Stem cell emergence and hemopoietic activity are incompatible in mouse intraembryonic sites

Spatiotemporal dynamics of cytokines expression dictate fetal liver hematopoiesis

During embryogenesis, yolk-sac and intra-embryonic-derived hematopoietic progenitors, comprising the precursors of adult hematopoietic stem cells, converge into the fetal liver. With a new staining strategy, we defined all non-hematopoietic components of the fetal liver and found that hepatoblasts are the major producers of hematopoietic growth factors. We identified mesothelial cells, a novel component of the stromal compartment, producing Kit ligand, a major hematopoietic cytokine. A high-definition imaging dataset analyzed using a deep-learning based pipeline allowed the unambiguous identification of hematopoietic and stromal populations, and enabled determining a neighboring network composition, at the single cell resolution. Throughout active hematopoiesis, progenitors preferentially associate with hepatoblasts, but not with stellate or endothelial cells. We found that, unlike yolk sac-derived progenitors, intra-embryonic progenitors respond to a chemokine gradient created by CXCL12-producing stellate cells. These results revealed that FL hematopoiesis is a spatiotemporal dynamic process, defined by an environment characterized by low cytokine concentrations.

How cell migration helps immune sentinels

The immune system relies on the migratory capacity of its cellular components, which must be mobile in order to defend the host from invading micro-organisms or malignant cells. This applies in particular to immune sentinels from the myeloid lineage, i.e. macrophages and dendritic cells. Cell migration is already at work during mammalian early development, when myeloid cell precursors migrate from the yolk sac, an extra embryonic structure, to colonize tissues and form the pool of tissue-resident macrophages. Later, this is accompanied by a migration wave of precursors and monocytes from the bone marrow to secondary lymphoid organs and the peripheral tissues. They differentiate into DCs and monocyte-derived macrophages. During adult life, cell migration endows immune cells with the ability to patrol their environment as well as to circulate between peripheral tissues and lymphoid organs. Hence migration of immune cells is key to building an efficient defense system for an organism. In this review, we will describe how cell migratory capacity regulates the various stages in the life of myeloid cells from development to tissue patrolling, and migration to lymph nodes. We will focus on the role of the actin cytoskeletal machinery and its regulators, and how it contributes to the establishment and function of the immune system.

Embryonic Origins of the Hematopoietic System: Hierarchies and Heterogeneity

The hierarchical framework of the adult blood system as we know it from current medical and hematology textbooks, displays a linear branching network of dividing and differentiated cells essential for the growth and maintenance of the healthy organism. This view of the hierarchy has evolved over the last 75 years. An amazing increase in cellular complexity has been realized; however, innovative single-cell technologies continue to uncover essential cell types and functions in animal models and the human blood system. The most potent cell of the hematopoietic hierarchy is the hematopoietic stem cell. Stem cells for adult tissues are the long-lived self-renewing cellular component, which ensure that differentiated tissue-specific cells are maintained and replaced through the entire adult lifespan. Although much blood research is focused on hematopoietic tissue homeostasis, replacement and regeneration during adult life, embryological studies have widened and enriched our understanding of additional developmental hierarchies and interacting cells of this life-sustaining tissue. Here, we review the current state of knowledge of the hierarchical organization and the vast heterogeneity of the hematopoietic system from embryonic to adult stages.

Megakaryocyte production is sustained by direct differentiation from erythromyeloid progenitors in the yolk sac until midgestation

The extra-embryonic yolk sac contains the first definitive multipotent hematopoietic cells, denominated erythromyeloid progenitors. They originate in situ prior to the emergence of hematopoietic stem cells and give rise to erythroid, monocytes, granulocytes, mast cells and macrophages, the latter in a Myb transcription factor-independent manner. We uncovered here the heterogeneity of yolk sac erythromyeloid progenitors, at the single cell level, and discriminated multipotent from committed progenitors, prior to fetal liver colonization. We identified two temporally distinct megakaryocyte differentiation pathways. The first occurs in the yolk sac, bypasses intermediate bipotent megakaryocyte-erythroid progenitors and, similar to the differentiation of macrophages, is Myb-independent. By contrast, the second originates later, from Myb-dependent bipotent progenitors expressing Csf2rb and colonize the fetal liver, where they give rise to megakaryocytes and to large numbers of erythrocytes. Understanding megakaryocyte development is crucial as they play key functions during vascular development, in particular in separating blood and lymphatic networks.

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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.

The mechanisms of pathological extramedullary hematopoiesis in diseases

Extramedullary hematopoiesis (EMH) is the expansion and differentiation of hematopoietic stem and progenitor cells outside of the bone marrow. In postnatal life, as a compensatory mechanism for ineffective hematopoiesis of the bone marrow, pathological EMH is triggered by hematopoietic disorders, insufficient hematopoietic compensation, and other pathological stress conditions, such as infection, advanced tumors, anemia, and metabolic stress. Pathological EMH has been reported in many organs, and the sites of pathological EMH may be related to reactivation of the embryonic hematopoietic structure in these organs. As a double-edged sword (blood and immune cell supplementation as well as clinical complications), pathological EMH has been widely studied in recent years. In particular, pathological EMH induced by late-stage tumors contributes to tumor immunosuppression. Thus, a deeper understanding of the mechanism of pathological EMH may be conducive to the development of therapies against the pathological processes that induce EMH. This article reviews the recent progress of research on the cellular and molecular mechanisms of pathological EMH in specific diseases.

Bone marrow contribution to the heart from development to adulthood

Cardiac chamber walls contain large numbers of non-contractile interstitial cells, including fibroblasts, endothelial cells, pericytes and significant populations of blood lineage-derived cells. Blood cells first colonize heart tissues a few days before birth, although their recruitment from the bloodstream to the cardiac interstitium is continuous and extends throughout adult life. The bone marrow, as the major hematopoietic site of adult individuals, is in charge of renewing all circulating cell types, and it therefore plays a pivotal role in the incorporation of blood cells to the heart. Bone marrow-derived cells are instrumental to tissue homeostasis in the steady-state heart, and are major effectors in cardiac disease progression. This review will provide a comprehensive approach to bone marrow-derived blood cell functions in the heart, and discuss aspects related to hot topics in the cardiovascular field like cell-based heart regeneration strategies.

Activation of the vitamin D receptor transcription factor stimulates the growth of definitive erythroid progenitors

The pathways that regulate the growth of erythroid progenitors are incompletely understood. In a computational analysis of gene expression changes during erythroid ontogeny, the vitamin D receptor (Vdr) nuclear hormone receptor transcription factor gene was identified in fetal and adult stages, but not at the embryonic stage of development. Vdr was expressed in definitive erythroid (EryD) progenitors and was downregulated during their maturation. Activation of Vdr signaling by the vitamin D3 agonist calcitriol increased the outgrowth of EryD colonies from fetal liver and adult bone marrow, maintained progenitor potential, and delayed erythroid maturation, as revealed by clonogenic assays, suspension culture, cell surface phenotype, and gene expression analyses. The early (cKit⁺CD71^lo/neg), but not the late (cKit⁺CD71^hi), EryD progenitor subset of Lin^negcKit⁺ cells was responsive to calcitriol. Culture of cKit⁺CD71^lo/neg progenitors in the presence of both vitamin D3 and glucocorticoid receptor ligands resulted in an increase in proliferation that was at least additive compared with either ligand alone. Lentivirus shRNA-mediated knockdown of Vdr expression abrogated the stimulation of early erythroid progenitor growth by calcitriol. These findings suggest that Vdr has a cell-intrinsic function in early erythroid progenitors. Targeting of downstream components of the Vdr signaling pathway may lead to new approaches for the expansion of erythroid progenitors ex vivo.

Embryonic Intra-Aortic Clusters Undergo Myeloid Differentiation Mediated by Mesonephros-Derived CSF1 in Mouse

The aorta-gonad-mesonephros (AGM) region contains intra-aortic clusters (IACs) thought to have acquired hematopoietic stem cell (HSC) potential in vertebrate embryos. To assess extrinsic regulation of IACs in the AGM region, we employed mouse embryos harboring a Sall1-GFP reporter gene, which allows identification of mesonephros cells based on GFP expression. Analysis of AGM region tissue sections confirmed mesonephros GFP expression. Mesonephric cells sorted at E10.5 expressed mRNA encoding Csf1, a hematopoietic cytokine, and corresponding protein, based on real-time PCR and immunocytochemistry, respectively. Further analysis indicated that some IACs express the CSF1 receptor, CSF1R. Expression of Cebpa and Irf8 mRNAs was higher in CSF1R-positive IACs, whereas that of Cebpε and Gfi1 mRNAs was lower relative to CSF1R-negative IACs, suggesting that CSF1/CSF1R signaling functions in IAC myeloid differentiation by modulating expression of these transcription factors. Colony formation assays using CSF1R-positive IACs revealed increased numbers of myeloid colonies in the presence of CSF1. Analysis using an intra-cellular signaling array indicated the greatest fold increase of Cleaved Caspase-3 in AGM cells in the presence of CSF1. Immunohistochemistry revealed that Cleaved Caspase-3 is primarily expressed in IACs in the AGM region, and incubation of IACs with CSF1 up-regulated Cleaved Caspase-3. Overall, our findings suggest that CSF1 secreted from mesonephros accelerates IAC myeloid differentiation in the AGM region, possibly via Caspase-3 cleavage.

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.

Critical requirement of VEGF-C in transition to fetal erythropoiesis

Vegfc is essential for mobilization, maturation, and enucleation of primitive erythroblasts. Vegfc deletion compromises liver colonization by erythro-myeloid progenitors and subsequent macrophage/erythroid expansion.

Mll-AF4 Confers Enhanced Self-Renewal and Lymphoid Potential during a Restricted Window in Development

MLL-AF4+ infant B cell acute lymphoblastic leukemia is characterized by an early onset and dismal survival. It initiates before birth, and very little is known about the early stages of the disease’s development. Using a conditional Mll-AF4-expressing mouse model in which fusion expression is targeted to the earliest definitive hematopoietic cells generated in the mouse embryo, we demonstrate that Mll-AF4 imparts enhanced B lymphoid potential and increases repopulation and self-renewal capacity during a putative pre-leukemic state. This occurs between embryonic days 12 and 14 and manifests itself most strongly in the lymphoid-primed multipotent progenitor population, thus pointing to a window of opportunity and a potential cell of origin. However, this state alone is insufficient to generate disease, with the mice succumbing to B cell lymphomas only after a long latency. Future analysis of the molecular details of this pre-leukemic state will shed light on additional events required for progression to acute leukemia.

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Mll-AF4 confers enhanced B cell potential and causes an expansion of pro-B cells

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Mll-AF4 increases self-renewal potential

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Mll-AF4 exerts its effects in a restricted developmental window

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The LMPP is a potential cell of origin for Mll-AF4-associated disease

Tracing haematopoietic stem cell formation at single-cell resolution

Haematopoietic stem cells (HSCs) are derived early from embryonic precursors, such as haemogenic endothelial cells and pre-haematopoietic stem cells (pre-HSCs), the molecular identity of which still remains elusive. Here we use potent surface markers to capture the nascent pre-HSCs at high purity, as rigorously validated by single-cell-initiated serial transplantation. Then we apply single-cell RNA sequencing to analyse endothelial cells, CD45(-) and CD45(+) pre-HSCs in the aorta-gonad-mesonephros region, and HSCs in fetal liver. Pre-HSCs show unique features in transcriptional machinery, arterial signature, metabolism state, signalling pathway, and transcription factor network. Functionally, activation of mechanistic targets of rapamycin (mTOR) is shown to be indispensable for the emergence of HSCs but not haematopoietic progenitors. Transcriptome data-based functional analysis reveals remarkable heterogeneity in cell-cycle status of pre-HSCs. Finally, the core molecular signature of pre-HSCs is identified. Collectively, our work paves the way for dissection of complex molecular mechanisms regulating stepwise generation of HSCs in vivo, informing future efforts to engineer HSCs for clinical applications.

Development and differentiation of the erythroid lineage in mammals

The red blood cell (RBC) is responsible for performing the highly specialized function of oxygen transport, making it essential for survival during gestation and postnatal life. Establishment of sufficient RBC numbers, therefore, has evolved to be a major priority of the postimplantation embryo. The "primitive" erythroid lineage is the first to be specified in the developing embryo proper. Significant resources are dedicated to producing RBCs throughout gestation. Two transient and morphologically distinct waves of hematopoietic progenitor-derived erythropoiesis are observed in development before hematopoietic stem cells (HSCs) take over to produce "definitive" RBCs in the fetal liver. Toward the end of gestation, HSCs migrate to the bone marrow, which becomes the primary site of RBC production in the adult. Erythropoiesis is regulated at various stages of erythroid cell maturation to ensure sufficient production of RBCs in response to physiological demands. Here, we highlight key aspects of mammalian erythroid development and maturation as well as differences among the primitive and definitive erythroid cell lineages.

Human definitive haemogenic endothelium and arterial vascular endothelium represent distinct lineages

The generation of haematopoietic stem cells (HSCs) from human pluripotent stem cells (hPSCs) will depend on the accurate recapitulation of embryonic haematopoiesis. In the early embryo, HSCs develop from the haemogenic endothelium (HE) and are specified in a Notch-dependent manner through a process named endothelial-to-haematopoietic transition (EHT). As HE is associated with arteries, it is assumed that it represents a subpopulation of arterial vascular endothelium (VE). Here we demonstrate at a clonal level that hPSC-derived HE and VE represent separate lineages. HE is restricted to the CD34⁺CD73⁻CD184⁻ fraction of day 8 embryoid bodies (EBs) and it undergoes a NOTCH-dependent EHT to generate RUNX1C⁺ cells with multilineage potential. Arterial and venous VE progenitors, by contrast, segregate to the CD34⁺CD73^medCD184⁺ and CD34⁺CD73^hiCD184⁻ fractions, respectively. Together, these findings identify HE as distinct from VE and provide a platform for defining the signalling pathways that regulate their specification to functional HSCs.

Localized SCF and IGF-1 secretion enhances erythropoiesis in the spleen of murine embryos

Fetal spleen is a major hematopoietic site prior to initiation of bone marrow hematopoiesis. Morphologic analysis suggested erythropoietic activity in fetal spleen, but it remained unclear how erythropoiesis was regulated. To address this question, we performed flow cytometric analysis and observed that the number of spleen erythroid cells increased 18.6-fold from 16.5 to 19.5 days post-coitum (dpc). Among erythropoietic cytokines, SCF and IGF-1 were primarily expressed in hematopoietic, endothelial and mesenchymal-like fetal spleen cells. Cultures treated with SCF and/or IGF-1R inhibitors showed significantly decreased CD45−c-Kit−CD71+/−Ter119+ erythroid cells and downregulated Gata1, Klf1 and β-major globin expression. Administration of these inhibitors to pregnant mice significantly decreased the number of CD45−c-Kit−CD71+/−Ter119+ cells and downregulated β-major globin gene expression in embryos derived from these mice. We conclude that fetal spleen is a major erythropoietic site where endothelial and mesenchymal-like cells primarily accelerate erythropoietic activity through SCF and IGF-1 secretion.

Histologic Features of Postnatal Development of Immune System Organs in the Sprague-Dawley Rat

The immune system of the rat undergoes substantial functional and morphological development during the postnatal period. Some aspects of this development are genetically predetermined, while other aspects depend on environmental influences. Detailed information on postnatal development is important in the interpretation of histopathologic findings in juvenile toxicology and pubertal assay studies, as well as other studies conducted in juvenile rats. Studies were conducted to provide detailed characterization of histologic features of the major functional compartments of immune system organs in male and female Sprague-Dawley rats at weekly intervals from the day of birth through postnatal day (PND) 42. Maturation of the individual immune system organs occurred across a range of ages, with histologic maturation of T-cell-related compartments typically occurring prior to maturation of B-cell-related compartments. The sequence of histologic maturation was bone marrow and thymus on PND 14, mesenteric lymph node on PND 21, Peyer's patches and bronchus-associated lymphoid tissue on PND 28, mandibular lymph node, nasopharynx-associated lymphoid tissue, and diffuse mucosal mononuclear cell population of small intestine on PND 35, and spleen on PND 42. An estimation of functional maturation can be made based on the morphological indications of maturity of each compartment of immune system organs, but histologic indications of maturity do not confirm functional immunocompetence.

Extramedullary hematopoiesis (EMH) in laboratory animals: offering an insight into stem cell research

Extramedullary hematopoiesis (EMH) is a pathological process secondary to underlying bone marrow (BM) insufficiency in adults. It is characterized by the emergence of multipotent hematopoietic progenitors scattered around the affected tissue, most likely in the spleen, liver, and lymph node, etc. EMH in patients frequently receives less medical attention and is neglected unless a compressive or obstructive hematopoietic mass appears to endanger the patient's life. However, on a biological basis, EMH reflects the alteration of relationships among hematopoietic stem and progenitor cells (HSPCs) and their original and new microenvironments. The ability of hematopoietic stem cells (HSCs) to mobilize from the bone marrow and to accommodate and function in extramedullary tissues is rather complicated and far from our current understanding. Fortunately, many reports from the studies of drugs and genetics using animals have incidentally found EMH to be involved. Thereby, the molecular basis of EMH could further be elucidated from those animals after cross-comparison. A deeper understanding of the extramedullary hematopoietic niche could help expand stem cells in vitro and establish a better treatment in patients for stem cell transplantation.

Concise Review: early embryonic erythropoiesis: not so primitive after all

In the developing embryo, hematopoiesis begins with the formation of primitive erythroid cells (EryP), a distinct and transient red blood cell lineage. EryP play a vital role in oxygen delivery and in generating shear forces necessary for normal vascular development. Progenitors for EryP arise as a cohort within the blood islands of the mammalian yolk sac at the end of gastrulation. As a strong heartbeat is established, nucleated erythroblasts begin to circulate and to mature in a stepwise, nearly synchronous manner. Until relatively recently, these cells were thought to be "primitive" in that they seemed to more closely resemble the nucleated erythroid cells of lower vertebrates than the enucleated erythrocytes of mammals. It is now known that mammalian EryP do enucleate, but not until several days after entering the bloodstream. I will summarize the common and distinguishing characteristics of primitive versus definitive (adult-type) erythroid cells, review the development of EryP from the emergence of their progenitors through maturation and enucleation, and discuss pluripotent stem cells as models for erythropoiesis. Erythroid differentiation of both mouse and human pluripotent stem cells in vitro has thus far reproduced early but not late red blood cell ontogeny. Therefore, a deeper understanding of cellular and molecular mechanisms underlying the differences and similarities between the embryonic and adult erythroid lineages will be critical to improving methods for production of red blood cells for use in the clinic.

Two waves of distinct hematopoietic progenitor cells colonize the fetal thymus

The generation of T cells depends on the migration of hematopoietic progenitor cells to the thymus throughout life. The identity of the thymus-settling progenitor cells has been a matter of considerable debate. Here we found that thymopoiesis was initiated by a first wave of T cell lineage-restricted progenitor cells with limited capacity for population expansion but accelerated differentiation into mature T cells. They gave rise to αβ and γδ T cells that constituted Vγ3(+) dendritic epithelial T cells. Thymopoiesis was subsequently maintained by less-differentiated progenitor cells that retained the potential to develop into B cells and myeloid cells. In that second wave, which started before birth, progenitor cells had high proliferative capacity but delayed differentiation capacity and no longer gave rise to embryonic γδ T cells. Our work reconciles conflicting hypotheses on the nature of thymus-settling progenitor cells.

Erythroid development in the mammalian embryo

Erythropoiesis is the process by which progenitors for red blood cells are produced and terminally differentiate. In all vertebrates, two morphologically distinct erythroid lineages (primitive, embryonic, and definitive, fetal/adult) form successively within the yolk sac, fetal liver, and marrow and are essential for normal development. Red blood cells have evolved highly specialized functions in oxygen transport, defense against oxidation, and vascular remodeling. Here we review key features of the ontogeny of red blood cell development in mammals, highlight similarities and differences revealed by genetic and gene expression profiling studies, and discuss methods for identifying erythroid cells at different stages of development and differentiation.

The migration of hematopoietic progenitors from the fetal liver to the fetal bone marrow: lessons learned and possible clinical applications

The ontogeny of hematopoietic stem cells (HSCs) is complex, with multiple sites of embryonic origin as well as several locations of expansion and maturation in the embryo and the adult. Hematopoietic progenitors (HPs) with diverse developmental potential are first found in the yolk sac, aorta-gonad-mesonephros region and placenta. These progenitors then colonize the fetal liver (FL), where they undergo expansion and maturation. HSCs from the FL colonize the fetal bone marrow (FBM), governed by a complex orchestration of transcription programs including migratory molecules with chemotactic activity, adhesion molecules, and molecules that modulate the extracellular matrix. Understanding the mechanisms that regulate the patterns of HSC migration between FL and FBM could improve the engraftment potential of embryonic stem cell-derived HPs, because these cells might display a migratory behavior more similar to early HPs than to adult HSCs. Understanding the changes in migratory behavior in the context of FL to FBM HSC migration could lead to new approaches in the treatment of blood malignancies. We will review the current knowledge in the field of FL to the FBM HSCs migration during development, focusing on changes in expression of molecules important for this process and exploring its clinical applications.

Return to the hematopoietic stem cell origin

Studying embryonic hematopoiesis is complicated by diversity of its locations in the constantly changing anatomy and by the mobility of blood cell precursors. Embryonic hematopoietic progenitors are identified in traditional in vivo and in vitro cell potential assays. Profound epigenetic plasticity of mammalian embryonic cells combined with significant inductive capacity of the potential assays suggest that our understanding of hematopoietic ontogenesis is substantially distorted. Non-invasive in vivo cell tracing methodology offers a better insight into complex processes of blood cell specification. In contrast to the widely accepted view based on the cell potential assays, the genetic tracing approach identified the yolk sac as the source of adult hematopoietic stem cell lineage. Realistic knowledge of the blood origin is critical for safe and efficient recapitulation of hematopoietic development in culture.

Immature hematopoietic stem cells undergo maturation in the fetal liver

Hematopoietic stem cells (HSCs), which are defined by their capacity to reconstitute adult conventional mice, are first found in the dorsal aorta after 10.5 days post coitus (dpc) and in the fetal liver at 11 dpc. However, lympho-myeloid hematopoietic progenitors are detected in the dorsal aorta from 9 dpc, raising the issue of their role in establishing adult hematopoiesis. Here, we show that these progenitors are endowed with long-term reconstitution capacity, but only engraft natural killer (NK)-deficient Rag2γc(-/-) mice. This novel population, called here immature HSCs, evolves in culture with thrombopoietin and stromal cells, into HSCs, defined by acquisition of CD45 and MHC-1 expression and by the capacity to reconstitute NK-competent mice. This evolution occurs during ontogeny, as early colonization of fetal liver by immature HSCs precedes that of HSCs. Moreover, organ culture experiments show that immature HSCs acquire, in this environment, the features of HSCs.

Notch1 activation in embryonic VE-cadherin populations selectively blocks hematopoietic stem cell generation and fetal liver hematopoiesis

Hematopoietic stem cells (HSC) are found in several independent sites embryonically. Loss-of-function studies indicated that Notch1, but not Notch2 signaling was required for HSC emergence from the aortic-gonado-mesonephros (AGM) region. We previously showed that constitutive Notch1 activation impaired primitive erythroid differentiation, but its effects on HSC emergence from the AGM region were not studied. To further define specific roles of Notch receptors, we characterized HSC in mouse embryos expressing either Notch1 intracellular domain (ICD) or Notch4ICD in VE-cadherin or SM22α expressing populations. Although embryonic Notch1 activation in VE-cadherin populations led to lethality after E13.5, earlier defects in the fetal liver were observed. Embryos were analyzed at E12.5 to assess hematopoiesis and the phenotype of developing cells in the AGM region. We found that activation of Notch1 in the endothelial compartment in VE-cadherin expressing cells resulted in the absence of intra-aortic clusters and defects in fetal liver hematopoiesis. In contrast, although Notch4 expression is regulated during fetal hematopoiesis, activation of Notch4 in VE-cadherin expressing populations did not affect HSC phenotype, although later vascular remodeling was impaired. Likewise, activation of Notch1 in SM22α positive populations had no significant effect on hematopoiesis. Our results indicate a cell type-dependent activity and distinct features of Notch1 versus Notch4 signaling and their impact on HSC generation.

The expanding family of bone marrow homing factors for hematopoietic stem cells: stromal derived factor 1 is not the only player in the game

The α-chemokine stromal derived factor 1 (SDF-1), which binds to the CXCR4 and CXCR7 receptors, directs migration and homing of CXCR4⁺ hematopoietic stem/progenitor cells (HSPCs) to bone marrow (BM) and plays a crucial role in retention of these cells in stem cell niches. However, this unique role of SDF-1 has been recently challenged by several observations supporting SDF-1-CXCR4-independent BM homing. Specifically, it has been demonstrated that HSPCs respond robustly to some bioactive lipids, such as sphingosine-1-phosphate (S1P) and ceramide-1-phosphate (C1P), and migrate in response to gradients of certain extracellular nucleotides, including uridine triphosphate (UTP) and adenosine triphosphate (ATP). Moreover, the responsiveness of HSPCs to an SDF-1 gradient is enhanced by some elements of innate immunity (e.g., C3 complement cascade cleavage fragments and antimicrobial cationic peptides, such as cathelicidin/LL-37 or β2-defensin) as well as prostaglandin E2 (PGE2). Since all these factors are upregulated in BM after myeloblative conditioning for transplantation, a more complex picture of homing emerges that involves several factors supporting, and in some situations even replacing, the SDF-1-CXCR4 axis.

Hematopoiesis leading to a diversity of dendritic antigen-presenting cell types

Hematopoietic stem cells (HSCs) undergo expansion and differentiation, giving rise to all terminally differentiated blood cells throughout life. HSCs are found in distinct anatomical sites during development, and in adults, hematopoiesis occurs predominantly on the luminal side of the bone cavity in bone marrow. Millions of newly formed blood cells are generated per second to accommodate the short half-life of hematopoietic cells. For this to happen, HSCs must sustain their self-renewal capacity as well as their capability to commit and differentiate toward multiple cell lineages. Development of the hematopoietic system is finely regulated as the animal ages, so that it does not become exhausted or misdirected. This review covers aspects of hematopoietic development from the embryonic period through adult life in relation to development of dendritic cells. It also considers a role for HSCs in extramedullary sites and their possible role in myelopoiesis, with formation of tissue-specific antigen-presenting cells.

Notch signaling in mammalian hematopoietic stem cells

Notch is a crucial cell signaling pathway in metazoan development. By means of cell-cell interactions, Notch signaling regulates cellular identity, proliferation, differentiation and apoptosis. Within the last decade, numerous studies have shown an important role for this pathway in the development and homeostasis of mammalian stem cell populations. Hematopoietic stem cells (HSCs) constitute a well-defined population that shows self-renewal and multi-lineage differentiation potential, with the clinically relevant capacity to repopulate the hematopoietic system of an adult organism. Here, we review the emergence, development and maintenance of HSCs during mammalian embryogenesis and adulthood, with respect to the role of Notch signaling in hematopoietic biology.

A transient definitive erythroid lineage with unique regulation of the β-globin locus in the mammalian embryo

A transient erythromyeloid wave of definitive hematopoietic progenitors (erythroid/myeloid progenitors [EMPs]) emerges in the yolk sac beginning at embryonic day 8.25 (E8.25) and colonizes the liver by E10.5, before adult-repopulating hematopoietic stem cells. At E11.5, we observe all maturational stages of erythroid precursors in the liver and the first definitive erythrocytes in the circulation. These early fetal liver erythroblasts express predominantly adult β-globins and the definitive erythroid-specific transcriptional modifiers c-myb, Sox6, and Bcl11A. Surprisingly, they also express low levels of "embryonic" βH1-, but not εy-, globin transcripts. Consistent with these results, RNA polymerase and highly modified histones are found associated with βH1- and adult globin, but not εy-globin, genes. E11.5 definitive proerythroblasts from mice transgenic for the human β-globin locus, like human fetal erythroblasts, express predominately human γ-, low β-, and no ε-globin transcripts. Significantly, E9.5 yolk sac-derived EMPs cultured in vitro have similar murine and human transgenic globin expression patterns. Later liver proerythroblasts express low levels of γ-globin, while adult marrow proerythroblasts express only β-globin transcripts. We conclude that yolk sac-derived EMPs, the first of 2 origins of definitive erythropoiesis, express a unique pattern of globin genes as they generate the first definitive erythrocytes in the liver of the mammalian embryo.

Hematopoietic stem cell development in the placenta

The placenta is a highly vascularized organ that mediates fetal-maternal exchange during pregnancy and is thereby vital for the survival and growth of the developing embryo. In addition to having this well-established role in supporting pregnancy, the placenta was recently shown to function as a hematopoietic organ. The placenta is unique among other fetal hematopoietic organs, as it is capable of both generating multipotential hematopoietic cells de novo and establishing a major hematopoietic stem cell (HSC) pool in the conceptus, while protecting HSCs from premature differentiation. The mouse placenta contains two distinct vascular regions that support hematopoiesis: the large vessels in the chorionic plate where HSCs/progenitors are thought to emerge and the labyrinth vasculature where nascent HSCs/progenitors may colonize for expansion and possible functional maturation. Defining how this cytokine- and growth factor rich organ supports HSC generation, maturation and expansion may ultimately help to establish culture protocols for HSC expansion or de novo generation from pluripotent cells.

Innate immunity as orchestrator of stem cell mobilization

Hematopoietic stem and progenitor cells (HSPCs), as well as other types of stem cells, circulate under steady-state conditions at detectable levels in peripheral blood (PB), with their numbers increasing in response to stress, inflammation and tissue/organ injury. This mobilization process may be envisioned as a danger-sensing response mechanism triggered by hypoxia or mechanical or infection-induced tissue damage that recruits into PB different types of stem cells that have a role in immune surveillance and organ/tissue regeneration. Mobilization is also significantly enhanced by the administration of pharmacological agents, which has been exploited in hematological transplantology as a means to obtain HSPCs for hematopoietic reconstitution. In this review we will present mounting evidence that innate immunity orchestrates this evolutionarily conserved mechanism of HSPC mobilization.

The omentum is a site of stromal cell-derived factor 1alpha production and reservoir for CXC chemokine receptor 4-positive cell recruitment

BACKGROUND

The mechanism of the omental response to injury remains poorly defined. This study investigates the omental reaction to a foreign body, examining the role of a chemokine ligand/receptor pair known to play a crucial role in angiogenesis and wound healing.

METHODS

A ventral hernia, surgically created in the abdominal wall of 6 swine, was repaired with silicone sheeting to activate the omentum. Omental thickness was determined by ultrasonography. Serial stromal cell-derived factor 1alpha (SDF-1alpha) concentrations were measured in blood, wound, and peritoneal fluids by enzyme-linked immunosorbent assay.

RESULTS

During the 14-day study period, serial ultrasonography showed a 20-fold increase in omental thickness, and enzyme-linked immunosorbent assay revealed a 4-fold increase in SDF-1alpha concentration in local wound fluid. Omental vessel count and vascular surface area were 8- to 10-fold higher in reactive omentum. Immunohistochemistry showed nearly complete replacement of control omental fat with CXC chemokine receptor 4 (CXCR4)-positive cells by day 14.

CONCLUSIONS

Activated omentum, important in the SDF-1alpha/CXCR4 axis, may serve as an intraperitoneal reservoir for recruitment of circulating bone marrow-derived cells vital to healing.

New light on the biology and developmental potential of haematopoietic stem cells and progenitor cells

Even though stem cells have been identified in several tissues, one of the best understood somatic stem cells is the bone marrow residing haematopoietic stem cell (HSC). These cells are able to generate all types of blood cells found in the periphery over the lifetime of an animal, making them one of the most profound examples of tissue-restricted stem cells. HSC therapy also represents one of the absolutely most successful cell-based therapies applied both in the treatment of haematological disorders and cancer. However, to fully explore the clinical potential of HSCs we need to understand the molecular regulation of cell maturation and lineage commitment. The extensive research effort invested in this area has resulted in a rapid development of the understanding of the relationship between different blood cell lineages and increased understanding for how a balanced composition of blood cells can be generated. In this review, several of the basic features of HSCs, as well as their multipotent and lineage-restricted offspring, are addressed, providing a current view of the haematopoietic development tree. Some of the basic mechanisms believed to be involved in lineage restriction events including activities of permissive and instructive external signals are also discussed, besides transcription factor networks and epigenetic alterations to provide an up-to-date view of early haematopoiesis.

Definitive hematopoietic stem/progenitor cells manifest distinct differentiation output in the zebrafish VDA and PBI

One unique feature of vertebrate definitive hematopoiesis is the ontogenic switching of hematopoietic stem cells from one anatomical compartment or niche to another. In mice, hematopoietic stem cells are believed to originate in the aorta-gonad-mesonephros (AGM), subsequently migrate to the fetal liver (FL) and finally colonize the bone marrow (BM). Yet, the differentiation potential of hematopoietic stem cells within early niches such as the AGM and FL remains incompletely defined. Here, we present in vivo analysis to delineate the differentiation potential of definitive hematopoietic stem/progenitor cells (HSPCs) in the zebrafish AGM and FL analogies, namely the ventral wall of dorsal aorta (VDA) and the posterior blood island (PBI), respectively. Cell fate mapping and analysis of zebrafish runx1(w84x) and vlad tepes (vlt(m651)) mutants revealed that HSPCs in the PBI gave rise to both erythroid and myeloid lineages. However, we surprisingly found that HSPCs in the VDA were not quiescent but were uniquely adapted to generate myeloid but not erythroid lineage cells. We further showed that such distinct differentiation output of HSPCs was, at least in part, ascribed to the different micro-environments present in these two niches. Our results highlight the importance of niche in shaping the differentiation output of developing HSPCs.

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.

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.

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.

BMP4/Smad5 dependent stress erythropoiesis is required for the expansion of erythroid progenitors during fetal development

The rapid growth of the embryo places severe demands on the ability of the cardiovascular system to deliver oxygen to cells. To meet this need, erythroid progenitors rapidly expand in the fetal liver microenvironment such that by E14.5, erythropoiesis predominates in the fetal liver. In this report we show that the BMP4/Smad5 dependent stress erythropoiesis pathway plays a key role in the expansion of erythroid progenitors in the fetal liver. These data show that the fetal liver contains two populations of erythroid progenitors. One population resembles the steady state erythroid progenitors found in the adult bone marrow. While the second population exhibits the properties of stress erythroid progenitors found in adult spleen. Here we demonstrate that defects in BMP4/Smad5 signaling preferentially affect the expansion of the stress erythroid progenitors in the fetal liver leading to fetal anemia. These data suggest that steady state erythropoiesis is unable to generate sufficient erythrocytes to maintain the rapid growth of the embryo leading to the induction of the BMP4 dependent stress erythropoiesis pathway. These observations underscore the similarities between fetal erythropoiesis and stress erythropoiesis.

Ontogeny of erythropoiesis in the mammalian embryo

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

Bone morphogenetic protein 4 modulates c-Kit expression and differentiation potential in murine embryonic aorta-gonad-mesonephros haematopoiesis in vitro

The transforming growth factor-β-related factor bone morphogenetic protein 4 (BMP4) is expressed in the human embryonic aorta-gonad-mesonephros (AGM) coincident with the emergence of haematopoietic cells and influences postnatal mammalian haematopoietic stem cells in vitro. To investigate the role of BMP4 in mammalian embryonic haematopoiesis, cells were isolated from murine AGM and two populations of CD34⁺ cells with different levels of c-Kit expression and multipotency were identified. CD34⁺/c-Kit^high cells express CD45 and are haematopoietic-restricted progenitors. In contrast, CD34⁺/c-Kit^low cells are Flk1+/CD45^neg and generate adherent colonies in ex vivo culture that resemble haemangioblast colonies identified in other systems. The addition of BMP4 to AGM cells resulted in expansion of the CD34⁺/c-Kit^low cell pool within 48 h, via a combination of down modulation of the c-Kit receptor in CD34⁺/c-Kit^high cells and proliferation. In long-term culture, BMP4 increased the growth/survival of CD34⁺/c-Kit^high haematopoietic progenitors, effects that were blocked by BMP inhibitors. CD34⁺/c-Kit^high progenitors cultured with BMP4 also generated adherent colonies typical of c-Kit^low cells. These results suggest that BMP4 regulates c-Kit expression and differentiation potential in CD34⁺ AGM cells and supports a role for BMP signalling in the maintenance of multipotency during embryonic haematopoiesis, providing an insight into stem cell homeostasis within the mammalian haematopoietic niche.

Differential contributions of haematopoietic stem cells to foetal and adult haematopoiesis: insights from functional analysis of transcriptional regulators

An increasing number of molecules have been identified as candidate regulators of stem cell fates through their involvement in leukaemia or via post-genomic gene discovery approaches. A full understanding of the function of these molecules requires (1) detailed knowledge of the gene networks in which they participate and (2) an appreciation of how these networks vary as cells progress through the haematopoietic cell hierarchy. An additional layer of complexity is added by the occurrence of different haematopoietic cell hierarchies at different stages of ontogeny. Beyond these issues of cell context dependence, it is important from a mechanistic point of view to define the particular cell fate pathway impacted by any given regulator. Herein, we advance the notion that haematopoietic stem cells (HSC), which sustain haematopoiesis throughout adult life and are specified in foetal life, have a minimal or late contribution to foetal haematopoiesis but instead largely proliferate during the foetal period. In light of this notion, we revisit published data on mouse knockouts of haematopoietically-affiliated transcription factors highlighting novel insights that may be gained from taking such a view.

Definitive hematopoiesis initiates through a committed erythromyeloid progenitor in the zebrafish embryo

Shifting sites of blood cell production during development is common across widely divergent phyla. In zebrafish, like other vertebrates, hematopoietic development has been roughly divided into two waves, termed primitive and definitive. Primitive hematopoiesis is characterized by the generation of embryonic erythrocytes in the intermediate cell mass and a distinct population of macrophages that arises from cephalic mesoderm. Based on previous gene expression studies, definitive hematopoiesis has been suggested to begin with the generation of presumptive hematopoietic stem cells (HSCs) along the dorsal aorta that express c-myb and runx1. Here we show, using a combination of gene expression analyses, prospective isolation approaches, transplantation, and in vivo lineage-tracing experiments, that definitive hematopoiesis initiates through committed erythromyeloid progenitors (EMPs) in the posterior blood island (PBI) that arise independently of HSCs. EMPs isolated by coexpression of fluorescent transgenes driven by the lmo2 and gata1 promoters exhibit an immature, blastic morphology and express only erythroid and myeloid genes. Transplanted EMPs home to the PBI, show limited proliferative potential, and do not seed subsequent hematopoietic sites such as the thymus or pronephros. In vivo fate-mapping studies similarly demonstrate that EMPs possess only transient proliferative potential, with differentiated progeny remaining largely within caudal hematopoietic tissue. Additional fate mapping of mesodermal derivatives in mid-somitogenesis embryos suggests that EMPs are born directly in the PBI. These studies provide phenotypic and functional analyses of the first hematopoietic progenitors in the zebrafish embryo and demonstrate that definitive hematopoiesis proceeds through two distinct waves during embryonic development.

Concise Review: Role and Function of the Ubiquitin-Proteasome System in Mammalian Stem and Progenitor Cells

Highly ordered degradation of cell proteins by the ubiquitin-proteasome system, a sophisticated cellular proteolytic machinery, has been identified as a key regulatory mechanism in many eukaryotic cells. Accumulating evidence reveals that the ubiquitin-proteasome system is involved in the regulation of fundamental processes in mammalian stem and progenitor cells of embryonic, neural, hematopoietic, and mesenchymal origin. Such processes, including development, survival, differentiation, lineage commitment, migration, and homing, are directly controlled by the ubiquitin-proteasome system, either via proteolytic degradation of key regulatory proteins of signaling and gene expression pathways or via nonproteolytic mechanisms involving the proteasome itself or posttranslational modifications of target proteins by ubiquitin or other ubiquitin-like modifiers. Future characterization of the precise roles and functions of the ubiquitin-proteasome system in mammalian stem and early progenitor cells will improve our understanding of stem cell biology and may provide an experimental basis for the development of novel therapeutic strategies in regenerative medicine. Disclosure of potential conflicts of interest is found at the end of this article.

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.