Noggin and chordin have distinct activities in promoting lineage commitment of mouse embryonic stem (ES) cells

Deciphering and reconstitution of positional information in the human brain development

Organoid has become a novel in vitro model to research human development and relevant disorders in recent years. With many improvements on the culture protocols, current brain organoids could self-organize into a complicated three-dimensional organization that mimics most of the features of the real human brain at the molecular, cellular, and further physiological level. However, lacking positional information, an important characteristic conveyed by gradients of signaling molecules called morphogens, leads to the deficiency of spatiotemporally regulated cell arrangements and cell–cell interactions in the brain organoid development. In this review, we will overview the role of morphogen both in the vertebrate neural development in vivo as well as the brain organoid culture in vitro, the strategies to apply morphogen concentration gradients in the organoid system and future perspectives of the brain organoid technology.

Osteogenic Mechanisms of Basal Ganglia Calcification and its ex vivo Model in the Hypoparathyroid Milieu

CONTEXT

Basal-ganglia calcification (BGC) is common (70%) in patients with chronic hypoparathyroidism. Interestingly, cortical gray matter is spared from calcification. The mechanism of BGC, role of hyperphosphatemia, and modulation of osteogenic molecules by parathyroid hormone (PTH) in its pathogenesis is not clear.

OBJECTIVE

We assessed the expression of a large repertoire of molecules with proosteogenic or antiosteogenic effects, including neuroprogenitor cells in caudate, dentate, and cortical gray matter from normal autopsy tissues. The effect of high phosphate and PTH was assessed in an ex vivo model of BGC using striatum tissue culture of the Sprague-Dawley rat.

METHODS

The messenger RNA and protein expression of 39 molecules involved in multiple osteogenic pathways were assessed in 25 autopsy tissues using reverse-transcriptase polymerase chain reaction, Western blot, and immunofluorescence. The striatal culture was maintained in a hypoparathyroid milieu for 24 days with and without (a) high phosphate (10-mm β-glycerophosphate) and (b) PTH(1-34) (50 ng/mL Dulbecco's modified Eagle's medium-F12 media) for their effect on striatal calcification and osteogenic molecules.

RESULTS

Procalcification molecules (osteonectin, β-catenin, klotho, FZD4, NT5E, LRP5, WNT3A, collagen-1α, and SOX2-positive neuroprogenitor stem cells) had significantly higher expression in the caudate than gray matter. Caudate nuclei also had higher expression of antiosteogenic molecules (osteopontin, carbonic anhydrase-II [CA-II], MGP, sclerostin, ISG15, ENPP1, and USP18). In an ex vivo model, striatum culture showed an increased propensity for calcified nodules with mineral deposition similar to that of bone tissue on Fourier-transformed infrared spectroscopy, alizarin, and von Kossa stain. Mineralization in striatal culture was enhanced by high phosphate and decreased by exogenous PTH through increased expression of CA-II.

CONCLUSION

This study provides a conceptual advance on the molecular mechanisms of BGC and the possibility of PTH therapy to prevent this complication in a hypoparathyroid milieu.

Modeling Mammalian Commitment to the Neural Lineage Using Embryos and Embryonic Stem Cells

Early mammalian embryogenesis relies on a large range of cellular and molecular mechanisms to guide cell fate. In this highly complex interacting system, molecular circuitry tightly controls emergent properties, including cell differentiation, proliferation, morphology, migration, and communication. These molecular circuits include those responsible for the control of gene and protein expression, as well as metabolism and epigenetics. Due to the complexity of this circuitry and the relative inaccessibility of the mammalian embryo in utero, mammalian neural commitment remains one of the most challenging and poorly understood areas of developmental biology. In order to generate the nervous system, the embryo first produces two pluripotent populations, the inner cell mass and then the primitive ectoderm. The latter is the cellular substrate for gastrulation from which the three multipotent germ layers form. The germ layer definitive ectoderm, in turn, is the substrate for multipotent neurectoderm (neural plate and neural tube) formation, representing the first morphological signs of nervous system development. Subsequent patterning of the neural tube is then responsible for the formation of most of the central and peripheral nervous systems. While a large number of studies have assessed how a competent neurectoderm produces mature neural cells, less is known about the molecular signatures of definitive ectoderm and neurectoderm and the key molecular mechanisms driving their formation. Using pluripotent stem cells as a model, we will discuss the current understanding of how the pluripotent inner cell mass transitions to pluripotent primitive ectoderm and sequentially to the multipotent definitive ectoderm and neurectoderm. We will focus on the integration of cell signaling, gene activation, and epigenetic control that govern these developmental steps, and provide insight into the novel growth factor-like role that specific amino acids, such as L-proline, play in this process.

Functional comparison of distinct Brachyury+ states in a renal differentiation assay

Mesodermal populations can be generated in vitro from mouse embryonic stem cells (mESCs) using three-dimensional (3-D) aggregates called embryoid bodies or two-dimensional (2-D) monolayer culture systems. Here, we investigated whether Brachyury-expressing mesodermal cells generated using 3-D or 2-D culture systems are equivalent or, instead, have different properties. Using a Brachyury-GFP/E2-Crimson reporter mESC line, we isolated Brachyury-GFP ⁺ mesoderm cells using flow-activated cell sorting and compared their gene expression profiles and ex vivo differentiation patterns. Quantitative real-time polymerase chain reaction analysis showed significant up-regulation of Cdx2, Foxf1 and Hoxb1 in the Brachyury-GFP⁺ cells isolated from the 3-D system compared with those isolated from the 2-D system. Furthermore, using an ex vivo mouse kidney rudiment assay, we found that, irrespective of their source, Brachyury-GFP⁺ cells failed to integrate into developing nephrons, which are derived from the intermediate mesoderm. However, Brachyury-GFP⁺ cells isolated under 3-D conditions appeared to differentiate into endothelial-like cells within the kidney rudiments, whereas the Brachyury-GFP⁺ isolated from the 2-D conditions only did so to a limited degree. The high expression of Foxf1 in the 3-D Brachyury-GFP⁺ cells combined with their tendency to differentiate into endothelial-like cells suggests that these mesodermal cells may represent lateral plate mesoderm.

Derivation of Functional Dopamine Neurons from Embryonic Stem Cells

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the selective degeneration of dopaminergic (DA) neurons in the substantia nigra of the midbrain. Pharmacological treatment of PD has been a prevailing strategy. However, it has some limitations because its effectiveness gradually decreases and side effects develop. As an alternative, cell transplantation therapy has been tried. Although transplantation of fetal ventral mesencephalic cells looks promising for the treatment of PD in some cases, ethical and technical problems in obtaining large numbers of human fetal brain tissues also lead to difficulty in its clinical application. Our recent studies showed that a high yield of DA neurons could be derived from embryonic stem (ES) cells and they efficiently induced behavioral recovery in a PD animal model. Here we summarize methods for generation of functional DA neurons from ES cells for application to PD models.

Embryonic stem cell differentiation requires full length Chd1

The modulation of chromatin dynamics by ATP-dependent chromatin remodeling factors has been recognized as an important mechanism to regulate the balancing of self-renewal and pluripotency in embryonic stem cells (ESCs). Here we have studied the effects of a partial deletion of the gene encoding the chromatin remodeling factor Chd1 that generates an N-terminally truncated version of Chd1 in mouse ESCs in vitro as well as in vivo. We found that a previously uncharacterized serine-rich region (SRR) at the N-terminus is not required for chromatin assembly activity of Chd1 but that it is subject to phosphorylation. Expression of Chd1 lacking this region in ESCs resulted in aberrant differentiation properties of these cells. The self-renewal capacity and ESC chromatin structure, however, were not affected. Notably, we found that newly established ESCs derived from Chd1(Δ2/Δ2) mutant mice exhibited similar differentiation defects as in vitro generated mutant ESCs, even though the N-terminal truncation of Chd1 was fully compatible with embryogenesis and post-natal life in the mouse. These results underscore the importance of Chd1 for the regulation of pluripotency in ESCs and provide evidence for a hitherto unrecognized critical role of the phosphorylated N-terminal SRR for full functionality of Chd1.

The transcription factor Pou3f1 promotes neural fate commitment via activation of neural lineage genes and inhibition of external signaling pathways

The neural fate commitment of pluripotent stem cells requires the repression of extrinsic inhibitory signals and the activation of intrinsic positive transcription factors. However, how these two events are integrated to ensure appropriate neural conversion remains unclear. In this study, we showed that Pou3f1 is essential for the neural differentiation of mouse embryonic stem cells (ESCs), specifically during the transition from epiblast stem cells (EpiSCs) to neural progenitor cells (NPCs). Chimeric analysis showed that Pou3f1 knockdown leads to a markedly decreased incorporation of ESCs in the neuroectoderm. By contrast, Pou3f1-overexpressing ESC derivatives preferentially contribute to the neuroectoderm. Genome-wide ChIP-seq and RNA-seq analyses indicated that Pou3f1 is an upstream activator of neural lineage genes, and also is a repressor of BMP and Wnt signaling. Our results established that Pou3f1 promotes the neural fate commitment of pluripotent stem cells through a dual role, activating internal neural induction programs and antagonizing extrinsic neural inhibitory signals.

The positional identity of mouse ES cell-generated neurons is affected by BMP signaling

We investigated the effects of bone morphogenetic proteins (BMPs) in determining the positional identity of neurons generated in vitro from mouse embryonic stem cells (ESCs), an aspect that has been neglected thus far. Classical embryological studies in lower vertebrates indicate that BMPs inhibit the default fate of pluripotent embryonic cells, which is both neural and anterior. Moreover, mammalian ESCs generate neurons more efficiently when cultured in a minimal medium containing BMP inhibitors. In this paper, we show that mouse ESCs produce, secrete, and respond to BMPs during in vitro neural differentiation. After neuralization in a minimal medium, differentiated ESCs show a gene expression profile consistent with a midbrain identity, as evaluated by the analysis of a number of markers of anterior-posterior and dorsoventral identity. We found that BMPs endogenously produced during neural differentiation mainly act by inhibiting the expression of a telencephalic gene profile, which was revealed by the treatment with Noggin or with other BMP inhibitors. To better characterize the effect of BMPs on positional fate, we compared the global gene expression profiles of differentiated ESCs with those of embryonic forebrain, midbrain, and hindbrain. Both Noggin and retinoic acid (RA) support neuronal differentiation of ESCs, but they show different effects on their positional identity: whereas RA supports the typical gene expression profile of hindbrain neurons, Noggin induces a profile characteristic of dorsal telencephalic neurons. Our findings show that endogenously produced BMPs affect the positional identity of the neurons that ESCs spontaneously generate when differentiating in vitro in a minimal medium. The data also support the existence of an intrinsic program of neuronal differentiation with dorsal telencephalic identity. Our method of ESC neuralization allows for fast differentiation of neural cells via the same signals found during in vivo embryonic development and for the acquisition of cortical identity by the inhibition of BMP alone.

Geminin is required for epithelial to mesenchymal transition at gastrulation

Geminin is a multifunctional protein previously suggested to both maintain the bone morphogenetic protein inhibition required for neural induction and to control cell-cycle progression and cell fate in the early embryo. Since Geminin is required in the blastocyst on E3.5, we employed shRNA to examine its role during postimplantation development. Geminin knockdown inhibited the epithelial to mesenchymal transition (EMT) required at gastrulation and neural crest delamination, resulting in anterior-posterior axis and patterning defects, while overexpression promoted EMT at both locations. Geminin was negatively correlated with expression of E-cadherin, which is critically involved in controlling epithelial architecture. In addition, Geminin expression level was correlated with Wnt signaling and expression of the Wnt target gene Axin2 and with Msx2, and negatively correlated with the expression of Bmp4 and Neurog1 in quantitative reverse transcriptase-polymerase chain reaction analysis of RNAs from individual embryos. These results suggest that in addition to patterning the early embryo, Geminin plays a previously unrecognized role in EMT via its ability to affect Wnt signaling and E-cadherin expression.

Evo-devo: Hydra raises its Noggin

Noggin, along with other secreted bone morphogenetic protein (BMP) inhibitors, plays a crucial role in neural induction and neural tube patterning as well as in somitogenesis, cardiac morphogenesis and formation of the skeleton in vertebrates. The BMP signalling pathway is one of the seven fundamental pathways that drive embryonic development and pattern formation in animals. Understanding its evolutionary origin and role in pattern formation is, therefore, important to evolutionary developmental biology (evo-devo). We have studied the evolutionary origin of BMP-Noggin antagonism in hydra, which is a powerful diploblastic model to study evolution of pattern-forming mechanisms because of the unusual cellular dynamics during its pattern formation and its remarkable ability to regenerate. We cloned and characterized the noggin gene from hydra and found it to exhibit considerable similarity with its orthologues at the amino acid level. Microinjection of hydra Noggin mRNA led to duplication of the dorsoventral axis in Xenopus embryos, demonstrating its functional conservation across the taxa. Our data, along with those of others, indicate that the evolutionarily conserved antagonism between BMP and its inhibitors predates bilateral divergence. This article reviews the various roles of Noggin in different organisms and some of our recent work on hydra Noggin in the context of evolution of developmental signalling pathways.

The 3'-phosphoadenosine 5'-phosphosulfate transporters, PAPST1 and 2, contribute to the maintenance and differentiation of mouse embryonic stem cells

Recently, we have identified two 3'-phosphoadenosine 5'-phosphosulfate (PAPS) transporters (PAPST1 and PAPST2), which contribute to PAPS transport into the Golgi, in both human and Drosophila. Mutation and RNA interference (RNAi) of the Drosophila PAPST have shown the importance of PAPST-dependent sulfation of carbohydrates and proteins during development. However, the functional roles of PAPST in mammals are largely unknown. Here, we investigated whether PAPST-dependent sulfation is involved in regulating signaling pathways required for the maintenance of mouse embryonic stem cells (mESCs), differentiation into the three germ layers, and neurogenesis. By using a yeast expression system, mouse PAPST1 and PAPST2 proteins were shown to have PAPS transport activity with an apparent K(m) value of 1.54 microM or 1.49 microM, respectively. RNAi-mediated knockdown of each PAPST induced the reduction of chondroitin sulfate (CS) chain sulfation as well as heparan sulfate (HS) chain sulfation, and inhibited mESC self-renewal due to defects in several signaling pathways. However, we suggest that these effects were due to reduced HS, not CS, chain sulfation, because knockdown of mouse N-deacetylase/N-sulfotransferase, which catalyzes the first step of HS sulfation, in mESCs gave similar results to those observed in PAPST-knockdown mESCs, but depletion of CS chains did not. On the other hand, during embryoid body formation, PAPST-knockdown mESCs exhibited abnormal differentiation, in particular neurogenesis was promoted, presumably due to the observed defects in BMP, FGF and Wnt signaling. The latter were reduced as a result of the reduction in both HS and CS chain sulfation. We propose that PAPST-dependent sulfation of HS or CS chains, which is regulated developmentally, regulates the extrinsic signaling required for the maintenance and normal differentiation of mESCs.

Gene regulatory networks in embryonic stem cells and brain development

Embryonic stem cells (ESCs) are endowed with the ability to generate multiple cell lineages and carry great therapeutic potentials in regenerative medicine. Future application of ESCs in human health and diseases will embark on the delineation of molecular mechanisms that define the biology of ESCs. Here, we discuss how the finite ESC components mediate the intriguing task of brain development and exhibit biomedical potentials to cure diverse neurological disorders.

Bone morphogenetic proteins induce expression of metalloproteinases in melanoma cells and fibroblasts

Bone morphogenetic proteins are secreted growth factors which belong to the TGFbeta super family. In recent studies, we showed that the expression of BMP-4 and -7 is induced in melanoma cells in comparison to normal melanocytes. Functional analyses revealed that BMPs are inevitable factors for migration and invasion processes of melanoma cells; however, the role of BMPs in degradation and remodelling of the extracellular matrix remained unknown. We discovered that melanoma cell clones with reduced BMP activity, generated by stable transfection with an antisense BMP-4 construct or with the BMP inhibitor chordin, showed reduced expression of MMP-1, -2, -3 and -9. Moreover, BMPs displayed paracrine effects on stromal fibroblasts. Treatment of fibroblasts with BMP-2 or -4 led to increased MMP-1, -2, -3 and -13 expression. These data show that BMPs play an important role in dissemination of tumour cells from the primary tumour, either by enhancing the matrix degrading capacity of melanoma cells themselves or by stimulating tumour surrounding fibroblasts to induce expression of matrix metalloproteinases.

Coupled global and targeted proteomics of human embryonic stem cells during induced differentiation

Elucidating the complex combinations of growth factors and signaling molecules that maintain pluripotency or, alternatively, promote the controlled differentiation of human embryonic stem cells (hESCs) has important implications for the fundamental understanding of human development, devising cell replacement therapies, and cancer cell biology. hESCs are commonly grown on irradiated mouse embryonic fibroblasts (MEFs) or in conditioned medium from MEFs. These culture conditions interfere with many experimental conclusions and limit the ability to perform conclusive proteomics studies. The current investigation avoided the use of MEFs or MEF-conditioned medium for hESC culture, allowing global proteomics analysis without these confounding conditions, and elucidated neural cell-specific signaling pathways involved in noggin-induced hESC differentiation. Based on these analyses, we propose the following early markers of hESC neural differentiation: collapsin response mediator proteins 2 and 4 and the nuclear autoantigenic sperm protein as a marker of pluripotent hESCs. We then developed a directed mass spectrometry assay using multiple reaction monitoring (MRM) to identify and quantify these markers and in addition the epidermal ectoderm marker cytokeratin-8. Analysis of global proteomics, quantitative RT-PCR, and MRM data led to testing the isoform interference hypothesis where redundant peptides dilute quantification measurements of homologous proteins. These results show that targeted MRM analysis on non-redundant peptides provides more exact quantification of homologous proteins. This study describes the facile transition from discovery proteomics to targeted MRM analysis and allowed us to identify and verify several potential biomarkers for hESCs during noggin-induced neural and BMP4-induced epidermal ectoderm differentiation.

Derive and conquer: sourcing and differentiating stem cells for therapeutic applications

Although great progress has been made in the isolation and culture of stem cells, the future of stem-cell-based therapies and their productive use in drug discovery and regenerative medicine depends on two key factors: finding reliable sources of multipotent and pluripotent cells and the ability to control their differentiation to generate desired derivatives. It is essential for clinical applications to establish reliable sources of pathogen-free human embryonic stem cells (ESCs) and develop suitable differentiation techniques. Here, we address some of the problems associated with the sourcing of human ESCs and discuss the current status of stem-cell differentiation technology.

Differential gene expression in ES-derived neural stem cells by using RT-PCR

Embryonic stem (ES) cells hold promise to treat a variety of disease. The major obstacle is to determine the requirements that will drive these cells to a particular lineage. Two approaches to examine lineage commitment are the addition of growth factors or directed differentiation of ES cells. Although many neural genes have been identified, the cascade of gene expression that directs neural differentiation is not well understood. Today, with microarray technology, large data sets of differential gene expression patterns are used to identify genes that may be used as indicators of a particular cell lineage or tissue type. Semiquantitative polymerase chain reaction (PCR) can be carried out to verify the expression of individual genes, followed by quantitative PCR to precisely determine the level of mRNA expression. However, functional analysis of potential neurogenic genes must be done to identify those genes that play a critical role in neural lineage commitment.

Cell patterning chip for controlling the stem cell microenvironment

Cell-cell signaling is an important component of the stem cell microenvironment, affecting both differentiation and self-renewal. However, traditional cell-culture techniques do not provide precise control over cell-cell interactions, while existing cell-patterning technologies are limited when used with proliferating or motile cells. To address these limitations, we created the Bio Flip Chip (BFC), a microfabricated polymer chip containing thousands of microwells, each sized to trap down to a single stem cell. We have demonstrated the functionality of the BFC by patterning a 50 x 50 grid of murine embryonic stem cells (mESCs), with patterning efficiencies >75%, onto a variety of substrates--a cell-culture dish patterned with gelatin, a 3-D substrate, and even another layer of cells. We also used the BFC to pattern small groups of cells, with and without cell-cell contact, allowing incremental and independent control of contact-mediated signaling. We present quantitative evidence that cell-cell contact plays an important role in depressing mESC colony formation, and show that E-cadherin is involved in this negative regulatory pathway. Thus, by allowing exquisite control of the cellular microenvironment, we provide a technology that enables new applications in tissue engineering and regenerative medicine.

Germ layer induction in ESC--following the vertebrate roadmap

Controlled differentiation of pluripotential cells takes place routinely and with great success in developing vertebrate embryos. It therefore makes sense to take note of how this is achieved and use this knowledge to control the differentiation of embryonic stem cells (ESCs). An added advantage is that the differentiated cells resulting from this process in embryos have proven functionality and longevity. This unit reviews what is known about the embryonic signals that drive differentiation in one of the most informative of the vertebrate animal models of development, the amphibian Xenopus laevis. It summarizes their identities and the extent to which their activities are dose-dependent. The unit details what is known about the transcription factor responses to these signals, describing the networks of interactions that they generate. It then discusses the target genes of these transcription factors, the effectors of the differentiated state. Finally, how these same developmental programs operate during germ layer formation in the context of ESC differentiation is summarized.

In vitro differentiation of murine embryonic stem cells toward a renal lineage

Embryonic stem (ES) cells have the capacity to differentiate into all cells of the developing embryo and may provide a renewable resource for future cell replacement therapies. The addition of bone morphogenetic protein 4 (BMP4) to serum-free ES cell culture has previously been shown to induce transcription factors, signaling molecules, and cell adhesion proteins expressed during mesoderm specification of the embryo. Here, we show the dynamics of primitive streak mesoderm differentiation in ES cells is comparable between serum and serum-free embryoid body (EB) cultures, supplemented with BMP4. Furthermore, we show a delayed wave of expression of a cohort of genes (Pax2, WT1, podocalyxin, pod-1, and nephrin), which play important roles during embryonic kidney development. The paired box transcription factor, Pax2, is one of the earliest genes expressed during kidney organogenesis and is required for normal urogenital development. ES cell lines containing either a modified Pax2 promoter-lacZ or bacterial artificial chromosome-green fluorescent protein (GFP) transgene were generated, which enabled the quantitative analysis of kidney rather than neuronal Pax2 expression within EBs. Both beta-galactosidase activity and GFP expression were detected by immunohistochemical and flow cytometric analysis following 16 days of EB culture, which correlated with an increase in Pax2 transcript levels. Together, these results suggest a spontaneous kidney gene expression program develops in mature EBs grown in both serum and serum-free conditions, when supplemented with BMP4. Further, the recombinant growth factors BMP2, BMP4, and BMP7 strongly influence gene expression within mesoderm induced EBs. BMP4 promotes ventral (blood) and intermediate (kidney) mesoderm gene expression, whereas BMP2 and BMP7 promote kidney outcomes at the expense of hematopoietic commitment. This induction assay and these unique ES cell lines will be useful for the generation of mesoderm-derived cell populations with implications for future cell therapeutic/integration assays.

Progress and prospects: gene transfer into embryonic stem cells

With the isolation of human embryonic stem cells (hESCs) in 1998 came the realization of a long-sought aspiration for an unlimited source of human tissue. The difficulty of differentiating ESCs to pure, clinically exploitable cell populations to treat genetic and degenerative diseases is being solved in part with the help of genetically modified cell lines. With progress in genome editing and somatic cell nuclear transfer, it is theoretically possible to obtain genetically repaired isogenic cells. Moreover, the prospect of being able to select, isolate and expand a single cell to a vast population of cells could achieve a unique level of quality control, until now unattainable in the field of gene therapy. Most of the tools necessary to develop these strategies already exist in the mouse ESC system. We review here the advances accomplished in those fields and present some possible applications to hESC research.

Electrical stimulation modulates fate determination of differentiating embryonic stem cells

A clear understanding of cell fate regulation during differentiation is key in successfully using stem cells for therapeutic applications. Here, we report that mild electrical stimulation strongly influences embryonic stem cells to assume a neuronal fate. Although the resulting neuronal cells showed no sign of specific terminal differentiation in culture, they showed potential to differentiate into various types of neurons in vivo, and, in adult mice, contributed to the injured spinal cord as neuronal cells. Induction of calcium ion influx is significant in this differentiation system. This phenomenon opens up possibilities for understanding novel mechanisms underlying cellular differentiation and early development, and, perhaps more importantly, suggests possibilities for treatments in medical contexts.

Chondrogenic differentiation of human embryonic stem cells: the effect of the micro-environment

We have previously induced differentiation of embryonic stem cells (ESC) to specific phenotypes by manipulating the culture conditions, including the use of indirect co-culture. In this study, we hypothesized that co-culture with primary chondrocytes can induce human embryonic stem cells (hESC) to differentiate towards the chondrocyte lineage. Co-cultures of hESC and chondrocytes were established using well inserts, with control comprising hESC grown alone or with fibroblasts. After 28 days, after removal of the chondrocyte inserts, hESC differentiation was assessed, by morphology, immunocytochemistry, and reverse transcription polymerase chain reaction. hESC, co-cultured or grown alone, were also implanted into SCID mice on a poly-D, L-lactide scaffold, harvested 35 days later and assessed in the same way. hESC co-cultured with chondrocytes formed colonies and secreted extracellular matrix containing glycosaminoglycans (GAG). Quantitative assay showed increased synthesis of sulfated GAG in co-culture as compared with control hESC grown alone for the same period (p < 0.0001). In addition, co-cultured hESC expressed Sox 9 and collagen type II, unlike control hESC. Co-culture with fibroblasts did not induce chondrogenic differentiation. The implanted constructs with co-cultured hESC contained significantly more type II collagen (p < 0.01), type I collagen (p < 0.05), total collagen (p < 0.01), and GAG (p < 0.01) than those with hESC grown alone. Thus, we show for the first time differentiation of hESC to chondrocytes. Our results confirm the potential of the culture micro-environment to influence ESC differentiation and could provide the basis for future generation of chondrogenic cells for use in tissue repair and increase our understanding of the mechanisms that direct differentiation.

BMP-4 induces a Smad-dependent apoptotic cell death of mouse embryonic stem cell-derived neural precursors

Embryonic ectoderm is fated to become either neural or epidermal, depending on patterning processes that occur before and during gastrulation. It has been stated that epidermal commitment proceeds from a bone morphogenetic protein-4 (BMP-4)-dependent inhibition of dorsal ectoderm neuralization. We recently demonstrated that murine embryonic stem (ES) cells treated with BMP-4 undergo effective keratinocyte commitment and epidermogenesis. Focusing on the precise role of BMP-4 in the early choice between neural and epidermal commitment, we show here that BMP-4 treatment of ES cells leads to a dramatic apoptotic death of Sox-1+ neural precursors with concomitant epidermal engagement. In addition, neutralization of the Smad pathway prevents both the BMP-4 apoptotic process and the inhibition of neural differentiation. Our results suggest that, in mammals, BMP-4, as an active inducer of epidermal commitment, interferes with the survival of neural precursors through induction of their apoptotic cell death.

Isolation, characterization, gene modification, and nuclear reprogramming of porcine mesenchymal stem cells

Bone marrow mesenchymal stem cells (MSCs) are adult pluripotent cells that are considered to be an important resource for human cell-based therapies. Understanding the clinical potential of MSCs may require their use in preclinical large-animal models, such as pigs. The objectives of the present study were 1) to establish porcine MSC (pMSC) cultures; 2) to optimize in vitro pMSC culture conditions, 3) to investigate whether pMSCs are amenable to genetic manipulation, and 4) to determine pMSC reprogramming potential using somatic cell nuclear transfer (SCNT). The pMSCs isolated from bone marrow grew, attached to plastic with a fibroblast-like morphology, and expressed the mesenchymal surface marker THY1 but not the hematopoietic marker ITGAM. Furthermore, pMSCs underwent lipogenic, chondrogenic, and osteogenic differentiation when exposed to specific inducing conditions. The pMSCs grew well in a variety of media, and proliferative capacity was enhanced by culture under low oxygen atmosphere. Transient transduction of pMSCs and isogenic skin fibroblasts (SFs) with a human adenovirus carrying the gene for green fluorescent protein (GFP; Ad5-F35eGFP) resulted in more pMSCs expressing GFP compared with SFs. Cell lines with stable genetic modifications and extended expression of transgene were obtained when pMSCs were transfected with a plasmid containing the GFP gene. Infection of pMSC and SF cell lines by an adeno-associated virus resulted in approximately 12% transgenic cells, which formed transgenic clonal lines after propagation as single cells. The pMSCs can be expanded in vitro and used as nuclear donors to produce SCNT embryos. Thus, pMSCs are an attractive cell type for large-animal autologous and allogenic cell therapy models and for SCNT transgenesis.

Germ layer induction from embryonic stem cells

Embryonic stem (ES) cells have the potential to develop into all cell types of the adult body. This capability provides the basis for considering the ES cell system as a novel and unlimited source of cells for replacement therapies for the treatment of a wide range of diseases. Before the cell-based therapy potential of ES cells can be realized, a better understanding of the pathways regulating lineage-specific differentiation is required. Current studies suggest that the bone morphogenic protein, transforming growth factor-beta, Wnt, and fibroblast growth factor pathways that are required for gastrulation and germ layer induction in the embryo are also essential for differentiation of ES cells in culture. The current understanding of how these factors influence germ layer induction in both the embryo and in the ES cell differentiation system is addressed in this review.

Embryonic stem cell differentiation: emergence of a new era in biology and medicine

The discovery of mouse embryonic stem (ES) cells >20 years ago represented a major advance in biology and experimental medicine, as it enabled the routine manipulation of the mouse genome. Along with the capacity to induce genetic modifications, ES cells provided the basis for establishing an in vitro model of early mammalian development and represented a putative new source of differentiated cell types for cell replacement therapy. While ES cells have been used extensively for creating mouse mutants for more than a decade, their application as a model for developmental biology has been limited and their use in cell replacement therapy remains a goal for many in the field. Recent advances in our understanding of ES cell differentiation, detailed in this review, have provided new insights essential for establishing ES cell-based developmental models and for the generation of clinically relevant populations for cell therapy.

Noggin and bFGF cooperate to maintain the pluripotency of human embryonic stem cells in the absence of feeder layers

Human embryonic stem (hES) cells are typically maintained on mouse embryonic fibroblast (MEF) feeders or with MEF-conditioned medium. However, these xenosupport systems greatly limit the therapeutic applications of hES cells because of the risk of cross-transfer of animal pathogens. Here we showed that the bone morphogenetic protein antagonist noggin is critical in preventing differentiation of hES cells in culture. Furthermore, we found that the combination of noggin and basic fibroblast growth factor (bFGF) was sufficient to maintain the prolonged growth of hES cells while retaining all hES cell features. Since both noggin and bFGF are expressed in MEF, our findings suggest that they may be important factors secreted by MEF for maintaining undifferentiated pluripotent hES cells. Our data provide new insight into the mechanism how hES cell self-renewal is regulated. The newly developed feeder-free culture system will provide a more reliable alternative for future therapeutic applications of hES cells.

Musculoskeletal Differentiation of Cells Derived from Human Embryonic Germ Cells

Stem cells have the potential to significantly improve cell and tissue regeneration therapies, but little is understood about how to control their behavior. We investigated the potential differentiation capability of cells derived from human embryonic germ (EG) cells into musculoskeletal lineages by providing a three-dimensional environment with increased cell-cell contact and growth factors. Cells were clustered into pellets to mimic the mesenchyme condensation process during limb development. LVEC cells, an embryoid body-derived (EBD) cell culture generated from EG cells, were cultured in micromass pellets for 21 days in the presence of bone morphogenetic protein 2 (BMP2) and/or transforming growth factor beta-3 (TGFbeta3). Gene expression for cartilage-, bone-, and muscle-specific matrix proteins--including collagen types I, II, III, IX, X; aggrecan; cartilage proteoglycan link protein; cartilage oligomeric protein; chondroitin sulfate-4-S; and myf5--was upregulated in the pellets treated with TGFbeta3, while mRNAs for neurofilament heavy (NFH), a neuron marker, and flk-1, a hematopoietic marker, decreased. Total collagen and proteoglycan production exhibited a time-dependent increase in the pellets treated with TGFbeta3, further confirming the expression of characteristic musculoskeletal markers. Furthermore, our results indicate the ability to select or differentiate stem cells toward a musculoskeletal lineage from a heterogenous EBD cell line.

Retinoic-acid-concentration-dependent acquisition of neural cell identity during in vitro differentiation of mouse embryonic stem cells

Retinoic acid (RA) is one of the most important morphogens, and its embryonic distribution correlates with neural differentiation and positional specification in the developing central nervous system. To investigate the concentration-dependent effects of RA on neural differentiation of mouse embryonic stem cells (ES cells), we investigated the precise expression profiles of neural and regional specific genes by ES cells aggregated into embryoid bodies (EBs) exposed to various concentrations of RA or the BMP antagonist Noggin. RA promoted both neural differentiation and caudalization in a concentration-dependent manner, and the concentration of RA was found to regulate dorso-ventral identity, i.e., higher concentrations of RA induced a dorsal phenotype, and lower concentrations of RA induced a more ventral phenotype. The induction of the more ventral phenotype was due to the higher expression level of the N-terminus of sonic hedgehog protein (Shh-N) when treated with low concentration RA, as it was abrogated by an inhibitor of Shh signaling, cyclopamine. These findings suggest that the concentration of RA strictly and simultaneously regulates the neuralization and positional specification during differentiation of mouse ES cells and that it may be possible to use it to establish a strategy for controlling the identity of ES-cell-derived neural cells.

Self-renewal vs. Differentiation of Mouse Embryonic Stem Cells1

Embryonic stem (ES) cells are typically derived from the inner cell mass of the preimplantation blastocyst and can both self-renew and differentiate into all the cells and tissues of the embryo. Because they are pluripotent, ES cells have been used extensively to analyze gene function in development via gene targeting. The embryonic stem cell is also an unsurpassed starting material to begin to understand a critical, largely inaccessible period of development. If their differentiation could be controlled, they would also be an important source of cells for transplantation to replace cells lost through disease or injury or to replace missing hormones or genes. Traditionally, ES cells have been differentiated in suspension culture as embryoid bodies, named because of their similarity to the early postimplantation-staged embryo. Unlike the pristine organization of the early embryo, differentiation in embryoid bodies appears to be largely unpatterned, although multiple cell types form. It has recently been possible to separate the desired cell types from differentiating ES cells in embryoid bodies by using cell-type-restricted promoters driving expression of either antibiotic resistance genes or fluorophores such as EGFP. In combination with growth factor exposure, highly differentiated cell types have successfully been derived from ES cells. Recent technological advances such as RNA interference to knock down gene expression in ES cells are also producing enriched populations of cells and elucidating gene function in early development.

SVZa neural stem cells differentiate into distinct lineages in response to BMP4

Neural stem cells (NSCs) reside in the anterior portion of the forebrain subventricular zone (SVZa) and generate the progenitors which will differentiate into neurons, and via a tangential migratory pathway, known as the rostral migratory stream (RMS), migrate to the olfactory bulbs (OB). Bone morphogenetic proteins (BMPs) play significant roles in neural development at different stages and locations, but their roles have not been determined in the SVZa. To explore possible roles of BMPs in SVZa NSCs, BMP4 at various concentrations were tested for their capacity to induce SVZa NSCs. The expression of BMP4 was also examined in living cells using a reportor vector, in which the BMP4 promotor was conjugated with red fluorescent protein (RFP). In the meantime, the differentiation of SVZa NSCs was dynamically monitored by using reportor vectors of the Nestin enhancer and the promoters of TH and GFAP. In the OB, high expression of BMP4 was found using both promoter activity analysis and in situ hybridization. However, low BMP4 expression was found in the RMS and only moderate expression of BMP4 was displayed in the SVZa. The results also demonstrated that low concentrations (1-5 ng/ml) of BMP4 promoted the proliferation of SVZa NSCs but high concentrations (10-100 ng/ml) of BMP4 inhibited this proliferation. BMP4 enhanced neuron commitment before 4 days but inhibited it after 4 days. As the antagonist of BMP4, Noggin almost completely blocked all these BMP4 responses. Thus, our findings indicate that BMP4 promotes the exit from the cell cycle and triggers the differentiation of neuron progenitors in the OB. BMP4 also promotes the proliferation of the committed neuron progenitors in the RMS, but in the SVZa, BMP4 may facilitate the commitment of NSCs into astrocytes.

Beta-catenin signaling is required for neural differentiation of embryonic stem cells

Culture of embryonic stem (ES) cells at high density inhibits both beta-catenin signaling and neural differentiation. ES cell density does not influence beta-catenin expression, but a greater proportion of beta-catenin is targeted for degradation in high-density cultures. Moreover, in high-density cultures, beta-catenin is preferentially localized to the membrane further reducing beta-catenin signaling. Increasing beta-catenin signaling by treatment with Wnt3a-conditioned medium, by overexpression of beta-catenin, or by overexpression of a dominant-negative form of E-cadherin promotes neurogenesis. Furthermore, beta-catenin signaling is sufficient to induce neurogenesis in high-density cultures even in the absence of retinoic acid (RA), although RA potentiates the effects of beta-catenin. By contrast, RA does not induce neurogenesis in high-density cultures in the absence of beta-catenin signaling. Truncation of the armadillo domain of beta-catenin, but not the C terminus or the N terminus, eliminates its proneural effects. The proneural effects of beta-catenin reflect enhanced lineage commitment rather than proliferation of neural progenitor cells. Neurons induced by beta-catenin overexpression either alone or in association with RA express the caudal neuronal marker Hoxc4. However, RA treatment inhibits the beta-catenin-mediated generation of tyrosine hydroxylase-positive neurons, suggesting that not all of the effects of RA are dependent upon beta-catenin signaling. These observations suggest that beta-catenin signaling promotes neural lineage commitment by ES cells, and that beta-catenin signaling may be a necessary co-factor for RA-mediated neuronal differentiation. Further, enhancement of beta-catenin signaling with RA treatment significantly increases the numbers of neurons generated from ES cells, thus suggesting a method for obtaining large numbers of neural species for possible use in for ES cell transplantation.

In VitroDifferentiation andIn VivoMineralization of Osteogenic Cells Derived from Human Embryonic Stem Cells

The first report of the derivation of embryonic stem (ES) cell lines from human blastocysts had major implications for research into developmental biology and regenerative medicine. Finding efficient and reproducible methods to derive therapeutically useful cells from an ES cell source is a key feature of many regenerative medicine strategies. We have previously demonstrated that it is possible to induce osteogenic differentiation of murine ES cells by supplementing the culture medium with ascorbic acid, beta-glycerophosphate, and dexamethasone. This study investigated whether methods for driving osteogenic differentiation developed with murine ES cells could be applied successfully to human ES cells. The H1 line was propagated in vitro on murine feeder layers and shown to be pluripotent by expression of the markers Oct-4 and SSEA-4. Subsequently, differentiation was initiated via embryoid body (EB) formation and, after 5 days in suspension culture, cells harvested from EBs were replated in a medium containing osteogenic supplements. We found that the treatment regimen previously identified as optimal for murine ES cells, and in particular the addition of dexamethasone at specific time points, also induced the greatest osteogenic response from human ES cells. We identified mineralizing cells in vitro that immunostained positively for osteocalcin and found an increase in expression of an essential bone transcription factor, Runx2. When implanted into SCID mice on a poly-D, L-lactide (PDLLA) scaffold, the cells had the capacity to give rise to mineralized tissue in vivo. After 35 days of implantation, regions of mineralized tissue could be identified within the scaffold by von Kossa staining and immunoexpression of the human form of osteocalcin. We did not see any evidence of teratoma formation. These data therefore demonstrate the derivation of osteoblasts from pluripotent human ES cells with the capacity to form mineralized tissue both in vitro and in vivo. We have also shown that a culture methodology established for differentiation of murine ES cells was entirely transferable to human ES cells. Further development of this technology will result in the capacity to generate sufficient yields of osteogenic cells for use in skeletal tissue repair.

Spatio-temporal activation of Smad1 and Smad5 in vivo: monitoring transcriptional activity of Smad proteins

Signaling by bone morphogenetic proteins is essential for a wide variety of developmental processes. Receptor-regulated Smad proteins, Smads 1 and 5, are intracellular mediators of bone morphogenetic protein signaling. Together with Smad4, these proteins translocate to the nucleus and modulate transcription by binding to specific sequences on the promoters of target genes. We sought to map transcriptional Smad1/5 activity in development by generating embryonic stem cell lines carrying a Smad1/5-specific response element derived from the Id1 promoter coupled to beta-galactosidase or luciferase as reporters. Three independent lines (BRE-lac1, BRE-lac2 and BRE-luc) have shown the existence of an autocrine bone morphogenetic protein signaling pathway in mouse embryonic stem cells. Reporter activity was detected in chimeric embryos, suggesting sensitivity to physiological concentrations of bone morphogenetic protein. Reporter activity in embryos from transgenic mouse lines was detected in tissues where an essential role for active bone morphogenetic protein signaling via Smads 1 or 5 had been previously established. We have thus generated, for the first time, an in vivo readout for studying the role of Smad1/5-mediated transcriptional activity in development.

Neural Differentiation from Embryonic Stem Cells: Which Way?

Embryonic stem (ES) cells can in theory produce all cell types of a living organism while renewing themselves with a stable genetic background. These unique features make ES cells a favorable tool for biomedical researches as well as a potential source for therapeutic application. A first step for approaching to ES cells is the directed differentiation to cells of interest, such as the neural cell lineage. Here, we summarize the up and down sides of each category of neural differentiation protocols that have so far been used in mouse and human ES cells, and introduce an efficient and plausible method used in our laboratory for derivation of neuroectodermal cells from human ES cells. This synthesis has led to our suggestions on issues for future design of neural differentiation protocols.

Noggin inhibits chondrogenic but not osteogenic differentiation in mesodermal stem cell line C1 and skeletal cells

Osteoblasts and chondroblasts are derived from common mesenchymal progenitors. Although bone morphogenetic protein induces mesenchymal differentiation into both osteogenic and chodrogenic lineage cells in vitro, its inhibitor, Noggin, is expressed exclusively during chondrogenic but not osteogenic differentiation in an embryonal carcinoma-derived mesodermal cell line, C1. We hypothesized that Noggin may regulate cell differentiation in a lineage-specific manner. To test this hypothesis, Noggin was overexpressed using recombinant adenovirus (Ad/Noggin) in mesodermal C1 cells to examine whether Noggin specifically inhibits chondrogenic differentiation. Noggin overexpression by recombinant adenovirus infection reduced Sox9, patched, Ihh, and type II, X, and XI collagen mRNA expression levels in C1 cell aggregates that were induced to differentiate into chondrocyte lineage by culturing in differentiation medium. In contrast, Noggin overexpression did not affect osteogenic differentiation in C1 cells because osteoblast phenotypic markers such as osteocalcin and alkaline phosphatase mRNA levels were not altered. We further examined whether Noggin also differentially affects chondrogenesis and osteogenesis in limb development by using organ cultures of long bone. Ad/Noggin infection into 15.5 d post conception limb skeletal rudiments that were cultured on filter membrane in vitro or on the chorioallantoic membranes in ovo inhibited the levels of chondrogenesis, which were evaluated based on alcian blue staining. These results suggest that Noggin specifically blocks chondrogenic differentiation, rather than osteogenic differentiation, in mesodermal stem cell line C1 and skeletal cells.

Differentiation of embryonic stem cells to a neural fate: A route to re-building the nervous system?

The many and varied proposed applications of cell replacement therapies in the treatment of human disease states, particularly those arising from cell loss or dysfunction, have been discussed widely in both the scientific and popular press. Although an attractive concept, cell therapies require the development of a readily available source of donor cells suitable for transplantation. Embryonic stem (ES) cells, with proven ability to differentiate to all cell populations of the embryo and adult in vitro, provide a potential source of therapeutic cells. The differentiation capability of mouse ES cells in vitro has been studied extensively over the last 20 years and the formation of neural precursors and neural cell lineages from mouse ES cells is well established. Cell populations highly enriched/homogenous in neural precursors have been achieved using a variety of chemical or biological inducing agents coupled with selective growth conditions. Preliminary reports suggest that similar neural enrichment is seen when these methodologies are applied to primate and human ES cells. ES cell-derived neural precursors have been analyzed in vitro and in vivo and found to be functionally normal and, after introduction into rodent models of human neurodegenerative diseases, capable of effecting measurable disease recovery. We review progress in the formation of neural precursors from mouse ES cells, particularly the recent reports of directed differentiation of ES in response to biological inductive factors, and assess the transfer of these approaches to human ES cells.

Over‐expression of bHLH genes facilitate neural formation of mouse embryonic stem (ES) cells in vitro

Mouse embryonic stem (ES) cells are useful tools for investigating differentiation into neurons and glial cells in vitro. In order to induce ES cells to differentiate into neural cells, many researchers have investigated the efficiency of induction. Embryoid body (EB) formation and retinoic acid are potent differentiation inducers known to be a trigger at the early stage of development. Basic helix-loop-helix (bHLH) is one of the important transcription factors, which is essential for premature neural formation. In NeuroD2 and Mash1-transfected cells, neural formation was observed at day 6 after the plating of embryoid bodies in culture. Nestin was detected in NeuroD2- and Mash1-transfected cells at day 10, and strong signal was detected in Mash1 transfectants by RT-PCR analysis. Map2 and Nurr1 were also detected strongly at the early stage in transfected cells compared with the wild type control, especially in the Mash1 transfectant. In immunocytochemical analysis, Tuj1-positive neurons were detected at high frequency in Mash1 transfectants and some cells were stained by tyrosine hydrogenase (TH), a marker of dopaminergic neurons. These results demonstrate that bHLH has a potential activity at an early stage for ES cells and can induce effective and rapid neural differentiation in vitro.

Lineage choice and differentiation in mouse embryos and embryonic stem cells

The use of embryonic stem (ES) cells for generating healthy tissues has the potential to revolutionize therapies for human disease or injury, for which there are currently no effective treatments. Strategies for manipulating stem cell differentiation should be based on knowledge of the mechanisms by which lineage decisions are made during early embryogenesis. Here, we review current research into the factors influencing lineage differentiation in the mouse embryo and the application of this knowledge to in vitro differentiation of ES cells. In the mouse embryo, specification of tissue lineages requires cell-cell interactions that are influenced by coordinated cell migration and cellular neighborhood mediated by the key WNT, FGF, and TGFbeta signaling pathways. Mimicking the cellular interactions of the embryo by providing appropriate signaling molecules in culture has enabled the differentiation of ES cells to be directed predominately toward particular lineages. Multistep strategies incorporating the provision of soluble factors known to influence lineage choices in the embryo, coculture with other cells or tissues, genetic modification, and selection for desirable cell types have allowed the production of ES cell derivatives that produce beneficial effects in animal models. Increasing the efficiency of this process can only result from a better understanding of the molecular control of cell lineage determination in the embryo.

Origin of metazoan stem cell system in sponges: first approach to establish the model (Suberites domuncula)

It is established that Porifera (sponges) represent the earliest phylum which branched off from the common ancestor of all multicellular animals, the Urmetazoa. In the present study, the hypothesis is tested if, during this transition, pluripotent stem cells were formed which are provided-similar to the totipotent cells (archaeocytes/germ cells)-with a self-renewal capacity. As a model system, primmorphs from the sponge Suberites domuncula were used. These 3D-cell aggregates were cultivated in medium (RPMI 1640/seawater) either lacking silicate and ferric iron or in medium which was supplemented with these 'morphogenetic' factors. As molecular markers for the potential existence of stem cells in primmorphs, two genes which encode proteins found in stem cells of higher metazoan species, were cloned from S. domuncula. First, the noggin gene, which is present in the Spemann organizer of amphibians and whose translation product acts during the formation of dorsal mesoderm derivatives. The second gene encodes the mesenchymal stem cell-like protein. Both cDNAs were used to study their expression in primmorphs in dependence on the incubation conditions. It was found that noggin expression is strongly upregulated in primmorphs kept in the presence of silicate and ferric iron, while the expression of the mesenchymal stem cell-like protein was downregulated. These data are discussed with respect to the existence of stem cells in sponges.

The Chemokine Networks in Sponges: Potential Roles in Morphogenesis, Immunity and Stem Cell Formation

Porifera (sponges) are now well accepted as the phylum which branched off first from the common ancestor of all metazoans, the Urmetazoa. The transition to the Metazoa became possible because during this phase, cell-cell as well as cell-matrix adhesion molecules evolved which allowed the formation of a colonial stage of animals. The next prerequisite for the evolution to the Urmetazoa was the establishment of an effective immune system which, flanked by apoptosis, allowed the formation of a first level of individuation. In sponges (with the model Suberites domuncula and Geodia cydonium), the main mediators of the immune responses are the chemokines. Since sponges lack a vascular system and consequently blood cells (in the narrow sense), we have used the term chemokines (in a broad sense) to highlight that the complex network of intercellular mediators initiates besides differentiation processes also cell movement. In the present review, the cDNAs encoding the following chemokines were described and the roles of their deduced proteins during self-self and nonself recognition outlined: the allograft inflammatory factor, the glutathione peroxidase, the endothelial-monocyte-activating polypeptide, the pre-B-cell colony-enhancing factor and the myotrophin as well as an enzyme, the (2-5)A synthetase, which is involved in cytokine response in vertebrates. A further step required to reach the evolutionary step of the integrated stage of the Urmetazoa was the acquisition of a stem cell system. In this review, first markers for stem cells (mesenchymal stem cell-like protein) as well as for chemokines involved in the maintenance of stem cells (noggin and glia maturation factor) are described at the molecular level, and a first functional analysis is approached. Taken together, it is outlined that the chemokine network was essential for the establishment of metazoans, which evolved approximately 600 to 800 million years ago.