Intraaortic hemopoietic cells are derived from endothelial cells during ontogeny

Three-dimensional cartography of hematopoietic clusters in the vasculature of whole mouse embryos

Hematopoietic cell clusters in the aorta of vertebrate embryos play a pivotal role in the formation of the adult blood system. Despite their importance, hematopoietic clusters have not been systematically quantitated or mapped because of technical limitations posed by the opaqueness of whole mouse embryos. Here, we combine an approach to make whole mouse embryos transparent, with multicolor marking, to allow observation of hematopoietic clusters using high-resolution 3-dimensional confocal microscopy. Our method provides the first complete map and temporal quantitation of all hematopoietic clusters in the mouse embryonic vasculature. We show that clusters peak in number at embryonic day 10.5, localize to specific vascular subregions and are heterogeneous, indicating a basal endothelial to non-basal (outer cluster) hematopoietic cell transition. Clusters enriched with the c-Kit(+)CD31(+)SSEA1(-) cell population contain functional hematopoietic progenitors and stem cells. Thus, three-dimensional cartography of transparent mouse embryos provides novel insight into the vascular subregions instrumental in hematopoietic progenitor/stem cell development, and represents an important technological advancement for comprehensive in situ hematopoietic cluster analysis.

Visualizing blood cell emergence from aortic endothelium

Three recent Nature papers use time-lapse confocal imaging to visualize the birth of blood cells from the aortic endothelium. Two studies (Bertrand et al., 2010; Kissa and Herbomel, 2010) utilize the zebrafish embryo, while the third (Boisset et al., 2010) develops a novel technique to image the mouse aorta.

Live imaging of Runx1 expression in the dorsal aorta tracks the emergence of blood progenitors from endothelial cells

Blood cells of an adult vertebrate are continuously generated by hematopoietic stem cells (HSCs) that originate during embryonic life within the aorta-gonad-mesonephros region. There is now compelling in vivo evidence that HSCs are generated from aortic endothelial cells and that this process is critically regulated by the transcription factor Runx1. By time-lapse microscopy of Runx1-enhanced green fluorescent protein transgenic zebrafish embryos, we were able to capture a subset of cells within the ventral endothelium of the dorsal aorta, as they acquire hemogenic properties and directly emerge as presumptive HSCs. These nascent hematopoietic cells assume a rounded morphology, transiently occupy the subaortic space, and eventually enter the circulation via the caudal vein. Cell tracing showed that these cells subsequently populated the sites of definitive hematopoiesis (thymus and kidney), consistent with an HSC identity. HSC numbers depended on activity of the transcription factor Runx1, on blood flow, and on proper development of the dorsal aorta (features in common with mammals). This study captures the earliest events of the transition of endothelial cells to a hemogenic endothelium and demonstrates that embryonic hematopoietic progenitors directly differentiate from endothelial cells within a living organism.

Vascular remodeling of the vitelline artery initiates extravascular emergence of hematopoietic clusters

The vitelline artery is a temporary structure that undergoes extensive remodeling during midgestation to eventually become the superior mesenteric artery (also called the cranial mesenteric artery, in the mouse). Here we show that, during this remodeling process, large clusters of hematopoietic progenitors emerge via extravascular budding and form structures that resemble previously described mesenteric blood islands. We demonstrate through fate mapping of vascular endothelium that these mesenteric blood islands are derived from the endothelium of the vitelline artery. We further show that the vitelline arterial endothelium and subsequent blood island structures originate from a lateral plate mesodermal population. Lineage tracing of the lateral plate mesoderm demonstrates contribution to all hemogenic vascular beds in the embryo, and eventually, all hematopoietic cells in the adult. The intraembryonic hematopoietic cell clusters contain viable, proliferative cells that exhibit hematopoietic stem cell markers and are able to further differentiate into myeloid and erythroid lineages. Vitelline artery-derived hematopoietic progenitor clusters appear between embryonic day 10 and embryonic day 10.75 in the caudal half of the midgut mesentery, but by embryonic day 11.0 are sporadically found on the cranial side of the midgut, thus suggesting possible extravascular migration aided by midgut rotation.

Vessel-associated stem cells from skeletal muscle: From biology to future uses in cell therapy

Over the last years, the existence of different stem cells with myogenic potential has been widely investigated. Besides the classical skeletal muscle progenitors represented by satellite cells, numerous multipotent and embryologically unrelated progenitors with a potential role in muscle differentiation and repair have been identified. In order to conceive a therapeutic approach for degenerative muscle disorders, it is of primary importance to identify an ideal stem cell endowed with all the features for a possible use in vivo. Among all emerging populations, vessel-associated stem cells are a novel and promising class of multipotent progenitors of mesodermal origin and with high myogenic potential which seem to best fit all the requirements for a possible cell therapy. In vitro and in vivostudies have already tested the effectiveness and safety of vessel-associated stem cells in animal models. This leads to the concrete possibility in the future to start pilot human clinical trials, hopefully opening the way to a turning point in the treatment of genetic and acquired muscle disorders.

Hey2 acts upstream of Notch in hematopoietic stem cell specification in zebrafish embryos

Hematopoietic stem cells (HSCs) are essential for homeostasis and injury-induced regeneration of the vertebrate blood system. Although HSC transplantations constitute the most common type of stem cell therapy applied in the clinic, we know relatively little about the molecular programming of HSCs during vertebrate embryogenesis. In vertebrate embryos, HSCs form in close association with the ventral wall of the dorsal aorta. We have shown previously that in zebrafish, HSC formation depends on the presence of a signaling cascade that involves Hedgehog, vascular endothelial growth factor, and Notch signaling. Here, we reveal that Hey2, a hairy/enhancer-of-split-related basic helix-loop-helix transcription factor often believed to act downstream of Notch, is also required for HSC formation. In dorsal aorta progenitors, Hey2 expression is induced downstream of cloche and the transcription factor Scl/Tal1, and is maintained by Hedgehog and vascular endothelial growth factor signaling. Whereas knockdown of Hey2 expression results in a loss of Notch receptor expression in dorsal aorta angioblasts, activation of Notch signaling in hey2 morphants rescues HSC formation in zebrafish embryos. These results establish an essential role for Hey2 upstream of Notch in HSC formation.

Nonredundant roles for Runx1 alternative promoters reflect their activity at discrete stages of developmental hematopoiesis

The transcription factor Runx1 is a pivotal regulator of definitive hematopoiesis in mouse ontogeny. Vertebrate Runx1 is transcribed from 2 promoters, the distal P1 and proximal P2, which provide a paradigm of the complex transcriptional and translational control of Runx1 function. However, very little is known about the biologic relevance of alternative Runx1 promoter usage in definitive hematopoietic cell emergence. Here we report that both promoters are active at the very onset of definitive hematopoiesis, with a skewing toward the P2. Moreover, functional and morphologic analysis of a novel P1-null and an attenuated P2 mouse model revealed that although both promoters play important nonredundant roles in the emergence of definitive hematopoietic cells, the proximal P2 was most critically required for this. The nature of the observed phenotypes is indicative of a differential contribution of the P1 and P2 promoters to the control of overall Runx1 levels, where and when this is most critically required. In addition, the dynamic expression of P1-Runx1 and P2-Runx1 points at a requirement for Runx1 early in development, when the P2 is still the prevalent promoter in the emerging hemogenic endothelium and/or first committed hematopoietic cells.

Differentiation of mesodermal cells from pluripotent stem cells

The pluripotency of embryonic stem cells has been well demonstrated by a vast variety of studies showing the induction of differentiation into desired cell types that have the potential to be used not only in basic studies but also in medical applications. The induction of mesodermal cells, especially blood cells, from embryonic stem cells is notable from the point of view of transplantation, and the methods for this induction have improved over the last few years, with more defined culture conditions in place. Concurrently, the generation of induced pluripotent stem cells from somatic cells opens the possibility of autologous transplantation. In fact, there are a growing number of reports demonstrating that several mesodermal cells can be differentiated from induced pluripotent stem cells using the same methods used for embryonic stem cells. This review summarizes recent advances in the differentiation of mesodermal cells from embryonic stem cells and induced pluripotent stem cells.

Haematopoietic stem cells derive directly from aortic endothelium during development

A major goal of regenerative medicine is to instruct formation of multipotent, tissue-specific stem cells from induced pluripotent stem cells (iPSCs) for cell replacement therapies. Generation of haematopoietic stem cells (HSCs) from iPSCs or embryonic stem cells (ESCs) is not currently possible, however, necessitating a better understanding of how HSCs normally arise during embryonic development. We previously showed that haematopoiesis occurs through four distinct waves during zebrafish development, with HSCs arising in the final wave in close association with the dorsal aorta. Recent reports have suggested that murine HSCs derive from haemogenic endothelial cells (ECs) lining the aortic floor. Additional in vitro studies have similarly indicated that the haematopoietic progeny of ESCs arise through intermediates with endothelial potential. Here we have used the unique strengths of the zebrafish embryo to image directly the generation of HSCs from the ventral wall of the dorsal aorta. Using combinations of fluorescent reporter transgenes, confocal time-lapse microscopy and flow cytometry, we have identified and isolated the stepwise intermediates as aortic haemogenic endothelium transitions to nascent HSCs. Finally, using a permanent lineage tracing strategy, we demonstrate that the HSCs generated from haemogenic endothelium are the lineal founders of the adult haematopoietic system.

Blood cell generation from the hemangioblast

Understanding how blood cells are generated is important from a biological perspective but also has potential implications in the treatment of blood diseases. Such knowledge could potentially lead to defining new conditions to amplify hematopoietic stem cells (HSCs) or could translate into new methods to produce HSCs, or other types of blood cells, from human embryonic stem cells or induced pluripotent stem cells. Additionally, as most key transcription factors regulating early hematopoietic development have also been implicated in various types of leukemia, understanding their function during normal development could result in a better comprehension of their roles during abnormal hematopoiesis in leukemia. In this review, we discuss our current understanding of the molecular and cellular mechanisms of blood development from the earliest hematopoietic precursor, the hemangioblast, a precursor for both endothelial and hematopoietic cell lineages.

Development of the endocardium

The endocardium, the endothelial lining of the heart, plays complex and critical roles in heart development, particularly in the formation of the cardiac valves and septa, the division of the truncus arteriosus into the aortic and pulmonary trunks, the development of Purkinje fibers that form the cardiac conduction system, and the formation of trabecular myocardium. Current data suggest that the endocardium is a regionally specialized endothelium that arises through a process of de novo vasculogenesis from a distinct population of mesodermal cardiogenic precursors in the cardiac crescent. In this article, we review recent developments in the understanding of the embryonic origins of the endocardium. Specifically, we summarize vasculogenesis and specification of endothelial cells from mesodermal precursors, and we review the transcriptional pathways involved in these processes. We discuss the lineage relationships between the endocardium and other endothelial populations and between the endocardium and the myocardium. Finally, we explore unresolved questions about the lineage relationships between the endocardium and the myocardium. One of the central questions involves the timing with which mesodermal cells, which arise in the primitive streak and migrate to the cardiac crescent, become committed to an endocardial fate. Two competing conceptual models of endocardial specification have been proposed. In the first, mesodermal precursor cells in the cardiac crescent are prespecified to become either endocardial or myocardial cells, while in the second, fate plasticity is retained by bipotential cardiogenic cells in the cardiac crescent. We propose a third model that reconciles these two views and suggest future experiments that might resolve this question.

Notch signalling and haematopoietic stem cell formation during embryogenesis

The Notch signalling pathway is repeatedly employed during embryonic development and adult homeostasis of a variety of tissues. In particular, its frequent involvement in the regulation of stem and progenitor cell maintenance and proliferation, as well as its role in binary fate decisions in cells that are destined to differentiate, is remarkable. Here, we review its role in the development of haematopoietic stem cells during vertebrate embryogenesis and put it into the context of Notch's functions in arterial specification, angiogenic vessel sprouting and vessel maintenance. We further discuss interactions with other signalling cascades, and pinpoint open questions and some of the challenges that lie ahead.

The differential activities of Runx1 promoters define milestones during embryonic hematopoiesis

The transcription factor RUNX1/AML1 is a master regulator of hematopoietic development. Its spatiotemporal expression is tightly regulated during embryonic development and is under the control of 2 alternative promoters, distal and proximal. Despite the functional significance of Runx1, the relative and specific activities of these 2 promoters remain largely uncharacterized. To investigate these activities, we introduced 2 reporter genes under the control of the proximal and distal promoters in embryonic stem cell and transgenic mouse lines. Our study reveals that both in vitro and in vivo the proximal Runx1 isoform marks a hemogenic endothelium cell population, whereas the subsequent expression of distal Runx1 defines fully committed definitive hematopoietic progenitors. Interestingly, hematopoietic commitment in distal Runx1 knockout embryos appears normal. Altogether, our data demonstrate that the differential activities of the 2 Runx1 promoters define milestones of hematopoietic development and suggest that the proximal isoform plays a critical role in the generation of hematopoietic cells from hemogenic endothelium. Identification and access to the discrete stages of hematopoietic development defined by the activities of the Runx1 promoters will provide the opportunity to further explore the cellular and molecular mechanisms of hematopoietic development.

Ontogeny of haematopoiesis: recent advances and open questions

Unravelling the embryonic origins of the haematopoietic system has been the subject of sustained research for more than a century. Nevertheless, many important questions are still either unanswered or remain a matter of intense debate. Recent progress in mouse and embryonic stem cell model systems as well as imaging and post-genomic technologies has provided new insights into many of these open questions. Here we place into context recent reports on the anatomical site of blood stem cell emergence and, using red blood cells as an example, illustrate how the development of stem cells and the other blood lineages is both temporally and spatially decoupled. In addition, we outline how embryonic stem cell assays are increasingly used as a powerful surrogate for studying lineage relationships and developmental potential of early embryonic blood progenitors. Finally, we review how recent progress in the reconstruction of transcriptional regulatory networks is beginning to define the connectivity between key regulators that control early blood development. In light of these rapid recent advances, research into the embryonic origins of the haematopoietic system should remain one of the most vibrant disciplines within the wider field of haematology for the foreseeable future.

Hematopoietic cell development in the zebrafish embryo

PURPOSE OF REVIEW

A wealth of new experimental evidence has been published over the past year that has helped refine our models of blood cell development. We will review this information, discuss the current models of hematopoietic ontogeny and provide perspective on current and future research directions, with an emphasis on how studies in the zebrafish are helping us better understand how hematopoietic stem cells are formed in the vertebrate embryo.

RECENT FINDINGS

Several important studies have been published recently addressing the embryonic development of hematopoietic stem cells. These studies have helped clarify several controversial topics in developmental hematopoiesis, including the concepts of the hemangioblast and hemogenic endothelium. In particular, the postulate that hematopoietic stem cells arise through hemogenic endothelial intermediates has been greatly strengthened by a collection of convincing publications reviewed below.

SUMMARY

A precise understanding of how hematopoietic stem cells are patterned during development has important implications for both developmental biology and regenerative medicine. Since hematopoietic stem cells are the only hematopoietic cells capable of lifelong, multilineage blood cell production, understanding the stepwise, molecular processes of their instruction from mesoderm is key to replicating these events in vitro from pluripotent embryonic stem cells.

Conditional Cre/LoxP strategies for the study of hematopoietic stem cell formation

Some of the questions that have intrigued developmental biologists studying blood cell formation are: where do blood cells form, what are their precursors, and what signals are required for their emergence. Elegant embryonic grafting experiments in non-mammalian vertebrates, transplantation assays in mouse, and genetic analyses in zebrafish and mouse have been brought to bear on these problems, with enormous success. More recently investigators have applied conditional gene deletion and replacement strategies to refine our knowledge of this process in mammals. Here we describe several studies that have used the Cre/LoxP system to study blood cell formation, and what has been learned as a result.

Restoration of Runx1 expression in the Tie2 cell compartment rescues definitive hematopoietic stem cells and extends life of Runx1 knockout animals until birth

Mice deficient in the runt homology domain transcription factor Runx1/AML1 fail to generate functional clonogenic hematopoietic cells and die in utero by embryonic day 12.5. We previously generated Runx1 reversible knockout mice, in which the Runx1 locus can be restored by Cre-mediated recombination. We show here that selective restoration of the Runx1 locus in the Tie2 cell compartment rescues clonogenic hematopoietic progenitors in early Runx1-null embryos and rescues lymphoid and myeloid lineages during fetal development. Furthermore, fetal liver cells isolated from reactivated Runx1 embryos are capable of long-term multilineage lymphomyeloid reconstitution of adult irradiated recipients, demonstrating the rescue of definitive hematopoietic stem cells. However, this rescue of the definitive hematopoietic hierarchy is not sufficient to rescue the viability of animals beyond birth, pointing to an essential role for Runx1 in other vital developmental processes.

[The aortic endothelium in the embryo: genesis and role in hematopoiesis]

Intra-aortic haematopoiesis is a transient phenomenon, characterised by the emergence of Hematopoietic Stem Cells (HSC) from the ventral aortic endothelium through an endothelial cell (EC) to HSC lineage switch. HSC differentiation is followed by the colonization of definitive haematopoietic organs. Since intra-aortic haematopoiesis is born from EC of the aortic floor, we wondered how vascular integrity was maintained during hematopoietic production. We have used interspecific quail to chick grafts to study the aortic morphogenesis during hematopoiesis. We have demonstrated that: 1) before haematopoiesis, the aortic endothelium, originally entirely from splanchnic origin, was colonized by somitic EC, creating a new roof and sides derived from the somite, whereas the floor was contributed by splanchnopleural-derived EC. 2) As haematopoiesis proceeded, somite-derived EC colonized the aortic floor, where they settled underneath the HSC clusters. 3) After haematopoiesis, splanchnopleural ECs have disappeared from the aortic floor and have been replaced by somite-derived EC. At this stage, the whole aortic endothelium originated from somitic cells. 4) We have identified that the somite contributed to the vascular smooth muscle cells (VSMC). 5) Using grafts of either single quail dermomyotome or sclerotome in the chick, we showed that EC originated from the dermomyotome whereas the vascular smooth muscle cells originated from the sclerotome. Taken together, our results bring about new insights on aorta morphogenesis and the time-restricted production of HSCs.

Decoding the hemogenic endothelium in mammals

A collection of recent Nature papers examines the relationship between endothelial precursors and hematopoietic cells. Two of these studies (Eilken et al., 2009; Lancrin et al., 2009) use time-lapse imaging with live markers and genetic analysis of differentiating ESCs to reveal that even non-aortic-derived endothelial cells are hemogenic.

Increase of hematopoietic progenitor and suppression of endothelial gene expression by Runx1 expression during in vitro ES differentiation

OBJECTIVE

Runx1 is essential for both the establishment of hematopoiesis during development and maintenance of adult hematopoiesis. To reveal the roles of Runx1, we examined how and when Runx1 functions during development of hematopoiesis, and revealed the genes controlled by Runx1.

MATERIALS AND METHODS

A combined in vitro approach involving in vitro hematopoietic differentiation of embryonic stem cells and conditional gene expression of Runx1 was utilized for this study. Then we analyzed the effects of Runx1 on the differentiation and proliferation of hematopoietic cells and carried out DNA microarray analysis.

RESULTS

Pulse expression of Runx1 prior to the emergence of hematopoietic cells caused immature hematopoietic cell increase but did not have any effects on the induction of hemogenic cells. During this process, the mRNA level of several endothelial cell-specific genes was downregulated.

CONCLUSION

Runx1 expression play important roles on the proliferation of emerging immature hematopoietic progenitors or the transition process from endothelial to hematopoietic cells presumably by suppressing the genes related to endothelial phenotype.

The haemangioblast generates haematopoietic cells through a haemogenic endothelium stage

It has been proposed that during embryonic development haematopoietic cells arise from a mesodermal progenitor with both endothelial and haematopoietic potential called the haemangioblast. A conflicting theory instead associates the first haematopoietic cells with a phenotypically differentiated endothelial cell that has haematopoietic potential (that is, a haemogenic endothelium). Support for the haemangioblast concept was initially provided by the identification during mouse embryonic stem cell differentiation of a clonal precursor, the blast colony-forming cell (BL-CFC), which gives rise to blast colonies with both endothelial and haematopoietic components. Although recent studies have now provided evidence for the presence of this bipotential precursor in vivo, the precise mechanism for generation of haematopoietic cells from the haemangioblast still remains completely unknown. Here we demonstrate that the haemangioblast generates haematopoietic cells through the formation of a haemogenic endothelium intermediate, providing the first direct link between these two precursor populations. The cell population containing the haemogenic endothelium is transiently generated during BL-CFC development. This cell population is also present in gastrulating mouse embryos and generates haematopoietic cells on further culture. At the molecular level, we demonstrate that the transcription factor Tal1 (also known as Scl; ref. 10) is indispensable for the establishment of this haemogenic endothelium population whereas the core binding factor Runx1 (also known as AML1; ref. 11) is critical for generation of definitive haematopoietic cells from haemogenic endothelium. Together our results merge the two a priori conflicting theories on the origin of haematopoietic development into a single linear developmental process.

Runx1 is required for the endothelial to haematopoietic cell transition but not thereafter

Haematopoietic stem cells (HSCs) are the founder cells of the adult haematopoietic system, and thus knowledge of the molecular program directing their generation during development is important for regenerative haematopoietic strategies. Runx1 is a pivotal transcription factor required for HSC generation in the vascular regions of the mouse conceptus-the aorta, vitelline and umbilical arteries, yolk sac and placenta. It is thought that HSCs emerge from vascular endothelial cells through the formation of intra-arterial clusters and that Runx1 functions during the transition from 'haemogenic endothelium' to HSCs. Here we show by conditional deletion that Runx1 activity in vascular-endothelial-cadherin-positive endothelial cells is indeed essential for intra-arterial cluster, haematopoietic progenitor and HSC formation in mice. In contrast, Runx1 is not required in cells expressing Vav1, one of the first pan-haematopoietic genes expressed in HSCs. Collectively these data show that Runx1 function is essential in endothelial cells for haematopoietic progenitor and HSC formation from the vasculature, but its requirement ends once or before Vav is expressed.

Resident vascular progenitor cells: an emerging role for non-terminally differentiated vessel-resident cells in vascular biology

Throughout development and adult life the vasculature exhibits a remarkably dynamic capacity for growth and repair. The vasculature also plays a pivotal role in the execution of other diverse biologic processes, such as the provisioning of early hematopoietic stem cells during embryonic development or the regulation of vascular tone and blood pressure. Adding to this importance, from an anatomical perspective, the vasculature is clearly an omnipresent organ, with few areas of the body that it does not penetrate. Given these impressive characteristics, it is perhaps to be expected that the vasculature should require, or at least be associated with, a ready supply of stem and progenitor cells. However, somewhat surprisingly, it is only now just beginning to be broadly appreciated that the vasculature plays host to a range of vessel-resident stem and progenitor cells. The possibility that these vessel-resident cells are implicated in processes as diverse as tumor vascularization and adaptive vascular remodeling appears likely, and several exciting avenues for clinical translation are already under investigation. This review explores the various stem and progenitor cell populations that are resident in the microvasculature, endothelium, and vessel walls and vessel-resident cells capable of phenotypic transformation.

Fate tracing reveals the endothelial origin of hematopoietic stem cells

Hematopoietic stem cells (HSCs) originate within the aortic-gonado-mesonephros (AGM) region of the midgestation embryo, but the cell type responsible for their emergence is unknown since critical hematopoietic factors are expressed in both the AGM endothelium and its underlying mesenchyme. Here we employ a temporally restricted genetic tracing strategy to selectively label the endothelium, and separately its underlying mesenchyme, during AGM development. Lineage tracing endothelium, via an inducible VE-cadherin Cre line, reveals that the endothelium is capable of HSC emergence. The endothelial progeny migrate to the fetal liver, and later to the bone marrow, and are capable of expansion, self-renewal, and multilineage hematopoietic differentiation. HSC capacity is exclusively endothelial, as ex vivo analyses demonstrate lack of VE-cadherin Cre induction in circulating and fetal liver hematopoietic populations. Moreover, AGM mesenchyme, as selectively traced via a myocardin Cre line, is incapable of hematopoiesis. Our genetic tracing strategy therefore reveals an endothelial origin of HSCs.

New morphological aspects of blood islands formation in the embryonic mouse hearts

Vasculogenesis in embryonic hearts proceeds by formation of aggregates consisting of erythroblasts and endothelial cells. These aggregates are called blood-islands or blood-island-like structures. We aimed to characterize blood islands in mouse embryonic hearts at stages spanning from 11 dpc through 13 dpc, i.e. prior to the establishment of the coronary circulation. Our observations suggested that there are two types of blood islands. One formed by migrating nucleated erythroblasts, which associated with migrating endothelial cell and the second by in situ emergence of two kinds of cells belonging to separate populations: one resembling an erythroblast progenitor and the second resembling an endothelial-cell progenitor. The subepicardial blood islands contain nucleated erythroblasts, undifferentiated mesenchymal cells, platelets, and early lymphocytes. The subepicardial blood islands resemble vesicles with protruding prongs directed toward the myocardium. Ahead of the prongs, angiogenic sprouting and degradation of fibronectin is observed. Vesicles gradually change their shape from spherical to tubular at 13 dpc and grow and extend along the interventricular sulcuses forming vascular tubes. We presume that the vascular tubes located within the interventricular sulcuses are precursors of coronary veins. Our data seems to indicate that embryonic heart vasculogenesis is accompanied by hematopoiesis.

Runx1 Interactions in Stem Cell Biology

The study of developmental hematopoiesis has provided important insights into the molecules that establish and sustain this process throughout adult life. At the base of the hematopoietic hierarchy is the hematopoietic stem cell (HSC), which emerges in the mouse conceptus starting at 10.5 days post coitus (≥ 34 somite pair stages).1 HSCs have been found in several distinct sites: the yolk sac, umbilical and vitelline arteries, the dorsal aorta in the aorta/gonad/mesonephros (AGM) region, fetal liver, and, more recently, the placenta.1–4 HSCs emerge from these sites (yolk sac, umbilical and vitelline arteries, and AGM region) through the formation of intra-aortic hematopoietic clusters that develop from endothelium.5–7 Studies in mouse, zebrafish, chick, and frog embryos established that Runx1 (AML1) is the earliest specific marker of all definitive hematopoietic sites in the conceptus. Runx1 is expressed in endothelial and mesenchymal cells and in intraaortic hematopoietic clusters, and marks all committed HPs and HSCs in both the embryo and the adult.6,8–10 It has been proposed that Runx1 functions during the transition from a “hemogenic endothelium” to intra-aortic clusters and HSCs.8 Here, we show that deletion of Runx1 in vascular endothelial cadherin (VEC) positive cells blocks the emergence of intra-aortic hematopoietic clusters, HPs, and HSCs. Greater than 95% of adult bone marrow cells are marked when VEC-Cre is used to delete a Rosa26 reporter allele, demonstrating that almost all blood cells have transited through a VEC+ intermediate at some point in their life. On the other hand, Runx1 deletion with Vav-Cre, which occurs in fetal liver HPs and HSCs, does not block hematopoiesis. Collectively, these data demonstrate that Runx1 is absolutely required in endothelial cells for hematopoietic cluster, HP, and HSC formation, but after HSCs are born from endothelium, Runx1 is no longer required to maintain them.

A perivascular origin for mesenchymal stem cells in multiple human organs

Mesenchymal stem cells (MSCs), the archetypal multipotent progenitor cells derived in cultures of developed organs, are of unknown identity and native distribution. We have prospectively identified perivascular cells, principally pericytes, in multiple human organs including skeletal muscle, pancreas, adipose tissue, and placenta, on CD146, NG2, and PDGF-Rbeta expression and absence of hematopoietic, endothelial, and myogenic cell markers. Perivascular cells purified from skeletal muscle or nonmuscle tissues were myogenic in culture and in vivo. Irrespective of their tissue origin, long-term cultured perivascular cells retained myogenicity; exhibited at the clonal level osteogenic, chondrogenic, and adipogenic potentials; expressed MSC markers; and migrated in a culture model of chemotaxis. Expression of MSC markers was also detected at the surface of native, noncultured perivascular cells. Thus, blood vessel walls harbor a reserve of progenitor cells that may be integral to the origin of the elusive MSCs and other related adult stem cells.

Impaired embryonic haematopoiesis yet normal arterial development in the absence of the Notch ligand Jagged1

Specific deletion of Notch1 and RBPjkappa in the mouse results in abrogation of definitive haematopoiesis concomitant with the loss of arterial identity at embryonic stage. As prior arterial determination is likely to be required for the generation of embryonic haematopoiesis, it is difficult to establish the specific haematopoietic role of Notch in these mutants. By analysing different Notch-ligand-null embryos, we now show that Jagged1 is not required for the establishment of the arterial fate but it is required for the correct execution of the definitive haematopoietic programme, including expression of GATA2 in the dorsal aorta. Moreover, successful haematopoietic rescue of the Jagged1-null AGM cells was obtained by culturing them with Jagged1-expressing stromal cells or by lentiviral-mediated transduction of the GATA2 gene. Taken together, our results indicate that Jagged1-mediated activation of Notch1 is responsible for regulating GATA2 expression in the AGM, which in turn is essential for definitive haematopoiesis in the mouse.

Sonic hedgehog is required for the assembly and remodeling of branchial arch blood vessels

Sonic hedgehog (Shh) is a morphogen involved in many developmental processes. Injection of cells (5E1) that produce a Shh-blocking antibody causes an attenuation of the Shh response, and this causes vascular malformations and impaired remodeling characterized by hemorrhages and protrusions of the anterior cardinal vein and outflow tract, delayed fusion of the dorsal aortae, impaired branching of the internal carotid artery, and delayed remodeling of the aortic arches. Distribution of smooth muscle cells in the vessel wall is unchanged. In 5E1-injected embryos, we also observed impaired assembly of endothelial cells into vascular tubes, particularly in the sixth branchial arch, around the anterior cardinal vein and around the dorsal aorta. In 5E1-treated embryos, increased numbers of macrophage-like cells, apoptotic cells, and a decreased level of proliferation were observed in head mesenchyme. Together, these observations show that Shh signaling is required at multiple stages for proper vessel formation and remodeling.

Mind bomb-1 is essential for intraembryonic hematopoiesis in the aortic endothelium and the subaortic patches

Intraembryonic hematopoiesis occurs at two different sites, the floor of the aorta and subaortic patches (SAPs) of the para-aortic splanchnopleura (P-Sp)/aorta-gonad-mesonephros (AGM) region. Notch1 and RBP-jkappa are critical for the specification of hematopoietic stem cells (HSCs) in Notch signal-receiving cells. However, the mechanism by which Notch signaling is triggered from the Notch signal-sending cells to support embryonic hematopoiesis remains to be determined. We previously reported that Mind bomb-1 (Mib1) regulates Notch ligands in the Notch signal-sending cells (B. K. Koo, M. J. Yoon, K. J. Yoon, S. K. Im, Y. Y. Kim, C. H. Kim, P. G. Suh, Y. N. Jan, and Y. Y. Kong, PLoS ONE 2:e1221, 2007). Here, we show that intraembryonic hematopoietic progenitors were absent in the P-Sp of Mib1(-/-) embryos, whereas they were partly preserved in the Tie2-cre; Mib1(f)(/f) P-Sps, suggesting that Mib1 plays a role in the endothelium and the SAPs. Interestingly, dll1 and dll4/Jag1 are expressed in the SAPs and the endothelium of the AGM, respectively, where mib1 is detected. Indeed, Notch signaling was activated in the nascent HSCs at both sites. In the P-Sp explant culture, the overexpression of Dll1 in OP9 stromal cells rescued the failed production of hematopoietic progenitors in the Mib1(-/-) P-Sp, while its activity was abolished by Mib1 knockdown. These results suggest that Mib1 is important for intraembryonic hematopoiesis not only in the aortic endothelium but also in the SAPs.

Molecular and developmental biology of the hemangioblast

The hemangioblast hypothesis was proposed a century ago. The existence of hemangioblasts is now demonstrated in mouse and human embryonic stem cell (ESC)-derived embryoid bodies (EBs), in the mouse and zebrafish gastrula, and in adults. The hemangioblast is believed to derive from mesodermal cells, and is enriched in the Bry+Flk1+ and Flk1+Scl+ cell populations in EBs and in the posterior primitive streak of the mouse gastrula and in the ventral mesoderm of the zebrafish gastrula. However, recent studies suggest that the hemangioblast does not give rise to all endothelial and hematopoietic lineages in mouse and zebrafish embryos. Although several signaling pathways are known to involve the generation of hemangioblasts, it remains largely unknown how the hemangioblast is formed and what are the master genes controlling hemangioblast development. This review will summarize our current knowledge, challenges, and future directions on molecular and developmental aspects of the hemangioblast.

Angiotensin-converting enzyme (CD143) marks hematopoietic stem cells in human embryonic, fetal, and adult hematopoietic tissues

Previous studies revealed that mAb BB9 reacts with a subset of CD34+ human BM cells with hematopoietic stem cell (HSC) characteristics. Here we map BB9 expression throughout hematopoietic development and show that the earliest definitive HSCs that arise at the ventral wall of the aorta and surrounding endothelial cells are BB9+. Thereafter, BB9 is expressed by primitive hematopoietic cells in fetal liver and in umbilical cord blood (UCB). BB9+CD34+ UCB cells transplanted into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice contribute 10-fold higher numbers of multilineage blood cells than their CD34+BB9− counterparts and contain a significantly higher incidence of SCID-repopulating cells than the unfractionated CD34+ population. Protein microsequencing of the 160-kDa band corresponding to the BB9 protein established its identity as that of somatic angiotensin-converting enzyme (ACE). Although the role of ACE on human HSCs remains to be determined, these studies designate ACE as a hitherto unrecognized marker of human HSCs throughout hematopoietic ontogeny and adulthood.

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.

Embryonic stromal clones reveal developmental regulators of definitive hematopoietic stem cells

Hematopoietic stem cell (HSC) self-renewal and differentiation is regulated by cellular and molecular interactions with the surrounding microenvironment. During ontogeny, the aorta-gonad-mesonephros (AGM) region autonomously generates the first HSCs and serves as the first HSC-supportive microenvironment. Because the molecular identity of the AGM microenvironment is as yet unclear, we examined two closely related AGM stromal clones that differentially support HSCs. Expression analyses identified three putative HSC regulatory factors, beta-NGF (a neurotrophic factor), MIP-1gamma (a C-C chemokine family member) and Bmp4 (a TGF-beta family member). We show here that these three factors, when added to AGM explant cultures, enhance the in vivo repopulating ability of AGM HSCs. The effects of Bmp4 on AGM HSCs were further studied because this factor acts at the mesodermal and primitive erythropoietic stages in the mouse embryo. In this report, we show that enriched E11 AGM HSCs express Bmp receptors and can be inhibited in their activity by gremlin, a Bmp antagonist. Moreover, our results reveal a focal point of Bmp4 expression in the mesenchyme underlying HSC containing aortic clusters at E11. We suggest that Bmp4 plays a relatively late role in the regulation of HSCs as they emerge in the midgestation AGM.

Sclerotomal origin of smooth muscle cells in the wall of the avian dorsal aorta

The dorsal aorta is the earliest formed intraembryonic blood vessel. It is composed of an inner lining consisting of endothelial cells and an outer wall consisting of smooth muscle cells (SMCs) and fibrocytes. Aortic SMCs have been suggested to arise from several developmental lineages. Cephalic neural crest provides SMCs of the proximal part of the aorta, and SMCs of the distal part are derived from the paraxial mesoderm. Here, we show by using quail-chick chimerization that in the avian embryo, SMCs in the wall of the dorsal aorta at trunk level arise from the sclerotome. Our findings indicate a two-step process of aortic wall formation. First, non-paraxial mesoderm-derived mural cells accumulate at the floor of the aorta. We refer to these cells as primary SMCs. Second, SMCs from the sclerotome are recruited to the roof and sides of the aorta, eventually replacing the primary SMCs in the aortic floor.

The hematopoietic stem cell and its niche: a comparative view

Stem cells have been identified as a source of virtually all highly differentiated cells that are replenished during the lifetime of an animal. The critical balance between stem and differentiated cell populations is crucial for the long-term maintenance of functional tissue types. Stem cells maintain this balance by choosing one of several alternate fates: self-renewal, commitment to differentiate, and senescence or cell death. These characteristics comprise the core criteria by which these cells are usually defined. The self-renewal property is important, as it allows for extended production of the corresponding differentiated cells throughout the life span of the animal. A microenvironment that is supportive of stem cells is commonly referred to as a stem cell niche. In this review, we first present some general concepts regarding stem cells and their niches, comparing stem cells of many different kinds from diverse organisms, and in the second part, we compare specific aspects of hematopoiesis and the niches that support hematopoiesis in Drosophila, zebrafish and mouse.

Gata2, Fli1, and Scl form a recursively wired gene-regulatory circuit during early hematopoietic development

Conservation of the vertebrate body plan has been attributed to the evolutionary stability of gene-regulatory networks (GRNs). We describe a regulatory circuit made up of Gata2, Fli1, and Scl/Tal1 and their enhancers, Gata2-3, Fli1+12, and Scl+19, that operates during specification of hematopoiesis in the mouse embryo. We show that the Fli1+12 enhancer, like the Gata2-3 and Scl+19 enhancers, targets hematopoietic stem cells (HSCs) and relies on a combination of Ets, Gata, and E-Box motifs. We show that the Gata2-3 enhancer also uses a similar cluster of motifs and that Gata2, Fli1, and Scl are expressed in embryonic day-11.5 dorsal aorta where HSCs originate and in fetal liver where they multiply. The three HSC enhancers in these tissues and in ES cell-derived hemangioblast equivalents are bound by each of these transcription factors (TFs) and form a fully connected triad that constitutes a previously undescribed example of both this network motif in mammalian development and a GRN kernel operating during the specification of a mammalian stem cell.

Hemogenic endothelial progenitor cells isolated from human umbilical cord blood

Hemogenic endothelium has been identified in embryonic dorsal aorta and in tissues generated from mouse embryonic stem cells, but to date there is no evidence for such bipotential cells in postnatal tissues or blood. Here we identify a cell population from human umbilical cord blood that gives rise to both endothelial cells and hematopoietic progenitors in vitro. Cord blood CD34+/CD133+ cells plated at high density in an endothelial basal medium formed an endothelial monolayer and a nonadherent cell population after 14-21 days. AML-1, a factor required for definitive hematopoiesis, was detected at low levels in adherent cells and at high levels in nonadherent cells. Nonadherent cells coexpressed the endothelial marker vascular endothelial (VE)-cadherin and the hematopoietic marker CD45, whereas adherent cells were composed primarily of VE-cadherin+/CD45- cells and a smaller fraction of VE-cadherin+/CD45+ cells. Both nonadherent and adherent cells produced hematopoietic colonies in methylcellulose, with the adherent cells yielding more colony-forming units (CFU)-GEMM compared with the nonadherent cells. To determine whether the adherent endothelial cells were producing hematopoietic progenitors, single cells from the adherent population were expanded in 96-well dishes for 14 days. The clonal populations expressed VE-cadherin, and a subset expressed AML-1, epsilon-globin, and gamma-globin. Three of 17 clonal cell populations gave rise to early CFU-GEMM hematopoietic progenitors and burst-forming unit-erythroid progenitors. These results provide evidence for hemogenic endothelial cells in human umbilical cord blood.

Prospective identification of myogenic endothelial cells in human skeletal muscle

We document anatomic, molecular and developmental relationships between endothelial and myogenic cells within human skeletal muscle. Cells coexpressing myogenic and endothelial cell markers (CD56, CD34, CD144) were identified by immunohistochemistry and flow cytometry. These myoendothelial cells regenerate myofibers in the injured skeletal muscle of severe combined immunodeficiency mice more effectively than CD56+ myogenic progenitors. They proliferate long term, retain a normal karyotype, are not tumorigenic and survive better under oxidative stress than CD56+ myogenic cells. Clonally derived myoendothelial cells differentiate into myogenic, osteogenic and chondrogenic cells in culture. Myoendothelial cells are amenable to biotechnological handling, including purification by flow cytometry and long-term expansion in vitro, and may have potential for the treatment of human muscle disease.

Directing human embryonic stem cells to generate vascular progenitor cells

Pluripotent human embryonic stem cells (hESCs) differentiate into most of the cell types of the adult human body, including vascular cells. Vascular cells, such as endothelial cells and vascular smooth muscle cells (SMCs) are significant contributors to tissue repair and regeneration. In addition to their potential applications for treatment of vascular diseases and stimulation of ischemic tissue growth, it is also possible that endothelial cells and SMCs derived from hESCs can be used to engineer artificial vessels to repair damaged vessels and form vessel networks in engineered tissues. Here we review the current status of directing hESCs to differentiate to vascular cells.

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.

Emergence of human angiohematopoietic cells in normal development and from cultured embryonic stem cells

Human hematopoiesis proceeds transiently in the extraembryonic yolk sac and embryonic, then fetal liver before being stabilized in the bone marrow during the third month of gestation. In addition to this classic developmental sequence, we have previously shown that the aorta-gonad-mesonephros (AGM) embryonic territory produces stem cells for definitive hematopoiesis from 27 to 40 days of human development, through an intermediate blood-forming endothelium stage. These studies have relied on the use of traditional markers of human hematopoietic and endothelial cells. In addition, we have recently identified and characterized a novel surface molecule, BB9, which typifies the earliest founders of the human angiohematopoietic system. BB9, which was initially identified with a monoclonal antibody raised to Stro-1(+) bone marrow stromal cells, recognizes in the adult the most primitive Thy-1(+) CD133(+) Lin(-), non-obese diabetic--severe combined immunodeficiency disease (NOD-SCID) mouse engrating hematopoietic stem cells (HSCs). In the 3- to 4-week embryo, BB9 expression typifies a subset of splanchnopleural mesodermal cells that migrate dorsally and colonize the ventral aspect of the aorta where they establish a population of hemogenic endothelial cells. We have indeed confirmed that hematopoietic potential in the human embryo, as assessed by long-term culture-initiating cell (LTC-IC) and SCID mouse reconstituting cell (SRC) activities, is confined to BB9-expressing cells. We have further validated these results in the model of human embryonic stem cells (hESCs) in which we have modeled, through the development of hematopoietic embryoid bodies (EBs), primitive and definitive hematopoieses. In this setting, we have documented the emergence of BB9(+) hemangioblast-like clonogenic angiohematopoietic progenitors that currently represent the earliest known founders of the human vascular and blood systems.

Functional identification of the hematopoietic stem cell niche in the ventral domain of the embryonic dorsal aorta

The first definitive/adult-type hematopoietic stem cells (HSCs) in the mouse aorta-gonad-mesonephros region emerge between embryonic days 10.5 and 11.5. The discovery of clusters of hematopoietic cells on the ventral luminal surface of the dorsal aorta in various vertebrate species has led to speculation that the floor of the dorsal aorta may play an essential role for the development of the definitive hematopoietic system. Here, we functionally show affiliation of definitive HSCs with the ventral floor of the dorsal aorta, whereas colony-forming hematopoietic activity is associated with both ventral and dorsal domains. We show that a rare population of PECAM1(high)CD45(+) cells, within which definitive HSCs reside, is predominantly localized to intraaortic clusters. Furthermore, using ex vivo culture analysis, we demonstrate that the ventral domain of the dorsal aorta has an exclusive functional capacity of inducing and expanding definitive HSCs.

SCL-GFP transgenic zebrafish: in vivo imaging of blood and endothelial development and identification of the initial site of definitive hematopoiesis

The bHLH transcription factor SCL plays a central role in the generation of hematopoietic cells in vertebrates. We modified a PAC containing the whole zebrafish scl locus, inserting GFP into the first coding exon of scl. In germline-transgenic zebrafish generated using this construct, GFP expression completely recapitulates the endogenous expression of scl in blood, endothelium and CNS. We performed in vivo timelapse imaging of blood and endothelial precursor migration at the single-cell level and show that these cells migrate from the posterior lateral plate mesoderm to their site of differentiation in the intermediate cell mass. The anterior lateral plate domain of GFP expression gives rise to primitive macrophages and the blood vessels of the head. In later embryos, GFP expression identifies clusters of hematopoietic cells that develop between the dorsal aorta and posterior cardinal veins after primitive erythrocytes have entered circulation. Two treatments that block definitive hematopoiesis (treatment with dioxin (TCDD), and injection of an antisense morpholino oligonucleotide targeted to runx1) ablate these hematopoietic clusters. This indicates that these clusters represent the first site of definitive hematopoiesis in zebrafish. This site is anatomically homologous to the proposed source of hematopoietic stem cells in amniotes, the aorta-gonad-mesonephros (AGM) region. A second transgenic line, containing the promoter of scl driving GFP, lacks expression in the definitive clusters.

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.