A phase and electron microscopic study of vasculogenesis and erythropoiesis in the yolk sac of the mouse

Extraembryonic hematopoietic lineages-to macrophages and beyond

The first blood and immune cells in vertebrates emerge in the extraembryonic yolk sac. Throughout the last century, it has become evident that this extraembryonic tissue gives rise to transient primitive and definitive hematopoiesis but not hematopoietic stem cells. More recently, studies have elucidated that yolk sac-derived blood and immune cells are present far longer than originally expected. These cells take over essential roles for the survival and proper organogenesis of the developing fetus up until birth. In this review, we discuss the most recent findings and views on extraembryonic hematopoiesis in mice and humans.

In vivo clonal tracking reveals evidence of haemangioblast and haematomesoblast contribution to yolk sac haematopoiesis

During embryogenesis, haematopoietic and endothelial lineages emerge closely in time and space. It is thought that the first blood and endothelium derive from a common clonal ancestor, the haemangioblast. However, investigation of candidate haemangioblasts in vitro revealed the capacity for mesenchymal differentiation, a feature more compatible with an earlier mesodermal precursor. To date, no evidence for an in vivo haemangioblast has been discovered. Using single cell RNA-Sequencing and in vivo cellular barcoding, we have unravelled the ancestral relationships that give rise to the haematopoietic lineages of the yolk sac, the endothelium, and the mesenchyme. We show that the mesodermal derivatives of the yolk sac are produced by three distinct precursors with dual-lineage outcomes: the haemangioblast, the mesenchymoangioblast, and a previously undescribed cell type: the haematomesoblast. Between E5.5 and E7.5, this trio of precursors seeds haematopoietic, endothelial, and mesenchymal trajectories.

Cdx regulates gene expression through PRC2-mediated epigenetic mechanisms

The extra-embryonic yolk sac contains adjacent layers of mesoderm and visceral endoderm. The mesodermal layer serves as the first site of embryonic hematopoiesis, while the visceral endoderm provides a means of exchanging nutrients and waste until the development of the chorioallantoic placenta. While defects in chorioallantoic fusion and yolk sac hematopoiesis have been described in Cdx mutant mouse models, little is known about the gene targets and molecular mechanisms through which Cdx members regulate these processes. To this end, we used RNA-seq to examine Cdx-dependent gene expression changes in the yolk sac. We find that loss of Cdx function impacts the expression of genes involved in yolk sac hematopoiesis, as previously described, as well as novel Cdx2 target genes. In addition, we observed Cdx-dependent changes in PRC2 subunit expression accompanied by altered H3K27me3 deposition at a subset of Cdx target genes as early as E7.5 in the embryo proper. This study identifies additional Cdx target genes and provides further evidence for Cdx-dependent epigenetic regulation of gene expression in the early embryo, and that this regulation is required to maintain gene expression programs in the extra-embryonic yolk sac at later developmental stages.

Diverse cellular origins of adult blood vascular endothelial cells

During embryonic stages, vascular endothelial cells (ECs) originate from the mesoderm, at specific extraembryonic and embryonic regions, through a process called vasculogenesis. In the adult, EC renewal/replacement mostly depend on local resident ECs or endothelial progenitor cells (EPCs). Nevertheless, contribution from circulating ECs/EPCs was also reported. In addition, cells lacking from EC/EPC markers with in vitro extended plasticity were shown to originate endothelial-like cells (ELCs). Most of these cells consist of mesenchymal stromal progenitors, which would eventually get mobilized from the bone marrow after injury. Based on that, current knowledge on different mouse and human bone marrow stromal cell (BM-SC) subpopulations, able to contribute with mesenchymal stromal/stem cells (MSCs), is herein reviewed. Such analyses underline an unexpected heterogeneity among sinusoidal LepR⁺ stromal/CAR cells. For instance, in a recent report a subgroup of LepR⁺ stromal/CAR progenitors, which express GLAST and is traced in Wnt1^Cre;R26R^Tom mice, was found to contribute with ELCs in vivo. These GLAST ​⁺ ​Wnt1⁺ BM-SCs were shown to get mobilized to the peripheral blood and to contribute with liver regeneration. Other sources of ELCs, such as adipose, neural and dental pulp tissues, were also published. Finally, mechanisms likely involved in the enhanced cellular plasticity properties of bone marrow/adipose tissue stromal cells, able to originate ELCs, are assessed. In the future, strategies to analyze the in vivo expression profile of stromal cells, with MSC properties, in combination with screening of active genomic regions at the single cell-level, during early postnatal development and/or after injury, will likely help understanding properties of these ELC sources.

Is extra-embryonic endoderm a source of placental blood cells?

The extra-embryonic hypoblast/visceral endoderm of Placentalia carries out a variety of functions during gestation, including hematopoietic induction. Results of decades-old and recent experiments have provided compelling evidence that, in addition to its inducing properties, hypoblast/visceral endoderm itself is a source of placental blood cells. Those observations that highlight extra-embryonic endoderm's role as an overlooked source of placental blood cells across species are briefly discussed here, with suggestions for future exploration.

Continuous Mesothelial Injury and Regeneration during long Term Peritoneal Dialysis

This study reconstructs the whole sequence of mesothelial injury and regeneration in patients on longterm peritoneal dialysis. Our observations indicate that peritoneal dialysis induces a process of continuous mesothelial injury and regeneration. New mesothelial cells seem to originate from wandering mesothelial cells of the peritoneal fluid, as well as from mesothelial cell precursors localized in the sub-mesothelial connective tissue.

Chasing Mavericks: The quest for defining developmental waves of hematopoiesis

Hematopoiesis is the process by which mature blood and immune cells are produced from hematopoietic stem and progenitor cells (HSCs and HSPCs). The last several decades of research have shed light on the origin of HSCs, as well as the heterogeneous pools of fetal progenitors that contribute to lifelong hematopoiesis. The overarching concept that hematopoiesis occurs in dynamic, overlapping waves throughout development, with each wave contributing to both continuous and developmentally limited cell types, has been solidified over the years. However, recent advances in our ability to track the production of hematopoietic cells in vivo have challenged several long-held dogmas on the origin and persistence of distinct hematopoietic cell types. In this review, we highlight emerging concepts in hematopoietic development and identify unanswered questions.

Etv2 as an essential regulator of mesodermal lineage development

The 'master regulatory factors' that position at the top of the genetic hierarchy of lineage determination have been a focus of intense interest, and have been investigated in various systems. Etv2/Etsrp71/ER71 is such a factor that is both necessary and sufficient for the development of haematopoietic and endothelial lineages. As such, genetic ablation of Etv2 leads to complete loss of blood and vessels, and overexpression can convert non-endothelial cells to the endothelial lineage. Understanding such master regulatory role of a lineage is not only a fundamental quest in developmental biology, but also holds immense possibilities in regenerative medicine. To harness its activity and utility for therapeutic interventions, it is essential to understand the regulatory mechanisms, molecular function, and networks that surround Etv2. In this review, we provide a comprehensive overview of Etv2 biology focused on mouse and human systems.

Exosomes Derived from Hypoxia-Treated Human Adipose Mesenchymal Stem Cells Enhance Angiogenesis Through the PKA Signaling Pathway

Angiogenesis is a complicated and sequential process that plays an important role in different physiological processes. Mesenchymal stem cells (MSCs), which are pluripotent stem cells, are widely used for the treatment of ischemic and traumatic diseases, and exosomes derived from these cells can also promote angiogenesis. Therefore, we aimed to uncover mechanisms to improve MSC exosome-mediated angiogenesis. For this study, we isolated human adipose-derived MSCs (hAD-MSCs) and assessed differentiation ability and markers. Cells were divided into hypoxia-treated MSCs (H-MSCs) and normoxia-treated MSCs (N-MSC), and exosomes were extracted by ultrafiltration. Exosomes (100 μg/mL) from H-MSCs and N-MSCs were added to human umbilical vein endothelial cells (HUVECs). Exosome uptake and the ability of endothelial cells to form tubes were detected in real time. Protein samples were collected at different time points to detect the expression of inhibitors (Vash1) and enhancers (Angpt1 and Flk1) of angiogenesis; we also assessed their related signaling pathways. We found that exosomes from the hypoxia group were more easily taken up by HUVECs; furthermore, their angiogenesis stimulatory activity was also significantly enhanced compared to that with exosomes from the normoxia group. HUVECs exposed to exosomes from H-MSCs significantly upregulated angiogenesis-stimulating genes and deregulated angiogenesis-inhibitory genes. The expression of vascular endothelial growth factor (VEGF) and activation of the protein kinase A (PKA) signaling pathway in HUVECs were significantly increased by hypoxia-exposed exosomes. Moreover, a PKA inhibitor was shown to significantly suppress angiogenesis. Finally, we concluded that hypoxia-exposed exosomes derived from hAD-MSCs can improve angiogenesis by activating the PKA signaling pathway and promoting the expression of VEGF. These results could be used to uncover safe and effective treatments for traumatic diseases.

An Introduction to Erythropoiesis Approaches

Many experimental models have been used to study erythropoiesis. Even prior to the advent of the genetic manipulation of animal models, erythropoiesis was examined in the mouse, chicken, sheep, goat, and rabbit, among other vertebrates. Erythroid cell lines derived from human blood cancers were also very useful, as they could be genetically manipulated more easily than whole animals. Genetic models in the mouse, zebrafish, and frog have provided a plethora of information advancing our understanding of erythropoiesis, and remain gold standards in the field for studies of hemoglobin switching, and experiments to study authentic blood cell development. Mouse and human embryonic stem (ES) and induced pluripotent (iPS) cells can be differentiated to erythroid cells in culture, though their use is somewhat limited by their propensity to express only the embryonic and fetal globin genes. Some very useful cell lines have been developed by manipulating ES or fetal liver erythroid progenitor cells from knockout mouse models. In recent years, our understanding of erythropoiesis has improved, due to the ability to knock down genes in native human hematopoietic stem and progenitor cells derived from umbilical cord blood or bone marrow, and differentiate them ex vivo to the erythroid lineage. These native cells, and cell lines derived from them, are now providing essential information about human erythropoiesis, which is complementary to that obtained from animal studies. This review provides some perspective about the cell and animal models used to study erythropoiesis over the years.

LYVE1 Marks the Divergence of Yolk Sac Definitive Hemogenic Endothelium from the Primitive Erythroid Lineage

The contribution of the different waves and sites of developmental hematopoiesis to fetal and adult blood production remains unclear. Here, we identify lymphatic vessel endothelial hyaluronan receptor-1 (LYVE1) as a marker of yolk sac (YS) endothelium and definitive hematopoietic stem and progenitor cells (HSPCs). Endothelium in mid-gestation YS and vitelline vessels, but not the dorsal aorta and placenta, were labeled by Lyve1-Cre. Most YS HSPCs and erythro-myeloid progenitors were Lyve1-Cre lineage traced, but primitive erythroid cells were not, suggesting that they represent distinct lineages. Fetal liver (FL) and adult HSPCs showed 35%-40% Lyve1-Cre marking. Analysis of circulation-deficient Ncx1^-/- concepti identified the YS as a major source of Lyve1-Cre labeled HSPCs. FL proerythroblast marking was extensive at embryonic day (E) 11.5-13.5, but decreased to hematopoietic stem cell (HSC) levels by E16.5, suggesting that HSCs from multiple sources became responsible for erythropoiesis. Lyve1-Cre thus marks the divergence between YS primitive and definitive hematopoiesis and provides a tool for targeting YS definitive hematopoiesis and FL colonization.

Describing the Stem Cell Potency: The Various Methods of Functional Assessment and In silico Diagnostics

Stem cells are defined by their capabilities to self-renew and give rise to various types of differentiated cells depending on their potency. They are classified as pluripotent, multipotent, and unipotent as demonstrated through their potential to generate the variety of cell lineages. While pluripotent stem cells may give rise to all types of cells in an organism, Multipotent and Unipotent stem cells remain restricted to the particular tissue or lineages. The potency of these stem cells can be defined by using a number of functional assays along with the evaluation of various molecular markers. These molecular markers include diagnosis of transcriptional, epigenetic, and metabolic states of stem cells. Many reports are defining the particular set of different functional assays, and molecular marker used to demonstrate the developmental states and functional capacities of stem cells. The careful evaluation of all these methods could help in generating standard identifying procedures/markers for them.

Hematopoietic (stem) cell development - how divergent are the roads taken?

The development of the hematopoietic system during early embryonic stages occurs in spatially and temporally distinct waves. Hematopoietic stem cells (HSC), the most potent and self‐renewing cells of this system, are produced in the final ‘definitive’ wave of hematopoietic cell generation. In contrast to HSCs in the adult, which differentiate via intermediate progenitor populations to produce functional blood cells, the generation of hematopoietic cells in the embryo prior to HSC generation occurs in the early waves by producing blood cells without intermediate progenitors (such as the ‘primitive’ hematopoietic cells). The lineage relationship between the early hematopoietic cells and the cells giving rise to HSCs, the genetic networks controlling their emergence, and the precise temporal determination of HSC fate remain topics of intense research and debate. This Review article discusses the current knowledge on the step‐wise embryonic establishment of the adult hematopoietic system, examines the roles of pivotal intrinsic regulators in this process, and raises questions concerning the temporal onset of HSC fate determination.

Specification and function of hemogenic endothelium during embryogenesis

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

The ETS Factor, ETV2: a Master Regulator for Vascular Endothelial Cell Development

Appropriate vessel development and its coordinated function is essential for proper embryogenesis and homeostasis in the adult. Defects in vessels cause birth defects and are an important etiology of diseases such as cardiovascular disease, tumor and diabetes retinopathy. The accumulative data indicate that ETV2, an ETS transcription factor, performs a potent and indispensable function in mediating vessel development. This review discusses the recent progress of the study of ETV2 with special focus on its regulatory mechanisms and cell fate determining role in developing mouse embryos as well as somatic cells.

DNA methyltransferase 1 functions through C/ebpa to maintain hematopoietic stem and progenitor cells in zebrafish

Background

DNA methyltransferase 1 (Dnmt1) regulates expression of many critical genes through maintaining parental DNA methylation patterns on daughter DNA strands during mitosis. It is essential for embryonic development and diverse biological processes, including maintenance of hematopoietic stem and progenitor cells (HSPCs). However, the precise molecular mechanism of how Dnmt1 is involved in HSPC maintenance remains unexplored.

Methods

An N-ethyl-N-nitrosourea (ENU)-based genetic screening was performed to identify putative mutants with defects in definitive HSPCs during hematopoiesis in zebrafish. The expression of hematopoietic markers was analyzed via whole mount in situ hybridization assay (WISH). Positional cloning approach was carried out to identify the gene responsible for the defective definitive hematopoiesis in the mutants. Analyses of the mechanism were conducted by morpholino-mediated gene knockdown, mRNA injection rescue assays, anti-phosphorylated histone H3 (pH3) immunostaining and TUNEL assay, quantitative real-time PCR, and bisulfite sequencing analysis.

Results

A heritable mutant line with impaired HSPCs of definitive hematopoiesis was identified. Positional cloning demonstrated that a stop codon mutation was introduced in dnmt1 which resulted in a predicted truncated Dnmt1 lacking the DNA methylation catalytic domain. Molecular analysis revealed that expression of CCAAT/enhancer-binding protein alpha (C/ebpa) was upregulated, which correlated with hypomethylation of CpG islands in the regulation regions of cebpa gene in Dnmt1 deficient HSPCs. Overexpression of a transcriptional repressive SUMO-C/ebpa fusion protein could rescue hematological defects in the dnmt1 mutants. Finally, dnmt1 and cebpa double null embryos exhibited no obvious abnormal hematopoiesis indicated that the HSPC defects triggered by dnmt1 mutation were C/ebpa dependent.

Conclusions

Dnmt1 is required for HSPC maintenance via cebpa regulation during definitive hematopoiesis in zebrafish.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-015-0115-7) contains supplementary material, which is available to authorized users.

Differentiation potential of o bombay human-induced pluripotent stem cells and human embryonic stem cells into fetal erythroid-like cells

Objective

There is constant difficulty in obtaining adequate supplies of blood components, as well as disappointing performance of "universal" red blood cells. Advances in somatic cell reprogramming of human-induced pluripotent stem cells (hiPSCs) have provided a valuable alternative source to differentiate into any desired cell type as a therapeutic promise to cure many human disease.

Materials and Methods

In this experimental study, we examined the erythroid differentiation potential of normal Bombay hiPSCs (B-hiPSCs) and compared results to human embryonic stem cell (hESC) lines. Because of lacking ABO blood group expression in B-hiPSCs, it has been highlighted as a valuable source to produce any cell type in vitro.

Results

Similar to hESC lines, hemangioblasts derived from B-hiPSCs expressed approximately 9% KDR⁺CD31⁺ and approximately 5% CD31⁺CD34⁺. In semisolid media, iPSC and hESC-derived hemangioblast formed mixed type of hematopoietic colony. In mixed colonies, erythroid progenitors were capable to express CD71⁺GPA⁺HbF⁺ and accompanied by endothelial cells differentiation.

Conclusion

Finally, iPS and ES cells have been directly induced to erythropoiesis without hemangioblast formation that produced CD71⁺HbF⁺ erythroid cells. Although we observed some variations in the efficiency of hematopoietic differentiation between iPSC and ES cells, the pattern of differentiation was similar among all three tested lines.

HOIP deficiency causes embryonic lethality by aberrant TNFR1-mediated endothelial cell death

Linear ubiquitination is crucial for innate and adaptive immunity. The linear ubiquitin chain assembly complex (LUBAC), consisting of HOIL-1, HOIP, and SHARPIN, is the only known ubiquitin ligase that generates linear ubiquitin linkages. HOIP is the catalytically active LUBAC component. Here, we show that both constitutive and Tie2-Cre-driven HOIP deletion lead to aberrant endothelial cell death, resulting in defective vascularization and embryonic lethality at midgestation. Ablation of tumor necrosis factor receptor 1 (TNFR1) prevents cell death, vascularization defects, and death at midgestation. HOIP-deficient cells are more sensitive to death induction by both tumor necrosis factor (TNF) and lymphotoxin-α (LT-α), and aberrant complex-II formation is responsible for sensitization to TNFR1-mediated cell death in the absence of HOIP. Finally, we show that HOIP's catalytic activity is necessary for preventing TNF-induced cell death. Hence, LUBAC and its linear-ubiquitin-forming activity are required for maintaining vascular integrity during embryogenesis by preventing TNFR1-mediated endothelial cell death.

Krüppel-like transcription factors KLF1 and KLF2 have unique and coordinate roles in regulating embryonic erythroid precursor maturation

The Krüppel-like transcription factors KLF1 and KLF2 are essential for embryonic erythropoiesis. They can partially compensate for each other during mouse development, and coordinately regulate numerous erythroid genes, including the β-like globins. Simultaneous ablation of KLF1 and KLF2 results in earlier embryonic lethality and severe anemia. In this study, we determine that this anemia is caused by a paucity of blood cells, and exacerbated by diminished β-like globin gene expression. The anemia phenotype is dose-dependent, and, interestingly, can be ameliorated by a single copy of the KLF2, but not the KLF1 gene. The roles of KLF1 and KLF2 in maintaining normal peripheral blood cell numbers and globin mRNA amounts are erythroid cell-specific. Mechanistic studies led to the discovery that KLF2 has an essential function in erythroid precursor maintenance. KLF1 can partially compensate for KLF2 in this role, but is uniquely crucial for erythroid precursor proliferation through its regulation of G1- to S-phase cell cycle transition. A more drastic impairment of primitive erythroid colony formation from embryonic progenitor cells occurs with simultaneous loss of KLF1 and KLF2 than with loss of a single factor. KLF1 and KLF2 coordinately regulate several proliferation-associated genes, including Foxm1. Differential expression of FoxM1, in particular, correlates with the observed KLF1 and KLF2 gene dosage effects on anemia. Furthermore, KLF1 binds to the FoxM1 gene promoter in blood cells. Thus KLF1 and KLF2 coordinately regulate embryonic erythroid precursor maturation through the regulation of multiple homeostasis-associated genes, and KLF2 has a novel and essential role in this process.

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

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

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

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

Common developmental pathway for primitive erythrocytes and multipotent hematopoietic progenitors in early mouse development

Development of the hematopoietic system proceeds in a multistep manner. Primitive erythrocytes are the first hematopoietic cells to be observed that were produced transiently in developing embryos. Multilineage lymphohematopoiesis occurs after the primitive erythropoiesis. However, the lineage relationship of cells that comprise embryonic hematopoietic system is not well characterized. To clarify this process, careful analyses of the embryonic cells that differentiate into these cell lineages are necessary. We identified the common precursors of primitive erythrocytes and multipotent hematopoietic cells in mouse embryonic stem cell cultures and mouse embryos. A subset defined as CD45⁻CD41⁺AA4.1⁻ cells showed bipotential capability to produce primitive erythrocytes and lymphomyeloid cells at the single-cell level. The cell population was present in vivo before hematopoietic stem cells (HSCs) appeared. Our results show that primitive erythrocytes and lymphomyeloid cells are not completely separate cell lineages, and these precursors comprise the embryonic hematopoietic system before HSC emergence.

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Primitive erythrocytes and lymphomyeloid progenitors have bipotent precursors

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The precursors form primitive erythroid and lymphomyeloid cells in distinct waves

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The common precursors are noted in the yolk sac and embryo proper

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Primitive erythrocytes and lymphocytes can be derived from a single precursor cell

The modern primitives: applying new technological approaches to explore the biology of the earliest red blood cells

One of the most critical stages in mammalian embryogenesis is the independent production of the embryo's own circulating, functional red blood cells. Correspondingly, erythrocytes are the first cell type to become functionally mature during embryogenesis. Failure to achieve this invariably leads to in utero lethality. The recent application of technologies such as transcriptome analysis, flow cytometry, mutant embryo analysis, and transgenic fluorescent gene expression reporter systems has shed new light on the distinct erythroid lineages that arise early in development. Here, I will describe the similarities and differences between the distinct erythroid populations that must form for the embryo to survive. While much of the focus of this review will be the poorly understood primitive erythroid lineage, a discussion of other erythroid and hematopoietic lineages, as well as the cell types making up the different niches that give rise to these lineages, is essential for presenting an appropriate developmental context of these cells.

A microscopic and macroscopic study of aging collagen on its molecular structure, mechanical properties, and cellular response

During aging, collagen structure changes, detrimentally affecting tissues' biophysical and biomechanical properties due to an accumulation of advanced glycation end-products (AGEs). In this investigation, we conducted a parallel study of microscopic and macroscopic properties of different-aged collagens from newborn to 2-yr-old rats, to examine the effect of aging on fibrillogenesis, mechanical and contractile properties of reconstituted hydrogels from these collagens seeded with or without fibroblasts. In addition to fibrillogenesis of collagen under the conventional conditions, some fibrillogenesis was conducted alongside a 12-T magnetic field, and gelation rate and AGE content were measured. A nondestructive indentation technique and optical coherence tomography were used to determine the elastic modulus and dimensional changes, respectively. It was revealed that in comparison to younger specimens, older collagens exhibited higher viscosity, faster gelation rates, and a higher AGE-specific fluorescence. Exceptionally, only young collagens formed highly aligned fibrils under magnetic fields. The youngest collagen demonstrated a higher elastic modulus and contraction in comparison to the older collagen. We conclude that aging changes collagen monomer structure, which considerably affects the fibrillogenesis process, the architecture of the resulting collagen fibers and the global network, and the macroscopic properties of the formed constructs.

Transcriptional hierarchies regulating early blood cell development

Hematopoiesis represents one of the paradigmatic systems for studying stem cell biology, but our understanding of how the hematopoietic system develops during embryogenesis is still incomplete. While many lessons have been learned from studying the mouse embryo, embryonic stem cells have come to the fore as an alternative and more tractable model to recapitulate hematopoietic development. Here we review what is known about the embryonic origin of blood from these complementary systems and how transcription factor networks regulate the emergence of hematopoietic tissue from the mesoderm. Furthermore, we have performed an integrated analysis of genome-wide microarray and ChIP-seq data sets from mouse embryos and embryonic stem (ES) cell lines deficient in key regulators and demonstrate how this type of analysis can be used to reconstruct regulatory hierarchies that both confirm existing regulatory linkages and suggest additional interactions.

Erythroid development in the mammalian embryo

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

SUMO1-activating enzyme subunit 1 is essential for the survival of hematopoietic stem/progenitor cells in zebrafish

In vertebrates, establishment of the hematopoietic stem/progenitor cell (HSPC) pool involves mobilization of these cells in successive developmental hematopoietic niches. In zebrafish, HSPCs originate from the ventral wall of the dorsal aorta (VDA), the equivalent of the mammalian aorta-gonad-mesonephros (AGM). The HSPCs subsequently migrate to the caudal hematopoietic tissue (CHT) for transitory expansion and differentiation during the larval stage, and they finally colonize the kidney, where hematopoiesis takes place in adult fish. Here, we report the isolation and characterization of a zebrafish mutant, tango(hkz5), which shows defects of definitive hematopoiesis. In tango(hkz5) mutants, HSPCs initiate normally in the AGM and subsequently colonize the CHT. However, definitive hematopoiesis is not sustained in the CHT owing to accelerated apoptosis and diminished proliferation of HSPCs. Positional cloning reveals that tango(hkz5) encodes SUMO1-activating enzyme subunit 1 (Sae1). A chimera generation experiment and biochemistry analysis reveal that sae1 is cell-autonomously required for definitive hematopoiesis and that the tango(hkz5) mutation produces a truncated Sae1 protein (ΔSae1), resulting in systemic reduction of sumoylation. Our findings demonstrate that sae1 is essential for the maintenance of HSPCs during fetal hematopoiesis in zebrafish.

Embryonic development of hematopoietic stem cells: implications for clinical use

Hematopoietic stem cell (HSC) transplantation is an important treatment modality for hematological malignancies or to correct congenital immunodeficiency disorders. Several stem cell sources are currently applied clinically, with a recent increased application of umbilical cord blood. The low number of HSCs available, particularly in umbilical cord blood, is a limiting factor, and different lines of research are ongoing to circumvent this issue. In this review, we will describe the research strategies developed to expand adult HSCs in vitro and to generate new HSCs from pluripotent stem cell lines. We will also discuss the importance of studying the embryonic microenvironment since it allows both generation and extensive expansion of HSCs. Understanding the mechanisms that underlie HSC production, self-renewal and differentiation is necessary for the establishment of optimal in vitro HSC cultures, where a limitless and manipulatable resource of HSCs would be available for both clinical and fundamental research.

The embryonic origins of erythropoiesis in mammals

Erythroid (red blood) cells are the first cell type to be specified in the postimplantation mammalian embryo and serve highly specialized, essential functions throughout gestation and postnatal life. The existence of 2 developmentally and morphologically distinct erythroid lineages, primitive (embryonic) and definitive (adult), was described for the mammalian embryo more than a century ago. Cells of the primitive erythroid lineage support the transition from rapidly growing embryo to fetus, whereas definitive erythrocytes function during the transition from fetal life to birth and continue to be crucial for a variety of normal physiologic processes. Over the past few years, it has become apparent that the ontogeny and maturation of these lineages are more complex than previously appreciated. In this review, we highlight some common and distinguishing features of the red blood cell lineages and summarize advances in our understanding of how these cells develop and differentiate throughout mammalian ontogeny.

Extramedullary hematopoiesis: a new look at the underlying stem cell niche, theories of development, and occurrence in animals

Extramedullary hematopoiesis (EMH) is the formation and development of blood cells outside the medullary spaces of the bone marrow. Although widely considered an epiphenomenon, secondary to underlying primary disease and lacking serious clinical or diagnostic implications, the presence of EMH is far from incidental on a molecular basis; rather, it reflects a well-choreographed suite of changes involving stem cells and their microenvironment (the stem cell niche). The goals of this review are to reconsider the molecular basis of EMH based on current knowledge of stem cell niches and to examine its role in the pathophysiologic mechanisms of EMH in animals. The ability of blood cells to home, proliferate, and mature in extramedullary tissues of adult animals reflects embryonic patterns of hematopoiesis and establishment or reactivation of a stem cell niche. This involves pathophysiologic alterations in hematopoietic stem cells, extracellular matrix, stromal cells, and local and systemic chemokines. Four major theories involving changes in stem cells and/or their microenvironment can explain the development of most occurrences of EMH: (1) severe bone marrow failure; (2) myelostimulation; (3) tissue inflammation, injury, and repair; and (4) abnormal chemokine production. EMH has also been reported within many types of neoplasms. Understanding the concepts and factors involved in stem cell niches enhances our understanding of the occurrence of EMH in animals and its relationship to underlying disease. In turn, a better understanding of the prevalence and distribution of EMH in animals and its molecular basis could further inform our understanding of the hematopoietic stem cell niche.

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

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

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.

Sumoylation of CCAAT/enhancer–binding protein α promotes the biased primitive hematopoiesis of zebrafish

Hematopoiesis is evolutionarily conserved from zebrafish to mammals, and this includes both primitive and definitive waves during embryogenesis. Primitive hematopoiesis is dominated by erythropoiesis with limited myelopoiesis. Protein sumoylation, a ubiquitination-like posttranslational protein modification, is implicated in a variety of biochemical processes, most notably in transcriptional repression. We show here that the loss of 6 small ubiquitin-related modifier (SUMO) paralogs triggers a sharp up-regulation of the myeloid-specific marker mpo and down-regulation of the erythroid-specific marker gata1 in myelo-erythroid progenitor cells (MPCs) in the intermediate cell mass (ICM) during primitive hematopoiesis. Accordingly, in transgenic zebrafish lines, hyposumoylation expands myelopoiesis at the expense of erythropoiesis. A SUMO–CCAAT/enhancer–binding protein α (SUMO-C/ebpα) fusion restores the normal myelopoiesis/erythropoiesis balance, suggesting that sumoylation status of C/ebpα contributes to myelo-erythroid lineage determination. Our results therefore implicate sumoylation in early lineage determination and reveal the possible molecular mechanism underlying the puzzling biased primitive hematopoiesis in vertebrates.

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.

Mesothelium of the murine allantois exhibits distinct regional properties

The rodent allantois is thought to be unique amongst mammals in not having an endodermal component. Here, we have investigated the mesothelium, or outer surface, of murine umbilical precursor tissue, the allantois (∼7.25-8.5 days postcoitum, dpc) to discover whether it exhibits the properties of an epithelium. A combination of morphology, challenge with biotinylated dextran amines (BDAs), and immunohistochemistry revealed that the mesothelium of the mouse allantois exhibits distinct regional properties. By headfold stages (∼7.75-8.0 dpc), distal mesothelium was generally squamous in shape, and highly permeable to BDA challenge, whereas ventral proximal mesothelium, referred to as "ventral cuboidal mesothelium" (VCM) for the characteristic cuboidal shape of its cells, was relatively impermeable. Although "dorsal cuboidal mesothelium" (DCM) resembled the VCM in cell shape, its permeability to BDA was intermediate between the other two regions. Results of immunostaining for Zonula Occludens-1 (ZO-1) and Epithelial-cadherin (E-cadherin), together with transmission electron microscopy (TEM), suggested that impermeability in the VCM may be due to greater cellular contact area between cells and close packing rather than to maturity of tight junctions, the latter of which, by comparison with the visceral yolk sac, appeared to be rare or absent from the allantoic surface. Both VCM and DCM exhibited an ultrastructure more favorable for protein synthesis than did the distal squamous mesothelium; however, at most stages, VCM exhibited robust afadin (AF-6), whereas the DCM uniquely contained alpha-4-integrin. These observations demonstrate that the allantoic mesothelium is not a conventional epithelium but possesses regional ultrastructural, functional and molecular differences that may play important roles in the correct deployment of the umbilical cord and its associated vascular, hematopoietic, and other cell types.

Single-lineage transcriptome analysis reveals key regulatory pathways in primitive erythroid progenitors in the mouse embryo

Primitive erythroid (EryP) progenitors are the first cell type specified from the mesoderm late in gastrulation. We used a transgenic reporter to image and purify the earliest blood progenitors and their descendants from developing mouse embryos. EryP progenitors exhibited remarkable proliferative capacity in the yolk sac immediately before the onset of circulation, when these cells comprise nearly half of all cells of the embryo. Global expression profiles generated at 24-hour intervals from embryonic day 7.5 through 2.5 revealed 2 abrupt changes in transcript diversity that coincided with the entry of EryPs into the circulation and with their late maturation and enucleation, respectively. These changes were paralleled by the expression of critical regulatory factors. Experiments designed to test predictions from these data demonstrated that the Wnt-signaling pathway is active in EryP progenitors, which display an aerobic glycolytic profile and the numbers of which are regulated by transforming growth factor-β1 and hypoxia. This is the first transcriptome assembled for a single hematopoietic lineage of the embryo over the course of its differentiation.

Expression of AA4.1 marks lymphohematopoietic progenitors in early mouse development

The hematopoietic system of mice is established during the early to midgestational stage of development. However, the earliest lymphohematopoietic progenitors that appear during mouse development have been less well characterized compared with the hematopoietic stem cell compartment of fetal liver and bone marrow. We isolated the earliest lymphohematopoietic progenitors by using embryonic stem (ES) cell culture in vitro. Cells with the c-Kit(+)Lin(-) cell surface phenotype were present abundantly in ES cells cocultured with stromal cell lines. We further separated the cells into two distinct cell subsets based on AA4.1 expression. Although AA4.1(+) and AA4.1(-) cells had equivalent potency to generate myeloid cell lineages, the lymphoid potential in ES-cell-derived cells was largely restricted to the cells expressing AA4.1. The same cell type was present abundantly in the early yolk sac and in fewer numbers (approximately 5% of that in the yolk sac) in the caudal half of the developing embryos. These data suggest that AA4.1 is a cell surface marker that can identify the earliest lymphohematopoietic progenitors in mouse development.

Biomechanical forces promote embryonic haematopoiesis

Biomechanical forces are emerging as critical regulators of embryogenesis, particularly in the developing cardiovascular system. After initiation of the heartbeat in vertebrates, cells lining the ventral aspect of the dorsal aorta, the placental vessels, and the umbilical and vitelline arteries initiate expression of the transcription factor Runx1 (refs 3-5), a master regulator of haematopoiesis, and give rise to haematopoietic cells. It remains unknown whether the biomechanical forces imposed on the vascular wall at this developmental stage act as a determinant of haematopoietic potential. Here, using mouse embryonic stem cells differentiated in vitro, we show that fluid shear stress increases the expression of Runx1 in CD41(+)c-Kit(+) haematopoietic progenitor cells, concomitantly augmenting their haematopoietic colony-forming potential. Moreover, we find that shear stress increases haematopoietic colony-forming potential and expression of haematopoietic markers in the para-aortic splanchnopleura/aorta-gonads-mesonephros of mouse embryos and that abrogation of nitric oxide, a mediator of shear-stress-induced signalling, compromises haematopoietic potential in vitro and in vivo. Collectively, these data reveal a critical role for biomechanical forces in haematopoietic development.

Role of Ceacam1 in VEGF induced vasculogenesis of murine embryonic stem cell-derived embryoid bodies in 3D culture

CEACAM1 (carcinoembryonic antigen-related cell adhesion molecule 1), a type I transmembrane glycoprotein involved in cell-cell adhesion has been shown to act as an angiogenic factor for mouse and human endothelial cells. Based on the ability of CEACAM1 to initiate lumen formation in human mammary epithelial cells grown in 3D culture (Matrigel), we hypothesized that murine CEACAM1 may play a similar role in vasculogenesis. In order to test this hypothesis, murine embryonic stem (ES) cells stimulated with VEGF were differentiated into embryoid bodies (EB) for 8 days (-8-0 d) and transferred to Matrigel in the presence or absence of anti-CEACAM1 antibody for an additional 12 days (0-12 d). In the absence of anti-CEACAM1 antibody or in the presence of an isotype control antibody, the EB in Matrigel underwent extensive sprouting, generating lengthy vascular structures with well-defined lumina as demonstrated by confocal microscopy, electron microscopy, and immunohistochemical analysis. Both the length and architecture of the vascular tubes were inhibited by anti-CEACAM1 mAb CC1, a mAb that blocks the cell-cell adhesion functions of CEACAM1, thus demonstrating a critical role for this cell-cell adhesion molecule in generating and maintaining vasculogenesis. QRT-PCR analysis of the VEGF treated ES cells grown under conditions that convert them to EB revealed expression of Ceacam1 as early as -5 to -3 d reaching a maximum at day 0 at which time EBs were transferred to Matrigel, thereafter levels at first declined and then increased over time. Other markers of vasculogenesis including Pecam1, VE-Cad, and Tie-1 were not detected until day 0 when EBs were transferred to Matrigel followed by a steady increase in levels, indicating later roles in vasculogenesis. In contrast, Tie-2 and Flk-1 (VEGFR2) were detected on day five of EB formation reaching a maximum at day 0 on transfer to Matrigel, similar to Ceacam1, but after which Tie-2 declined over time, while Flk-1 increased over time. QRT-PCR analysis of the anti-CEACAM1 treated ES cells revealed a significant decrease in the expression of Ceacam1, Pecam1, Tie-1, and Flk-1, while VE-Cad and Tie-2 expression were unaffected. These results suggest that the expression and signaling of CEACAM1 may affect the expression of other factors known to play critical roles in vasculogenesis. Furthermore this 3D model of vasculogenesis in an environment of extracellular matrix may be a useful model for comparison to existing models of angiogenesis.

The Allantoic Core Domain: new insights into development of the murine allantois and its relation to the primitive streak

The whereabouts and properties of the posterior end of the primitive streak have not been identified in any species. In the mouse, the streak's posterior terminus is assumed to be confined to the embryonic compartment, and to give rise to the allantois, which links the embryo to its mother during pregnancy. In this study, we have refined our understanding of the biology of the murine posterior primitive streak and its relation to the allantois. Through a combination of immunostaining and morphology, we demonstrate that the primitive streak spans the posterior extraembryonic and embryonic regions at the onset of the neural plate stage ( approximately 7.0 days postcoitum, dpc). Several hours later, the allantoic bud emerges from the extraembryonic component of the primitive streak (XPS). Then, possibly in collaboration with overlying allantois-associated extraembryonic visceral endoderm, the XPS establishes a germinal center within the allantois, named here the Allantoic Core Domain (ACD). Microsurgical removal of the ACD beyond headfold (HF) stages resulted in the formation of allantoic regenerates that lacked the ACD and failed to elongate; nevertheless, vasculogenesis and vascular patterning proceeded. In situ and transplantation fate mapping demonstrated that, from HF stages onward, the ACD's progenitor pool contributed to the allantois exclusive of the proximal flanks. By contrast, the posterior intraembryonic primitive streak (IPS) provided the flanks. Grafting the ACD into T(C)/T(C) hosts, whose allantoises are significantly foreshortened, restored allantoic elongation. These results revealed that the ACD is essential for allantoic elongation, but the cues required for vascularization lie outside of it. On the basis of these and previous findings, we conclude that the posterior primitive streak of the mouse conceptus is far more complex than was previously believed. Our results provide new directives for addressing the origin and development of the umbilical cord, and establish a novel paradigm for investigating the fetal/placental relationship.

Stem cell sources for regenerative medicine

Tissue-resident stem cells or primitive progenitors play an integral role in homeostasis of most organ systems. Recent developments in methodologies to isolate and culture embryonic and somatic stem cells have many new applications poised for clinical and preclinical trials, which will enable the potential of regenerative medicine to be realized. Here, we overview the current progress in therapeutic applications of various stem cells and discuss technical and social hurdles that must be overcome for their potential to be realized.

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.

Human fetal aorta-derived vascular progenitor cells: identification and potential application in ischemic diseases

Vasculogenesis, the formation of blood vessels in embryonic or fetal tissue mediated by immature vascular cells (i.e., angioblasts), is poorly understood. Here we report a summary of our recent studies on the identification of a population of vascular progenitor cells (VPCs) in human fetal aorta. These undifferentiated mesenchymal cells co-express endothelial and myogenic markers (CD133+, CD34+, KDR+, desmin+) and are localized in outer layer of the aortic stroma of 11-12 weeks old human fetuses. Under stimulation with VEGF-A or PDGF-BB, VPCs give origin to a mixed population of mature endothelial and mural cells, respectively. When embedded in a three-dimensional collagen gel, VPCs organize into cohesive cellular cords that resembled mature vascular structures. The therapeutic efficacy of a small number of VPCs transplanted into ischemic limb muscle was demonstrated in immunodeficient mice. Investigation of the effect of VPCs on experimental heart ischemia and on diabetic ischemic ulcers in mice is in progress and seems to confirm their efficacy. On the whole, fetal aorta represents an important source for the investigation of phenotypic and functional features of human vascular progenitor cells.

Numb mediates the interaction between Wnt and Notch to modulate primitive erythropoietic specification from the hemangioblast

During embryonic development, the establishment of the primitive erythroid lineage in the yolk sac is a temporally and spatially restricted program that defines the onset of hematopoiesis. In this report, we have used the embryonic stem cell differentiation system to investigate the regulation of primitive erythroid development at the level of the hemangioblast. We show that the combination of Wnt signaling with inhibition of the Notch pathway is required for the development of this lineage. Inhibition of Notch signaling at this stage appears to be mediated by the transient expression of Numb in the hemangioblast-derived blast cell colonies. Activation of the Notch pathway was found to inhibit primitive erythropoiesis efficiently through the upregulation of inhibitors of the Wnt pathway. Together, these findings demonstrate that specification of the primitive erythroid lineage is controlled, in part, by the coordinated interaction of the Wnt and Notch pathways, and position Numb as a key mediator of this process.