Hematopoietic differentiation of human embryonic stem cells progresses through sequential hematoendothelial, primitive, and definitive stages resembling human yolk sac development

Injection of embryonic stem cell derived macrophages ameliorates fibrosis in a murine model of liver injury

Chronic liver injury can be caused by viral hepatitis, alcohol, obesity, and metabolic disorders resulting in fibrosis, hepatic scarring, and cirrhosis. Novel therapies are urgently required and previous work has demonstrated that treatment with bone marrow derived macrophages can improve liver regeneration and reduce fibrosis in a murine model of hepatic injury and fibrosis. Here, we describe a protocol whereby pure populations of therapeutic macrophages can be produced in vitro from murine embryonic stem cells on a large scale. Embryonic stem cell derived macrophages display comparable morphology and cell surface markers to bone marrow derived macrophages but our novel imaging technique revealed that their phagocytic index was significantly lower. Differences were also observed in their response to classical induction protocols with embryonic stem cell derived macrophages having a reduced response to lipopolysaccharide and interferon gamma and an enhanced response to IL4 compared to bone marrow derived macrophages. When their therapeutic potential was assessed in a murine, carbon tetrachloride-induced injury and fibrosis model, embryonic stem cell derived macrophages significantly reduced the amount of hepatic fibrosis to 50% of controls, down-regulated the number of fibrogenic myofibroblasts and activated liver progenitor cells. To our knowledge, this is the first study that demonstrates a therapeutic effect of macrophages derived in vitro from pluripotent stem cells in a model of liver injury. We also found that embryonic stem cell derived macrophages repopulated the Kupffer cell compartment of clodronate-treated mice more efficiently than bone marrow derived macrophages, and expressed comparatively lower levels of Myb and Ccr2, indicating that their phenotype is more comparable to tissue-resident rather than monocyte-derived macrophages.

Runx1 Structure and Function in Blood Cell Development

RUNX transcription factors belong to a highly conserved class of transcriptional regulators which play various roles in the development of the majority of metazoans. In this review we focus on the founding member of the family, RUNX1, and its role in the transcriptional control of blood cell development in mammals. We summarize data showing that RUNX1 functions both as activator and repressor within a chromatin environment, a feature that requires its interaction with multiple other transcription factors and co-factors. Furthermore, we outline how RUNX1 works together with other factors to reshape the epigenetic landscape and the three-dimensional structure of gene loci within the nucleus. Finally, we review how aberrant forms of RUNX1 deregulate blood cell development and cause hematopoietic malignancies.

A view of human haematopoietic development from the Petri dish

Human pluripotent stem cells (hPSCs) provide an unparalleled opportunity to establish in vitro differentiation models that will transform our approach to the study of human development. In the case of the blood system, these models will enable investigation of the earliest stages of human embryonic haematopoiesis that was previously not possible. In addition, they will provide platforms for studying the origins of human blood cell diseases and for generating de novo haematopoietic stem and progenitor cell populations for cell-based regenerative therapies.

Hematopoietic stem cell-independent hematopoiesis: emergence of erythroid, megakaryocyte, and myeloid potential in the mammalian embryo

Steady-state production of all circulating blood cells in the adult ultimately depends on hematopoietic stem cells (HSCs), which first arise in small numbers beginning at embryonic day (E) 10.5 in large arterial vessels of the murine embryo. However, blood cell synthesis first begins in the yolk sac beginning at E7.25 and consists of two waves of hematopoietic progenitors. The first wave consists of primitive erythroid, megakaryocyte, and macrophage progenitors that rapidly give rise to maturing blood cells of all three lineages. This 'primitive' wave of progenitors is followed by a partially overlapping wave of 'erythro-myeloid progenitors', which contain definitive erythroid, megakaryocyte, macrophage, neutrophil, and mast cell progenitors that seed the fetal liver and jump-start hematopoiesis before the engraftment and expansion of HSCs. These two waves of progenitors that arise in the yolk sac are necessary and even sufficient to sustain the survival of the mouse embryo until birth in the absence of HSCs. They provide key signals to support HSC emergence. Finally, HSC-independent hematopoiesis also provides long-lived tissue-resident macrophage populations that function in multiple adult organs.

Distinct gene expression program dynamics during erythropoiesis from human induced pluripotent stem cells compared with adult and cord blood progenitors

BACKGROUND

Human-induced pluripotent stem cells (hiPSCs) are a potentially invaluable resource for regenerative medicine, including the in vitro manufacture of blood products. HiPSC-derived red blood cells are an attractive therapeutic option in hematology, yet exhibit unexplained proliferation and enucleation defects that presently preclude such applications. We hypothesised that substantial differential regulation of gene expression during erythroid development accounts for these important differences between hiPSC-derived cells and those from adult or cord-blood progenitors. We thus cultured erythroblasts from each source for transcriptomic analysis to investigate differential gene expression underlying these functional defects.

RESULTS

Our high resolution transcriptional view of definitive erythropoiesis captures the regulation of genes relevant to cell-cycle control and confers statistical power to deploy novel bioinformatics methods. Whilst the dynamics of erythroid program elaboration from adult and cord blood progenitors were very similar, the emerging erythroid transcriptome in hiPSCs revealed radically different program elaboration compared to adult and cord blood cells. We explored the function of differentially expressed genes in hiPSC-specific clusters defined by our novel tunable clustering algorithms (SMART and Bi-CoPaM). HiPSCs show reduced expression of c-KIT and key erythroid transcription factors SOX6, MYB and BCL11A, strong HBZ-induction, and aberrant expression of genes involved in protein degradation, lysosomal clearance and cell-cycle regulation.

CONCLUSIONS

Together, these data suggest that hiPSC-derived cells may be specified to a primitive erythroid fate, and implies that definitive specification may more accurately reflect adult development. We have therefore identified, for the first time, distinct gene expression dynamics during erythroblast differentiation from hiPSCs which may cause reduced proliferation and enucleation of hiPSC-derived erythroid cells. The data suggest several mechanistic defects which may partially explain the observed aberrant erythroid differentiation from hiPSCs.

Development: For cloche the Bell Tolls

A recent publication identifies npas4l as the gene defective in the well-known cloche mutant that lacks most endothelial as well as hematopoietic cells. This work poses intriguing questions as to the genetic and molecular nature of the origin of hemato-vascular lineages during early embryogenesis.

Identification of embryo-fetal cells in celomic fluid using morphological and short-tandem repeats analysis

OBJECTIVE

The main problem to wide acceptability of celocentesis as earlier prenatal diagnosis is contamination of the sample by maternal cells. The objective of this study was to investigate the cellular composition of celomic fluid for morphological discrimination between maternal and embryo-fetal cells.

METHOD

Celomic fluids were aspired by ultrasound-guided transcervical celocentesis at 7-9 weeks' gestation from singleton pregnancies before surgical termination for psychological reasons. DNA extracted from celomic fluid cells showed the same morphology, and quantitative fluorescent polymerase chain reaction (PCR) assay was performed to evaluate their fetal or maternal origin.

RESULTS

Six different types of non-hematological maternal and four different types of embryo-fetal cells were detected. The most common maternal cells were of epithelial origin. The majority of embryo-fetal cells were roundish with a nucleus located in an eccentric position near the wall. These cells were considered to be erythroblasts, probably derived from the yolk sac that serves as the initial site of erythropoiesis.

CONCLUSIONS

The combined use of morphology and DNA analysis makes it possible to select and isolate embryo-fetal cells, even when maternal contamination is high. This development provides the opportunity for the use of celocentesis for early prenatal diagnosis of genetic diseases and application of array comparative genomic hybridization. © 2016 John Wiley & Sons, Ltd.

Superior Red Blood Cell Generation from Human Pluripotent Stem Cells Through a Novel Microcarrier-Based Embryoid Body Platform

In vitro generation of red blood cells (RBCs) from human embryonic stem cells and human induced pluripotent stem cells appears to be a promising alternate approach to circumvent shortages in donor-derived blood supplies for clinical applications. Conventional methods for hematopoietic differentiation of human pluripotent stem cells (hPSC) rely on embryoid body (EB) formation and/or coculture with xenogeneic cell lines. However, most current methods for hPSC expansion and EB formation are not amenable for scale-up to levels required for large-scale RBC generation. Moreover, differentiation methods that rely on xenogenic cell lines would face obstacles for future clinical translation. In this study, we report the development of a serum-free and chemically defined microcarrier-based suspension culture platform for scalable hPSC expansion and EB formation. Improved survival and better quality EBs generated with the microcarrier-based method resulted in significantly improved mesoderm induction and, when combined with hematopoietic differentiation, resulted in at least a 6-fold improvement in hematopoietic precursor expansion, potentially culminating in a 80-fold improvement in the yield of RBC generation compared to a conventional EB-based differentiation method. In addition, we report efficient terminal maturation and generation of mature enucleated RBCs using a coculture system that comprised primary human mesenchymal stromal cells. The microcarrier-based platform could prove to be an appealing strategy for future scale-up of hPSC culture, EB generation, and large-scale generation of RBCs under defined and xeno-free conditions.

High-Efficiency Serum-Free Feeder-Free Erythroid Differentiation of Human Pluripotent Stem Cells Using Small Molecules

This article describes a highly efficient, fully feeder-free, serum-free method for erythroid differentiation of induced pluripotent stem cells and human embryonic stem cells, including a clinical-grade line, that is amenable to scale-up and as such will be of significant value for basic and translational studies of hematopoiesis and erythropoiesis.

This article describes a good manufacturing practice (GMP)-compatible, feeder-free and serum-free method to produce large numbers of erythroid cells from human pluripotent stem cells (hPSCs), either embryonic or induced. This multistep protocol combines cytokines and small molecules to mimic and surpass the early stages of development. It produces, without any selection or sorting step, a population of cells in which 91.8% ± 5.4% express CD34 at day 7, 98.6% ± 1.3% express CD43 at day 10, and 99.1% ± 0.95% of cells are CD235a positive by day 31 of the differentiation process. Moreover, this differentiation protocol supports extensive expansion, with a single hPSC producing up to 150 hematopoietic progenitor cells by day 10 and 50,000–200,000 erythroid cells by day 31. The erythroid cells produced exhibit a definitive fetal hematopoietic type, with 90%–95% fetal globin and variable proportion of embryonic and adult globin at the protein level. The presence of small molecules during the differentiation protocol has quantitative and qualitative effects; it increases the proportion of adult globin and decreases the proportion of embryonic globin. Given its level of definition, this system provides a powerful tool for investigation of the mechanisms governing early hematopoiesis and erythropoiesis, including globin switching and enucleation. The early stages of the differentiation protocol could also serve as a starting point for the production of endothelial cells and other hematopoietic cells, or to investigate the production of long-term reconstituting hematopoietic stem cells from hPSCs.

Significance

This differentiation protocol allows the production of a large amount of erythroid cells from pluripotent stem cells. Its efficiency is compatible with that of in vitro red blood cell production, and it can be a considerable asset for studying developmental erythropoiesis and red blood cell enucleation, thereby aiding both basic and translational research. In addition to red cells, the early stages of the protocol could also be used as a starting point for the large-scale production of other hematopoietic cell types, including the ultimate goal of generating long-term reconstituting hematopoietic stem cells.

Medial HOXA genes demarcate haematopoietic stem cell fate during human development

Pluripotent stem cells (PSC) may provide a potential source of haematopoietic stem/progenitor cells (HSPCs) for transplantation; however, unknown molecular barriers prevent the self-renewal of PSC-HSPCs. Using two-step differentiation, human embryonic stem cells (hESCs) differentiated in vitro into multipotent haematopoietic cells that had CD34⁺CD38^−/loCD90⁺CD45⁺GPI-80⁺ foetal liver (FL) HSC immunophenotype, but displayed poor expansion potential and engraftment ability. Transcriptome analysis of immunophenotypic hESC-HSPCs revealed that, despite their molecular resemblance to FL-HSPCs, medial HOXA genes remained suppressed. Knockdown of HOXA7 disrupted FL-HSPC function and caused transcriptome dysregulation that resembled hESC-derived progenitors. Overexpression of medial HOXA genes prolonged FL-HSPC maintenance but was insufficient to confer self-renewal to hESC-HSPCs. Stimulation of retinoic acid signalling during endothelial-to-haematopoietic transition induced the HOXA cluster and other HSC/definitive haemogenic endothelium genes, and prolonged HSPC maintenance in culture. Thus, retinoic acid signalling-induced medial HOXA gene expression marks the establishment of the definitive HSC fate and controls HSC identity and function.

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.

Epsilon globin gene expression in developing human fetal tissues

OBJECTIVE

The discovery of free fetal DNA in plasma of pregnant women has opened a new avenue for non-invasive prenatal diagnosis. We hypothesized that epsilon (ɛ)-globin gene expression could serve as a positive control for the presence of fetal nucleic acid.

STUDY DESIGN

We measured ɛ-globin mRNA in human fetal tissues and compared concentrations with that measured in adult non-pregnant and pregnant samples. Total RNA was isolated from fetal marrow, liver, blood, and placenta (10-24 weeks gestation), from adult peripheral blood mononuclear cells, and from maternal plasma. RNA was reverse transcribed and quantitative polymerase chain reaction performed for ɛ-globin expression.

RESULTS

ɛ-globin gene expression was detected in all fetal samples, was detected in plasma of pregnant women, but was negligible in non-pregnant samples. Relative ɛ-globin gene expression was significantly greater in fetal blood compared to fetal liver, and was minimally expressed in placenta. ɛ-globin gene expression decreased at the highest gestational ages in fetal blood, while expression was greatest at 15-19 weeks in fetal marrow.

CONCLUSION

Fetal ɛ-globin gene expression is significantly greater than adult expression and is increased in maternal plasma compared to non-pregnant samples. ɛ-globin gene expression might serve as a positive control when determining the presence of fetal nucleic acid in total nucleic acid isolated from maternal plasma.

Concise review: programming human pluripotent stem cells into blood

Blood disorders are treated with cell therapies including haematopoietic stem cell (HSC) transplantation as well as platelet and red blood cell transfusions. However the source of cells is entirely dependent on donors, procedures are susceptible to transfusion-transmitted infections and serious complications can arise in recipients due to immunological incompatibility. These problems could be alleviated if it was possible to produce haematopoietic cells in vitro from an autologous and renewable cell source. The production of haematopoietic cells in the laboratory from human induced pluripotent stem cells (iPSCs) may provide a route to realize this goal but it has proven challenging to generate long-term reconstituting HSCs. To date, the optimization of differentiation protocols has mostly relied on the manipulation of extrinsic signals to mimic the in vivo environment. We review studies that have taken an alternative approach to modulate intrinsic signals by enforced expression of transcription factors. Single and combinations of multiple transcription factors have been used in a variety of contexts to enhance the production of haematopoietic cells from human pluripotent stem cells. This programming approach, together with the recent advances in the production and use of synthetic transcription factors, holds great promise for the production of fully functional HSCs in the future.

3D high throughput screening and profiling of embryoid bodies in thermoformed microwell plates

3D organoids using stem cells to study development and disease are now widespread. These models are powerful to mimic in vivo situations but are currently associated with high variability and low throughput. For biomedical research, platforms are thus necessary to increase reproducibility and allow high-throughput screens (HTS). Here, we introduce a microwell platform, integrated in standard culture plates, for functional HTS. Using micro-thermoforming, we form round-bottom microwell arrays from optically clear cyclic olefin polymer films, and assemble them with bottom-less 96-well plates. We show that embryonic stem cells aggregate faster and more reproducibly (centricity, circularity) as compared to a state-of-the-art microwell array. We then run a screen of a chemical library to direct differentiation into primitive endoderm (PrE) and, using on-chip high content imaging (HCI), we identify molecules, including regulators of the cAMP pathway, regulating tissue size, morphology and PrE gene activity. We propose that this platform will benefit to the systematic study of organogenesis in vitro.

Development of hematopoietic stem and progenitor cells from human pluripotent stem cells

Human pluripotent stem cells (hPSCs), including human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs), provide a new cell source for regenerative medicine, disease modeling, drug discovery, and preclinical toxicity screening. Understanding of the onset and the sequential process of hematopoietic cells from differentiated hPSCs will enable the achievement of personalized medicine and provide an in vitro platform for studying of human hematopoietic development and disease. During embryogenesis, hemogenic endothelial cells, a specified subset of endothelial cells in embryonic endothelium, are the primary source of multipotent hematopoietic stem cells. In this review, we discuss current status in the generation of multipotent hematopoietic stem and progenitor cells from hPSCs via hemogenic endothelial cells. We also review the achievements in direct reprogramming from non-hematopoietic cells to hematopoietic stem and progenitor cells. Further characterization of hematopoietic differentiation in hPSCs will improve our understanding of blood development and expedite the development of hPSC-derived blood products for therapeutic purpose.

Signaling Control of Differentiation of Embryonic Stem Cells toward Mesendoderm

Mesendoderm (ME) refers to the primitive streak in mammalian embryos, which has the ability to further differentiate into mesoderm and endoderm. A better understanding on the regulatory networks of ME differentiation of embryonic stem (ES) cells would provide important insights on early embryo patterning and a possible guidance for ES applications in regenerative medicine. Studies on developmental biology and embryology have offered a great deal of knowledge about key signaling pathways involved in primitive streak formation. Recently, various chemically defined recipes have been formulated to induce differentiation of ES cells toward ME in vitro, which greatly facilitate the elucidation of the regulatory mechanisms of different signals involved in ME specification. Among the extrinsic signals, transforming growth factor-β/Activin signaling and Wnt signaling have been shown to be the most critical ones. On another side, intrinsic epigenetic regulation has been indicated to be important in ME determination. In this review, we summarize the current understanding on the extrinsic and intrinsic regulations of ES cells-to-ME differentiation and the crosstalk among them, aiming to get a general overview on ME specification and primitive streak formation.

Induced pluripotent stem cell derived macrophages as a cellular system to study salmonella and other pathogens

A number of pathogens, including several human-restricted organisms, persist and replicate within macrophages (Mφs) as a key step in pathogenesis. The mechanisms underpinning such host-restricted intracellular adaptations are poorly understood, in part, due to a lack of appropriate model systems. Here we explore the potential of human induced pluripotent stem cell derived macrophages (iPSDMs) to study such pathogen interactions. We show iPSDMs express a panel of established Mφ-specific markers, produce cytokines, and polarise into classical and alternative activation states in response to IFN-γ and IL-4 stimulation, respectively. iPSDMs also efficiently phagocytosed inactivated bacterial particles as well as live Salmonella Typhi and S. Typhimurium and were able to kill these pathogens. We conclude that iPSDMs can support productive Salmonella infection and propose this as a flexible system to study host/pathogen interactions. Furthermore, iPSDMs can provide a flexible and practical cellular platform for assessing host responses in multiple genetic backgrounds.

From marrow to matrix: novel gene and cell therapies for epidermolysis bullosa

Epidermolysis bullosa encompasses a group of inherited connective tissue disorders that range from mild to lethal. There is no cure, and current treatment is limited to palliative care that is largely ineffective in treating the systemic, life-threatening pathology associated with the most severe forms of the disease. Although allogeneic cell- and protein-based therapies have shown promise, both novel and combinatorial approaches will undoubtedly be required to totally alleviate the disorder. Progress in the development of next-generation therapies that synergize targeted gene-correction and induced pluripotent stem cell technologies offers exciting prospects for personalized, off-the-shelf treatment options that could avoid many of the limitations associated with current allogeneic cell-based therapies. Although no single therapeutic avenue has achieved complete success, each has substantially increased our collective understanding of the complex biology underlying the disease, both providing mechanistic insights and uncovering new hurdles that must be overcome.

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

Objective

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

Materials and Methods

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

Results

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

Conclusion

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

Progress and challenges in generating functional hematopoietic stem/progenitor cells from human pluripotent stem cells

The generation of hematopoietic stem cells (HSCs) from human pluripotent stem cells (hPSCs) in vitro holds great potential for providing alternative sources of donor cells for clinical HSC transplantation. However, the low efficiency of current protocols for generating blood lineages and the dysfunction identified in hPSC-derived hematopoietic cells limit their use for full hematopoietic reconstitution in clinics. This review outlines the current understanding of in vitro hematopoietic differentiation from hPSCs, emphasizes the intrinsic and extrinsic molecular mechanisms that are attributed to the aberrant phenotype and function in hPSC-derived hematopoietic cells, pinpoints the current challenges to develop the truly functional HSCs from hPSCs for clinical applications and explores their potential solutions.

Advances in cellular technology in the hematology field: What have we learned so far?

Despite the advances in the hematology field, blood transfusion-related iatrogenesis is still a major issue to be considered during such procedures due to blood antigenic incompatibility. This places pluripotent stem cells as a possible ally in the production of more suitable blood products. The present review article aims to provide a comprehensive summary of the state-of-the-art concerning the differentiation of both embryonic stem cells and induced pluripotent stem cells to hematopoietic cell lines. Here, we review the most recently published protocols to achieve the production of blood cells for future application in hemotherapy, cancer therapy and basic research.

Modeling murine yolk sac hematopoiesis with embryonic stem cell culture systems

The onset of hematopoiesis in mammals is defined by generation of primitive erythrocytes and macrophage progenitors in embryonic yolk sac. Laboratories have met the challenge of transient and swiftly changing specification events from ventral mesoderm through multipotent progenitors and maturing lineage-restricted hematopoietic subtypes, by developing powerful in vitro experimental models to interrogate hematopoietic ontogeny. Most importantly, studies of differentiating embryonic stem cell derivatives in embryoid body and stromal coculture systems have identified crucial roles for transcription factor networks (e.g. Gata1, Runx1, Scl) and signaling pathways (e.g. BMP, VEGF, WNT) in controlling stem and progenitor cell output. These and other relevant pathways have pleiotropic biological effects, and are often associated with early embryonic lethality in knockout mice. Further refinement in subsequent studies has allowed conditional expression of key regulatory genes, and isolation of progenitors via cell surface markers (e.g. FLK1) and reporter-tagged constructs, with the purpose of measuring their primitive and definitive hematopoietic potential. These observations continue to inform attempts to direct the differentiation, and augment the expansion, of progenitors in human cell culture systems that may prove useful in cell replacement therapies for hematopoietic deficiencies. The purpose of this review is to survey the extant literature on the use of differentiating murine embryonic stem cells in culture to model the developmental process of yolk sac hematopoiesis.

Expression and function of the ID1 gene during transforming growth factor-β1-induced differentiation of human embryonic stem cells to endothelial cells

ID1 can mediate transforming growth factor-β (TGF-β)/activin receptor-like kinase-1 (ALK1)-induced (and Smad-dependent) migration in endothelial cells (ECs). However, the role that ID1 plays during differentiation of human embryonic stem cells (hESCs) into ECs induced by TGF-β1 remains unclear. In this study, a hESC differentiation model that recapitulates the developmental steps of vasculogenesis during the early stages of embryonic development was used to explore this question. We found that TGF-β1 increases endothelial cell differentiation and inhibits endothelial tube formation. Furthermore, at an early stage of differentiation, TGF-β1 may induce in vitro differentiation of hESCs into ECs by inhibiting expression of ID1, while at a later stage of differentiation, TGF-β1 may stimulate the proliferation and migration of ECs via the ALK1/Smad1/5/ID1 pathway. Downregulation of ID1 by gene silencing can lead to acceleration of TGF-β1-induced hESC differentiation into ECs and inhibition of proliferation and migration of ECs. This study may reveal some mechanisms of in vivo vasculogenesis in the early stages of embryonic development.

Generation of multipotent early lymphoid progenitors from human embryonic stem cells

During human embryonic stem cell (ESC) hematopoietic differentiation, the description of the initial steps of lymphopoiesis remains elusive. Using a two-step culture procedure, we identified two original populations of ESC-derived hematopoietic progenitor cells (HPCs) with CD34(+)CD45RA(+)CD7(-) and CD34(+)CD45RA(+)CD7(+) phenotypes. Bulk cultures and limiting dilution assays, culture with MS5 cells in the presence of Notch ligand Delta-like-1 (DL-1), and ex vivo colonization tests using fetal thymic organ cultures showed that although CD34(+)CD45RA(+)CD7(-) HPCs could generate cells of the three lymphoid lineages, their potential was skewed toward the B cell lineages. In contrast, CD34(+)CD45RA(+)CD7(+) HPCs predominantly exhibited a T/natural killer (NK) cell differentiation potential. Furthermore these cells could differentiate equivalently into cells of the granulo-macrophagic lineage and dendritic cells and lacked erythroid potential. Expression profiling of 18 markers by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) revealed that CD34(+)CD45RA(+)CD7(-) and CD34(+)CD45RA(+)CD7(+) HPCs express genes of the lymphoid specification and that CD34(+)CD45RA(+)CD7(-) cells express B-cell-associated genes, while CD34(+)CD45RA(+)CD7(+) HPCs display a T-cell molecular profile. Altogether, these findings indicate that CD34(+)CD45RA(+)CD7(-) and CD34(+)CD45RA(+)CD7(+) HPCs correspond to candidate multipotent early lymphoid progenitors polarized toward either the B or T/NK lineage, respectively. This work should improve our understanding of the early steps of lymphopoiesis from pluripotent stem cells and pave the way for the production of lymphocytes for cell-based immunotherapy and lymphoid development studies.

Efficient differentiation of human pluripotent stem cells to endothelial progenitors via small-molecule activation of WNT signaling

Human pluripotent stem cell (hPSC)-derived endothelial cells and their progenitors may provide the means for vascularization of tissue-engineered constructs and can serve as models to study vascular development and disease. Here, we report a method to efficiently produce endothelial cells from hPSCs via GSK3 inhibition and culture in defined media to direct hPSC differentiation to CD34⁺CD31⁺ endothelial progenitors. Exogenous vascular endothelial growth factor (VEGF) treatment was dispensable, and endothelial progenitor differentiation was β-catenin dependent. Furthermore, by clonal analysis, we showed that CD34⁺CD31⁺CD117⁺TIE-2⁺ endothelial progenitors were multipotent, capable of differentiating into calponin-expressing smooth muscle cells and CD31⁺CD144⁺vWF⁺I-CAM1⁺ endothelial cells. These endothelial cells were capable of 20 population doublings, formed tube-like structures, imported acetylated low-density lipoprotein, and maintained a dynamic barrier function. This study provides a rapid and efficient method for production of hPSC-derived endothelial progenitors and endothelial cells and identifies WNT/β-catenin signaling as a primary regulator for generating vascular cells from hPSCs.

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WNT pathway activation directs hPSC differentiation to endothelial progenitors

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hPSC-derived endothelial progenitors can differentiate to endothelial cells

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Purified hPSC-derived endothelial cells are capable of 20 population doublings

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WNT pathway activation permits defined production of endothelial cells from hPSCs

let-7 miRNAs can act through notch to regulate human gliogenesis

It is clear that neural differentiation from human pluripotent stem cells generates cells that are developmentally immature. Here, we show that the let-7 plays a functional role in the developmental decision making of human neural progenitors, controlling whether these cells make neurons or glia. Through gain- and loss-of-function studies on both tissue and pluripotent derived cells, our data show that let-7 specifically regulates decision making in this context by regulation of a key chromatin-associated protein, HMGA2. Furthermore, we provide evidence that the let-7/HMGA2 circuit acts on HES5, a NOTCH effector and well-established node that regulates fate decisions in the nervous system. These data link the let-7 circuit to NOTCH signaling and suggest that this interaction serves to regulate human developmental progression.

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let-7 miRNAs influence developmental maturity of neural progenitors

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let-7 miRNAs act through HMGA2 and NOTCH to regulate gliogenesis

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HMGA2 expression regulates access of NICD to HES5 promoter

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Induction of let-7 miRNAs can accelerate oligodendrogenesis

HOXA9 promotes hematopoietic commitment of human embryonic stem cells

The molecular determinants regulating the specification of human embryonic stem cells (hESCs) into hematopoietic cells remain elusive. HOXA9 plays a relevant role in leukemogenesis and hematopoiesis. It is highly expressed in hematopoietic stem and progenitor cells (HSPCs) and is downregulated upon differentiation. Hoxa9-deficient mice display impaired hematopoietic development, and deregulation of HOXA9 expression is frequently associated with acute leukemia. Analysis of the genes differentially expressed in cord blood HSPCs vs hESC-derived HSPCs identified HOXA9 as the most downregulated gene in hESC-derived HSPCs, suggesting that expression levels of HOXA9 may be crucial for hematopoietic differentiation of hESC. Here we show that during hematopoietic differentiation of hESCs, HOXA9 expression parallels hematopoietic development, but is restricted to the hemogenic precursors (HEP) (CD31(+)CD34(+)CD45(-)), and diminishes as HEPs differentiate into blood cells (CD45(+)). Different gain-of-function and loss-of-function studies reveal that HOXA9 enhances hematopoietic differentiation of hESCs by specifically promoting the commitment of HEPs into primitive and total CD45(+) blood cells. Gene expression analysis suggests that nuclear factor-κB signaling could be collaborating with HOXA9 to increase hematopoietic commitment. However, HOXA9 on its own is not sufficient to confer in vivo long-term engraftment potential to hESC-hematopoietic derivatives, reinforcing the idea that additional molecular regulators are needed for the generation of definitive in vivo functional HSPCs from hESC.

Differentiation of hESCs into Mesodermal Subtypes: Vascular-, Hematopoietic- and Mesenchymal-lineage Cells

To date, studies on the application of mesodermally derived mesenchymal-, hematopoietic- and vascular-lineage cells for cell therapy have provided either poor or insufficient data. The results are equivocal with regard to therapeutic efficiency and yield. Since the establishment of human embryonic stem cells (hESCs) in 1998, the capacity of hESCs to differentiate into various mesodermal lineages has sparked considerable interest in the regenerative medicine community, a group interested in generating specialized cells to treat patients suffering from degenerative diseases. Even though hESCs are sensitive, effective methods for guiding the differentiation of hESCs into specific mesodermal cell types are still being developed. In addition, to understand the functional properties of hESC derivatives, numerous animal model studies have been performed by many research groups over the last decade. In this review, we describe and summarize the protocols currently used for differentiation of hESCs into multiple mesodermal lineages and their therapeutic efficiency in different animal models. Furthermore, we discuss the technical hurdles associated with each protocol and the safety of hESC derivatives for therapeutic applications. Technical improvement of the methods used to produce hESC derivatives for therapeutic use in patients with degenerative diseases should remain an objective of future studies, as should the development of effective and stable induction systems.

Broad T-cell receptor repertoire in T-lymphocytes derived from human induced pluripotent stem cells

Human induced pluripotent stem cells (hiPSCs) have enormous potential for the treatment of inherited and acquired disorders. Recently, antigen-specific T lymphocytes derived from hiPSCs have been reported. However, T lymphocyte populations with broad T cell receptor (TCR) diversity have not been generated. We report that hiPSCs derived from skin biopsy are capable of producing T lymphocyte populations with a broad TCR repertoire. In vitro T cell differentiation follows a similar developmental program as observed in vivo, indicated by sequential expression of CD7, intracellular CD3 and surface CD3. The γδ TCR locus is rearranged first and is followed by rearrangement of the αβ locus. Both γδ and αβ T cells display a diverse TCR repertoire. Upon activation, the cells express CD25, CD69, cytokines (TNF-α, IFN-γ, IL-2) and cytolytic proteins (Perforin and Granzyme-B). These results suggest that most, if not all, mechanisms required to generate functional T cells with a broad TCR repertoire are intact in our in vitro differentiation protocol. These data provide a foundation for production of patient-specific T cells for the treatment of acquired or inherited immune disorders and for cancer immunotherapy.

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.

Tracking the elusive fibrocyte: identification and characterization of collagen-producing hematopoietic lineage cells during murine wound healing

Fibrocytes are a unique population of circulating cells reported to exhibit characteristics of both hematopoietic and mesenchymal cells, and play an important role in wound healing. However, putative fibrocytes have been found to lose expression of hematopoietic surface markers such as CD45 during differentiation, making it difficult to track these cells in vivo with conventional methodologies. In this study, to distinguish hematopoietic and nonhematopoietic cells without surface markers, we took advantage of the gene vav 1, which is expressed solely on hematopoietic cells but not on other cell types, and established a novel transgenic mouse, in which hematopoietic cells are irreversibly labeled with green fluorescent protein and nonhematopoietic cells with red fluorescent protein. Use of single-cell transcriptional analysis in this mouse model revealed two discrete types of collagen I (Col I) expressing cells of hematopoietic lineage recruited into excisional skin wounds. We confirmed this finding on a protein level, with one subset of these Col I synthesizing cells being CD45+ and CD11b+, consistent with the traditional definition of a fibrocyte, while another was CD45- and Cd11b-, representing a previously unidentified population. Both cell types were found to initially peak, then reduce posthealing, consistent with a disappearance from the wound site and not a loss of identifying surface marker expression. Taken together, we have unambiguously identified two cells of hematopoietic origin that are recruited to the wound site and deposit collagen, definitively confirming the existence and natural time course of fibrocytes in cutaneous healing.

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

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

Primitive and definitive erythropoiesis in mammals

Red blood cells (RBCs), which constitute the most abundant cell type in the body, come in two distinct flavors- primitive and definitive. Definitive RBCs in mammals circulate as smaller, anucleate cells during fetal and postnatal life, while primitive RBCs circulate transiently in the early embryo as large, nucleated cells before ultimately enucleating. Both cell types are formed from lineage-committed progenitors that generate a series of morphologically identifiable precursors that enucleate to form mature RBCs. While definitive erythroid precursors mature extravascularly in the fetal liver and postnatal marrow in association with macrophage cells, primitive erythroid precursors mature as a semi-synchronous cohort in the embryonic bloodstream. While the cytoskeletal network is critical for the maintenance of cell shape and the deformability of definitive RBCs, little is known about the components and function of the cytoskeleton in primitive erythroblasts. Erythropoietin (EPO) is a critical regulator of late-stage definitive, but not primitive, erythroid progenitor survival. However, recent studies indicate that EPO regulates multiple aspects of terminal maturation of primitive murine and human erythroid precursors, including cell survival, proliferation, and the rate of terminal maturation. Primitive and definitive erythropoiesis share central transcriptional regulators, including Gata1 and Klf1, but are also characterized by the differential expression and function of other regulators, including myb, Sox6, and Bcl11A. Flow cytometry-based methodologies, developed to purify murine and human stage-specific erythroid precursors, have enabled comparative global gene expression studies and are providing new insights into the biology of erythroid maturation.

Considerations on the possible origins of fetal hemoglobin cells produced in developing tumors

Although tumor angiogenesis in relation to cancer therapy has been widely investigated for more than four decades, its counterpart tumor hematopoiesis has not been equally considered. In that respect, in our long-term immunohistochemical examination of fetal hemoglobin (HbF) cells in various solid tumors, we have observed signs of fetal hematopoiesis in situ within the tumors. We hypothesize that this observed fetal hematopoiesis, involving angiogenesis, mirrors mammalian blood system development in the embryo and the fetus; this is consistent with the concept of the hemogenic endothelial progenitor, common to endothelial and hemopoietic cells. Based on this assumption, there should exist in tumors at least two routes of hematoangiogenesis: one of fetal (HbF) hematopoiesis and the other of adult (HbA) hematopoiesis, each one deserving a different therapeutic approach. In the fetal route, HbF should support tumor growth by virtue of its high oxygen affinity.

Hematopoietic differentiation of pluripotent stem cells in culture

This chapter describes a two-dimensional "monolayer" system for differentiating human pluripotent stem cells (PSCs) into "primitive" hematopoietic progenitor cells (HPCs) resembling those produced in vivo by the early embryonic yolk sac. This experimental system utilizes defined conditions without serum or feeder cells. Cytokines are added sequentially to stimulate the formation of mesoderm and its subsequent patterning to hematopoietic progenitors. The HPCs produced by this protocol have multi-lineage potential (erythroid, megakaryocyte, and myeloid) and can be isolated as a homogeneous population for use in standard hematopoietic studies including liquid expansion to mature lineages and colony assays. In addition, the HPCs can be cryopreserved for distribution or analysis at later times. The HPCs generated by this protocol have been used successfully to better define intrinsic variation in hematopoietic potential between different PSC lines and to model human hematopoietic diseases using patient-derived induced pluripotent stem cells.

The negative impact of Wnt signaling on megakaryocyte and primitive erythroid progenitors derived from human embryonic stem cells

The Wnt gene family consists of structurally related genes encoding secreted signaling molecules that have been implicated in many developmental processes, including regulation of cell fate and patterning during embryogenesis. Previously, we found that Wnt signaling is required for primitive or yolk sac-derived-erythropoiesis using the murine embryonic stem cell (ESC) system. Here, we examine the effect of Wnt signaling on the formation of early hematopoietic progenitors derived from human ESCs. The first hematopoietic progenitor cells in the human ESC system express the pan-hematopoietic marker CD41 and the erythrocyte marker, glycophorin A or CD235. We have developed a novel serum-free, feeder-free, adherent differentiation system that can efficiently generate large numbers of CD41+CD235+ cells. We demonstrate that this cell population contains progenitors not just for primitive erythroid and megakaryocyte cells but for the myeloid lineage as well and term this population the primitive common myeloid progenitor (CMP). Treatment of mesoderm-specified cells with Wnt3a led to a loss of hematopoietic colony-forming ability while the inhibition of canonical Wnt signaling with DKK1 led to an increase in the number of primitive CMPs. Canonical Wnt signaling also inhibits the expansion and/or survival of primitive erythrocytes and megakaryocytes, but not myeloid cells, derived from this progenitor population. These findings are in contrast to the role of Wnt signaling during mouse ESC differentiation and demonstrate the importance of the human ESC system in studying species-specific differences in development.

Engineering the human pluripotent stem cell microenvironment to direct cell fate

Human pluripotent stem cells (hPSCs), including both embryonic stem cells and induced pluripotent stem cells, offer a potential cell source for research, drug screening, and regenerative medicine applications due to their unique ability to self-renew or differentiate to any somatic cell type. Before the full potential of hPSCs can be realized, robust protocols must be developed to direct their fate. Cell fate decisions are based on components of the surrounding microenvironment, including soluble factors, substrate or extracellular matrix, cell-cell interactions, mechanical forces, and 2D or 3D architecture. Depending on their spatio-temporal context, these components can signal hPSCs to either self-renew or differentiate to cell types of the ectoderm, mesoderm, or endoderm. Researchers working at the interface of engineering and biology have identified various factors which can affect hPSC fate, often based on lessons from embryonic development, and they have utilized this information to design in vitro niches which can reproducibly direct hPSC fate. This review highlights culture systems that have been engineered to promote self-renewal or differentiation of hPSCs, with a focus on studies that have elucidated the contributions of specific microenvironmental cues in the context of those culture systems. We propose the use of microsystem technologies for high-throughput screening of spatial-temporal presentation of cues, as this has been demonstrated to be a powerful approach for differentiating hPSCs to desired cell types.

Vascular progenitors from cord blood-derived induced pluripotent stem cells possess augmented capacity for regenerating ischemic retinal vasculature

BACKGROUND

The generation of vascular progenitors (VPs) from human induced pluripotent stem cells (hiPSCs) has great potential for treating vascular disorders such as ischemic retinopathies. However, long-term in vivo engraftment of hiPSC-derived VPs into the retina has not yet been reported. This goal may be limited by the low differentiation yield, greater senescence, and poor proliferation of hiPSC-derived vascular cells. To evaluate the potential of hiPSCs for treating ischemic retinopathies, we generated VPs from a repertoire of viral-integrated and nonintegrated fibroblast and cord blood (CB)-derived hiPSC lines and tested their capacity for homing and engrafting into murine retina in an ischemia-reperfusion model.

METHODS AND RESULTS

VPs from human embryonic stem cells and hiPSCs were generated with an optimized vascular differentiation system. Fluorescence-activated cell sorting purification of human embryoid body cells differentially expressing endothelial/pericytic markers identified a CD31(+)CD146(+) VP population with high vascular potency. Episomal CB-induced pluripotent stem cells (iPSCs) generated these VPs with higher efficiencies than fibroblast-iPSC. Moreover, in contrast to fibroblast-iPSC-VPs, CB-iPSC-VPs maintained expression signatures more comparable to human embryonic stem cell VPs, expressed higher levels of immature vascular markers, demonstrated less culture senescence and sensitivity to DNA damage, and possessed fewer transmitted reprogramming errors. Luciferase transgene-marked VPs from human embryonic stem cells, CB-iPSCs, and fibroblast-iPSCs were injected systemically or directly into the vitreous of retinal ischemia-reperfusion-injured adult nonobese diabetic-severe combined immunodeficient mice. Only human embryonic stem cell- and CB-iPSC-derived VPs reliably homed and engrafted into injured retinal capillaries, with incorporation into damaged vessels for up to 45 days.

CONCLUSIONS

VPs generated from CB-iPSCs possessed augmented capacity to home, integrate into, and repair damaged retinal vasculature.

DNA methylation of Runx1 regulatory regions correlates with transition from primitive to definitive hematopoietic potential in vitro and in vivo

The transcription factor Runx1 (AML1) is a central regulator of hematopoiesis and is required for the formation of definitive hematopoietic stem cells (HSCs). Runx1 is alternatively expressed from two promoters: the proximal (P2) prevails during primitive hematopoiesis, while the distal (P1) dominates in definitive HSCs. Although some transcription factor binding sites and cis-regulatory elements have been identified, a mechanistic explanation for the alternative promoter usage remains elusive. We investigated DNA methylation of known Runx1 cis-elements at stages of hematopoietic development in vivo and during differentiation of murine embryonic stem cells (ESCs) in vitro. In vivo, we find loss of methylation correlated with the primitive to definitive transition at the P1 promoter. In vitro, hypomethylation, acquisition of active chromatin modifications, and increased transcriptional activity at P1 are promoted by direct interaction with HOXB4, a transcription factor that confers definitive repopulation status on primitive hematopoietic progenitors. These data demonstrate a novel role for DNA methylation in the alternative promoter usage at the Runx1 locus and identify HOXB4 as a direct activator of the P1 promoter. This epigenetic signature should serve as a novel biomarker of HSC potential in vivo, and during ESC differentiation in vitro.

Hematopoietic specification from human pluripotent stem cells: current advances and challenges toward de novo generation of hematopoietic stem cells

Significant advances in cellular reprogramming technologies and hematopoietic differentiation from human pluripotent stem cells (hPSCs) have already enabled the routine production of multiple lineages of blood cells in vitro and opened novel opportunities to study hematopoietic development, model genetic blood diseases, and manufacture immunologically matched cells for transfusion and cancer immunotherapy. However, the generation of hematopoietic cells with robust and sustained multilineage engraftment has not been achieved. Here, we highlight the recent advances in understanding the molecular and cellular pathways leading to blood development from hPSCs and discuss potential approaches that can be taken to facilitate the development of technologies for de novo production of hematopoietic stem cells.

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.

Origin and differentiation of microglia

Microglia are the resident macrophage population of the central nervous system (CNS). Adequate microglial function is crucial for a healthy CNS. Microglia are not only the first immune sentinels of infection, contributing to both innate and adaptive immune responses locally, but are also involved in the maintenance of brain homeostasis. Emerging data are showing new and fundamental roles for microglia in the control of neuronal proliferation and differentiation, as well as in the formation of synaptic connections. While microglia have been studied for decades, a long history of experimental misinterpretation meant that their true origins remained debated. However, recent studies on microglial origin indicate that these cells in fact arise early during development from progenitors in the embryonic yolk sac (YS) that seed the brain rudiment and, remarkably, appear to persist there into adulthood. Here, we review the history of microglial cells and discuss the latest advances in our understanding of their origin, differentiation, and homeostasis, which provides new insights into their roles in health and disease.

Concise review: chemical approaches for modulating lineage-specific stem cells and progenitors

Generation and manipulation of lineage-restricted stem and progenitor cells in vitro and/or in vivo are critical for the development of stem cell-based clinical therapeutics. Lineage-restricted stem and progenitor cells have many advantageous qualities, including being able to efficiently engraft and differentiate into desirable cell types in vivo after transplantation, and they are much less tumorigenic than pluripotent cells. Generation of lineage-restricted stem and progenitor cells can be achieved by directed differentiation from pluripotent stem cells or lineage conversion from easily obtained somatic cells. Small molecules can be very helpful in these processes since they offer several important benefits. For example, the risk of tumorigenesis is greatly reduced when small molecules are used to replace integrated transcription factors, which are widely used in cell fate conversion. Furthermore, small molecules are relatively easy to apply, optimize, and manufacture, and they can more readily be developed into conventional pharmaceuticals. Alternatively, small molecules can be used to expand or selectively control the differentiation of lineage-restricted stem and progenitor cells for desirable therapeutics purposes in vitro or in vivo. Here we summarize recent progress in the use of small molecules for the expansion and generation of desirable lineage-restricted stem and progenitor cells in vitro and for selectively controlling cell fate of lineage-restricted stem and progenitor cells in vivo, thereby facilitating stem cell-based clinical applications.