Lineage tracing of human development through somatic mutations

The ontogeny of the human haematopoietic system during fetal development has previously been characterized mainly through careful microscopic observations1. Here we reconstruct a phylogenetic tree of blood development using whole-genome sequencing of 511 single-cell-derived haematopoietic colonies from healthy human fetuses at 8 and 18 weeks after conception, coupled with deep targeted sequencing of tissues of known embryonic origin. We found that, in healthy fetuses, individual haematopoietic progenitors acquire tens of somatic mutations by 18 weeks after conception. We used these mutations as barcodes and timed the divergence of embryonic and extra-embryonic tissues during development, and estimated the number of blood antecedents at different stages of embryonic development. Our data support a hypoblast origin of the extra-embryonic mesoderm and primitive blood in humans.

Stem Cell Transplantation in the Fetus

BACKGROUND

In utero transplantation (IUT) of hematopoietic stem cells has the potential to treat a large number of hematologic and metabolic diseases amenable to partial replacement of the hematopoietic system.

METHODS

A review of the literature was conducted that focused on the clinical and experimental experience with IUT and, in this context, the development of the hematopoietic and immune systems.

RESULTS

Successful application of IUT has been limited to the treatment of various types of immunodeficiencies that affect lymphocyte development and function. Other congenital defects such as the thalassemias have not resulted in clinically significant engraftment. Recent efforts at understanding and overcoming the barriers to engraftment in the fetus have focused on providing a selective advantage to donor stem cells and fostering immune tolerance toward the donor cells. The critical cellular components of the graft that promote engraftment and tolerance induction are being evaluated in animal models. Improvements in engraftment have resulted from the inclusion of T cells and/or dendritic cells in the graft, as well as a strategy of combined prenatal and postnatal transplantation.

CONCLUSIONS

The advantages, necessity, and benefits of early treatment will continue to encourage development of IUT as a means to treat hematopoietic and other types of birth defects.

Endothelium and NOTCH specify and amplify aorta-gonad-mesonephros-derived hematopoietic stem cells

Hematopoietic stem cells (HSCs) first emerge during embryonic development within vessels such as the dorsal aorta of the aorta-gonad-mesonephros (AGM) region, suggesting that signals from the vascular microenvironment are critical for HSC development. Here, we demonstrated that AGM-derived endothelial cells (ECs) engineered to constitutively express AKT (AGM AKT-ECs) can provide an in vitro niche that recapitulates embryonic HSC specification and amplification. Specifically, nonengrafting embryonic precursors, including the VE-cadherin-expressing population that lacks hematopoietic surface markers, cocultured with AGM AKT-ECs specified into long-term, adult-engrafting HSCs, establishing that a vascular niche is sufficient to induce the endothelial-to-HSC transition in vitro. Subsequent to hematopoietic induction, coculture with AGM AKT-ECs also substantially increased the numbers of HSCs derived from VE-cadherin⁺CD45⁺ AGM hematopoietic cells, consistent with a role in supporting further HSC maturation and self-renewal. We also identified conditions that included NOTCH activation with an immobilized NOTCH ligand that were sufficient to amplify AGM-derived HSCs following their specification in the absence of AGM AKT-ECs. Together, these studies begin to define the critical niche components and resident signals required for HSC induction and self-renewal ex vivo, and thus provide insight for development of defined in vitro systems targeted toward HSC generation for therapeutic applications.

Transcriptome analysis of the zebrafish model of Diamond-Blackfan anemia from RPS19 deficiency via p53-dependent and -independent pathways

Diamond-Blackfan anemia (DBA) is a rare inherited bone marrow failure syndrome that is characterized by pure red-cell aplasia and associated physical deformities. It has been proven that defects of ribosomal proteins can lead to this disease and that RPS19 is the most frequently mutated gene in DBA patients. Previous studies suggest that p53-dependent genes and pathways play important roles in RPS19-deficient embryos. However, whether there are other vital factors linked to DBA has not been fully clarified. In this study, we compared the whole genome RNA-Seq data of zebrafish embryos injected with RPS19 morpholino (RPS19 MO), RPS19 and p53 morpholino simultaneously (RPS19+p53 MO) and control morpholino (control). We found that genes enriched in the functions of hematological systems, nervous system development and skeletal and muscular disorders had significant differential expression in RPS19 MO embryos compared with controls. Co-inhibition of p53 partially alleviates the abnormalities for RPS19-deficient embryos. However, the hematopoietic genes, which were down-regulated significantly in RPS19 MO embryos, were not completely recovered by the co-inhibition of p53. Furthermore, we identified the genome-wide p53-dependent and -independent genes and pathways. These results indicate that not only p53 family members but also other factors have important impacts on RPS19-deficient embryos. The detection of potential pathogenic genes and pathways provides us a new paradigm for future research on DBA, which is a systematic and complex hereditary disease.

RUNX1: A microRNA hub in normal and malignant hematopoiesis

Hematopoietic development is orchestrated by gene regulatory networks that progressively induce lineage-specific transcriptional programs. To guarantee the appropriate level of complexity, flexibility, and robustness, these networks rely on transcriptional and post-transcriptional circuits involving both transcription factors (TFs) and microRNAs (miRNAs). The focus of this review is on RUNX1 (AML1), a master hematopoietic transcription factor which is at the center of miRNA circuits necessary for both embryonic and post-natal hematopoiesis. Interference with components of these circuits can perturb RUNX1-controlled coding and non-coding transcriptional programs in leukemia.

[Analyzing hematopoietic development of mouse embryonic AGM region by tissue culture]

To analyze hematopoietic kinetics of mouse embryonic aorta-gonad-mesonephros (AGM) region, an in vitro tissue culture method was developed in this study, partly based on previous reports. After 2 days of tissue culture, a significant number of erythro myeloid progenitors, as quantitated by colony forming assay was detected in the AGM region. Moreover, the cells from cultured E10.5 AGM region could generate 10.8 ± 3.5 colony-forming unit in spleen (CFU-S) per tissue on average. Transplantation of cultured E10.5-E11.0 AGM cells resulted in efficient (85.7% repopulated) and long-term (> 4 months) reconstitution of lethally irradiated adult recipients with remarkable chimerism [(51.12 ± 21.17)%]. The multilineage contribution of donor cells was validated by significant engraftment of myeloid and/or lymphoid cells in peripheral blood, bone marrow, spleen and thymus of recipients. Taken together, the tissue culture method can enable us to manipulate the AGM region in vitro, fulfilling a systematic evaluation of developmental kinetics of various hematopoietic precursor cells, particularly HSC, in normal and mutant mid-gestation mouse embryos.

Bcl-2 expression and apoptosis in the regulation of hematopoietic stem cells

Apoptosis or programmed cell death plays a pivotal role in regulating tissue homeostasis in the adult and in tissue remodeling during embryogenesis. As in other tissues, apoptosis plays an important role within the hematopoietic system in removing aged and non-functional cells. It plays a particularly important role in regulating the cells of the immune system. The signals and molecules regulating apoptosis in these immune cells have been intensely investigated over the years, providing great insight into the mechanisms involved. In contrast, much less is known about the regulation and role of apoptosis in the cells that produce differentiated hematopoietic cells, namely the hematopoietic stem cells (HSCs). It is appreciated that HSCs are under tight regulatory control, as either excessive proliferation or apoptosis will result in too many or too few hematopoietic cells (for example, leukemia or anemia). Apoptosis thus plays an essential role in maintaining the appropriate balance of HSC and mature blood cells and in protecting the HSC pool for life-long hematopoiesis. This review summarizes the current knowledge concerning apoptosis and its role in the physiology of the hematopoietic system, especially within the HSC compartment.

Regulation of stem cells in the zebra fish hematopoietic system

Hematopoietic stem cells (HSCs) have been used extensively as a model for stem cell biology. Stem cells share the ability to self-renew and differentiate into multiple cell types, making them ideal candidates for tissue regeneration or replacement therapies. Current applications of stem cell technology are limited by our knowledge of the molecular mechanisms that control their proliferation and differentiation, and various model organisms have been used to fill these gaps. This chapter focuses on the contributions of the zebra fish model to our understanding of stem cell regulation within the hematopoietic system. Studies in zebra fish have been valuable for identifying new genetic and signaling factors that affect HSC formation and development with important implications for humans, and new advances in the zebra fish toolbox will allow other aspects of HSC behavior to be investigated as well, including migration, homing, and engraftment.

Differential H3K4 methylation identifies developmentally poised hematopoietic genes

Throughout development, cell fate decisions are converted into epigenetic information that determines cellular identity. Covalent histone modifications are heritable epigenetic marks and are hypothesized to play a central role in this process. In this report, we assess the concordance of histone H3 lysine 4 dimethylation (H3K4me2) and trimethylation (H3K4me3) on a genome-wide scale in erythroid development by analyzing pluripotent, multipotent, and unipotent cell types. Although H3K4me2 and H3K4me3 are concordant at most genes, multipotential hematopoietic cells have a subset of genes that are differentially methylated (H3K4me2+/me3-). These genes are transcriptionally silent, highly enriched in lineage-specific hematopoietic genes, and uniquely susceptible to differentiation-induced H3K4 demethylation. Self-renewing embryonic stem cells, which restrict H3K4 methylation to genes that contain CpG islands (CGIs), lack H3K4me2+/me3- genes. These data reveal distinct epigenetic regulation of CGI and non-CGI genes during development and indicate an interactive relationship between DNA sequence and differential H3K4 methylation in lineage-specific differentiation.

The regulatory role of stromal microenvironments in fetal hematopoietic ontogeny

Fetal hematopoietic development occurs through the successive expansion and differentiation of hematopoietic stem cells in distinct anatomic sites. The temporal pattern of fetal hematopoietic ontogeny suggests a coordinated developmental sequence whereby the preceding organ sustains the basic, immediate hematopoietic needs of the embryo allowing time for the development of niches within the subsequent organ with more complex supportive functions. We examine the hypothesis that there is a period of stromal genesis and circulating mesenchymal precursor cells, which gives rise to specialized niches within each of the definitive fetal hematopoietic organs, and these niches regulate hematopoietic stem cells fate determination. This article reviews fetal hematopoietic and stromal development and the current understanding of the development, composition, and regulation of the fetal stem cell niche.

Identification of a crystal cell-specific enhancer of the black cells prophenoloxidase gene in Drosophila

In Drosophila, Black cells (Bc) encodes a Prophenoloxidase and is expressed late in the maturation of crystal cells, which are blood cells involved in wound healing and immune encapsulation. Enhancer analysis of Bc revealed a 1,025-bp upstream sequence that regulates gene expression in a crystal cell exclusive pattern. Expression of this fragment is altered by mutations in the GATA family serpent (srp) and RUNX family lozenge (lz) genes; Srp and Lz are required for crystal cell specification. Deletional analysis uncovered a 330-bp crystal cell-specific sequence, which contains two GATA and three Lz binding sites. Mutational analysis revealed that both GATA sites are necessary, but not sufficient for crystal cell expression. However, one of the Lz sites is essential for crystal cell expression. Thus, Srp and Lz do not just specify the crystal cell lineage, but also regulate the later differentiation of these cells. Additionally, we now have a sensitive tool for marking crystal cells in live animals.

The cholinergic system is involved in regulation of the development of the hematopoietic system

Gene expression profiling demonstrated that components of the cholinergic system, including choline acetyltransferase, acetylcholinesterase and nicotinic acetylcholine receptors (nAChRs), are expressed in embryonic stem cells and differentiating embryoid bodies (EBs). Triggering of nAChRs expressed in EBs by nicotine resulted in activation of MAPK and shifts of spontaneous differentiation toward hemangioblast. In vivo, non-neural nAChRs are detected early during development in fetal sites of hematopoiesis. Similarly, in vivo exposure of the developing embryo to nicotine resulted in higher numbers of hematopoietic progenitors in fetal liver. However postpartum, the number of hematopoietic stem/progenitor cells (HSPC) was decreased, suggesting an impaired colonization of the fetal bone marrow with HSPCs. This correlated with increased number of circulating HSPC and decreased expression of CXCR4 that mediates migration of circulating cells into the bone marrow regulatory niche. In addition, protein microarrays demonstrated that nicotine changed the profile of cytokines produced in the niche. While the levels of IL1alpha, IL1beta, IL2, IL9 and IL10 were not changed, the production of hematopoiesis-supportive cytokines including G-CSF, GM-CSF, IL3, IL6 and IGFBP-3 was decreased. This correlated with the decreased repopulating ability of HSPC in vivo and diminished hematopoietic activity in bone marrow cultures treated with nicotine. Interestingly, nicotine stimulated the production of IL4 and IL5, implying a possible role of the cholinergic system in pathogenesis of allergic diseases. Our data provide evidence that the nicotine-induced imbalance of the cholinergic system during gestation interferes with normal development and provides the basis for negative health outcomes postpartum in active and passive smokers.

Production and purification of recombinant human granulocyte–macrophage colony stimulating factor (GM-CSF) from high cell density cultures of Pichia pastoris

Granulocyte-macrophage colony stimulating factor (GM-CSF) is a hematopoietic growth factor, which has been used as a therapeutic agent in clinical cases like neutropenia. In this study, we report the production of recombinant human GM-CSF in the methylotrophic yeast Pichia pastoris through secretory expression using the inducible AOX1 promoter. Recombinant P. pastoris GS115 cells were grown in fed batch cultures to obtain a biomass density of 55.6 gDCW L(-1) and a high volumetric activity of 131 mg L(-1) of GM-CSF. The protein migrated as a diffuse band on SDS-PAGE at the range of 28-35 kDa indicating differential glycosylation. The secreted protein was purified to 95% in two steps using cation exchange and size exclusion chromatography.

Identification of a new intrinsically timed developmental checkpoint that reprograms key hematopoietic stem cell properties

Hematopoietic stem cells (HSCs) execute self-renewal divisions throughout fetal and adult life, although some of their properties do alter. Here we analyzed the magnitude and timing of changes in the self-renewal properties and differentiated cell outputs of transplanted HSCs obtained from different sources during development. We also assessed the expression of several "stem cell" genes in corresponding populations of highly purified HSCs. Fetal and adult HSCs displayed marked differences in their self-renewal, differentiated cell output, and gene expression properties, with persistence of a fetal phenotype until 3 weeks after birth. Then, 1 week later, the HSCs became functionally indistinguishable from adult HSCs. The same schedule of changes in HSC properties occurred when HSCs from fetal or 3-week-old donors were transplanted into adult recipients. These findings point to the existence of a previously unrecognized, intrinsically regulated master switch that effects a developmental change in key HSC properties.

Requirement of the LIM homeodomain transcription factor tailup for normal heart and hematopoietic organ formation in Drosophila melanogaster

Dorsal vessel morphogenesis in Drosophila melanogaster serves as a superb system with which to study the cellular and genetic bases of heart tube formation. We used a cardioblast-expressed Toll-GFP transgene to screen for additional genes involved in heart development and identified tailup as a locus essential for normal dorsal vessel formation. tailup, related to vertebrate islet1, encodes a LIM homeodomain transcription factor expressed in all cardioblasts and pericardial cells of the heart tube as well as in associated lymph gland hematopoietic organs and alary muscles that attach the dorsal vessel to the epidermis. A transcriptional enhancer regulating expression in these four cell types was identified and used as a tailup-GFP transgene with additional markers to characterize dorsal vessel defects resulting from gene mutations. Two reproducible phenotypes were observed in mutant embryos: hypoplastic heart tubes with misaligned cardioblasts and the absence of most lymph gland and pericardial cells. Conversely, a significant expansion of the lymph glands and abnormal morphology of the heart were observed when tailup was overexpressed in the mesoderm. Tailup was shown to bind to two DNA recognition sequences in the dorsal vessel enhancer of the Hand basic helix-loop-helix transcription factor gene, with one site proven to be essential for the lymph gland, pericardial cell, and Svp/Doc cardioblast expression of Hand. Together, these results establish Tailup as being a critical new transcription factor in dorsal vessel morphogenesis and lymph gland formation and place this regulator directly upstream of Hand in these developmental processes.

Hematopoietic stem cells proliferate until after birth and show a reversible phase-specific engraftment defect

The regulation of HSC proliferation and engraftment of the BM is an important but poorly understood process, particularly during ontogeny. Here we show that in mice, all HSCs are cycling until 3 weeks after birth. Then, within 1 week, most became quiescent. Prior to 4 weeks of age, the proliferating HSCs with long-term multilineage repopulating activity displayed an engraftment defect when transiting S/G2/M. During these cell cycle phases, their expression of CXC chemokine ligand 12 (CXCL12; also referred to as stromal cell-derived factor 1 [SDF-1]) transiently increased. The defective engrafting activity of HSCs in S/G2/M was reversed when cells were allowed to progress into G1 prior to injection or when the hosts (but not the cells) were pretreated with a CXCL12 antagonist. Interestingly, the enhancing effect of CXCL12 antagonist pretreatment was exclusive to transplants of long-term multilineage repopulating HSCs in S/G2/M. These results demonstrate what we believe to be a new HSC regulatory checkpoint during development. They also suggest an ability of HSCs to express CXCL12 in a fashion that changes with cell cycle progression and is associated with a defective engraftment that can be overcome by in vivo administration of a CXCL12 antagonist.

Children are not little adults: just ask their hematopoietic stem cells

HSCs differ during ontogeny in some important parameters, including anatomic site of residence and cell cycling characteristics. In this issue of the JCI, Bowie et al. show that postnatal HSCs as well as fetal liver HSCs in mice are active in the cell cycle at much higher rates than that of adult HSCs; however, this increased frequency of cycling abruptly ceases 4 weeks after birth (see the related article beginning on page 2808). The cycling postnatal HSCs expressed high levels of CXC chemokine ligand 12 (CXCL12, also known as stromal cell-derived factor 1 [SDF-1]), a chemokine previously implicated in stem cell trafficking to the marrow cavity and shown to be expressed by cells within the hematopoietic microenvironment. These cells also possessed an engraftment defect impeding reconstitution in irradiated recipient mice, which was reversible by pretransplant administration of antagonists of the CXCL12 receptor, CXCR4. Such agents are currently clinically available, suggesting that this approach could be used to improve stem cell transplantation and engraftment.

Analysis of integrin functions in peri-implantation embryos, hematopoietic system, and skin

Integrins mediate cell adhesion, permit traction forces important for cell migration, and cross-talk with growth factor receptors to regulate cell proliferation, cell survival, and cell differentiation. The plethora of functions explains their central role for development and disease. The progress in mouse genetics and the ease with which the mouse genome can be manipulated enormously contributed to our understanding of how integrins exert their functions at the molecular level. In the present chapter, we describe tests that are routinely used in our laboratory to investigate embryos, organs, and cells (peri-implantation embryos, hematopoietic system, epidermis, and hair follicles) that lack the expression of integrins or integrin-associated proteins.

Tracing Hematopoietic Precursor Migration to Successive Hematopoietic Organs during Zebrafish Development

Although the ontogeny of hematopoietic stem cells (HSCs) in vertebrates has been studied intensely, a lineage relationship between the HSCs found in the developmentally successive hematopoietic organs remains to be shown. By using an in situ photoactivatable cell tracer in the transparent zebrafish embryo, we demonstrated that definitive blood precursors appeared between the dorsal aorta and axial vein, validating the homology of this tissue with the AGM (aorta-gonad-mesonephros) of amniotes. These cells first migrated through the blood to a previously undescribed caudal hematopoietic tissue (CHT), where they differentiated, expanded, and further migrated to seed the definitive hematopoietic organs, the thymus and kidney. Immigrants on the way to the thymus expressed c-myb and ikaros but not rag1; they were probably no longer HSCs, however, because they lacked scl and runx1 expression, unlike immigrants to the kidney. The CHT thus has a hematopoietic function similar to that of the mammalian fetal liver.

Syk and Slp-76 mutant mice reveal a cell-autonomous hematopoietic cell contribution to vascular development

Developmental studies support a common origin for blood and endothelial cells, while studies of adult angiogenic responses suggest that the hematopoietic system can be a source of endothelial cells later in life. Whether hematopoietic tissue is a source of endothelial cells during normal vascular development is unknown. Mouse embryos lacking the signaling proteins Syk and Slp-76 develop abnormal blood-lymphatic endothelial connections. Here we demonstrate that expression of GFPSlp-76 in a subset of hematopoietic cells rescues this phenotype, and that deficient cells confer focal vascular phenotypes in chimeric embryos consistent with a cell-autonomous mechanism. Endogenous Syk and Slp-76, as well as transgenic GFPSlp-76, are expressed in circulating cells previously proposed to be endothelial precursors, supporting a causal role for these cells. These studies provide genetic evidence for hematopoietic contribution to vascular development and suggest that hematopoietic tissue can provide a source of vascular endothelial progenitor cells throughout life.

lyl-1 and tal-1/scl, two genes encoding closely related bHLH transcription factors, display highly overlapping expression patterns during cardiovascular and hematopoietic ontogeny

The TAL-1/SCL and LYL-1 genes encode two closely related basic helix-loop-helix transcription factors involved in child T-acute lymphoblastic leukemia through chromosomal rearrangements and transcriptional deregulation. During ontogeny, Tal-1/SCL is required for hematopoietic cell generation, both in the yolk sac, where erythro-myeloid cells are first produced, then in the intra-embryonic compartment, where hematopoietic stem cells independently arise. We describe here the expression pattern of lyl-1 in mouse embryos from 7 to 14 days post coitus using in situ hybridization, as well as beta-Galactosidase (beta-Gal) expression in lyl-1-lacZ knock-in embryos, which express a C-terminally truncated Lyl-1 protein fused to the beta-Galactosidase (Lyl-1Delta/beta-Gal). In addition, we compare lyl-1 expression pattern with that of tal-1/scl. Similar to Tal-1/SCL, Lyl-1 mRNA expression occurs in the developing cardiovascular and hematopoietic systems. However, contrary to tal-1/scl, lyl-1 is not expressed in the developing nervous system. In lyl-1-lacZ knock-in heterozygous and homozygous embryos, beta-Gal expression completely correlates with Lyl-1 mRNA expression in the intra-embryonic compartment and is present: (1) in the developing hematopoietic system, precisely where hematopoietic stem cells emerge, and thereafter in the fetal liver; (2) in the developing vascular system; and (3) in the endocardium. In contrast, whereas Lyl-1 mRNA is expressed in yolk sac-derived endothelial and hematopoietic cells, Lyl-1Delta/beta-Gal is either absent or poorly expressed in these cell types, thus differing from Tal-1/SCL, which is highly expressed there at both mRNA and protein levels.

Evidence of Haematopoiesis within the Developing Human Diencephalon

AIM

The sites of haematopoiesis during human ontogeny can be correlated to the sites where haematopoiesis occurs in vertebrate phylogeny. As haematopoiesis has been described in the diencephalon and pituitary gland of water-inhabiting vertebrates we wanted to find out whether such a phenomenon also occurs in human embryos.

MATERIAL AND METHODS

Paraffin-embedded specimens from the diencephalon and pituitary gland of human embryos at the 7th to 22nd gestational week and from adults were investigated by conventional histology and immunohistology for the presence of haematopoietic cells.

RESULTS

Cellular accumulations predominantly of erythroid and megakaryocytic lineage were identified in the floor of the developing diencephalon of the 7th/8th gestational week. At the older developmental stages of the 18th to 22nd gestational week loose aggregates of haematopoietic cells within the leptomeningeal spaces adjacent to the hypophyseal infundibulum were detected in 2 out of 7 cases analyzed.

CONCLUSIONS

As it has been proposed that lymphohaematopoietic clusters occasionally occur within the brain in bone marrow-less vertebrates as a response to noxious agents, we speculate that this temporal appearance of haematopoietic cell clusters within the diencephalon floor in early human ontogeny could also be due to fetal immunomodulations.

Puromycin insensitive leucyl-specific aminopeptidase (PILSAP) is required for the development of vascular as well as hematopoietic system in embryoid bodies

We have shown that puromycin insensitive leucyl-specific aminopeptidase (PILSAP) is required for regulation of angiogenesis. However, it remains unclear whether PILSAP plays a role in endothelial cell (EC) differentiation. We examined the role of PILSAP by using an embryoid bodies (EBs) culture system. Fms-like tyrosine kinase-1 (Flk-1) showed two expression peaks on days 4 and 10 of culture. These two peaks represent populations of mesodermal precursors and mature ECs, respectively. Endothelial markers such as VE-cadherin, CD34, CD31 and Tie2 followed the first peak of Flk-1. Interestingly, the expression of PILSAP showed a pattern similar to that of Flk-1. ES cells transfected with mutant PILSAP (mtPILSAP) cDNA of a dominant negative activity organized less vascular structure and showed decreased levels of vascular lineage markers. The similar results were obtained in EBs treated with leucinethiol, a specific inhibitor of leucine aminopeptidase or siRNA for PILSAP. However, Flk-1 expression was unaffected on day 4. The expression of markers for hematopoietic lineage and muscle cells in mtPILSAP-EBs was also reduced. These results suggest that although PILSAP may not function in the initial generation of Flk-1 positive mesodermal precursors, it dose play a role in growth of vascular, hematopoietic, and muscular lineage population from those precursors.

Regulation of the lmo2 promoter during hematopoietic and vascular development in zebrafish

The Lmo2 transcription factor, a T-cell oncoprotein, is required for both hematopoiesis and angiogenesis. To investigate the fate of lmo2-expressing cells and the transcriptional regulation of lmo2 in vivo, we generated stable transgenic zebrafish that express green fluorescent protein (EGFP) or DsRed under the control of an lmo2 promoter. A 2.5-kb fragment contains the cis-regulatory elements required to recapitulate endogenous lmo2 expression in embryonic hematopoietic and vascular tissues. We further characterized embryonic Lmo2+ cells through transplantation into vlad tepes (vlt), an erythropoietic mutant. These Lmo2+ primitive wave donor cells differentiated into circulating hematopoietic cells and extended the life span of vlt recipients, but did not demonstrate long-term repopulation of the erythroid lineage. Promoter analysis identified a 174-bp proximal promoter that was sufficient to recapitulate lmo2 expression. This element contains critical ETS-binding sites conserved between zebrafish and pufferfish. Furthermore, we show that ets1 is coexpressed with lmo2, and overexpression experiments indicate that ets1 can activate the lmo2 promoter through this element. Our studies elucidate the transcriptional regulation of this key transcription factor, and provide a transgenic system for the functional analysis of blood and blood vessels in zebrafish.

Global analysis of hematopoietic and vascular endothelial gene expression by tissue specific microarray profiling in zebrafish

In this study, we utilize fluorescent activated cell sorting (FACS) of cells from transgenic zebrafish coupled with microarray analysis to globally analyze expression of cell type specific genes. We find that it is possible to isolate cell populations from Tg(fli1:egfp)(y1) zebrafish embryos that are enriched in vascular, hematopoietic and pharyngeal arch cell types. Microarray analysis of GFP+ versus GFP- cells isolated from Tg(fli1:egfp)(y1) embryos identifies genes expressed in hematopoietic, vascular and pharyngeal arch tissue, consistent with the expression of the fli1:egfp transgene in these cell types. Comparison of expression profiles from GFP+ cells isolated from embryos at two different time points reveals that genes expressed in different fli1+ cell types display distinct temporal expression profiles. We also demonstrate the utility of this approach for gene discovery by identifying numerous previously uncharacterized genes that we find are expressed in fli1:egfp-positive cells, including new markers of blood, endothelial and pharyngeal arch cell types. In parallel, we have developed a database to allow easy access to both our microarray and in situ results. Our results demonstrate that this is a robust approach for identification of cell type specific genes as well as for global analysis of cell type specific gene expression in zebrafish embryos.

The embryonic aorta-gonad-mesonephros region as a generator of haematopoietic stem cells

During mammalian embryonic development the definitive haematopoietic stem cells (HSCs) may arise either in the extra-embryonic mesoderm or in the aorta-gonad-mesonephros (AGM) region that forms in close proximity to the assembling urogenital system, generating the gonad, cortex of the adrenal gland and metanephros. Researchers have been attempting for a long time to define the region of importance for generating the definitive HSCs that colonize the fetal liver and bone marrow, the two major sites where haematopoiesis takes place in the adult. The fetal liver might gain HSCs from both of the primary haematopoietic sources, but the extra-embryonic HSCs seem not to be able to colonize adult bone marrow directly. It is known that the microenvironment around the HSCs is important for directing cell fates, but we do not yet have much idea about the cell-cell interactions, tissue interactions and molecules that regulate cell behaviour in the AGM. We will here discuss the contribution of the AGM to definitive haematopoiesis in mammals and review some of the cell-cell interactions and associated signalling systems involved in the development of AGM stem cells.

Developmental regulation of the immune system

Term newborns have a higher frequency of microbial infections than older children and adults. Extremely premature newborns (<28 weeks gestation) have an even higher frequency. Quantitative and qualitative differences in the development of the immune system have been identified as a partial explanation for the increase in the incidence of infectious sequelae in these two patient populations. A less studied population of patients is late preterm newborns that are 34 to 35 6/7 weeks gestation. In general, this subset of patients is frequently grouped with term newborns. However, recent studies have provided data suggesting a potential unrecognized risk to health in this population, including at least a clinical suspicion for an increased risk of sepsis. Although little specific data on the host-defense capability of the near-term newborn exist, recent advancements in developmental immunology provide a framework for understanding the mechanisms underlying the propensity of infections in the preterm, near-term, and term newborn.

Ontogeny of the common carp (Cyprinus carpio L.) innate immune system

The ontogeny of the teleost innate immune system was studied in carp using cellular, histological and quantitative molecular techniques. Carp myeloid cells first appeared ventro-lateral of the aorta at 2 days post fertilization (the start of hatching), and subsequently around the sinuses of the vena cardinalis (or posterior blood islet), head kidney and trunk kidney. In addition, the hematopoietic tissue around the sinuses of the vena cardinalis transformed into that of the trunk kidney, which is the first description of the ontogeny of the trunk kidney hematopoietic tissue in teleosts. The mAb's used in this study reacted with carp myeloid surface molecules that are already transcribed and processed from the first appearance of myeloid cells, and thus serve a significant role in unraveling ontogenetic processes of teleost immunology. Finally, this study associated the first appearance of myeloid cells with an immune response on the molecular level: 2 days post fertilization embryos responded to LPS injection with upregulation of interleukin 1-beta, inducible nitric oxide synthase and serum amyloid A, and down-regulation of complement factor 3 and alpha2-macroglobulin, implying a functional embryonic innate defense system.

The embryonic origins of hematopoietic stem cells: a tale of hemangioblast and hemogenic endothelium

The developmental origin of hematopoietic stem cells has been for decades the subject of great interest. Once thought to emerge from the yolk sac, hematopoietic stem cells have now been shown to originate from the embryonic aorta. Increasing evidence suggests that hematopoietic stem cells are produced from an endothelial intermediate designated by the authors as hemangioblast or hemogenic endothelium. Recently, the allantois in the avian embryo and the placenta in the mouse embryo were shown to be a site of hematopoietic cell production/expansion and thus appear to play a critical role in the formation of the hematopoietic system. In this review we shall give an overview of the data obtained from human, mouse and avian models on the cellular origins of the hematopoietic system and discuss some aspects of the molecular mechanisms controlling hematopoietic cell production.

A role for niches in hematopoietic cell development

Stem cells reside in a physical niche, a particular microenvironment. The organization of cellular niches has been shown to play a key role in regulating normal stem cell differentiation, maintenance and regeneration. Hematopoietic stem cells (HSC) emerge at distinct allocation territories during ontogenesis, notably the aorto-gonadal region, the fetal liver. Adult HSC expand and differentiate exclusively in the bone marrow (BM). They can be mobilized into the blood stream. This implies that stem cells are not autonomous units of development; rather, tissue specific niches control their destiny. Interaction of HSCs with their stem cell niches is critical for adult hematopoiesis in the BM. A niche is composed of stromal cells, which either through direct cell-to-cell contact or via release of soluble factors maintain the typical features of stem cells, mainly stem cell quiescence, maintenance or expansion. HSCs are keeping the balance of the quiescence and the self-renewal in the stem cell niche, and are maintaining long-term hematopoiesis.Therefore, an understanding of cellular and chemical architecture of the stem cell niche is vital in understanding stem cell behavior. This review summarizes the recent developments in our understanding of the stem cell niche with particular focus on the HSC niche.

Oncogenic transcription factor Evi1 regulates hematopoietic stem cell proliferation through GATA-2 expression

The ecotropic viral integration site-1 (Evi1) is an oncogenic transcription factor in murine and human myeloid leukemia. We herein show that Evi1 is predominantly expressed in hematopoietic stem cells (HSCs) in embryos and adult bone marrows, suggesting a physiological role of Evi1 in HSCs. We therefore investigate the role and authentic target genes of Evi1 in hematopoiesis using Evi1-/- mice, which die at embryonic day 10.5. HSCs in Evi1-/- embryos are markedly decreased in numbers in vivo with defective self-renewing proliferation and repopulating capacity. Notably, expression rate of GATA-2 mRNA, which is essential for proliferation of definitive HSCs, is profoundly reduced in HSCs of Evi1-/- embryos. Restoration of the Evi1 or GATA-2 expression in Evi1-/- HSCs could prevent the failure of in vitro maintenance and proliferation of HSC through upregulation of GATA-2 expression. An analysis of the GATA-2 promoter region revealed that Evi1 directly binds to GATA-2 promoter as an enhancer. Our results reveal that GATA-2 is presumably one of critical targets for Evi1 and that transcription factors regulate the HSC pool hierarchically.

Mouse embryonic explant culture system for analysis of hematopoietic and vascular development

In vertebrates, the earliest differentiated cell types (hematopoietic and endothelial) arise from mesoderm induced during the process of gastrulation. These cells become organized into the blood islands of the extraembryonic yolk sac and are morphologically apparent by around d 7.5 in the mouse. Additional waves of hematopoietic and vasculogenic/angiogenic activity later result in the development of definitive hematopoietic lineages and in the formation of the allantois and cardiovascular system of the embryo proper. In part because of the limited accessibility of the mammalian embryo to experimental manipulation in vivo, regulation of these events is still not well understood. Both in the yolk sac and within the embryo proper, prospective hematovascular mesoderm differentiates in the vicinity of endodermal tissues. Here we describe an embryonic explant culture system that permits the dissection of mesodermal and endodermal contributions to hematopoietic and endothelial cell formation during gastrulation. This system can be used to assay for soluble or endodermal cell-associated molecules involved in mediating critical interactions between mesoderm and endoderm in the formation of the hematopoietic and endothelial lineages during embryonic development.

Finding the Hematopoietic Stem Cell Niche in the Placenta

The origin of definitive hematopoiesis poses a fundamental biological question. In this issue of Developmental Cell, two groups have independently found a novel hematopoietic stem cell (HSC) niche in the extraembryonic placenta, in addition to previously identified alternative locations of hematopoiesis at different developmental stages.

The cell cycle associated protein, HTm4, is expressed in differentiating cellsof the hematopoietic and central nervous system in mice

HTm4 is a member of a newly defined family of human and murine proteins, the MS4 (membrane-spanning four) protein group, which has a distinctive four-transmembrane structure. MS4 protein functions include roles as cell surface signaling receptors and intracellular adapter proteins. We have previously demonstrated that HTm4 regulates the function of the KAP phosphatase, a key regulator of cell cycle progression. In humans, the expression of HTm4 is largely restricted to cells of the hematopoietic lineage, possibly reflecting a causal role for this molecule in differentiation/proliferation of hematopoietic lineage cells. In this study, we show that, like the human homologue, murine HTm4 is also predominantly a hematopoietic protein with distinctive expression patterns in developing murine embryos and in adult animals. In addition, we observed that murine HTm4 is highly expressed in the developing and adult murine nervous system, suggesting a previously unrecognized role in central and peripheral nervous system development.

[AGM region and hematopoiesis during ontogeny--review]

During mammalian ontogeny, hematopoietic activity can be found in distinct anatomical sites, which con-tribute to primitive or definite hematopoiesis. The origin of the hematopoietic stem cell (HSC) has been a controversial issue in the field of hematopoiesis. It has long been believed that the origin derives from the extra-embryonic yolk sac. However, there is now considerable evidence that the first adult repopulating HSC is autonomously generated from a distinct region within the embryonic mesoderm, the aorta-gonad-mesonephros (AGM) region. This review describes the origin and precise location of HSC in the embryo and in AGM region, the hematopoietic microenvironment and the hematopoietic regulatory mechanisms in AGM region.

Genetic programs regulating HSC specification, maintenance and expansion

All mature blood cells originate from a small population of self-renewing pluripotent hematopoietic stem cells (HSCs). The capacity to self-renew characterizes all stem cells, whether normal or neoplastic. Interestingly, recent studies suggest that self-renewal is essential for tumor cell maintenance, implicating that this process has therapeutic relevance. Unfortunately, the molecular bases for self-renewal of vertebrate cells remain poorly defined. This article will focus on the developmental mechanisms underlying fetal and adult HSC homeostasis. Specifically, distinctions between genetic programs regulating HSC specification (identity), self-renewal (in both fetal and adult) and differentiation/commitment will be discussed with a special emphasis on transcriptional and chromatin regulators.

Generation of HSCs in the embryo and assays to detect them

The precise temporal and spatial emergence of hematopoietic stem cells (HSC) in the murine embryo has been somewhat controversial largely due to differences in the assays utilized to demonstrate HSC repopulating ability. One strategy is to determine where and when one can first detect HSC that engraft upon transplantation into lethally irradiated adult mice. However, knowing that the primary sites and patterns of hematopoiesis change during ontogeny, an alternative strategy is to select transplantation models where the recipient subjects more closely mirror the stage of development of the donor cells. In this regard, perhaps the most relevant assay to determine the presence of HSC activity in the early embryo is to transplant the donor cells in utero into recipient embryos. Other recipient models that may permit engraftment of embryonic cells include the use of submyeloablated or genetically HSC deficient newborn mice. Additional informative strategies have included co-culturing embryonic tissues that appear to lack HSC activity, with stromal cells derived from different developmental sites of hematopoiesis to induce HSC emergence, followed by transplantation as a means to determine which embryonic tissues possess HSC potential. This review will highlight some of the various transplantation assays used to identify HSC from embryonic tissues.

Histochemical in situ identification of bovine embryonic blood cells reveals differences to the adult haematopoietic system and suggests a close relationship between haematopoietic stem cells and primordial germ cells

Cryostat sections of bovine embryos of exactly known age (obtained from artificial insemination), ranging from 32 to 60 days post-insemination, were treated with a wide range of antibodies directed against cell surface antigens or lineage-specific factors in order to demonstrate different types of fetal blood cells and their precursors. An antibody specific to bovine c-kit (bk-1) stained not only presumptive haematopoietic stem cells in the dorsal aorta and the embryonic liver, but also a subpopulation of putative primordial germ cells in the gonadal anlage, the latter being further characterised by a positive labelling with the lectins STA, WFA and WGA and a histochemical reaction for alkaline phosphatase. The antibody against CD 45, commonly regarded as a pan-leukocyte marker, reacted in the bovine embryo with different types of blood cells, as well as with presumptive vasculogenetic cells and a subpopulation of putative primordial germ cells. CD 61 immunoreaction proved to be a useful tool for demonstrating megakaryocytopoiesis in the embryonic liver, in addition to the lumen of blood vessels and the mesonephros. Staining with BM-2 was restricted to a single population of medium-sized, round to oval cells, forming small groups within the parenchymal strands of the liver. Characterised furthermore by a U-shaped nucleus, this BM-2-positive cell type apparently represents a developmental stage in the granulopoietic lineage. B-lymphocytopoiesis in the bovine liver was detected with antibodies directed against WC-4 and IgM, but not until day 58 post-insemination. Using antibodies to CD 14, no positive results could be obtained in embryonic tissues, although anti-CD 14-positive macrophages were easily recognised in lymph nodes of adult bovines. The antibody against CD 68, however, identified two populations of primitive macrophages in our samples. One population was located in parenchymal strands of the embryonic liver, probably acting as nursing cells for haematopoietic foci, and the other was observed intravasally in the sinusoids of the liver, most probably representing primitive Kupffer cells.

Circulation and chemotaxis of fetal hematopoietic stem cells

The major site of hematopoiesis transitions from the fetal liver to the spleen and bone marrow late in fetal development. To date, experiments have not been performed to evaluate functionally the migration and seeding of hematopoietic stem cells (HSCs) during this period in ontogeny. It has been proposed that developmentally timed waves of HSCs enter the bloodstream only during distinct windows to seed the newly forming hematopoietic organs. Using competitive reconstitution assays to measure HSC activity, we determined the localization of HSCs in the mid-to-late gestation fetus. We found that multilineage reconstituting HSCs are present at low numbers in the blood at all timepoints measured. Seeding of fetal bone marrow and spleen occurred over several days, possibly while stem cell niches formed. In addition, using dual-chamber migration assays, we determined that like bone marrow HSCs, fetal liver HSCs migrate in response to stromal cell-derived factor-1α (SDF-1α); however, unlike bone marrow HSCs, the migratory response of fetal liver HSCs to SDF-1α is greatly increased in the presence of Steel factor (SLF), suggesting an important role for SLF in HSC homing to and seeding of the fetal hematopoietic tissues. Together, these data demonstrate that seeding of fetal organs by fetal liver HSCs does not require large fluxes of HSCs entering the fetal bloodstream, and that HSCs constitutively circulate at low levels during the gestational period from 12 to 17 days postconception. Newly forming hematopoietic tissues are seeded gradually by HSCs, suggesting initial seeding is occurring as hematopoietic niches in the spleen and bone marrow form and become capable of supporting HSC self-renewal. We demonstrate that fetal and adult HSCs exhibit specific differences in chemotactic behavior. While both migrate in response to SDF-1α, fetal HSCs also respond significantly to the cytokine SLF. In addition, the combination of SDF-1α and SLF results in substantially enhanced migration of fetal HSCs, leading to migration of nearly all fetal HSCs in this assay. This finding indicates the importance of the combined effects of SLF and SDF-1α in the migration of fetal HSCs, and is, to our knowledge, the first demonstration of a synergistic effect of two chemoattractive agents on HSCs.

New results on the migratory behavior of blood cell precursors in the early embryo might be relevant to bone marrow transplants and other clinical therapies

Origins of mammalian hematopoiesis: in vivo paradigms and in vitro models

Though a topic of medical interest for centuries, our understanding of vertebrate hematopoietic or "blood-forming" tissue development has improved greatly only in recent years and given a series of scientific and technical milestones. Key among these observations was the description of procedures that allowed the transplantation of blood-forming activity. Beyond this, other advances include the creation of a variety of knock-out animals (mice and more recently zebrafish), microdissection of embryonic and fetal blood-forming tissues, hematopoietic stem (HSC) and progenitor cell (HPC) colony-forming assays, the discovery of cytokines with defined hematopoietic activities, gene transfer technologies, and the description of lineage-specific surface antigens for the identification and purification of pluripotent and differentiated blood cells. The availability of both murine and human embryonic stem cells (ESC) and the delineation of in vitro systems to direct their differentiation have now been added to this analytical arsenal. Such tools have allowed researchers to interrogate the complex developmental processes behind both primitive (yolk sac or extraembryonic) and definitive (intraembryonic) hematopoietic tissue formation. Using ES cells, we hope to not only gain additional basic insights into hematopoietic development but also to develop platforms for therapeutic use in patients suffering from hematological disease. In this review, we will focus on points of convergence and divergence between murine and human hematopoiesis in vivo and in vitro, and use these observations to evaluate the literature regarding attempts to create hematopoietic tissue from embryonic stem cells, the pitfalls encountered therein, and what challenges remain.

Transplantation and in vivo imaging of multilineage engraftment in zebrafish bloodless mutants

The zebrafish is firmly established as a genetic model for the study of vertebrate blood development. Here we have characterized the blood-forming system of adult zebrafish. Each major blood lineage can be isolated by flow cytometry, and with these lineal profiles, defects in zebrafish blood mutants can be quantified. We developed hematopoietic cell transplantation to study cell autonomy of mutant gene function and to establish a hematopoietic stem cell assay. Hematopoietic cell transplantation can rescue multilineage hematopoiesis in embryonic lethal gata1-/- mutants for over 6 months. Direct visualization of fluorescent donor cells in embryonic recipients allows engraftment and homing events to be imaged in real time. These results provide a cellular context in which to study the genetics of hematopoiesis.

Developmental hemostasis: pro- and anticoagulant systems during childhood

An understanding of developmental hemostasis is pivotal for optimal prevention, diagnosis, and treatment of hemostatic problems during childhood. The development of microassays in the early 1980s enabled researchers to delineate age-dependent features of the coagulation system and to establish reference ranges for healthy children of all age groups, from premature infants to adolescents. Based on the results from these studies, the hemostatic system in the young can be described as evolving, and yet functional, since healthy fetuses, infants, and children do not suffer hemorrhagic nor thromboembolic complications spontaneously or in the presence of minor challenges. Plasma concentrations of most pro- and anticoagulant proteins are decreased throughout childhood but provide an effective hemostatic balance on a lower level compared with adults. The current article describes the development of pro- and anticoagulant systems throughout childhood.

Radiosensitivity of the developing haemopoietic system in mammals and its adult consequences: animal studies

The haemopoietic system in the developing mammal is very sensitive to the damaging effects of ionizing radiation. Epidemiological studies have established a strong association between obstetric exposure to diagnostic radiation and an increase in the incidence of childhood leukaemia and between low dose gamma irradiation during the early fetal period and mental retardation in children. It has been suggested that insufficient oxygen supply to the developing brain due to radiation induced damage to fetal haemopoietic tissue has a role in inducing the severe mental retardation observed in the Japanese children exposed to atom bomb radiation in utero. Experimental studies have shown that X- and gamma irradiation of pregnant mice with <1 Gy during the late organogenesis or fetal period caused chromosome damage and significant depletion in the fetal haemopoietic progenitor cells and led to haematological disorders in the adults. The present paper reviews the experimental findings on the effect of pre-natal irradiation on the fetal haemopoietic system and its long-term consequences.

The beta3 integrin gene is expressed at high levels in the major haematopoietic and lymphoid organs, vascular system, and skeleton during mouse embryo development

Integrins are a family of cell surface molecules that mediate the attachment of cells to the extracellular matrix (ECM). These alphabeta heterodimers are involved in many biological processes. We used northern blotting and in situ hybridization to study the pattern of beta3 integrin gene expression during mouse embryogenesis. Northern blotting detected two species of beta3 mRNA from 7 to 17 days post coitum (dpc). These transcripts were abundant in the adult testis, kidney, liver, spleen, and heart. In situ hybridization experiments detected high levels of beta3 in the major haematopoietic and lymphoid organs: yolk sac, liver, and thymus. Moreover, beta3 transcripts were also detected in the vascular system, where beta3 integrin probably plays a key role in angiogenesis and vasculogenesis. We also detected a hybridization signal in the gut, the bronchioles of the lungs, and the bladder wall. beta3 transcripts were also present in the medullary regions of the adrenal glands and in the developing skeleton. Our study shows that beta3 gene expression is not restricted to the liver and gut during mouse development. We also detected beta3 integrin mRNA in the yolk sac, vessels, lung, bladder, and developing bones. Our data suggest that beta3 integrin plays a key role in many important physiological processes like haematopoiesis, angiogenesis, phagocytosis, and bone resorption.

Prenatal and postnatal myeloid cells demonstrate stepwise progression in the pathogenesis of MLL fusion gene leukemia

The steps to leukemia following an in utero fusion of MLL (HRX, ALL-1) to a partner gene in humans are not known. Introduction of the Mll-AF9 fusion gene into embryonic stem cells results in leukemia in mice with cell-type specificity similar to humans. In this study we used myeloid colony assays, immunophenotyping, and transplantation to evaluate myelopoiesis in Mll-AF9 mice. Colony assays demonstrated that both prenatal and postnatal Mll-AF9 tissues have significantly increased numbers of CD11b(+)/CD117(+)/Gr-1(+/-) myeloid cells, often in compact clusters. The self-renewal capacity of prenatal myeloid progenitors was found to decrease following serial replating of colony-forming cells. In contrast, early postnatal myeloid progenitors increased following replating; however, the enhanced self-renewal of early postnatal myeloid progenitor cells was limited and did not result in long-term cell lines or leukemia in vivo. Unlimited replating, long-term CD11b/Gr-1(+) myeloid cell lines, and the ability to produce early leukemia in vivo in transplantation experiments, were found only in mice with overt leukemia. Prenatal Mll-AF9 tissues had reduced total (mature and progenitor) CD11b/Gr-1(+) cells compared with wild-type tissues. Colony replating, immunophenotyping, and cytochemistry suggest that any perturbation of cellular differentiation from the prenatal stage onward is partial and largely reversible. We describe a novel informative in vitro and in vivo model system that permits study of the stages in the pathogenesis of Mll fusion gene leukemia, beginning in prenatal myeloid cells, progressing to a second stage in the postnatal period and, finally, resulting in overt leukemia in adult animals.

The origin and development of the immune system with a view to stem cell therapy

Careful study of the phylogeny and ontogeny of the three components of the immune system reveals that the macrophage, lymphatic, and hematopoietic systems originate independently of each other. Chronologically, the most ancient is the macrophage system, which arises in the coelomic cavity as mesenchymal ameboid cells having the properties to recognize self from non-self and to ingest foreign particles. The lymphatic system later develops from the endoderm of pharyngeal pouches, where the thymic anlage differentiates. The lymphocytes that originate here seed all lymphatic organs and retain the ability to divide and thereby form multiple colonies (lymphatic nodules) in the respiratory and digestive tract; further diversification of lymphocytes follows after confrontation with antigens. The last component of the immune system to appear is the hematopoietic system, which originates from the splanchnic mesoderm of the yolk sac as hematogenic tissue, containing hemangioblasts. The hematogenic tissue remains attached to the outer wall of the vitelline vessels, which provides an efficient mechanism for introducing the hematogenic tissue into the embryo. In an appropriate microenvironment, the hemangioblasts give rise to sinusoidal endothelium and to hemocytoblasts - the bone marrow stem cells for erythrocytes, myeloid cells, and megakaryocytes. The facts and opinions presented in this article are not in agreement with the currently accepted dogma that a common "hematolymphatic stem cell" localized in the marrow generates all of the cellular components of blood and the immune system.

Erythropoietin and hypoxia inducible factor-1 expression in the mid-trimester human fetus

AIM

Infants born prematurely lack a normal response to anemia and fail to increase erythropoietin (Epo) production despite an apparent need for improved tissue oxygenation. This anemia may involve a deficiency in the fetal and premature kidney to produce Epo. To evaluate fetal Epo production, Epo and hypoxia inducible factor-1 (HIF) mRNA expression was measured in the mid-trimester human fetus.

METHODS

Fetal liver and kidney samples were obtained at 11-22 wk of gestation. RNA was isolated and reverse transcribed from snap-frozen specimens. Epo and HIF cDNA concentrations were determined using real-time polymerase chain reaction (PRISM). Epo cDNA concentrations were standardized to HIF concentrations present in each sample.

RESULTS

HIF concentrations remained constant during gestation in kidney and liver samples. Epo cDNA concentrations in kidney did not change from 12 to 22 wk (8.4 +/- 3.4 fg Epo pg(-1) HIF cDNA, 4.8 +/- 1.4, 2.6 +/- 0.4, and 4.2 +/- 1.8 at 11-14, 15-16, 17-19, and 20-22 wk of gestation, respectively), while Epo cDNA concentrations in liver increased with gestation (74.5 +/- 31.9 fg pg(-1) HIF, 23.8 +/- 6.5, 96.4 +/- 19.2 and 276.1 +/- 28.5 at 12-14, 15-16, 17-19 and 20-22 wk of gestation, respectively, p < 0.05, 20-22 wk of gestation liver samples vs all other gestations). Concentrations were 5-20-fold higher in liver than in kidney in each gestational group (p < 0.01, liver vs kidney).

CONCLUSION

HIF concentrations did not change with gestation in liver or kidney. The human fetal kidney produced approximately 5% of the total Epo mRNA measured during the second trimester. It remains to be determined how Epo production by these tissues is affected by premature birth.

Developmental biology of thrombopoietin in the human fetus and neonate

This review focuses on the biology of thrombopoietin (Tpo) during human development. It summarizes the current understanding on molecular biological aspects of Tpo, cellular effects of Tpo on fetal and neonatal megakaryopoiesis, regulation of Tpo production, and circulating Tpo concentrations in human fetuses and neonates. Some important aspects on the developmental biology of Tpo are highlighted. They include the finding of high Tpo gene expression in the bone marrow during the onset of medullary hematopoiesis, higher circulating Tpo concentrations in fetuses and neonates than in children or adults, and a higher sensitivity of neonatal megakaryocyte progenitor cells to Tpo. However, other aspects of the developmental biology of Tpo are incompletely understood.

CONCLUSION

More carefully designed studies are needed to provide the necessary background for an optimal treatment of fetal and neonatal thrombocytopenia.

Embryonic hematopoiesis in mice and humans

AIM

The aim of this report is to review briefly the ontogeny of hematopoiesis in mice and humans and to discuss recent evidence for an intraembryonic source of hematopoietic stem cells.

METHODS

The hematopoiesis overview summarizes information present in the PubMed online database and from experiments conducted in our laboratories.

RESULTS

The major sites of hematopoiesis change throughout development in mice and humans. Recent evidence suggests that hematopoietic cells may emerge from mesoderm precursors within the embryo as well as in the yolk sac.

CONCLUSION

The ontogeny of hematopoiesis is similar in mice and humans. The murine system is a useful model to study the earliest events involved in forming hematopoietic stem cells.