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Abstract: The revision of the bioethics law of 2004 must occur in a five year''s time. For this revision, the authorities decided to organize general states of bioethics and requested the production of contributions by the companies, institutions or associations. These texts tackle various subjects, like the Assisted Reproductive Technologies, research on the embryo and the stem cells and the banks of umbilical cord blood. Certain opinions converge, others differ, but all take part in the great debate which will take place at the time of the general conference. [Copyright &y& Elsevier]

Evolution of development is discussed with emphasis on stem cells, particularly the hematopoietic ones (HSCs), the best-characterized of the stem cells. It is proposed that stem cell-based organogenesis and regeneration, stem cell mobility, and the coevolution of polymorphic histocompatibility that limits stem cells to self or close relatives are interrelated themes and are coselected in an evolutionary sense.

The interaction between cytotrophoblasts and L-selectin to form a system to support the placenta during early pregnancy in mammals is studied. Cytotrophoblasts are stem cells in the chorionic villous and L-selectin is an adhesion molecule.

A study examines the origin and development of neural stem cells' functioning in the brain of zebrafish.

A study on the behavior of neural stem cells in the brain of zebrafish is discussed. Immunodetection of markers is done in the study.

A study examines the role of human embryonic stem cells when they are engrafted into mouse blastocyts. It is revealed that the blastocyts proliferate.

Study of stem cell activity in testis of Drosophila for sex determination and development of germline stem cell niche in Drosophila is presented.

Discrete perinuclear dispersion feature of histone lysine trimethylation in Plzf-expressing spermatogonia is studied.

The article proposes that bivalent domains silence developmental genes in embryonic stem (ES) cells while keeping them poised for activation and a striking correspondence is observed between genome sequence and histone methylation in ES cells, which become notably weaker in differentiated cells. These results highlight the importance of DNA sequence in defining the initial epigenetic landscape and suggest a novel chromatin-based mechanism for maintaining pluripotency.

Polycomb group proteins are essential for early development in metazoans, and it is found that Polycomb Repressive Complex 2 (PRC2) target genes are preferentially activated during embryonic stem (ES) cell differentiation. The results indicate that PRC2 occupies a special set of developmental genes in ES cells that must be repressed to maintain pluripotency and that are poised for activation during ES cell differentiation.

Stem cells encapsulate the fundamental problem of metazoan biology in miniature, and increasing evidence indicates that stem cell chromatin activates proliferation genes and represses differentiation genes. Understanding how Polycomb group proteins stabilize these configurations will advance the understanding of embryonic development, tissue homeostasis, regeneration, aging and oncogenesis.

Role of caudal-related homeobox genes cdx1a and cdx4 on formation of hematopoietic stem cells and homeo box gene regulation in zebrafish is presented.

A study on pluripotency of embryonic stem cells based on wingless-type MMTV integration site family gene and beta catenin pathways is presented.

A computational tool known as enhancer element locator is developed to identify mammalian enhancer elements regulated by Hedge-hog (Hh), Wnt and Ras/MAPK signaling pathways. Predictions validated in transgenic mouse embryos reveal the presence of multiple tissue-specific enhancers in mouse c-Myc and N-Myc genes.

The efficacy of nuclei from embryonic cells or embryonic stem cells in spindle chromosome complex formation in cloning experiments of amphibians and mammals is studied.

The local reduction of expression of the RETINOBLASTOMA-RELATED (RBR) gene in Arabidopsis roots increases the amount of stem cells without affecting cell cycle duration in mitotically active cells. Expression analysis and genetic interactions position RBR-mediated regulation of the stem cell state downstream of the patterning gene SCARECROW.

Tetraploid (4n) mouse embryos die at variable developmental stages. By examining 4n embryos from F2 hybrid and outbred mice, we show that 4n developmental potential is influenced by genetic background. The imprinted inactivation of an X chromosome-linked eGFP transgene in extraembryonic tissues occurred correctly in 4n embryos. A decrease of the cleavage rate in 4n preimplantation embryos compared to diploid (2n) embryos was revealed by real-time imaging, using a histone H2b:eGFP reporter. It has previously been known that mouse chimeras produced by the combination of diploid (2n) embryos with embryonic stem (ES) cells result in mixtures of the two components in epiblast-derived tissues. In contrast, the use of 4n host embryos with ES cells restricts 4n cells from the embryonic regions of chimeras, resulting in mice that are believed to be completely ES-derived. Using H2b:eGFP transgenic mice and ES cells, the behavior of 4n cells was determined at single cell resolution in 4n:2n injection and aggregation chimeras. We found a significant contribution of 4n cells to the embryonic ectoderm at gastrulation in every chimera analyzed. We show that the transition of the embryonic regions from a chimeric tissue to a predominantly 2n tissue occurs after gastrulation and that tetraploid cells may persist to midgestation. These findings suggest that the results of previously published tetraploid complementation assays may be influenced by the presence of tetraploid cells in the otherwise diploid embryonic regions. Keywords: Tetraploid; Genome duplication; Gastmlation; Embryonic stem cells; GFP; Live embryo imaging

The formation of the primitive endoderm covering the inner cell mass of early mouse embryos can be simulated in vitro by the differentiation of mouse embryonic stem (ES) cells in culture following either aggregation of suspended cells or stimulation of cell monolayers with retinoic acid. The developmentally regulated transcription factors GATA-4 and GATA-6 have determining role in mouse extraembryonic endoderm development. We analyzed the in vitro differentiation of mouse embryonic stem cells deficient of GATA factors and conclude that GATA-4 is required for ES cells to perceive a cell positioning (cell aggregation) signal and GATA-6 is required to sense morphogenic (retinoic acid) signal. The collaboration between GATA-6 and GATA-4, or GATA-6 and GATA-5 which can substitute for GATA-4, is involved in the perception of differentiation cues by embryonic stem cells in their determination of endoderm lineage. This study indicates that the lineage differentiation of ES cells can be manipulated by the expression of GATA factors. Keywords: Disabled-2; GATA factors; Primitive endoderm; Retinoic acid; Cell growth suppression; Differentiation

Notch signaling plays various key roles in cell fate determination during CNS development in a context-dependent fashion. However, its precise physiological role and the localization of its target cells remain unclear. To address this issue, we developed a new reporter system for assessing the RBP-J-mediated activation of Notch signaling target genes in living cells and tissues using a fluorescent protein Venus. Our reporter system revealed that Notch signaling is selectively activated in neurosphere-initiating multipotent neural stem cells in vitro and in radial glia in the embryonic forebrain in vivo. Furthermore, the activation of Notch signaling occurs during gliogenesis and is required in the early stage of astroglial development. Consistent with these findings, the persistent activation of Notch signaling inhibits the differentiation of GFAP-positive astrocytes. Thus, the development of our RBP-J-dependent live reporter system, which is activated upon Notch activation, together with a stage-dependent gain-of-function analysis allowed us to gain further insight into the complexity of Notch signaling in mammalian CNS development. Keywords: Notch; Hes1; RBP-J; Neural stem ceil; Neural progenitors; Astrocyte; EGFP; Venus; Self-renewal; Gliogenesis

Genomescale location analysis is used to identify the target genes of transcription factors OCT4, SOX2, and NANOG regulators in vivo. The result provides new insights into the transcriptional regulation of stem cells and reveal how OCT4, SOX2, and NANOG contribute to pluripotency and self-renewal.

The gene expression profiles of highly purified hematopoietic stem cells (HSCs) and non-self-renewing multipotent hematopoietic progenitors (MPPs) are compared to improve the ability to identify HSCs and their localization in vivo. Cell surface receptors of the SLAM family, including CD150, CD244, and CD48, are differentially expressed among functionally distinct progenitors and HSCs occupy multiple niches, including sinusoidal endothelium in diverse tissues.

The mechanisms and regulation of stem cell fate decisions in the blood-forming (hematopoietic) system are reported. A simple and broadly applicable method is provided to isolate these cells and to visualize them in their normal environment.

A regional pulmonary stem cell population, termed as bronchioalveolar stem cells (BASCs) is isolated and identified at the bronchioalveolar duct junction. Data support the hypothesis that BASCs are a stem cell population that maintains the bronchiolar Clara cells and alveolar cells of the distal lung and that their transformed counterparts give rise to adenocarcinoma.

Although communications between mammalian oocytes and their surrounding granulosa cells mediated by the Kit--Kit ligand (KL, or stem cell factor, SCF) system have been proven to be crucial for follicular development, Kit downstream signaling pathways in mammalian oocytes are largely unknown. In this study, by using ovaries and isolated oocytes from postnatal mice and rats, we demonstrated for the first time that components of the PI3 kinase pathway, the serine/threonine kinase Akt (PKB) which enhances cellular proliferation and survival, and an Akt substrate FKHRL1 which is a transcription factor that leads to apoptosis and cell cycle arrest, are expressed in mammalian oocytes. By using an in vitro oocytes culture system, we found that oocytes-derived Akt and FKHRL1 are regulated by SCF. Treatment of cultured oocytes with SCF cannot only rapidly phosphorylate and activate Akt, but also simultaneously phosphorylate and may therefore functionally suppress FKHRL1, through the action of PI3 kinase. Together with our in situ hybridization and immunohistochemistry data that Akt and FKHRL1 are mostly expressed in oocytes in primordial and primary ovaries and reports that FKHRL1 gene-deficient mice exhibited excessive activation from primordial to primary follicles as well as enlarged oocyte sizes, we suppose that in mammalian oocytes, actions of granulosa cell derived SCF on primordial to primary follicle transition and subsequent follicle development may involve activation of Akt and inhibition of FKHRL1 activities in oocytes. The role of oocyte's Akt may be to enhance follicle development and the role of oocyte's FKHRL1 may be to inhibit follicle development. We propose that the cascade from granulosa cell SCF to oocyte Kit-PI3 kinase-Akt-FKHRL1 may play an important role to regulate the growth rate of mammalian oocytes and hypothetically also the oocyte secretion of factors that may regulate the activation and early development of ovarian follicles. Keywords: Mammalian oocytes; Stem cell factor (SCF); Kit; PI3 kinase; Akt; FKHRL1

PW1 is a mediator of p53 and TNF[alpha] signaling pathways previously identified in a screen to isolate muscle stem cell regulators. We generated transgenic mice carrying a C-terminal deleted form of PW1 ([DELTA]PW1) which blocks p53-mediated cell death and TNF[alpha]-mediated NF[kappa]B activation fused to the myogenin promoter. Embryonic/fetal muscle development appears normal during transgene expression, however, postnatal transgenic pups display severe phenotypes including runtism, reduced muscle mass and fiber diameters resembling atrophy. Atrogin-1, a marker of skeletal muscle atrophy, is expressed postnatally in transgenic mice. Electron microscopic analyses of transgenic muscle reveal a marked decrease in quiescent muscle satellite cells suggesting a deregulation of postnatal stem cells. Furthermore, transgenic primary myoblasts show a resistance to the effects of TNF[alpha] upon differentiation. Taken together, our data support a role for PW1 and related stress pathways in mediating skeletal muscle stem cell behavior which in turn is critical for postnatal muscle growth and homeostasis. In addition, these data reveal that postnatal stem cell behavior is likely specified during early muscle development. Keywords: Satellite cells; Stem cells; Skeletal muscle; Development; Mouse; PW1/Peg3; p53; TNF; Atrophy

In a large-scale forward-genetic screen, we discovered that a limited number of genes are required for the regulation of retinal stem cells after embryogenesis in zebrafish. In 18 mutants out of almost 2000 F2 families screened, the eye undergoes normal embryonic development, but fails to continue growth from the ciliary marginal zone (CMZ), the post-embryonic stem-cell niche. Class I-A mutants (5 loci) display lower amounts of proliferation in the CMZ, while nearly all cells in the retina appear differentiated. Class I-B mutants (2 loci) have a reduced CMZ with a concomitant expansion in the retinal pigmented epithelium (RPE), suggesting a common post-embryonic stem cell is the source for these neighboring cell types. Class II encompasses three distinct types of mutants (11 loci) with expanded CMZ, in which the progenitor population is arrested in the cell cycle. We also show that in at least one combination, the reduced CMZ phenotype is genetically epistatic to the expanded CMZ phenotype, suggesting that Class I genes are more likely to affect the stem cells and Class II the progenitor cells. Finally, a comparative mapping analysis demonstrates that the new genes isolated do not correspond to genes previously implicated in stem-cell regulation. Our study suggests that embryonic and post-embryonic stem cells utilize separable genetic programs in the zebrafish retina. Keywords: Stem cell; Cell cycle; Neural retina; Retinal pigmented epithelium; Ciliary marginal zone; Danio rerio

Complex intrinsic and extrinsic mechanisms determine neural cell fate during development of the nervous system. Using Id4 deficient mice, we show that Id4 is required for normal development of the central nervous system (CNS), timing neural differentiation in the developing forebrain. In the absence of Id4, the ventricular zone of the neocortex, future hippocampus as well as lateral and medial ganglionic eminences exhibited a 20-30% reduction in mitotic neural precursor cells (NPCs). Although the number of apoptotic cells was significantly increased, the neocortex of [Id4.sup.-/-] embryos was consistently thicker due to premature neuronal differentiation, which resulted in an increase in early-born neurons in the adult [Id4.sup.-/-] cortex. Late-born cortical neurons and astrocytes in the cortex, septum, hippocampus and caudate putamen of [Id4.sup.-/-] adult brains were decreased, however, likely due to the depletion of the NPC pool. Consequently, adult [Id4.sup.-/-] brains were smaller and exhibited enlarged ventricles. In vitro analysis of neurosphere cultures revealed that proliferation of Id4-deficient NPCs was impaired and that BMP2-mediated astrocyte differentiation was accelerated in the absence of Id4. Together, these in vivo and in vitro data suggest a crucial role for Id4 in regulating NPC proliferation and differentiation. Keywords: Stem cells; Neuroepithelial cells; Neuronal differentiation; Neurospheres; BMP2-mediated astrocyte differentiation

The causes for hair graying, an obvious sign of human aging, are little known. However, evidence indicates that hair graying is due to incomplete melanocyte stem cell maintenance and Pax3 and Miff are key molecules that help regulate the balance between melanocyte stem cell maintenance and differentiation.

Adult bone marrow-derived stem cells (BMDC) have been shown to contribute to numerous tissues after transplantation into a new host. However, whether the participation of these cells is part of the normal response to injury remains a matter of debate. Using parabiotically joined pairs of genetically labeled and wildtype mice, we show here that irradiation-induced damage of the target tissue, injection of bone marrow into the circulation, and immunological perturbation that are consequences of bone marrow transplantation are not necessary for bone marrow contribution to myofibers. Moreover, severe toxin-induced damage is not a prerequisite, as BMDC contribution to muscle is enhanced in response to increased muscle activity resulting from muscle overloading or forced exercise. Indeed, these two forms of muscle stress result in much more rapid contribution (within 1 month) than voluntary running (6 months). These results indicate that BMDC contribute to myofibers in response to physiologic stresses encountered by healthy organisms throughout life. Keywords: Adult stem cells; Bone marrow-derived cells; Muscle; Transplantation; Irradiation; Regeneration; Damage; Exercise; Parabiosis

The identification and physical isolation of testis stem cells, a subset of type A spermatogonia, is critical to our understanding of their growth regulation during the first steps of spermatogenesis. These stem cells remain poorly characterized because of the paucity of specific molecular markers that permit us to distinguish them from other germ cells. Thus, the molecular mechanisms driving the first steps of spermatogenesis are still unknown. We show in the present study that GFR[alpha]-1, the receptor for GDNF (glial cell line-derived neurotrophic factor), is strongly expressed by a subset of type A spermatogonia in the basal part of the seminiferous epithelium. Using this characteristic, we devised a method to specifically isolate these GFR[alpha]-1-positive cells from immature mouse testes. The isolated cells express Ret, a tyrosine kinase transmembrane receptor that mediates the intracellular response to GDNF via GFR[alpha]-1. After stimulation with rGDNF, the isolated cells proliferated in culture and underwent the first steps of germ cell differentiation. Microarray analysis revealed that GDNF induces the differential expression of a total of 1124 genes. Among the genes upregulated by GDNF were many genes involved in early mammalian development, differentiation, and the cell cycle. This report describes the first isolation of a pure population of GFR[alpha]-1-positive cells in the testis and identifies signaling pathways that may play a crucial role in maintaining germ-line stem cell proliferation and/or renewal. Keywords: Type A spermatogonia; Stem cell; GFR[alpha]-1; GDNF; FGF2; Affymetrix microarray

The regenerative capacity of the heart following injury is debated. It is argued that the physiologic role and therapeutic potential of cardiac resident progenitor cells remain unclear.

Recent reports of stem cell plasticity have led to the suggestion that there are few intrinsic differences between precursor cells, and that environment dictates fundamental cellular properties such as differentiation potential. This suggestion has been buoyed by other work suggesting that apparent in vivo differences between neural precursor cells are lost when placed in a culture environment. We sought to further test this hypothesis by comparing neural precursors present in various neural tissues during the early postnatal period. Precursors from three postnatal actively neurogenic regions and three postneurogenic regions (cerebral cortex, lateral striatum, and optic nerve) were assayed at postnatal day 1, day 10, and adulthood, and compared to well-characterized ventricular subependymal neural stem cells. In contrast to stem cells that remain multipotential throughout life, the progenitor cells become restricted in a time- and region- dependent manner to an exclusively glial-producing phenotype, a phenomenon that occurs both in vitro and in vivo. Transcription factors associated with neural precursor identity are expressed regardless of brain region of origin or time in vitro. Environmental coculture manipulations are only able to rescue neurogenesis in olfactory bulb precursors but not other restricted progenitors. Thus, in contrast to the views that the in vitro environment has a homogenizing effect on distinct neural precursors, our data suggest that robust intrinsic differences with respect to self-renewal and continued neuron production exist between neural precursors from different brain regions. These differences are evident in vitro and in vivo. Keywords: Progenitor; Stem cell; Neuron; Neurogenesis; Postnatal; Optic nerve; Intrinsic

Breast epithelial stem cells are thought to be the primary targets in the etiology of breast cancer. Since breast cancers mostly express estrogen and progesterone receptor (ER[alpha] and PR), we examined the biology of these ER[alpha]/PR-positive cells and their relationship to stem cells in normal human breast epithelium. We employed several complementary approaches to identify putative stem cell markers, to characterise an isolated stem cell population and to relate these to cells expressing the steroid receptors ER[alpha] and PR. Using DNA radiolabelling in human tissue implanted into athymic nude mice, a population of label-retaining cells were shown to be enriched for the putative stem cell markers [p21.sup.CIP1] and Msi-1, the human homolog of Drosophila Musashi. Steroid receptor-positive cells were found to co-express these stem cell markers together with cytokeratin 19, another putative stem cell marker in the breast. Human breast epithelial cells with Hoechst dye-effluxing 'side population' (SP) properties characteristic of mammary stem cells in mice were demonstrated to be undifferentiated 'intermediate' cells by lack of expression of myoepithelial and luminal apical membrane markers. These SP cells were 6-fold enriched for ER[alpha]-positive cells and expressed several fold higher levels of the ER[alpha], [p21.sup.CIP1] and Msi1 genes than non-SP cells. In contrast to non-SP cells, SP cells formed branching structures in matrigel which included cells of both luminal and myoepithelial lineages. The data suggest a model where scattered steroid receptor-positive cells are stem cells that self-renew through asymmetric cell division and generate patches of transit amplifying and differentiated cells. Keywords: Normal breast; Stem cells; Differentiation; Epithelium; Cancer; Steroid receptors; [P21.sup.CIP1]; Msi1: Side population cells

Germline cells that can form different embryonic stem (ES) as well as embryonic germ (EG) cells are obtained only from embryonic tissues, hence tests were conducted to obtain such cells from other sources. Researchers conducted tests to obtain similar ES-like cells from neonatal mouse testis and these cells were similar to the ES/EG cells except that in their genomic imprinting pattern.

Therapeutic cloning or nuclear transfer for stem cells (NTSC) seeks to overcome immune rejection through the development of embryonic stem cells (ES cells) derived from cloned blastocysts. The successful derivation of a human embryonic stem cell (hESC) line from blastocysts generated by somatic cell nuclear transfer (SCNT) provides proof-of-principle for 'therapeutic cloning,' though immune matching of the differentiated NT-hES remains to be established. Here, in nonhuman primates (NHPs; rhesus and cynomologus macaques), the strategies used with human SCNT improve NHP-SCNT development significantly. Protocol improvements include the following: enucleation just prior to metaphase-II arrest; extrusion rather than extraction of the meiotic spindle-chromosome complex (SCC); nuclear transfer by electrofusion with simultaneous cytoplast activation; and sequential media. Embryo transfers (ET) of 135 SCNT-NHP into 25 staged surrogates did not result in convincing evidence of pregnancies after 30 days post-ET. These results demonstrate that (i) protocols optimized in humans generate preimplantation embryos in nonhuman primates; (ii) some, though perhaps not yet all, hurdles in deriving NT-nhpES cells from cloned macaque embryos (therapeutic cloning) have been overcome; (iii) reproductive cloning with SCNT-NHP embryos appears significantly less efficient than with fertilized embryos; (iv) therapeutic cloning with matured metaphase-II oocytes, aged oocytes, or 'fertilization failures' might remain difficult since enucleation is optimally performed prior to metaphase-II arrest; and (v) challenges remain for producing reproductive successes since NT embryos appear inferior to fertilized ones due to spindle detects resulting from centrosome and motor deficiencies that produce aneuploid preimplantation embryos, among other anomalies including genomic imprinting, mitochondrial and cytoplasmic heterogeneities, cell cycle asynchronies, and improper nuclear reprogramming. Keywords: Embryogenesis; Blastocyst; Meiotic spindle extraction; Nuclear transtier; Stem cells; Molecular motors: NuMA

The precise lineage between neural stem cells and mature astrocytes remains poorly defined. To examine astrocyte development, we have characterized glial precursors from neural tissue derived from early embryonic ages. We show that CD44 identifies an astrocyte-restricted precursor cell (ARP) that is committed to generating astrocytes in vitro and in vivo in both rodent and human tissue. CD44+ cells arise later in development than neuronal-restricted precursors (NRPs) or tripotential glial-restricted precursors (GRPs). ARPs are distinguished from GRP and NRP cells by their antigenic profile and differentiation ability. ARPs can be generated from GRP cells in mass or clonal cultures and in vivo after transplantation, suggesting a sequential differentiation of neuroepithelial stem cells (NEPs) to GRPs to ARPs and then to astrocytes. The properties of A RPs are different from other astrocyte precursors described previously in their expression of CD44 and S-100 [beta] and absence of other lineage markers. Using a CD44 misexpression transgenic mouse model (CNP-CD44 mouse), we show that CD44 overexpression in vivo and in vitro decreases the number of mature glia and increases the number of O4+/GFAP+ cells tenfold. Misexpression of CD44 in culture inhibits oligodendrocytes and arrests cells at the precursor state. In summary, our data provide strong evidence for the existence of a CD44+ ARP in the developing nervous system. Keywords: Stem cells; Neuroepithelial cells; Glial-restricted precursors; Differentiation; Extracellular matrix protein

Genetic studies in fish, amphibia, and mice have shown that deficiency of Nodal signaling blocks differentiation into mesoderm and endoderm. Thus, Nodal is considered as a major inducer of mesendoderm during gastrulation. On this basis, Nodal is a candidate for controlling differentiation of pluripotent human embryonic stem cells (hESCs) into tissue lineages with potential clinical value. We have investigated the effect of Nodal, both as a recombinant protein and as a constitutively expressed transgene, on differentiation of hESCs. When control hESCs were grown in chemically defined medium, their expression of markers of pluripotency progressively decreased, while expression of neuroectoderm markers was strongly upregulated, thus revealing a neuroectodermal default mechanism for differentiation in this system, hESCs cultured in recombinant Nodal, by contrast, showed prolonged expression of pluripotency marker genes and reduced induction of neuroectoderm markers. These Nodal effects were accentuated in hESCs expressing a Nodal transgene, with striking morphogenetic consequences. Nodal-expressing hESCs developing as embryoid bodies contained an outer layer of visceral endoderm-like cells surrounding an inner layer of epiblast-like cells, each layer having distinct gene expression patterns. Markers of neuroectoderm were not upregulated during development of Nodal-expressing embryoid bodies, nor was there induction of markers for definitive mesoderm or endoderm differentiation. Moreover, the inner layer expressed markers of pluripotency, characteristic of undifferentiated hESCs and of epiblast in mouse embryos. These results could be accounted for by an inhibitory effect of Nodal-induced visceral endoderm on pluripotent cell differentiation into mesoderm and endoderm, with a concomitant inhibition of neuroectoderm differentiation by Nodal itself. There could also be a direct effect of Nodal in the maintenance of pluripotency. In summary, analysis of the Nodal-expressing phenotype suggests a function for the transforming growth factor-beta (TGF-[beta],) growth factor superfamily in pluripotency and in early cell fate decisions leading to primary tissue layers during in vitro development of pluripotent human stem cells. The effects of Nodal on early differentiation illustrate how hESCs can augment mouse embryos as a model for analyzing mechanisms of early mammalian development. Keywords: Nodal; TGF-[beta] family; Human embryonic stem cells; Visceral endoderm; Anterior visceral endoderm; Neuroectoderm; Pluripotency; Primary germ layers

The past few years have seen remarkable progress in our understanding of embryonic stem cell (ES cell) biology. The necessity of examining human ES cells in culture, coupled with the wealth of genomic data and the multiplicity of cell lines available, has enabled researchers to identify critical conserved pathways regulating self-renewal and identify markers that tightly correlate with the ES cell state. Comparison across species has suggested additional pathways likely to be important in long-term self-renewal of ES cells including heterochronic genes, microRNAs, genes involved in telomeric regulation, and polycomb repressors. In this review, we have discussed information on molecules known to be important in ES cell self-renewal or blastocyst development and highlighted known differences between mouse and human ES cells. We suggest that several additional pathways required for self-renewal remain to be discovered and these likely include genes involved in antisense regulation, microRNAs, as well as additional global repressive pathways and novel genes. We suggest that cross species comparisons using large-scale genomic analysis tools are likely to reveal conserved and divergent paths required for ES cell self-renewal and will allow us to derive ES lines from species and strains where this has been difficult. Keywords: Human embryonic stem cells; Self-renewal; Conserved and divergent paths

The AP-1 transcription factor JunB regulates the numbers of hematopoietic stem cells (HSC). JunB overexpression decreases the frequency of long-term HSC, while JunB inactivation specifically expands the number of long term HSC and granulocyte/macrophage progenitors resulting in chronic myeloproliferative disorder (MPD).