A phase and electron microscopic study of vasculogenesis and erythropoiesis in the yolk sac of the mouse

Identification of erythroid-enriched gene expression in the mouse embryonic yolk sac using microdissected cells

Little is known about the genes that control the embryonic erythroid program. Laser capture microdissection was used to isolate primitive erythroid precursors and epithelial cells from frozen sections of the embryonic day 9.5 yolk sac. The RNA samples were amplified and labeled for hybridization to Affymetrix GeneChip Mouse Genome 430A 2.0 arrays. Ninety-one genes are expressed significantly higher in erythroid than in epithelial cells. Ingenuity pathway analysis indicates that many of these erythroid-enriched genes cluster in highly significant biological networks. One of these networks contains RBTN2/LMO2, SCL/TAL1, and EKLF/KLF1, three of the very few genes required for primitive erythropoiesis. Quantitative real-time polymerase chain reaction was used to verify that platelet factor 4, reelin, thrombospondin-1, and muscleblind-like 1 mRNA is erythroid-enriched. These genes have established roles in development or differentiation in other systems, and are, therefore, good candidates for regulating primitive erythropoiesis. These results provide a catalog of genes expressed during primitive erythropoiesis.

Activation of hypoxic response in human embryonic stem cell-derived embryoid bodies

Oxygen tension can provide an important determinant for differentiation and development of many cells and tissues. Genetic regulation of hemato-endothelial commitment is known to respond to oxygen deprivation via stimulation of hypoxia inducible factors (HIFs). Here, we use a closed bioreactor system to monitor and control the dissolved oxygen during differentiation of human embryonic stem cells (hESCs) via formation of embryoid bodies (hEBs). Exposing hESC-derived EBs to ambient oxygen at or below 5% results in stabilization of HIF-1alpha and increased transcription of hypoxic responsive genes. Interestingly, we find that rather than HIF-1alpha expression being stable over prolonged (7-16 days) culture in hypoxic conditions, HIF-1alpha expression peaks after approximately 48 hours of hypoxic exposure, and then declines to near undetectable levels, despite constant hypoxic exposure. This transient stabilization of HIF-1alpha during hESC-derived EB culture is demonstrated for four distinct stages of differentiation. Furthermore, we demonstrate hEB cell expansion is slowed by hypoxic exposure, with increased apoptosis. However, hEB cell proliferation returns to normal rates upon return to normoxic conditions. Therefore, although hypoxia effectively stimulates hypoxic responsive genes, this single variable was not sufficient to improve development of hemato-endothelial cells from hESCs.

Ontogeny of erythropoiesis in the mammalian embryo

Red cells are required not only for adult well-being but also for survival and growth of the mammalian embryo beyond early postimplantation stages of development. The embryo's first "primitive" erythroid cells, derived from a transient wave of committed progenitors, emerge from the yolk sac as immature precursors and differentiate as a semisynchronous cohort in the bloodstream. Surprisingly, this maturational process in the mammalian embryo is characterized by globin gene switching and ultimately by enucleation. The yolk sac also synthesizes a second transient wave of "definitive" erythroid progenitors that enter the bloodstream and seed the liver of the fetus. At the same time, hematopoietic stem cells within the embryo also seed the liver and are the presumed source of long-term erythroid potential. Fetal definitive erythroid precursors mature in macrophage islands within the liver, enucleate, and enter the bloodstream as erythrocytes. Toward the end of gestation, definitive erythropoiesis shifts to its final location, the bone marrow. It has recently been recognized that the yolk sac-derived primitive and fetal definitive erythroid lineages, like their adult definitive erythroid counterpart, are each hierarchically associated with the megakaryocyte lineage. Continued comparative studies of primitive and definitive erythropoiesis in mammalian and nonmammalian embryos will lead to an improved understanding of terminal erythroid maturation and globin gene regulation.

Regulation of hematopoiesis and the hematopoietic stem cell niche by Wnt signaling pathways

Hematopoietic stem cells (HSCs) are a rare population of cells that are responsible for life-long generation of blood cells of all lineages. In order to maintain their numbers, HSCs must establish a balance between the opposing cell fates of self-renewal (in which the ability to function as HSCs is retained) and initiation of hematopoietic differentiation. Multiple signaling pathways have been implicated in the regulation of HSC cell fate. One such set of pathways are those activated by the Wnt family of ligands. Wnt signaling pathways play a crucial role during embryogenesis and deregulation of these pathways has been implicated in the formation of solid tumors. Wnt signaling also plays a role in the regulation of stem cells from multiple tissues, such as embryonic, epidermal, and intestinal stem cells. However, the function of Wnt signaling in HSC biology is still controversial. In this review, we will discuss the basic characteristics of the adult HSC and its regulatory microenvironment, the "niche", focusing on the regulation of the HSC and its niche by the Wnt signaling pathways.

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.

Involvement of marrow-derived endothelial cells in vascularization

Until recently, the adult neovasculature was thought to arise only through angiogenesis, the mechanism by which new blood vessels form from preexisting vessels through endothelial cell migration and proliferation. However, recent studies have provided evidence that postnatal neovasculature can also arise though vasculogenesis, a process by which endothelial progenitor cells are recruited and differentiate into mature endothelial cells to form new blood vessels. Evidence for the existence of endothelial progenitors has come from studies demonstrating the ability of bone marrow-derived cells to incorporate into adult vasculature. However, the exact nature of endothelial progenitor cells remains controversial. Because of the lack of definitive markers of endothelial progenitors, the in vivo contribution of progenitor cells to physiological and pathological neovascularization remains unclear. Early studies reported that endothelial progenitor cells actively integrate into the adult vasculature and are critical in the development of many types of vascular-dependent disorders such as neoplastic progression. Moreover, it has been suggested that endothelial progenitor cells can be used as a therapeutic strategy aimed at promoting vascular growth in a variety of ischemic diseases. However, increasing numbers of studies have reported no clear contribution of endothelial progenitors in physiological or pathological angiogenesis. In this chapter, we discuss the origin of the endothelial progenitor cell in the embryo and adult, and we discuss the cell's link to the primitive hematopoietic stem cell. We also review the potential significance of endothelial progenitor cells in the formation of a postnatal vascular network and discuss the factors that may account for the current lack of consensus of the scientific community on this important issue.

Development of the hemangioblast defines the onset of hematopoiesis in human ES cell differentiation cultures

The onset of hematopoiesis in the mouse embryo and in the embryonic stem (ES) cell differentiation model is defined by the emergence of the hemangioblast, a progenitor with both hematopoietic and vascular potential. While there is evidence for the existence of a hemangioblast in the mouse, it is unclear if this progenitor develops during the establishment of the human hematopoietic system. In this report, we have mapped hematopoietic development in human ES cell (hESC) differentiation cultures and demonstrated that a comparable hemangioblast population exists. The human hemangioblasts were identified by their capacity to generate blast colonies that display both hematopoietic and vascular potential. These colony-forming cells express the receptor tyrosine kinase KDR (VEGF receptor 2) and represent a transient population that develops in BMP-4-stimulated embryoid bodies (EBs) between 72 and 96 hours of differentiation, prior to the onset of the primitive erythroid program. Two distinct types of hemangioblasts were identified, those that give rise to primitive erythroid cells, macrophages, and endothelial cells and those that generate only the primitive erythroid population and endothelial cells. These findings demonstrate for the first time the existence of the human hemangioblast and in doing so identify the earliest stage of hematopoietic commitment.

Subcellular transport of EKLF and switch-on of murine adult beta maj globin gene transcription

Erythroid Krüppel-like factor (EKLF) is an essential transcription factor for mammalian beta-like globin gene switching, and it specifically activates transcription of the adult beta globin gene through binding of its zinc fingers to the promoter. It has been a puzzle that in the mouse, despite its expression throughout the erythroid development, EKLF activates the adult beta(maj) globin promoter only in erythroid cells beyond the stage of embryonic day 10.5 (E10.5) but not before. We show here that expression of the mouse beta(maj) globin gene in the aorta-gonad-mesonephros region of E10.5 embryos and in the E14.5 fetal liver is accompanied by predominantly nuclear localization of EKLF. In contrast, EKLF is mainly cytoplasmic in the erythroid cells of E9.5 blood islands in which beta(maj) is silenced. Remarkably, in a cultured mouse adult erythroleukemic (MEL) cell line, the activation of the beta(maj) globin gene by dimethyl sulfoxide (DMSO) or hexamethylene-bis-acetamide (HMBA) induction is also paralleled by a shift of the subcellular location of EKLF from the cytoplasm to the nucleus. Blockage of the nuclear import of EKLF in DMSO-induced MEL cells with a nuclear export inhibitor repressed the transcription of the beta(maj) globin gene. Transient transfection experiments further indicated that the full-sequence context of EKLF was required for the regulation of its subcellular locations in MEL cells during DMSO induction. Finally, in both the E14.5 fetal liver cells and induced MEL cells, the beta-like globin locus is colocalized the PML oncogene domain nuclear body, and concentrated with EKLF, RNA polymerase II, and the splicing factor SC35. These data together provide the first evidence that developmental stage- and differentiation state-specific regulation of the nuclear transport of EKLF might be one of the steps necessary for the switch-on of the mammalian adult beta globin gene transcription.

Characterization of GATA-1(+) hemangioblastic cells in the mouse embryo

Hemangioblasts are thought to be one of the sources of hematopoietic progenitors, yet little is known about their localization and fate in the mouse embryo. We show here that a subset of cells co-expressing the hematopoietic marker GATA-1 and the endothelial marker VE-cadherin localize on the yolk sac blood islands at embryonic day 7.5. Clonal analysis demonstrated that GATA-1(+) cells isolated from E7.0-7.5 embryos include a common precursor for hematopoietic and endothelial cells. Moreover, this precursor possesses primitive and definitive hematopoietic bipotential. By using a transgenic complementation rescue approach, GATA-1(+) cell-derived progenitors were selectively restored in Runx1-deficient mice. In the rescued mice, definitive erythropoiesis was recovered but the rescued progenitors did not display multilineage hematopoiesis or intra-aortic hematopoietic clusters. These results provide evidence of the presence of GATA-1(+) hemangioblastic cells in the extra-embryonic region and also their functional contribution to hematopoiesis in the embryo.

Wnt2 coordinates the commitment of mesoderm to hematopoietic, endothelial, and cardiac lineages in embryoid bodies

Our recent gene expression profiling analyses demonstrated that Wnt2 is highly expressed in Flk1(+) cells, which serve as common progenitors of endothelial cells, blood cells, and mural cells. In this report, we characterize the role of Wnt2 in mesoderm development during embryonic stem (ES) cell differentiation by creating ES cell lines in which Wnt2 was deleted. Wnt2(-/-) embryoid bodies (EBs) generated increased numbers of Flk1(+) cells and blast colony-forming cells compared with wild-type EBs, and had higher Flk1 expression at comparable stages of differentiation. Although Flk1(+) cells were increased, we found that endothelial cell and terminal cardiomyocyte differentiation was impaired, but hematopoietic cell differentiation was enhanced and smooth muscle cell differentiation was unchanged in Wnt2(-/-) EBs. Later stage Wnt2(-/-) EBs had either lower or undetectable expression of endothelial and cardiac genes compared with wild-type EBs. Consistently, vascular plexi were poorly formed and neither beating cardiomyocytes nor alpha-actinin-staining cells were detectable in later stage Wnt2(-/-) EBs. In contrast, hematopoietic cell gene expression was upregulated, and the number of hematopoietic progenitor colonies was significantly enhanced in Wnt2(-/-) EBs. Our data indicate that Wnt2 functions at multiple stages of development during ES cell differentiation and during the commitment and diversification of mesoderm: as a negative regulator for hemangioblast differentiation and hematopoiesis but alternatively as a positive regulator for endothelial and terminal cardiomyocyte differentiation.

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.

Microvascular tubes derived from embryonic stem cells sustain blood flow

Since the introduction of somatic cell nuclear transfer (SCNT), therapeutic cloning has been brought closer to reality. Among the potential applications of therapeutic cloning is therapeutic angiogenesis. Although recent progress has been made with clinical therapeutic angiogenesis, it has met with limited success. One reason for this limitation has been the cell types used to generate the collateral vessels used for shunting around coronary blockages. Consequently, we developed a procedure using the embryonic stem (ES) cell model system to generate microvascular tubes similar to small vessels found in vivo. We then evaluated their ability to graft and sustain blood flow by transplanting them onto enhanced green fluorescent protein (eGFP)-expressing embryonic day-9 (E9) embryo hearts. Microvascular tubes generated from ES cells have not been thoroughly tested for their ability to graft and function within the heart, primarily because of issues including immune rejection of the foreign cells comprising collateral vessels and limited methodologies to prevent teratoma risk. However, because recent therapeutic cloning techniques have provided evidence of diminished risk of immune rejection, we improved the methodology for generating and isolating tubes from ES cells to evaluate their applicability for therapeutic angiogenesis. Here, we demonstrate that microvascular tubes generated from ES cells are capable of grafting onto E9-day embryo hearts and sustaining the flow of blood cells as verified by eGFP-expressing blood cells within non-eGFP ES cell-derived microvascular tubes.

Discordant developmental waves of angioblasts and hemangioblasts in the early gastrulating mouse embryo

Vasculogenesis and hematopoiesis are thought to arise in hemangioblasts,the common progenitors of cells in vessels and in blood. This scheme was challenged by kinetic analysis of vascular endothelial and hematopoietic progenitors in early gastrulating mouse embryos. The OP-9 co-culture system with a combination of cytokines permitted the detection of endothelial progenitors, as well as stroma-dependent hematopoietic progenitors. Endothelial progenitors were detected as early as embryonic day (E) 5.50,after which time their numbers increased. Stroma-dependent hematopoietic progenitors were detected at E6.75, the time point when hemangioblasts reportedly emerge. Colony-forming units in culture (CFU-c), most likely generated from stroma-dependent hematopoietic progenitors via contact with the microenvironment, were detected at E7.50, concomitant with the onset of primitive hematopoiesis in the yolk sac. The presence of nucleated erythrocytes and the expression of an embryonic-type globin in erythroid colonies derived from stroma-dependent hematopoietic progenitors and from CFU-c support the notion that these progenitors coordinately establish primitive hematopoiesis. Using Oct3/4 promoter-driven GFP transgenic mice,early endothelial progenitors, stroma-dependent hematopoietic progenitors, and CFU-c were all shown to express the Oct3/4 transcription factor. Among Oct3/4-positive cells, both endothelial and hematopoietic progenitors were present in the CD31-positive fraction, leaving a subset of endothelial progenitors in the CD31-negative fraction. These data imply that Oct3/4-positive mesoderm gives rise to CD31-negative angioblasts,CD31-positive angiboblasts and CD31-positive hemangioblasts. We propose a distinct developmental pathway in which the angioblast lineage directly diverges from mesoderm prior to and independent of hemangioblast development.

Isolation of erythroid cells from the mouse embryonic yolk sac by laser capture microdissection and subsequent microarray hybridization

Erythropoietic tissues are complex, containing both erythroid and other cells. The embryonic yolk sac in particular contains primitive erythroid cells in low abundance. Laser capture microdissection (LCM) was performed to isolate erythroid cells, and epithelial cells, from mouse embryonic day 10 (E10) yolk sac. Quantitative RT-PCR was performed to confirm that enriched cell populations were obtained. epsilony- and betaH1-globin mRNAs were enriched in the erythroid compared to the epithelial fraction, and villin mRNA was enriched in the epithelial compared to the erythroid fraction. RNA isolated from the microdissected erythroid cells was of high quality as indicated by capillary electrophoresis. The RNA from the LCM erythroid fraction was linearly amplified with T7 RNA polymerase and hybridized to a Mouse 430A 2.0 Affymetrix array. Forty-eight percent of genes were present in the microarray assays, including low abundance transcripts such as erythroid transcription factors and enzymes involved in heme synthesis. With the LCM/microarray strategy, it will be possible to identify genes that are differentially regulated in native primitive and definitive erythroid cells.

Endothelial FAK is essential for vascular network stability, cell survival, and lamellipodial formation

Morphogenesis of a vascular network requires dynamic vessel growth and regression. To investigate the cellular mechanism underlying this process, we deleted focal adhesion kinase (FAK), a key signaling mediator, in endothelial cells (ECs) using Tie2-Cre mice. Targeted FAK depletion occurred efficiently early in development, where mutants exhibited a distinctive and irregular vasculature, resulting in hemorrhage and lethality between embryonic day (e) 10.5 and 11.5. Capillaries and intercapillary spaces in yolk sacs were dilated before any other detectable abnormalities at e9.5, and explants demonstrate that the defects resulted from the loss of FAK and not from organ failure. Time-lapse microscopy monitoring EC behavior during vascular formation in explants revealed no apparent decrease in proliferation or migration but revealed increases in cell retraction and death leading to reduced vessel growth and increased vessel regression. Consistent with this phenotype, ECs derived from mutant embryos exhibited aberrant lamellipodial extensions, altered actin cytoskeleton, and nonpolarized cell movement. This study reveals that FAK is crucial for vascular morphogenesis and the regulation of EC survival and morphology.

Vasculogenesis of the embryonic heart: Origin of blood island-like structures

The earliest vascular structures (blood island-like) in the embryonic heart are clusters of angioblasts and nucleated red blood cells (NRBCs), which differentiate into endothelial cells and erythrocytes, respectively. Our purpose was to define the area and chronology of NRBC appearance in the mouse embryonic heart at the stages before a patency between coronary vessels and peripheral circulation is established (10.5-13.5 dpc). Before and at the onset of vascularization, NBCs were not present within the proepicardium; however, Ter/119+ differentiating erythroblasts and single scattered CD45+ were found in the heart beginning from 10.5 dpc. The Ter/119+ cells were in close apposition to angioblasts (PECAM1+) and were recognized as components of blood island-like structures or vascular vesicles in transmission electron microscope and were located mostly in the subepicardium. Some of the NRBCs were not accompanied by angioblasts and located close to the endocardial endothelium or at the border of the endocardial endothelium or in the subepicardium. These erythroblasts were beginning to assemble with angioblasts. CD34+ NBCs as well as progenitor cells of erythroid lineage were not detected in the heart at these stages of development. The state of differentiation of NRBCs of blood islands was similar/the same as the morphology of circulating blood cells at the respective stages of embryo development. The presence of mature NRBCs in the subendocardial area and lack of progenitor cells of erythroid lineage within the heart indicate that erythroid commitment occurs outside the heart. We suggest that NRBCs enter the heart from the blood stream at 10.5-12 dpc independently from angioblasts.

Erythropoiesis and red cell function in vertebrate embryos

All vertebrate embryos produce a specific erythroid cell population--primitive erythrocytes--early in development. These cells are characterized by expression of the specific embryonic haemoglobins. Many aspects of primitive erythropoiesis and the physiological function of primitive red cells are still enigmatic. Nevertheless, recent years have seen intensive efforts to characterize in greater detail the molecular events underlying the initiation of erythropoiesis in vertebrate embryos. Several key genes have been identified that are necessary for primitive and the subsequent definitive erythropoiesis, which differs in several aspect from primitive erythropoiesis. This review gives in its first part a short overview dealing with comparative aspects of primitive and early definitive erythropoiesis in higher and lower vertebrates and in the second part we discuss the physiological function of primitive red cells based mainly on results from mammalian and avian embryos.

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.

"Maturational" globin switching in primary primitive erythroid cells

Mammals have 2 distinct erythroid lineages. The primitive erythroid lineage originates in the yolk sac and generates a cohort of large erythroblasts that terminally differentiate in the bloodstream. The definitive erythroid lineage generates smaller enucleated erythrocytes that become the predominant cell in fetal and postnatal circulation. These lineages also have distinct globin expression patterns. Our studies in primary murine primitive erythroid cells indicate that betaH1 is the predominant beta-globin transcript in the early yolk sac. Thus, unlike the human, murine beta-globin genes are not up-regulated in the order of their chromosomal arrangement. As primitive erythroblasts mature from proerythroblasts to reticulocytes, they undergo a betaH1- to epsilony-globin switch, up-regulate adult beta1- and beta2-globins, and down-regulate zeta-globin. These changes in transcript levels correlate with changes in RNA polymerase II density at their promoters and transcribed regions. Furthermore, the epsilony- and betaH1-globin genes in primitive erythroblasts reside within a single large hyperacetylated domain. These data suggest that this "maturational" betaH1- to epsilony-globin switch is dynamically regulated at the transcriptional level. Globin switching during ontogeny is due not only to the sequential appearance of primitive and definitive lineages but also to changes in globin expression as primitive erythroblasts mature in the bloodstream.

Designer blood: creating hematopoietic lineages from embryonic stem cells

Embryonic stem (ES) cells exhibit the remarkable capacity to become virtually any differentiated tissue upon appropriate manipulation in culture, a property that has been beneficial for studies of hematopoiesis. Until recently, the majority of this work used murine ES cells for basic research to elucidate fundamental properties of blood-cell development and establish methods to derive specific mature lineages. Now, the advent of human ES cells sets the stage for more applied pursuits to generate transplantable cells for treating blood disorders. Current efforts are directed toward adapting in vitro hematopoietic differentiation methods developed for murine ES cells to human lines, identifying the key interspecies differences in biologic properties of ES cells, and generating ES cell-derived hematopoietic stem cells that are competent to repopulate adult hosts. The ultimate medical goal is to create patient-specific and generic ES cell lines that can be expanded in vitro, genetically altered, and differentiated into cell types that can be used to treat hematopoietic diseases.

Loss of Tie2 receptor compromises embryonic stem cell-derived endothelial but not hematopoietic cell survival

Tie2 is a receptor-type tyrosine kinase expressed on hematopoietic stem cells and endothelial cells. We used cultured embryonic stem (ES) cells to determine the function of Tie2 during early vascular development and hematopoiesis. Upon differentiation, the ES cell-derived Tie2+ Flk1+ fraction was enriched for hematopoietic and endothelial progenitor cells. To investigate lymphatic differentiation, we used a monoclonal antibody against LYVE-1 and found that LYVE-1+ cells derived from Tie2+ Flk1+ cells possessed various characteristics of lymphatic endothelial cells. To determine whether Tie2 played a role in this process, we analyzed differentiation of Tie2-/- ES cells. Although the initial numbers of LYVE-1+ and PECAM-1+ cells derived from Tie2-/- cells did not vary significantly, the number of both decreased dramatically upon extended culturing. Such decreases were rescued by treatment with a caspase inhibitor, suggesting that reductions were due to apoptosis as a consequence of a lack of Tie2 signaling. Interestingly, Tie2-/- ES cells did not show measurable defects in development of the hematopoietic system, suggesting that Tie2 is not essential for hematopoietic cell development.

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.

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.

Genesis of hematopoietic stem cells in vitro and in vivo: new insights into developmental maturation

Hematopoietic stem cells first arise in the mammalian embryo in a primitive state, not capable of reconstituting hematopoiesis in irradiated adult recipients. As development proceeds, these cells eventually mature to acquire definitive, adult characteristics, including adult reconstitution ability. Mouse embryonic stem cells induced to undergo hematopoiesis in vitro readily generate primitive hematopoietic stem cells but rarely generate the definitive type. Recent work has stimulated a new appreciation of the events involved in the developmental maturation of hematopoietic stem cells. Application of this knowledge to in vitro differentiation systems will be critical to the successful development of hematopoietic therapies from embryonic stem cells.

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.

Effect of ethanol on the development of visceral yolk sac

BACKGROUND

Prenatal ethanol exposure can cause development retardation and malformations in human offspring. Before the formation of chorioallantoic placenta, yolk sac plays an important role in transporting nutrients from the mother to the embryo. Functional suppression of yolk sac is found to be relevant to the malformations in mammalian embryos.

METHODS

Female 8.5-day C57BL/6J mouse embryos were cultured in vitro and exposed to different doses of ethanol. The development of visceral yolk sac (VYS) was examined with light and electron microscopes. The expression profiles of some vasculogenesis-related genes were detected with reverse transcription-PCR.

RESULTS

A dose-dependent toxicity to the VYS was found, including reduced diameter, decreased protein and DNA contents, and suppressed development of vitelline vessels. The hypogenesis of VYS agreed with the retarded development and/or malformations found in the embryos. Histological and functional alterations were found in the ethanol-exposed VYS endodermal cells. The expressions of vasculogenesis-related genes, fetal liver kinase 1 (Flk1) and tyrosine kinase with immunoglobulin and epidermal growth factor homology domains 2 (Tie2), were repressed by ethanol.

CONCLUSIONS

Impaired structural and functional development of VYS may contribute to the teratogenic action of ethanol in mice, which may also provide a clue to the study of fetal alcohol syndrome in humans.

Zebrafish Genetics and Formation of Embryonic Vasculature

The embryonic vasculature develops in a conserved manner in all vertebrates. Endothelial progenitor cells differentiate from mesodermal cells, then migrate and assemble into the dorsal aorta and the cardinal vein. This primitive circulatory loop undergoes sprouting and branching via a two-step navigation mechanism to form the trunk vascular network. Various studies using several model systems have uncovered a number of signaling mechanisms that regulate these complex processes. A genetic approach in zebrafish has led to identification of mutations and molecules that are responsible for specification of endothelial progenitor cells, differentiation of arterial and venous cells, and patterning of the dorsal aorta and intersegmental vessels. These studies highlight the unique utilities and benefits of the zebrafish system for studying development of embryonic blood vessels.

Transplanted human bone marrow contributes to vascular endothelium

Recent evidence indicates that bone marrow is a source of endothelial progenitor cells that are mobilized into the peripheral blood in response to cytokines or tissue injury. Previously, we showed that functional endothelial cells (ECs) can be clonally derived from phenotypically defined hematopoietic stem cells. To determine the EC potential of human bone marrow and peripheral blood stem cells, blood vessels in sex-mismatched transplant recipients were evaluated. EC outcomes were identified by using a combination of immunohistochemistry and XY interphase FISH. Donor-derived ECs were detected in the skin and gut of transplant recipients with a mean frequency of 2% and could readily be distinguished from CD45-expressing hematopoietic stem cells. None of the >4,000 ECs examined had more than two sex chromosomes, consistent with an absence of cell fusion. Y chromosome signals were not detected in sex-matched female recipients, excluding the vertical transmission of male cells. None of the recipients evaluated before hematopoietic engraftment demonstrated donor-derived ECs, indicating a close linkage between the recovery of hematopoiesis and EC outcomes. Transplantable bone marrow-derived endothelial progenitor cells may represent novel therapeutic targets for hematopoietic and vascular disease.

Desmoplakin is required for microvascular tube formation in culture

Desmoplakin (DP) is a key component of cellular adhesion junctions known as desmosomes; however, recent investigations have revealed a novel location for DP in junctions separate from desmosomes termed complexus adherens junctions. These junctions are found at contact sites between endothelial cells that line capillaries. Few studies have focused on the function of DP in de novo capillary formation (vasculogenesis) and branching (angiogenesis) during tumorigenesis, embryonic development, cardiovascular development or wound healing. Only recently have investigations begun to determine the effect the loss of DP has on capillaries during embryogenesis (i.e. in DP-/- mice). Evidence shows that the loss of desmoplakin in vivo results in leaky capillaries and/or capillary malformation. Consequently, the goal of this study was to determine the function of DP in complexus adherens junctions during capillary formation. To accomplish this goal, we used siRNA technology to knock down desmoplakin expression in endothelial cells before they were induced to form microvascular tubes on matrigel. DP siRNA treated cells sent out filopodia and came in close contact with each other when plated onto matrigel; however, in most cases they failed to form tubes as compared with control endothelial cells. Interestingly, after siRNA degradation, endothelial cells were then capable of forming microvascular tubes. In depth analyses into the function of DP in capillary formation were not previously possible because the tools and experimental approaches only recently have become available (i.e. siRNA). Consequently, fully understanding the role of desmoplakin in capillary formation may lead to a novel approach for inhibiting vasculo- and angiogenesis in tumor formation.

Yolk sac-derived primitive erythroblasts enucleate during mammalian embryogenesis

The enucleated definitive erythrocytes of mammals are unique in the animal kingdom. The observation that yolk sac-derived primitive erythroid cells in mammals circulate as nucleated cells has led to the conjecture that they are related to the red cells of fish, amphibians, and birds that remain nucleated throughout their life span. In mice, primitive red cells express both embryonic and adult hemoglobins, whereas definitive erythroblasts accumulate only adult hemoglobins. We investigated the terminal differentiation of murine primitive red cells with use of antibodies raised to embryonic beta H1-globin. Primitive erythroblasts progressively enucleate between embryonic days 12.5 and 16.5, generating mature primitive erythrocytes that are similar in size to their nucleated counterparts. These enucleated primitive erythrocytes circulate as late as 5 days after birth. The enucleation of primitive red cells in the mouse embryo has not previously been well recognized because it coincides with the emergence of exponentially expanding numbers of definitive erythrocytes from the fetal liver. Our studies establish a new paradigm in the understanding of primitive erythropoiesis and support the concept that primitive erythropoiesis in mice shares many similarities with definitive erythropoiesis of mammals.

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.

Ephrin receptor, EphB4, regulates ES cell differentiation of primitive mammalian hemangioblasts, blood, cardiomyocytes, and blood vessels

Differentiation of pluripotent embryonic stem (ES) cells is associated with expression of fate-specifying gene products. Coordinated development, however, must involve modifying factors that enable differentiation and growth to adjust in response to local microenvironmental determinants. We report here that the ephrin receptor, EphB4, known to be spatially restricted in expression and critical for organized vessel formation, modifies the rate and magnitude of ES cells acquiring genotypic and phenotypic characteristics of mesodermal tissues. Hemangioblast, blood cell, cardiomyocyte, and vascular differentiation was impaired in EphB4-/- ES cells in conjunction with decreased expression of mesoderm-associated, but not neuroectoderm-associated, genes. Therefore, EphB4 modulates the response to mesoderm induction signals. These data add differentiation kinetics to the known effects of ephrin receptors on mammalian cell migration and adhesion. We propose that modifying sensitivity to differentiation cues is a further means for ephrin receptors to contribute to tissue patterning and organization.

Origins of mammalian hematopoiesis: in vivo paradigms and in vitro models

Though a topic of medical interest for centuries, our understanding of vertebrate hematopoietic or "blood-forming" tissue development has improved greatly only in recent years and given a series of scientific and technical milestones. Key among these observations was the description of procedures that allowed the transplantation of blood-forming activity. Beyond this, other advances include the creation of a variety of knock-out animals (mice and more recently zebrafish), microdissection of embryonic and fetal blood-forming tissues, hematopoietic stem (HSC) and progenitor cell (HPC) colony-forming assays, the discovery of cytokines with defined hematopoietic activities, gene transfer technologies, and the description of lineage-specific surface antigens for the identification and purification of pluripotent and differentiated blood cells. The availability of both murine and human embryonic stem cells (ESC) and the delineation of in vitro systems to direct their differentiation have now been added to this analytical arsenal. Such tools have allowed researchers to interrogate the complex developmental processes behind both primitive (yolk sac or extraembryonic) and definitive (intraembryonic) hematopoietic tissue formation. Using ES cells, we hope to not only gain additional basic insights into hematopoietic development but also to develop platforms for therapeutic use in patients suffering from hematological disease. In this review, we will focus on points of convergence and divergence between murine and human hematopoiesis in vivo and in vitro, and use these observations to evaluate the literature regarding attempts to create hematopoietic tissue from embryonic stem cells, the pitfalls encountered therein, and what challenges remain.

Ontogeny of plasma cells in the early rat yolk sac

The present study investigated the development of plasma cells in the early rat yolk sac (days 10-16 of gestation) by light microscopy, transmission electron microscopy, immunoelectron microscopy, and indirect immunofluoresce techniques. Cells delineating the morphology of plasma cells in the yolk sac were observed as early as 12 days of embryonic life. As for positive immune staining for the intra-cytoplasmic immunoglobulin (Ig) production (IgA, IgM and IgG), the intensity of the immune staining was very weak on days 10 and 11 of gestation, while it turned very dense on day 12 of gestation. At 14 days of gestation, the number of positive cells was markedly reduced. Immunoelectron microscopy visualized products of the immune reaction in cisterns of the rough endoplasmic reticulum. Conventional electron microscopic examination of 12, 13, and 16-day yolk sacs confirmed the development and differentiation of plasma cells with their well-known ultrastructural features, making this the first study to demonstrate these in the early rat yolk sac. The development of plasma cells in the early yolk sac implies the ability of the yolk sac to effect a humoral immune response at this stage of fetal life. The probable role of plasma cells in the yolk sac is also discussed.

Primary endothelial cells isolated from the yolk sac and para-aortic splanchnopleura support the expansion of adult marrow stem cells in vitro

The embryonic origin and development of hematopoietic and endothelial cells is highly interdependent. We hypothesized that primary endothelial cells from murine yolk sac and para-aortic splanchnopleura (P-Sp) may possess the capacity to expand hematopoietic stem cells (HSCs) and progenitor cells ex vivo. Using Tie2-GFP transgenic mice in combination with fluorochrome-conjugated monoclonal antibodies to vascular endothelial growth factor receptor-2 (Flk1) and CD41, we have successfully isolated pure populations of primary endothelial cells from 9.5-days after coitus (dpc) yolk sac and P-Sp. Adult murine bone marrow Sca-1+c-Kit+lin- cells were cocultured with yolk sac or P-Sp Tie2-GFP+Flk-1+CD41- endothelial cell monolayers for 7 days and the total number of nonadherent cells increased 47- and 295-fold, respectively, and hematopoietic progenitor counts increased 9.4- and 11.4-fold, respectively. Both the yolk sac and P-Sp endothelial cell cocultures facilitated long-term (> 6 months) HSC competitive repopulating ability (2.8- to 9.8-fold increases, respectively). These data suggest that 9.5-dpc yolk sac- and P-Sp-derived primary Tie2-GFP+Flk-1+CD41- endothelial cells possess the capacity to expand adult bone marrow hematopoietic progenitor cell and HSC repopulating ability ex vivo.

Phenotype and hematopoietic potential of side population cells throughout embryonic development

Adult murine bone marrow hematopoietic stem cells (HSCs) can be purified by sorting Hoechst 33342-extruding side population (SP) cells. Herein we investigated whether SP cells reside within embryonic tissues and exhibit hematopoietic progenitor activity. We isolated yolk sac (YS) and embryonic tissues 7.5 to 11.5 days after coitus (dpc), resolved an SP in each, and demonstrated that these SP cells exhibit distinct phenotypic and functional characteristics throughout development. YS and embryonic SP isolated 8.0 dpc expressed vascular endothelial-cadherin (VE-cadherin) and vascular endothelial receptor 2 (Flk-1), markers not expressed by bone marrow SP but expressed by endothelial cells and progenitors. SP at this stage did not express CD45 or produce hematopoietic colonies in vitro. In contrast, SP isolated 9.5 to 11.5 dpc contained a significantly higher proportion of cells expressing cKit and CD45, markers highly expressed by bone marrow SP. Furthermore, YS SP isolated 9.5 to 11.5 dpc demonstrated 40- to 90-fold enrichment for hematopoietic progenitor activity over unfractionated tissue. Our data indicate that YS and embryonic SP cells detected prior to the onset of circulation express the highest levels of endothelial markers and do not generate blood cells in vitro; however, as development progresses, they acquire hematopoietic potential and phenotypic characteristics similar to those of bone marrow SP.

Identification of mature plasma cells in early rat yolk sac. A possible origin from the endodermal cell layer: immunohistochemistry and immunoelectron microscopic study

Plasma cells play a pivotal role in the immune system and are responsible for the synthesis and release of immunoglobulins. Numerous in vitro culture experiments on the yolk sac demonstrated the generation of mature cells of the myeloid and lymphoid lineages under appropriate culture conditions. However, there are no reports describing the development of mature lymphoid cells in the yolk sac so far. For this reason, we undertook this study to investigate the development of antibody-containing plasma cells during early yolk sac haematopoiesis. Immunohistochemistry and immunoelectron microscopy were employed in the study. Results of this work demonstrated very weak immune staining for the intracytoplasmic IgA, IgG, and IgM at days 10 and 11 of embryonic life, while dark staining was obtained at 12 days. Positive staining was localized to the endodermal cell layer. Electron microscopic examinations revealed the existence of cells with the typical characteristics of plasma cells inside the endodermal cell layer, which may suggest their endodermal origin. To further verify the nature of these cells, intracytoplasmic immunoglobulins were demonstrated by immunoelectron microscopy. The present study demonstrated emergence of mature functioning plasma cells in early rat yolk sac. In a previous work we hypothesized the possibility of endodermal origin of yolk sac macrophages. This study adds additional evidence to support that hypothesis. The possible role of plasma cells in the yolk sac is discussed.

Biology of hematopoietic stem cells and progenitors: implications for clinical application

Stem cell biology is scientifically, clinically, and politically a current topic. The hematopoietic stem cell, the common ancestor of all types of blood cells, is one of the best-characterized stem cells in the body and the only stem cell that is clinically applied in the treatment of diseases such as breast cancer, leukemias, and congenital immunodeficiencies. Multicolor cell sorting enables the purification not only of hematopoietic stem cells, but also of their downstream progenitors such as common lymphoid progenitors and common myeloid progenitors. Recent genetic approaches including gene chip technology have been used to elucidate the gene expression profile of hematopoietic stem cells and other progenitors. Although the mechanisms that control self-renewal and lineage commitment of hematopoietic stem cells are still ambiguous, recent rapid advances in understanding the biological nature of hematopoietic stem and progenitor cells have broadened the potential application of these cells in the treatment of diseases.

Ontogeny of hematopoiesis: examining the emergence of hematopoietic cells in the vertebrate embryo

Hematopoietic stem cells (HSCs) are responsible for generating all the lineages of the blood. During vertebrate development, waves of hematopoietic activity can be found in distinct anatomical sites, and they contribute to both embryonic and adult hematopoiesis. The origin of the HSCs that ultimately give rise to all the adult blood lineages has been a controversial issue in the field of hematopoiesis. Studies of amniotes have linked HSC activity to the aorta-gonad-mesonephros (AGM) region, whereas others suggest that the yolk sac is the true source of HSCs. This review describes both primitive and definitive hematopoiesis in mice, humans, chicks, frogs, and zebrafish and examines the current debate over the embryonic origins of HSCs.

Appearance of erythrocyte-like globules in the mouse visceral yolk sac endodermal cells on embryonic day 12, with special reference to blood islands

The mouse visceral yolk sac (VYS) is widely known to play an important role as erythropoietic tissue during embryonic periods. Mouse VYS from embryonic days 9 to 12 was examined by light microscopy, electron microscopy and histochemical analysis with benzidine to detect the presence of hemoglobin with special reference to the development of VYS, the disappearance of the blood islands in VYS, and the appearance of a novel structure in the form of erythrocyte-like globules in VYS endodermal cells. The villous appearance of VYS became complicated by the development of VYS endodermal cells. The blood islands positive for the benzidine reaction were light microscopically detected on embryonic days 9, 10, and 11. They disappeared on embryonic day 12, however. Erythrocyte-like globules positive for the benzidine reaction were not observed in VYS endodermal cells on embryonic days 9, 10, and 11, but then appeared on embryonic day 12, by light and electron microscopy. Erythrocyte-like globules in VYS endodermal cells, which appear after the disappearance of blood islands in VYS, may participate in erythropoiesis during embryonic development.

Circulation is established in a stepwise pattern in the mammalian embryo

To better understand the relationship between the embryonic hematopoietic and vascular systems, we investigated the establishment of circulation in mouse embryos by examining the redistribution of yolk sac-derived primitive erythroblasts and definitive hematopoietic progenitors. Our studies revealed that small numbers of erythroblasts first enter the embryo proper at 4 to 8 somite pairs (sp) (embryonic day 8.25 [E8.25]), concomitant with the proposed onset of cardiac function. Hours later (E8.5), most red cells remained in the yolk sac. Although the number of red cells expanded rapidly in the embryo proper, a steady state of approximately 40% red cells was not reached until 26 to 30 sp (E10). Additionally, erythroblasts were unevenly distributed within the embryo's vasculature before 35 sp. These data suggest that fully functional circulation is established after E10. This timing correlated with vascular remodeling, suggesting that vessel arborization, smooth muscle recruitment, or both are required. We also examined the distribution of committed hematopoietic progenitors during early embryogenesis. Before E8.0, all progenitors were found in the yolk sac. When normalized to circulating erythroblasts, there was a significant enrichment (20- to 5-fold) of progenitors in the yolk sac compared with the embryo proper from E9.5 to E10.5. These results indicated that the yolk sac vascular network remains a site of progenitor production and preferential adhesion even as the fetal liver becomes a hematopoietic organ. We conclude that a functional vascular system develops gradually and that specialized vascular-hematopoietic environments exist after circulation becomes fully established.

Thrombopoietin promotes mixed lineage and megakaryocytic colony-forming cell growth but inhibits primitive and definitive erythropoiesis in cells isolated from early murine yolk sacs

The role of thrombopoietin (Tpo) in promoting hematopoiesis has been extensively studied in late fetal, neonatal, and adult mice. However, the effects of Tpo on early yolk sac hematopoiesis have been largely unexplored. We examined whole embryos or the cells isolated from embryo proper and yolk sacs and identified both Tpo and c-mpl (Tpo receptor) mRNA transcripts in tissues as early as embryonic day 6.5 (E6.5). Presomite whole embryos and somite-staged yolk sac and embryo proper cells were plated in methylcellulose cultures and treated with selected hematopoietic growth factors in the presence or absence of Tpo. Tpo alone failed to promote colony-forming unit (CFU) formation. However, in the presence of other growth factors, Tpo caused a substantial dose-dependent reduction in primitive and definitive erythroid CFU growth in cultures containing E7.5 and E8.0 whole embryos and E8.25 to 9.5 yolk sac-derived cells. Meanwhile, Tpo treatment resulted in a substantial dose-dependent increase in CFU-mixed lineage (CFU-Mix) and CFU-megakaryocyte (CFU-Meg) formation in cultures containing cells from similar staged tissues. Addition of Tpo to cultures of sorted E9.5 yolk sac c-Kit(+)CD34(+) hematopoietic progenitors also inhibited erythroid CFU growth but augmented CFU-Mix and CFU-Meg activity. Effects of Tpo on CFU growth were blocked in the presence of a monoclonal antibody with Tpo-neutralizing activity but not with control antibody. Thus, under certain growth factor conditions, Tpo directly inhibits early yolk sac erythroid CFU growth but facilitates megakaryocyte and mixed lineage colony formation.

Putative intermediate precursor between hematogenic endothelial cells and blood cells in the developing embryo

During embryogenesis, endothelial cells are a source of hematopoietic cells. Vascular endothelial (VE)-cadherin modulates adherens junctions between endothelial cells. How endothelial cells, integrated into the vascular bed via adherens junctions, give rise to free-floating hematopoietic cells has been examined. Contrary to our previous reports, in this report a cell type simultaneously expressing VE-cadherin and the hematopoietic marker CD45 was identified, without rigorous enzymatic dissociation of embryonic tissues. In spite of expressing several other endothelial markers such as endothelial cell nitrous oxide synthase (ECNOS) and MECA-32, this newly defined population failed to produce endothelial colonies when cultured on OP9 stroma, in direct contrast to enzymatically dissociated VE-cadherin+ cells. When isolated from 9.5 days post coitus (d.p.c.) embryos, VE-cadherin+ CD45+ cells generated erythroid, myeloid, but not B lymphoid, cells, also in contrast to VE-cadherin+ cells obtained by enzymatic dissociation. Runx1 null mutant embryos lacked this novel population. Collectively, these results introduce a novel VE-cadherin+ population within the developing embryo, which may represent an intermediate cell type in the transition of hemogenic endothelial cells into blood.

Expression of CD41 marks the initiation of definitive hematopoiesis in the mouse embryo

Murine hematopoietic stem cells (HSCs) originate from mesoderm in a process that requires the transcription factor SCL/Tal1. To define steps in the commitment to blood cell fate, we compared wild-type and SCL(-/-) embryonic stem cell differentiation in vitro and identified CD41 (GpIIb) as the earliest surface marker missing from SCL(-/-) embryoid bodies (EBs). Culture of fluorescence-activated cell sorter (FACS) purified cells from EBs showed that definitive hematopoietic progenitors were highly enriched in the CD41(+) fraction, whereas endothelial cells developed from CD41(-) cells. In the mouse embryo, expression of CD41 was detected in yolk sac blood islands and in fetal liver. In yolk sac and EBs, the panhematopoietic marker CD45 appeared in a subpopulation of CD41(+) cells. However, multilineage hematopoietic colonies developed not only from CD45(+)CD41(+) cells but also from CD45(-)CD41(+) cells, suggesting that CD41 rather than CD45 marks the definitive culture colony-forming unit (CFU-C) at the embryonic stage. In contrast, fetal liver CFU-C was CD45(+), and only a subfraction expressed CD41, demonstrating down-regulation of CD41 by the fetal liver stage. In yolk sac and EBs, CD41 was coexpressed with embryonic HSC markers c-kit and CD34. Sorting for CD41 and c-kit expression resulted in enrichment of definitive hematopoietic progenitors. Furthermore, the CD41(+) c-kit(+) population was missing from runx1/AML1(-/-) EBs that lack definitive hematopoiesis. These results suggest that the expression of CD41, a candidate target gene of SCL/Tal1, and c-kit define the divergence of definitive hematopoiesis from endothelial cells during development. Although CD41 is commonly referred to as megakaryocyte-platelet integrin in adult hematopoiesis, these results implicate a wider role for CD41 during murine ontogeny.

Hypoxia-inducible factor and the development of stem cells of the cardiovascular system

Decreased oxygen (O2) levels activate hypoxia-inducible factor (HIF-1) to induce genes involved in glycolysis, glucose transport, erythropoiesis, and angiogenesis. Mutations in various HIF-1 subunits have contributed to our understanding of the role hypoxia plays during early embryonic development in general and the cardiovascular system in particular. We propose that HIF-1 is important for the generation, proliferation, maintenance, and differentiation of the early cardiovascular system. Understanding aberrations in these hypoxic responses is important since they contribute to serious human disease such as ischemia and tumorigenesis. In this review we will focus on the critical role of O2 in regulating cardiovascular events during early embryonic development.

Molecular regulation of embryonic hematopoiesis and vascular development: a novel pathway

In all vertebrate animals, the first blood and vascular endothelial cells are formed during gastrulation, a process in which the mesoderm of the embryo is induced and then patterned by molecules whose identity is still largely unknown. Clusters of developing blood cells surrounded by a layer of endothelial cells comprise the "blood islands" and form in the visceral yolk sac, external to the developing embryo proper. Despite the identification of genes, such as Flk1, SCL/tal-1, Cbfa2/Runx1/AML1, and CD34, that are expressed during the induction of primitive hematopoiesis and vasculogenesis, the early molecular and cellular events involved in these processes are not well understood. Recent work has demonstrated that extracellular signals secreted by a layer of visceral endoderm surrounding the embryo are essential for the initiation of these events. A member of the Hedgehog family of signaling molecules is produced by visceral endoderm and is required for formation of blood and endothelial cells in explant cultures. Hedgehog proteins also stimulate proliferation of definitive hematopoietic stem/progenitor cells. Therefore, these findings may have important medical implications for regulating hematopoiesis and vascular development for therapeutic purposes and for the development of new sources of hematopoietic stem cells for transplantation and as targets for gene therapy.

Induction of embryonic hematopoietic and endothelial stem/progenitor cells by hedgehog-mediated signals

Blood and vascular endothelial cells form in all vertebrates during gastrulation, a process in which the mesoderm of the embryo is induced and then patterned by molecules whose identity is still largely unknown. Blood islands' of primitive hematopoietic cell clusters surrounded by a layer of endothelial cells form in the yolk sac, external to the developing embryo proper. These lineages arise from a layer of extraembryonic mesoderm that is closely apposed with a layer of primitive (visceral) endoderm. Despite the identification of genes such as Flk1, SCL/tal-1, Cbfa2/Runx1/AML1 and CD34 that are expressed during the induction of primitive hematopoiesis and vasculogenesis, the early molecular and cellular events involved in these processes are not well understood. Recent work has demonstrated that extracellular signals secreted by visceral endoderm surrounding the embryo are essential for the initiation of these events. A member of the Hedgehog family of signaling molecules (Indian hedgehog) is produced by visceral endoderm, can induce formation of blood and endothelial cells in explant cultures and can reprogram prospective neurectoderm along hematopoietic and endothelial cell lineages. Hedgehog proteins also stimulate proliferation of definitive hematopoietic stem/progenitor cells. These findings may have important implications for regulating hematopoiesis and vascular development for therapeutic purposes in humans and for the development of new sources of stem cells for transplantation and gene therapy.