Yolk sac hematopoiesis: does it contribute to the adult hematopoietic system?

Prenatal PM2.5 exposure affects embryonic hematopoietic development through SOX2-regulated gene expression

Fine particulate matter (PM2.5) is one of the most concerning air pollutants, with emerging evidence indicating that it can negatively impact embryonic development and lead to adverse birth outcomes. Hematopoiesis is a critical process essential for the survival and normal development of the embryo, consisting of three temporally overlapping stages and involving multiple hematopoietic loci, including the yolk sac and fetal liver. Therefore, we hypothesized that abnormal embryonic hematopoietic development can significantly influence developmental outcomes. In this study, we established a prenatal PM2.5 exposure model and observed decreased embryo weights and elevated platelet counts at embryonic day 18.5 (E18.5). Additionally, we employed flow cytometry and colony-forming unit assays, which revealed a significant decrease in the proliferative differentiation potential of erythro-myeloid progenitors in the E10.5 yolk sac, as well as a reduction in both the number and function of hematopoietic stem progenitor cells in the E14.5 fetal liver. Through bioinformatic analysis, we identified that these alterations are associated with several typical biological processes and genes regarding cell proliferation, cell differentiation, response to hypoxia, and regulation of hematopoiesis. Importantly, via quantitative real-time PCR, chromatin immunoprecipitation, and immunofluorescence, we further elucidated that prenatal exposure to PM2.5 affects embryonic hematopoiesis by regulating the expression of SOX2, an important transcription factor involved in embryonic development, along with its related genes. Collectively, these findings provide experimental evidence supporting the necessity for controlling regional PM2.5 exposure to promote child well-being in polluted areas.

Endothelial heterogeneity in bone marrow: insights across development, adult life and leukemia

The central role of the endothelial microenvironment in orchestrating bone marrow (BM) homeostasis and hematopoietic support has been confirmed at various developmental stages and in adult life. The BM vasculature is crucial in mediating communication between BM parenchyma and circulating blood, displaying remarkable heterogeneity in structure and function. While vascular cell diversity in other tissues has long been recognized, the molecular basis of this phenomenon in BM is just now emerging. Over the past decade, single-cell approaches and microscopic observations have expanded our understanding of BM vasculature. While solely characterized for their paracrine properties in the past, recent advances have revolutionized our perception of endothelial function, revealing distinct anatomical locations associated with diverse endothelial cell states. The identification of phenotypic differences between normal and pathological conditions has therefore deepened our understanding of vascular dynamics and their impact on hematopoiesis in health and disease. In this review, we highlight key milestones and recent advances in understanding vascular heterogeneity within BM microenvironment during development, adulthood and aging. We also explore how leukemia affects this heterogeneity and how we can take this knowledge forward to improve clinical practices. By synthesizing existing literature, we aim to address unresolved questions and outline future research directions.

NR4A1 and NR4A2 orphan nuclear receptors regulate endothelial-to-hematopoietic transition in mouse hematopoietic stem cell specification

Hematopoietic stem cells (HSCs) sustain life-long hematopoiesis and emerge during mid-gestation from hemogenic endothelial progenitors via an endothelial-to-hematopoietic transition (EHT). The full scope of molecular mechanisms governing this process remains unclear. The NR4A subfamily of orphan nuclear receptors act as tumor suppressors in myeloid leukemogenesis and have never been implicated in HSC specification. Here, we report that Nr4a1 and Nr4a2 expression is upregulated in hemogenic endothelium during EHT. Progressive genetic ablation of Nr4a gene dosage results in a gradual decrease in numbers of nascent c-Kit+ hematopoietic progenitors in developing embryos, c-Kit+ cell cluster size in the dorsal aorta, and a block in HSC maturation, revealed by an accumulation of pro-HSCs and pre-HSC-type I cells and decreased numbers of pre-HSC-type II cells. Consistent with these observations, cells isolated from embryonic day 11.5 Nr4a1-/-; Nr4a2-/- aorta-gonads-mesonephros are devoid of in vivo long-term hematopoietic repopulating potential. Molecularly, employing spatial transcriptomic analysis we determined that the genetic ablation of Nr4a1 and Nr4a2 prevents Notch signaling from being downregulated in intra-aortic clusters and thus for pro-HSCs to mature into HSCs. Interestingly, this defect is partially rescued by ex vivo culture of dissected aorta-gonads-mesonephros with SCF, IL3 and FLT3L, which may bypass Notch-dependent regulation. Overall, our data reveal a role for the NR4A family of orphan nuclear receptors in EHT.

Hematopoietic and leukemic stem cells homeostasis: the role of bone marrow niche

The bone marrow microenvironment (BMM) has highly specialized anatomical characteristics that provide a sanctuary place for hematopoietic stem cells (HSCs) that allow appropriate proliferation, maintenance, and self-renewal capacity. Several cell types contribute to the constitution and function of the bone marrow niche. Interestingly, uncovering the secrets of BMM and its interaction with HSCs in health paved the road for research aiming at better understanding the concept of leukemic stem cells (LSCs) and their altered niche. In fact, they share many signals that are responsible for interactions between LSCs and the bone marrow niche, due to several biological similarities between LSCs and HSCs. On the other hand, LSCs differ from HSCs in their abnormal activation of important signaling pathways that regulate survival, proliferation, drug resistance, invasion, and spread. Targeting these altered niches can help in better treatment choices for hematological malignancies and bone marrow disorders in general and acute myeloid leukemia (AML) in particular. Moreover, targeting those niches may help in decreasing the emergence of drug resistance and lower the relapse rate. In this article, the authors reviewed the most recent literature on bone marrow niches and their relations with either normal HSCs and AML cells/LSC, by focusing on pathogenetic and therapeutic implications.

Modelling post-implantation human development to yolk sac blood emergence

Implantation of the human embryo begins a critical developmental stage that comprises profound events including axis formation, gastrulation and the emergence of haematopoietic system1,2. Our mechanistic knowledge of this window of human life remains limited due to restricted access to in vivo samples for both technical and ethical reasons3-5. Stem cell models of human embryo have emerged to help unlock the mysteries of this stage6-16. Here we present a genetically inducible stem cell-derived embryoid model of early post-implantation human embryogenesis that captures the reciprocal codevelopment of embryonic tissue and the extra-embryonic endoderm and mesoderm niche with early haematopoiesis. This model is produced from induced pluripotent stem cells and shows unanticipated self-organizing cellular programmes similar to those that occur in embryogenesis, including the formation of amniotic cavity and bilaminar disc morphologies as well as the generation of an anterior hypoblast pole and posterior domain. The extra-embryonic layer in these embryoids lacks trophoblast and shows advanced multilineage yolk sac tissue-like morphogenesis that harbours a process similar to distinct waves of haematopoiesis, including the emergence of erythroid-, megakaryocyte-, myeloid- and lymphoid-like cells. This model presents an easy-to-use, high-throughput, reproducible and scalable platform to probe multifaceted aspects of human development and blood formation at the early post-implantation stage. It will provide a tractable human-based model for drug testing and disease modelling.

Hematopoietic stem cells through the ages: A lifetime of adaptation to organismal demands

Hematopoietic stem cells (HSCs), which govern the production of all blood lineages, transition through a series of functional states characterized by expansion during fetal development, functional quiescence in adulthood, and decline upon aging. We describe central features of HSC regulation during ontogeny to contextualize how adaptive responses over the life of the organism ultimately form the basis for HSC functional degradation with age. We particularly focus on the role of cell cycle regulation, inflammatory response pathways, epigenetic changes, and metabolic regulation. We then explore how the knowledge of age-related changes in HSC regulation can inform strategies for the rejuvenation of old HSCs.

Yolk sac development, function and role in rodent pregnancy

During the early phases of embryonic development, the yolk sac serves as an initial placenta in many animal species. While in some, this role subsides around the end of active organogenesis, it continues to have important functions in rodents, alongside the chorio-allantoic placenta. The yolk sac is the initial site of hematopoiesis in many animal species including primates. Cells of epiblastic origin form blood islands that are the forerunners of hematopoietic cells and of the primitive endothelial cells that form the vitelline circulation. The yolk sac is also a major route of embryonic and fetal nutrition apparently as long as it functions. In mammals and especially rodents, macro and micronutrients are absorbed by active pinocytosis into the visceral yolk sac, degraded and the degradation products (i.e., amino acids) are then transferred to the embryo. Interference with the yolk sac function may directly reflect on embryonic growth and development, inducing congenital malformations or in extreme damage, causing embryonic and fetal death. In rodents, many agents were found to damage the yolk sac (i.e., anti-yolk sac antibodies or toxic substances interfering with yolk sac pinocytosis) subsequently affecting the embryo/fetus. Often, the damage to the yolk sac is transient while embryonic damage persists. In humans, decreased yolk sac diameter was associated with diabetic pregnancies and increased diameter was associated with pregnancy loss. In addition, culture of rat yolk sacs in serum obtained from pregnant diabetic women or from women with autoimmune diseases induced severe damage to the visceral yolk sac epithelium and embryonic malformations. It can be concluded that as a result of the crucial role of the yolk sac in the well-being of the early embryo, any damage to its normal function may severely and irreversibly affect further development of the embryo/fetus.

Ontogenetically distinct neutrophils differ in function and transcriptional profile in zebrafish

The current view of hematopoiesis considers leukocytes on a continuum with distinct developmental origins, and which exert non-overlapping functions. However, there is less known about the function and phenotype of ontogenetically distinct neutrophil populations. In this work, using a photoconvertible transgenic zebrafish line; Tg(mpx:Dendra2), we selectively label rostral blood island-derived and caudal hematopoietic tissue-derived neutrophils in vivo during steady state or upon injury. By comparing the migratory properties and single-cell expression profiles of both neutrophil populations at steady state we show that rostral neutrophils show higher csf3b expression and migration capacity than caudal neutrophils. Upon injury, both populations share a core transcriptional profile as well as subset-specific transcriptional signatures. Accordingly, both rostral and caudal neutrophils are recruited to the wound independently of their distance to the injury. While rostral neutrophils respond uniformly, caudal neutrophils respond heterogeneously. Collectively, our results reveal that co-existing neutrophils populations with ontogenically distinct origin display functional differences.

Modelling Human Post-Implantation Development via Extra-Embryonic Niche Engineering

Implantation of the human embryo commences a critical developmental stage that comprises profound morphogenetic alteration of embryonic and extra-embryonic tissues, axis formation, and gastrulation events. Our mechanistic knowledge of this window of human life remains limited due to restricted access to in vivo samples for both technical and ethical reasons. Additionally, human stem cell models of early post-implantation development with both embryonic and extra-embryonic tissue morphogenesis are lacking. Here, we present iDiscoid, produced from human induced pluripotent stem cells via an engineered a synthetic gene circuit. iDiscoids exhibit reciprocal co-development of human embryonic tissue and engineered extra-embryonic niche in a model of human post-implantation. They exhibit unanticipated self-organization and tissue boundary formation that recapitulates yolk sac-like tissue specification with extra-embryonic mesoderm and hematopoietic characteristics, the formation of bilaminar disc-like embryonic morphology, the development of an amniotic-like cavity, and acquisition of an anterior-like hypoblast pole and posterior-like axis. iDiscoids offer an easy-to-use, high-throughput, reproducible, and scalable platform to probe multifaceted aspects of human early post-implantation development. Thus, they have the potential to provide a tractable human model for drug testing, developmental toxicology, and disease modeling.

Increasing Complexity of Molecular Landscapes in Human Hematopoietic Stem and Progenitor Cells during Development and Aging

The past five decades have seen significant progress in our understanding of human hematopoiesis. This has in part been due to the unprecedented development of advanced technologies, which have allowed the identification and characterization of rare subsets of human hematopoietic stem and progenitor cells and their lineage trajectories from embryonic through to adult life. Additionally, surrogate in vitro and in vivo models, although not fully recapitulating human hematopoiesis, have spurred on these scientific advances. These approaches have heightened our knowledge of hematological disorders and diseases and have led to their improved diagnosis and therapies. Here, we review human hematopoiesis at each end of the age spectrum, during embryonic and fetal development and on aging, providing exemplars of recent progress in deciphering the increasingly complex cellular and molecular hematopoietic landscapes in health and disease. This review concludes by highlighting links between chronic inflammation and metabolic and epigenetic changes associated with aging and in the development of clonal hematopoiesis.

One Size Does Not Fit All: Heterogeneity in Developmental Hematopoiesis

Our knowledge of the complexity of the developing hematopoietic system has dramatically expanded over the course of the last few decades. We now know that, while hematopoietic stem cells (HSCs) firmly reside at the top of the adult hematopoietic hierarchy, multiple HSC-independent progenitor populations play variegated and fundamental roles during fetal life, which reflect on adult physiology and can lead to disease if subject to perturbations. The importance of obtaining a high-resolution picture of the mechanisms by which the developing embryo establishes a functional hematopoietic system is demonstrated by many recent indications showing that ontogeny is a primary determinant of function of multiple critical cell types. This review will specifically focus on exploring the diversity of hematopoietic stem and progenitor cells unique to embryonic and fetal life. We will initially examine the evidence demonstrating heterogeneity within the hemogenic endothelium, precursor to all definitive hematopoietic cells. Next, we will summarize the dynamics and characteristics of the so-called "hematopoietic waves" taking place during vertebrate development. For each of these waves, we will define the cellular identities of their components, the extent and relevance of their respective contributions as well as potential drivers of heterogeneity.

Clearing the Haze: How Does Nicotine Affect Hematopoiesis before and after Birth?

Hematopoiesis is a tightly regulated process orchestrated by cell-intrinsic and cell-extrinsic cues. Over the past several decades, much effort has been focused on understanding how these cues regulate hematopoietic stem cell (HSC) function. Many endogenous key regulators of hematopoiesis have been identified and extensively characterized. Less is known about the mechanisms of long-term effects of environmental toxic compounds on hematopoietic stem and progenitor cells (HSPCs) and their mature immune cell progeny. Research over the past several decades has demonstrated that tobacco products are extremely toxic and pose huge risks to human health by causing diseases like cancer, respiratory illnesses, strokes, and more. Recently, electronic cigarettes have been promoted as a safer alternative to traditional tobacco products and have become increasingly popular among younger generations. Nicotine, the highly toxic compound found in many traditional tobacco products, is also found in most electronic cigarettes, calling into question their purported "safety". Although it is known that nicotine is toxic, the pathophysiology of disease in exposed people remains under investigation. One plausible contributor to altered disease susceptibility is altered hematopoiesis and associated immune dysfunction. In this review, we focus on research that has addressed how HSCs and mature blood cells respond to nicotine, as well as identify remaining questions.

Stem Cell-Based Disease Models for Inborn Errors of Immunity

The intrinsic capacity of human hematopoietic stem cells (hHSCs) to reconstitute myeloid and lymphoid lineages combined with their self-renewal capacity hold enormous promises for gene therapy as a viable treatment option for a number of immune-mediated diseases, most prominently for inborn errors of immunity (IEI). The current development of such therapies relies on disease models, both in vitro and in vivo, which allow the study of human pathophysiology in great detail. Here, we discuss the current challenges with regards to developmental origin, heterogeneity and the subsequent implications for disease modeling. We review models based on induced pluripotent stem cell technology and those relaying on use of adult hHSCs. We critically review the advantages and limitations of current models for IEI both in vitro and in vivo. We conclude that existing and future stem cell-based models are necessary tools for developing next generation therapies for IEI.

Lyl-1 regulates primitive macrophages and microglia development

During ontogeny, macrophage populations emerge in the Yolk Sac (YS) via two distinct progenitor waves, prior to hematopoietic stem cell development. Macrophage progenitors from the primitive/"early EMP" and transient-definitive/"late EMP" waves both contribute to various resident primitive macrophage populations in the developing embryonic organs. Identifying factors that modulates early stages of macrophage progenitor development may lead to a better understanding of defective function of specific resident macrophage subsets. Here we show that YS primitive macrophage progenitors express Lyl-1, a bHLH transcription factor related to SCL/Tal-1. Transcriptomic analysis of YS macrophage progenitors indicate that primitive macrophage progenitors present at embryonic day 9 are clearly distinct from those present at later stages. Disruption of Lyl-1 basic helix-loop-helix domain leads initially to an increased emergence of primitive macrophage progenitors, and later to their defective differentiation. These defects are associated with a disrupted expression of gene sets related to embryonic patterning and neurodevelopment. Lyl-1-deficiency also induce a reduced production of mature macrophages/microglia in the early brain, as well as a transient reduction of the microglia pool at midgestation and in the newborn. We thus identify Lyl-1 as a critical regulator of primitive macrophages and microglia development, which disruption may impair resident-macrophage function during organogenesis.

Zebrafish models of acute leukemias: Current models and future directions

Acute myeloid leukemias (AML) and acute lymphoid leukemias (ALL) are heterogenous diseases encompassing a wide array of genetic mutations with both loss and gain of function phenotypes. Ultimately, these both result in the clonal overgrowth of blast cells in the bone marrow, peripheral blood, and other tissues. As a consequence of this, normal hematopoietic stem cell function is severely hampered. Technologies allowing for the early detection of genetic alterations and understanding of these varied molecular pathologies have helped to advance our treatment regimens toward personalized targeted therapies. In spite of this, both AML and ALL continue to be a major cause of morbidity and mortality worldwide, in part because molecular therapies for the plethora of genetic abnormalities have not been developed. This underscores the current need for better model systems for therapy development. This article reviews the current zebrafish models of AML and ALL and discusses how novel gene editing tools can be implemented to generate better models of acute leukemias. This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cells and Disease Technologies > Perturbing Genes and Generating Modified Animals.

Contributions of Embryonic HSC-Independent Hematopoiesis to Organogenesis and the Adult Hematopoietic System

During ontogeny, the establishment of the hematopoietic system takes place in several phases, separated both in time and location. The process is initiated extra-embryonically in the yolk sac (YS) and concludes in the main arteries of the embryo with the formation of hematopoietic stem cells (HSC). Initially, it was thought that HSC-independent hematopoietic YS cells were transient, and only required to bridge the gap to HSC activity. However, in recent years it has become clear that these cells also contribute to embryonic organogenesis, including the emergence of HSCs. Furthermore, some of these early HSC-independent YS cells persist into adulthood as distinct hematopoietic populations. These previously unrecognized abilities of embryonic HSC-independent hematopoietic cells constitute a new field of interest. Here, we aim to provide a succinct overview of the current knowledge regarding the contribution of YS-derived hematopoietic cells to the development of the embryo and the adult hematopoietic system.

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

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