Embryonic origin of the adult hematopoietic system: advances and questions

Hematopoiesis

Enormous numbers of adult blood cells are constantly regenerated throughout life from hematopoietic stem cells through a series of progenitor stages. Accessibility, robust functional assays, well-established prospective isolation, and successful clinical application made hematopoiesis the classical mammalian stem cell system. Most of the basic concepts of stem cell biology have been defined in this system. At the same time, many long-standing disputes in hematopoiesis research illustrate our still limited understanding. Here we discuss the embryonic development and lifelong maintenance of the hematopoietic system, its cellular components, and some of the hypotheses about the molecular mechanisms involved in controlling hematopoietic cell fates.

The biochemistry of hematopoietic stem cell development

BACKGROUND

The cornerstone of the adult hematopoietic system and clinical treatments for blood-related disease is the cohort of hematopoietic stem cells (HSC) that is harbored in the adult bone marrow microenvironment. Interestingly, this cohort of HSCs is generated only during a short window of developmental time. In mammalian embryos, hematopoietic progenitor and HSC generation occurs within several extra- and intraembryonic microenvironments, most notably from 'hemogenic' endothelial cells lining the major vasculature. HSCs are made through a remarkable transdifferentiation of endothelial cells to a hematopoietic fate that is long-lived and self-renewable. Recent studies are beginning to provide an understanding of the biochemical signaling pathways and transcription factors/complexes that promote their generation.

SCOPE OF REVIEW

The focus of this review is on the biochemistry behind the generation of these potent long-lived self-renewing stem cells of the blood system. Both the intrinsic (master transcription factors) and extrinsic regulators (morphogens and growth factors) that affect the generation, maintenance and expansion of HSCs in the embryo will be discussed.

MAJOR CONCLUSIONS

The generation of HSCs is a stepwise process involving many developmental signaling pathways, morphogens and cytokines. Pivotal hematopoietic transcription factors are required for their generation. Interestingly, whereas these factors are necessary for HSC generation, their expression in adult bone marrow HSCs is oftentimes not required. Thus, the biochemistry and molecular regulation of HSC development in the embryo are overlapping, but differ significantly from the regulation of HSCs in the adult.

GENERAL SIGNIFICANCE

HSC numbers for clinical use are limiting, and despite much research into the molecular basis of HSC regulation in the adult bone marrow, no panel of growth factors, interleukins and/or morphogens has been found to sufficiently increase the number of these important stem cells. An understanding of the biochemistry of HSC generation in the developing embryo provides important new knowledge on how these complex stem cells are made, sustained and expanded in the embryo to give rise to the complete adult hematopoietic system, thus stimulating novel strategies for producing increased numbers of clinically useful HSCs. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

The biochemistry of hematopoietic stem cell development

BACKGROUND

The cornerstone of the adult hematopoietic system and clinical treatments for blood-related disease is the cohort of hematopoietic stem cells (HSC) that is harbored in the adult bone marrow microenvironment. Interestingly, this cohort of HSCs is generated only during a short window of developmental time. In mammalian embryos, hematopoietic progenitor and HSC generation occurs within several extra- and intraembryonic microenvironments, most notably from 'hemogenic' endothelial cells lining the major vasculature. HSCs are made through a remarkable transdifferentiation of endothelial cells to a hematopoietic fate that is long-lived and self-renewable. Recent studies are beginning to provide an understanding of the biochemical signaling pathways and transcription factors/complexes that promote their generation.

SCOPE OF REVIEW

The focus of this review is on the biochemistry behind the generation of these potent long-lived self-renewing stem cells of the blood system. Both the intrinsic (master transcription factors) and extrinsic regulators (morphogens and growth factors) that affect the generation, maintenance and expansion of HSCs in the embryo will be discussed.

MAJOR CONCLUSIONS

The generation of HSCs is a stepwise process involving many developmental signaling pathways, morphogens and cytokines. Pivotal hematopoietic transcription factors are required for their generation. Interestingly, whereas these factors are necessary for HSC generation, their expression in adult bone marrow HSCs is oftentimes not required. Thus, the biochemistry and molecular regulation of HSC development in the embryo are overlapping, but differ significantly from the regulation of HSCs in the adult.

GENERAL SIGNIFICANCE

HSC numbers for clinical use are limiting, and despite much research into the molecular basis of HSC regulation in the adult bone marrow, no panel of growth factors, interleukins and/or morphogens has been found to sufficiently increase the number of these important stem cells. An understanding of the biochemistry of HSC generation in the developing embryo provides important new knowledge on how these complex stem cells are made, sustained and expanded in the embryo to give rise to the complete adult hematopoietic system, thus stimulating novel strategies for producing increased numbers of clinically useful HSCs. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

A new ontology (structured hierarchy) of human developmental anatomy for the first 7 weeks (Carnegie stages 1-20)

This paper describes a new ontology of human developmental anatomy covering the first 49 days [Carnegie stages (CS)1-20], primarily structured around the parts of organ systems and their development. The ontology includes more than 2000 anatomical entities (AEs) that range from the whole embryo, through organ systems and organ parts down to simple or leaf tissues (groups of cells with the same morphological phenotype), as well as features such as cavities. Each AE has assigned to it a set of facts of the form <AE><relationship><parent>, with the relationships including starts_at and ends_at (CSs), part_of (there can be several parents) and is_a (this gives the type of tissue, from an organ system down to one of ~ 80 simple tissues predominantly composed of a single cell kind, which is also specified). Leaf tissues also have a develops_from link to its parent tissue. The ontology includes ~14 000 such facts, which are mainly from the literature and an earlier ontology of human developmental anatomy (EHDAA, now withdrawn). The relationships enable these facts to be integrated into a single, complex hierarchy (or mathematical graph) that was made and can be viewed in the OBO-Edit browser (oboedit.org). Each AE has an EHDAA2 ID that may be useful in an informatics context, while the ontology as a whole can be used for organizing databases of human development. It is also a knowledge resource: a user can trace the lineage of any tissue back to the egg, study the changes in cell phenotype that occur as a tissue develops, and use the structure to add further (e.g. molecular) information. The ontology may be downloaded from www.obofoundry.org. Queries and corrections should be sent to j.bard@ed.ac.uk.

Identification of functional progenitor cells in the pulmonary vasculature

The pulmonary vasculature comprises a complex network of branching arteries and veins all functioning to reoxygenate the blood for circulation around the body. The cell types of the pulmonary artery are able to respond to changes in oxygen tension in order to match ventilation to perfusion. Stem and progenitor cells in the pulmonary vasculature are also involved, be it in angiogenesis, endothelial dysfunction or formation of vascular lesions. Stem and progenitor cells may be circulating around the body, residing in the pulmonary artery wall or stimulated for release from a central niche like the bone marrow and home to the pulmonary vasculature along a chemotactic gradient. There may currently be some controversy over the pathogenic versus therapeutic roles of stem and progenitor cells and, indeed, it is likely both chains of evidence are correct due to the specific influence of the immediate environmental niche a progenitor cell may be in. Due to their great plasticity and a lack of specific markers for stem and progenitor cells, they can be difficult to precisely identify. This review discusses the methodological approaches used to validate the presence of and subtype of progenitors cells in the pulmonary vasculature while putting it in context of the current knowledge of the therapeutic and pathogenic roles for such progenitor cells.

PDCD2 controls hematopoietic stem cell differentiation during development

Programmed cell death 2 (Pdcd2) is a highly conserved protein of undefined function, and is widely expressed in embryonic and adult tissues. The observations that knockout of Pdcd2 in the mouse is embryonic lethal at preimplantation stages, and that in Drosophila, Zfrp8, the ortholog of Pdcd2, is required for normal lymph gland development suggest that Pdcd2 is important for regulating hematopoietic development. Through genetic and functional studies, we investigated pdcd2 function during the zebrafish ontogeny. Knockdown of pdcd2 expression in zebrafish embryos resulted in defects in embryonic hematopoietic development. Loss of pdcd2 function caused increased expression of progenitor markers, and accumulation of erythroid progenitors during primitive hematopoiesis. Additionally, hematopoietic stem cells (HSCs) failed to appear in the aorta-gonad mesonephros, and were not able to terminally differentiate or reconstitute hematopoiesis. Pdcd2 effects on HSC emergence were cell autonomous and P53-independent, and loss of pdcd2 function was associated with mitotic defects and apoptosis. Restoration of runx1 function(s) and modulation of apoptosis through the inhibition of Jak/Stat signaling rescued the hematopoietic and erythroid defects resulting from pdcd2 knockdown. Our studies suggest that pdcd2 plays a critical role in regulating the transcriptional hierarchy controlling hematopoietic lineage determination. Furthermore, the effects of pdcd2 in regulating mitotic cell death may contribute to its role(s) in directing hematopoietic differentiation during development.

A long way to stemness

Comment on: Tanaka Y, et al. Proc Natl Acad Sci USA 2012; 109:4515-20.

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.

Ion channels in hematopoietic and mesenchymal stem cells

Hematopoietic stem cells (HSCs) reside in bone marrow niches and give rise to hematopoietic precursor cells (HPCs). These have more restricted lineage potential and eventually differentiate into specific blood cell types. Bone marrow also contains mesenchymal stromal cells (MSCs), which present multilineage differentiation potential toward mesodermal cell types. In bone marrow niches, stem cell interaction with the extracellular matrix is mediated by integrin receptors. Ion channels regulate cell proliferation and differentiation by controlling intracellular Ca(2+), cell volume, release of growth factors, and so forth. Although little evidence is available about the ion channel roles in true HSCs, increasing information is available about HPCs and MSCs, which present a complex pattern of K(+) channel expression. K(+) channels cooperate with Ca(2+) and Cl(-) channels in regulating calcium entry and cell volume during mitosis. Other K(+) channels modulate the integrin-dependent interaction between leukemic progenitor cells and the niche stroma. These channels can also regulate leukemia cell interaction with MSCs, which also involves integrin receptors and affects the MSC-mediated protection from chemotherapy. Ligand-gated channels are also implicated in these processes. Nicotinic acetylcholine receptors regulate cell proliferation and migration in HSCs and MSCs and may be implicated in the harmful effects of smoking.

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.

Novel insights into the genetic controls of primitive and definitive hematopoiesis from zebrafish models

Hematopoiesis is a dynamic process where initiation and maintenance of hematopoietic stem cells, as well as their differentiation into erythroid, myeloid and lymphoid lineages, are tightly regulated by a network of transcription factors. Understanding the genetic controls of hematopoiesis is crucial as perturbations in hematopoiesis lead to diseases such as anemia, thrombocytopenia, or cancers, including leukemias and lymphomas. Animal models, particularly conventional and conditional knockout mice, have played major roles in our understanding of the genetic controls of hematopoiesis. However, knockout mice for most of the hematopoietic transcription factors are embryonic lethal, thus precluding the analysis of their roles during the transition from embryonic to adult hematopoiesis. Zebrafish are an ideal model organism to determine the function of a gene during embryonic-to-adult transition of hematopoiesis since bloodless zebrafish embryos can develop normally into early larval stage by obtaining oxygen through diffusion. In this review, we discuss the current status of the ontogeny and regulation of hematopoiesis in zebrafish. By providing specific examples of zebrafish morphants and mutants, we have highlighted the contributions of the zebrafish model to our overall understanding of the roles of transcription factors in regulation of primitive and definitive hematopoiesis.

Dlk1 is a negative regulator of emerging hematopoietic stem and progenitor cells

The first mouse adult-repopulating hematopoietic stem cells emerge in the aorta-gonad-mesonephros region at embryonic day (E) 10.5. Their numbers in this region increase thereafter and begin to decline at E12.5, thus pointing to the possible existence of both positive and negative regulators of emerging hematopoietic stem cells. Our recent expression analysis of the aorta-gonad-mesonephros region showed that the Delta-like homologue 1 (Dlk1) gene is up-regulated in the region of the aorta-gonad-mesonephros where hematopoietic stem cells are preferentially located. To analyze its function, we studied Dlk1 expression in wild-type and hematopoietic stem cell-deficient embryos and determined hematopoietic stem and progenitor cell activity in Dlk1 knockout and overexpressing mice. Its role in hematopoietic support was studied in co-culture experiments using stromal cell lines that express varying levels of Dlk1. We show here that Dlk1 is expressed in the smooth muscle layer of the dorsal aorta and the ventral sub-aortic mesenchyme, where its expression is dependent on the hematopoietic transcription factor Runx1. We further demonstrate that Dlk1 has a negative impact on hematopoietic stem and progenitor cell activity in the aorta-gonad-mesonephros region in vivo, which is recapitulated in co-cultures of hematopoietic stem cells on stromal cells that express varying levels of Dlk1. This negative effect of Dlk1 on hematopoietic stem and progenitor cell activity requires the membrane-bound form of the protein and cannot be recapitulated by soluble Dlk1. Together, these data suggest that Dlk1 expression by cells of the aorta-gonad-mesonephros hematopoietic microenvironment limits hematopoietic stem cell expansion and is, to our knowledge, the first description of such a negative regulator in this tissue.

GFI1 and GFI1B control the loss of endothelial identity of hemogenic endothelium during hematopoietic commitment

Recent studies have established that during embryonic development, hematopoietic progenitors and stem cells are generated from hemogenic endothelium precursors through a process termed endothelial to hematopoietic transition (EHT). The transcription factor RUNX1 is essential for this process, but its main downstream effectors remain largely unknown. Here, we report the identification of Gfi1 and Gfi1b as direct targets of RUNX1 and critical regulators of EHT. GFI1 and GFI1B are able to trigger, in the absence of RUNX1, the down-regulation of endothelial markers and the formation of round cells, a morphologic change characteristic of EHT. Conversely, blood progenitors in Gfi1- and Gfi1b-deficient embryos maintain the expression of endothelial genes. Moreover, those cells are not released from the yolk sac and disseminated into embryonic tissues. Taken together, our findings demonstrate a critical and specific role of the GFI1 transcription factors in the first steps of the process leading to the generation of hematopoietic progenitors from hemogenic endothelium.

Hematopoietic stem and progenitor cells as effectors in innate immunity

Recent research has shed light on novel functions of hematopoietic stem and progenitor cells (HSPC). While they are critical for maintenance and replenishment of blood cells in the bone marrow, these cells are not limited to the bone marrow compartment and function beyond their role in hematopoiesis. HSPC can leave bone marrow and circulate in peripheral blood and lymph, a process often manipulated therapeutically for the purpose of transplantation. Additionally, these cells preferentially home to extramedullary sites of inflammation where they can differentiate to more mature effector cells. HSPC are susceptible to various pathogens, though they may participate in the innate immune response without being directly infected. They express pattern recognition receptors for detection of endogenous and exogenous danger-associated molecular patterns and respond not only by the formation of daughter cells but can themselves secrete powerful cytokines. This paper summarizes the functional and phenotypic characterization of HSPC, their niche within and outside of the bone marrow, and what is known regarding their role in the innate immune response.

Hematopoietic stem cell development and regulatory signaling in zebrafish

BACKGROUND

Hematopoietic stem cells (HSCs) are a population of multipotent cells that can self-renew and differentiate into all blood lineages. HSC development must be tightly controlled from cell fate determination to self-maintenance during adulthood. This involves a panel of important developmental signaling pathways and other factors which act synergistically within the HSC population and/or in the HSC niche. Genetically conserved processes of HSC development plus many other developmental advantages make the zebrafish an ideal model organism to elucidate the regulatory mechanisms underlying HSC programming.

SCOPE OF REVIEW

This review summarizes recent progress on zebrafish HSCs with particular focus on how developmental signaling controls hemogenic endothelium-derived HSC development. We also describe the interaction of different signaling pathways during these processes.

MAJOR CONCLUSIONS

The hematopoietic stem cell system is a paradigm for stem cell studies. Use of the zebrafish model to study signaling regulation of HSCs in vivo has resulted in a great deal of information concerning HSC biology in vertebrates.

GENERAL SIGNIFICANCE

These new findings facilitate a better understanding of molecular mechanisms of HSC programming, and will provide possible new strategies for the treatment of HSC-related hematological diseases, such as leukemia. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

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.

SCL/TAL1 regulates hematopoietic specification from human embryonic stem cells

Determining the molecular regulators/pathways responsible for the specification of human embryonic stem cells (hESCs) into hematopoietic precursors has far-reaching implications for potential cell therapies and disease modeling. Mouse models lacking SCL/TAL1 (stem cell leukemia/T-cell acute lymphocytic leukemia 1) do not survive beyond early embryogenesis because of complete absence of hematopoiesis, indicating that SCL is a master early hematopoietic regulator. SCL is commonly found rearranged in human leukemias. However, there is barely information on the role of SCL on human embryonic hematopoietic development. Differentiation and sorting assays show that endogenous SCL expression parallels hematopoietic specification of hESCs and that SCL is specifically expressed in hematoendothelial progenitors (CD45(-)CD31(+)CD34(+)) and, to a lesser extent, on CD45(+) hematopoietic cells. Enforced expression of SCL in hESCs accelerates the emergence of hematoendothelial progenitors and robustly promotes subsequent differentiation into primitive (CD34(+)CD45(+)) and total (CD45(+)) blood cells with higher clonogenic potential. Short-hairpin RNA-based silencing of endogenous SCL abrogates hematopoietic specification of hESCs, confirming the early hematopoiesis-promoting effect of SCL. Unfortunately, SCL expression on its own is not sufficient to confer in vivo engraftment to hESC-derived hematopoietic cells, suggesting that additional yet undefined master regulators are required to orchestrate the stepwise hematopoietic developmental process leading to the generation of definitive in vivo functional hematopoiesis from hESCs.

B-1 B cell development in the fetus and adult

Models of hematopoiesis often depict lymphocyte production as a uniform process in which a homogenous population of hematopoietic stem cells (HSCs) generates progenitors from which all types of lymphocytes are derived. However, it is increasingly evident that these schemes are too simplistic and that the lymphoid potential of HSCs and precursors arising in the embryo, fetus, neonate, and adult is remarkably distinct. We review recent findings regarding the development of B lymphocytes, and the B-1 B cell lineage in particular, as a case in point. These studies show that B-1 and B-2 B cells involved in innate and adaptive immune responses, respectively, arise in staggered waves of development from distinct progenitors. We discuss the implications of this layered model of B cell development for understanding normal and dysregulated B lymphopoiesis.

Hemogenic endothelium during development and beyond

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

Early ontogenic origin of the hematopoietic stem cell lineage

Several lines of evidence suggest that the adult hematopoietic system has multiple developmental origins, but the ontogenic relationship between nascent hematopoietic populations under this scheme is poorly understood. In an alternative theory, the earliest definitive blood precursors arise from a single anatomical location, which constitutes the cellular source for subsequent hematopoietic populations. To deconvolute hematopoietic ontogeny, we designed an embryo-rescue system in which the key hematopoietic factor Runx1 is reactivated in Runx1-null conceptuses at specific developmental stages. Using complementary in vivo and ex vivo approaches, we provide evidence that definitive hematopoiesis and adult-type hematopoietic stem cells originate predominantly in the nascent extraembryonic mesoderm. Our data also suggest that other anatomical sites that have been proposed to be sources of the definitive hematopoietic hierarchy are unlikely to play a substantial role in de novo blood generation.

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.

Whole-mount three-dimensional imaging of internally localized immunostained cells within mouse embryos

We describe a three-dimensional (3D) confocal imaging technique to characterize and enumerate rare, newly emerging hematopoietic cells located within the vasculature of whole-mount preparations of mouse embryos. However, the methodology is broadly applicable for examining the development and 3D architecture of other tissues. Previously, direct whole-mount imaging has been limited to external tissue layers owing to poor laser penetration of dense, opaque tissue. Our whole-embryo imaging method enables detailed quantitative and qualitative analysis of cells within the dorsal aorta of embryonic day (E) 10.5-11.5 embryos after the removal of only the head and body walls. In this protocol we describe the whole-mount fixation and multimarker staining procedure, the tissue transparency treatment, microscopy and the analysis of resulting images. A typical two-color staining experiment can be performed and analyzed in ∼6 d.

Hematopoietic stem cells: to be or Notch to be

Notch is a well-conserved signaling pathway and its function in cell fate determination is crucial in embryonic development and in the maintenance of tissue homeostasis during adult life. Notch activation depends on cell-cell interactions that are essential for the generation of cell diversity from initially equivalent cell populations. In the adult hematopoiesis, Notch is undoubtedly a very efficient promoter of T-cell differentiation, and this has masked for a long time the effects of Notch on other blood lineages, which are gradually being identified. However, the adult hematopoietic stem cell (HSC) remains mostly refractory to Notch intervention in experimental systems. In contrast, Notch is essential for the generation of the HSCs, which takes place during embryonic development. This review summarizes the knowledge accumulated in recent years regarding the role of the Notch pathway in the different stages of HSC ontology from embryonic life to fetal and adult bone marrow stem cells. In addition, we briefly examine other systems where Notch regulates specific stem cell capacities, in an attempt to understand how Notch functions in stem cell biology.

Cell cycle regulation in hematopoietic stem cells

Hematopoietic stem cells (HSCs) give rise to all lineages of blood cells. Because HSCs must persist for a lifetime, the balance between their proliferation and quiescence is carefully regulated to ensure blood homeostasis while limiting cellular damage. Cell cycle regulation therefore plays a critical role in controlling HSC function during both fetal life and in the adult. The cell cycle activity of HSCs is carefully modulated by a complex interplay between cell-intrinsic mechanisms and cell-extrinsic factors produced by the microenvironment. This fine-tuned regulatory network may become altered with age, leading to aberrant HSC cell cycle regulation, degraded HSC function, and hematological malignancy.

Endothelio-hematopoietic relationship: getting closer to the beginnings

The close association between hematopoietic and endothelial cells during embryonic development led to the proposal that they may originate from a common ancestor - the hemangioblast. Due to a lack of unique specific markers for in vivo cell fate tracking studies, evidence supporting this theory derives mainly from in vitro differentiation studies. Teixeira and colleagues describe a novel enhancer that drives specific eGFP expression in blood islands of the electroporated chick embryo, thereby presenting a tool potentially suitable for analysis of hemangioblast differentiation and development of blood islands.

See research article: http://www.biomedcentral.com/1471-213X/11/76

Highly potent human hematopoietic stem cells first emerge in the intraembryonic aorta-gonad-mesonephros region

Human HSCs appear first in the embryonic dorsal aorta, and only later in the yolk sac, liver, and placenta; a single human AGM region HSC can generate at least 300 daughter HSCs that are retransplantable into secondary recipient mice.

Hematopoietic stem cells (HSCs) emerge during embryogenesis and maintain hematopoiesis in the adult organism. Little is known about the embryonic development of human HSCs. We demonstrate that human HSCs emerge first in the aorta-gonad-mesonephros (AGM) region, specifically in the dorsal aorta, and only later appear in the yolk sac, liver, and placenta. AGM region cells transplanted into immunodeficient mice provide long-term high level multilineage hematopoietic repopulation. Human AGM region HSCs, although present in low numbers, exhibit a very high self-renewal potential. A single HSC derived from the AGM region generates at least 300 daughter HSCs in primary recipients, which disseminate throughout the entire recipient bone marrow and are retransplantable. These findings highlight the vast regenerative potential of the earliest human HSCs and set a new standard for in vitro generation of HSCs from pluripotent stem cells for the purpose of regenerative medicine.

BMP4 signaling directs primitive endoderm-derived XEN cells to an extraembryonic visceral endoderm identity

The visceral endoderm (VE) is an epithelial tissue in the early postimplantation mouse embryo that encapsulates the pluripotent epiblast distally and the extraembryonic ectoderm proximally. In addition to facilitating nutrient exchange before the establishment of a circulation, the VE is critical for patterning the epiblast. Since VE is derived from the primitive endoderm (PrE) of the blastocyst, and PrE-derived eXtraembryonic ENdoderm (XEN) cells can be propagated in vitro, XEN cells should provide an important tool for identifying factors that direct VE differentiation. In this study, we demonstrated that BMP4 signaling induces the formation of a polarized epithelium in XEN cells. This morphological transition was reversible, and was associated with the acquisition of a molecular signature comparable to extraembryonic (ex) VE. Resembling exVE which will form the endoderm of the visceral yolk sac, BMP4-treated XEN cells regulated hematopoiesis by stimulating the expansion of primitive erythroid progenitors. We also observed that LIF exerted an antagonistic effect on BMP4-induced XEN cell differentiation, thereby impacting the extrinsic conditions used for the isolation and maintenance of XEN cells in an undifferentiated state. Taken together, our data suggest that XEN cells can be differentiated towards an exVE identity upon BMP4 stimulation and therefore represent a valuable tool for investigating PrE lineage differentiation.

Hematopoietic stem and progenitor cell trafficking

Migration of hematopoietic stem cells (HSCs) is essential during embryonic development and throughout adult life. During embryogenesis, trafficking of HSCs is responsible for the sequential colonization of different hematopoietic organs by blood-producing cells. In adulthood, circulation of HSCs maintains homeostasis of the hematopoietic system and participates in innate immune responses. HSC trafficking is also crucial in clinical settings such as bone marrow (BM) and stem cell transplantation. This review provides an overview of the molecular and cellular signals that control and fine-tune trafficking of HSCs and hematopoietic progenitor cells in embryogenesis and during postnatal life. We also discuss the potential clinical utility of therapeutic approaches to modulate HSC trafficking in patients.

Hierarchical organization and early hematopoietic specification of the developing HSC lineage in the AGM region

The aorta-gonad-mesonephros region plays an important role in hematopoietic stem cell (HSC) development during mouse embryogenesis. The vascular endothelial cadherin⁺ CD45⁺ (VE-cad⁺CD45⁺) population contains the major type of immature pre-HSCs capable of developing into long-term repopulating definitive HSCs. In this study, we developed a new coaggregation culture system, which supports maturation of a novel population of CD45-negative (VE-cad⁺CD45⁻CD41⁺) pre-HSCs into definitive HSCs. The appearance of these pre-HSCs precedes development of the VE-cad⁺CD45⁺ pre-HSCs (termed here type I and type II pre-HSCs, respectively), thus establishing a hierarchical directionality in the developing HSC lineage. By labeling the luminal surface of the dorsal aorta, we show that both type I and type II pre-HSCs are distributed broadly within the endothelial and subendothelial aortic layers, in contrast to mature definitive HSCs which localize to the aortic endothelial layer. In agreement with expression of CD41 in pre-HSCs, in vivo CD41-Cre-mediated genetic tagging occurs in embryonic pre-HSCs and persists in all lymphomyeloid lineages of the adult animal.