Clonal analysis of early mammalian development

In vitro models of human hypoblast and mouse primitive endoderm

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

Perspectives in Genome-Editing Techniques for Livestock

Genome editing of farm animals has undeniable practical applications. It helps to improve production traits, enhances the economic value of livestock, and increases disease resistance. Gene-modified animals are also used for biomedical research and drug production and demonstrate the potential to be used as xenograft donors for humans. The recent discovery of site-specific nucleases that allow precision genome editing of a single-cell embryo (or embryonic stem cells) and the development of new embryological delivery manipulations have revolutionized the transgenesis field. These relatively new approaches have already proven to be efficient and reliable for genome engineering and have wide potential for use in agriculture. A number of advanced methodologies have been tested in laboratory models and might be considered for application in livestock animals. At the same time, these methods must meet the requirements of safety, efficiency and availability of their application for a wide range of farm animals. This review aims at covering a brief history of livestock animal genome engineering and outlines possible future directions to design optimal and cost-effective tools for transgenesis in farm species.

Derivation of Porcine Extra-Embryonic Endoderm Cell Lines Reveals Distinct Signaling Pathway and Multipotency States

The inner cell mass of the pre-implantation blastocyst consists of the epiblast and hypoblast from which embryonic stem cells (ESCs) and extra-embryonic endoderm (XEN) stem cells, respectively, can be derived. Importantly, each stem cell type retains the defining properties and lineage restriction of its in vivo tissue origin. We have developed a novel approach for deriving porcine XEN (pXEN) cells via culturing the blastocysts with a chemical cocktail culture system. The pXEN cells were positive for XEN markers, including Gata4, Gata6, Sox17, and Sall4, but not for pluripotent markers Oct4, Sox2, and Nanog. The pXEN cells also retained the ability to undergo visceral endoderm (VE) and parietal endoderm (PE) differentiation in vitro. The maintenance of pXEN required FGF/MEK+TGFβ signaling pathways. The pXEN cells showed a stable phenotype through more than 50 passages in culture and could be established repeatedly from blastocysts or converted from the naïve-like ESCs established in our lab. These cells provide a new tool for exploring the pathways of porcine embryo development and differentiation and providing further reference to the establishment of porcine ESCs with potency of germline chimerism and gamete development.

Capture of Mouse and Human Stem Cells with Features of Formative Pluripotency

Pluripotent cells emerge as a naive founder population in the blastocyst, acquire capacity for germline and soma formation, and then undergo lineage priming. Mouse embryonic stem cells (ESCs) and epiblast-derived stem cells (EpiSCs) represent the initial naive and final primed phases of pluripotency, respectively. Here, we investigate the intermediate formative stage. Using minimal exposure to specification cues, we derive stem cells from formative mouse epiblast. Unlike ESCs or EpiSCs, formative stem (FS) cells respond directly to germ cell induction. They colonize somatic tissues and germline in chimeras. Whole-transcriptome analyses show similarity to pre-gastrulation formative epiblast. Signal responsiveness and chromatin accessibility features reflect lineage capacitation. Furthermore, FS cells show distinct transcription factor dependencies, relying critically on Otx2. Finally, FS cell culture conditions applied to human naive cells or embryos support expansion of similar stem cells, consistent with a conserved staging post on the trajectory of mammalian pluripotency.

Inositol 1,4,5-trisphosphate receptors are essential for fetal-maternal connection and embryo viability

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are a family of intracellular Ca2+ release channels located on the ER membrane, which in mammals consist of 3 different subtypes (IP3R1, IP3R2, and IP3R3) encoded by 3 genes, Itpr1, Itpr2, and Itpr3, respectively. Studies utilizing genetic knockout mouse models have demonstrated that IP3Rs are essential for embryonic survival in a redundant manner. Deletion of both IP3R1 and IP3R2 has been shown to cause cardiovascular defects and embryonic lethality. However, it remains unknown which cell types account for the cardiovascular defects in IP3R1 and IP3R2 double knockout (DKO) mice. In this study, we generated conditional IP3R1 and IP3R2 knockout mouse models with both genes deleted in specific cardiovascular cell lineages. Our results revealed that deletion of IP3R1 and IP3R2 in cardiomyocytes by TnT-Cre, in endothelial / hematopoietic cells by Tie2-Cre and Flk1-Cre, or in early precursors of the cardiovascular lineages by Mesp1-Cre, resulted in no phenotypes. This demonstrated that deletion of both IP3R genes in cardiovascular cell lineages cannot account for the cardiovascular defects and embryonic lethality observed in DKO mice. We then revisited and performed more detailed phenotypic analysis in DKO embryos, and found that DKO embryos developed cardiovascular defects including reduced size of aortas, enlarged cardiac chambers, as well as growth retardation at embryonic day (E) 9.5, but in varied degrees of severity. Interestingly, we also observed allantoic-placental defects including reduced sizes of umbilical vessels and reduced depth of placental labyrinth in DKO embryos, which could occur independently from other phenotypes in DKO embryos even without obvious growth retardation. Furthermore, deletion of both IP3R1 and IP3R2 by the epiblast-specific Meox2-Cre, which targets all the fetal tissues and extraembryonic mesoderm but not extraembryonic trophoblast cells, also resulted in embryonic lethality and similar allantoic-placental defects. Taken together, our results demonstrated that IP3R1 and IP3R2 play an essential and redundant role in maintaining the integrity of fetal-maternal connection and embryonic viability.

Blastocyst Microinjection with Embryonic Stem Cells

The ability to delete the function of an endogenous gene in the mouse was made possible by the development of embryonic stem (ES) cells, pluripotent cells that retain the ability to develop into all tissues of a developing embryo. The ability to genetically modify these cells followed, allowing targeted mutation of ES cells in vitro and the deletion of specific gene function. However, regardless of the simplicity or complexity of the genetic modification, all ES cells require injection into host embryos to establish pregnancies and result in chimeric mice. Blastocysts are commonly used as the host embryos for this purpose, as it is relatively easy to inject cells into the blastocoel cavity of the developing embryo. This chapter describes the procedure for injection of ES cells into blastocyst stage embryos for the generation of knockout mice.

Building Principles for Constructing a Mammalian Blastocyst Embryo

The self-organisation of a fertilised egg to form a blastocyst structure, which consists of three distinct cell lineages (trophoblast, epiblast and hypoblast) arranged around an off-centre cavity, is unique to mammals. While the starting point (the zygote) and endpoint (the blastocyst) are similar in all mammals, the intervening events have diverged. This review examines and compares the descriptive and functional data surrounding embryonic gene activation, symmetry-breaking, first and second lineage establishment, and fate commitment in a wide range of mammalian orders. The exquisite detail known from mouse embryogenesis, embryonic stem cell studies and the wealth of recent single cell transcriptomic experiments are used to highlight the building principles underlying early mammalian embryonic development.

Cellular aspect ratio and cell division mechanics underlie the patterning of cell progeny in diverse mammalian epithelia

Cell division is essential to expand, shape, and replenish epithelia. In the adult small intestine, cells from a common progenitor intermix with other lineages, whereas cell progeny in many other epithelia form contiguous patches. The mechanisms that generate these distinct patterns of progeny are poorly understood. Using light sheet and confocal imaging of intestinal organoids, we show that lineages intersperse during cytokinesis, when elongated interphase cells insert between apically displaced daughters. Reducing the cellular aspect ratio to minimize the height difference between interphase and mitotic cells disrupts interspersion, producing contiguous patches. Cellular aspect ratio is similarly a key parameter for division-coupled interspersion in the early mouse embryo, suggesting that this physical mechanism for patterning progeny may pertain to many mammalian epithelia. Our results reveal that the process of cytokinesis in elongated mammalian epithelia allows lineages to intermix and that cellular aspect ratio is a critical modulator of the progeny pattern.

Neural stem cell heterogeneity in the mammalian forebrain

The brain was long considered an organ that underwent very little change after development. It is now well established that the mammalian central nervous system contains neural stem cells that generate progeny that are capable of making new neurons, astrocytes, and oligodendrocytes throughout life. The field has advanced rapidly as it strives to understand the basic biology of these precursor cells, and explore their potential to promote brain repair. The purpose of this review is to present current knowledge about the diversity of neural stem cells in vitro and in vivo, and highlight distinctions between neural stem cell populations, throughout development, and within the niche. A comprehensive understanding of neural stem cell heterogeneity will provide insights into the cellular and molecular regulation of neural development and lifelong neurogenesis, and will guide the development of novel strategies to promote regeneration and neural repair.

Developmentally regulated expression of mil-1 and mil-2, mouse interferon-induced transmembrane protein like genes, during formation and differentiation of primordial germ cells

In all multicellular organisms, germ cells originating from a fertilized egg have the highly specialized role of transmitting genetic information to the next generation. In many animal species, the establishment of the germ cell lineage is regulated by the maternally inherited germplasm. In mammals, however, germline determination is not based on the unequal distribution of maternal determinants. In the processes of mammalian germ cell formation and subsequent differentiation, the molecular basis of the acquisition of germ cell status is not well understood. Since migrating primordial germ cells (PGCs) are lineage-restricted to the germline, they have already acquired a germ cell specific fate distinct from that of pluri/multi-potent stem cells. However, there have been no molecules known to be expressed in migrating PGCs but not in the inner cell mass of blastocysts. Such molecules should be involved in early germ cell development, and they should make good markers for following the process of PGC formation. To identify such molecules, we performed a subtracted cDNA screening with migrating PGCs and blastocysts in mice, and isolated 11 clones preferentially expressed in PGCs. Here, we report the identification of two genes with similarity to human interferon-induced transmembrane protein (Ifitm) genes, and expression patterns of these genes in forming and in differentiating PGCs. During germ cell formation, mouse Ifitm like (mil)-1 was expressed in putative PGC ancestors in embryos at 6.5-7.5 days post coitum. In migrating PGCs, mil-1 expression was continuously observed and mil-2 expression was first detected during germ cell differentiation.

Resolving Heterogeneity: Fluorescence-Activated Cell Sorting of Dynamic Cell Populations from Feeder-Free Mouse Embryonic Stem Cell Culture

Embryonic stem cell (ESC) culture comprises a mixture of cells that are primed to differentiate into different lineages. In conditions where ESCs self-renew, these primed populations continuously interconvert and consequently show highly dynamic coordinated changes in their expression of different sets of pluripotency and differentiation markers. It has become increasingly apparent that this transcriptional heterogeneity is an important characteristic of ESC culture. By sorting for specific populations of ESCs it is possible to enrich for cells with a capacity to colonize the embryo proper or the extra-embryonic lineages such as the descendents of the primitive endoderm or trophoblast. Here, we describe a method of isolating specific sub-sets of ESCs from the pluripotent cells present in in vitro ESC culture using SSEA1 antibody staining in combination with reporter lines and fluorescence activated cell sorting (FACS).

Germline and Pluripotent Stem Cells

Epigenetic mechanisms play an essential role in the germline and imprinting cycle. Germ cells show extensive epigenetic programming in preparation for the generation of the totipotent state, which in turn leads to the establishment of pluripotent cells in blastocysts. The latter are the cells from which pluripotent embryonic stem cells are derived and maintained in culture. Following blastocyst implantation, postimplantation epiblast cells develop, which give rise to all somatic cells as well as primordial germ cells, the precursors of sperm and eggs. Pluripotent stem cells in culture can be induced to undergo differentiation into somatic cells and germ cells in culture. Understanding the natural cycles of epigenetic reprogramming that occur in the germline will allow the generation of better and more versatile stem cells for both therapeutic and research purposes.

Elf5 counteracts precocious trophoblast differentiation by maintaining Sox2 and 3 and inhibiting Hand1 expression

In mice the transcription factor Elf5 is necessary for correct trophoblast development. Upon knockdown of Elf5, TS cells display neither a decrease in proliferation nor an increase in cell death but rather an increased propensity to differentiate. Such cells rapidly lose Sox2 and 3 expression, while transiently upregulating the giant cell differentiation determinant gene Hand1. Other genes affected within 24h of Elf5 knock-down, many of which have not previously been implicated in trophoblast development, exhibited in vivo expression domains and in vitro expression responses consistent with Elf5 having a role in counteracting trophoblast differentiation. In an ES to TS differentiation assay using Cdx2 overexpression with Elf5 loss of function cell lines, it was shown that Elf5 is necessary to prevent terminal trophoblast differentiation. This data thus suggest that Elf5 is a gatekeeper for the TS to differentiated trophoblast transition thereby preventing the precocious differentiation of the undifferentiated extraembryonic ectoderm.

On the fate of primordial germ cells injected into early mouse embryos

Primordial germ cells (PGCs) are the founder cells of the germline. Via gametogenesis and fertilisation this lineage generates a new embryo in the next generation. PGCs are also the cell of origin of multilineage teratocarcinomas. In vitro, mouse PGCs can give rise to embryonic germ (EG) cells – pluripotent stem cells that can contribute to primary chimaeras when introduced into pre-implantation embryos. Thus, PGCs can give rise to pluripotent cells in the course of the developmental cycle, during teratocarcinogenesis and by in vitro culture. However, there is no evidence that PGCs can differentiate directly into somatic cell types. Furthermore, it is generally assumed that PGCs do not contribute to chimaeras following injection into the early mouse embryo. However, these data have never been formally published. Here, we present the primary data from the original PGC-injection experiments performed 40 years ago, alongside results from more recent studies in three separate laboratories. These results have informed and influenced current models of the relationship between pluripotency and the germline cycle. Current technologies allow further experiments to confirm and expand upon these findings and allow definitive conclusions as to the developmental potency of PGCs.

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Primordial germ cells (PGCs) are the founder cells of the germline.

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They express pluripotency factors and can form pluripotent cells in vivo and in vitro.

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To assess if they are pluripotent, PGCs have been injected into early mouse embryos.

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Here report that to date contribution to chimaeras has not been demonstrated.

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Further experiments are required to determine the developmental potency of PGCs.

Reprogramming DNA methylation in the mammalian life cycle: building and breaking epigenetic barriers

In mammalian development, epigenetic modifications, including DNA methylation patterns, play a crucial role in defining cell fate but also represent epigenetic barriers that restrict developmental potential. At two points in the life cycle, DNA methylation marks are reprogrammed on a global scale, concomitant with restoration of developmental potency. DNA methylation patterns are subsequently re-established with the commitment towards a distinct cell fate. This reprogramming of DNA methylation takes place firstly on fertilization in the zygote, and secondly in primordial germ cells (PGCs), which are the direct progenitors of sperm or oocyte. In each reprogramming window, a unique set of mechanisms regulates DNA methylation erasure and re-establishment. Recent advances have uncovered roles for the TET3 hydroxylase and passive demethylation, together with base excision repair (BER) and the elongator complex, in methylation erasure from the zygote. Deamination by AID, BER and passive demethylation have been implicated in reprogramming in PGCs, but the process in its entirety is still poorly understood. In this review, we discuss the dynamics of DNA methylation reprogramming in PGCs and the zygote, the mechanisms involved and the biological significance of these events. Advances in our understanding of such natural epigenetic reprogramming are beginning to aid enhancement of experimental reprogramming in which the role of potential mechanisms can be investigated in vitro. Conversely, insights into in vitro reprogramming techniques may aid our understanding of epigenetic reprogramming in the germline and supply important clues in reprogramming for therapies in regenerative 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.

Trophoblast differentiation of human embryonic stem cells

Molecular mechanisms regulating human trophoblast differentiation remain poorly understood due to difficulties in obtaining primary tissues from very early developmental stages in humans. Therefore, the use of human embryonic stem cells (hESCs) as a source for generating trophoblast tissues is of significant interest. Trophoblast-like cells have been obtained through treatment of hESCs with bone morphogenetic protein (BMP) or inhibitors of activin/nodal/transforming growth factor-β signaling, or through protocols involving formation of embryoid bodies (EBs); however, there is controversy over whether hESC-derived cells are indeed analogous to true trophoblasts found in vivo. In this review, we provide an overview of previously described efforts to obtain trophoblasts from hESCs. We also discuss the merits and limitations of hESCs as a source of trophoblast derivatives.

Conversion from mouse embryonic to extra-embryonic endoderm stem cells reveals distinct differentiation capacities of pluripotent stem cell states

The inner cell mass of the mouse pre-implantation blastocyst comprises epiblast progenitor and primitive endoderm cells of which cognate embryonic (mESCs) or extra-embryonic (XEN) stem cell lines can be derived. Importantly, each stem cell type retains the defining properties and lineage restriction of their in vivo tissue of origin. Recently, we demonstrated that XEN-like cells arise within mESC cultures. This raises the possibility that mESCs can generate self-renewing XEN cells without the requirement for gene manipulation. We have developed a novel approach to convert mESCs to XEN cells (cXEN) using growth factors. We confirm that the downregulation of the pluripotency transcription factor Nanog and the expression of primitive endoderm-associated genes Gata6, Gata4, Sox17 and Pdgfra are necessary for cXEN cell derivation. This approach highlights an important function for Fgf4 in cXEN cell derivation. Paracrine FGF signalling compensates for the loss of endogenous Fgf4, which is necessary to exit mESC self-renewal, but not for XEN cell maintenance. Our cXEN protocol also reveals that distinct pluripotent stem cells respond uniquely to differentiation promoting signals. cXEN cells can be derived from mESCs cultured with Erk and Gsk3 inhibitors (2i), and LIF, similar to conventional mESCs. However, we find that epiblast stem cells (EpiSCs) derived from the post-implantation embryo are refractory to cXEN cell establishment, consistent with the hypothesis that EpiSCs represent a pluripotent state distinct from mESCs. In all, these findings suggest that the potential of mESCs includes the capacity to give rise to both extra-embryonic and embryonic lineages.

A system to enrich for primitive streak-derivatives, definitive endoderm and mesoderm, from pluripotent cells in culture

Two lineages of endoderm develop during mammalian embryogenesis, the primitive endoderm in the pre-implantation blastocyst and the definitive endoderm at gastrulation. This complexity of endoderm cell populations is mirrored during pluripotent cell differentiation in vitro and has hindered the identification and purification of the definitive endoderm for use as a substrate for further differentiation. The aggregation and differentiation of early primitive ectoderm-like (EPL) cells, resulting in the formation of EPL-cell derived embryoid bodies (EPLEBs), is a model of gastrulation that progresses through the sequential formation of primitive streak-like intermediates to nascent mesoderm and more differentiated mesoderm populations. EPL cell-derived EBs have been further analysed for the formation of definitive endoderm by detailed morphological studies, gene expression and a protein uptake assay. In comparison to embryoid bodies derived from ES cells, which form primitive and definitive endoderm, the endoderm compartment of embryoid bodies formed from EPL cells was comprised almost exclusively of definitive endoderm. Definitive endoderm was defined as a population of squamous cells that expressed Sox17, CXCR4 and Trh, which formed without the prior formation of primitive endoderm and was unable to endocytose horseradish peroxidase from the medium. Definitive endoderm formed in EPLEBs provides a substrate for further differentiation into specific endoderm lineages; these lineages can be used as research tools for understanding the mechanisms controlling lineage establishment and the nature of the transient intermediates formed. The similarity between mouse EPL cells and human ES cells suggests EPLEBs can be used as a model system for the development of technologies to enrich for the formation of human ES cell-derived definitive endoderm in the future.

The hypoblast (visceral endoderm): an evo-devo perspective

When amniotes appeared during evolution, embryos freed themselves from intracellular nutrition; development slowed, the mid-blastula transition was lost and maternal components became less important for polarity. Extra-embryonic tissues emerged to provide nutrition and other innovations. One such tissue, the hypoblast (visceral endoderm in mouse), acquired a role in fixing the body plan: it controls epiblast cell movements leading to primitive streak formation, generating bilateral symmetry. It also transiently induces expression of pre-neural markers in the epiblast, which also contributes to delay streak formation. After gastrulation, the hypoblast might protect prospective forebrain cells from caudalizing signals. These functions separate mesendodermal and neuroectodermal domains by protecting cells against being caught up in the movements of gastrulation.

Mapping mouse hemangioblast maturation from headfold stages

The mouse posterior primitive streak at neural plate/headfold stages (NP/HF, ~7.5 dpc-8 dpc) represents an optimal window from which hemangioblasts can be isolated. We performed immunohistochemistry on this domain using established monoclonal antibodies for proteins that affect blood and endothelial fates. We demonstrate that HoxB4 and GATA1 are the first set of markers that segregate independently to endothelial or blood populations during NP/HF stages of mouse embryonic development. In a subset of cells, both proteins are co-expressed and immunoreactivities appear mutually excluded within nuclear spaces. We searched for this particular state at later sites of hematopoietic stem cell emergence, viz., the aorta-gonad-mesonephros (AGM) and the fetal liver at 10.5-11.5 dpc, and found that only a rare number of cells displayed this character. Based on this spatial-temporal argument, we propose that the earliest blood progenitors emerge either directly from the epiblast or through segregation within the allantoic core domain (ACD) through reduction of cell adhesion and pSmad1/5 nuclear signaling, followed by a stochastic decision toward a blood or endothelial fate that involves GATA1 and HoxB4, respectively. A third form in which binding distributions are balanced may represent a common condition shared by hemangioblasts and HSCs. We developed a heuristic model of hemangioblast maturation, in part, to be explicit about our assumptions.

Isolated mouse inner cell mass is unable to reconstruct trophectoderm

The ability of ICM to differentiate into TE is still a controversial issue. Many of authors have showed the reconstruction of TE from isolated ICMs. We showed that immunosurgical method is not 100% efficient and that the original TE cells very often remain on the surface of isolated ICMs. We also found that isolated ICM cells cultured in vitro do not express Cdx2, and that the TE is reconstituted from TE cells which have survived immunosurgery. This indicates that very soon after the formation of TE in the blastocyst, the cells of ICM lose the potency to differentiate into trophectoderm.

Functional heterogeneity of embryonic stem cells revealed through translational amplification of an early endodermal transcript

ES cells are defined as self-renewing, pluripotent cell lines derived from early embryos. Cultures of ES cells are also characterized by the expression of certain markers thought to represent the pluripotent state. However, despite the widespread expression of key markers such as Oct4 and the appearance of a characteristic undifferentiated morphology, functional ES cells may represent only a small fraction of the cultures grown under self-renewing conditions. Thus phenotypically "undifferentiated" cells may consist of a heterogeneous population of functionally distinct cell types. Here we use a transgenic allele designed to detect low level transcription in the primitive endoderm lineage as a tool to identify an immediate early endoderm-like ES cell state. This reporter employs a tandem array of internal ribosomal entry sites to drive translation of an enhanced Yellow Fluorescent Protein (Venus) from the transcript that normally encodes for the early endodermal marker Hex. Expression of this Venus transgene reports on single cells with low Hex transcript levels and reveals the existence of distinct populations of Oct4 positive undifferentiated ES cells. One of these cells types, characterized by both the expression of the Venus transgene and the ES cells marker SSEA-1 (V(+)S(+)), appears to represent an early step in primitive endoderm specification. We show that the fraction of cells present within this state is influenced by factors that both promote and suppress primitive endoderm differentiation, but conditions that support ES cell self-renewal prevent their progression into differentiation and support an equilibrium between this state and at least one other that resembles the Nanog positive inner cell mass of the mammalian blastocysts. Interestingly, while these subpopulations are equivalently and clonally interconvertible under self-renewing conditions, when induced to differentiate both in vivo and in vitro they exhibit different behaviours. Most strikingly when introduced back into morulae or blastocysts, the V(+)S(+) population is not effective at contributing to the epiblast and can contribute to the extra-embryonic visceral and parietal endoderm, while the V(-)S(+) population generates high contribution chimeras. Taken together our data support a model in which ES cell culture has trapped a set of interconvertible cell states reminiscent of the early stages in blastocyst differentiation that may exist only transiently in the early embryo.

Isolation of Oct4-expressing extraembryonic endoderm precursor cell lines

BACKGROUND

The extraembryonic endoderm (ExEn) defines the yolk sac, a set of membranes that provide essential support for mammalian embryos. Recent findings suggest that the committed ExEn precursor is present already in the embryonic Inner Cell Mass (ICM) as a group of cells that intermingles with the closely related epiblast precursor. All ICM cells contain Oct4, a key transcription factor that is first expressed at the morula stage. In vitro, the epiblast precursor is most closely represented by the well-characterized embryonic stem (ES) cell lines that maintain the expression of Oct4, but analogous ExEn precursor cell lines are not known and it is unclear if they would express Oct4.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report the isolation and characterization of permanently proliferating Oct4-expressing rat cell lines ("XEN-P cell lines"), which closely resemble the ExEn precursor. We isolated the XEN-P cell lines from blastocysts and characterized them by plating and gene expression assays as well as by injection into embryos. Like ES cells, the XEN-P cells express Oct4 and SSEA1 at high levels and their growth is stimulated by leukemia inhibitory factor, but instead of the epiblast determinant Nanog, they express the ExEn determinants Gata6 and Gata4. Further, they lack markers characteristic of the more differentiated primitive/visceral and parietal ExEn stages, but exclusively differentiate into these stages in vitro and contribute to them in vivo.

CONCLUSIONS/SIGNIFICANCE

Our findings (i) suggest strongly that the ExEn precursor is a self-renewable entity, (ii) indicate that active Oct4 gene expression (transcription plus translation) is part of its molecular identity, and (iii) provide an in vitro model of early ExEn differentiation.

Epigenetic restriction of embryonic cell lineage fate by methylation of Elf5

Mouse ES cells can differentiate into all three germ layers of the embryo but are generally excluded from the trophoblast lineage. Here we show that ES cells deficient in DNA methylation can differentiate efficiently into trophoblast derivatives. In a genome-wide screen we identify the transcription factor Elf5 as methylated and repressed in ES cells, and hypomethylated and expressed in TS and methylation-deficient ES cells. Elf5 creates a positive feedback loop with TS cell determinants Cdx2 and Eomes that is restricted to the trophoblast lineage by epigenetic regulation of Elf5. Importantly, the late-acting function of Elf5 allows initial plasticity and regulation in the early blastocyst. Thus, Elf5 acts downstream of initial lineage determination as a gatekeeper to reinforce commitment to the trophoblast lineage, or to abort this pathway in epiblast cells. This epigenetic restriction of cell lineage fate provides a molecular mechanism for Waddington’s concept of canalization of developmental pathways.

Cellular features of the extra-embryonic endoderm during elongation in the ovine conceptus

The extraembryonic endoderm of the elongating ovine conceptus was analyzed by scanning and transmission electron microscopy and by whole mount actin staining and immunofluorescence. Morphological and functional differences between the visceral endoderm (VE), the founding cell layer, and the parietal endoderm (PE) are presented. During the elongation process, the PE differentiated to fusiform multinucleated cells aligned parallel to the elongation axis of the conceptus, whereas the VE cells retained the aspect of typical epithelial cells. The multinucleated PE cells however, expressed cellular and nuclear markers typical of endodermal and polarized epithelial cells. The proteins of the extracellular matrix, laminin, and fibronectin, were specifically expressed in the PE. The presence of pairs of nuclei linked by mid-bodies positively stained with tubulin antibodies, indicated that the syncytial differentiation of the PE was due to karyokinesis which was not followed by cytokinesis rather than by cell fusion.

Pluripotency of embryonic stem cells

Embryonic stem (ES) cells derived from pre-implantation embryos have the potential to differentiate into any cell type derived from the three germ layers of ectoderm (epidermal tissues and nerves), mesoderm (muscle, bone, blood), and endoderm (liver, pancreas, gastrointestinal tract, lungs), including fetal and adult cells. Alone, these cells do not develop into a viable fetus or adult animal because they do not retain the potential to contribute to extraembryonic tissue, and in vitro, they lack spatial and temporal signaling cues essential to normal in vivo development. The basis of pluripotentiality resides in conserved regulatory networks composed of numerous transcription factors and multiple signaling cascades. Together, these regulatory networks maintain ES cells in a pluripotent and undifferentiated form; however, alterations in the stoichiometry of these signals promote differentiation. By taking advantage of this differentiation capacity in vitro, ES cells have clearly been shown to possess the potential to generate multipotent stem and progenitor cells capable of differentiating into a limited number of cell fates. These latter types of cells may prove to be therapeutically viable, but perhaps more importantly, the studies of these cells have led to a greater understanding of mammalian development.

Early Lineage Segregation between Epiblast and Primitive Endoderm in Mouse Blastocysts through the Grb2-MAPK Pathway

It has been thought that early inner cell mass (ICM) is a homogeneous population and that cell position in the ICM leads to the formation of two lineages, epiblast (EPI) and primitive endoderm (PE), by E4.5. Here, however, we show that the ICM at E3.5 is already heterogeneous. The EPI- and PE-specific transcription factors, Nanog and Gata6, were expressed in the ICM in a random "salt and pepper" pattern, as early as E3.5, in a mutually exclusive manner. Lineage tracing showed predominant lineage restriction of single ICM cells at E3.5 to either lineage. In embryos lacking Grb2 where no PE forms, Gata6 expression was lost and all ICM cells were Nanog positive. We propose a model in which the ICM develops as a mosaic of EPI and PE progenitors at E3.5, dependent on Grb2-Ras-MAP kinase signaling, followed by later segregation of the progenitors into the appropriate cell layers.

Lineage allocation and asymmetries in the early mouse embryo

The mouse blastocyst, at the time of implantation, has three distinct cell lineages: epiblast (EPI), trophoblast and primitive endoderm (PE). Interactions between these three lineages and their directional growth and migration are critical for establishing the initial asymmetries that result in anterior-posterior patterning of the embryo proper. We have re-investigated the timing of specification of the three lineages in relation to the differential allocation of progeny of the first two blastomeres to the embryonic versus abembryonic axis of the blastocyst. We find that the majority of cells of the inner cell mass (ICM) are specified to be EPI or PE by the mid 3.5 day blastocyst and that this is associated with localized expression of GATA-6 in the ICM. We propose a model for molecular specification of the blastocyst lineages in which a combination of cell division order, signal transduction differences between inner and outer cells and segregation of key transcription factors can produce a blastocyst in which all three lineages are normally set up in an ordered, lineage-dependent manner, but which can also reconstruct a blastocyst when division order or cell interactions are disturbed.

Mosaic analysis of small intestinal development using the spf(ash)-heterozygous female mouse

Mosaic analysis using the spf(ash)-heterozygous female mouse was performed to clarify the cell lineage and cell behavior during small intestinal development with special attention given to the villus and crypt formation. The spf(ash) mutation, located on the X-chromosome, causes ornithine transcarbamylase (OTC) deficiency, which leads to mosaic expression of this enzyme in the small intestine of the heterozygous female mouse. In the small intestine in heterozygous fetuses, very small patches, which were aggregates of OTC-positive cells or negative cells, with no definite orientation to the villus structures were observed. In the neonatal small intestine, the intervillus region (the presumptive crypts) was polyclonal, and the majority of crypts were comprised exclusively cells of either genotype in 2-week-old small intestine. These results suggest that extensive migration and cell mixing of small intestinal epithelial cells, which have no definite correlation with the villus formation, occur in fetal stages of development, and that the crypt morphogenesis commences after birth independently of the monoclonality of the epithelial cells. Our data with the mosaic mice also reconfirmed the monoclonality of the adult small intestinal crypts demonstrated in mouse aggregation chimeras.

Quantitative analysis of cell allocation during liver development, using the spf(ash)-heterozygous female mouse

Mosaicism of ornithine transcarbamylase (OTC) expression in hepatocytes was quantitatively analyzed during liver development of the spf(ash)-heterozygous female mouse. Because the mosaic patterns depend on cell migration and cell mixing, such analysis could give insights on the growth pattern or allocation pattern of hepatocytes during liver development. Complex mosaic patterns of OTC-positive and -negative hepatocytes were observed in sections of fetal and postnatal livers. Sizes of patches, which were aggregates of OTC-positive or -negative hepatocytes, increased during development. Patches were slender and comparatively simple in 15.5- and 17.5-day fetal and neonatal livers. Quantitative analysis of patch shapes demonstrated that undulation of patches was maximal at 7 postnatal days. Patches with nodular shapes also started to increase in number at this stage. Isolated patches in sections of fetal livers and postnatal livers three-dimensionally connected with one another. However, especially in fetal livers, in which OTC-positive patches were minor, due to the presence of abundant hemopoietic cells, isolated three-dimensional patches consisting of approximately 5 to 70 cells were often found. They were shaped like slender branching or zigzag-shaped cords, but no definite orientation such as portal-central was observed in them at any stage. These results suggest that hepatocytes contiguously allocate their daughter cells as zigzag-shaped or branching cords at younger stages. Some hepatocytes grow with nodular formation after 7 postnatal days. Migration and mixing of hepatocytes appear to be more extensive at fetal stages than in the adult liver. Immunohistochemical analysis of intercellular junction proteins (E-cadherin, connexins 26 and 32, occludin, and ZO-1) also revealed that their expression and distribution changed in hepatocytes during development, which may be correlated with the OTC mosaic patterns.

Biallelic expression of imprinted genes in the mouse germ line: implications for erasure, establishment, and mechanisms of genomic imprinting

Genomic imprinting in mammals determines parental-specific (monoallelic) expression of a relatively small number of genes during development. Imprinting must logically be imparted in the germ line, where inherited maternal and paternal imprinting is erased and new imprinting established according to the individual's sex. We have assessed the allele-specific expression of four imprinted genes, two of which exhibit maternal-specific (H19 and Igf2r) and two of which exhibit paternal-specific (Igf2 and Snrpn) monoallelic somatic expression, in the germ line of F1 hybrid mice utilizing quantitative RT-PCR single-nucleotide primer extension assays. The expression of each gene was biallelic in the female and male germ line from the time that migratory mitotic PGCs entered the embryonic genital ridge and throughout gametogenesis, except that H19 RNA was not detected late in gametogenesis. These findings demonstrate that inherited imprinting is erased, or not recognized, in germ cells by the time of genital ridge colonization; also that new imprinting may not be established until late in gametogenesis, or that it is incomplete or not recognized at this stage. Regardless of imprinting status, a generalized neutralization of imprinting is evident in the germ line, associated with the totipotent state of this unique cell lineage.

Quantitative analysis of protein synthesis in mouse embryos. II: Differentiation of endoderm, mesoderm, and ectoderm

The changes in protein synthesis that occur during differentiation of the primitive germ layers were examined by high-resolution, two-dimensional gel electrophoresis of proteins synthesized in 6.5 and 7.5 days postcoitum (d.p.c.) mouse embryos. For 6.5 d.p.c. embryos, protein synthesis patterns were compared between whole extraembryonic and embryonic regions and between embryonic visceral endoderm and embryonic ectoderm. For 7.5 d.p.c. embryos, comparisons were made between extraembryonic and embryonic regions and between isolated embryonic endoderm, mesoderm, and ectoderm. Each of the isolated 7.5 d.p.c. germ layers was divided into anterior and posterior fragments in order to evaluate possible regional differences in gene expression along the anterior-posterior axis. Comparisons of protein synthesis patterns revealed the greatest difference between isolated endoderm and ectoderm, indicating that by as early as 6.5 d.p.c. patterns of gene expression differ significantly between these tissues. The greatest similarities were found between ectoderm and whole embryonic regions and between endoderm and whole extraembryonic regions, which most likely reflects the overall cellular compositions of the embryonic and extraembryonic regions. Based on their patterns of synthesis, four groups of proteins were identified that were preferentially synthesized in either endoderm or ectoderm. These provide useful markers for studying differentiation in these tissues. One other protein, migrating at the position expected for vimentin, was synthesized at an elevated rate in isolated mesoderm. We also observed differences in rates of synthesis of alpha-tubulin and tropomyosin-5 indicative of potential differences in cytoskeletal composition among the germ layers beyond those previously described. The difference in overall protein synthesis patterns between anterior and posterior regions was greatest in the embryonic endoderm, indicating that differentiation along the anterior-posterior axis may be initiated sooner or may proceed more rapidly in the endoderm than in the other germ layers. These data provide the first quantitative evaluation of the degree to which differentiation of the three primitive germ layers affects protein synthesis patterns and reveal potentially useful markers of endoderm and ectoderm differentiation.

Anatomy and physiology of spiking local and intersegmental interneurons in the median neuroblast lineage of the grasshopper

The range of anatomical and physiological properties in the adult progeny of an identified neuroblast was investigated. Some 80-90 adult neurons constitute the dorsal unpaired median (DUM) group of the grasshopper metathoracic ganglion. Within the group are efferent, octopaminergic neurons with large cell bodies and overshooting action potentials. Our objective was to determine the properties of the neurons with small cell bodies that make up the majority of the clone, some 60-70 neurons, about which scant information was available. The small DUM neurons have cell body diameters of 10-20 microns and stain with antibodies to GABA (Thompson and Siegler, '89: Proc. Soc. Neurosci. 15:1296 (abstr.); Witten and Truman, '89: Proc. Soc. Neurosci. 15:365 (abstr.)). By employing intracellular electrophysiological and morphological techniques, we have established that the small DUM neurons are spiking interneurons, expressing passively conducted action potentials in the cell body. They fall into two basic classes: local interneurons with bilateral branches in the auditory neuropiles, and intersegmental interneurons with bilateral branches widespread in the methathoracic ganglion and axons traveling in both anterior connectives. The local interneurons typically respond to sound, whereas the intersegmental interneurons selectively respond to wind on the head or to generalized movements by the animal. Primary neurites of small and large DUM neurons enter the neuropil in a bundle, but the neurites of DUM interneurons are more posterior and have a separate trajectory from those of the efferent DUM neurons once in the ganglion core. A model is presented for the sequential development of efferent, local, and intersegmental DUM neurons from the median neuroblast.

Use of triple tissue blastocyst reconstitution to study the development of diploid parthenogenetic primitive ectoderm in combination with fertilization-derived trophectoderm and primitive endoderm

Diploid mouse conceptuses lacking a paternal genome can form morphologically normal but small fetuses of up to 25 somites, but they invariably fail to develop beyond mid-gestation. Such conceptuses differ from normal most notably in the poor development of extra-embryonic tissues which are largely of trophectodermal and primitive endodermal origin. However, it is not clear whether the demise of diploid parthenogenetic (P) or gynogenetic (G) conceptuses is attributable entirely to the defective development of these two tissues or whether differentiation of the primitive ectoderm, the precursor of the foetus, extra-embryonic mesoderm and amnion, is also impaired by the absence of a paternal genome. Therefore, a new blastocyst reconstitution technique was used which enabled primitive ectoderm from P blastocysts to be combined with primitive endoderm and trophectoderm from fertilization-derived (F) blastocysts. One third of the 'triple tissue' reconstituted blastocysts that implanted yielded foetuses. However, all foetuses recovered on the 11th or 12th day of gestation were small and, with one exception, either obviously retarded or arrested in development. The exception was a living 44 somite specimen which is the most advanced P foetus yet recorded. Foetuses were invariably degenerating in conceptuses recovered on the 13th day. In contrast, at least 16% of control reconstituted blastocysts with primitive ectoderm as well as primitive endoderm and trophectoderm of F origin developed normally on the 13th day of gestation or to term. Hence, the presence of a paternal genome seems to be essential for normal differentiation of all 3 primary tissues of the mouse blastocyst. The P foetuses that developed from reconstituted blastocysts were so closely invested by their membranes that they often showed abnormal flexure of the posterior region of the body. Several also showed a deficiency of allantoic tissue. Therefore, the possibility that the defect in development of P primitive ectoderms resided in their extra-embryonic tissues was investigated by analysing a series of chimaeras produced by injecting them into intact F blastocysts. The foregoing anomalies were not discernible even when P cells made a large contribution to the extra-embryonic mesoderm or amnion plus umbilical cord. Furthermore, selection against P cells was no greater in extra-embryonic derivatives of the primitive ectoderm than in the foetus itself.

Comparison of epidermal patch size in X-chromosome-linked mosaic and dizygotic chimeric mice

Mosaic animals can be made by aggregating embryonic tissues of distinguishable strains or they will occur spontaneously in eutherian mammals as a result of X-chromosome inactivation. Tissues of mosaic animals comprise aggregates of cells of similar lineage called 'patches'. The patch size of isolated epidermis from chimeras and X-chromosome-linked mosaics was compared in a quantitative fashion. Patch size was determined in the isolated epidermis of skin from aggregation chimeras between BALB/c and C3H/He strains of mice variant at the Gpi-1 locus and from the skin of X-chromosome-linked mosaic female BALB/c x C3H/He a mice heterozygous at the Pgk-1 locus. Patch size in this isolated tissue was not significantly different in these two types of mosaic animals. The results suggest that mechanisms in patch formation are primarily mechanical, dependent on cell division patterns.

A mouse embryonic stem cell line showing pluripotency of differentiation in early embryos and ubiquitous beta-galactosidase expression

For analysis of chimeric mice made by injecting embryonic stem (ES) cells into host blastocysts, it is very desirable if the ES cells have a good cell marker that can distinguish them from host cells. It is ideal if the marker can be easily visualized in every type of cell and tissue throughout the embryogenesis. We tried to produce such ES cell lines by introducing an E. coli beta-galactosidase (beta-gal) gene construct by electroporation. One of the transformant lines (MS1-EL4) showed beta-gal activity in every undifferentiated stem cell. After being induced to differentiate in vitro, cells with various morphologies showed beta-gal activity. We also detected beta-gal activity in a wide variety of tissue elements in solid tumors made by injecting the MS1-EL4 cells into syngeneic mice. Then we produced chimeric embryos by injecting the MS1-EL4 cells into blastocysts and recovering the embryos at various developmental stages. We found that the MS1-EL4 cells contributed to various tissues and expressed beta-gal activity, including not only descendants of the inner cell mass but also the trophectoderm-derived extraembryonic ectoderm.

Mosaic pattern and lineage analysis in chimeras

Mammalian chimeras have been used in a number of developmental studies over the years. A major limitation in these studies has been the lack of in situ procedures for establishing mosaic pattern in the tissues of these animals. Recently, a number of procedures have become available for the histochemical demonstration of mosaicism in chimeras. These include the elucidation of various enzymes, receptors, or surface antigens, which have variant expression between strains. The observation of pattern in organs of mosaic animals can suggest possible modes of organogenesis and organ maintenance. Experimentation with such animals can be used to establish some mechanisms of pathogenesis as well.

Effect of delayed implantation on differentiation of the extra-embryonic endoderm in the mouse blastocyst

Delayed implanting blastocysts recovered from both ovariectomized and lactating pregnant mice were examined for the presence of primitive endoderm. One or more of three different criteria were used to identify this tissue which was found to be present in nearly all such blastocysts, regardless of the length of time for which their implantation had been delayed. Furthermore, the tissue appeared to differentiate in blastocysts from ovariectomized females at approximately the same postcoital stage as in those from sham-operated controls. Formation of the parietal endoderm layer was not observed in a single instance during delay, but began approximately 10 h after it had been terminated by injecting ovariectomized females with oestradiol benzoate. Cells in mitosis were found both at longer intervals after the onset of implantation delay and at shorter intervals after its termination than reported previously. It is concluded that, contrary to what might have been anticipated from certain earlier studies, there seem to be no obvious advantages in using delayed implanting rather than nondelayed blastocysts for investigating initial steps in differentiation of the primitive endoderm. Delayed blastocysts may, nevertheless, be of value in elucidating factors controlling the differentiation or onset of migration of parietal endoderm cells.