Differential plasticity of epiblast and primitive endoderm precursors within the ICM of the early mouse embryo

Survival of the fattest

Experiments on the social amoeba Dictyostelium discoideum show that the origins of lineage bias in this system lie in the nutritional history of individual cells. Clues to the molecular basis for this process suggest similar forces may be at work in early mammalian development.

Concise review: Bone marrow meets blastocyst: lessons from an unlikely encounter

This article discusses the implications of the recent discovery that rat bone marrow-derived multipotent adult progenitor cells (rMAPCs), a cell type with broad somatic differentiation potential but of uncertain lineage identity, are similar to rat blastocyst-derived extraembryonic endoderm precursor (rXENP) cells, which appear to represent the committed extraembryonic endoderm precursor of the blastocyst. It was found that under rMAPC culture conditions, rXENP cells can be homogeneously cultured and similar cells, named rat hypoblast stem cells (rHypoSCs), can be derived from rat blastocysts more rapidly and directly. The detailed comparison of rHypoSCs, rXENP cells, and rMAPCs revealed highly similar gene expression profiles and developmental potentials. The significance of these findings for embryology, stem cell biology, and medicine is discussed. Specifically, the results assign a lineage identity to rMAPCs, indicate that rMAPCs originated by environmental reprogramming, and imply that HypoSCs, XENP cells, and MAPCs possess lineage plasticity. The MAPC-HypoSC relation also strengthens the consistency of rat and mouse embryology and consequently the idea that HypoSCs represent the committed extraembryonic endoderm precursor of the blastocyst. On this basis, it is argued that the direct comparison of HypoSCs (now available in pure form) with embryonic stem cells will be highly useful for the understanding of pluripotency and plasticity. Finally, the new findings suggest an explanation for an obscure observation on stem cell-induced transplantation tolerance. Thus, the HypoSC/XENP/MAPC phenotype provides a unique but broadly instructive model with which to study stem cell plasticity, reprogramming, and transplantation tolerance, all central themes in regenerative medicine.

Culture of mouse amniotic fluid-derived cells on irradiated STO feeders results in the generation of primitive endoderm cell lines capable of self-renewal in vitro

The cells present in amniotic fluid (AF) are currently used for prenatal diagnosis of fetal anomalies but are also a potential source of cells for cell therapy. To better characterize putative progenitor cell populations present in AF, we used culture conditions that support self-renewal to determine if these promoted the generation of stable cell lines from AF-derived cells (AFC). Cells isolated from E11.5 mouse were cultured on irradiated STO fibroblast feeder layers in human embryonic germ cell derivation conditions. The cultures grew multicellular epithelial colonies that could be repropagated from single cells. Reverse transcription semiquantitative polymerase chain reaction of established cell lines revealed that they belonged to the extraembryonic endoderm (ExEn) expressing high levels of Gata6, Gata4, Sox17, Foxa2 and Sox7 mRNA. Hierarchical clustering based on the whole transcriptome expression profile of the AFC lines (AFCL) shows significant correlation between transcription profiles of AFCL and blastocyst-derived XEN, an ExEn cell line. In vitro differentiation of AFCL results in the generation of cells expressing albumin and α-fetoprotein (AFP), while intramuscular injection of AFCL into immunodeficient mice produced AFP-positive tumors with primitive endodermal appearance. Hence, E11.5 mouse AF contains cells that efficiently produce XEN lines. These AF-derived XEN lines do not spontaneously differentiate into embryonic-type cells but are phenotypically stable and have the capacity for extensive expansion. The lack of requirement for reprogramming factors to turn AF-derived progenitor cells into stable cell lines capable of massive expansion together with the known ability of ExEn to contribute to embryonic tissue suggests that this cell type may be a candidate for banking for cell therapies.

Atypical protein kinase C couples cell sorting with primitive endoderm maturation in the mouse blastocyst

During mouse pre-implantation development, extra-embryonic primitive endoderm (PrE) and pluripotent epiblast precursors are specified in the inner cell mass (ICM) of the early blastocyst in a ‘salt and pepper’ manner, and are subsequently sorted into two distinct layers. Positional cues provided by the blastocyst cavity are thought to be instrumental for cell sorting; however, the sequence of events and the mechanisms that control this segregation remain unknown. Here, we show that atypical protein kinase C (aPKC), a protein associated with apicobasal polarity, is specifically enriched in PrE precursors in the ICM prior to cell sorting and prior to overt signs of cell polarisation. aPKC adopts a polarised localisation in PrE cells only after they reach the blastocyst cavity and form a mature epithelium, in a process that is dependent on FGF signalling. To assess the role of aPKC in PrE formation, we interfered with its activity using either chemical inhibition or RNAi knockdown. We show that inhibition of aPKC from the mid blastocyst stage not only prevents sorting of PrE precursors into a polarised monolayer but concomitantly affects the maturation of PrE precursors. Our results suggest that the processes of PrE and epiblast segregation, and cell fate progression are interdependent, and place aPKC as a central player in the segregation of epiblast and PrE progenitors in the mouse blastocyst.

Allocation of inner cells to epiblast vs primitive endoderm in the mouse embryo is biased but not determined by the round of asymmetric divisions (8→16- and 16→32-cells)

The epiblast (EPI) and the primitive endoderm (PE), which constitute foundations for the future embryo body and yolk sac, build respectively deep and surface layers of the inner cell mass (ICM) of the blastocyst. Before reaching their target localization within the ICM, the PE and EPI precursor cells, which display distinct lineage-specific markers, are intermingled randomly. Since the ICM cells are produced in two successive rounds of asymmetric divisions at the 8→16 (primary inner cells) and 16→32 cell stage (secondary inner cells) it has been suggested that the fate of inner cells (decision to become EPI or PE) may depend on the time of their origin. Our method of dual labeling of embryos allowed us to distinguish between primary and secondary inner cells contributing ultimately to ICM. Our results show that the presence of two generations of inner cells in the 32-cell stage embryo is the source of heterogeneity within the ICM. We found some bias concerning the level of Fgf4 and Fgfr2 expression between primary and secondary inner cells, resulting from the distinct number of cells expressing these genes. Analysis of experimental aggregates constructed using different ratios of inner cells surrounded by outer cells revealed that the fate of cells does not depend exclusively on the timing of their generation, but also on the number of cells generated in each wave of asymmetric division. Taking together, the observed regulatory mechanism adjusting the proportion of outer to inner cells within the embryo may be mediated by FGF signaling.

A molecular basis for developmental plasticity in early mammalian embryos

Early mammalian embryos exhibit remarkable plasticity, as highlighted by the ability of separated early blastomeres to produce a whole organism. Recent work in the mouse implicates a network of transcription factors in governing the establishment of the primary embryonic lineages. A combination of genetics and embryology has uncovered the organisation and function of the components of this network, revealing a gradual resolution from ubiquitous to lineage-specific expression through a combination of defined regulatory relationships, spatially organised signalling, and biases from mechanical inputs. Here, we summarise this information, link it to classical embryology and propose a molecular framework for the establishment and regulation of developmental plasticity.

Morphogenetic analysis of peri-implantation development

BACKGROUND

Although successful implantation is required for development in placental mammals, the molecular and morphogenetic events that define peri-implantation remain largely unexplored.

RESULTS

Here we present detailed morphological and immunohistochemical analysis of mouse embryos between embryonic day 3.75 and 5.25 of gestation, during the implantation process in vivo. We examined expression patterns of key transcription factors (Sox2, Oct4, Nanog, Cdx2, Gata6, Sox17, and Yy1) during pre- and postimplantation development. Additionally, we examined morphogenetic changes through analysis of ZO-1, Laminin, and E-Cadherin localization. The results presented reveal novel changes in gene expression and morphogenetic events during peri-implantation in utero. Here we show: (1) molecular and morphological changes in primitive endoderm cells as they transition from a salt and pepper distribution to a sheet covering the inner cell mass; (2) tissue-specific GATA6 levels; and (3) a striking pattern of SOX17 that is suggestive of a functional role either directing or permitting implantation at specific sites in the uterine epithelium.

CONCLUSIONS

A growing number of knockout mice display peri-implantation lethality, and the data presented herein identify key morphogenetic landmarks that can be used to characterize mutant phenotypes, as well as further our basic understanding of peri-implantation development.

Totipotent embryonic stem cells arise in ground-state culture conditions

Embryonic stem cells (ESCs) are derived from mammalian embryos during the transition from totipotency, when individual blastomeres can make all lineages, to pluripotency, when they are competent to make only embryonic lineages. ESCs maintained with inhibitors of MEK and GSK3 (2i) are thought to represent an embryonically restricted ground state. However, we observed heterogeneous expression of the extraembryonic endoderm marker Hex in 2i-cultured embryos, suggesting that 2i blocked development prior to epiblast commitment. Similarly, 2i ESC cultures were heterogeneous and contained a Hex-positive fraction primed to differentiate into trophoblast and extraembryonic endoderm. Single Hex-positive ESCs coexpressed epiblast and extraembryonic genes and contributed to all lineages in chimeras. The cytokine LIF, necessary for ESC self-renewal, supported the expansion of this population but did not directly support Nanog-positive epiblast-like ESCs. Thus, 2i and LIF support a totipotent state comparable to early embryonic cells that coexpress embryonic and extraembryonic determinants.

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Embryos cultured in 2i are blocked before extraembryonic lineage segregation

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ESCs cultured in 2i express extraembryonic markers heterogeneously

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Single 2i ESCs differentiate into both embryonic and extraembryonic lineages

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LIF promotes proliferation of extraembryonically primed ESC populations

Ectogenesis: what could be learned from novel in-vitro culture systems?

Early mammalian development consists of two distinct phases separated by the event of implantation. Whereas much has been discovered about developmental mechanisms prior to implantation, the inability to culture and follow in real time cell behaviour over the period of implantation means that the second phase has not been explored in as much detail. Recently, a novel in-vitro culture system was described that permits continuous culture and time-lapse observations through the peri- and early post-implantation stages. This system has already delivered detailed information on the cellular processes accompanying early morphogenesis and allowed direct connections to be established between events occurring at the two developmental phases. This review discusses the potential of this novel technology and its possible applications that could have not only impact on basic science but also practical implications for human medicine.

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.

Early cell lineage specification in a marsupial: a case for diverse mechanisms among mammals

Early cell lineage specification in eutherian mammals results in the formation of a pluripotent inner cell mass (ICM) and trophoblast. By contrast, marsupials have no ICM. Here, we present the first molecular analysis of mechanisms of early cell lineage specification in a marsupial, the tammar wallaby. There was no overt differential localisation of key lineage-specific transcription factors in cleavage and early unilaminar blastocyst stages. Pluriblast cells (equivalent to the ICM) became distinguishable from trophoblast cells by differential expression of POU5F1 and, to a greater extent, POU2, a paralogue of POU5F1. Unlike in the mouse, pluriblast-trophoblast differentiation coincided with a global nuclear-to-cytoplasmic transition of CDX2 localisation. Also unlike in the mouse, Hippo pathway factors YAP and WWTR1 showed mutually distinct localisation patterns that suggest non-redundant roles. NANOG and GATA6 were conserved as markers of epiblast and hypoblast, respectively, but some differences to the mouse were found in their mode of differentiation. Our results suggest that there is considerable evolutionary plasticity in the mechanisms regulating early lineage specification in mammals.

Anatomy of a blastocyst: cell behaviors driving cell fate choice and morphogenesis in the early mouse embryo

The preimplantation period of mouse early embryonic development is devoted to the specification of two extraembryonic tissues and their spatial segregation from the pluripotent epiblast. During this period two cell fate decisions are made while cells gradually lose their totipotency. The first fate decision involves the segregation of the extraembryonic trophectoderm (TE) lineage from the inner cell mass (ICM); the second occurs within the ICM and involves the segregation of the extraembryonic primitive endoderm (PrE) lineage from the pluripotent epiblast (EPI) lineage, which eventually gives rise to the embryo proper. Multiple determinants, such as differential cellular properties, signaling cues and the activity of transcriptional regulators, influence lineage choice in the early embryo. Here, we provide an overview of our current understanding of the mechanisms governing these cell fate decisions ensuring proper lineage allocation and segregation, while at the same time providing the embryo with an inherent flexibility to adjust when perturbed.

Extraembryonic endoderm cells as a model of endoderm development

In recent years the multipotent extraembryonic endoderm (XEN) stem cells have been the center of much attention. In vivo, XEN cells contribute to the formation of the extraembryonic endoderm, visceral and parietal endoderm and later on, the yolk sac. Recent data have shown that the distinction between embryonic and extraembryonic endoderm is not as strict as previously thought due to the integration, and not the displacement, of the visceral endoderm into the definitive embryonic endoderm. Therefore, cells from the extraembryonic endoderm also contribute to definitive endoderm. Many research groups focused on unraveling the potential and ability of XEN cells to both support differentiation and/or differentiate into endoderm-like tissues as an alternative to embryonic stem (ES) cells. Moreover, the conversion of ES to XEN cells, shown recently without genetic manipulations, uncovers significant and novel molecular mechanisms involved in extraembryonic endoderm and definitive endoderm development. XEN cell lines provide a unique model for an early mammalian lineage that complements the established ES and trophoblast stem cell lines. Through the study of essential genes and signaling requirements for XEN cells in vitro, insights will be gained about the developmental program of the extraembryonic and embryonic endodermal lineage in vivo. This review will provide an overview on the current literature focusing on XEN cells as a model for primitive endoderm and possibly definitive endoderm as well as the potential of using these cells for therapeutic applications.

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.

Intercellular interactions, position, and polarity in establishing blastocyst cell lineages and embryonic axes

The formation of the three lineages of the mouse blastocyst provides a powerful model system to study interactions among cell behavior, cell signaling, and lineage development. Hippo signaling differences between the inner and outer cells of the early cleavage stages, combined with establishment of a stably polarized outer epithelium, lead to the establishment of the inner cell mass and the trophectoderm, whereas FGF signaling differences among the individual cells of the ICM lead to gradual separation and segregation of the epiblast and primitive endoderm lineages. Events in the late blastocyst lead to the formation of a special subset of cells from the primitive endoderm that are key sources for the signals that establish the subsequent body axis. The slow pace of mouse early development, the ability to culture embryos over this time period, the increasing availability of live cell imaging tools, and the ability to modify gene expression at will are providing increasing insights into the cell biology of early cell fate decisions.

Computational multiscale modeling of embryo development

Recent advances in live imaging and genetics of mammalian development which integrate observations of biochemical activity, cell-cell signaling and mechanical interactions between cells pave the way for predictive mathematical multi-scale modeling. In early mammalian embryo development, two of the most critical events which lead to tissue patterning involve changes in gene expression as well as mechanical interactions between cells. We discuss the relevance of mathematical modeling of multi-cellular systems and in particular in simulating these patterns and describe some of the technical challenges one encounters. Many of these issues are not unique for the embryonic system but are shared by other multi-cellular modeling areas.

Stochastic processes in the development of pluripotency in vivo

The divergence of the pluripotent inner cell mass and extraembryonic trophectoderm from an apparently homogenous population of cells is a decisive event in mammalian preimplantation development. While three models have been proposed to explain early cellular differentiation in the mouse embryo, the initial cue generating asymmetry within the embryo remains elusive. Recently, unexpected heterogeneity in the expression of crucial transcription factors within the blastocyst has raised the intriguing possibility that a stochastic component is involved in lineage divergence. Unraveling the molecular dynamics and developmental function of the observed heterogeneity awaits further investigations at the single-cell level using quantitative live-imaging with appropriate reporter lines. The possible involvement of dynamic heterogeneity in the establishment, maintenance and resolution of pluripotency makes this topic highly relevant not only to developmental biology, but also to stem cell research and regenerative medicine. In this review, we discuss the possible involvement of stochastic processes in lineage divergence and the establishment of pluripotency in vivo, based on recent data from mouse embryology and stem cell research.

Cell lineage allocation within the inner cell mass of the mouse blastocyst

At the time of implantation, the early mouse embryo consists of three distinct cell lineages: the epiblast (EPI), primitive endoderm (PrE), and trophectoderm (TE). Here we will focus on the EPI and PrE cell lineages, which arise within the inner cell mass (ICM) of the blastocyst. Though still poorly understood, our current understanding of the mechanisms underlying this lineage allocation will be discussed. It was originally thought that lineage choice was strictly controlled by the position of a cell within the ICM. However, it is now believed that the EPI and PrE lineages are defined both by their position and by the expression of lineage-specific transcription factors. Interestingly, these lineage-specific transcription factors are initially co-expressed in early ICM cells, suggesting an initial multi-lineage priming state. Thereafter, lineage-specific transcription factors display a mutually exclusive salt-and-pepper distribution that reflects cell specification of the EPI or PrE fates. Later on, lineage segregation and likely commitment are completed with the sequestration of PrE cells to the surface of the ICM, which lies at the blastocyst cavity roof. We discuss recent advances that have focused on elucidating how the salt-and-pepper pattern is established and then resolved within the ICM, leading to the correct apposition of cell lineages in preparation for implantation.