripotent stem cells (iPSCs) proceeds in a stepwise manner with reprogramming factor binding, transcription, and chromatin states changing during transitions. Evidence is emerging that epigenetic priming events early in the process may be critical for pluripotency induction later. Chromatin and its regulators are important controllers of reprogramming, and reprogramming factor levels, stoichiometry, and extracellular conditions influence the outcome. The rapid progress in characterizing reprogramming is benefiting applications of iPSCs and is already enabling the rational design of novel reprogramming factor cocktails. However, recent studies have also uncovered an epigenetic instability of the X chromosome in human iPSCs that warrants careful consideration.

Regulatory microRNA network identification in bovine blastocyst development

Mammalian blastocyst formation is characterized by two lineage segregations resulting in the formation of the trophectoderm, the hypoblast, and the epiblast cell lineages. Cell fate determination during these early lineage segregations is associated with changes in the expression of specific transcription factors. In addition to the transcription factor-based control, it has become clear that also microRNAs (miRNAs) play an important role in the post-transcriptional regulation of pluripotency and differentiation. To elucidate the role of miRNAs in early lineage segregation, we compared the miRNA expression in early bovine blastocysts with the more advanced stage of hatched blastocysts. Reverse transcription-quantitative PCR-based miRNA expression profiling revealed eight upregulated miRNAs (miR-127, miR-130a, miR-155, miR-196a, miR-203, miR-28, miR-29c, and miR-376a) and four downregulated miRNAs (miR-135a, miR-218, miR-335, and miR-449b) in hatched blastocysts. Through an integrative analysis of matching miRNA and mRNA expression data, candidate miRNA-mRNA interaction pairs were prioritized for validation. Using an in vitro luciferase reporter assay, we confirmed a direct interaction between miR-218 and CDH2, miR-218 and NANOG, and miR-449b and NOTCH1. By interfering with the FGF signaling pathway, we found functional evidence that miR-218, mainly expressed in the inner cell mass, regulates the NANOG expression in the bovine blastocyst in response to FGF signaling. The results of this study expand our knowledge about the miRNA signature of the bovine blastocyst and of the interactions between miRNAs and cell fate regulating transcription factors.

Analysis of human embryos from zygote to blastocyst reveals distinct gene expression patterns relative to the mouse

Early mammalian embryogenesis is controlled by mechanisms governing the balance between pluripotency and differentiation. The expression of early lineage-specific genes can vary significantly between species, with implications for developmental control and stem cell derivation. However, the mechanisms involved in patterning the human embryo are still unclear. We analyzed the appearance and localization of lineage-specific transcription factors in staged preimplantation human embryos from the zygote until the blastocyst. We observed that the pluripotency-associated transcription factor OCT4 was initially expressed in 8-cell embryos at 3 days post-fertilization (dpf), and restricted to the inner cell mass (ICM) in 128-256 cell blastocysts (6dpf), approximately 2 days later than the mouse. The trophectoderm (TE)-associated transcription factor CDX2 was upregulated in 5dpf blastocysts and initially coincident with OCT4, indicating a lag in CDX2 initiation in the TE lineage, relative to the mouse. Once established, the TE expressed intracellular and cell-surface proteins cytokeratin-7 (CK7) and fibroblast growth factor receptor-1 (FGFR1), which are thought to be specific to post-implantation human trophoblast progenitor cells. The primitive endoderm (PE)-associated transcription factor SOX17 was initially heterogeneously expressed in the ICM where it co-localized with a sub-set of OCT4 expressing cells at 4-5dpf. SOX17 was progressively restricted to the PE adjacent to the blastocoel cavity together with the transcription factor GATA6 by 6dpf. We observed low levels of Laminin expression in the human PE, though this basement membrane component is thought to play an important role in mouse PE cell sorting, suggesting divergence in differentiation mechanisms between species. Additionally, while stem cell lines representing the three distinct cell types that comprise a mouse blastocyst have been established, the identity of cell types that emerge during early human embryonic stem cell derivation is unclear. We observed that derivation from plating intact human blastocysts resulted predominantly in the outgrowth of TE-like cells, which impairs human embryonic stem cell derivation. Altogether, our findings provide important insight into developmental patterning of preimplantation human embryos with potential consequences for stem cell derivation.

Early cell fate decisions in the mouse embryo

During mammalian preimplantation development, the fertilised egg gives rise to a group of pluripotent embryonic cells, the epiblast, and to the extraembryonic lineages that support the development of the foetus during subsequent phases of development. This preimplantation period not only accommodates the first cell fate decisions in a mammal's life but also the transition from a totipotent cell, the zygote, capable of producing any cell type in the animal, to cells with a restricted developmental potential. The cellular and molecular mechanisms governing the balance between developmental potential and lineage specification have intrigued developmental biologists for decades. The preimplantation mouse embryo offers an invaluable system to study cell differentiation as well as the emergence and maintenance of pluripotency in the embryo. Here we review the most recent findings on the mechanisms controlling these early cell fate decisions. The model that emerges from the current evidence indicates that cell differentiation in the preimplantation embryo depends on cellular interaction and intercellular communication. This strategy underlies the plasticity of the early mouse embryo and ensures the correct specification of the first mammalian cell lineages.

The expression of fibroblast growth factor receptors during early bovine conceptus development and pharmacological analysis of their actions on trophoblast growth in vitro

The overall aim of this work was to examine the expression profiles for fibroblast growth factor receptors (FGFRs) and describe their biological importance during bovine pre- and peri-implantation conceptus development. FGFR1 and FGFR2 mRNAs were detected at 1-, 2-, 8-cell, morula and blastocyst stages whereas FGFR3 and FGFR4 mRNAs were detected after the 8-cell stage but not earlier. The abundance of FGFR1, FGFR3, and FGFR4 mRNAs increased at the morula and blastocyst stages. Immunofluorescence microscopy detected FGFR2 and FGFR4 exclusively in trophoblast cells whereas FGFR1 and FGFR3 were detected in both trophoblast cells and inner cell mass in blastocysts. Neither transcripts for FGF10 nor its receptor (FGFR2b) were temporally related to interferon τ (IFNT) transcript profile during peri- and postimplantation bovine conceptus development. A series of studies used a chemical inhibitor of FGFR kinase function (PD173074) to examine FGFR activation requirements during bovine embryo development. Exposing embryos to the inhibitor (1 μM) beginning on day 5 post-fertilization did not alter the percentage of embryos that developed into blastocysts or blastocyst cell numbers. The inhibitor did not alter the abundance of CDX2 mRNA but decreased (P<0.05) the relative abundance of IFNT mRNA in blastocysts. Exposing blastocysts to the inhibitor from days 8 to 11 post-fertilization reduced (P<0.05) the percentage of blastocysts that formed outgrowths after transfer to Matrigel-coated plates. In conclusion, each FGFR was detected in bovine embryos, and FGFR activation is needed to maximize IFNT expression and permit outgrowth formation.

Why is it so difficult to derive pluripotent stem cells in domestic ungulates?

Pluripotent stem cells are the focus of an extremely active field of investigation that is bringing new light on our understanding of the mechanisms that control pluripotency and differentiation. Rodent and primates are the only species where true, or bona fide, pluripotent stem cells have been derived. The attempts to derive pluripotent stem cells from domestic ungulates have been going on for more than 20 years with little progress. Cell lines from these species present a series of limitations that have precluded their use for both basic and clinically oriented studies. However, in the last 3 years, some substantial progress have been made making the currently available ungulate pluripotent stem cells closest than ever before to their human and mouse counterpart. This result has been achieved through both conceptual and technical progress that will be illustrated and discussed in this review.

The transcriptional regulation of pluripotency

The defining features of embryonic stem cells (ESCs) are their self-renewing and pluripotent capacities. Indeed, the ability to give rise into all cell types within the organism not only allows ESCs to function as an ideal in vitro tool to study embryonic development, but also offers great therapeutic potential within the field of regenerative medicine. However, it is also this same remarkable developmental plasticity that makes the efficient control of ESC differentiation into the desired cell type very difficult. Therefore, in order to harness ESCs for clinical applications, a detailed understanding of the molecular and cellular mechanisms controlling ESC pluripotency and lineage commitment is necessary. In this respect, through a variety of transcriptomic approaches, ESC pluripotency has been found to be regulated by a system of ESC-associated transcription factors; and the external signalling environment also acts as a key factor in modulating the ESC transcriptome. Here in this review, we summarize our current understanding of the transcriptional regulatory network in ESCs, discuss how the control of various signalling pathways could influence pluripotency, and provide a future outlook of ESC research.

FGF4 is required for lineage restriction and salt-and-pepper distribution of primitive endoderm factors but not their initial expression in the mouse

The emergence of pluripotent epiblast (EPI) and primitive endoderm (PrE) lineages within the inner cell mass (ICM) of the mouse blastocyst involves initial co-expression of lineage-associated markers followed by mutual exclusion and salt-and-pepper distribution of lineage-biased cells. Precisely how EPI and PrE cell fate commitment occurs is not entirely clear; however, previous studies in mice have implicated FGF/ERK signaling in this process. Here, we investigated the phenotype resulting from zygotic and maternal/zygotic inactivation of Fgf4. Fgf4 heterozygous blastocysts exhibited increased numbers of NANOG-positive EPI cells and reduced numbers of GATA6-positive PrE cells, suggesting that FGF signaling is tightly regulated to ensure specification of the appropriate numbers of cells for each lineage. Although the size of the ICM was unaffected in Fgf4 null mutant embryos, it entirely lacked a PrE layer and exclusively comprised NANOG-expressing cells at the time of implantation. An initial period of widespread EPI and PrE marker co-expression was however established even in the absence of FGF4. Thus, Fgf4 mutant embryos initiated the PrE program but exhibited defects in its restriction phase, when lineage bias is acquired. Consistent with this, XEN cells could be derived from Fgf4 mutant embryos in which PrE had been restored and these cells appeared indistinguishable from wild-type cells. Sustained exogenous FGF failed to rescue the mutant phenotype. Instead, depending on concentration, we noted no effect or conversion of all ICM cells to GATA6-positive PrE. We propose that heterogeneities in the availability of FGF produce the salt-and-pepper distribution of lineage-biased cells.