Primordial germ cell specification: a context-dependent cellular differentiation event [corrected].

During embryonic development, the foundation of the germline is laid by the specification of primordial germ cells (PGCs) from the postimplantation epiblast via bone morphogenetic protein (BMP) and WNT signalling. While the majority of epiblast cells undergo differentiation towards somatic cell lineages, PGCs initiate a unique cellular programme driven by the cooperation of the transcription factors BLIMP1, PRDM14 and AP2γ. These factors synergistically suppress the ongoing somatic differentiation and drive the re-expression of pluripotency and germ cell-specific genes accompanied by global epigenetic changes. However, an unresolved question is how postimplantation epiblast cells acquire the developmental competence for the PGC fate downstream of BMP/WNT signalling. One emerging concept is that transcriptional enhancers might play a central role in the establishment of developmental competence and the execution of cell fate determination. Here, we discuss recent advances on the specification and reprogramming of PGCs thereby highlighting the concept of enhancer function.U.G. is supported by a Marie Curie Intra-European fellowship. E.M. is supported by the Icelandic Research Fund. M.A.S. is supported by the Wellcome Trust (WT096738).This is the final version. It was first published by Royal Society Publishing at http://rstb.royalsocietypublishing.org/content/369/1657/2013054

Conserved features of non-primate bilaminar disc embryos and the germline.

Post-implantation embryo development commences with a bilaminar disc in most mammals, including humans. Whereas access to early human embryos is limited and subject to greater ethical scrutiny, studies on non-primate embryos developing as bilaminar discs offer exceptional opportunities for advances in gastrulation, the germline, and the basis for evolutionary divergence applicable to human development. Here, we discuss the advantages of investigations in the pig embryo as an exemplar of development of a bilaminar disc embryo with relevance to early human development. Besides, the pig has the potential for the creation of humanized organs for xenotransplantation. Precise genetic engineering approaches, imaging, and single-cell analysis are cost effective and efficient, enabling research into some outstanding questions on human development and for developing authentic models of early human development with stem cells

The unfolding body plan of primate embryos in culture.

Improved culture of non-human primate embryos reveals the establishment of the crucial framework for subsequent development of bodily tissues and the germline in fetuses, bringing us closer to comprehending the elusive development of early human embryos

Germ cell specification and pluripotency in mammals: a perspective from early embryogenesis.

Germ cells are unique cell types that generate a totipotent zygote upon fertilization, giving rise to the next generation in mammals and many other multicellular organisms. How germ cells acquire this ability has been of considerable interest. In mammals, primordial germ cells (PGCs), the precursors of sperm and oocytes, are specified around the time of gastrulation. PGCs are induced by signals from the surrounding extra-embryonic tissues to the equipotent epiblast cells that give rise to all cell types. Currently, the mechanism of PGC specification in mammals is best understood from studies in mice. Following implantation, the epiblast cells develop as an egg cylinder while the extra-embryonic ectoderm cells which are the source of important signals for PGC specification are located over the egg cylinder. However, in most cases, including humans, the epiblast cells develop as a planar disc, which alters the organization and the source of the signaling for cell fates. This, in turn, might have an effect on the precise mechanism of PGC specification in vivo as well as in vitro using pluripotent embryonic stem cells. Here, we discuss how the key early embryonic differences between rodents and other mammals may affect the establishment of the pluripotency network in vivo and in vitro, and consequently the basis for PGC specification, particularly from pluripotent embryonic stem cells in vitro

ERG-associated protein with SET domain (ESET)-Oct4 interaction regulates pluripotency and represses the trophectoderm lineage.

BACKGROUND: Pluripotency, the capacity for indefinite self-renewal and differentiation into diverse cell types is a unique state exhibited by embryonic stem (ES) cells. Transcriptional regulators, such as Oct4, are critical for pluripotency, but the role of epigenetic modifiers remains to be fully elucidated. RESULTS: Here, we show that ERG-associated protein with SET domain (ESET), a histone methyltransferase enzyme, maintains pluripotency through repression of Cdx2, a key trophectoderm determinant, by histone H3 lysine 9 trimethylation (H3K9me3) of the promoter region. Notably, this repression is mediated through the synergistic function of small ubiquitin-related modifier (SUMO)ylated ESET and Oct4. ESET localises to the promyelocytic leukaemia (PML) nuclear bodies and is SUMOylated in ES cells. Interaction of ESET with Oct4 depends on a SUMO-interacting motif (SIM) in Oct4, which is critical for the repression of Cdx2. CONCLUSION: Loss of ESET or Oct4 results in strikingly similar phenotypes both in ES cells with their differentiation into trophectoderm cells, and in early embryos where there is a failure of development of the pluripotent inner cell mass (ICM) of blastocysts. We propose that SUMOylated ESET-Oct4 complex is critical for both the initiation and maintenance of pluripotency through repression of differentiation, particularly of the trophectoderm lineage by epigenetic silencing of Cdx2.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

Proximal visceral endoderm and extraembryonic ectoderm regulate the formation of primordial germ cell precursors.

BACKGROUND: The extraembryonic tissues, visceral endoderm (VE) and extraembryonic ectoderm (ExE) are known to be important for the induction of primordial germ cells (PGCs) in mice via activation of the bone morphogenetic protein (BMP) signalling pathway. We investigated whether the VE and ExE have a direct role in the specification of PGCs, or in an earlier event, namely the induction of the PGC precursors in the proximal posterior epiblast cells. RESULTS: We cultured embryonic day (E) 5.75 to E7.0 mouse embryos in an explant-assay with or without extraembryonic tissues. The reconstituted pieces of embryonic and extraembryonic tissues were assessed for the formation of both PGC precursors and specified PGCs. For this, Blimp1:gfp and Stella:gfp transgenic mouse lines were used to distinguish between PGC precursors and specified PGC, respectively. We observed that the VE regulates formation of an appropriate number of PGC precursors between E6.25-E7.25, but it is not essential for the subsequent specification of PGCs from the precursor cells. Furthermore, we show that the ExE has a different role from that of the VE, which is to restrict localization of PGC precursors to the posterior part of the embryo. CONCLUSION: We show that the VE and ExE have distinct roles in the induction of PGC precursors, namely the formation of a normal number of PGC precursors, and their appropriate localization during early development. However, these tissues do not have a direct role during the final stages of specification of the founder population of PGCs.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

Essential role for Argonaute2 protein in mouse oogenesis.

BACKGROUND: Argonaute2 protein (Ago2) is a key component of RNA-induced gene silencing complex, which is crucial for microRNA-mediated repression of target genes. The function of Ago2 in the mouse oocyte and early embryonic development is less well characterized but it is likely to have an important role in regulating maternally inherited mRNA. We have examined the role of Ago2 by conditional deletion of the gene in developing oocytes. RESULTS: Ago2 was deleted specifically in the growing oocytes. Although the Ago2-deficient oocytes are able to develop to mature oocytes, they have abnormal spindles and chromosomes that are unable to cluster together properly. This phenotype is very similar to the phenotype of Dicer-deficient oocytes. We examined the microRNA expression profile in the Ago2-deficient oocyte and found that the expression of most microRNAs was reduced by more than 80%. To determine the downstream genes that are regulated by Ago2, we used microarray analysis on Ago2-deficient oocytes and found that 512 genes were upregulated and 1,073 genes were downregulated (FC > 2, P < 0.05). CONCLUSION: Our study shows that Ago2 has a key function in the mouse oocyte through global regulation of microRNA stability, and through this mechanism it affects gene expression in developing oocytes.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

Membrane-Bound Steel Factor Maintains a High Local Concentration for Mouse Primordial Germ Cell Motility, and Defines the Region of Their Migration

Steel factor, the protein product of the Steel locus in the mouse, is a multifunctional signal for the primordial germ cell population. We have shown previously that its expression accompanies the germ cells during migration to the gonads, forming a “travelling niche” that controls their survival, motility, and proliferation. Here we show that these functions are distributed between the alternatively spliced membrane-bound and soluble forms of Steel factor. The germ cells normally migrate as individuals from E7.5 to E11.5, when they aggregate together in the embryonic gonads. Movie analysis of Steel-dickie mutant embryos, which make only the soluble form, at E7.5, showed that the germ cells fail to migrate normally, and undergo “premature aggregation” in the base of the allantois. Survival and directionality of movement is not affected. Addition of excess soluble Steel factor to Steel-dickie embryos rescued germ cell motility, and addition of Steel factor to germ cells in vitro showed that a fourfold higher dose was required to increase motility, compared to survival. These data show that soluble Steel factor is sufficient for germ cell survival, and suggest that the membrane-bound form provides a higher local concentration of Steel factor that controls the balance between germ cell motility and aggregation. This hypothesis was tested by addition of excess soluble Steel factor to slice cultures of E11.5 embryos, when migration usually ceases, and the germ cells aggregate. This reversed the aggregation process, and caused increased motility of the germ cells. We conclude that the two forms of Steel factor control different aspects of germ cell behavior, and that membrane-bound Steel factor controls germ cell motility within a “motility niche” that moves through the embryo with the germ cells. Escape from this niche causes cessation of motility and death by apoptosis of the ectopic germ cells