Generation of definitive hematopoietic stem cells from murine early yolk sac and paraaortic splanchnopleures by aorta-gonad-mesonephros region-derived stromal cells

Haematopoietic development and HSC formation in vitro: promise and limitations of gastruloid models

Haematopoietic stem cells (HSCs) are the most extensively studied adult stem cells. Yet, six decades after their first description, reproducible and translatable generation of HSC in vitro remains an unmet challenge. HSC production in vitro is confounded by the multi-stage nature of blood production during development. Specification of HSC is a late event in embryonic blood production and depends on physical and chemical cues which remain incompletely characterised. The precise molecular composition of the HSC themselves is incompletely understood, limiting approaches to track their origin in situ in the appropriate cellular, chemical and mechanical context. Embryonic material at the point of HSC emergence is limiting, highlighting the need for an in vitro model of embryonic haematopoietic development in which current knowledge gaps can be addressed and exploited to enable HSC production. Gastruloids are pluripotent stem cell-derived 3-dimensional (3D) cellular aggregates which recapitulate developmental events in gastrulation and early organogenesis with spatial and temporal precision. Gastruloids self-organise multi-tissue structures upon minimal and controlled external cues, and are amenable to live imaging, screening, scaling and physicochemical manipulation to understand and translate tissue formation. In this review, we consider the haematopoietic potential of gastruloids and review early strategies to enhance blood progenitor and HSC production. We highlight possible strategies to achieve HSC production from gastruloids, and discuss the potential of gastruloid systems in illuminating current knowledge gaps in HSC specification.

Multiple waves of fetal-derived immune cells constitute adult immune system

It has been over three decades since Drs. Herzenberg and Herzenberg proposed the layered immune system hypothesis, suggesting that different types of stem cells with distinct hematopoietic potential produce specific immune cells. This layering of immune system development is now supported by recent studies showing the presence of fetal-derived immune cells that function in adults. It has been shown that various immune cells arise at different embryonic ages via multiple waves of hematopoiesis from special endothelial cells (ECs), referred to as hemogenic ECs. However, it remains unknown whether these fetal-derived immune cells are produced by hematopoietic stem cells (HSCs) during the fetal to neonatal period. To address this question, many advanced tools have been used, including lineage-tracing mouse models, cellular barcoding techniques, clonal assays, and transplantation assays at the single-cell level. In this review, we will review the history of the search for the origins of HSCs, B-1a progenitors, and mast cells in the mouse embryo. HSCs can produce both B-1a and mast cells within a very limited time window, and this ability declines after embryonic day (E) 14.5. Furthermore, the latest data have revealed that HSC-independent adaptive immune cells exist in adult mice, which implies more complicated developmental pathways of immune cells. We propose revised road maps of immune cell development.

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.

The distinct effects of P18 overexpression on different stages of hematopoiesis involve TGF-β and NF-κB signaling

Deficiency of P18 can significantly improve the self-renewal potential of hematopoietic stem cells (HSC) and the success of long-term engraftment. However, the effects of P18 overexpression, which is involved in the inhibitory effects of RUNX1b at the early stage of hematopoiesis, have not been examined in detail. In this study, we established inducible P18/hESC lines and monitored the effects of P18 overexpression on hematopoietic differentiation. Induction of P18 from day 0 (D0) dramatically decreased production of CD34highCD43- cells and derivative populations, but not that of CD34lowCD43- cells, changed the cell cycle status and apoptosis of KDR+ cells and downregulated the key hematopoietic genes at D4, which might cause the severe blockage of hematopoietic differentiation at the early stage. By contrast, induction of P18 from D10 dramatically increased production of classic hematopoietic populations and changed the cell cycle status and apoptosis of CD45+ cells at D14. These effects can be counteracted by inhibition of TGF-β or NF-κB signaling respectively. This is the first evidence that P18 promotes hematopoiesis, a rare property among cyclin-dependent kinase inhibitors (CKIs).

Adult-repopulating lymphoid potential of yolk sac blood vessels is not confined to arterial endothelial cells

During embryogenesis, hematopoietic stem progenitor cells (HSPCs) are believed to be derived from hemogenic endothelial cells (HECs). Moreover, arterial feature is proposed to be a prerequisite for HECs to generate HSPCs with lymphoid potential. Although the molecular basis of hematopoietic stem cell-competent HECs has been delicately elucidated within the embryo proper, the functional and molecular characteristics of HECs in the extraembryonic yolk sac (YS) remain largely unresolved. In this study, we initially identified six molecularly different endothelial populations in the midgestational YS through integrated analysis of several single-cell RNA sequencing (scRNA-seq) datasets and validated the arterial vasculature distribution of Gja5⁺ ECs using a Gja5-EGFP reporter mouse model. Further, we explored the hemogenic potential of different EC populations based on their Gja5-EGFP and CD44 expression levels. The hemogenic potential was ubiquitously detected in spatiotemporally different vascular beds on embryonic days (E)8.5-E9.5 and gradually concentrated in CD44-positive ECs from E10.0. Unexpectedly, B-lymphoid potential was detected in the YS ECs as early as E8.5 regardless of their arterial features. Furthermore, the capacity for generating hematopoietic progenitors with in vivo lymphoid potential was found in nonarterial as well as arterial YS ECs on E10.0-E10.5. Importantly, the distinct identities of E10.0-E10.5 HECs between YS and intraembryonic caudal region were revealed by further scRNA-seq analysis. Cumulatively, these findings extend our knowledge regarding the hemogenic potential of ECs from anatomically and molecularly different vascular beds, providing a theoretical basis for better understanding the sources of HSPCs during mammalian development.

Fibroblasts as confederates of the immune system

Fibroblastic stromal cells are as diverse, in origin and function, as the niches they fashion in the mammalian body. This cellular variety impacts the spectrum of responses elicited by the immune system. Fibroblast influence on the immune system keeps evolving our perspective on fibroblast roles and functions beyond just a passive structural part of organs. This review discusses the foundations of fibroblastic stromal-immune crosstalk, under the scope of stromal heterogeneity as a basis for tissue-specific tutoring of the immune system. Focusing on the skin as a relevant immunological organ, we detail the complex interactions between distinct fibroblast populations and immune cells that occur during homeostasis, injury repair, scarring, and disease. We further review the relevance of fibroblastic stromal cell heterogeneity and how this heterogeneity is central to regulate the immune system from its inception during embryonic development into adulthood.

Paracrine CCL17 and CCL22 signaling regulates hematopoietic stem/progenitor cell migration and retention in mouse fetal liver

Fetal liver (FL) is the major embryonic hematopoietic organ and a site where circulating hematopoietic stem/progenitor cells (HSPCs) reside. However, HSPC migration/retention mechanisms in FL remain unclear. A chemokine screen revealed that the CCR4 ligands CCL17 and CCL22 are highly expressed in mouse embryonic day (E) 12.5 FL. Flow cytometric analysis confirmed CCR4 expression in FL HSPCs. To identify sources of CCL17 and CCL22, we fractionated FL into various cell types and found that Ccl17 and Ccl22 were predominantly expressed in HPCs/matured HCs. In vitro cell migration analysis confirmed enhanced HSPC migration in the presence of HPCs/matured HCs. Furthermore, exo-utero injection of anti-CCR4 neutralizing antibody into pregnant mice significantly reduced the number of FL HSPCs in embryos. These data demonstrate a paracrine mechanism by which HSPC migration/retention is regulated by CCL17 and CCL22 secreted from HPCs or matured HCs in FL.

Hematopoietic Stem and Progenitor Cells (HSPCs)

Hematopoietic stem/progenitor cells (HSPCs) isolated from bone marrow have been successfully employed for 50 years in hematological transplantations. Currently, these cells are more frequently isolated from mobilized peripheral blood or umbilical cord blood. In this chapter, we overview several topics related to these cells including their phenotype, methods for isolation, and in vitro and in vivo assays to evaluate their proliferative potential. The successful clinical application of HSPCs is widely understood to have helped establish the rationale for the development of stem cell therapies and regenerative medicine.

Induction of developmental hematopoiesis mediated by transcription factors and the hematopoietic microenvironment

The induction of hematopoiesis in various cell types via transcription factor (TF) reprogramming has been demonstrated by several strategies. The eventual goal of these approaches is to generate a product for unmet needs in hematopoietic cell transplantation therapies. The most successful strategies hew closely to clues provided from developmental hematopoiesis in terms of factor expression and environmental cues. In this review, we aim to summarize the TFs that play important roles in developmental hematopoiesis primarily and to also touch on adult hematopoiesis. Several aspects of cellular and molecular biology coalesce in this process, with TFs and surrounding cellular signals playing a major role in the overall development of the hematopoietic lineage. We attempt to put these elements into the context of reprogramming and highlight their roles.

Hematopoietic stem cell-independent hematopoiesis and the origins of innate-like B lymphocytes

The current paradigm that a single long-term hematopoietic stem cell can regenerate all components of the mammalian immune system has been challenged by recent findings in mice. These findings show that adult tissue-resident macrophages and innate-like lymphocytes develop early in fetal hematopoiesis from progenitors that emerge prior to, and apparently independently of, conventional long-term hematopoietic stem cells. Here, we discuss these recent findings, which show that an early and distinct wave of hematopoiesis occurs for all major hematopoietic lineages. These data provide evidence that fetal hematopoietic progenitors not derived from the bona fide long-term hematopoietic stem cells give rise to tissue-resident immune cells that persist throughout adulthood. We also discuss recent insights into B lymphocyte development and attempt to synthesize seemingly contradictory recent findings on the origins of innate-like B-1a lymphocytes during fetal hematopoiesis.

Overexpression of GATA2 Enhances Development and Maintenance of Human Embryonic Stem Cell-Derived Hematopoietic Stem Cell-like Progenitors

GATA2 is essential for the endothelial-to-hematopoietic transition (EHT) and generation of hematopoietic stem cells (HSCs). It is poorly understood how GATA2 controls the development of human pluripotent stem cell (hPSC)-derived HS-like cells. Here, using human embryonic stem cells (hESCs) in which GATA2 overexpression was induced by doxycycline (Dox), we elucidated the dual functions of GATA2 in definitive hematopoiesis before and after the emergence of CD34⁺CD45⁺CD90⁺CD38^- HS-like cells. Specifically, GATA2 promoted expansion of hemogenic precursors via the EHT and then helped to maintain HS-like cells in a quiescent state by regulating cell cycle. RNA sequencing showed that hPSC-derived HS-like cells were very similar to human fetal liver-derived HSCs. Our findings will help to elucidate the mechanism that controls the early stages of human definitive hematopoiesis and may help to develop a strategy to generate hPSC-derived HSCs.

Long-Term Engraftment of ESC-Derived B-1 Progenitor Cells Supports HSC-Independent Lymphopoiesis

It is generally considered that mouse embryonic stem cell (ESC) differentiation into blood cells in vitro recapitulates yolk sac (YS) hematopoiesis. As such, similar to YS-derived B-progenitors, we demonstrate here that ESC-derived B-progenitors differentiate into B-1 and marginal zone B cells, but not B-2 cells in immunodeficient mice after transplantation. ESC-derived B-1 cells were maintained in the recipients for more than 6 months, secreting natural IgM antibodies in vivo. Gene expression profiling displayed a close relationship between ESC- and YS-derived B-1 progenitors. Because there are no hematopoietic stem cells (HSCs) detectable in our ESC differentiation culture, successful long-term engraftment of ESC-derived functional B-1 cells supports the presence of HSC-independent B-1 cell development.

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ESC-derived B-progenitors mature into B-1 cells and MZ B cells in vivo

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ESC-derived B-1 cells engrafted in vivo long-term and secrete natural antibodies

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ESC-derived B-progenitors are molecularly similar to YS-derived B-progenitors

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Long-term B-1 cell engraftment represents HSC-independent lymphopoiesis

Murine hematopoietic stem cell activity is derived from pre-circulation embryos but not yolk sacs

The embryonic site of definitive hematopoietic stem cell (dHSC) origination has been debated for decades. Although an intra-embryonic origin is well supported, the yolk sac (YS) contribution to adult hematopoiesis remains controversial. The same developmental origin makes it difficult to identify specific markers that discern between an intraembryonic versus YS-origin using a lineage trace approach. Additionally, the highly migratory nature of blood cells and the inability of pre-circulatory embryonic cells (i.e., 5-7 somite pairs (sp)) to robustly engraft in transplantation, even after culture, has precluded scientists from properly answering these questions. Here we report robust, multi-lineage and serially transplantable dHSC activity from cultured 2-7sp murine embryonic explants (Em-Ex). dHSC are undetectable in 2-7sp YS explants. Additionally, the engraftment from Em-Ex is confined to an emerging CD31⁺CD45⁺c-Kit⁺CD41^- population. In sum, our work supports a model in which the embryo, not the YS, is the major source of lifelong definitive hematopoiesis.

What do the lineage tracing studies tell us? Consideration for hematopoietic stem cell origin, dynamics, and leukemia-initiating cells

The recent advance of technologies enables us to trace the cell fate in vivo by marking the cells that express the gene of interest or by barcoding them at a single cell level. Various tamoxifen-inducible Cre-recombinase mice combined with Rosa-floxed lines are utilized. In this review, with the results revealed by lineage tracing assays, we re-visit the long-standing debate for the origin of hematopoietic stem cells in the mouse embryo, and introduce the view of native hematopoiesis, and possible leukemic-initiating cells emerged during fetal stages.

Cells with hematopoietic potential reside within mouse proepicardium

During embryonic development, hematopoietic cells are present in areas of blood-vessel differentiation. These hematopoietic cells emerge from a specific subpopulation of endothelial cells called the hemogenic endothelium. We have previously found that mouse proepicardium contained its own population of endothelial cells forming a network of vascular tubules. We hypothesize that this EC population contains cells of hematopoietic potential. Therefore, we investigated an in vitro hematopoietic potential of proepicardial cell populations. The CD31+/CD45-/CD71- cell population cultured for 10 days in MethocultTM gave numerous colonies of CFU-GEMM, CFU-GM, and CFU-E type. These colonies consisted of various cell types. Flk-1+/CD31-/CD45-/CD71-, and CD45+ and/or CD71+ cell populations produced CFU-GEMM and CFU-GM, or CFU-GM and CFU-E colonies, respectively. Immunohistochemical evaluations of smears prepared from colonies revealed the presence of cells of different hematopoietic lineages. These cells were characterized by labeling with various combinations of antibodies directed against CD31, CD41, CD71, c-kit, Mpl, Fli1, Gata-2, and Zeb1 markers. Furthermore, we found that proepicardium-specific marker WT1 co-localized with Runx1 and Zeb1 and that single endothelial cells bearing CD31 molecule expressed Runx1 in the proepicardial area of embryonic tissue sections. We have shown that cells of endothelial and/or hematopoietic phenotypes isolated from mouse proepicardium possess hematopoietic potential in vitro and in situ. These results are supported by RT-PCR analyses of proepicardial extract, which revealed the expression of mRNA for crucial regulatory factors for hemogenic endothelium specification, i.e., Runx1, Notch1, Gata2, and Sox17. Our data are in line with previous observation on hemangioblast derivation from the quail PE.

The hematopoietic stem cell niche: from embryo to adult

Hematopoietic stem cells (HSCs) develop in discrete anatomical niches, migrating during embryogenesis from the aorta-gonad-mesonephros (AGM) region to the fetal liver, and finally to the bone marrow, where most HSCs reside throughout adult life. These niches provide supportive microenvironments that specify, expand and maintain HSCs. Understanding the constituents and molecular regulation of HSC niches is of considerable importance as it could shed new light on the mechanistic principles of HSC emergence and maintenance, and provide novel strategies for regenerative medicine. However, controversy exists concerning the cellular complexity of the bone marrow niche, and our understanding of the different HSC niches during development remains limited. In this Review, we summarize and discuss what is known about the heterogeneity of the HSC niches at distinct stages of their ontogeny, from the embryo to the adult bone marrow, drawing predominantly on data from mouse studies.

Progress towards generation of human haematopoietic stem cells

De novo generation of haematopoietic stem cells from different human pluripotent stem cell sources remains a high priority for haematology and regenerative medicine. At present, efficient derivation of functional haematopoietic stem cells with the capability for definitive in vivo engraftment and multi-lineage potential remains challenging. Here, we discuss recent progress and strategies to overcome obstacles that have thwarted past efforts. In addition, we review promising advances in the generation of mature blood lineages and the potential of induced pluripotent stem cells.

Early Development of Definitive Erythroblasts from Human Pluripotent Stem Cells Defined by Expression of Glycophorin A/CD235a, CD34, and CD36

The development of human erythroid cells has been mostly examined in models of adult hematopoiesis, while their early derivation during embryonic and fetal stages is largely unknown. We observed the development and maturation of erythroblasts derived from human pluripotent stem cells (hPSCs) by an efficient co-culture system. These hPSC-derived early erythroblasts initially showed definitive characteristics with a glycophorin A⁺ (GPA⁺) CD34^lowCD36⁻ phenotype and were distinct from adult CD34⁺ cell-derived ones. After losing CD34 expression, early GPA⁺CD36⁻ erythroblasts matured into GPA⁺CD36^low/+ stage as the latter expressed higher levels of β-globin along with a gradual loss of mesodermal and endothelial properties, and terminally suppressed CD36. We establish a unique in vitro model to trace the early development of hPSC-derived erythroblasts by serial expression of CD34, GPA, and CD36. Our findings may provide insight into the understanding of human early erythropoiesis and, ultimately, therapeutic potential.

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The hPSC/AGM-S3 co-culture system generates considerable definitive erythroblasts

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hPSC-derived erythroblasts initiate from a unique GPA⁺CD34^lowCD36⁻ fraction

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Human early erythropoiesis can be traced by serial expression of CD34, GPA, and CD36

De novo generation of HSCs from somatic and pluripotent stem cell sources

Generating human hematopoietic stem cells (HSCs) from autologous tissues, when coupled with genome editing technologies, is a promising approach for cellular transplantation therapy and for in vitro disease modeling, drug discovery, and toxicology studies. Human pluripotent stem cells (hPSCs) represent a potentially inexhaustible supply of autologous tissue; however, to date, directed differentiation from hPSCs has yielded hematopoietic cells that lack robust and sustained multilineage potential. Cellular reprogramming technologies represent an alternative platform for the de novo generation of HSCs via direct conversion from heterologous cell types. In this review, we discuss the latest advancements in HSC generation by directed differentiation from hPSCs or direct conversion from somatic cells, and highlight their applications in research and prospects for therapy.

Lymphoid progenitor emergence in the murine embryo and yolk sac precedes stem cell detection

Mammalian embryos produce several waves of hematopoietic cells before the establishment of the hematopoietic stem cell (HSC) hierarchy. These early waves of embryonic hematopoiesis present a reversed hierarchy in which hematopoietic potential is first displayed by highly specialized cells that are derived from transient uni- and bipotent progenitor cells. Hematopoiesis progresses through multilineage erythro-myeloid progenitor cells that lack self-renewal potential and, subsequently, to make distinct lymphoid progenitor cells before culminating in detectable definitive HSC. This review provides an overview of the stepwise development of embryonic hematopoiesis. We focus on recent progress in demonstrating that lymphoid lineages emerge from hemogenic endothelial cells before the presence of definitive HSC activity and discuss the implications of these findings.

Erythro-myeloid progenitors: "definitive" hematopoiesis in the conceptus prior to the emergence of hematopoietic stem cells

Erythro-myeloid progenitors (EMP) serve as a major source of hematopoiesis in the developing conceptus prior to the formation of a permanent blood system. In this review, we summarize the current knowledge regarding the emergence, fate, and potential of this hematopoietic stem cell (HSC)-independent wave of hematopoietic progenitors, focusing on the murine embryo as a model system. A better understanding of the temporal and spatial control of hematopoietic emergence in the embryo will ultimately improve our ability to derive hematopoietic stem and progenitor cells from embryonic stem cells and induced pluripotent stem cells to serve therapeutic purposes.

Haematopoietic differentiation is inhibited when Notch activity is enhanced in FLK1(+) mesoderm progenitors

Notch signalling has been implicated during haematopoietic development in vivo and in the differentiation of haematopoietic cells from pluripotent cells in vitro. However interpretation of data from many of these studies has been complicated by the heterogeneous nature of cell populations under study and by the fact that the Notch pathway is active during embryogenesis prior to the development of the haematopoietic system. To define the role of Notch signalling in more precise cell populations during the early stages of haematopoietic development within the aorta-gonad-mesonephros (AGM) microenvironment we co-cultured differentiating ESCs on a stromal cell line derived from this region of the embryo. Our co-culture system had no effect on the production of FLK1(+) mesoderm progenitor cells but promoted their subsequent haematopoietic differentiation. We assessed the role of Notch signalling on haematopoietic differentiation of isolated FLK1(+) cells. Notch activity is dynamic and drops to basal levels as FLK1(+) cells commit to a haematopoietic fate. Further reduction of Notch activity by the inducible expression of dominant negative MAML had no functional consequences. In contrast, induction of Notch activity using an inducible NotchIC expression system had an inhibitory effect on haematopoietic differentiation. We used a Cre-mediated recombination strategy whereby NotchIC-expressing cells were marked with the hCD2 receptor and observed a reduction in the number of multi-lineage and myeloid colonies derived from NotchIC(+) compared to NotchIC(-) FLK1(+) cells isolated from the same culture. We believe that our culture system represents a good model for haematopoietic development within the AGM microenvironment and our data suggest that haematopoietic commitment of FLK1(+) cells in this setting occurs when Notch activity is below a specific threshold.

Comparison of toxicity of benzene metabolite hydroquinone in hematopoietic stem cells derived from murine embryonic yolk sac and adult bone marrow

Benzene is an occupational toxicant and an environmental pollutant that potentially causes hematotoxicity and leukemia in exposed populations. Epidemiological studies suggest an association between an increased incidence of childhood leukemia and benzene exposure during the early stages of pregnancy. However, experimental evidence supporting the association is lacking at the present time. It is believed that benzene and its metabolites target hematopoietic stem cells (HSCs) to cause toxicity and cancer in the hematopoietic system. In the current study, we compared the effects of hydroquinone (HQ), a major metabolite of benzene in humans and animals, on mouse embryonic yolk sac hematopoietic stem cells (YS-HSCs) and adult bone marrow hematopoietic stem cells (BM-HSCs). YS-HSCs and BM-HSCs were isolated and enriched, and were exposed to HQ at increasing concentrations. HQ reduced the proliferation and the differentiation and colony formation, but increased the apoptosis of both YS-HSCs and BM-HSCs. However, the cytotoxic and apoptotic effects of HQ were more apparent and reduction of colony formation by HQ was more severe in YS-HSCs than in BM-HSCs. Differences in gene expression profiles were observed in HQ-treated YS-HSCs and BM-HSCs. Cyp4f18 was induced by HQ both in YS-HSCs and BM-HSCs, whereas DNA-PKcs was induced in BM-HSCs only. The results revealed differential effects of benzene metabolites on embryonic and adult HSCs. The study established an experimental system for comparison of the hematopoietic toxicity and leukemogenicity of benzene and metabolites during mouse embryonic development and adulthood.

Hematopoietic cell differentiation from embryonic and induced pluripotent stem cells

Pluripotent stem cells, both embryonic stem cells and induced pluripotent stem cells, are undifferentiated cells that can self-renew and potentially differentiate into all hematopoietic lineages, such as hematopoietic stem cells (HSCs), hematopoietic progenitor cells and mature hematopoietic cells in the presence of a suitable culture system. Establishment of pluripotent stem cells provides a comprehensive model to study early hematopoietic development and has emerged as a powerful research tool to explore regenerative medicine. Nowadays, HSC transplantation and hematopoietic cell transfusion have successfully cured some patients, especially in malignant hematological diseases. Owing to a shortage of donors and a limited number of the cells, hematopoietic cell induction from pluripotent stem cells has been regarded as an alternative source of HSCs and mature hematopoietic cells for intended therapeutic purposes. Pluripotent stem cells are therefore extensively utilized to facilitate better understanding in hematopoietic development by recapitulating embryonic development in vivo, in which efficient strategies can be easily designed and deployed for the generation of hematopoietic lineages in vitro. We hereby review the current progress of hematopoietic cell induction from embryonic stem/induced pluripotent stem cells.

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.

Isolation of embryonic hematopoietic niche cells by flow cytometry and laser capture microdissection

Hematopoietic stem cells (HSCs) can differentiate into several types of hematopoietic cells, such as erythrocytes, megakaryocytes, lymphocytes, neutrophils, or macrophages, and also undergo self-renewal to sustain hematopoiesis throughout an organism's lifetime. HSCs emerge and expand during mouse embryogenesis. HSC regulation is governed by two types of activity: intrinsic activity programmed primarily by cell autonomous gene expression, and extrinsic factors, which originate from the so-called niche cells surrounding HSCs. Previously, we reported that endothelial niche cells regulate HSC generation at aorta-gonad-mesonephros region and placenta, and that hepatoblastic niche cells regulate HSC differentiation in mouse embryonic liver. In the course of those studies, we employed immunohistochemistry, flow cytometry, and the laser capture microdissection system to assess embryonic regulation of the mouse hematopoietic niche.

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.

Serum- and stromal cell-free hypoxic generation of embryonic stem cell-derived hematopoietic cells in vitro, capable of multilineage repopulation of immunocompetent mice

Induced pluripotent stem cells (iPSCs) may become a promising source for the generation of patient-specific hematopoietic stem cells (HSCs) in vitro. A crucial prerequisite will be the availability of reliable protocols for the directed and efficient differentiation toward HSCs. So far, the most robust strategy for generating HSCs from pluripotent cells in vitro has been established in the mouse model involving ectopic expression of the human transcription factor HOXB4. However, most differentiation protocols include coculture on a xenogenic stroma cell line and the use of animal serum. Involvement of any of both would pose a major barrier to the translation of those protocols to human autologous iPSCs intended for clinical use. Therefore, we asked whether long-term repopulating HSCs can, in principle, be generated from embryonic stem cells without stroma cells or serum. Here, we showed that long-term multilineage engraftment could be accomplished in immunocompetent mice when HSCs were generated in serum-free medium without stroma cell support and when hypoxic conditions were used. Under those conditions, HOXB4(+) embryonic stem cell-derived hematopoietic stem and progenitor cells were immunophenotypically similar to definitive bone marrow resident E-SLAM(+) (CD150(+)CD48(-)CD45(+)CD201(+)) HSCs. Thus, our findings may ease the development of definitive, adult-type HSCs from pluripotent stem cells, entirely in vitro.

Mechanism of action of HOXB4 on the hematopoietic differentiation of embryonic stem cells

Pluripotent stem cells can be differentiated into hematopoietic lineages in vitro and hold promise for the future treatment of hematological disease. Differentiation strategies involving defined factors in serum-free conditions have been successful in producing hematopoietic progenitors and some mature cell types from mouse and human embryonic stem cells and induced pluripotent cells. However, these precisely defined protocols are relatively inefficient and have not been used successfully to produce hematopoietic stem cells capable of multilineage long-term reconstitution of the hematopoietic system. More complex differentiation induction strategies including coculture with stromal cells derived from sites of hematopoietic activity in vivo and enforced expression of reprogramming transcription factors, such as HOXB4, have been required to increase the efficiency of the differentiation procedure and to produce these most potent hematopoietic stem cells. We review the studies that have used HOXB4 to improve hematopoietic differentiation from pluripotent cells focusing on studies that have provided some insight into its mechanism of action. A better understanding of the molecular pathways involved in the action of HOXB4 might lead to more defined culture systems and safer protocols for clinical translation.

Origin of blood cells and HSC production in the embryo

Hematopoietic stem cells (HSCs) are capable of self-renewal and differentiation into all blood cell types. During adult life, they reside in the bone marrow in a quiescent state. By contrast, in the growing embryo hematopoiesis is sequentially found in several developmental niches. This review provides an overview of the still controversial contribution of each of these embryonic sites to the final pool of adult HSCs and discusses new insights into the cellular origin and the molecular regulation implicated in the generation of blood progenitor cells. A better understanding of HSC development during ontogeny is essential to develop new strategies to amplify HSCs or to generate them from embryonic stem cells or by somatic cell reprogramming.

Characterization of hemangioblast in umbilical arteries of mid-gestation mouse embryos

Hemangioblasts are the common precursors of hematopoietic and vascular cells, and are characterized as blast colony-forming cells (BL-CFCs) in vitro. We previously identified BL-CFCs in the mouse aorta-gonads-mesonephros (AGM) region, but not yolk sac, placenta, circulation, or fetal liver. Here, we aim to determine whether BL-CFCs develop in the umbilical arteries (UA) that link the dorsal aorta (sub-region of AGM) and placenta. We find that the UA cells of E11.5 mouse embryos were capable of generating typical blast colonies. On replating, these colonies produced erythroid/myeloid progenitors and B220(+) B lymphocytes in vitro, corroborating their definitive hematopoietic nature. They also generated CD31(+) or endomucin(+) tube-like structures on OP9 stromal cells, showing their endothelial potential. The proximal and distal regions of UA had equal numbers of BL-CFCs. To evaluate whether BL-CFCs can be autonomously maintained or expanded in UA or AGM, in vitro organ culture was performed. Interestingly, the BL-CFC pool in the AGM was significantly amplified, in striking contrast to a decrease in the UA. Taken together, our findings indicate that in addition to the AGM the UA serves as an important, but less supportive, niche for hemangioblast development.

Mapping mouse hemangioblast maturation from headfold stages

The mouse posterior primitive streak at neural plate/headfold stages (NP/HF, ~7.5 dpc-8 dpc) represents an optimal window from which hemangioblasts can be isolated. We performed immunohistochemistry on this domain using established monoclonal antibodies for proteins that affect blood and endothelial fates. We demonstrate that HoxB4 and GATA1 are the first set of markers that segregate independently to endothelial or blood populations during NP/HF stages of mouse embryonic development. In a subset of cells, both proteins are co-expressed and immunoreactivities appear mutually excluded within nuclear spaces. We searched for this particular state at later sites of hematopoietic stem cell emergence, viz., the aorta-gonad-mesonephros (AGM) and the fetal liver at 10.5-11.5 dpc, and found that only a rare number of cells displayed this character. Based on this spatial-temporal argument, we propose that the earliest blood progenitors emerge either directly from the epiblast or through segregation within the allantoic core domain (ACD) through reduction of cell adhesion and pSmad1/5 nuclear signaling, followed by a stochastic decision toward a blood or endothelial fate that involves GATA1 and HoxB4, respectively. A third form in which binding distributions are balanced may represent a common condition shared by hemangioblasts and HSCs. We developed a heuristic model of hemangioblast maturation, in part, to be explicit about our assumptions.

Concise review: expanding roles for hematopoietic cellular therapy and the blood transfusion services

Hematopoietic stem cells (HSCs) have remained at the forefront of stem cell research for the past 50 years, since the therapeutic potential of bone marrow transplantation was realized. Uniquely, among stem and progenitor cells, research progress has been made in parallel between the laboratory benchtop and hospital bedside during this period. Integral to this work has been the role of the transfusion medicine services in the collection, storage, and processing of HSCs. The next decade promises to bring further developments: with new fields of cellular therapies, stem cell vaccination, and stem cell drug testing opening up. This article summarizes exciting areas of research concerning the behavior and potential clinical applications of HSCs. For the purposes of clarity, we describe in turn the trafficking and transfer of HSCs; ex vivo expansion of HSC units from different sources; and finally, applications of specifically selected subsets of hematopoietic cells and their progeny.

The benefits and risks of stem cell technology

The potential impact of stem cell technology on medical and dental practice is vast. Stem cell research will not only provide the foundation for future therapies, but also reveal unique insights into basic disease mechanisms. Therefore, an understanding of stem cell technology will be necessary for clinicians in the future. Herein, we give a basic overview of stem cell biology and therapeutics for the practicing clinician.

Brief report: efficient generation of hematopoietic precursors and progenitors from human pluripotent stem cell lines

By mimicking embryonic development of the hematopoietic system, we have developed an optimized in vitro differentiation protocol for the generation of precursors of hematopoietic lineages and primitive hematopoietic cells from human embryonic stem cells (ESC) and induced pluripotent stem cells (iPSCs). Factors such as cytokines, extra cellular matrix components, and small molecules as well as the temporal association and concentration of these factors were tested on seven different human ESC and iPSC lines. We report the differentiation of up to 84% human CD45+ cells (average 41% ± 16%, from seven pluripotent lines) from the differentiation culture, including significant numbers of primitive CD45+/CD34+ and CD45+/CD34+/CD38- hematopoietic progenitors. Moreover, the numbers of hematopoietic progenitor cells generated, as measured by colony forming unit assays, were comparable to numbers obtained from fresh umbilical cord blood mononuclear cell isolates on a per CD45+ cell basis. Our approach demonstrates highly efficient generation of multipotent hematopoietic progenitors with among the highest efficiencies reported to date (CD45+/CD34+) using a single standardized differentiation protocol on several human ESC and iPSC lines. Our data add to the cumulating evidence for the existence of an in vitro derived precursor to the hematopoietic stem cell (HSC) with limited engrafting ability in transplanted mice but with multipotent hematopoietic potential. Because this protocol efficiently expands the preblood precursors and hematopoietic progenitors, it is ideal for testing novel factors for the generation and expansion of definitive HSCs with long-term repopulating ability.

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.

Hepatoblasts comprise a niche for fetal liver erythropoiesis through cytokine production

In mammals, definitive erythropoiesis first occurs in fetal liver (FL), although little is known about how the process is regulated. FL consists of hepatoblasts, sinusoid endothelial cells and hematopoietic cells. To determine niche cells for fetal liver erythropoiesis, we isolated each FL component by flow cytometry. mRNA analysis suggested that Dlk-1-expressing hepatoblasts primarily expressed EPO and SCF, genes encoding erythropoietic cytokines. EPO protein was detected predominantly in hepatoblasts, as assessed by ELISA and immunohistochemistry, and was not detected in sinusoid endothelial cells and hematopoietic cells. To characterize hepatoblast function in FL, we analyzed Map2k4(-/-) mouse embryos, which lack hepatoblasts, and observed down-regulation of EPO and SCF expression in FL relative to wild-type mice. Our observations demonstrate that hepatoblasts comprise a niche for erythropoiesis through cytokine secretion.

Establishment and regulation of the HSC niche: Roles of osteoblastic and vascular compartments

Hematopoietic stem cells (HSC) are multi-potent cells that function to generate a lifelong supply of all blood cell types. During mammalian embryogenesis, sites of hematopoiesis change over the course of gestation: from extraembryonic yolk sac and placenta, to embryonic aorta-gonad-mesonephros region, fetal liver, and finally fetal bond marrow where HSC reside postnatally. These tissues provide microenviroments for de novo HSC formation, as well as HSC maturation and expansion. Within adult bone marrow, HSC self-renewal and differentiation are thought to be regulated by two major cellular components within their so-called niche: osteoblasts and vascular endothelial cells. This review focuses on HSC generation within, and migration to, different tissues during development, and also provides a summary of major regulatory factors provided by osteoblasts and vascular endothelial cells within the adult bone marrow niche.

Novel methods for determining hematopoietic stem and progenitor cell emergence in the murine yolk sac

The mammalian yolk sac is known to play a prominent role in emergence of the hematopoietic system. The extent of this contribution has been a subject of debate in recent years largely due to effects of the early circulation that obscures the site of origin of hematopoietic stem and progenitor cells. This review discusses the limitations of some of the standard assays currently employed to study hematopoietic stem and progenitor cell emergence and highlights several recently reported novel methods that address this problem from new perspectives. Two methods directly alter the circulation by either preventing it from occurring in the first place or by removing vascular connections between the embryo and the yolk sac. Other approaches have altered the ability of hematopoietic cells to interact with their environment, resulting in the lack of migration or an inability to bind to potential hematopoietic niches. A third set of experiments utilize lineage tracing techniques to follow the migration of early progenitors once they enter the circulation. Taken together, these novel methods provide new evidence for the contribution of yolk sac hematopoietic stem and progenitor cells to the adult hematopoietic system.

Challenges and strategies for generating therapeutic patient-specific hemangioblasts and hematopoietic stem cells from human pluripotent stem cells

Recent characterization of hemangioblasts differentiated from human embryonic stem cells (hESC) has further confirmed evidence from murine, zebrafish and avian experimental systems that hematopoietic and endothelial lineages arise from a common progenitor. Such progenitors may provide a valuable resource for delineating the initial developmental steps of human hemato-endotheliogenesis, which is a process normally difficult to study due to the very limited accessibility of early human embryonic/fetal tissues. Moreover, efficient hemangioblast and hematopoietic stem cell (HSC) generation from patient-specific pluripotent stem cells has enormous potential for regenerative medicine, since it could lead to strategies for treating a multitude of hematologic and vascular disorders. However, significant scientific challenges remain in achieving these goals, and the generation of transplantable hemangioblasts and HSC derived from hESC currently remains elusive. Our previous work has suggested that the failure to derive engraftable HSC from hESC is due to the fact that current methodologies for differentiating hESC produce hematopoietic progenitors developmentally similar to those found in the human yolk sac, and are therefore too immature to provide adult-type hematopoietic reconstitution. Herein, we outline the nature of this challenge and propose targeted strategies for generating engraftable human pluripotent stem cell-derived HSC from primitive hemangioblasts using a developmental approach. We also focus on methods by which reprogrammed somatic cells could be used to derive autologous pluripotent stem cells, which in turn could provide unlimited sources of patient-specific hemangioblasts and HSC.

Regulation of hematopoietic cell clusters in the placental niche through SCF/Kit signaling in embryonic mouse

Hematopoietic stem cells (HSCs) emerge from and expand in the mouse placenta at mid-gestation. To determine their compartment of origin and define extrinsic signals governing their commitment to this lineage, we identified hematopoietic cell (HC) clusters in mouse placenta, defined as cells expressing the embryonic HSC markers CD31, CD34 and Kit, by immunohistochemistry. HC clusters were first observed in the placenta at 9.5 days post coitum (dpc). To determine their origin, we tagged the allantoic region with CM-DiI at 8.25 dpc, prior to placenta formation, and cultured embryos in a whole embryo culture (WEC) system. CM-DiI-positive HC clusters were observed 42 hours later. To determine how clusters are extrinsically regulated, we isolated niche cells using laser capture micro-dissection and assayed them for expression of genes encoding hematopoietic cytokines. Among a panel of candidates assayed, only stem cell factor (SCF) was expressed in niche cells. To define niche cells, endothelial and mesenchymal cells were sorted by flow cytometry from dissociated placenta and hematopoietic cytokine gene expression was investigated. The endothelial cell compartment predominantly expressed SCF mRNA and protein. To determine whether SCF/Kit signaling regulates placental HC cluster proliferation, we injected anti-Kit neutralizing antibody into 10.25 dpc embryos and assayed cultured embryos for expression of hematopoietic transcription factors. Runx1, Myb and Gata2 were downregulated in the placental HC cluster fraction relative to controls. These observations demonstrate that placental HC clusters originate from the allantois and are regulated by endothelial niche cells through SCF/Kit signaling.

Induction of hematopoietic differentiation of mouse embryonic stem cells by an AGM-derived stromal cell line is not further enhanced by overexpression of HOXB4

Hematopoietic differentiation of embryonic stem (ES) cells can be enhanced by co-culture with stromal cells derived from hematopoietic tissues and by overexpression of the transcription factor HOXB4. In this study, we compare the hematopoietic inductive effects of stromal cell lines derived from different subregions of the embryonic aorta-gonad-mesonephros tissue with the commonly used OP9 stromal cell line and with HOXB4 activation. We show that stromal cell lines derived from the aorta and surrounding mesenchyme (AM) act at an earlier stage of the differentiation process compared with the commonly used OP9 stromal cells. AM stromal cells were able to promote the further differentiation of isolated brachyury-GFP(+) mesodermal cells into hematopoietic progenitors, whereas the OP9 stromal cells could not support the differentiation of these cells. Co-culture and analyses of individual embryoid bodies support the hypothesis that the AM stromal cell lines could enhance the de novo production of hematopoietic progenitors, lending support to the idea that AM stromal cells might act on prehematopoietic mesoderm. The induction level observed for AM stromal cells was comparable to HOXB4 activation, but no additive effect was observed when these 2 inductive strategies were combined. Addition of a γ-secretase inhibitor reduced the inductive effects of both the stromal cell line and HOXB4, providing clues to possible shared molecular mechanisms.

Tachykinins and Neurokinin Receptors in Bone Marrow Functions: Neural-Hematopoietic Link

After many decades of neuropeptide research, advances in the field of tachykinins have considerably increased and shown their implications in several physiological processes. In this review we focus on the role of the tachykinins in the regulation of hematopoietic functions. Evidence has shown that neural control of this process is emerging as a significant category in hematopoietic modulation. In the context of this regulation, we discuss the existence of a complex network involving the neurokinin receptors, tachykinins and cytokines. This network is tightly regulated by each of its components.

Orderly hematopoietic development of induced pluripotent stem cells via Flk-1(+) hemoangiogenic progenitors

Induced pluripotent stem (iPS) cells, reprogrammed somatic cells with embryonic stem (ES) cell-like characteristics, are generated by the introduction of combinations of specific transcription factors. Little is known about the differentiation of iPS cells in vitro. Here we demonstrate that murine iPS cells produce various hematopoietic cell lineages when incubated on a layer of OP9 stromal cells. During this differentiation, iPS cells went through an intermediate stage consisting of progenitor cells that were positive for the early mesodermal marker Flk-1 and for the sequential expression of other genes that are associated with hematopoietic and endothelial development. Flk-1(+) cells differentiated into primitive and definitive hematopoietic cells, as well as into endothelial cells. Furthermore, Flk-1(+) populations contained common bilineage progenitors that could generate both hematopoietic and endothelial lineages from single cells. Our results demonstrate that iPS cell-derived cells, like ES cells, can follow a similar hematopoietic route to that seen in normal embryogenesis. This finding highlights the potential use of iPS cells in clinical areas such as regenerative medicine, disease investigation, and drug screening.

Overcoming obstacles in the search for the site of hematopoietic stem cell emergence

The murine embryo has become a valuable tool to examine the ontogeny of hematopoiesis. However, the onset of the systemic circulation has long been a confounding developmental variable that may mask the site of blood cell emergence. This Minireview examines some approaches that have been applied to overcome this obstacle.

Hematopoiesis from Human Embryonic Stem Cells: Overcoming the Immune Barrier in Stem Cell Therapies

The multipotency and proliferative capacity of human embryonic stem cells (hESCs) make them a promising source of stem cells for transplant therapies and of vital importance given the shortage in organ donation. Recent studies suggest some immune privilege associated with hESC-derived tissues. However, the adaptability of the immune system makes it unlikely that fully differentiated tissues will permanently evade immune rejection. One promising solution is to induce a state of immune tolerance to a hESC line using tolerogenic hematopoietic cells derived from it. This could provide acceptance of other differentiated tissues from the same line. However, this approach will require efficient multilineage hematopoiesis from hESCs. This review proposes that more efficient differentiation of hESCs to the tolerogenic cell types required is most likely to occur through applying knowledge gained of the ontogeny of complex regulatory signals used by the embryo for definitive hematopoietic development in vivo. Stepwise formation of mesoderm, induction of definitive hematopoietic stem cells, and the application of factors key to their self-renewal may improve in vitro production both quantitatively and qualitatively.

Generation of functional erythrocytes from human embryonic stem cell-derived definitive hematopoiesis

A critical issue for clinical utilization of human ES cells (hESCs) is whether they can generate terminally mature progenies with normal function. We recently developed a method for efficient production of hematopoietic progenitors from hESCs by coculture with murine fetal liver-derived stromal cells. Large numbers of hESCs-derived erythroid progenitors generated by the coculture enabled us to analyze the development of erythropoiesis at a clone level and investigate their function. The results showed that the globin expression in the erythroid cells in individual clones changed in a time-dependent manner. In particular, embryonic epsilon-globin-expressing erythroid cells from individual clones decreased, whereas adult-type beta-globin-expressing cells increased to approximately 100% in all clones we examined, indicating that the cells undergo definitive hematopoiesis. Enucleated erythrocytes also appeared among the clonal progeny. A comparison analysis showed that hESC-derived erythroid cells took a similar differentiation pathway to human cord blood CD34(+) progenitor-derived cells when examined for the expression of glycophorin A, CD71 and CD81. Furthermore, these hESC-derived erythroid cells could function as oxygen carriers and had a sufficient glucose-6-phosphate dehydrogenase activity. The present study should provide an experimental model for exploring early development of human erythropoiesis and hemoglobin switching and may help in the discovery of drugs for hereditary diseases in erythrocyte development.