Emergence of haematopoietic stem cells during development

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.

Endocardial Cell Plasticity in Cardiac Development, Diseases and Regeneration

Endocardial cells are specialized endothelial cells that form the innermost layer of the heart wall. By virtue of genetic lineage-tracing technology, many of the unexpected roles of endocardium during murine heart development, diseases, and regeneration have been identified recently. In addition to heart valves developed from the well-known endothelial to mesenchymal transition, recent fate-mapping studies using mouse models reveal that multiple cardiac cell lineages are also originated from the endocardium. This review focuses on a variety of different cell types that are recently reported to be endocardium derived during murine heart development, diseases, and regeneration. These multiple cell fates underpin the unprecedented roles of endocardial progenitors in function, pathological progression, and regeneration of the heart. Because emerging studies suggest that developmental mechanisms can be redeployed and recapitulated in promoting heart disease development and also cardiac repair and regeneration, understanding the mechanistic regulation of endocardial plasticity and modulation of their cell fate conversion may uncover new therapeutic potential in facilitating heart regeneration.

Endocardially Derived Macrophages Are Essential for Valvular Remodeling

During mammalian embryogenesis, de novo hematopoiesis occurs transiently in multiple anatomical sites including the yolk sac, dorsal aorta, and heart tube. A long-unanswered question is whether these local transient hematopoietic mechanisms are essential for embryonic growth. Here, we show that endocardial hematopoiesis is critical for cardiac valve remodeling as a source of tissue macrophages. Colony formation assay from explanted heart tubes and genetic lineage tracing with the endocardial specific Nfatc1-Cre mouse revealed that hemogenic endocardium is a de novo source of tissue macrophages in the endocardial cushion, the primordium of the cardiac valves. Surface marker characterization, gene expression profiling, and ex vivo phagocytosis assay revealed that the endocardially derived cardiac tissue macrophages play a phagocytic and antigen presenting role. Indeed, genetic ablation of endocardially derived macrophages caused severe valve malformation. Together, these data suggest that transient hemogenic activity in the endocardium is indispensable for the valvular tissue remodeling in the heart.

Lack of Phenotypical and Morphological Evidences of Endothelial to Hematopoietic Transition in the Murine Embryonic Head during Hematopoietic Stem Cell Emergence

During mouse ontogeny, hematopoietic cells arise from specialized endothelial cells, i.e., the hemogenic endothelium, and form clusters in the lumen of arterial vessels. Hemogenic endothelial cells have been observed in several embryonic tissues, such as the dorsal aorta, the placenta and the yolk sac. Recent work suggests that the mouse embryonic head also produces hematopoietic stem cells (HSCs)/progenitors. However, a histological basis for HSC generation in the head has not yet been determined because the hematopoietic clusters and hemogenic endothelium in the head region have not been well characterized. In this study, we used whole-mount immunostaining and 3D confocal reconstruction techniques to analyze both c-Kit⁺ hematopoietic clusters and Runx1⁺ hemogenic endothelium in the whole-head vasculature. The number of c-Kit⁺ hematopoietic cells was 20-fold less in the head arteries than in the dorsal aorta. In addition, apparent nascent hematopoietic cells, which are characterized by a “budding” structure and a Runx1⁺ hemogenic endothelium, were not observed in the head. These results suggest that head HSCs may not be or are rarely generated from the endothelium in the same manner as aortic HSCs.

The developmental origins and lineage contributions of endocardial endothelium

Endocardial development involves a complex orchestration of cell fate decisions that coordinate with endoderm formation and other mesodermal cell lineages. Historically, investigations into the contribution of endocardium in the developing embryo was constrained to the heart where these cells give rise to the inner lining of the myocardium and are a major contributor to valve formation. In recent years, studies have continued to elucidate the complexities of endocardial fate commitment revealing a much broader scope of lineage potential from developing endocardium. These studies cover a wide range of species and model systems and show direct contribution or fate potential of endocardium giving rise to cardiac vasculature, blood, fibroblast, and cardiomyocyte lineages. This review focuses on the marked expansion of knowledge in the area of endocardial fate potential. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

What do we know about the participation of hematopoietic stem cells in hematopoiesis?

The demonstrated presence in adult tissues of cells with sustained tissue regenerative potential has given rise to the concept of tissue stem cells. Assays to detect and measure such cells indicate that they have enormous proliferative potential and usually an ability to produce all or many of the mature cell types that define the specialized functionality of the tissue. In the hematopoietic system, one or only a few cells can restore lifelong hematopoiesis of the whole organism. To what extent is the maintenance of hematopoietic stem cells required during normal hematopoiesis? How does the constant maintenance of hematopoiesis occur and what is the behavior of the hematopoietic stem cells in the normal organism? How many of the hematopoietic stem cells are created during the development of the organism? How many hematopoietic stem cells are generating more mature progeny at any given moment? What happens to the population of hematopoietic stem cells in aging? This review will attempt to describe the results of recent research which contradict some of the ideas established over the past 30 years about how hematopoiesis is regulated.

Haemogenic endocardium contributes to transient definitive haematopoiesis

Haematopoietic cells arise from spatiotemporally restricted domains in the developing embryo. Although studies of non-mammalian animal and in vitro embryonic stem cell models suggest a close relationship among cardiac, endocardial and haematopoietic lineages, it remains unknown whether the mammalian heart tube serves as a haemogenic organ akin to the dorsal aorta. Here we examine the haemogenic activity of the developing endocardium. Mouse heart explants generate myeloid and erythroid colonies in the absence of circulation. Haemogenic activity arises from a subset of endocardial cells in the outflow cushion and atria earlier than in the aorta-gonad-mesonephros region, and is transient and definitive in nature. Interestingly, key cardiac transcription factors, Nkx2-5 and Isl1, are expressed in and required for the haemogenic population of the endocardium. Together, these data suggest that a subset of endocardial/endothelial cells serve as a de novo source for transient definitive haematopoietic progenitors.

Human CD34+ CD133+ hematopoietic stem cells cultured with growth factors including Angptl5 efficiently engraft adult NOD-SCID Il2rγ-/- (NSG) mice

Increasing demand for human hematopoietic stem cells (HSCs) in clinical and research applications necessitates expansion of HSCs in vitro. Before these cells can be used they must be carefully evaluated to assess their stem cell activity. Here, we expanded cord blood CD34⁺ CD133⁺ cells in a defined medium containing angiopoietin like 5 and insulin-like growth factor binding protein 2 and evaluated the cells for stem cell activity in NOD-SCID Il2rg^−/− (NSG) mice by multi-lineage engraftment, long term reconstitution, limiting dilution and serial reconstitution. The phenotype of expanded cells was characterized by flow cytometry during the course of expansion and following engraftment in mice. We show that the SCID repopulating activity resides in the CD34⁺ CD133⁺ fraction of expanded cells and that CD34⁺ CD133⁺ cell number correlates with SCID repopulating activity before and after culture. The expanded cells mediate long-term hematopoiesis and serial reconstitution in NSG mice. Furthermore, they efficiently reconstitute not only neonate but also adult NSG recipients, generating human blood cell populations similar to those reported in mice reconstituted with uncultured human HSCs. These findings suggest an expansion of long term HSCs in our culture and show that expression of CD34 and CD133 serves as a marker for HSC activity in human cord blood cell cultures. The ability to expand human HSCs in vitro should facilitate clinical use of HSCs and large-scale construction of humanized mice from the same donor for research applications.

Cells with hematopoietic activity in the mouse placenta reside in side population

The discovery of a major hematopoietic stem cell pool in midgestation mouse embryo has defined the placenta as an important hematopoietic anatomical site. In this study, we examined the flow cytometric pattern of mouse placenta cells on embryonic days (E) 10.5 to E15.5, in view of CD45 and c-Kit expression. We also determined which population of these cells shows differentiation potential toward multiple hematopoietic lineages by performing coculture with OP9 stromal cells and colony-forming assay in methylcellulose. Only CD45(+)c-Kit(+) population showed the ability to form hematopoietic colonies including multiple lineages. To distinguish which fraction of placenta cells have the hematopoietic activity, we used GFP transgenic mice in which the fetal part of the placenta is GFP positive and the maternal part is GFP negative. E11.5 and E13.5 CD45(+)c-Kit(+) placental cells that have ability to form hematopoietic colonies are the fetal GFP positive placental cells. E11.5 and E13.5 CD45(+)c-Kit(+) placental cells that have an ability to form hematopoietic colonies mainly reside in Hoechst dye-effluxing side population area (SP). Taken together, in the placenta of mouse embryo, we conclude that SP cells in the CD45(+)c-Kit(+) fetal placental cells have the ability to form hematopoietic colonies.

Mankind's first natural stem cell transplant

The timing of the umbilical cord clamping at birth is still controversial. In the modern era of medicine, the cord has been clamped early to facilitate resuscitation and stabilization of infants. However, recently delayed cord clamping has been supported by physicians because it allows for the physiological transfer of blood from the placenta to the infant. Many clinical studies have revealed that the delayed cord clamping elevates blood volume and haemoglobin and prevents anaemia in infants. Moreover, since it was known that umbilical cord blood contains various valuable stem cells such as haematopoietic stem cells, endothelial cell precursors, mesenchymal progenitors and multipotent/pluripotent lineage stem cells, the merit of delayed cord clamping has been magnified. In this review, we discuss the advantages and disadvantages of delayed cord clamping at birth. We highlight the importance of delayed cord clamping in realizing mankind's first stem cell transfer and propose that it should be encouraged in normal births.

Impaired embryonic haematopoiesis yet normal arterial development in the absence of the Notch ligand Jagged1

Specific deletion of Notch1 and RBPjkappa in the mouse results in abrogation of definitive haematopoiesis concomitant with the loss of arterial identity at embryonic stage. As prior arterial determination is likely to be required for the generation of embryonic haematopoiesis, it is difficult to establish the specific haematopoietic role of Notch in these mutants. By analysing different Notch-ligand-null embryos, we now show that Jagged1 is not required for the establishment of the arterial fate but it is required for the correct execution of the definitive haematopoietic programme, including expression of GATA2 in the dorsal aorta. Moreover, successful haematopoietic rescue of the Jagged1-null AGM cells was obtained by culturing them with Jagged1-expressing stromal cells or by lentiviral-mediated transduction of the GATA2 gene. Taken together, our results indicate that Jagged1-mediated activation of Notch1 is responsible for regulating GATA2 expression in the AGM, which in turn is essential for definitive haematopoiesis in the mouse.

Immunostimulatory effects of mesenchymal stem cell-derived neurons: implications for stem cell therapy in allogeneic transplantations

Mesenchymal stem cells (MSCs) differentiate along various lineages to specialized mesodermal cells and also transdifferentiate into cells such as ectodermal neurons. MSCs are among the leading adult stem cells for application in regenerative medicine. Advantages include their immune-suppressive properties and reduced ethical concerns. MSCs also show immune-enhancing functions. Major histocompatibility complex II (MHC-II) is expected to be downregulated in MSCs during neurogenesis. Ideally, "off the shelf" MSCs would be suited for rapid delivery into patients. The question is whether these MSC-derived neurons can reexpress MHC-II in a milieu of inflammation. Western analyses demonstrated gradual decrease in MHC-II during neurogenesis, which correlated with the expression of nuclear CIITA, the master regulator of MHC-II expression. MHC-II expression was reversed by exogenous IFNY. One-way mixed lymphocyte reaction with partly differentiated neurons showed a stimulatory effect, which was partly explained by the release of the proinflammatory neurotransmitter substance P (SP), cytokines, and decreases in miR-130a and miR-206. The anti-inflammatory neurotransmitters VIP and CGRP were decreased at the peak time of immune stimulation. In summary, MSC-derived neurons show decreased MHC-II expression, which could be reexpressed by IFNY. The release of neurotransmitters could be involved in initiating inflammation, underscoring the relevance of immune responses as consideration for stem cell therapies.