Diffuse intraembryonic hemopoiesis in normal and chimeric avian development

Embryonic Origins of the Hematopoietic System: Hierarchies and Heterogeneity

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

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

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

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

A Bird's Eye View on the Origin of Aortic Hemogenic Endothelial Cells

During early embryogenesis, the hemogenic endothelium of the developing dorsal aorta is the main source of definitive hematopoietic stem cells (HSCs), which will generate all blood cell lineages of the adult organism. The hemogenic endothelial cells (HECs) of the dorsal aorta are known to arise from the splanchnic lateral plate mesoderm. However, the specific cell lineages and developmental paths that give rise to aortic HECs are still unclear. Over the past half a century, the scientific debate on the origin of aortic HECs and HSCs has largely focused on two potential and apparently alternative birthplaces, the extraembryonic yolk sac blood islands and the intraembryonic splanchnic mesoderm. However, as we argue, both yolk sac blood islands and aortic HECs may have a common hemangioblastic origin. Further insight into aortic HEC development is being gained from fate-mapping studies that address the identity of progenitor cell lineages, rather than their physical location within the developing embryo. In this perspective article, we discuss the current knowledge on the origin of aortic HECs with a particular focus on the evidence provided by studies in the avian embryo, a model that pioneered the field of developmental hematopoiesis.

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

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

A view of human haematopoietic development from the Petri dish

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

The embryonic origins and genetic programming of emerging haematopoietic stem cells

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

Temporal transcriptome analysis of the chicken embryo yolk sac

Background

The yolk sac (YS) is an extra-embryonic tissue that surrounds the yolk and absorbs, digests and transports nutrients during incubation of the avian embryo as well as during early term mammalian embryonic development. Understanding YS functions and development may enhance the efficient transfer of nutrients and optimize embryo development. To identify temporal large-scale patterns of gene expression and gain insights into processes and mechanisms in the YS, we performed a transcriptome study of the YS of chick embryos on embryonic days (E) E13, E15, E17, E19, and E21 (hatch).

Results

3547 genes exhibited a significantly changed expression across days. Clustering and functional annotation of these genes as well as histological sectioning of the YS revealed that we monitored two cell types: the epithelial cells and the erythropoietic cells of the YS. We observed a significant up-regulation of epithelial genes involved in lipid transport and metabolism between E13 and E19. YS epithelial cells expressed a vast array of lipoprotein receptors and fatty acid transporters. Several lysosomal genes (CTSA, PSAP, NPC2) and apolipoproteins genes (apoA1, A2, B, C3) were among the highest expressed, reflecting the intensive digestion and re-synthesis of lipoproteins in YS epithelial cells. Genes associated with cytoskeletal structure were down-regulated between E17 and E21 supporting histological evidence of a degradation of YS epithelial cells towards hatch.

Expression patterns of hemoglobin synthesis genes indicated a high erythropoietic capacity of the YS between E13 and E15, which decreased towards hatch. YS histological sections confirmed these results. We also observed that YS epithelial cells expressed high levels of genes coding for plasma carrier proteins (ALB, AFP, LTF, TTR), normally produced by the liver.

Conclusions

Here we expand current knowledge on developmental, nutritional and molecular processes in the YS. We demonstrate that in the final week of chick embryonic development, the YS plays different roles to support or replace the functions of several organs that have not yet reached their full functional capacity. The YS has a similar functional role as the intestine in digestion and transport of nutrients, the liver in producing plasma carrier proteins and coagulation factors, and the bone marrow in synthesis of blood cells.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-690) contains supplementary material, which is available to authorized users.

How the avian model has pioneered the field of hematopoietic development

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

Developmental hematopoiesis: ontogeny, genetic programming and conservation

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

Rho GTPases control specific cytoskeleton-dependent functions of hematopoietic stem cells

The Rho family of guanosine triphosphatases (GTPases) is composed of members of the Ras superfamily of proteins. They are GTP-bound molecules with a modest intrinsic GTPase activity that can be accelerated upon activation/localization of specialized guanine nucleotide exchange factors. Members of this family act as molecular switches and are required for coordinated cytoskeletal rearrangements that are crucial in a set of specialized functions of mammalian stem cells. These functions include self-renewal, adhesion, and migration. Mouse gene-targeting studies have provided convincing evidence of the indispensable and dispensable roles of individual members of the Rho GTPase family and the putative upstream and downstream mediators in stem cell-specific functions. The role of Rho GTPases and related signaling pathways previously seen in other cell types and organisms have been confirmed in mammalian hematopoietic stem cells (HSCs), and new signaling pathways and unexpected functions unique to HSCs have been identified and dissected. This review summarizes our current understanding of the role of Rho family of GTPases on HSC and progenitor activity through cytoskeleton-mediated signaling pathways, providing insight about relevant signaling pathways that regulate mammalian stem cell self-renewal, adhesion, and migration.

Histological analyses demonstrate the temporary contribution of yolk sac, liver, and bone marrow to hematopoiesis during chicken development

The use of avian animal models has contributed to the understanding of many aspects of the ontogeny of the hematopoietic system in vertebrates. However, specific events that occur in the model itself are still unclear. There is a lack of consensus, among previous studies, about which is the intermediate site responsible for expansion and differentiation of hematopoietic cells, and the liver's contribution to the development of this system. Here we aimed to evaluate the presence of hematopoiesis in the yolk sac and liver in chickens, from the stages of intra-aortic clusters in the aorta-genital ridges-mesonephros (AGM) region until hatching, and how it relates to the establishment of the bone marrow. Gallus gallus domesticus L. embryos and their respective yolk sacs at embryonic day 3 (E3) and up to E21 were collected and processed according to standard histological techniques for paraffin embedding. The slides were stained with hematoxylin-eosin, Lennert's Giemsa, and Sirius Red at pH 10.2, and investigated by light microscopy. This study demonstrated that the yolk sac was a unique hematopoietic site between E4 and E12. Hematopoiesis occurred in the yolk sac and bone marrow between E13 and E20. The liver showed granulocytic differentiation in the connective tissue of portal spaces at E15 and onwards. The yolk sac showed expansion of erythrocytic and granulocytic lineages from E6 to E19, and E7 to E20, respectively. The results suggest that the yolk sac is the major intermediate erythropoietic and granulopoietic site where expansion and differentiation occur during chicken development. The hepatic hematopoiesis is restricted to the portal spaces and represented by the granulocytic lineage.

Deconvoluting the ontogeny of hematopoietic stem cells

Two different models describe the development of definitive hematopoiesis and hematopoietic stem cells (HSCs). In one of these, the visceral yolk sac serves as a starting point of relatively lengthy developmental process culminating in the fetal liver hematopoiesis. In another, the origin of adult hematopoiesis is split between the yolk sac and the dorsal aorta, which has a peculiar capacity to generate definitive HSCs. Despite a large amount of experimental data consistent with the latter view, it becomes increasingly unsustainable in the light of recent cell tracing studies. Moreover, analysis of the published studies supporting the aorta-centered version uncovers significant caveats in standard experimental approach and argumentation. As a result, the theory cannot offer feasible cellular mechanisms of the HSC emergence. This review summarizes key efforts to discern the developmental pathway of the adult-type HSCs and attempts to put forward a hypothesis on the inflammatory mechanisms of hematopoietic ontogenesis.

Hematopoiesis

Hematopoiesis - the process by which blood cells are formed - has been studied intensely for over a century using a variety of model systems. There is conservation of the overall hematopoietic process between vertebrates, although some differences do exist. Over the last decade, the zebrafish has come to the forefront as a new model in hematopoiesis research, as it allows the use of large-scale genetics, chemical screens and transgenics. This comparative approach to understanding hematopoiesis has led to fundamental knowledge about the process and to the development of new therapies for disease. Here, we provide a broad overview of vertebrate hematopoiesis. We also highlight the benefits of using zebrafish as a model.

Erythropoiesis: development and differentiation

Through their oxygen delivery function, red blood cells are pivotal to the healthy existence of all vertebrate organisms. These cells are required during all stages of life--embryonic, fetal, neonatal, adolescent, and adult. In the adult, red blood cells are the terminally differentiated end-product cells of a complex hierarchy of hematopoietic progenitors that become progressively restricted to the erythroid lineage. During this stepwise differentiation process, erythroid progenitors undergo enormous expansion, so as to fulfill the daily requirement of ~2 × 10(11) new erythrocytes. How the erythroid lineage is made has been a topic of intense research over the last decades. Developmental studies show that there are two types of red blood cells--embryonic and adult. They develop from distinct hemogenic/hematopoietic progenitors in different anatomical sites and show distinct genetic programs. This article highlights the developmental and differentiation events necessary in the production of hemoglobin-producing red blood cells.

Extraembryonic origin of circulating endothelial cells

Circulating endothelial cells (CEC) are contained in the bone marrow and peripheral blood of adult humans and participate to the revascularization of ischemic tissues. These cells represent attractive targets for cell or gene therapy aimed at improving ischemic revascularization or inhibition of tumor angiogenesis. The embryonic origin of CEC has not been addressed previously. Here we use quail-chick chimeras to study CEC origin and participation to the developing vasculature. CEC are traced with different markers, in particular the QH1 antibody recognizing only quail endothelial cells. Using yolk-sac chimeras, where quail embryos are grafted onto chick yolk sacs and vice-versa, we show that CEC are generated in the yolk sac. These cells are mobilized during wound healing, demonstrating their participation to angiogenic repair processes. Furthermore, we found that the allantois is also able to give rise to CEC in situ. In contrast to the yolk sac and allantois, the embryo proper does not produce CEC. Our results show that CEC exclusively originate from extra-embryonic territories made with splanchnopleural mesoderm and endoderm, while definitive hematopoietic stem cells and endothelial cells are of intra-embryonic origin.

The vascular origin of hematopoietic cells

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

On the origin of hematopoietic stem cells: progress and controversy

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

Hematopoietic stem cell emergence in the conceptus and the role of Runx1

Hematopoietic stem cells (HSCs) are functionally defined as cells that upon transplantation into irradiated or otherwise immunocompromised adult organisms provide long-term reconstitution of the entire hematopoietic system. They emerge in the vertebrate conceptus around midgestation. Genetic studies have identified a number of transcription factors and signaling molecules that act at the onset of hematopoiesis, and have begun to delineate the molecular mechanisms underlying the formation of HSCs. One molecule that has been a particularly useful marker of this developmental event in multiple species is Runx1 (also known as AML1, Pebp2alpha). Runx1 is a sequence-specific DNA-binding protein, that along with its homologues Runx2 and Runx3 and their shared non-DNA binding subunit CBFbeta, constitute a small family of transcription factors called core-binding factors (CBFs). Runx1 is famous for its role in HSC emergence, and notorious for its involvement in leukemia, as chromosomal rearrangements and inactivating mutations in the human RUNX1 gene are some of the most common events in de novo and therapy-related acute myelogenous leukemia, myelodysplastic syndrome and acute lymphocytic leukemia. Here we will review the role of Runx1 in HSC emergence in the mouse conceptus and describe some of the genetic pathways that operate upstream and downstream of this gene. Where relevant, we will include data obtained from other species and embryonic stem (ES) cell differentiation cultures.

Blood cell generation from the hemangioblast

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

Notch signalling and haematopoietic stem cell formation during embryogenesis

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

Autologous blood cell therapies from pluripotent stem cells

The discovery of human embryonic stem cells (hESCs) raised promises for a universal resource for cell based therapies in regenerative medicine. Recently, fast-paced progress has been made towards the generation of pluripotent stem cells (PSCs) amenable for clinical applications, culminating in reprogramming of adult somatic cells to autologous PSCs that can be indefinitely expanded in vitro. However, besides the efficient generation of bona fide, clinically safe PSCs (e.g., without the use of oncoproteins and gene transfer based on viruses inserting randomly into the genome), a major challenge in the field remains how to efficiently differentiate PSCs to specific lineages and how to select cells that will function normally upon transplantation in adults. In this review, we analyse the in vitro differentiation potential of PSCs to the hematopoietic lineage by discussing blood cell types that can be currently obtained, limitations in derivation of adult-type HSCs and prospects for clinical application of PSCs-derived blood cells.

Ventral embryonic tissues and Hedgehog proteins induce early AGM hematopoietic stem cell development

Hematopoiesis is initiated in several distinct tissues in the mouse conceptus. The aorta-gonad-mesonephros (AGM) region is of particular interest, as it autonomously generates the first adult type hematopoietic stem cells (HSCs). The ventral position of hematopoietic clusters closely associated with the aorta of most vertebrate embryos suggests a polarity in the specification of AGM HSCs. Since positional information plays an important role in the embryonic development of several tissue systems, we tested whether AGM HSC induction is influenced by the surrounding dorsal and ventral tissues. Our explant culture results at early and late embryonic day 10 show that ventral tissues induce and increase AGM HSC activity, whereas dorsal tissues decrease it. Chimeric explant cultures with genetically distinguishable AGM and ventral tissues show that the increase in HSC activity is not from ventral tissue-derived HSCs, precursors or primordial germ cells (as was previously suggested). Rather, it is due to instructive signaling from ventral tissues. Furthermore, we identify Hedgehog protein(s) as an HSC inducing signal.

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

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

The discovery of a source of adult hematopoietic cells in the embryo

This essay is about the 1975 JEEM paper by Françoise Dieterlen-Lièvre (Dieterlen-Lièvre, 1975) and the studies that followed it, which indicated that the adult hematopoietic system in the avian embryo originates, not from the blood islands of the extraembryonic yolk sac as was then believed, but from the body of the embryo itself. Dieterlen-Lièvre's 1975 paper created a paradigm shift in hematopoietic research, and provided a new and lasting focus on hematopoietic activity within the embryo body.

Circulating endothelial cells, bone marrow-derived endothelial progenitor cells and proangiogenic hematopoietic cells in cancer: From biology to therapy

Vascularization, a hallmark of tumorigenesis, is classically thought to occur exclusively through angiogenesis (i.e. endothelial sprouting). However, there is a growing body of evidence that endothelial progenitor cells (EPCs) and proangiogenic hematopoietic cells (HCs) are able to support the vascularization of tumors and may therefore play a synergistic role with angiogenesis. An additional cell type being studied in the field of tumor vascularization is the circulating endothelial cell (CEC), whose presence in elevated numbers reflects vascular injury. Levels of EPCs and CECs are reported to correlate with tumor stage and have been evaluated as biomarkers of the efficacy of anticancer/antiangiogenic treatments. Furthermore, because EPCs and subtypes of proangiogenic HCs are actively participating in capillary growth, these cells are attractive potential vehicles for delivering therapeutic molecules. The current paper provides an update on the biology of CECs, EPCs and proangiogenic HCs, and explores the utility of these cell populations for clinical oncology.

Cell signaling directing the formation and function of hemogenic endothelium during murine embryogenesis

During developmental hematopoiesis, multilineage hematopoietic progenitors are thought to derive from a subset of vascular endothelium. Herein, we define the phenotype of such hemogenic endothelial cells and demonstrate, on a clonal level, that they exhibit multilineage hematopoietic potential. Furthermore, we have begun to define the molecular signals that regulate their development. We found that the formation of yolk sac hemogenic endothelium and its hematopoietic potential were significantly impaired in the absence of retinoic acid (RA) signaling, and could be restored in RA-deficient (Raldh2(-/-)) embryos by provision of exogenous RA in utero. Thus, we identify a novel, critical role for RA signaling in the development of hemogenic endothelium that contributes to definitive hematopoiesis.

Rac1 is essential for intraembryonic hematopoiesis and for the initial seeding of fetal liver with definitive hematopoietic progenitor cells

Definitive hematopoietic stem and progenitor cells (HSCs/Ps) originating from the yolk sac and/or para-aorta-splanchno-pleura/aorta-gonad-mesonephros are hypothesized to colonize the fetal liver, but mechanisms involved are poorly defined. The Rac subfamily of Rho GTPases has been shown to play essential roles in HSC/P localization to the bone marrow following transplantation. Here, we study the role of Rac1 in HSC/P migration during ontogeny and seeding of fetal liver. Using a triple-transgenic approach, we have deleted Rac1 in HSCs/Ps during very early embryonic development. Without Rac1, there was a decrease in circulating HSCs/Ps in the blood of embryonic day (E) 10.5 embryos, while yolk sac definitive hematopoiesis was quantitatively normal. Intraembryonic hematopoiesis was significantly impaired in Rac1-deficient embryos, culminating with absence of intra-aortic clusters and fetal liver hematopoiesis. At E10.5, Rac1-deficient HSCs/Ps displayed decreased transwell migration and impaired inter-action with the microenvironment in migration-dependent assays. These data suggest that Rac1 plays an important role in HSC/P migration during embryonic development and is essential for the emergence of intraembryonic hematopoiesis.

Mouse lysocardiolipin acyltransferase controls the development of hematopoietic and endothelial lineages during in vitro embryonic stem-cell differentiation

The blast colony-forming cell (BL-CFC) was identified as an equivalent to the hemangioblast during in vitro embryonic stem (ES) cell differentiation. However, the molecular mechanisms underlying the generation of the BL-CFC remain largely unknown. Here we report the isolation of mouse lysocardiolipin acyltransferase (Lycat) based on homology to zebrafish lycat, a candidate gene for the cloche locus. Mouse Lycat is expressed in hematopoietic organs and is enriched in the Lin(-)C-Kit(+)Sca-1(+) hematopoietic stem cells in bone marrow and in the Flk1(+)/hCD4(+)(Scl(+)) hemangioblast population in embryoid bodies. The forced Lycat transgene leads to increased messenger RNA expression of hematopoietic and endothelial genes as well as increased blast colonies and their progenies, endothelial and hematopoietic lineages. The Lycat small interfering RNA transgene leads to a decrease expression of hematopoietic and endothelial genes. An unbiased genomewide microarray analysis further substantiates that the forced Lycat transgene specifically up-regulates a set of genes related to hemangioblasts and hematopoietic and endothelial lineages. Therefore, mouse Lycat plays an important role in the early specification of hematopoietic and endothelial cells, probably acting at the level of the hemangioblast.

Migratory path of definitive hematopoietic stem/progenitor cells during zebrafish development

The development of vertebrate definitive hematopoiesis is featured by temporally and spatially dynamic distribution of hematopoietic stem/progenitor cells (HSPCs). It is proposed that the migration of definitive HSPCs, at least in part, accounts for this unique characteristic; however, compelling in vivo lineage evidence is still lacking. Here we present an in vivo analysis to delineate the migration route of definitive HSPCs in the early zebrafish embryo. Cell-marking analysis was able to first map definitive HSPCs to the ventral wall of dorsal aorta (DA). These cells were subsequently found to migrate to a previously unappreciated organ, posterior blood island (PBI), located between the caudal artery and caudal vein, and finally populate the kidney, the adult hematopoietic organ. These findings demonstrate that the PBI acts as an intermediate hematopoietic organ in a manner analogous to the mammalian fetal liver to sustain definitive hematopoiesis before adult kidney hematopoiesis occurs. Thus our study unambiguously documents the in vivo trafficking of definitive HSPCs among developmentally successive hematopoietic compartments and underscores the ontogenic conservation of definitive hematopoiesis between zebrafish and mammals.

Ontogeny of the hematopoietic system

Blood cells are constantly produced in the bone marrow (BM) of adult mammals. This constant turnover ultimately depends on a rare population of progenitors that displays self-renewal and multilineage differentiation potential, the hematopoietic stem cells (HSCs). It is generally accepted that HSCs are generated during embryonic development and sequentially colonize the fetal liver, the spleen, and finally the BM. Here we discuss the experimental evidence that argues for the extrinsic origin of HSCs and the potential locations where HSC generation might occur. The identification of the cellular components playing a role in the generation process, in these precise locations, will be important in understanding the molecular mechanisms involved in HSC production from undifferentiated mesoderm.

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

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

Hypoxic adaptations of hemoglobin in Tibetan chick embryo: high oxygen-affinity mutation and selective expression

Tibetan chicks (Gallus gallus) survived with high hatchability (35.0%) and Recessive White Feather broilers (RWF) from low elevations survived rarely and with a low hatchability (3.0%) after simulated incubation under hypoxia of 13% O2. The functional mutation of Met-32D(B13)-Leu of alpha(D) globin chain was related with hypoxia based on allele distribution, homology model building and oxygen affinity assay. Whole embryos on days 3-8 and whole blood on days 9-18 were collected to investigate the stage expression profiles of all seven globins and HIF-1alpha by real-time PCR. Under hypoxia (12.0% O2) on days 3-8, HbE was overexpressed, HbA was expressed earlier and HbP expression was restricted, which completely overturned the expression profile under normoxia. The amount of hemoglobin expression in Tibetan chicks was remarkably higher than that of RWF. HIF-1alpha expression peaked early in both breeds, with. In conclusion, the special hypoxic expression profile on days 3-8 certainly is a common molecular mechanism of hypoxia tolerance in surviving Tibetan chick and RWF embryos; the mutation Met-32D(B13)-Leu and increasing hemoglobins are important mechanisms of hypoxia adaptation in Tibetan chick embryos, and we suggest that HIF-1alpha could be responsible for the hypoxic expression profile.

Blood Cells and Blood Cell Development in the Animal Kingdom

Recent findings strongly suggest that the molecular pathways involved in the development and function of blood cells are highly conserved among vertebrates and various invertebrate phyla. This has led to a renewed interest regarding homologies between blood cell types and their developmental origin among different animals. One way to address these areas of inquiry is to shed more light on the biology of blood cells in extant invertebrate taxa that have branched off the bilaterian tree in between insects and vertebrates. This review attempts, in a broadly comparative manner, to update the existing literature that deals with early blood cell development. I begin by providing a brief survey of the different types of blood cell lineages among metazoa. There is now good reason to believe that, in vertebrates and invertebrates alike, blood cell lineages diverge from a common type of progenitor cell, the hemocytoblast. I give a synopsis of the origin and determination of the hematocytoblast, beginning with a look at the hematopoietic organs that house hemocytoblasts in adult animals, followed by a more detailed overview of the embryonic development of the hematopoietic organ. Finally, I compare the process of blood lineage diversification in vertebrates and Drosophila.

Developmental relationship between hematopoietic and endothelial cells

Blood (hematopoietic cells) and blood vessels (endothelial cells) develop from mesoderm via a transitional progenitor known as the hemangioblast. Flk-1, a receptor tyrosine kinase, and Scl, a basic helix-loop-helix transcription factor, are two critical molecules functioning in this process. Recent studies have shown that Flk-1 expressing mesoderm contributes to the circulatory system, including hematopoietic, endothelial, smooth muscle, skeletal muscle, and cardiac muscle cells. Our studies suggest that hemangioblast specification within Flk-1 expressing mesoderm is regulated by Scl expression. Herein, we review studies that have utilized transgenic mouse models as well as an in vitro model of embryonic stem cell differentiation, both of which have greatly contributed to the current understanding of the cellular and molecular pathways regulating hemangioblast development and differentiation.

Role of hematopoietic lineage cells as accessory components in blood vessel formation

In adults, the vasculature is normally quiescent, due to the dominant influence of endogenous angiogenesis inhibitors over angiogenic stimuli. However, blood vessels in adults retain the capacity for brisk initiation of angiogenesis, the growth of new vessels from pre-existing vessels, during tissue repair and in numerous diseases, including inflammation and cancer. Because of the role of angiogenesis in tumor growth, many new cancer therapies are being conducted against tumor angiogenesis. It is thought that these anti-angiogenic therapies destroy the tumor vessels, thereby depriving the tumor of oxygen and nutrients. Therefore, a better understanding of the molecular mechanisms in the process of sprouting angiogenesis may lead to more effective therapies not only for cancer but also for diseases involving abnormal vasculature. It is widely believed that after birth, endothelial cells (EC) in new blood vessels are derived from resident EC of pre-existing vessels. However, evidence is now emerging that cells derived from the bone marrow may also contribute to postnatal angiogenesis. Most studies have focused initially on the contribution of endothelial progenitor cells in this process. However, we have proposed a concept in which cells of the hematopoietic lineage are mobilized and then entrapped in peripheral tissues, where they function as accessory cells that promote the sprouting of resident EC by releasing angiogenic signals. Most recently we found that hematopoietic cells play major roles in tumor angiogenesis by initiating sprouting angiogenesis and also in maturation of blood vessels in the fibrous cap of tumors. Therefore, manipulating these entrapment signals may offer therapeutic opportunities to stimulate or inhibit angiogenesis.

Aminopeptidase N during the ontogeny of the chick

Little is known about the production and function of metallopeptidases in embryonic development. One such enzyme, aminopeptidase N (APN), is present in several epithelia, the brain and angiogenic vessels in adults. APN promotes vascular growth and endothelial cell proliferation in physiological and pathological models of angiogenesis. However, its possible role in embryonic angiogenesis or other developmental processes is unknown. Its expression profile in the early phase of embryonic development has not been reported. We report here the expression of this enzyme during the early development of the chick embryo, using complementary techniques for monitoring APN mRNA, protein, and enzymatic activity. We detected APN in the embryo as early as gastrulation. In addition to the known sites of APN production identified in both adults and rat fetuses toward the end of gestation, APN was found in unexpected sites, such as the primitive streak, the dorsal folds of the neural tube, the somites, and the primordia of several organs. APN was present mostly in the cardiovascular compartment during the first 13 days of incubation, and in the hematopoietic compartment (yolk sac and aorta-gonad-mesonephros region) early in development. This study provides clues as to the possible role of APN in embryonic development.

Antibodies, immunoglobulin genes and the bursa of Fabricius in chicken B cell development

The bursa of Fabricius is critical for the normal development of B lymphocytes in birds. It is productively colonized during embryonic life by a limited number of B cell precursors that have undergone the immunoglobulin gene rearrangements required for expression of cell surface immunoglobulin. Immunoglobulin gene rearrangement occurs in the absence of terminal deoxynucleotidyl transferase and generates minimal antibody diversity. In addition, observations that immunoglobulin heavy and light chain variable gene rearrangement occur at the same time and that allelic exclusion of immunoglobulin expression is regulated at the level of variable region gene rearrangement provide a striking contrast to rodent and primate models of immunoglobulin gene assembly. Following productive colonization of the bursa, developing B cells undergo rapid proliferation and the immunoglobulin V region genes that generate the specificity of the B cell surface immunoglobulin receptor undergo diversification. Immunoglobulin diversity in birds is generated by somatic gene conversion events in which sequences derived from upstream families of pseudogenes replace homologous sequences in unique and functionally rearranged immunoglobulin heavy and light chain variable region genes. This mechanism is distinct from and much more efficient than mechanisms of antibody diversification seen in rodents and primates. While the bursal microenvironment is not required for immunoglobulin gene rearrangement and expression, it is essential for the generation of antibody diversity by gene conversion. Following hatch, gut derived antigens are taken up by the bursa. While bursal development prior to hatch occurs in the absence of exogenous antigen, chicken B cell development after hatch may therefore be influenced by the presence of environmental antigen. This review focuses on the differences between B cell development in the chicken as compared to rodent and primate models.

Erythropoiesis and red cell function in vertebrate embryos

All vertebrate embryos produce a specific erythroid cell population--primitive erythrocytes--early in development. These cells are characterized by expression of the specific embryonic haemoglobins. Many aspects of primitive erythropoiesis and the physiological function of primitive red cells are still enigmatic. Nevertheless, recent years have seen intensive efforts to characterize in greater detail the molecular events underlying the initiation of erythropoiesis in vertebrate embryos. Several key genes have been identified that are necessary for primitive and the subsequent definitive erythropoiesis, which differs in several aspect from primitive erythropoiesis. This review gives in its first part a short overview dealing with comparative aspects of primitive and early definitive erythropoiesis in higher and lower vertebrates and in the second part we discuss the physiological function of primitive red cells based mainly on results from mammalian and avian embryos.

Stepwise commitment from embryonic stem to hematopoietic and endothelial cells

There is great excitement in generating different types of somatic cells from in vitro differentiated embryonic stem (ES) cells, because they can potentially be utilized for therapies for human diseases for which there are currently no effective treatments. Successful generation and application of ES-derived somatic cells requires better understanding of molecular mechanisms that regulate self-renewal and lineage commitment. Accordingly, many studies are aimed toward understanding mechanisms for maintaining the stem cell state and pathways leading to lineage specification. In this chapter we discuss recent studies that examine molecules that are critical for ES cell self-renewal, as well as hematopoietic and endothelial cell lineage differentiation from ES cells.

GATA motifs regulate early hematopoietic lineage-specific expression of the Gata2 gene

Transcription factor GATA-2 is essential for definitive hematopoiesis, which developmentally emerges from the para-aortic splanchnopleura (P-Sp). The expression of a green fluorescent protein (GFP) reporter placed under the control of a 3.1-kbp Gata2 gene regulatory domain 5' to the distal first exon (IS) mirrored that of the endogenous Gata2 gene within the P-Sp and yolk sac (YS) blood islands of embryonic day (E) 9.5 murine embryos. The P-Sp- and YS-derived GFP(+) fraction of flow-sorted cells dissociated from E9.5 transgenic embryos contained far more CD34(+)/c-Kit(+) cells than the GFP(-) fraction did. When cultured in vitro, the P-Sp GFP(+) cells generated both immature hematopoietic and endothelial cell clusters. Detailed transgenic mouse reporter expression analyses demonstrate that five GATA motifs within the 3.1-kbp Gata2 early hematopoietic regulatory domain (G2-EHRD) were essential for GFP expression within the dorsal aortic wall, where hemangioblasts, the earliest precursors possessing both hematopoietic and vascular developmental potential, are thought to reside. These results thus show that the Gata2 gene IS promoter is regulated by a GATA factor(s) and selectively marks putative hematopoietic/endothelial precursor cells within the P-Sp.

From hemangioblast to hematopoietic stem cell: An endothelial connection?

The developmental origin of hematopoietic stem cells has been the subject of much research. Now that the developmental link between the hematopoietic system and the vasculature has been well established, questions remain regarding the precise cellular origin of definitive hematopoietic cells and at what point they branch off from the endothelial lineage. Do they emerge directly from a hemangioblast-type cell, similar to what is proposed for primitive yolk sac hematopoiesis, or are they generated via an endothelial intermediate, the hemogenic endothelium? In this review, we will give an overview of the data obtained from the mouse and avian models on the cellular origins of the hematopoietic system.

Placenta as a site for hematopoietic stem cell development

The discovery of a major hematopoietic stem cell (HSC) pool in mid-gestation mouse placenta has defined the placenta as yet another important anatomical site that participates in HSC development. Placental HSC activity starts in parallel with the AGM region, before HSCs are found in circulation or have colonized the fetal liver. Moreover, placental hematopoietic activity culminates in a rapid expansion of the definitive HSC pool, which occurs during the time when the fetal liver HSC reservoir begins to grow. Furthermore, hematopoietic cells in mid-gestation mouse placenta are not instructed for differentiation along the myeloerythroid lineage, as in the fetal liver. These findings suggest that the placenta provides a supportive niche where the definitive hematopoietic stem cell pool can be temporarily established during development. Future studies are needed to characterize the developmental events that lead to the establishment of placental HSC pool, and to define the microenvironmental signals that support this process. Furthermore, if the stem cell-promoting properties of the placental niche can be harnessed in vitro to support HSC formation, maturation, and/or expansion in culture, these assets may greatly improve hematopoietic stem cell-based therapies in the future.