Tracing the progeny of the aortic hemangioblast in the avian embryo

Unravelling early hematoendothelial development through the chick model: Insights and future perspectives

The chicken embryo has been an important model in advancing our understanding of early hematoendothelial development, particularly in the formation of hematopoietic stem cells (HSCs) and the endothelial-to-hematopoietic transition (EHT). The accessibility and ease of manipulation of chicken embryos have made them an invaluable tool for researching development of blood and endothelial cells. Early research using this model provided pivotal insights, demonstrating that intra-embryonic regions, such as the dorsal aorta (DA), are primary sources of HSCs, rather than the yolk sac (YS), as previously believed. The identification of intra-aortic hematopoietic clusters (IAHCs) and the process of EHT in the chicken embryo laid the foundation for similar discoveries in other vertebrate species, including mice and zebrafish. Recent advances in genetic tools, such as transgenic chickens expressing fluorescent proteins, have further enhanced the precision of cell lineage tracing and real-time imaging of dynamic cellular processes. This review highlights both historical contributions and contemporary advancements facilitated by the chicken model, underscoring its continued relevance in developmental biology. By examining key findings and methodological innovations, we aim to demonstrate the importance of the chicken embryo as a model system for understanding hematoendothelial development and its potential for informing therapeutic applications in regenerative medicine and blood disorders. Finally, we will underscore potential applications of the chicken model for comparative and omics-level studies in conjunction with other model systems and what future directions lie ahead.

Thymus Ontogeny and Development

The thymus is a primary lymphoid organ composed of a three-dimensional (3D) epithelial network that provides a specialized microenvironment for the phenotypical and functional maturation of lymphoid progenitors. The specification of the pharyngeal endoderm to thymus fate occurs during the early stages of thymic organogenesis, independent of the expression of the transcription factor Foxn1. However, Foxn1 governs the later organogenesis of thymus together with the colonizing lymphoid cells. In the present chapter, we will review recent evidence on the topic covered in our original chapter (Muñoz and Zapata 2019). It described the early development of thymus and its resemblance to the development of endoderm-derived epithelial organs based on tubulogenesis and branching morphogenesis as well as the molecules known to be involved in these processes.

Endothelial heterogeneity in bone marrow: insights across development, adult life and leukemia

The central role of the endothelial microenvironment in orchestrating bone marrow (BM) homeostasis and hematopoietic support has been confirmed at various developmental stages and in adult life. The BM vasculature is crucial in mediating communication between BM parenchyma and circulating blood, displaying remarkable heterogeneity in structure and function. While vascular cell diversity in other tissues has long been recognized, the molecular basis of this phenomenon in BM is just now emerging. Over the past decade, single-cell approaches and microscopic observations have expanded our understanding of BM vasculature. While solely characterized for their paracrine properties in the past, recent advances have revolutionized our perception of endothelial function, revealing distinct anatomical locations associated with diverse endothelial cell states. The identification of phenotypic differences between normal and pathological conditions has therefore deepened our understanding of vascular dynamics and their impact on hematopoiesis in health and disease. In this review, we highlight key milestones and recent advances in understanding vascular heterogeneity within BM microenvironment during development, adulthood and aging. We also explore how leukemia affects this heterogeneity and how we can take this knowledge forward to improve clinical practices. By synthesizing existing literature, we aim to address unresolved questions and outline future research directions.

Capturing embryonic hematopoiesis in temporal and spatial dimensions

Hematopoietic stem cells (HSCs) possess the ability to sustain the continuous production of all blood cell types throughout an organism's lifespan. Although primarily located in the bone marrow of adults, HSCs originate during embryonic development. Visualization of the birth of HSCs, their developmental trajectory, and the specific interactions with their successive niches have significantly contributed to our understanding of the biology and mechanics governing HSC formation and expansion. Intravital techniques applied to live embryos or non-fixed samples have remarkably provided invaluable insights into the cellular and anatomical origins of HSCs. These imaging technologies have also shed light on the dynamic interactions between HSCs and neighboring cell types within the surrounding microenvironment or niche, such as endothelial cells or macrophages. This review delves into the advancements made in understanding the origin, production, and cellular interactions of HSCs, particularly during the embryonic development of mice and zebrafish, focusing on studies employing (live) imaging analysis.

CD41+ extracellular vesicles produced by avian thrombocytes contain microRNAs

Avians have thrombocytes in their blood circulation rather than mammalian platelets. However, many details of thrombocyte characteristics have not been determined. Here, chicken thrombocytes were isolated, and extracellular vesicle (EV) production was investigated. The thrombocyte-specific markers cd41 and cd61 were expressed in the yolk sac at 24 h. According to the embryonic developmental stage, the cd41-expressing tissues changed from the yolk sac to the bone marrow and spleen. Accordingly, the bone marrow and spleen were the main tissues producing thrombocytes in adult chickens. Avian thrombocytes were separated from adult spleen cells through a combination of discontinuous density gradient centrifugation, phagocytic cell removal, and fluorescence-activated cell sorting. Isolated thrombocytes produced CD41+ EVs (CD41+ EVs), and the CD41+ EVs also expressed CD9. Microarray analysis revealed that CD41+ EVs contain many microRNAs. Macrophage lines (RAW264.7) phagocytosed CD41+ EVs, and their phagocytosis and migration activity were suppressed. Microarray analysis also revealed that EVs altered gene expression in macrophages. These data indicated that the CD41+ EV was a carrier of microRNAs produced from thrombocytes and affected the cell characteristics of the received cells. Therefore, the CD41+ EVs of avians worked as a communication tool.

The genesis of human hematopoietic stem cells

Developmental hematopoiesis consists of multiple, partially overlapping hematopoietic waves that generate the differentiated blood cells required for embryonic development while establishing a pool of undifferentiated hematopoietic stem cells (HSCs) for postnatal life. This multilayered design in which active hematopoiesis migrates through diverse extra and intraembryonic tissues has made it difficult to define a roadmap for generating HSCs vs non-self-renewing progenitors, especially in humans. Recent single-cell studies have helped in identifying the rare human HSCs at stages when functional assays are unsuitable for distinguishing them from progenitors. This approach has made it possible to track the origin of human HSCs to the unique type of arterial endothelium in the aorta-gonad-mesonephros region and document novel benchmarks for HSC migration and maturation in the conceptus. These studies have delivered new insights into the intricate process of HSC generation and provided tools to inform the in vitro efforts to replicate the physiological developmental journey from pluripotent stem cells via distinct mesodermal and endothelial intermediates to HSCs.

Characterization and functional properties of a novel monoclonal antibody which identifies a B cell subpopulation in bursa of Fabricius

The bursa of Fabricius (BF) plays a central role in the development of B lymphocytes in birds. During embryonic development the BF primordium is colonized by myeloid and lymphoid prebursal stem cells to form the follicle buds, which ultimately develop into lymphoid follicles with a central medullary and an outer cortical region. Lympho-myeloid differentiation within the medulla is fundamental to normal B cell development. In contrast, the complexity of the cellular composition of the follicular cortex and its role in B cell differentiation has only recently begun to be studied. As an effort to characterize the different bursal cells we have produced a large panel of monoclonal antibodies (mAbs) by immunizing mice with a BF cell suspension of guinea fowl (Numida meleagris). One of these antibodies (clone: 7H3) was found to recognize a 80 kDa cell surface antigen expressed first in the yolk sac blood island of 2-day-old guinea fowl and chicken embryos, and later detected in the embryonic circulation and primary lymphoid organs. Double immunofluorescence revealed that chB6+ (Bu-1+) B cells of embryonic BF co-express the 7H3 antigen. 7H3 immunoreactivity of the bursal follicles gradually diminished after hatching and only a subpopulation of cortical B cells expressed the 7H3 antigen. In addition, in post-hatched birds 7H3 mAb recognizes all T lymphocytes of the thymus, peripheral lymphoid organs and blood. Embryonic BF injected with the 7H3 mAb showed a near complete block of lymphoid follicle formation

In conclusion, 7H3 mAb labels a new differentiation antigen specific for avian hematopoietic cells, which migrate through the embryonic mesenchyme, colonize the developing BF lymphoid follicles, and differentiate into a subpopulation of cortical B cells. The staining pattern of the 7H3 mAb and the correlation of expression with cell migration suggest that the antigen will serve as valuable immunological marker for studying the ontogeny of avian B cells.

Hapln1b, a central organizer of the extracellular matrix, modulates kit signalling to control developmental haematopoiesis

During early vertebrate development, hematopoietic stem and progenitor cells (HSPCs) are produced from hemogenic endothelium located in the dorsal aorta, before they migrate to a transient niche where they expand, the fetal liver and the caudal hematopoietic tissue (CHT), in mammals and zebrafish, respectively. In zebrafish, previous studies have shown that the extracellular matrix (ECM) around the aorta needs to be degraded to allow HSPCs to leave the aortic floor and reach blood circulation. However, the role of the ECM components in HSPC specification has never been addressed. We show here that hapln1b, a key component of the ECM is specifically expressed in hematopoietic sites in the zebrafish embryo. Gain- and loss-of-function experiments all resulted in the absence of HSPCs in the early embryo, showing that hapln1b is required, at the correct level, to specify HSPCs in the hemogenic endothelium. Furthermore, we show that the expression of hapln1b is necessary to maintain the integrity of the ECM through its link domain. By combining functional analyses and computer modelling, we show that kitlgb interacts with the ECM to specify HSPCs. We demonstrate that the ECM is an integral component of the microenvironment and mediates cytokine signalling that is required for HSPC specification.

The hemogenic endothelium: a critical source for the generation of PSC-derived hematopoietic stem and progenitor cells

In vitro generation of hematopoietic cells and especially hematopoietic stem cells (HSCs) from human pluripotent stem cells (PSCs) are subject to intensive research in recent decades, as these cells hold great potential for regenerative medicine and autologous cell replacement therapies. Despite many attempts, in vitro, de novo generation of bona fide HSCs remains challenging, and we are still far away from their clinical use, due to insufficient functionality and quantity of the produced HSCs. The challenges of generating PSC-derived HSCs are already apparent in early stages of hemato-endothelial specification with the limitation of recapitulating complex, dynamic processes of embryonic hematopoietic ontogeny in vitro. Further, these current shortcomings imply the incompleteness of our understanding of human ontogenetic processes from embryonic mesoderm over an intermediate, specialized hemogenic endothelium (HE) to their immediate progeny, the HSCs. In this review, we examine the recent investigations of hemato-endothelial ontogeny and recently reported progress for the conversion of PSCs and other promising somatic cell types towards HSCs with the focus on the crucial and inevitable role of the HE to achieve the long-standing goal—to generate therapeutically applicable PSC-derived HSCs in vitro.

Hematopoiesis: A Layered Organization Across Chordate Species

The identification of distinct waves of progenitors during development, each corresponding to a specific time, space, and function, provided the basis for the concept of a "layered" organization in development. The concept of a layered hematopoiesis was established by classical embryology studies in birds and amphibians. Recent progress in generating reliable lineage tracing models together with transcriptional and proteomic analyses in single cells revealed that, also in mammals, the hematopoietic system evolves in successive waves of progenitors with distinct properties and fate. During embryogenesis, sequential waves of hematopoietic progenitors emerge at different anatomic sites, generating specific cell types with distinct functions and tissue homing capacities. The first progenitors originate in the yolk sac before the emergence of hematopoietic stem cells, some giving rise to progenies that persist throughout life. Hematopoietic stem cell-derived cells that protect organisms against environmental pathogens follow the same sequential strategy, with subsets of lymphoid cells being only produced during embryonic development. Growing evidence indicates that fetal immune cells contribute to the proper development of the organs they seed and later ensure life-long tissue homeostasis and immune protection. They include macrophages, mast cells, some γδ T cells, B-1 B cells, and innate lymphoid cells, which have "non-redundant" functions, and early perturbations in their development or function affect immunity in the adult. These observations challenged the view that all hematopoietic cells found in the adult result from constant and monotonous production from bone marrow-resident hematopoietic stem cells. In this review, we evaluate evidence for a layered hematopoietic system across species. We discuss mechanisms and selective pressures leading to the temporal generation of different cell types. We elaborate on the consequences of disturbing fetal immune cells on tissue homeostasis and immune development later in life.

Regulation of Hemogenic Endothelial Cell Development and Function

Embryonic definitive hematopoiesis generates hematopoietic stem and progenitor cells (HSPCs) essential for establishment and maintenance of the adult blood system. This process requires the specification of a subset of vascular endothelial cells to become blood-forming, or hemogenic, and the subsequent endothelial-to-hematopoietic transition to generate HSPCs therefrom. The mechanisms that regulate these processes are under intensive investigation, as their recapitulation in vitro from human pluripotent stem cells has the potential to generate autologous HSPCs for clinical applications. In this review, we provide an overview of hemogenic endothelial cell development and highlight the molecular events that govern hemogenic specification of vascular endothelial cells and the generation of multilineage HSPCs from hemogenic endothelium. We also discuss the impact of hemogenic endothelial cell development on adult hematopoiesis.

Discovering New Progenitor Cell Populations through Lineage Tracing and In Vivo Imaging

Identification of progenitor cells that generate differentiated cell types during development, regeneration, and disease states is central to understanding the mechanisms governing such transitions. For more than a century, different lineage-tracing strategies have been developed, which helped disentangle the complex relationship between progenitor cells and their progenies. In this review, we discuss how lineage-tracing analyses have evolved alongside technological advances, and how this approach has contributed to the identification of progenitor cells in different contexts of cell differentiation. We also highlight a few examples in which lineage-tracing experiments have been instrumental for resolving long-standing debates and for identifying unexpected cellular origins. This discussion emphasizes how this century-old quest to delineate cellular lineage relationships is still active, and new discoveries are being made with the development of newer methodologies.

In vivo generation of haematopoietic stem/progenitor cells from bone marrow-derived haemogenic endothelium

It is well established that haematopoietic stem and progenitor cells (HSPCs) are generated from a transient subset of specialized endothelial cells termed haemogenic, present in the yolk sac, placenta and aorta, through an endothelial-to-haematopoietic transition (EHT). HSPC generation via EHT is thought to be restricted to the early stages of development. By using experimental embryology and genetic approaches in birds and mice, respectively, we document here the discovery of a bone marrow haemogenic endothelium in the late fetus/young adult. These cells are capable of de novo producing a cohort of HSPCs in situ that harbour a very specific molecular signature close to that of aortic endothelial cells undergoing EHT or their immediate progenies, i.e., recently emerged HSPCs. Taken together, our results reveal that HSPCs can be generated de novo past embryonic stages. Understanding the molecular events controlling this production will be critical for devising innovative therapies.

A novel hiPSC-derived system for hematoendothelial and myeloid blood toxicity screens identifies compounds promoting and inhibiting endothelial-to-hematopoietic transition

The exposure to toxic environmental and pharmaceutical substances can pose a long-term risk to human's health. In this study, we sought to investigate the potential of our recently developed method for induction of myeloid hematoendothelial and blood cells by overexpression of two transcription factors, GATA2 and ETV2, in human induced pluripotent stem cells (hiPSCs) for toxicity screening. For the primary screen in a high-throughput format, we selected twenty-two chemicals with various degrees of cytotoxicity available from the NIEHS National Toxicology Program (Tox21). The compounds were applied during the endothelial-to-hematopoietic transition and to differentiated myeloid progenitors growing in suspension. The system was capable of identifying compounds with both inhibitory and favorable effects on hematopoietic network, changes in expression of hematopoietic markers, and mitochondrial and cytotoxicity. The findings were confirmed and further investigated by secondary screens, colony forming cell assay, and gene expression profiling. The hematoendothelial toxicity of 5-fluorouracil, berberine chloride, and benzo(a)pyrene is characterized by the inhibition of cell division and a shift of hematopoietic programming to non-hemogenic endothelial and mesenchymal fates. This study demonstrates the feasibility of transcription factor (TF)-based differentiation systems to monitor endothelial and hematotoxicity and serves as an informative platform for screening myelosuppressive or stimulatory drugs and mechanistic studies of their action.

How HSCs Colonize and Expand in the Fetal Niche of the Vertebrate Embryo: An Evolutionary Perspective

Rare hematopoietic stem cells (HSCs) can self-renew, establish the entire blood system and represent the basis of regenerative medicine applied to hematological disorders. Clinical use of HSCs is however limited by their inefficient expansion ex vivo, creating a need to further understand HSC expansion in vivo. After embryonic HSCs are born from the hemogenic endothelium, they migrate to the embryonic/fetal niche, where the future adult HSC pool is established by considerable expansion. This takes place at different anatomical sites and is controlled by numerous signals. HSCs then migrate to their adult niche, where they are maintained throughout adulthood. Exactly how HSC expansion is controlled during embryogenesis remains to be characterized and is an important step to improve the therapeutic use of HSCs. We will review the current knowledge of HSC expansion in the different fetal niches across several model organisms and highlight possible clinical applications.

Zebrafish disease models in hematology: Highlights on biological and translational impact

Zebrafish (Danio rerio) has proven to be a versatile and reliable in vivo experimental model to study human hematopoiesis and hematological malignancies. As vertebrates, zebrafish has significant anatomical and biological similarities to humans, including the hematopoietic system. The powerful genome editing and genome-wide forward genetic screening tools have generated models that recapitulate human malignant hematopoietic pathologies in zebrafish and unravel cellular mechanisms involved in these diseases. Moreover, the use of zebrafish models in large-scale chemical screens has allowed the identification of new molecular targets and the design of alternative therapies. In this review we summarize the recent achievements in hematological research that highlight the power of the zebrafish model for discovery of new therapeutic molecules. We believe that the model is ready to give an immediate translational impact into the clinic.

Hif-1α and Hif-2α regulate hemogenic endothelium and hematopoietic stem cell formation in zebrafish

During development, hematopoietic stem cells (HSCs) derive from specialized endothelial cells (ECs) called hemogenic endothelium (HE) via a process called endothelial-to-hematopoietic transition (EHT). Hypoxia-inducible factor-1α (HIF-1α) has been reported to positively modulate EHT in vivo, but current data indicate the existence of other regulators of this process. Here we show that in zebrafish, Hif-2α also positively modulates HSC formation. Specifically, HSC marker gene expression is strongly decreased in hif-1aa;hif-1ab (hif-1α) and in hif-2aa;hif-2ab (hif-2α) zebrafish mutants and morphants. Moreover, live imaging studies reveal a positive role for hif-1α and hif-2α in regulating HE specification. Knockdown of hif-2α in hif-1α mutants leads to a greater decrease in HSC formation, indicating that hif-1α and hif-2α have partially overlapping roles in EHT. Furthermore, hypoxic conditions, which strongly stimulate HSC formation in wild-type animals, have little effect in the combined absence of Hif-1α and Hif-2α function. In addition, we present evidence for Hif and Notch working in the same pathway upstream of EHT. Both notch1a and notch1b mutants display impaired EHT, which cannot be rescued by hypoxia. However, overexpression of the Notch intracellular domain in ECs is sufficient to rescue the hif-1α and hif-2α morphant EHT phenotype, suggesting that Notch signaling functions downstream of the Hif pathway during HSC formation. Altogether, our data provide genetic evidence that both Hif-1α and Hif-2α regulate EHT upstream of Notch signaling.

The TGFβ pathway is a key player for the endothelial-to-hematopoietic transition in the embryonic aorta

The embryonic aorta produces hematopoietic stem and progenitor cells from a hemogenic endothelium localized in the aortic floor through an endothelial to hematopoietic transition. It has been long proposed that the Bone Morphogenetic Protein (BMP)/Transforming Growth Factor ß (TGFß) signaling pathway was implicated in aortic hematopoiesis but the very nature of the signal was unknown. Here, using thorough expression analysis of the BMP/TGFß signaling pathway members in the endothelial and hematopoietic compartments of the aorta at pre-hematopoietic and hematopoietic stages, we show that the TGFß pathway is preferentially balanced with a prominent role of Alk1/TgfßR2/Smad1 and 5 on both chicken and mouse species. Functional analysis using embryonic stem cells mutated for Acvrl1 revealed an enhanced propensity to produce hematopoietic cells. Collectively, we reveal that TGFß through the Alk1/TgfßR2 receptor axis is acting on endothelial cells to produce hematopoiesis.

Human haematopoietic stem cell development: from the embryo to the dish

Haematopoietic stem cells (HSCs) emerge during embryogenesis and give rise to the adult haematopoietic system. Understanding how early haematopoietic development occurs is of fundamental importance for basic biology and medical sciences, but our knowledge is still limited compared with what we know of adult HSCs and their microenvironment. This is particularly true for human haematopoiesis, and is reflected in our current inability to recapitulate the development of HSCs from pluripotent stem cells in vitro In this Review, we discuss what is known of human haematopoietic development: the anatomical sites at which it occurs, the different temporal waves of haematopoiesis, the emergence of the first HSCs and the signalling landscape of the haematopoietic niche. We also discuss the extent to which in vitro differentiation of human pluripotent stem cells recapitulates bona fide human developmental haematopoiesis, and outline some future directions in the field.

Restricted intra-embryonic origin of bona fide hematopoietic stem cells in the chicken

Hematopoietic stem cells (HSCs), which are responsible for blood cell production, are generated during embryonic development. Human and chicken embryos share features that position the chicken as a reliable and accessible alternative model to study developmental hematopoiesis. However, the existence of HSCs has never been formally proven in chicken embryos. Here, we have established a complete cartography and quantification of hematopoietic cells in the aorta during development. We demonstrate the existence of bona fide HSCs, originating from the chicken embryo aorta (and not the yolk sac, allantois or head), through an in vivo transplantation assay. Embryos transplanted in ovo with GFP embryonic tissues on the chorio-allantoic membrane provided multilineage reconstitution in adulthood. Historically, most breakthrough discoveries in the field of developmental hematopoiesis were first made in birds and later extended to mammals. Our study sheds new light on the avian model as a valuable system to study HSC production and regulation in vivo.

Regulation of the hematopoietic stem cell lifecycle by the endothelial niche

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) predominantly reside either in direct contact or in close proximity to the vascular endothelium throughout their lifespan. From the moment of HSC embryonic specification from hemogenic endothelium, endothelial cells (ECs) act as a critical cellular-hub that regulates a vast repertoire of biological processes crucial for HSC maintenance throughout its lifespan. In this review, we will discuss recent findings in endothelial niche-mediated regulation of HSC function during development, aging and regenerative conditions.

RECENT FINDINGS

Studies employing genetic vascular models have unequivocally confirmed that ECs provide the essential instructive cues for HSC emergence during embryonic development as well as adult HSC maintenance during homeostasis and regeneration. Aging of ECs may impair their ability to maintain HSC function contributing to the development of aging-associated hematopoietic deficiencies. These findings have opened up new avenues to explore the therapeutic application of ECs. ECs can be adapted to serve as an instructive platform to expand bona fide HSCs and also utilized as a cellular therapy to promote regeneration of the hematopoietic system following myelosuppressive and myeloablative injuries.

SUMMARY

ECs provide a fertile niche for maintenance of functional HSCs throughout their lifecycle. An improved understanding of the EC-HSC cross-talk will pave the way for development of EC-directed strategies for improving HSC function during aging.

A view of human haematopoietic development from the Petri dish

Human pluripotent stem cells (hPSCs) provide an unparalleled opportunity to establish in vitro differentiation models that will transform our approach to the study of human development. In the case of the blood system, these models will enable investigation of the earliest stages of human embryonic haematopoiesis that was previously not possible. In addition, they will provide platforms for studying the origins of human blood cell diseases and for generating de novo haematopoietic stem and progenitor cell populations for cell-based regenerative therapies.

The embryonic origins and genetic programming of emerging haematopoietic stem cells

Haematopoietic stem cells (HSCs) emerge from the haemogenic endothelium (HE) localised in the ventral wall of the embryonic dorsal aorta (DA). The HE generates HSCs through a process known as the endothelial to haematopoietic transition (EHT), which has been visualised in live embryos and is currently under intense study. However, EHT is the culmination of multiple programming events, which are as yet poorly understood, that take place before the specification of HE. A number of haematopoietic precursor cells have been described before the emergence of definitive HSCs, but only one haematovascular progenitor, the definitive haemangioblast (DH), gives rise to the DA, HE and HSCs. DHs emerge in the lateral plate mesoderm (LPM) and have a distinct origin and genetic programme compared to other, previously described haematovascular progenitors. Although DHs have so far only been established in Xenopus embryos, evidence for their existence in the LPM of mouse and chicken embryos is discussed here. We also review the current knowledge of the origins, lineage relationships, genetic programming and differentiation of the DHs that leads to the generation of HSCs. Importantly, we discuss the significance of the gene regulatory network (GRN) that controls the programming of DHs, a better understanding of which may aid in the establishment of protocols for the de novo generation of HSCs in vitro.

Extravascular endothelial and hematopoietic islands form through multiple pathways in midgestation mouse embryos

The de novo generation of hematopoietic cells occurs during midgestation when a population of endothelial cells called hemogenic endothelium transitions into hematopoietic progenitors and stem cells. In mammalian embryos, the newly formed hematopoietic cells form clusters in the lumens of the major arteries in the embryo proper and in the vascular plexus of the yolk sac. Small clusters of hematopoietic cells that are independent of the vasculature (referred to here as extravascular islands) were shown to form in the mesentery during vascular remodeling of the vitelline artery. Using three-dimensional imaging of whole mouse embryos we demonstrate that extravascular budding of hematopoietic clusters is a more widespread phenomenon that occurs from the vitelline and the umbilical arteries both proximal to the embryo proper and distal in the extraembryonic yolk sac and placenta. Furthermore, we show that there are several mechanisms by which hematopoietic clusters leave the arteries, including vascular remodeling and extrusion. Lastly, we provide static images suggesting that extravascular islands contribute to the formation of new blood vessels. Thus, extravascular islands may represent a novel mechanism of vasculogenesis whereby established vessels contribute endothelial and hematopoietic cells to developing vascular beds.

Specification and function of hemogenic endothelium during embryogenesis

Hemogenic endothelium is a specialized subset of developing vascular endothelium that acquires hematopoietic potential and can give rise to multilineage hematopoietic stem and progenitor cells during a narrow developmental window in tissues such as the extraembryonic yolk sac and embryonic aorta-gonad-mesonephros. Herein, we review current knowledge about the historical and developmental origins of hemogenic endothelium, the molecular events that govern hemogenic specification of vascular endothelial cells, the generation of multilineage hematopoietic stem and progenitor cells from hemogenic endothelium, and the potential for translational applications of knowledge gained from further study of these processes.

Regulation of Blood Stem Cell Development

Understanding how the blood system is formed is an ongoing fundamental research challenge. Developmental biology has provided many insights into the molecules and processes that affect the formation of the blood tissues, both in health and disease. It is of particular interest for clinical transplantation therapies to understand how hematopoietic stem cells (HSCs)-the self-renewing purveyors of the adult blood system that produce over 10 different functionally specialized cell lineages and over 10(11) cells daily-are generated during embryonic stages. Recent successes to reprogram the fate of adult differentiated cells to pluripotency and to other cell lineages now highlight the importance of identifying the cells and molecules that affect the in vivo developmental initiation of rare and robust transplantable HSCs. The close association of the developing hematopoietic and vascular system, hematopoietic cell mobility through the circulation, and the essential role of the embryonic hematopoietic system in adult hematopoietic cell development make this a formidable study. This chapter reviews the advances, controversies, and current state of our knowledge of the growing field of hematopoietic development, with a special focus on the regulation of the natural transdifferentiation of endothelial cells to HSCs within the developing embryo.

Complex regulation of HSC emergence by the Notch signaling pathway

Hematopoietic stem cells are formed during embryonic development, and serve as the foundation of the definitive blood program for life. Notch signaling has been well established as an essential direct contributor to HSC specification. However, several recent studies have indicated that the contribution of Notch signaling is complex. HSC specification requires multiple Notch signaling inputs, some received directly by hematopoietic precursors, and others that occur indirectly within neighboring somites. Of note, proinflammatory signals provided by primitive myeloid cells are needed for HSC specification via upregulation of the Notch pathway in hemogenic endothelium. In addition to multiple requirements for Notch activation, recent studies indicate that Notch signaling must subsequently be repressed to permit HSC emergence. Finally, Notch must then be reactivated to maintain HSC fate. In this review, we discuss the growing understanding of the dynamic contributions of Notch signaling to the establishment of hematopoiesis during development.

Gata2b is a restricted early regulator of hemogenic endothelium in the zebrafish embryo

The adult blood system is established by hematopoietic stem cells (HSCs), which arise during development from an endothelial-to-hematopoietic transition of cells comprising the floor of the dorsal aorta. Expression of aortic runx1 has served as an early marker of HSC commitment in the zebrafish embryo, but recent studies have suggested that HSC specification begins during the convergence of posterior lateral plate mesoderm (PLM), well before aorta formation and runx1 transcription. Further understanding of the earliest stages of HSC specification necessitates an earlier marker of hemogenic endothelium. Studies in mice have suggested that GATA2 might function at early stages within hemogenic endothelium. Two orthologs of Gata2 exist in zebrafish: gata2a and gata2b. Here, we report that gata2b expression initiates during the convergence of PLM, becoming restricted to emerging HSCs. We observe Notch-dependent gata2b expression within the hemogenic subcompartment of the dorsal aorta that is in turn required to initiate runx1 expression. Our results indicate that Gata2b functions within hemogenic endothelium from an early stage, whereas Gata2a functions more broadly throughout the vascular system.

Tenascin C promotes hematoendothelial development and T lymphoid commitment from human pluripotent stem cells in chemically defined conditions

The recent identification of hemogenic endothelium (HE) in human pluripotent stem cell (hPSC) cultures presents opportunities to investigate signaling pathways that are essential for blood development from endothelium and provides an exploratory platform for de novo generation of hematopoietic stem cells (HSCs). However, the use of poorly defined human or animal components limits the utility of the current differentiation systems for studying specific growth factors required for HE induction and manufacturing clinical-grade therapeutic blood cells. Here, we identified chemically defined conditions required to produce HE from hPSCs growing in Essential 8 (E8) medium and showed that Tenascin C (TenC), an extracellular matrix protein associated with HSC niches, strongly promotes HE and definitive hematopoiesis in this system. hPSCs differentiated in chemically defined conditions undergo stages of development similar to those previously described in hPSCs cocultured on OP9 feeders, including the formation of VE-Cadherin⁺CD73⁻CD235a/CD43⁻ HE and hematopoietic progenitors with myeloid and T lymphoid potential.

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Hemogenic endothelium is generated in a completely defined xenogen-free system

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The system reproduces all stages of hematopoietic development

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Tenascin C enhances hematoendothelial development from pluripotent stem cells

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Tenesacin C uniquely supports T cell specification

Direct induction of haematoendothelial programs in human pluripotent stem cells by transcriptional regulators

Advancing pluripotent stem cell technologies for modeling hematopoietic stem cell development and blood therapies requires identifying key regulators of hematopoietic commitment from human pluripotent stem cells (hPSCs). Here, by screening the effect of 27 candidate factors, we reveal two groups of transcriptional regulators capable of inducing distinct hematopoietic programs from hPSCs: panmyeloid (ETV2 and GATA2) and erythro-megakaryocytic (GATA2 and TAL1). In both cases, these transcription factors directly convert hPSCs to endothelium, which subsequently transforms into blood cells with pan-myeloid or erythromegakaryocytic potential. These data demonstrate that two distinct genetic programs regulate the hematopoietic development from hPSCs and that both of these programs specify hPSCs directly to hemogenic endothelial cells. Additionally, this study provides a novel method for the efficient induction of blood and endothelial cells from hPSCs via overexpression of modified mRNA for the selected transcription factors.

How the avian model has pioneered the field of hematopoietic development

The chicken embryo has a long history as a key model in developmental biology. Because of its distinctive developmental characteristics, it has contributed to major breakthroughs in the field of hematopoiesis. Among these, the discovery of B lymphocytes and the three rounds of thymus colonization; the embryonic origin of hematopoietic stem cells and the traffic between different hematopoietic organs; and the existence of two distinct endothelial cell lineages one angioblastic, restricted to endothelial cell production, and another, hemangioblastic, able to produce both endothelial and hematopoietic cells, should be cited. The avian model has also contributed to substantiate the endothelial-to-hematopoietic transition associated with aortic hematopoiesis and the existence of the allantois as a hematopoietic organ. Because the immune system develops relatively late in aves, the avian embryo is used to probe the tissue-forming potential of mouse tissues through mouse-into-chicken chimeras, providing insights into early mouse development by circumventing the lethality associated with some genetic strains. Finally, the avian embryo can be used to investigate the differentiation potential of human ES cells in the context of a whole organism. The combinations of classic approaches with the development of powerful genetic tools make the avian embryo a great and versatile model.

Developmental hematopoiesis: ontogeny, genetic programming and conservation

Hematopoietic stem cells (HSCs) sustain blood production throughout life and are of pivotal importance in regenerative medicine. Although HSC generation from pluripotent stem cells would resolve their shortage for clinical applications, this has not yet been achieved mainly because of the poor mechanistic understanding of their programming. Bone marrow HSCs are first created during embryogenesis in the dorsal aorta (DA) of the midgestation conceptus, from where they migrate to the fetal liver and, eventually, the bone marrow. It is currently accepted that HSCs emerge from specialized endothelium, the hemogenic endothelium, localized in the ventral wall of the DA through an evolutionarily conserved process called the endothelial-to-hematopoietic transition. However, the endothelial-to-hematopoietic transition represents one of the last steps in HSC creation, and an understanding of earlier events in the specification of their progenitors is required if we are to create them from naïve pluripotent cells. Because of their ready availability and external development, zebrafish and Xenopus embryos have enormously facilitated our understanding of the early developmental processes leading to the programming of HSCs from nascent lateral plate mesoderm to hemogenic endothelium in the DA. The amenity of the Xenopus model to lineage tracing experiments has also contributed to the establishment of the distinct origins of embryonic (yolk sac) and adult (HSC) hematopoiesis, whereas the transparency of the zebrafish has allowed in vivo imaging of developing blood cells, particularly during and after the emergence of HSCs in the DA. Here, we discuss the key contributions of these model organisms to our understanding of developmental hematopoiesis.

A short history of hemogenic endothelium

Definitive hematopoietic cells are generated de novo during ontogeny from a specialized subset of endothelium, the so-called hemogenic endothelium. In this review we give a brief overview of the identification of hemogenic endothelium, explore its links with the HSC lineage, and summarize recent insights into the nature of hemogenic endothelium and the microenvironmental and intrinsic regulators contributing to its transition into blood. Ultimately, a better understanding of the processes controlling the transition of endothelium into blood will advance the generation and expansion of hematopoietic stem cells for therapeutic purposes.

Deciphering the hierarchy of angiohematopoietic progenitors from human pluripotent stem cells

Identification of sequential progenitors leading to blood formation from pluripotent stem cells (PSCs) will be essential for understanding the molecular mechanisms of hematopoietic lineage specification and for development of technologies for in vitro production of hematopoietic stem cells (HSCs). It is well established that during development, blood and endothelial cells in the extraembryonic and embryonic compartments are formed in parallel from precursors with angiogenic and hematopoietic potentials. However, the identity and hierarchy of these precursors in human PSC (hPSC) cultures remain obscure. Using developmental stage-specific mesodermal and endothelial markers and functional assays, we recently identified discrete populations of angiohematopoietic progenitors from hPSCs, including mesodermal precursors and hemogenic endothelial cells with primitive and definitive hematopoietic potentials. In addition, we discovered a novel population of multipotent hematopoietic progenitors with an erythroid phenotype, which retain angiogenic potential. Here we introduce our recent findings and discuss their implication for defining putative HSC precursor and factors required for activation of self-renewal potential in hematopoietic cells emerging from endothelium.

How studies on the avian embryo have opened new avenues in the understanding of development: a view about the neural and hematopoietic systems

The chick embryo is as ancient a source of knowledge on animal development as the very beginning of embryology. Already, at the time of Caspar Friedrich Wolff, contemplating the strikingly beautiful scenario of the germ deploying on the yellow background of the yolk inspired and supported the tenants of epigenesis at the expense of the preformation theory. In this article, we shall mention some of the many problems of developmental biology that were successfully clarified by research on chick embryos. Two topics, the development of the neural system and that of blood and blood vessels, familiar to the authors, will be discussed in more detail.

Signaling from the sympathetic nervous system regulates hematopoietic stem cell emergence during embryogenesis

The first adult-repopulating hematopoietic stem cells (HSCs) emerge in the aorta-gonads-mesonephros (AGM) region of the embryo. We have recently identified the transcription factor Gata3 as being upregulated in this tissue specifically at the time of HSC emergence. We now demonstrate that the production of functional and phenotypic HSCs in the AGM is impaired in the absence of Gata3. Furthermore, we show that this effect on HSC generation is secondary to the role of Gata3 in the production of catecholamines, the mediators of the sympathetic nervous system (SNS), thus making these molecules key components of the AGM HSC niche. These findings demonstrate that the recently described functional interplay between the hematopoietic system and the SNS extends to the earliest stages of their codevelopment and highlight the fact that HSC development needs to be viewed in the context of the development of other organs.

Identification of the hemogenic endothelial progenitor and its direct precursor in human pluripotent stem cell differentiation cultures

Hemogenic endothelium (HE) has been recognized as a source of hematopoietic stem cells (HSCs) in the embryo. Access to human HE progenitors (HEPs) is essential for enabling the investigation of the molecular determinants of HSC specification. Here, we show that HEPs capable of generating definitive hematopoietic cells can be obtained from human pluripotent stem cells (hPSCs) and identified precisely by a VE-cadherin(+)CD73(-)CD235a/CD43(-) phenotype. This phenotype discriminates true HEPs from VE-cadherin(+)CD73(+) non-HEPs and VE-cadherin(+)CD235a(+)CD41a(-) early hematopoietic cells with endothelial and FGF2-dependent hematopoietic colony-forming potential. We found that HEPs arise at the post-primitive-streak stage of differentiation directly from VE-cadherin-negative KDR(bright)APLNR(+)PDGFRα(low/-) hematovascular mesodermal precursors (HVMPs). In contrast, hemangioblasts, which are capable of forming endothelium and primitive blood cells, originate from more immature APLNR(+)PDGFRα(+) mesoderm. The demarcation of HEPs and HVMPs provides a platform for modeling blood development from endothelium with a goal of facilitating the generation of HSCs from hPSCs.

Hemogenic endothelium during development and beyond

During embryonic development, multilineage HSCs/progenitor cells are derived from specialized endothelial cells, termed hemogenic endothelium, within the yolk sac, placenta, and aorta. Whether hemogenic endothelial cells contribute to blood cell development at other sites of definitive hematopoiesis, such as in the fetal liver and fetal bone marrow, is not known. Also unknown is whether such cells exist within the vasculature of adult bone marrow and generate hematopoietic stem cells after birth. These issues and their clinical relevance are discussed herein.

Notch signaling and development of the hematopoietic system

Notch signaling exerts multiple important functions in the hematopoietic system. Notch1-mediated signals are essential to induce the onset of definitive hematopoiesis within specialized domains of hemogenic endothelium in the fetal dorsal aorta. In contrast, Notch is dispensable for the subsequent maintenance of hematopoietic stem cells in the adult bone marrow. Notch is a key regulator of early T-cell development in the thymus. An expanding number of hematopoietic and lymphoid cell types have been reported to receive context-dependent inputs from the Notch pathway that regulate their differentiation and function. Progress in the field will continue to bring fundamental information about hematopoiesis and practical insights into the potential to modulate Notch signaling for therapeutic purposes.

Hemogenic endothelium: origins, regulation, and implications for vascular biology

The study of endothelial development has been intertwined with hematopoiesis since the early 20th century when a bi-potential cell (hemangioblast) was noted to produce both endothelial and hematopoietic cells. Since then, ideas regarding the nature of connection between the vascular and hematopoietic systems have ranged from a tenuous association to direct lineage origination. In this review, historical data that spans hematopoietic development is examined within the context of hemogenic endothelium. Hemogenic endothelium, a specialized endothelial population capable of hematopoiesis, is an emerging theory that has recently gained momentum. Evidence across species and decades are reviewed, as are the possible modulators of the phenomenon, which include pathways that specify definitive hematopoiesis (Runx1), arterial identity (Notch1), as well as physiological and developmental factors.

The vascular origin of hematopoietic cells

More than a century ago, several embryologists described sites of hematopoietic activity in the vascular wall of mid-gestation vertebrate embryos, and postulated the transient existence of a blood generating endothelium during ontogeny. This hypothesis gained significant attention in the 1970s when orthotopic transplantation experiments between quail and chick embryos revealed specific vascular areas as the site of the origin of definitive hematopoiesis. However, the vascular origin of hematopoietic precursors remained elusive and controversial for decades. Only recently, multiple experimental approaches have clearly documented that during vertebrate development definitive hematopoietic precursors arise from a subset of vascular endothelial cells. Interestingly, this differentiation is promoted by the intravascular fluid mechanical forces generated by the establishment of blood flow upon the initiation of heartbeat, and it is therefore connected with cardiovascular development in several critical aspects. In this review we present our current understanding of the relationship between vascular and definitive hematopoietic development through an historical analysis of the scientific evidence produced in this area of investigation.

On the origin of hematopoietic stem cells: progress and controversy

Hematopoietic Stem Cells (HSCs) are responsible for the production and replenishment of all blood cell types during the entire life of an organism. Generated during embryonic development, HSCs transit through different anatomical niches where they will expand before colonizing in the bone marrow, where they will reside during adult life. Although the existence of HSCs has been known for more than fifty years and despite extensive research performed in different animal models, there is still uncertainty with respect to the precise origins of HSCs. We review the current knowledge on embryonic hematopoiesis and highlight the remaining questions regarding the anatomical and cellular identities of HSC precursors.