The orderly allocation of mesodermal cells to the extraembryonic structures and the anteroposterior axis during gastrulation of the mouse embryo

Of lineage and legacy: the development of mammalian hematopoietic stem cells

The hematopoietic system is one of the first complex tissues to develop in the mammalian conceptus. Of particular interest in the field of developmental hematopoiesis is the origin of adult bone marrow hematopoietic stem cells. Tracing their origin is complicated because blood is a mobile tissue and because hematopoietic cells emerge from many embryonic sites. The origin of the adult mammalian blood system remains a topic of lively discussion and intense research. Interest is also focused on developmental signals that induce the adult hematopoietic stem cell program, as these may prove useful for generating and expanding these clinically important cell populations ex vivo. This review presents a historical overview of and the most recent data on the developmental origins of hematopoiesis.

Genetics of Wnt signaling during early mammalian development

Proper cell-cell communication is necessary to orchestrate the cell fate determination, proliferation, movement, and differentiation that occurs during the development of a complex, multicellular organism. Members of the Wnt family of secreted signaling molecules regulate these processes in virtually every embryonic tissue and during the homeostatic maintenance of adult tissues. Mammalian genetic studies have been particularly useful in illustrating the specific roles that Wnt signaling pathways play in embryonic development, and in the etiology of diseases such as cancer. This chapter will largely focus on the functional roles that Wnts, signaling through the Wnt/-catenin pathway, play during early mammalian development.

Quantitative proteomics analysis demonstrates post-transcriptional regulation of embryonic stem cell differentiation to hematopoiesis

Embryonic stem (ES) cells can differentiate in vitro to produce the endothelial and hematopoietic precursor, the hemangioblasts, which are derived from the mesoderm germ layer. Differentiation of Bry(GFP/+) ES cell to hemangioblasts can be followed by the expression of the Bry(GFP/+) and Flk1 genes. Proteomic and transcriptomic changes during this differentiation process were analyzed to identify mechanisms for phenotypic change during early differentiation. Three populations of differentiating Bry(GFP) ES cells were obtained by flow cytometric sorting, GFP-Flk1- (epiblast), GFP+Flk1- (mesoderm), and GFP+Flk1+ (hemangioblast). Microarray analyses and relative quantification two-dimensional LCLC-MS/MS on nuclear extracts were performed. We identified and quantified 2389 proteins, 1057 of which were associated to their microarray probe set. These included a variety of low abundance transcription factors, e.g. UTF1, Sox2, Oct4, and E2F4, demonstrating a high level of proteomic penetrance. When paired comparisons of changes in the mRNA and protein expression levels were performed low levels of correlation were found. A strong correlation between isobaric tag-derived relative quantification and Western blot analysis was found for a number of nuclear proteins. Pathway and ontology analysis identified proteins known to be involved in the regulation of stem cell differentiation, and proteins with no described function in early ES cell development were also shown to change markedly at the proteome level only. ES cell development is regulated at the mRNA and protein level.

Targeting a GFP reporter gene to the MIXL1 locus of human embryonic stem cells identifies human primitive streak-like cells and enables isolation of primitive hematopoietic precursors

Differentiating human embryonic stem cells (HESCs) represent an experimental platform for establishing the relationships between the earliest lineages that emerge during human development. Here we report the targeted insertion in HESCs of sequences encoding green fluorescent protein (GFP) into the locus of MIXL1, a gene transiently expressed in the primitive streak during embryogenesis.(1,2) GFP fluorescence in MIXL1(GFP/)(w) HESCs differentiated in the presence of BMP4 reported the expression of MIXL1, permitting the identification of viable human primitive streak-like cells. The use of GFP as a reporter for MIXL1 combined with cell surface staining for platelet-derived growth factor receptor alpha (PDGFRalpha) enabled the isolation of a cell population that was highly enriched in primitive hematopoietic precursors, the earliest derivatives of the primitive streak. These experiments demonstrate the utility of MIXL1(GFP/w) HESCs for analyzing the previously inaccessible events surrounding the development of human primitive streak-like cells and their subsequent commitment to hematopoiesis.

Multiple mesoderm subsets give rise to endothelial cells, whereas hematopoietic cells are differentiated only from a restricted subset in embryonic stem cell differentiation culture

In the developing mouse, vascular endothelial cell (EC) and hematopoietic cell (HPC) lineages are two initial cell lineages that diverge from mesodermal cells, which have been roughly subdivided into three subtypes according to their geographical location: the organizer, embryonic mesoderm in the primitive streak, and extraembryonic mesoderm during gastrulation. Although the initial progenitors that become the two lineages appear in both vascular endothelial growth factor receptor 2(+) (VEGFR2(+)) lateral and extraembryonic mesoderm, little is known about the underlying molecular events that regulate the derivation of ECs and HPCs. Here, we describe an experimental system consisting of two types of embryonic stem cell lines capable of distinguishing between organizer and the middle section of the primitive streak region. Using this system, we were able to establish a defined culture condition that can separately induce distinct types of mesoderm. Although we were able to differentiate ECs from all mesoderm subsets, however, the potential of HPCs was restricted to the VEGFR2(+) cells derived from primitive streak-type mesodermal cells. We also show that the culture condition for the progenitors of primitive erythrocytes is separated from that for the progenitors of definitive erythrocytes. These results suggest the dominant role of extrinsic regulation during diversification of mesoderm.

Regulation of cardiomyocyte differentiation of embryonic stem cells by extracellular signalling

Investigating the signalling pathways that regulate heart development is essential if stem cells are to become an effective source of cardiomyocytes that can be used for studying cardiac physiology and pharmacology and eventually developing cell-based therapies for heart repair. Here, we briefly describe current understanding of heart development in vertebrates and review the signalling pathways thought to be involved in cardiomyogenesis in multiple species. We discuss how this might be applied to stem cells currently thought to have cardiomyogenic potential by considering the factors relevant for each differentiation step from the undifferentiated cell to nascent mesoderm, cardiac progenitors and finally a fully determined cardiomyocyte. We focus particularly on how this is being applied to human embryonic stem cells and provide recent examples from both our own work and that of others.

The murine allantois: emerging paradigms in development of the mammalian umbilical cord and its relation to the fetus

The fertilized egg of the mammal gives rise to the embryo and its extraembryonic structures, all of which develop in intimate relation with each other. Yet, whilst the past several decades have witnessed a vast number of studies on the embryonic component of the conceptus, study of the extraembryonic tissues and their relation to the fetus have been largely ignored. The allantois, precursor tissue of the mature umbilical cord, is a universal feature of all placental mammals that establishes the vital vascular bridge between the fetus and its mother. The allantois differentiates into the umbilical blood vessels, which become secured onto the chorionic component of the placenta at one end and onto the fetus at the other. In this way, fetal blood is channeled through the umbilical cord for exchange with the mother. Despite the importance of this vascular bridge, little is known about how it is made. The aim of this review is to address current understanding of the biology of the allantois in the mouse and genetic control of its features and functions, and to highlight new paradigms concerning the developmental relationship between the fetus and its umbilical cord.

Ontogeny of the hematopoietic system

Blood cells are constantly produced in the bone marrow (BM) of adult mammals. This constant turnover ultimately depends on a rare population of progenitors that displays self-renewal and multilineage differentiation potential, the hematopoietic stem cells (HSCs). It is generally accepted that HSCs are generated during embryonic development and sequentially colonize the fetal liver, the spleen, and finally the BM. Here we discuss the experimental evidence that argues for the extrinsic origin of HSCs and the potential locations where HSC generation might occur. The identification of the cellular components playing a role in the generation process, in these precise locations, will be important in understanding the molecular mechanisms involved in HSC production from undifferentiated mesoderm.

Gene function in mouse embryogenesis: get set for gastrulation

During early mouse embryogenesis, temporal and spatial regulation of gene expression and cell signalling influences lineage specification, embryonic polarity, the patterning of tissue progenitors and the morphogenetic movement of cells and tissues. Uniquely in mammals, the extraembryonic tissues are the source of signals for lineage specification and tissue patterning. Here we discuss recent discoveries about the lead up to gastrulation, including early manifestations of asymmetry, coordination of cell and tissue movements and the interactions of transcription factors and signalling activity for lineage allocation and germ-layer specification.

Multipotent flk-1+ cardiovascular progenitor cells give rise to the cardiomyocyte, endothelial, and vascular smooth muscle lineages

Cell-tracing studies in the mouse indicate that the cardiac lineage arises from a population that expresses the vascular endothelial growth factor receptor 2 (VEGFR2, Flk-1), suggesting that it may develop from a progenitor with vascular potential. Using the embryonic stem (ES) cell differentiation model, we have identified a cardiovascular progenitor based on the temporal expression of the primitive streak (PS) marker brachyury and Flk-1. Comparable progenitors could also be isolated from head-fold stage embryos. When cultured with cytokines known to function during cardiogenesis, individual cardiovascular progenitors generated colonies that displayed cardiomyocyte, endothelial, and vascular smooth muscle (VSM) potential. Isolation and characterization of this previously unidentified population suggests that the mammalian cardiovascular system develops from multipotential progenitors.

Wnt and TGF-beta signaling are required for the induction of an in vitro model of primitive streak formation using embryonic stem cells

The establishment of the primitive streak and its derivative germ layers, mesoderm and endoderm, are prerequisite steps in the formation of many tissues. To model these developmental stages in vitro, an ES cell line was established that expresses CD4 from the foxa2 locus in addition to GFP from the brachyury locus. A GFP-Bry(+) population expressing variable levels of CD4-Foxa2 developed upon differentiation of this ES cell line. Analysis of gene-expression patterns and developmental potential revealed that the CD4-Foxa2(hi)GFP-Bry(+) population displays characteristics of the anterior primitive streak, whereas the CD4-Foxa2(lo)GFP-Bry(+) cells resemble the posterior streak. Using this model, we were able to demonstrate that Wnt and TGF-beta/nodal/activin signaling simultaneously were required for the generation of the CD4-Foxa2(+)GFP-Bry(+) population. Wnt or low levels of activin-induced a posterior primitive streak population, whereas high levels of activin resulted in an anterior streak fate. Finally, sustained activin signaling was found to stimulate endoderm commitment from the CD4-Foxa2(+)GFP-Bry(+) ES cell population. These findings demonstrate that the early developmental events involved in germ-layer induction in the embryo are recapitulated in the ES cell model and uncover insights into the signaling pathways involved in the establishment of mesoderm and endoderm.

Clonal Analysis of Mouse Development Reveals a Polyclonal Origin for Yolk Sac Blood Islands

Direct clonal analysis of tissue and organ maturation in vivo is a critical step in the interpretation of in vitro cell precursor-progeny relationships. We have developed a method to analyze clonal progenitor contributions in vivo using ES cells stably expressing separate fluorescent proteins and placed into normal blastocysts to form tetrachimeras. Here we applied this method to the analysis of embryonic yolk sac blood islands. In most vertebrates, yolk sac blood islands are the initial sites of appearance of hematopoietic and endothelial cells. It has been proposed that these lineages arise from a common clonal progenitor, the hemangioblast, but this hypothesis has not been tested directly in physiological development in vivo. Our analysis shows that each island has contributions from multiple progenitors. Moreover, contribution by individual hemangioblast progenitors to both endothelial and hematopoietic lineages within an island, if it happens at all, is an infrequent event.

A common progenitor for haematopoietic and endothelial lineages in the zebrafish gastrula

It has been proposed that haematopoietic and endothelial cells share a common progenitor, termed the haemangioblast. This idea was initially conceived as a result of the observation that these two cell types develop in close proximity to each other within the embryo. Support for this hypothesis was provided by studies on single-cell-derived colonies that can produce both haematopoietic and endothelial cells in vitro. Although these data point towards the existence of a common progenitor for these two lineages, the presence of a bipotential progenitor cell has yet to be demonstrated in vivo. Through the construction of single-cell-resolution fate maps of the zebrafish late blastula and gastrula, we demonstrate that individual cells can give rise to both haematopoietic and endothelial cells. These bipotential progenitors arise along the entire extent of the ventral mesoderm and contribute solely to haematopoietic and endothelial cells. We also find that only a subset of haematopoietic and endothelial cells arise from haemangioblasts. The endothelial descendants of the haemangioblasts all clustered in a specific region of the axial vessels regardless of the location of their progenitors. Our results provide in vivo evidence supporting the existence of the haemangioblast and reveal distinct features of this cell population.

Primitive erythropoiesis from mesodermal precursors expressing VE-cadherin, PECAM-1, Tie2, endoglin, and CD34 in the mouse embryo

Vascular endothelial (VE) cadherin, PECAM-1 (platelet endothelial cell adhesion molecule-1, CD31), Tie2, CD34, and endoglin are established markers for adult and embryonic endothelial cells (ECs). Here, we report that the expression of these EC markers is initiated in the extraembryonic region at the late-streak stage (nominal stage E6.75). Immunohistochemical analysis shows that EC marker-positive cells arise in a subset of Flk1 (VEGF-R2) mesodermal cells. In contrast, GATA1, a marker for primitive erythropoietic progenitors, is expressed in a more restricted subset of Flk1-positive cells. Using flow cytometry, we observed that the GATA1-positive cell population existed as a subset of the EC marker-positive cell. Consistent with this notion, we showed with the primitive hematopoietic colony assay that primitive erythropoietic progenitors are enriched in PECAM-1- and Tie2-positive cells. These results suggest that primitive hematopoietic cells arise from EC marker-positive cells. Thus, VE-cadherin, PECAM-1, CD34, endoglin, and Tie2 are expressed not only in adult and embryonic ECs but in extraembryonic Flk1-positive cells during gastrulation. The latter cell population includes progenitors that give rise to primitive hematopoietic cells, suggesting that primitive and definitive hematopoietic cells in the mouse embryo arise from EC marker-positive cells.

Transcriptional profiling of mouse and human ES cells identifies SLAIN1, a novel stem cell gene

We analyzed the transcriptional profiles of differentiating mouse embryonic stem cells (mESCs) and show that embryoid bodies (EBs) sequentially expressed genes associated with the epiblast, primitive streak, mesoderm and endoderm of the developing embryo, validating ESCs as a model system for identifying cohorts of genes marking specific stages of embryogenesis. By comparing the transcriptional profiles of undifferentiated ESCs to those of their differentiated progeny, we identified 503 mESC and 983 hESC genes selectively expressed in undifferentiated ES cells. Over 75% of the mESC genes were expressed in hESC and vice versa, attesting to the underlying similarity of mESCs and hESCs. The expression of a cohort of 68 genes decreased greater than 2-fold during differentiation in both mESCs and hESCs. As well as containing many validated ESC genes such as Oct4 [Pou5f1], Nanog and Nodal, this cohort included an uncharacterised gene (FLJ30046), which we designated SLAIN1/Slain1. Slain1 was expressed at the stem cell and epiblast stages of ESC differentiation and in the epiblast, nervous system, tailbud and somites of the developing mouse embryo. SLAIN1 and its more widely expressed homologue SLAIN2 comprise a new family of structurally unique genes conserved throughout vertebrate evolution.

Localization of Brachyury (T) in embryonic and extraembryonic tissues during mouse gastrulation

T-box gene family members have important roles during murine embryogenesis, gastrulation, and organogenesis. Although relatively little is known about how T-box genes are regulated, published gene expression studies have revealed dynamic and specific patterns in both embryonic and extraembryonic tissues of the mouse conceptus. Mutant alleles of the T-box gene Brachyury (T) have identified roles in formation of mesoderm and its derivatives, such as somites and the allantois. However, given the cell autonomous nature of T gene activity and conflicting results of gene expression studies, it has been difficult to attribute a primary function to T in normal allantoic development. We report localization of T protein by sectional immunohistochemistry in both embryonic and extraembryonic tissues during mouse gastrulation, emphasizing T localization within the allantois. T was detected in all previously reported sites within the conceptus, including the primitive streak and its derivatives, nascent embryonic mesoderm, the node and notochord, as well as notochord-associated endoderm and posterior neurectoderm. In addition, we have clarified T within the allantois, where it was first detected in the proximal midline of the late allantoic bud (approximately 7.5 days postcoitum, dpc) and persisted within an expanded midline domain until 6-somite pairs (s; approximately 8.5 dpc). Lastly, we have discovered several novel T sites, including the developing heart, visceral endoderm, extraembryonic ectoderm, and its derivative, chorionic ectoderm. Together, these data provide a unified picture of T in the mammalian conceptus, and demonstrate T's presence in unrelated cell types and tissues in highly dynamic spatiotemporal patterns in both embryonic and extraembryonic tissues.

Amniotic Fluid and Placental Stem Cells

Human amniotic fluid has been used in prenatal diagnosis for more than 70 years. It has proven to be a safe, reliable, and simple screening tool for a wide variety of developmental and genetic diseases. However, there is now evidence that amniotic fluid may have more use than only as a diagnostic tool and may be the source of a powerful therapy for a multitude of congenital and adult disorders. A subset of cells found in amniotic fluid and placenta has been isolated and found to be capable of maintaining prolonged undifferentiated proliferation as well as able to differentiate into multiple tissue types encompassing the three germ layers. It is possible that in the near future, we will see the development of therapies using progenitor cells isolated from amniotic fluid and placenta for the treatment of newborns with congenital malformations as well as of adults, using cryopreserved amniotic fluid and placental stem cells. In this chapter, we describe a number of experiments that have isolated and characterized pluripotent progenitor cells from amniotic fluid and placenta. We also discuss various cell lines derived from amniotic fluid and placenta and future directions for this area of research.

Mesenchymal cells

Human embryonic stem cells (hESC) provide a potentially unlimited source of specialized cell types for regenerative medicine. Nonetheless, one of the key requirements used to fulfill this potential is the ability to direct the differentiation of hESC to selective fates in vitro. Studies have reported the development of culture strategies to derive multipotent mesenchymal precursors from hESCs in vitro. This chapter reviews the techniques that allow the selective derivation of such precursors and their differentiation toward various mesenchymal cell types. It also discusses current limitations and future perspectives on the use of hESC-derived mesenchymal tissues.

Core binding factor in the early avian embryo: cloning of Cbfβ and combinatorial expression patterns with Runx1

We have isolated the avian ortholog for CBFbeta, the common non-DNA binding subunit of the core binding factor (CBF) that has important regulatory roles in major developmental pathways. CBFbeta forms heterodimers with the DNA-binding Runx proteins and increases their affinity for DNA and their protein stability. Here, we describe the Cbfbeta expression pattern during the first 4 days of chick embryo development, with a special interest in the developing hematopoietic system. We have compared its expression pattern to that of Runx1, which is crucial for the generation of definitive hematopoietic cells, and to other hematopoietic- or endothelial-specific markers (c-Myb, Pu.1, CD45, c-Ets-1 and VE-Cadherin). Initially, Cbfbeta is widely expressed in the early mesoderm in both the yolk sac and the embryo proper, but later its expression becomes restricted to specific organs or cell types. We have found that Cbfbeta expression overlaps with Runx1 in the hematopoietic system and neural tube. The somitic and mesonephric structures, however, express Cbfbeta in the absence of detectable Runx1. Finally, Cbfbeta and Runx1 display multiple combinatorial patterns in the endoderm and in specific nerves or ganglia. Taken together, we show that Cbfbeta exhibits a dynamic expression pattern that varies according to the organ, cell type or developmental stage. By revealing multiple combinatorial patterns between Cbfbeta and Runx1, these data provide new insights into the role of CBF during early development.

Hematopoiesis in the yolk sac: more than meets the eye

The first blood cells observed in the embryo are large nucleated erythroblasts generated in blood islands of the extraembryonic yolk sac. These unique red cells have been termed primitive because of their resemblance to nucleated erythroblasts of nonmammalian species. It is now widely assumed that hematopoiesis in the yolk sac is "primitive" and that "definitive" hematopoiesis has its origins in the aorta/gonad/mesonephros (AGM) region. Recent studies of yolk sac hematopoiesis have challenged several aspects of this paradigm. First, primitive erythropoiesis in mammals shares many features with definitive erythropoiesis, including progressive erythroblast maturation leading to the circulation of enucleated erythrocytes. Second, the emergence of primitive erythroid progenitors in the yolk sac prior to somitogenesis may be associated with the macrophage and megakaryocyte lineages, raising the possibility that "primitive" hematopoiesis may be multilineage in nature. Third, a second wave of hematopoietic progenitors emerge from the yolk sac during early somitogenesis that consists of multiple myeloid lineages that are temporally and spatially associated with definitive erythroid progenitors. These "definitive" hematopoietic progenitors expand in numbers in the yolk sac and are thought to seed the fetal liver and generate the first definitive blood cells that rapidly emerge from the liver. Recent findings support a model of hematopoietic ontogeny in which the conceptus' first maturing blood cells and committed progenitors are provided by the yolk sac, allowing survival until AGM-derived hematopoietic stem cells can emerge, seed the liver and differentiate into mature blood cells.

The embryonic origins of hematopoietic stem cells: a tale of hemangioblast and hemogenic endothelium

The developmental origin of hematopoietic stem cells has been for decades the subject of great interest. Once thought to emerge from the yolk sac, hematopoietic stem cells have now been shown to originate from the embryonic aorta. Increasing evidence suggests that hematopoietic stem cells are produced from an endothelial intermediate designated by the authors as hemangioblast or hemogenic endothelium. Recently, the allantois in the avian embryo and the placenta in the mouse embryo were shown to be a site of hematopoietic cell production/expansion and thus appear to play a critical role in the formation of the hematopoietic system. In this review we shall give an overview of the data obtained from human, mouse and avian models on the cellular origins of the hematopoietic system and discuss some aspects of the molecular mechanisms controlling hematopoietic cell production.

Deletion of the selection cassette, but not cis-acting elements, in targeted Flk1-lacZ allele reveals Flk1 expression in multipotent mesodermal progenitors

Flk1, the gene encoding the vascular endothelial growth factor receptor 2 (VEGFR-2), is a well-known marker for vascular and hematopoietic progenitors and is indispensable for normal hematopoiesis and vasculogenesis. Here we show that Flk1 expression in the early mouse embryo marks a broad spectrum of mesodermal progenitors exiting the primitive streak as well as later mesodermal cell types including some cardiomyocytes, portions of the somites, and all extraembryonic mesoderm cells. These findings made use of an Flk1-lacZ knock-in allele in which the neomycin selection cassette was removed, which resulted in full replication of the endogenous expression of Flk1. Targeted deletion of a region in intron 1 that has been proposed to direct endothelial expression produced no alteration in either endothelial or broader mesodermal expression of the Flk1-lacZ allele. Examination of lacZ expression in homozygotes for the Flk1lacZ neo-out allele revealed that lacZ-expressing mesodermal cells persisted in nonvascular regions. Thus, Flk1 expression marks progenitors with broad mesodermal potential but is not absolutely required for the development of all mesodermal lineages in which it is expressed.

Blood island formation: longstanding observations and modern interpretations

The yolk sac blood islands have long been recognized as the first site for blood cell emergence during embryonic development. Much of our understanding of how blood islands form is derived from studies conducted in chick embryos. Similar processes are thought to be active during murine blood island development. But, how strong is the evidence that the process of blood island formation in the chick coincides with that documented in the mouse? This review will discuss some of the principle studies that led to two widely accepted definitions of blood islands and will examine more recent studies performed in murine embryos to determine whether either of these popular definitions can be directly applied.

From hemangioblast to hematopoietic stem cell: an endothelial connection?

The developmental origin of hematopoietic stem cells has been the subject of much research. Now that the developmental link between the hematopoietic system and the vasculature has been well established, questions remain regarding the precise cellular origin of definitive hematopoietic cells and at what point they branch off from the endothelial lineage. Do they emerge directly from a hemangioblast-type cell, similar to what is proposed for primitive yolk sac hematopoiesis, or are they generated via an endothelial intermediate, the hemogenic endothelium? In this review, we will give an overview of the data obtained from the mouse and avian models on the cellular origins of the hematopoietic system.

Early patterning of the mouse embryo: Implications for hematopoietic commitment and differentiation

Prior to and during gastrulation, reciprocal interactions between embryonic and extraembryonic lineages are crucial for the correct patterning of the embryo. Several lines of investigation have underscored the importance of extraembryonic ectoderm and primitive endodermal in establishing the anterior-posterior axis of the embryo. Signals from these tissues help to position the primitive streak, from which mesoderm will emerge, within the epiblast (embryo proper). Molecules secreted by the visceral endoderm are required for activation of hematopoietic and endothelial cell development, but the pathways involved and their target tissue (e.g., posterior epiblast versus extraembryonic mesoderm) remain obscure. Recent evidence suggests that commitment of mesodermal progenitors to the hematopoietic and endothelial lineages begins earlier than previously anticipated, within or shortly after these cells emerge from the primitive streak.

Germ layer induction from embryonic stem cells

Embryonic stem (ES) cells have the potential to develop into all cell types of the adult body. This capability provides the basis for considering the ES cell system as a novel and unlimited source of cells for replacement therapies for the treatment of a wide range of diseases. Before the cell-based therapy potential of ES cells can be realized, a better understanding of the pathways regulating lineage-specific differentiation is required. Current studies suggest that the bone morphogenic protein, transforming growth factor-beta, Wnt, and fibroblast growth factor pathways that are required for gastrulation and germ layer induction in the embryo are also essential for differentiation of ES cells in culture. The current understanding of how these factors influence germ layer induction in both the embryo and in the ES cell differentiation system is addressed in this review.

Embryonic stem cell differentiation: emergence of a new era in biology and medicine

The discovery of mouse embryonic stem (ES) cells >20 years ago represented a major advance in biology and experimental medicine, as it enabled the routine manipulation of the mouse genome. Along with the capacity to induce genetic modifications, ES cells provided the basis for establishing an in vitro model of early mammalian development and represented a putative new source of differentiated cell types for cell replacement therapy. While ES cells have been used extensively for creating mouse mutants for more than a decade, their application as a model for developmental biology has been limited and their use in cell replacement therapy remains a goal for many in the field. Recent advances in our understanding of ES cell differentiation, detailed in this review, have provided new insights essential for establishing ES cell-based developmental models and for the generation of clinically relevant populations for cell therapy.

Distinct populations of endoderm cells converge to generate the embryonic liver bud and ventral foregut tissues

The location and movement of mammalian gut tissue progenitors, prior to the expression of tissue-specific genes, has been unknown, but this knowledge is essential to identify transitions that lead to cell type specification. To address this, we used vital dyes to label exposed anterior endoderm cells of early somite stage mouse embryos, cultured the embryos into the tissue bud phase of development, and determined the tissue fate of the dye labeled cells. This approach was performed at three embryonic stages that are prior to, or coincident with, foregut tissue patterning (1-3 somites, 4-6 somites, and 7-10 somites). Short-term labeling experiments tracked the movement of tissue progenitor cells during foregut closure. Surprisingly, we found that two distinct types of endoderm-progenitor cells, lateral and medial, arising from three spatially separated embryonic domains, converge to generate the epithelial cells of the liver bud. Whereas the lateral endoderm-progenitors give rise to descendants that are constrained in tissue fate and position along the anterior-posterior axis of the gut, the medial gut endoderm-progenitors give rise to descendants that stream along the anterior-posterior axis at the ventral midline and contribute to multiple gut tissues. The fate map reveals extensive morphogenetic movement of progenitors prior to tissue specification, it permits a detailed analysis of endoderm tissue patterning, and it illustrates that diverse progenitor domains can give rise to individual tissue cell types.

Tracking mesoderm formation and specification to the hemangioblast in vitro

During development of the mouse embryo, blood cells are generated from mesodermal precursors at specific times and locations. Using various in vivo and in vitro systems, we are now starting to understand the cascade of molecular events leading to the commitment of mesoderm and the formation of the first blood precursors, the hemangioblast. The in vitro differentiation of embryonic stem (ES) cells has proved to be an invaluable model to study lineage commitment, because one can access and easily manipulate large quantities of early progenitor cells. To help us track mesodermal subpopulations and study their specification toward blood lineages, we have engineered an ES cell line that expresses the green fluorescence protein exclusively in the mesoderm germ layer, under the control of the Brachyury regulatory sequences.

Endothelial progenitor cells: a source for therapeutic vasculogenesis?

Angiogenesis has been defined as sprouting of blood vessels from pre-existing vascular structures. Risau and co-workers defined the term vasculogenesis while studying the formation of new blood vessels in embryoid bodies. This process is characterized by the recruitment of endothelial progenitor cells (EPC) to sites of new vessel formation with subsequent differentiation of EPC into mature endothelial cells, extensively proliferating in situ. Data from recent years provided evidence that EPC also exist in the adult and contribute to new vessel formation, a process called post-natal vasculogenesis. The existence of EPC has been convincingly shown in both, animals and humans. They represent a perfect cellular progenitor cell population for the ex vivo generation of EC, which in turn serve as cellular source for therapeutic vasculogenesis or tumor targeting. This review provides an overview on this hot topic of cellular-based therapeutic concepts and the therapeutic potential of ex vivo generated EPC.

Vascular endothelial growth factor A (VEGF-A) is involved in guidance of VEGF receptor-positive cells to the anterior portion of early embryos

The hemangioblast in the mesoderm gives rise to both angioblasts and hematopoietic stem cells. The movement of hemangioblast precursor cells in the fetal trunk is a critical event in early embryogenesis. Vascular endothelial growth factor (VEGF) signaling is likely involved in this migration given the partial disturbance of VEGF receptor (VEGFR)-positive cell accumulation and migration in VEGFR2 null mice or mice with a truncated VEGFR1. However, it is not clear how the VEGF system regulates this migration or its direction. We show here that the expression of VEGF-A is dominant in the anterior portion of the embryo, whereas VEGFR1 and VEGFR2 are expressed in the posterior portion of the embryo. An inhibitor of VEGFR kinase blocked the migration of VEGFR-positive cells in a whole-embryo culture system. In addition, VEGFR-positive cells migrated toward a VEGFR1- or VEGFR2-specific ligand in vitro. Furthermore, VEGFR-positive cells derived from wild-type or VEGFR2(+/-) mice moved rapidly anteriorly, whereas cells derived from VEGFR2(+/-) mice carrying a truncated VEGFR1 [VEGFR1(TM-TK)(-/-)] migrated little when injected into wild-type mice. These results suggest that the VEGF-A protein concentrated in the anterior region plays an important role in the guidance of VEGFR-positive cells from the posterior portion to the head region by interacting with VEGFR in the mouse embryo.

Haemangioblast commitment is initiated in the primitive streak of the mouse embryo

Haematopoietic and vascular cells are thought to arise from a common progenitor called the haemangioblast. Support for this concept has been provided by embryonic stem (ES) cell differentiation studies that identified the blast colony-forming cell (BL-CFC), a progenitor with both haematopoietic and vascular potential. Using conditions that support the growth of BL-CFCs, we identify comparable progenitors that can form blast cell colonies (displaying haematopoietic and vascular potential) in gastrulating mouse embryos. Cell mixing and limiting dilution analyses provide evidence that these colonies are clonal, indicating that they develop from a progenitor with haemangioblast potential. Embryo-derived haemangioblasts are first detected at the mid-streak stage of gastrulation and peak in number during the neural plate stage. Analysis of embryos carrying complementary DNA of the green fluorescent protein targeted to the brachyury locus demonstrates that the haemangioblast is a subpopulation of mesoderm that co-expresses brachyury (also known as T) and Flk-1 (also known as Kdr). Detailed mapping studies reveal that haemangioblasts are found at highest frequency in the posterior region of the primitive streak, indicating that initial stages of haematopoietic and vascular commitment occur before blood island development in the yolk sac.

A requirement for Notch1 distinguishes 2 phases of definitive hematopoiesis during development

Notch1 is known to play a critical role in regulating fates in numerous cell types, including those of the hematopoietic lineage. Multiple defects exhibited by Notch1-deficient embryos confound the determination of Notch1 function in early hematopoietic development in vivo. To overcome this limitation, we examined the developmental potential of Notch1(-/-) embryonic stem (ES) cells by in vitro differentiation and by in vivo chimera analysis. Notch1 was found to affect primitive erythropoiesis differentially during ES cell differentiation and in vivo, and this result reflected an important difference in the regulation of Notch1 expression during ES cell differentiation relative to the developing mouse embryo. Notch1 was dispensable for the onset of definitive hematopoiesis both in vitro and in vivo in that Notch1(-/-) definitive progenitors could be detected in differentiating ES cells as well as in the yolk sac and early fetal liver of chimeric mice. Despite the fact that Notch1(-/-) cells can give rise to multiple types of definitive progenitors in early development, Notch1(-/-) cells failed to contribute to long-term definitive hematopoiesis past the early fetal liver stage in the context of a wild-type environment in chimeric mice. Thus, Notch1 is required, in a cell-autonomous manner, for the establishment of long-term, definitive hematopoietic stem cells (HSCs).

Gastrula organiser and embryonic patterning in the mouse

Embryonic patterning of the mouse during gastrulation and early organogenesis engenders the specification of anterior versus posterior structures and body laterality by the interaction of signalling and modulating activities. A group of cells in the mouse gastrula, characterised by the expression of a repertoire of "organiser" genes, acts as a source and the conduit for allocation of the axial mesoderm, floor plate and definitive endoderm. The organiser and its derivatives provide the antagonistic activity that modulates WNT and TGFbeta signalling. Recent findings show that the organiser activity is augmented by morphogenetic activity of the extraembryonic and embryonic endoderm, suggesting embryonic patterning is not solely the function of the organiser.

Embryonic origins of mammalian hematopoiesis

Hematopoiesis and vasculogenesis in the mammalian embryo begin in the blood islands of the yolk sac and continue, somewhat later, within the embryo proper. A subset of the first endothelial and hematopoietic cells of the yolk sac arise in close spatial and temporal association, apparently from a common mesodermal progenitor, the "hemangioblast." The mechanisms that control formation of hemangioblast and embryonic hematopoietic and endothelial (angioblastic) stem/progenitor cells are still not well understood. Formation of these cell types from nascent mesoderm requires signals from an adjacent outer layer of primitive (visceral) endoderm. Indian hedgehog (Ihh), a member of the hedgehog family of extracellular morphogens, is secreted by visceral endoderm and alone is sufficient to induce hematopoiesis and vasculogenesis in explanted embryos. While gene targeting studies in mice support a role for hedgehog signaling in these processes in vivo, they also suggest that additional molecules (perhaps, for example, Wnt proteins) are required for induction and patterning of hematopoietic and vascular mesoderm. Indian hedgehog likely functions through upregulation of genes encoding other signaling molecules, such as bone morphogenetic protein (Bmp)-4, in the target tissue. This review will focus on hematopoietic and vascular development in the early mouse embryo and will discuss potential implications of recent studies for stem cell transplantation in humans.

Lineage choice and differentiation in mouse embryos and embryonic stem cells

The use of embryonic stem (ES) cells for generating healthy tissues has the potential to revolutionize therapies for human disease or injury, for which there are currently no effective treatments. Strategies for manipulating stem cell differentiation should be based on knowledge of the mechanisms by which lineage decisions are made during early embryogenesis. Here, we review current research into the factors influencing lineage differentiation in the mouse embryo and the application of this knowledge to in vitro differentiation of ES cells. In the mouse embryo, specification of tissue lineages requires cell-cell interactions that are influenced by coordinated cell migration and cellular neighborhood mediated by the key WNT, FGF, and TGFbeta signaling pathways. Mimicking the cellular interactions of the embryo by providing appropriate signaling molecules in culture has enabled the differentiation of ES cells to be directed predominately toward particular lineages. Multistep strategies incorporating the provision of soluble factors known to influence lineage choices in the embryo, coculture with other cells or tissues, genetic modification, and selection for desirable cell types have allowed the production of ES cell derivatives that produce beneficial effects in animal models. Increasing the efficiency of this process can only result from a better understanding of the molecular control of cell lineage determination in the embryo.

Gastrulation and angiogenesis, not endothelial specification, is sensitive to partial deficiency in vascular endothelial growth factor-a in mice

Mouse embryogenesis is dose sensitive to vascular endothelial growth factor-A (VEGF-A), and mouse embryos partially deficient in VEGF-A die in utero because of severe vascular defects. In this study, we investigate the possible causes that underlie this phenomenon. Although the development of vascular defects in VEGF-A-deficient embryos seems to suggest that endothelial differentiation depends on the presence of a sufficient level of VEGF-A, we were surprised to find that endothelial differentiation per se is insensitive to a significant loss of VEGF-A activity. Instead, the development of the multipotent mesenchymal cells, from which endothelial progenitors arise in the yolk sac, is most highly dependent on VEGF-A. As a result of VEGF-A deficiency, dramatically fewer multipotent mesenchymal cells are generated in the prospective yolk sac. However, among the small number of mesenchymal cells that do enter the prospective yolk sac, endothelial differentiation occurs at a normal frequency. In the embryo proper, vasculogenesis is initiated actively in spite of a significant VEGF-A deficiency, but the subsequent steps of vascular development are defective. We conclude that a full-level VEGF-A activity is not critical for endothelial specification but is important for two distinct processes before and after endothelial specification: the development of the yolk sac mesenchyme and angiogenic sprouting of blood vessels.

Technicolour transgenics: imaging tools for functional genomics in the mouse

Over the past decade, a battery of powerful tools that encompass forward and reverse genetic approaches have been developed to dissect the molecular and cellular processes that regulate development and disease. The advent of genetically-encoded fluorescent proteins that are expressed in wild type and mutant mice, together with advances in imaging technology, make it possible to study these biological processes in many dimensions. Importantly, these technologies allow direct visual access to complex events as they happen in their native environment, which provides greater insights into mammalian biology than ever before.

From mesoderm to blood islands: patterns of key molecules during yolk sac erythropoiesis

Several identified genes play key roles in the specification of the blood-forming system, from commitment of mesoderm to differentiation of hemopoietic and endothelial cells. We have thoroughly analyzed the expression dynamics of some of these genes during yolk sac erythropoiesis in the chick embryo. The study includes transcription factors which are known to participate in multimeric complexes: GATA-1, -2, SCL/tal-1 and Lmo2 (whose avian orthologue we have cloned), VEGF-R2, a critical regulator of hemopoietic and endothelial commitment, and hemoglobin used as a marker of the last step in erythroid differentiation. Several findings were unexpected. (1) Two distinct patterns were revealed for GATA-2, first: low expression, ubiquitous in all mesodermal cells, as soon as cells ingress through the primitive streak; secondly: high, blood island-specific expression. (2) VEGF-R2 is coexpressed with GATA-2 at the level of the primitive streak. (3) SCL and Lmo2 expression is restricted to presumptive hemangioblasts. (4) The up-regulation of GATA-2 in newly formed blood islands is shortly followed by GATA-1 expression. (5) Lmo2 is up-regulated in blood island angioblasts thus appearing as one of the earliest markers for endothelial cell commitment. VEGF-R2 is down-regulated in hemopoietic cells prior to GATA-2, SCL/tal-1, Lmo2 and GATA-1 in erythroblasts.

Conserved requirement of Lim1 function for cell movements during gastrulation

To investigate Lim1 function during gastrulation, we used transcript depletion through DEED antisense oligonucleotides in Xenopus and cell transplantation in mice. Xenopus embryos depleted of Lim1 lack anterior head structures and fail to form a proper axis as a result of a failure of gastrulation movements, even though mesodermal cell identities are specified. Similar disruption of cell movements in the mesoderm is also observed in Lim1(-/-) mice. Paraxial protocadherin (PAPC) expression is lost in the nascent mesoderm of Lim1(-/-) mouse embryos and in the organizer of Lim1-depleted Xenopus embryos; the latter can be rescued to a considerable extent by supplying PAPC exogenously. We conclude that a primary function of Lim1 in the early embryo is to enable proper cell movements during gastrulation.

The hypoblast of the chick embryo positions the primitive streak by antagonizing nodal signaling

The hypoblast (equivalent to the mouse anterior visceral endoderm) of the chick embryo plays a role in regulating embryonic polarity. Surprisingly, hypoblast removal causes multiple embryonic axes to form, suggesting that it emits an inhibitor of axis formation. We show that Cerberus (a multifunctional antagonist of Nodal, Wnt, and BMP signaling) is produced by the hypoblast and inhibits primitive streak formation. This activity is mimicked by Cerberus-Short (CerS), which only inhibits Nodal. Nodal misexpression can initiate an ectopic primitive streak, but only when the hypoblast is removed. We propose that, during normal development, the primitive streak forms only when the hypoblast is displaced away from the posterior margin by the endoblast, which lacks Cerberus.

In the absence of Rauber's sickle material, no blood islands are formed in the avian blastoderm

Using the quail-chick chimera technique, we followed the fate of Rauber's sickle cells in older whole blastoderms (cultured for approximately 2 days): after removal of the autochthonous Rauber's sickle from an unincubated chicken blastoderm, a quail Rauber's sickle was grafted isotopically and isochronically in its place. In transverse sections through these chimeras, the grafted quail Rauber's sickle cells were seen to have transformed into a broad row or ridge of quail junctional endoblast cells extending at the inner border of the area containing blood islands. After unilateral removal of the junctional endoblast from an intermediate streak chicken blastoderm (Stage 3; Hamburger and Hamilton [1951] J Morphol 88:49-92), we observed during further in vitro culture that at the operated side, in the area previously occupied by this junctional endoblast, blood islands no longer developed. If after such a unilateral removal of the chicken junctional endoblast quail junctional endoblast was apposed in its place, then blood islands reappeared in the operated area. The intimate contact between the apposed quail junctional endoblast and the recently formed blood islands, derived from peripherally migrating mesoderm, was very obvious on sections through such chimeras. We further demonstrate that Rauber's sickle vs. junctional endoblast is indispensable for the anlage of blood islands in avian blastoderms. Indeed, in the absence of Rauber's sickle material no blood islands develop (even when mesoderm is present after ingression of the upper layer via a primitive streak) in the isolated central region of the area centralis of unincubated chicken blastoderms after culture in vitro. Also, no junctional endoblast and no sickle canal appear in these explants. By contrast, if a Rauber's sickle fragment is placed on such an isolated central blastoderm region, then blood islands develop. These blood islands start to develop from peripherally migrating mesoderm in the neighborhood of the Rauber's sickle-derived junctional endoblast.

Ets2 is expressed during morphogenesis of the somite and limb in the mouse embryo

Ets2 is a member of the ETS family of transcription factors. In order to address the developmental function of Ets2, we have examined its expression pattern in E8.5 to E13.5 embryos using RNA whole-mount in situ hybridization. In the paraxial mesoderm, Ets2 is expressed uniformly in the presomitic mesoderm and then restricted to the cells in the rostral portion of the segmenting and the next two recently formed somites. In the developing limb, Ets2 expression in the mesenchyme reflects the progressive formation of the hand or foot plate and the digital skeleton. In addition, Ets2 is expressed in the otic vesicle and its derivatives, the dorsal (posterior) root ganglia, the neuroepithelium in the dorsal part of the caudal neural tube and the inter-segmental vasculature.

Expression of Notch genes and their ligands during gastrulation in the chicken embryo

Notch signalling is an important evolutionary conserved mechanism known to control cell fate choices through local interactions. Here, the patterns of expression of Notch-1 and -2 genes and their ligands Delta-1, Serrate-1 and -2, were established in the early blastodisc of the chicken embryo from the pre-streak to the first somite stages. Delta-1 was detected as early as the pre-streak stage in the posterior part of the embryo shortly followed in the same region by Notch-1 at the initial streak stage. Thereafter both were strongly expressed in the posterior part of the primitive streak until HH4. Notch-2 was also found at the level of the streak although at low levels. Notch-1 was homogeneously expressed by the epiblast and by mesodermal cells ingressing at the level of the streak whereas Delta-1 expression formed a 'salt and pepper' pattern. The difference between the two was clearly detected by double in situ hybridisation. From the mid-streak to the first somite stages, Notch-1 and Delta-1 expressions appeared in the anterior part of the embryo. Serrate-1 and -2 were not detected at these stages. Taken together, these results constitute a framework for analysing the role(s) for Notch signalling during gastrulation.

Vasculogenesis and the search for the hemangioblast

Embryonic endothelial cells (EC) are generated by two mechanisms, vasculogenesis and angiogenesis (1). The term vasculogenesis describes the de novo emergence of EC progenitors from the mesoderm, whereas angiogenesis corresponds to the generation of EC by sprouting from the pre-existing vascular network. Until recently, it was thought that vasculogenesis was restricted to the period of embryonic development, whereas in the adult, only angiogenesis contributed to EC proliferation. The discovery of circulating EC progenitors in adult bone marrow and peripheral blood has suggested that additional mechanisms besides angiogenesis can occur in the adult, and therefore have renewed interest in the embryonic origin and the development of these progenitor cells. Vasculogenesis in the chick embryo has been studied since the beginning of the 20th century. During early development, vasculogenesis is intimately linked to the emergence of hematopoietic cells (HC). The existence of a common precursor for both EC and HC, termed "hemangioblast," was postulated (2). The purpose of this review is to summarize the experimental evidence concerning the emergence of EC and HC during embryonic life.

Targeted mutagenesis of the Hira gene results in gastrulation defects and patterning abnormalities of mesoendodermal derivatives prior to early embryonic lethality

The Hira gene encodes a nuclear WD40 domain protein homologous to the yeast transcriptional corepressors Hir1p and Hir2p. Using targeted mutagenesis we demonstrate that Hira is essential for murine embryogenesis. Analysis of inbred 129Sv embryos carrying the null mutation revealed an initial requirement during gastrulation, with many mutant embryos having a distorted primitive streak. Mutant embryos recovered at later stages have a range of malformations with axial and paraxial mesendoderm being particularly affected, a finding consistent with the disruption of gastrulation seen earlier in development. This phenotype could be partially rescued by a CD1 genetic background, although the homozygous mutation was always lethal by embryonic day 11, with death probably resulting from abnormal placentation and failure of cardiac morphogenesis.

Absence of erythrogenesis and vasculogenesis in Plcg1-deficient mice

Mice nullizygous for Plcg1 cease growing at early to mid-gestation. An examination of carefully preserved wild-type embryos shows clear evidence of erythropoiesis, but erythropoiesis is not evident in Plcg1 nullizygous embryos at the same stage. The analyses of embryonic materials demonstrate that in the absence of Plcg1, erythroid progenitors cannot be detected in the yolk sac or embryo body by three different assays, burst-forming units, colony-forming units, and analysis for the developmental marker Ter119. However, non-erythroid granulocyte/macrophage colonies are produced by Plcg1 null embryos. Further analysis of these embryos demonstrates significantly diminished vasculogenesis in Plcg1 nullizygous embryos based on the lack of expression of the endothelial marker platelet endothelial cell adhesion molecule-1. In addition, Plcg1 nullizygous embryos express a greatly reduced level of vascular endothelial growth factor receptor-2/Flk-1, consistent with significantly impaired vasculogenesis and erythropoiesis. Interestingly, these early embryos do express phospholipase C-gamma2, however, it is unable to substitute for the absence of phospholipase C-gamma1, which can be detected in its tyrosine-phosphorylated state.

Heads or tails: Wnts and anterior-posterior patterning

Cell-cell communication is critical during embryogenesis for organizing the vertebrate body plan. Members of the Wnt family of secreted signaling molecules possess axis-inducing and posteriorizing activity when overexpressed. Wnt signals are modulated extracellularly by a diverse group of secreted Wnt antagonists and cofactors. Recent work has revealed that inhibition of posteriorly localized Wnt signaling by anteriorly localized Wnt antagonists is critical for inducing the anterior structures, forebrain and heart, from neural ectoderm and mesoderm, respectively. This review centers on the role that Wnts and Wnt antagonists play in the patterning of the vertebrate anterior-posterior axis.