Hematopoietic induction and respecification of A-P identity by visceral endoderm signaling in the mouse embryo

Extra-embryonic mesoderm during development and in in vitro models

Extra-embryonic tissues provide protection and nutrition in vertebrates, as well as a connection to the maternal tissues in mammals. The extra-embryonic mesoderm is an essential and understudied germ layer present in amniotes. It is involved in hematopoiesis, as well as in the formation of extra-embryonic structures such as the amnion, umbilical cord and placenta. The origin and specification of extra-embryonic mesoderm are not entirely conserved across species, and the molecular mechanisms governing its formation and function are not fully understood. This Review begins with an overview of the embryonic origin and function of extra-embryonic mesoderm in vertebrates from in vivo studies. We then compare in vitro models that generate extra-embryonic mesoderm-like cells. Finally, we discuss how insights from studying both embryos and in vitro systems can aid in designing even more advanced stem cell-based embryo models.

Targeted Deletion in the Basal Body Protein Talpid3 Leads to Loss of Primary Cilia in Embryonic Stem Cells and Defective Lineage-Specific Differentiation

Talpid3 is a basal body protein required for the formation of primary cilia, an organelle involved in signal transduction. Here, we asked if Talpid3 has a role in the regulation of differentiation and/or self-renewal of ES cells and whether cells lacking cilia due to a deletion in Talpid3 can be reprogrammed to induced pluripotent stem (iPS) cells. We show that mouse embryonic limb fibroblasts which lack primary cilia with a targeted deletion in the Talpid3 (Ta3) gene can be efficiently reprogrammed to iPS cells. Furthermore, vector-free Ta3-/- iPS cells retain ES cell features and are able to self-renew. However, both Ta3-/- iPS and ES cells are unable to form visceral endoderm and differentiate poorly into neurons. The observed defects are not a consequence of reprogramming since Ta3-/- ES cells also exhibit this phenotype. Thus, Talpid3 and primary cilia are required for some differentiation events but appear to be dispensable for stem cell self-renewal and reprogramming.

Erythropoiesis in the mammalian embryo

Red blood cells (RBCs) comprise a critical component of the cardiovascular network, which constitutes the first functional organ system of the developing mammalian embryo. Examination of circulating blood cells in mammalian embryos revealed two distinct types of erythroid cells: large, nucleated "primitive" erythroblasts followed by smaller, enucleated "definitive" erythrocytes. This review describes the current understanding of primitive and definitive erythropoiesis gleaned from studies of mouse and human embryos and induced pluripotent stem cells (iPSCs). Primitive erythropoiesis in the mouse embryo comprises a transient wave of committed primitive erythroid progenitors (primitive erythroid colony-forming cells, EryP-CFC) in the early yolk sac that generates a robust cohort of precursors that mature in the bloodstream and enucleate. In contrast, definitive erythropoiesis has two distinct developmental origins. The first comprises a transient wave of definitive erythroid progenitors (burst-forming units erythroid, BFU-E) that emerge in the yolk sac and seed the fetal liver where they terminally mature to provide the first definitive RBCs. The second comprises hematopoietic stem cell (HSC)-derived BFU-E that terminally mature at sites colonized by HSCs particularly the fetal liver and subsequently the bone marrow. Primitive and definitive erythropoiesis are derived from endothelial identity precursors with distinct developmental origins. Although they share prototypical transcriptional regulation, primitive and definitive erythropoiesis are also characterized by distinct lineage-specific factors. The exquisitely timed, sequential production of primitive and definitive erythroid cells is necessary for the survival and growth of the mammalian embryo.

The autotaxin-LPA axis promotes membrane trafficking and secretion in yolk sac visceral endoderm cells

Autotaxin, encoded by the Enpp2 gene, is an exoenzyme that produces lysophosphatidic acid, thereby regulating many biologic functions. We previously reported that Enpp2 mRNA was abundantly expressed in yolk sac visceral endoderm (VE) cells and that Enpp2-/- mice were lethal at embryonic day 9.5 owing to angiogenic defects in the yolk sac. Enpp2-/- mice showed lysosome fragmentation in VE cells and embryonic abnormalities including allantois malformation, neural tube defects, no axial turning, and head cavity formation. However, whether the defects in endocytic vesicle formation affect membrane trafficking in VE cells remained to be directly examined. In this study, we found that pinocytosis, transcytosis, and secretion of angiogenic factors such as vascular endothelial growth factor and transforming growth factor β1 were impaired in Enpp2-/- VE cells. Moreover, pharmacologic inhibition of membrane trafficking phenocopied the defects of Enpp2-/- mice. These findings demonstrate that Enpp2 promotes endocytosis and secretion of angiogenic factors in VE cells, thereby regulating angiogenesis/vasculogenesis and embryonic development.

Embryonic and extraembryonic tissues during mammalian development: shifting boundaries in time and space

The first few days of embryonic development in eutherian mammals are dedicated to the specification and elaboration of the extraembryonic tissues. However, where the fetus ends and its adnexa begins is not always as self-evident during the early stages of development, when the definitive body axes are still being laid down, the germ layers being specified and a discrete form or bodyplan is yet to emerge. Function, anatomy, histomorphology and molecular identities have been used through the history of embryology, to make this distinction. In this review, we explore them individually by using specific examples from the early embryo. While highlighting the challenges of drawing discrete boundaries between embryonic and extraembryonic tissues and the limitations of a binary categorization, we discuss how basing such identity on fate is the most universal and conceptually consistent. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

The mouse allantois: new insights at the embryonic-extraembryonic interface

During the early development of Placentalia, a distinctive projection emerges at the posterior embryonic-extraembryonic interface of the conceptus; its fingerlike shape presages maturation into the placental umbilical cord, whose major role is to shuttle fetal blood to and from the chorion for exchange with the mother during pregnancy. Until recently, the biology of the cord's vital vascular anlage, called the body stalk/allantois in humans and simply the allantois in rodents, has been largely unknown. Here, new insights into the development of the mouse allantois are featured, from its origin and mechanism of arterial patterning through its union with the chorion. Key to generating the allantois and its critical functions are the primitive streak and visceral endoderm, which together are sufficient to create the entire fetal-placental connection. Their newly discovered roles at the embryonic-extraembryonic interface challenge conventional wisdom, including the physical limits of the primitive streak, its function as sole purveyor of mesoderm in the mouse, potency of visceral endoderm, and the putative role of the allantois in the germ line. With this working model of allantois development, understanding a plethora of hitherto poorly understood orphan diseases in humans is now within reach. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Hormone regulation of thrombospondin-1 mRNA in porcine granulosa cells in vitro

Thrombospondin-1 (THBS1) is involved in the process of angiogenesis and is down-regulated by insulin-like growth factor 1 (IGF1) in porcine granulosa cells (GC), but what other hormones regulate GC THBS1 and its role in follicular growth is unclear. Thus, six experiments were conducted to determine the influence of other hormones on THBS1 gene expression in porcine GC, and to determine if THBS1 mRNA changes during follicular development. For Exp. 1-5, small (1-5 mm) follicles from ovaries of abattoir gilts were aspirated, GC collected and treated with FSH, IGF1, fibroblast growth factor 9 (FGF9), Sonic hedgehog (SHH), estradiol, cortisol, and/or prostaglandin E2 (PGE2). FSH, IGF1 and FGF9 each decreased (P < 0.05) THBS1 mRNA abundance. Alone, PGE2 increased (P < 0.05) THBS1 mRNA abundance. PGE2 significantly attenuated the FSH-induced inhibition of THBS1 mRNA expression. Estradiol, cortisol, and SHH had no effect on THBS1 mRNA abundance. In Exp. 6, small (1-3 mm), medium (4-6 mm) and large (7-14 mm) follicles were aspirated to measure abundance of THBS1 mRNA in GC which did not differ (P > 0.10) between small and medium-sized follicles but was threefold greater (P < 0.05) in large compared to small or medium follicles. We hypothesize that the inhibitory effects of FSH, IGF1 and FGF9 on the antiangiogenic gene THBS1 could contribute to promoting angiogenesis in the developing follicle, while stimulation of THBS1 mRNA by PGE2 may help reduce angiogenesis during the preovulatory period when PGE2 and THBS1 mRNA are at their greatest levels.

Gastruloids as in vitro models of embryonic blood development with spatial and temporal resolution

Gastruloids are three-dimensional embryonic organoids that reproduce key features of early mammalian development in vitro with unique scalability, accessibility, and spatiotemporal similarity to real embryos. Recently, we adapted the gastruloid culture conditions to promote cardiovascular development. In this work, we extended these conditions to capture features of embryonic blood development through a combination of immunophenotyping, detailed transcriptomics analysis, and identification of blood stem/progenitor cell potency. We uncovered the emergence of blood progenitor and erythroid-like cell populations in late gastruloids and showed the multipotent clonogenic capacity of these cells, both in vitro and after transplantation into irradiated mice. We also identified the spatial localization near a vessel-like plexus in the anterior portion of gastruloids with similarities to the emergence of blood stem cells in the mouse embryo. These results highlight the potential and applicability of gastruloids to the in vitro study of complex processes in embryonic blood development with spatiotemporal fidelity.

The fetal lineage is susceptible to Zika virus infection within days of fertilization

Adults contracting Zika virus (ZIKV) typically exhibit mild symptoms, yet ZIKV infection of pregnant individuals can cause miscarriage or birth defects in their offspring. Many studies have focused on maternal-to-fetal ZIKV transmission via blood and placenta. Notably, however, ZIKV is also transmitted sexually, raising the possibility that ZIKV could infect the embryo shortly after fertilization, long before the placenta is established. Here, we evaluate the consequences of ZIKV infection in mouse embryos during the first few days of embryogenesis. We show that divergent strains of ZIKV can infect the fetal lineage and can cause developmental arrest, raising concern for the developmental consequences of sexual ZIKV transmission.

This article has an associated ‘The people behind the papers’ interview.

Summary: Mouse preimplantation embryos are vulnerable to Zika virus-induced lethality even in the presence of the zona pellucida, highlighting a potential risk of sexually transmitted infection in early pregnancy.

Novel insights into alcoholic liver disease: Iron overload, iron sensing and hemolysis

The liver is the major target organ of continued alcohol consumption at risk and resulting alcoholic liver disease (ALD) is the most common liver disease worldwide. The underlying molecular mechanisms are still poorly understood despite decades of scientific effort limiting our abilities to identify those individuals who are at risk to develop the disease, to develop appropriate screening strategies and, in addition, to develop targeted therapeutic approaches. ALD is predestined for the newly evolving translational medicine, as conventional clinical and health care structures seem to be constrained to fully appreciate this disease. This concept paper aims at summarizing the 15 years translational experience at the Center of Alcohol Research in Heidelberg, namely based on the long-term prospective and detailed characterization of heavy drinkers with mortality data. In addition, novel experimental findings will be presented. A special focus will be the long-known hepatic iron accumulation, the somewhat overlooked role of the hematopoietic system and novel insights into iron sensing and the role of hepcidin. Our preliminary work indicates that enhanced red blood cell (RBC) turnover is critical for survival in ALD patients. RBC turnover is not primarily due to vitamin deficiency but rather to ethanol toxicity directly targeted to erythrocytes but also to the bone marrow stem cell compartment. These novel insights also help to explain long-known aspects of ALD such as mean corpuscular volume of erythrocytes (MCV) and elevated aspartate transaminase (GOT/AST) levels. This work also aims at identifying future projects, naming unresolved observations, and presenting novel hypothetical concepts still requiring future validation.

Strategies for Regenerative Vascular Tissue Engineering

Vascularization remains one of the key challenges in creating functional tissue-engineered constructs for therapeutic applications. This review aims to provide a developmental lens on the necessity of blood vessels in defining tissue function while exploring stem cells as a suitable source for vascular tissue engineering applications. The intersections of stem cell biology, material science, and engineering are explored as potential solutions for directing vascular assembly.

Nuclear Progesterone Receptor Expressed by the Cortical Thymic Epithelial Cells Dictates Thymus Involution in Murine Pregnancy

Progesterone is a gonadal pro-gestational hormone that is absolutely necessary for the success of pregnancy. Most notable actions of progesterone are observed in the female reproductive organs, the uterus and the ovary. Acting through the nuclear progesterone receptor (PGR), progesterone prepares the endometrium for implantation of the embryo. Interestingly, the maternal thymus also is a known expressor of Pgr; its absence is associated with murine pregnancy complications. However, the localization of its expression and its functional importance were not known. Here, we used a transgenic dual fluorescent reporter mouse model and genetic deletion of Pgr in Foxn1+ thymic epithelial cells (TEC) to demonstrate TEC-specific Pgr expression in pregnancy, especially in the cortex where thymocyte maturation occurs. Using our TEC-specific Pgr deletion mouse model, we demonstrate that TEC-specific Pgr is necessary for pregnancy-induced thymic involution in pregnancy. Our investigation reveals that PGR expression is upregulated in the cortical thymic epithelial cells during pregnancy, and that PGR expression is important for thymic involution during murine pregnancy.

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.

Specification and role of extraembryonic endoderm lineages in the periimplantation mouse embryo

During mammalian embryo development, the correct formation of the first extraembryonic endoderm lineages is fundamental for successful development. In the periimplantation blastocyst, the primitive endoderm (PrE) is formed, which gives rise to the parietal endoderm (PE) and visceral endoderm (VE) during further developmental stages. These PrE-derived lineages show significant differences in both their formation and roles. Whereas differentiation of the PE as a migratory lineage has been suggested to represent the first epithelial-to-mesenchymal transition (EMT) in development, organisation of the epithelial VE is of utmost importance for the correct axis definition and patterning of the embryo. Despite sharing a common origin, the striking differences between the VE and PE are indicative of their distinct roles in early development. However, there is a significant disparity in the current knowledge of each lineage, which reflects the need for a deeper understanding of their respective specification processes. In this review, we will discuss the origin and maturation of the PrE, PE, and VE during the periimplantation period using the mouse model as an example. Additionally, we consider the latest findings regarding the role of the PrE-derived lineages and early embryo morphogenesis, as obtained from the most recent in vitro models.

Embryonal erythropoiesis and aging exploit ferroptosis

Ferroptosis is a form of regulated cell necrosis, as a consequence of Fe(II)-dependent lipid peroxidation. Although ferroptosis has been linked to cancer cell death, neurodegeneration and reperfusion injury, physiological roles of ferroptosis have not been elucidated to date mostly due to the lack of appropriate methodologies. Here, we show that 4-hydroxy-2-nonenal (HNE)-modified proteins detected by a HNEJ-1 mouse monoclonal antibody is a robust immunohistochemical technology to locate ferroptosis in tissues in combination with morphological nuclear information, based on various models of ferroptosis, including erastin-induced cysteine-deprivation, conditional Gpx4 knockout and Fe(II)-dependent renal tubular injury, as well as other types of regulated cell death. Specificity of HNEJ-1 with ferroptosis was endorsed by non-selective identification of HNE-modified proteins in an Fe(II)-dependent renal tubular injury model. We further comprehensively searched for signs of ferroptosis in different developmental stages of Fischer-344 rats from E9.5-2.5 years of age. We observed that there was a significant age-dependent increase in ferroptosis in the kidney, spleen, liver, ovary, uterus, cerebellum and bone marrow, which was accompanied by iron accumulation. Not only phagocytic cells but also parenchymal cells were affected. Epidermal ferroptosis in ageing SAMP8 mice was significantly promoted by high-fat or carbohydrate-restricted diets. During embryogenesis of Fischer-344 rats, we found ferroptosis in nucleated erythrocytes at E13.5, which disappeared in enucleated erythrocytes at E18.5. Administration of a ferroptosis inhibitor, liproxstatin-1, significantly delayed erythrocyte enucleation. Therefore, our results demonstrate for the first time the involvement of ferroptosis in physiological processes, such as embryonic erythropoiesis and aging, suggesting the evolutionally acquired mechanism and the inevitable side effects, respectively.

Advances in understanding the crosstalk between mother and fetus on iron utilization

A full-term pregnancy comes with significant demand for iron. Not meeting this demand has adverse effects on maternal health and on the intrauterine and postnatal development of the infant. In the infant, some of these adverse effects cannot be reversed by postnatal iron supplementation, highlighting the need to tackle iron deficiency in utero. Achieving this requires sound understanding of the pathways that govern iron transfer at the fetomaternal interface. Two pathways are emerging as key players in this context; the hepcidin/ferroportin axis pathway and the iron regulatory protein (IRPs) pathway. In late gestation, suppression of maternal hepcidin, by as yet unknown factors, is required for increasing iron availability to the growing fetus. In the placenta, the rate of iron uptake by transferrin receptor TfR1 at the apical/maternal side and of iron release by ferroportin FPN at the basal/fetal side is controlled by IRP1. In fetal hepatocytes, build up of fetal iron stores requires post-translational inhibition of FPN by the cell-autonomous action of hepcidin. In the fetal liver, FPN is also subject to additional control at the transcriptional level, possibly by the action of hypoxia-inducible factor HIF2α. The rates of apical iron uptake and basal iron release in the placenta are modulated according to iron availability in the maternal blood and the placenta's own needs. This placental modulation ensures that the amount of iron delivered to the fetal circulation is maintained within a normal range, even in the face of mild maternal iron deficiency or overload. However, when maternal iron deficiency or overload are extreme, placental modulation is not sufficient to maintain normal iron supply to the fetus, resulting in fetal iron deficiency and overload respectively. Thus, the rate of iron transfer at the fetomaternal interface is subject to several regulatory signals operating simultaneously in the maternal liver, the placenta and the fetal liver. These regulatory signals act in concert to maintain normal iron supply to the fetus within a wide range of maternal iron states, but fail to do so when maternal iron deficiency or overload are extreme. The limitations of existing experimental models must be overcome if we are to gain better understanding of the role of these regulatory signals in normal and complicated pregnancy. Ultimately, that understanding could help identify better markers of fetal iron demand and underpin novel iron replacement strategies to treat maternal and fetal iron deficiency.

The transcription factor Rreb1 regulates epithelial architecture, invasiveness, and vasculogenesis in early mouse embryos

Ras-responsive element-binding protein 1 (Rreb1) is a zinc-finger transcription factor acting downstream of RAS signaling. Rreb1 has been implicated in cancer and Noonan-like RASopathies. However, little is known about its role in mammalian non-disease states. Here, we show that Rreb1 is essential for mouse embryonic development. Loss of Rreb1 led to a reduction in the expression of vasculogenic factors, cardiovascular defects, and embryonic lethality. During gastrulation, the absence of Rreb1 also resulted in the upregulation of cytoskeleton-associated genes, a change in the organization of F-ACTIN and adherens junctions within the pluripotent epiblast, and perturbed epithelial architecture. Moreover, Rreb1 mutant cells ectopically exited the epiblast epithelium through the underlying basement membrane, paralleling cell behaviors observed during metastasis. Thus, disentangling the function of Rreb1 in development should shed light on its role in cancer and other diseases involving loss of epithelial integrity.

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.

The mouse fetal-placental arterial connection: A paradigm involving the primitive streak and visceral endoderm with implications for human development

In Placentalia, the fetus depends upon an organized vascular connection with its mother for survival and development. Yet, this connection was, until recently, obscure. Here, we summarize how two unrelated tissues, the primitive streak, or body axis, and extraembryonic visceral endoderm collaborate to create and organize the fetal-placental arterial connection in the mouse gastrula. The primitive streak reaches into the extraembryonic space, where it marks the site of arterial union and creates a progenitor cell pool. Through contact with the streak, associated visceral endoderm undergoes an epithelial-to-mesenchymal transition, contributing extraembryonic mesoderm to the placental arterial vasculature, and to the allantois, or pre-umbilical tissue. In addition, visceral endoderm bifurcates into the allantois where, with the primitive streak, it organizes the nascent umbilical artery and promotes allantoic elongation to the chorion, the site of fetal-maternal exchange. Brachyury mediates streak extension and vascular patterning, while Hedgehog is involved in visceral endoderm's conversion to mesoderm. A unique CASPASE-3-positive cell separates streak- and non-streak-associated domains in visceral endoderm. Based on these new insights at the posterior embryonic-extraembryonic interface, we conclude by asking whether so-called primordial germ cells are truly antecedents to the germ line that segregate within the allantois, or whether they are placental progenitor cells. Incorporating these new working hypotheses into mutational analyses in which the placentae are affected will aid understanding a spectrum of disorders, including orphan diseases, which often include abnormalities of the umbilical cord, yolk sac, and hindgut, whose developmental relationship to each other has, until now, been poorly understood. This article is categorized under: Birth Defects > Associated with Preimplantation and Gastrulation Early Embryonic Development > Gastrulation and Neurulation.

Lymphatic Tissue Engineering and Regeneration

The lymphatic system is a major circulatory system within the body, responsible for the transport of interstitial fluid, waste products, immune cells, and proteins. Compared to other physiological systems, the molecular mechanisms and underlying disease pathology largely remain to be understood which has hindered advancements in therapeutic options for lymphatic disorders. Dysfunction of the lymphatic system is associated with a wide range of disease phenotypes and has also been speculated as a route to rescue healthy phenotypes in areas including cardiovascular disease, metabolic syndrome, and neurological conditions. This review will discuss lymphatic system functions and structure, cell sources for regenerating lymphatic vessels, current approaches for engineering lymphatic vessels, and specific therapeutic areas that would benefit from advances in lymphatic tissue engineering and regeneration.

Effect of experimentally induced hypothyroidism during gestation period on activity dependent neurotrophic factor (ADNF) in newborn rat brain tissue

Purpose The aim of the study was to evaluate the effects of prenatal hypothyroidism on neonatal rats by the way of activity-dependent neuroprotective factor (ADNF) expression. Methods Twenty-one Wistar albino neonatal rats were divided into two subgroups; a control group and neonatal rats with experimental maternal hypothyroidism. Hypothyroidism was induced by using propylthiouracil (PTU). Neonatal rats obtained PTU from breast milk continuously for 1 week after birth. The rats from the control group were fed only normal feed and water. After birth, body weight and blood thyroid hormone levels were tested. Glial fibrillary acidic protein (GFAP), Slug, Numb, Notch-1 and ADNF antibodies were used for immunohistochemical analysis. Real-time polymerase chain reaction (RT-PCR) and Western blotting analyses were used to evaluate ADNF gene expression levels from 1-week-old rat's brain. Results There was no difference between the two groups for birth weights. The thyroxine (T4) level from the experimental group was <0.4 ng/mL, and it was 0.8 ng/mL for the control group. It was shown that, the results from the experimental group samples had significantly lower ADNF mRNA levels than control group (p < 0.05). The increase from GFAP and Numb expression and decrease from Slug expression were shown in the experimental group. Local differences were identified for ADNF and a decrease was shown in both sides of brain. There was no difference for Notch-1 expression for both groups. Conclusion In this study, decreasing ADNF expression might contribute to developing neurological problems in congenital hypothyroidism.

Functional Heterogeneity of Endothelial Cells Derived from Human Pluripotent Stem Cells

Specification of endothelial cells (ECs) into arterial, venous, and lymphatic cells is a crucial process of vascular development, and expanding our knowledge about EC specification from human pluripotent stem cells (hPSCs) will aid the design of optimal strategies for producing desired types of ECs for therapies. In our prior studies, we revealed that hPSC-derived VE-cadherin(V)⁺CD31⁺CD34⁺ ECs are heterogeneous and include at least three major subsets with distinct hemogenic properties: V⁺CD43/235a^-CD73^- hemogenic endothelial progenitors (HEPs), V⁺CD43^loCD235a⁺73^- angiogenic hematopoietic progenitors (AHPs), and V⁺CD43/235a^-73⁺ non-HEPs. In this study, using angiogenesis assays, we demonstrated that ECs within these subsets have distinct endothelial colony- and tube-forming properties, proliferative and migratory properties, and endothelial nitric oxide synthase and inflammatory cytokine production potentials. Culture of isolated subsets in arterial, venous, and lymphatic conditions revealed that AHPs are skewed toward lymphatic, HEPs toward arterial, and non-HEPs toward venous differentiation in vitro. These findings suggest that selection and enhancement of production of a particular EC subset may aid in generating desirable EC populations with arterial, venous, or lymphatic properties from hPSCs.

Endothelial Cell Metabolism

Endothelial cells (ECs) are more than inert blood vessel lining material. Instead, they are active players in the formation of new blood vessels (angiogenesis) both in health and (life-threatening) diseases. Recently, a new concept arose by which EC metabolism drives angiogenesis in parallel to well-established angiogenic growth factors (e.g., vascular endothelial growth factor). 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3-driven glycolysis generates energy to sustain competitive behavior of the ECs at the tip of a growing vessel sprout, whereas carnitine palmitoyltransferase 1a-controlled fatty acid oxidation regulates nucleotide synthesis and proliferation of ECs in the stalk of the sprout. To maintain vascular homeostasis, ECs rely on an intricate metabolic wiring characterized by intracellular compartmentalization, use metabolites for epigenetic regulation of EC subtype differentiation, crosstalk through metabolite release with other cell types, and exhibit EC subtype-specific metabolic traits. Importantly, maladaptation of EC metabolism contributes to vascular disorders, through EC dysfunction or excess angiogenesis, and presents new opportunities for anti-angiogenic strategies. Here we provide a comprehensive overview of established as well as newly uncovered aspects of EC metabolism.

XEN and the Art of Stem Cell Maintenance: Molecular Mechanisms Maintaining Cell Fate and Self-Renewal in Extraembryonic Endoderm Stem (XEN) Cell Lines

The extraembryonic endoderm is one of the first cell types specified during mammalian development. This extraembryonic lineage is known to play multiple important roles throughout mammalian development, including guiding axial patterning and inducing formation of the first blood cells during embryogenesis. Moreover, recent studies have uncovered striking conservation between mouse and human embryos during the stages when extraembryonic endoderm cells are first specified, in terms of both gene expression and morphology. Therefore, mouse embryos serve as an excellent model for understanding the pathways that maintain extraembryonic endoderm cell fate. In addition, self-renewing multipotent stem cell lines, called XEN cells, have been derived from the extraembryonic endoderm of mouse embryos. Mouse XEN cell lines provide an additional tool for understanding the basic mechanisms that contribute to maintaining lineage potential, a resource for identifying how extraembryonic ectoderm specifies fetal cell types, and serve as a paradigm for efforts to establish human equivalents. Given the potential conservation of essential extraembryonic endoderm roles, human XEN cells would provide a considerable advance. However, XEN cell lines have not yet been successfully derived from human embryos. Given the potential utility of human XEN cell lines, this chapter focuses on reviewing the mechanisms known to govern the stem cell properties of mouse XEN, in hopes of facilitating new ways to establish human XEN cell lines.

Visceral endoderm and the primitive streak interact to build the fetal-placental interface of the mouse gastrula

Hypoblast/visceral endoderm assists in amniote nutrition, axial positioning and formation of the gut. Here, we provide evidence, currently limited to humans and non-human primates, that hypoblast is a purveyor of extraembryonic mesoderm in the mouse gastrula. Fate mapping a unique segment of axial extraembryonic visceral endoderm associated with the allantoic component of the primitive streak, and referred to as the "AX", revealed that visceral endoderm supplies the placentae with extraembryonic mesoderm. Exfoliation of the AX was dependent upon contact with the primitive streak, which modulated Hedgehog signaling. Resolution of the AX's epithelial-to-mesenchymal transition (EMT) by Hedgehog shaped the allantois into its characteristic projectile and individualized placental arterial vessels. A unique border cell separated the delaminating AX from the yolk sac blood islands which, situated beyond the limit of the streak, were not formed by an EMT. Over time, the AX became the hindgut lip, which contributed extensively to the posterior interface, including both embryonic and extraembryonic tissues. The AX, in turn, imparted antero-posterior (A-P) polarity on the primitive streak and promoted its elongation and differentiation into definitive endoderm. Results of heterotopic grafting supported mutually interactive functions of the AX and primitive streak, showing that together, they self-organized into a complete version of the fetal-placental interface, forming an elongated structure that exhibited A-P polarity and was composed of the allantois, an AX-derived rod-like axial extension reminiscent of the embryonic notochord, the placental arterial vasculature and visceral endoderm/hindgut.

Tril targets Smad7 for degradation to allow hematopoietic specification in Xenopus embryos

In Xenopus laevis, bone morphogenetic proteins (Bmps) induce expression of the transcription factor Gata2 during gastrulation, and Gata2 is required in both ectodermal and mesodermal cells to enable mesoderm to commit to a hematopoietic fate. Here, we identify tril as a Gata2 target gene that is required in both ectoderm and mesoderm for primitive hematopoiesis to occur. Tril is a transmembrane protein that functions as a co-receptor for Toll-like receptors to mediate innate immune responses in the adult brain, but developmental roles for this molecule have not been identified. We show that Tril function is required both upstream and downstream of Bmp receptor-mediated Smad1 phosphorylation for induction of Bmp target genes. Mechanistically, Tril triggers degradation of the Bmp inhibitor Smad7. Tril-dependent downregulation of Smad7 relieves repression of endogenous Bmp signaling during gastrulation and this enables mesodermal progenitors to commit to a blood fate. Thus, Tril is a novel component of a Bmp-Gata2 positive-feedback loop that plays an essential role in hematopoietic specification.

Stress-Induced Premature Senescence of Endothelial and Endothelial Progenitor Cells

This brief overview of premature senescence of dysfunctional endothelial and endothelial progenitor cells provides information on endothelial cell differentiation and specialization, their ontogeny, and controversies related to endothelial stem and progenitor cells. Stressors responsible for the dysfunction of endothelial and endothelial progenitor cells, as well as cellular mechanisms and consequences of endothelial cell dysfunction are presented. Metabolic signatures of dysfunctional endothelial cells and senescence pathways are described. Emerging strategies to rejuvenate endothelial and endothelial progenitor cells conclude the review.

Differentiation of functional endothelial cells from human induced pluripotent stem cells: A novel, highly efficient and cost effective method

Endothelial cells derived from human induced pluripotent stem cells (hiPSC- EC) are of significant value for research on human vascular development, in vitro disease models and drug screening. Here we report an alternative, highly efficient and cost-effective simple three step method (mesoderm induction, endothelial cell differentiation and endothelial cell expansion) to differentiate hiPSC directly into endothelial cells. We demonstrate that efficiency of described method to derive CD31+ and VE-Cadherin+ double positive cells is higher than 80% in 12 days. Most notably we established that hiPSC-EC differentiation efficacy depends on optimization of both mesoderm differentiation and endothelial cell differentiation steps.

Development and differentiation of the erythroid lineage in mammals

The red blood cell (RBC) is responsible for performing the highly specialized function of oxygen transport, making it essential for survival during gestation and postnatal life. Establishment of sufficient RBC numbers, therefore, has evolved to be a major priority of the postimplantation embryo. The "primitive" erythroid lineage is the first to be specified in the developing embryo proper. Significant resources are dedicated to producing RBCs throughout gestation. Two transient and morphologically distinct waves of hematopoietic progenitor-derived erythropoiesis are observed in development before hematopoietic stem cells (HSCs) take over to produce "definitive" RBCs in the fetal liver. Toward the end of gestation, HSCs migrate to the bone marrow, which becomes the primary site of RBC production in the adult. Erythropoiesis is regulated at various stages of erythroid cell maturation to ensure sufficient production of RBCs in response to physiological demands. Here, we highlight key aspects of mammalian erythroid development and maturation as well as differences among the primitive and definitive erythroid cell lineages.

BMP and Hedgehog Regulate Distinct AGM Hematopoietic Stem Cells Ex Vivo

Hematopoietic stem cells (HSC), the self-renewing cells of the adult blood differentiation hierarchy, are generated during embryonic stages. The first HSCs are produced in the aorta-gonad-mesonephros (AGM) region of the embryo through endothelial to a hematopoietic transition. BMP4 and Hedgehog affect their production and expansion, but it is unknown whether they act to affect the same HSCs. In this study using the BRE GFP reporter mouse strain that identifies BMP/Smad-activated cells, we find that the AGM harbors two types of adult-repopulating HSCs upon explant culture: One type is BMP-activated and the other is a non-BMP-activated HSC type that is indirectly controlled by Hedgehog signaling through the VEGF pathway. Transcriptomic analyses demonstrate that the two HSC types express distinct but overlapping genetic programs. These results revealing the bifurcation in HSC types at early embryonic stages in the AGM explant model suggest that their development is dependent upon the signaling molecules in the microenvironment.

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AGM explants contain two HSC types, BMP-activated and non-BMP-activated

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Non-BMP-activated HSCs are dependent on Hedgehog/VEGF

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Changes in the microenvironment ex vivo contribute to novel HSC composition

Expression pattern of bcar3, a downstream target of Gata2, and its binding partner, bcar1, during Xenopus development

Primitive hematopoiesis generates red blood cells that deliver oxygen to the developing embryo. Mesodermal cells commit to a primitive blood cell fate during gastrulation and, in order to do so the mesoderm must receive non-cell autonomous signals transmitted from other germ layers. In Xenopus, the transcription factor Gata2 functions in ectodermal cells to generate or transmit the non-cell autonomous signals. Here we have identified Breast Cancer Antiestrogen Resistance 3 (bcar3) as a gene that is induced in ectodermal cells downstream of Gata2. Bcar3 and its binding partner Bcar1 function to transduce integrin signaling, leading to changes in cellular morphology, motility and adhesion. We show that gata2, bcar3 and bcar1 are co-expressed in ventral ectoderm from early gastrula to early tailbud stages. At later stages of development, bcar3 and bcar1 are co-expressed in the spinal cord, notochord, fin mesenchyme and pronephros but each shows additional unique sites of expression. These co-expression and unique expression patterns suggest that Bcar3 and Bcar1 may function together but also independently during Xenopus development.

The DEK Oncoprotein Is a Critical Component of the EKLF/KLF1 Enhancer in Erythroid Cells

Understanding how transcriptional regulators are themselves controlled is important in attaining a complete picture of the intracellular effects that follow signaling cascades during early development and cell-restricted differentiation. We have addressed this issue by focusing on the regulation of EKLF/KLF1, a zinc finger transcription factor that plays a necessary role in the global regulation of erythroid gene expression. Using biochemical affinity purification, we have identified the DEK oncoprotein as a critical factor that interacts with an essential upstream enhancer element of the EKLF promoter and exerts a positive effect on EKLF levels. This element also binds a core set of erythroid transcription factors, suggesting that DEK is part of a tissue-restricted enhanceosome that contains BMP4-dependent and -independent components. Together with local enrichment of properly coded histones and an open chromatin domain, optimal transcriptional activation of the EKLF locus can be established.

GATA2 regulates Wnt signaling to promote primitive red blood cell fate

Primitive erythropoiesis is regulated in a non cell-autonomous fashion across evolution from frogs to mammals. In Xenopus laevis, signals from the overlying ectoderm are required to induce the mesoderm to adopt an erythroid fate. Previous studies in our lab identified the transcription factor GATA2 as a key regulator of this ectodermal signal. To identify GATA2 target genes in the ectoderm required for red blood cell formation in the mesoderm, we used microarray analysis to compare gene expression in ectoderm from GATA2 depleted and wild type embryos. Our analysis identified components of the non-canonical and canonical Wnt pathways as being reciprocally up- and down-regulated downstream of GATA2 in both mesoderm and ectoderm. We show that up-regulation of canonical Wnt signaling during gastrulation blocks commitment to a hematopoietic fate while down-regulation of non-canonical Wnt signaling impairs erythroid differentiation. Our results are consistent with a model in which GATA2 contributes to inhibition of canonical Wnt signaling, thereby permitting progenitors to exit the cell cycle and commit to a hematopoietic fate. Subsequently, activation of non-canonical Wnt signaling plays a later role in enabling these progenitors to differentiate as mature red blood cells.

Making Blood: The Haematopoietic Niche throughout Ontogeny

Approximately one-quarter of all cells in the adult human body are blood cells. The haematopoietic system is therefore massive in scale and requires exquisite regulation to be maintained under homeostatic conditions. It must also be able to respond when needed, such as during infection or following blood loss, to produce more blood cells. Supporting cells serve to maintain haematopoietic stem and progenitor cells during homeostatic and pathological conditions. This coalition of supportive cell types, organised in specific tissues, is termed the haematopoietic niche. Haematopoietic stem and progenitor cells are generated in a number of distinct locations during mammalian embryogenesis. These stem and progenitor cells migrate to a variety of anatomical locations through the conceptus until finally homing to the bone marrow shortly before birth. Under stress, extramedullary haematopoiesis can take place in regions that are typically lacking in blood-producing activity. Our aim in this review is to examine blood production throughout the embryo and adult, under normal and pathological conditions, to identify commonalities and distinctions between each niche. A clearer understanding of the mechanism underlying each haematopoietic niche can be applied to improving ex vivo cultures of haematopoietic stem cells and potentially lead to new directions for transplantation medicine.

Extra-embryonic Wnt3 regulates the establishment of the primitive streak in mice

The establishment of the head to tail axis at early stages of development is a fundamental aspect of vertebrate embryogenesis. In mice, experimental embryology, genetics and expression studies have suggested that the visceral endoderm, an extra-embryonic tissue, plays an important role in anteroposterior axial development. Here we show that absence of Wnt3 in the posterior visceral endoderm leads to delayed formation of the primitive streak and that interplay between anterior and posterior visceral endoderm restricts the position of the primitive streak. Embryos lacking Wnt3 in the visceral endoderm, however, appear normal by E9.5. Our results suggest a model for axial development in which multiple signals are required for anteroposterior axial development in mammals.

Type-1 pericytes accumulate after tissue injury and produce collagen in an organ-dependent manner

Introduction

Fibrosis, or scar formation, is a pathological condition characterized by excessive production and accumulation of collagen, loss of tissue architecture, and organ failure in response to uncontrolled wound healing. Several cellular populations have been implicated, including bone marrow-derived circulating fibrocytes, endothelial cells, resident fibroblasts, epithelial cells, and recently, perivascular cells called pericytes. We previously demonstrated pericyte functional heterogeneity in skeletal muscle. Whether pericyte subtypes are present in other tissues and whether a specific pericyte subset contributes to organ fibrosis are unknown.

Methods

Here, we report the presence of two pericyte subtypes, type-1 (Nestin-GFP-/NG2-DsRed+) and type-2 (Nestin-GFP+/NG2-DsRed+), surrounding blood vessels in lungs, kidneys, heart, spinal cord, and brain. Using Nestin-GFP/NG2-DsRed transgenic mice, we induced pulmonary, renal, cardiac, spinal cord, and cortical injuries to investigate the contributions of pericyte subtypes to fibrous tissue formation in vivo.

Results

A fraction of the lung’s collagen-producing cells corresponds to type-1 pericytes and kidney and heart pericytes do not produce collagen in pathological fibrosis. Note that type-1, but not type-2, pericytes increase and accumulate near the fibrotic tissue in all organs analyzed. Surprisingly, after CNS injury, type-1 pericytes differ from scar-forming PDGFRβ + cells.

Conclusions

Pericyte subpopulations respond differentially to tissue injury, and the production of collagen by type-1 pericytes is organ-dependent. Characterization of the mechanisms underlying scar formation generates cellular targets for future anti-fibrotic therapeutics.

Electronic supplementary material

The online version of this article (doi:10.1186/scrt512) contains supplementary material, which is available to authorized users.

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.

Mixl1 localizes to putative axial stem cell reservoirs and their posterior descendants in the mouse embryo

Mixl1 is thought to play important roles in formation of mesoderm and endoderm. Previously, Mixl1 expression was reported in the posterior primitive streak and allantois, but the precise spatiotemporal whereabouts of Mixl1 protein throughout gastrulation have not been elucidated. To localize Mixl1 protein, immunohistochemistry was carried out at 2-4 h intervals on mouse gastrulae between primitive streak and 16-somite pair (s) stages (~E6.5-9.5). Mixl1 localized to the entire primitive streak early in gastrulation. However, by headfold stages (~E7.75-8.0), Mixl1 diminished within the mid-streak but remained concentrated at either end of the streak, and localized throughout midline posterior visceral endoderm. At the streak's anterior end, Mixl1 was confined to the posterior crown cells of Hensen's node, which contribute to dorsal hindgut endoderm, and the posterior notochord. In the posterior streak, Mixl1 localized to the Allantoic Core Domain (ACD), which is the source of most of the allantois and contributes to the posterior embryonic-extraembryonic interface. In addition, Mix1 co-localized with the early hematopoietic marker, Runx1, in the allantois and visceral yolk sac blood islands. During hindgut invagination (4-16s, ~E8.5-9.5), Mixl1 localized to the hindgut lip, becoming concentrated within the midline anastomosis of the splanchnopleure, which appears to create the ventral component of the hindgut and omphalomesenteric artery. Surrounding the distal hindgut, Mixl1 identified midline cells within tailbud mesoderm. Mixl1 was also found in the posterior notochord. These findings provide a critical systematic, and tissue-level understanding of embryonic Mixl1 localization, and support its role in regulation of crucial posterior axial mesendodermal stem cell niches during embryogenesis.

Deconvoluting the ontogeny of hematopoietic stem cells

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

Effects of fibroblast growth factor 9 on steroidogenesis and control of FGFR2IIIc mRNA in porcine granulosa cells

The objectives of this study were to investigate the effects of fibroblast growth factor 9 (FGF9) on hormone-stimulated porcine granulosa cell proliferation and steroid production and to further elucidate the hormonal and developmental control of FGFR2IIIc gene expression in granulosa cells. Porcine ovaries were collected from a local slaughterhouse and granulosa cells were collected from small to medium (1 to 5 mm) follicles for 5 in vitro studies that were conducted. Cells were cultured for 48 h in 5% fetal calf serum plus 5% porcine serum and then treated with various combinations of FSH, IGF-I, FGF9, Sonic hedgehog (SHH), cortisol, PGE2, and/or wingless-type mouse mammary tumor virus integration site family member 5A (WNT5A) in serum-free medium for an additional 24 or 48 h. Medium was collected for analysis of steroid concentration via RIA, or RNA was collected for gene expression analysis of FGFR2IIIc via quantitative reverse transcription PCR. Fibroblast growth factor 9 stimulated (P < 0.05) IGF-I-induced estradiol production in the presence of FSH and testosterone. However, FGF9 had inconsistent effects on progesterone production, stimulating progesterone production in the presence of FSH and testosterone but inhibiting progesterone production in the presence of IGF-I, FSH, and testosterone. Cell numbers were increased (P < 0.05) by FGF9 in the presence of IGF-I and FSH but not in the presence of FSH and absence of IGF-I. For FGFR2IIIc mRNA studies, granulosa cells were treated with FSH, IGF-I, FGF9, SHH, cortisol, PGE2, or WNT5A. Follicle-stimulating hormone alone had no effect (P > 0.10) whereas IGF-I increased (P < 0.05) FGFR2IIIc mRNA abundance. Cortisol, PGE2, SHH, and WNT5A had no effect (P > 0.10) on FGFR2IIIc gene expression whereas FGF9 in the presence of FSH and IGF-I inhibited (P < 0.05) FGFR2IIIc gene expression. In an in vivo study, granulosa cells from large (7 to 14 mm) follicles had greater (P < 0.05) abundance of FGFR2IIIc mRNA than small (1 to 3 mm) or medium (4 to 6 mm) follicles. In conclusion, IGF-I-induced FGFR2IIIc mRNA may be a mechanism for increased responses to FGF9 in FSH plus IGF-I-treated granulosa cells. Fibroblast growth factor 9 and IGF-I may work together as amplifiers of follicular growth and granulosa cell differentiation by stimulating estradiol production and concomitantly stimulating granulosa cell growth in pigs.

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.

Analysis of primitive erythroid cell proliferation and enucleation using a cyan fluorescent reporter in transgenic mice

Primitive erythropoiesis is a vital process for mammalian embryonic development. Here we report the generation and characterization of a new transgenic mouse line that expresses a histone H2B-CFP fusion protein in the nuclei of primitive erythroid cells. We demonstrate the potential of this ε-globin-histone H2B-CFP line for multicolor imaging and flow cytometry analysis. The ε-globin-H2B-CFP line was used to analyze the cell cycle distribution and proliferation of CFP-expressing primitive erythroblasts from E8.5-E13.5. We also evaluated phagocytosis of extruded CFP-positive nuclei by macrophages in fetal liver and placenta. The ε-globin-H2B-CFP transgenic mouse line adds to the available tools for studying the development of the primitive erythroid lineage.

Regulation of endothelial cell differentiation and specification

The circulatory system is the first organ system to develop in the vertebrate embryo and is critical throughout gestation for the delivery of oxygen and nutrients to, as well as removal of metabolic waste products from, growing tissues. Endothelial cells, which constitute the luminal layer of all blood and lymphatic vessels, emerge de novo from the mesoderm in a process known as vasculogenesis. The vascular plexus that is initially formed is then remodeled and refined via proliferation, migration, and sprouting of endothelial cells to form new vessels from preexisting ones during angiogenesis. Mural cells are also recruited by endothelial cells to form the surrounding vessel wall. During this vascular remodeling process, primordial endothelial cells are specialized to acquire arterial, venous, and blood-forming hemogenic phenotypes and functions. A subset of venous endothelium is also specialized to become lymphatic endothelium later in development. The specialization of all endothelial cell subtypes requires extrinsic signals and intrinsic regulatory events, which will be discussed in this review.

Wnt3 function in the epiblast is required for the maintenance but not the initiation of gastrulation in mice

The formation of the anteroposterior axis in mice requires a Wnt3-dependent symmetry-breaking event that leads to the formation of the primitive streak and gastrulation. Wnt3 is expressed sequentially in two distinct areas of the mouse embryo before the appearance of the primitive streak; first in the posterior visceral endoderm and soon after in the adjacent posterior epiblast. Hence, although an axial requirement for Wnt3 is well established, its temporal and tissue specific requirements remain an open question. Here, we report the conditional inactivation of Wnt3 in the epiblast of developing mouse embryos. Contrary to previous studies, our data shows that embryos lacking Wnt3 specifically in the epiblast are able to initiate gastrulation and advance to late primitive streak stages but fail to thrive and are resorbed by E9.5. At the molecular level, we provide evidence that Wnt3 regulates its own expression and that of other primitive streak markers via activation of the canonical Wnt signaling pathway.

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.

The hypoblast (visceral endoderm): an evo-devo perspective

When amniotes appeared during evolution, embryos freed themselves from intracellular nutrition; development slowed, the mid-blastula transition was lost and maternal components became less important for polarity. Extra-embryonic tissues emerged to provide nutrition and other innovations. One such tissue, the hypoblast (visceral endoderm in mouse), acquired a role in fixing the body plan: it controls epiblast cell movements leading to primitive streak formation, generating bilateral symmetry. It also transiently induces expression of pre-neural markers in the epiblast, which also contributes to delay streak formation. After gastrulation, the hypoblast might protect prospective forebrain cells from caudalizing signals. These functions separate mesendodermal and neuroectodermal domains by protecting cells against being caught up in the movements of gastrulation.

Mapping mouse hemangioblast maturation from headfold stages

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

Embryonic regulation of the mouse hematopoietic niche

Hematopoietic stem cells (HSCs) can differentiate into several types of hematopoietic cells (HCs) (such as erythrocytes, megakaryocytes, lymphocytes, neutrophils, or macrophages) and also undergo self-renewal to sustain hematopoiesis throughout an organism's lifetime. HSCs are currently used clinically as transplantation therapy in regenerative medicine and are typically obtained from healthy donors or cord blood. However, problems remain in HSC transplantation, such as shortage of cells, donor risks, rejection, and graft-versus-host disease (GVHD). Thus, increased understanding of HSC regulation should enable us to improve HSC therapy and develop novel regenerative medicine techniques. HSC regulation is governed by two types of activity: intrinsic regulation, programmed primarily by cell autonomous gene expression, and extrinsic factors, which originate from so-called "niche cells" surrounding HSCs. Here, we focus on the latter and discuss HSC regulation with special emphasis on the role played by niche cells.

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

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

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.