The identification, origin, and migration of the primordial germ cells in the mouse embryo

Isolation and characterization of pluripotent human spermatogonial stem cell-derived cells

Several reports have documented the derivation of pluripotent cells (multipotent germline stem cells) from spermatogonial stem cells obtained from the adult mouse testis. These spermatogonia-derived stem cells express embryonic stem cell markers and differentiate to the three primary germ layers, as well as the germline. Data indicate that derivation may involve reprogramming of endogenous spermatogonia in culture. Here, we report the derivation of human multipotent germline stem cells (hMGSCs) from a testis biopsy. The cells express distinct markers of pluripotency, form embryoid bodies that contain derivatives of all three germ layers, maintain a normal XY karyotype, are hypomethylated at the H19 locus, and express high levels of telomerase. Teratoma assays indicate the presence of human cells 8 weeks post-transplantation but limited teratoma formation. Thus, these data suggest the potential to derive pluripotent cells from human testis biopsies but indicate a need for novel strategies to optimize hMGSC culture conditions and reprogramming.

Functional analysis of adult stem cells using Cre-mediated lineage tracing

Lineage-tracing has been used for decades to establish cell fate maps during development. Recently, with the advent of genetic lineage-tracing techniques (employing Cre-lox recombination), it has been possible to permanently mark progenitor/stem cell populations within somatic tissues. In addition, pulse-chase studies have shown that only stem cells are capable of producing labeled progeny after an extensive period of chase. This unit focuses on the protocols used to target putative adult stem cells in vivo. Using these techniques, one should be able to functionally confirm or deny the stem cell capacity of a given cell population.

Primordial germ cell specification from embryonic stem cells

Background

Primordial germ cell (PGC) specification is the first crucial step in germ line development. However, owing to significant challenges regarding the in vivo system, such as the complex cellular environment and potential problems with embryo manipulation, it is desirable to generate embryonic stem (ES) cells that are capable of overcoming these aforementioned limitations in order to provide a potential in vitro model to recapitulate the developmental processes in vivo.

Methodology and Principal Findings

Here, we studied the detailed process of PGC specification from stella-GFP ES cells. We first observed the heterogeneous expression of stella in ES cells. However, neither Stella-positive ES cells nor Stella-negative ES cells shared a similar gene expression pattern with either PGCs or PGC precursors. Second, we derived PGCs from ES cells using two differentiation methods, namely the attachment culture technique and the embryoid body (EB) method. Compared with PGCs derived via the attachment culture technique, PGCs derived via the EB method that had undergone the sequential erasure of Peg3 followed by Igf2r resulted in a cell line in which the expression dynamics of T, Fgf8 and Sox17, in addition to the expression of the epiblast markers, were more similar to the in vivo expression, thus demonstrating that the process of PGC derivation was more faithfully recapitulated using the EB method. Furthermore, we developed an in vitro model of PGC specification in a completely chemically defined medium (CDM) that indicated that BMP4 and Wnt3a promoted PGC derivation, whereas BMP8b and activinA had no observable effect on PGC derivation.

Conclusions and Significance

The in vitro model we have established can recapitulate the developmental processes in vivo and provides new insights into the mechanism of PGC specification.

Two-step oligoclonal development of male germ cells

During mouse development, primordial germ cells (PGCs) that give rise to the entire germ line are first identified within the proximal epiblast. However, long-term tracing of the fate of the cells has not been done wherein all cells in and around the germ-cell lineage are identified. Also, quantitative estimates of the number of founder PGCs using different models have come up with various numbers. Here, we use tetrachimeric mice to show that the progenitor numbers for the entire germ line in adult testis, and for the initiating embryonic PGCs, are both 4 cells. Although they proliferate to form polyclonal germ-cell populations in fetal and neonatal testes, germ cells that actually contribute to adult spermatogenesis originate from a small number of secondary founder cells that originate in the fetal period. The rest of the "deciduous" germ cells are lost, most likely by apoptosis, before the reproductive period. The second "actual" founder germ cells generally form small numbers of large monoclonal areas in testes by the reproductive period. Our results also demonstrate that there is no contribution of somatic cells to the male germ cell pool during development or in adulthood. These results suggest a model of 2-step oligoclonal development of male germ cells in mice, the second step distinguishing the heritable germ line from cells selected not to participate in forming the next generation.

The mitochondrial DNA genetic bottleneck results from replication of a subpopulation of genomes

In mammals, mitochondrial DNA (mtDNA) sequence variants are observed to segregate rapidly between generations despite the high mtDNA copy number in the oocyte. This has led to the concept of a genetic bottleneck for the transmission of mtDNA, but the mechanism remains contentious. Several studies have suggested that the bottleneck occurs during embryonic development, as a result of a marked reduction in germline mtDNA copy number. Mitotic segregation of mtDNAs during preimplantation, or during the expansion of primordial germ cells (PGCs) before they colonize the gonad, is thought to account for the increase in genotypic variance observed among mature oocytes from heteroplasmic mothers. This view has, however, been challenged by studies suggesting that the bottleneck occurs without a reduction in germline mtDNA content. To resolve this controversy, we measured mtDNA heteroplasmy and copy number in single germ cells isolated from heteroplasmic mice. By directly tracking the evolution of mtDNA genotypic variance during oogenesis, we show that the genetic bottleneck occurs during postnatal folliculogenesis and not during embryonic oogenesis.

Reprogramming primordial germ cells (PGC) to embryonic germ (EG) cells

In this unit we describe the derivation of pluripotent embryonic germ (EG) cells from mouse primordial germ cells (PGCs) isolated from both 8.5- and 11.5-days post-coitum (dpc) embryos. Once EG cells are derived we explain how to propagate and characterize the cell lines. We introduce readers to PGCs and explain differences between PGCs and their in vitro derivatives EG cells. Finally, we also compare mouse EG cells with ES cells. This unit will be of great interest to anyone interested in PGCs or studying the behavior of cultured PGCs or the derivation of new EG cell lines.

Developmental and Ultrastructual Characteristics of Mouse Oocytes Grown in Vitro from Primordial Germ Cells

The aim of this study was to clarify the developmental and ultrastructual characteristics of oocytes grown in vitro from primordial germ cells. The female genital ridges at 12.5 days post coitus were cultured for 18 days on an insert membrane in Waymouth's MB752/1 medium, supplemented with 15% fetal bovine serum and 1 mM sodium pyruvate; subsequently, the follicles isolated from the tissue were cultured for eight days in Waymouth's medium supplemented with 5 microg/ml insulin, 5 microg/ml transferrin, 5 ng/ml selenium, 10 mIU/ml follicle stimulating hormone, and 100 ng/ml stem cell factor. The primordial germ cells developed in vitro into oocytes of more than 60 microm in diameter. The transmission electron microscopic analysis indicated that the oocytes, which developed in vitro, showed no obvious abnormality in their ultrastructure and had organelles appropriate for the oocyte size. However, a delay in the progressive changes of morphology in some of the organelles during oocyte growth was often found when comparing them to oocytes grown in vivo.

A comprehensive, non-invasive visualization of primordial germ cell development in mice by the Prdm1-mVenus and Dppa3-ECFP double transgenic reporter

The ability to monitor the development of a given cell lineage in a non-invasive manner by fluorescent markers bothin vivoandin vitroprovides a great advantage for the analysis of the lineage of interest. To date, a number of transgenic or knock-in mouse strains, in which developing germ cells are marked with fluorescent reporters, have been generated. We here describe a novel double transgenic reporter mouse strain that expresses membrane-targeted Venus (mVenus), a brighter variant of yellow fluorescent protein (YFP), under the control ofPrdm1(Blimp1) regulatory elements and enhanced cyan fluorescent protein (ECFP) under the control ofDppa3(Stella/Pgc7). The double transgenic strain unambiguously markedPrdm1expression in the lineage-restricted precursors of primordial germ cells (PGCs) in the proximal epiblast at embryonic day (E) 6.25 and specifically illuminatedPrdm1- andDppa3-positive migrating PGCs after E8.5. The double transgenic reporter also precisely recapitulated dynamic embryonic expression ofPrdm1outside the germ cell lineage. Moreover, we derived ES cells that bore both transgenes. These cells made a robust contribution both to the germ and somatic cell lineages in chimeras with accuratePrdm1-mVenus andDppa3-ECFP expression. The transgenic strain and the ES cells will serve as valuable experimental materials not only for analyzing the origin and properties of the germ cell lineagein vivo, but also for establishing a culture system to efficiently induce proper germ cells with temporally coordinatedPrdm1andDppa3expressionin vitro.

Complex genome-wide transcription dynamics orchestrated by Blimp1 for the specification of the germ cell lineage in mice

Specification of germ cell fate is fundamental in development. With a highly representative single-cell microarray and rigorous quantitative PCR analysis, we defined the genome-wide transcription dynamics that create primordial germ cells (PGCs) from the epiblast, a process that exclusively segregates them from their somatic neighbors. We also analyzed the effect of the loss of Blimp1, a key transcriptional regulator, on these dynamics. Our analysis revealed that PGC specification involves complex, yet highly ordered regulation of a large number of genes, proceeding under the strong influence of mesoderm induction but specifically avoiding developmental programs such as the epithelial-mesenchymal transition, Hox cluster activation, cell cycle progression, and DNA methyltransferase machinery. Remarkably, Blimp1 is essential for repressing nearly all the genes normally down-regulated in PGCs relative to their somatic neighbors. In contrast, it is dispensable for the activation of approximately half of the genes up-regulated in PGCs, uncovering the Blimp1-independent events for PGC specification. Notably, however, highly PGC-specific genes exhibited distinct correlations to Blimp1 in wild-type embryos, and these correlations faithfully predicted their expression impairments in Blimp1 mutants. Moreover, their expression overlaps within single cells were severely damaged without Blimp1, demonstrating that Blimp1 exerts positive influence on their concerted activation. Thus, Blimp1 is not a single initiator but a dominant coordinator of the transcriptional program for the establishment of the germ cell fate in mice.

Modelling germ cell development in vitro

Germ cells have a critical role in mediating the generation of genetic diversity and transmitting this information across generations. Furthermore, gametogenesis is unique as a developmental process in that it generates highly-specialized haploid gametes from diploid precursor stem cells through meiosis. Despite the importance of this process, progress in elucidating the molecular mechanisms underpinning mammalian germ cell development has been retarded by the lack of an efficient and reproducible system of in vitro culture for the expansion and trans-meiotic differentiation of germline cells. The dearth of such a culture system has rendered the study of germ cell biology refractory to the application of new high-throughput technologies such as RNA interference, leaving in vivo gene-targeting approaches as the only option to determine the function of genes believed to be involved in gametogenesis. Recent reports detailing the derivation of gametes in vitro from stem cells may provide the first steps in developing new tools to solve this problem. This review considers the developments made in modelling germ cell development using stem cells, and some of the challenges that need to be overcome to make this a useful tool for studying gametogenesis and to realize any future clinical application.

Stem cells: a perspective on oocytes

Stem cells, with their unlimited self-renewal feature and their ability to differentiate into almost every mature cell type in the body, have enormous potential for research and therapeutic application. In this article, we review the formation of primordial germ cells, the precursors of adult gametocytes, from their specification to their migration to prospective gonads. We discuss recent studies that obtained germ cells from stem cells in vitro. We place special emphasis on studies that challenge the current dogma in reproductive biology that female mammals are born with a set number of nonrenewable germ cells in the ovary by showing germ cell renewal in the adult ovary.

Systematic localization of Oct-3/4 to the gastrulating mouse conceptus suggests manifold roles in mammalian development

Oct-3/4 was localized to the mouse conceptus between the onset of gastrulation and 16-somite pairs (-s; approximately 6.5-9.25 days postcoitum, dpc). Results revealed Oct-3/4 in a continuum of morphologically distinct epiblast-derived embryonic and extraembryonic tissues. In the allantois, distal-to-proximal diminution in the Oct-3/4 domain over time and co-localization with Flk-1 in angioblasts accorded with a role in vascular differentiation and the presence of a stem cell reservoir. In addition, visceral endoderm exhibited a dynamic salt-and-pepper distribution, which, combined with previous results of fate mapping and gene expression, suggested that Oct-3/4 is involved in the genesis of definitive endoderm. By 8-s, Oct-3/4 was globally down regulated in all but putative primordial germ cells (PGCs) and some allantoic cell clusters. Taken together, Oct-3/4's expression profile suggests unexpected and potentially far more versatile roles in development than have been previously appreciated.

THE OOGONIA OF MACROALGA UNDARIA PINNATIFIDA ARE ALKALINE-PHOSPHATASE POSITIVE AND CONTAIN GERMINAL BODY-LIKE STRUCTURES(1)

It was observed that in the female gametophyte of Undaria pinnatifida (Harv.) Suringar (Phaeophyta, Laminariales) gametangial initials and maturing oogonia demonstrated different levels of alkaline-phosphatase activity (APA). The oogonia exhibited a higher level of APA than in its initials. Electron-dense granular ovoid structures ∼0.5-0.6 μm were present in the cytoplasm of oogonia. These inclusions were not membrane bound and do not appear to be associated with any particular organelles. The number of the inclusions was 1 to 2 in a single section of the cell. In essential details, the specific APA and subcellular germinal body-like structure of the developing female gamete in U. pinnatifida were very similar to those in metazoan oocytes.

Oocyte quality and maternal control of development

The oocyte is a unique and highly specialized cell responsible for creating, activating, and controlling the embryonic genome, as well as supporting basic processes such as cellular homeostasis, metabolism, and cell cycle progression in the early embryo. During oogenesis, the oocyte accumulates a myriad of factors to execute these processes. Oogenesis is critically dependent upon correct oocyte-follicle cell interactions. Disruptions in oogenesis through environmental factors and changes in maternal health and physiology can compromise oocyte quality, leading to arrested development, reduced fertility, and epigenetic defects that affect long-term health of the offspring. Our expanding understanding of the molecular determinants of oocyte quality and how these determinants can be disrupted has revealed exciting new insights into the role of oocyte functions in development and evolution.

Development of germ cells in the mouse

In mammals, germ cells are induced from a population of cells at the base of the allantois. This regulative mechanism of germ line induction depends on Bmp signals and a combination of epigenetic changes that silence somatic differentiation genes and activate pluripotency genes. RNA binding proteins are a conserved feature of germ cell development in mammals, and play critical roles in the establishment and maintenance of pluripotency. After their specification, germ cells move through the gut to the gonads under the influence of migratory and attractive cues. In the gonad, germ cells initiate sex-specific differentiation. Germ cells that arrive in the ovary enter meiosis, whereas germ cells that arrive in the testis undergo mitotic arrest. Entry into meiosis is controlled by retinoic acid signals that are blocked in the testis. The signals regulating mitotic arrest in the testis are still not completely understood, but likely involve RNA-binding proteins. Epigenetic reprograming occurs during specification, migratory stages, and sex-specific stages, when maternal and paternal imprints are established. The facility of transitions between germ cells and stem cells suggests a close relationship among their genomic programs.

Isolated spermatogonia protrude active pseudopodia in vitro

When dispersed spermatogenic cells obtained by enzymatic digestion from prepuberal mice, adult male mice, nonazoospermic men and normospermic men were observed live using Normarski optics, it was found that, respectively, 47.4%, 1.4%, 5.1%, and 2.4% of them protruded active pseudopodia. These cells were 8 to 10 mum in diameter, had a high N/C ratio, and had one to two prominent nucleoli that were close to a distinct nuclear membrane. They showed low alkaline phosphatase activities and homogeneous nuclear immunoreactive patterns using gamma-H2AX, which suggests that they were spermatogonia.

Adult marrow-derived very small embryonic-like stem cells and tissue engineering

A population of CXCR4(+) lin(-) CD45(-) cells that express SSEA, Oct-4 and Nanog has been identified in adult bone marrow. These cells are very small and display several features typical for primary embryonic stem cells such as: i) a large nuclei surrounded by a narrow rim of cytoplasm; ii) open-type chromatin (euchromatin); and iii) high telomerase activity. These cells were named very small embryonic-like stem cells (VSEL-SC). The authors hypothesized that they are direct descendants of the germ lineage. Germ lineage, in order to pass genes on to the next generation, has to create soma and thus becomes a 'mother lineage' for all somatic cell lineages present in the adult body. Germ potential is established after conception in a totipotent zygote and retained subsequently during development in blastomers of morula, cells form the inner cell mass of blastocyst, epiblast and population of primordial germ cells. The authors envision that VSEL-SC are epiblast-derived pluripotent stem cells and could potentially become a less-controversial source of stem cells for regeneration.

Primordial germ cell migration

Primordial germ cells are migratory cells. They arise very early in embryogenesis and have a similar pattern of migration in Drosophila, Xenopus, chick and mouse. In each case the primordial germ cells associate with the developing gut from which they migrate to the gonads during organogenesis. Germ cells proliferate mitotically from the time they begin to migrate to the time they colonize the genital ridges. From the study of mouse primordial germ cells, we now know of a number of agents which affect primordial germ cell proliferation, migration and adhesion in vitro. More recently, we have studied the interactions between primordial germ cells and the cells and extracellular matrix molecules on their migratory route. By labelling germ cells in whole-mount preparations with an antibody to the germ cell marker SSEA-1, we have studied the spatial distribution of germ cells in situ using confocal microscopy. This study has revealed that germ cells link up with each other forming extensive networks during migration.

Clonal analysis of the origin of primordial germ cells in the mouse

Qualitative and quantitative clonal analysis has been used to answer three basic questions about the establishment of the germ cell lineage in the mouse. Where do primordial germ cells originate? What is the size of the founding population at the time of lineage restriction? When and where does lineage restriction occur? Single epiblast cells of 6.0 dpc and 6.5 dpc mouse embryos were injected with a short-term lineage label (lysinated rhodamine dextran, LRDX) and their descendants traced after 40 h embryo culture at neural plate and early somite stages, respectively. An objective matching technique was used to detect the lineage marker in primordial germ cells identified by their characteristic alkaline phosphatase staining. Precursors of the primordial germ cells were found in the proximal epiblast close to the extraembryonic ectoderm in both pregastrulation and early-streak stage embryos. They form part of the presumptive extraembryonic mesoderm and are not lineage restricted while in the epiblast. Quantitative analysis gives a best fit to a model of a founding population of 45 at the time of lineage restriction. The data indicate that the generation time lengthens at the time of allocation. Calculation of clonal histories gives a best fit of 16 h generation time after allocation compared with < 7 h before allocation, with lineage restriction occurring at the early midstreak stage, presumably in the region posterior to the streak in which primordial germ cells are first identifiable. Therefore primordial germ cells are probably allocated early during gastrulation in a group of > 40 cells already segregated in the extraembryonic mesoderm.

The murine allantois: emerging paradigms in development of the mammalian umbilical cord and its relation to the fetus

The fertilized egg of the mammal gives rise to the embryo and its extraembryonic structures, all of which develop in intimate relation with each other. Yet, whilst the past several decades have witnessed a vast number of studies on the embryonic component of the conceptus, study of the extraembryonic tissues and their relation to the fetus have been largely ignored. The allantois, precursor tissue of the mature umbilical cord, is a universal feature of all placental mammals that establishes the vital vascular bridge between the fetus and its mother. The allantois differentiates into the umbilical blood vessels, which become secured onto the chorionic component of the placenta at one end and onto the fetus at the other. In this way, fetal blood is channeled through the umbilical cord for exchange with the mother. Despite the importance of this vascular bridge, little is known about how it is made. The aim of this review is to address current understanding of the biology of the allantois in the mouse and genetic control of its features and functions, and to highlight new paradigms concerning the developmental relationship between the fetus and its umbilical cord.

Mitochondrial DNA and the Mammalian Oocyte

In mammals, mitochondria and mitochondrial DNA (mtDNA) are transmitted through the female germ line. Mature oocytes contain at least 100,000 copies of mtDNA, organized at 1-2 copies per organelle. Despite the high genome copy number, mtDNA sequence variants are observed to segregate rapidly between generations, and this has led to the concept of a developmental bottleneck for the transmission of mtDNA. Ultrastructural investigations of primordial germ cells show that they contain approximately 10 mitochondria, suggesting that mitochondrial biogenesis is arrested during early embryogenesis, and that the mitochondria contributing to the germ cell precursors are simply apportioned from those present in the zygote. Thus, as few as 0.01% of the mitochondria in the oocyte actually contribute to the offspring of the next generation. Mitochondrial replication restarts in the migrating primordial germ cells, and mitochondrial numbers steadily increase to a few thousand in primordial oocytes. Genetic evidence from both heteroplasmic mice and human pedigrees suggests that segregation of mtDNA sequence variants is largely a stochastic process that occurs during the mitotic divisions of the germ cell precursors. This process is essentially complete by the time the primary oocyte population is differentiated in fetal life. Analysis of the distribution of pathogenic mtDNA mutations in the offspring of carrier mothers shows that risk of inheriting a pathogenic mutation increases with the proportion in the mother, but there is no bias toward transmitting more or less of the mutant mtDNAs. This implies that there is no strong selection against oocytes carrying pathogenic mutations and that atresia is not a filter for oocyte quality based on oxidative phosphorylation capacity. The large number of mitochondria and mtDNAs present in the oocyte may simply represent a genetic mechanism to ensure their distribution to the gametes and somatic cells of the next generation. If true, mtDNA copy number, and by inference mitochondrial number, may be the most important determinant of oocyte quality, not because of the effects on oocyte metabolism, but because too few would result in a maldistribution in the early embryo.

Activation of protein kinases A and C promoted proliferation of chicken primordial germ cells

Many growth factors or cytokines regulate cell proliferation via different intracellular signaling pathways. The mechanisms remained quite unclear in avian primordial germ cells (PGCs). In the present study, two major protein kinases, PKA and PKC, were investigated to be involved in signal transduction of PGC proliferation. PGCs were isolated from genital ridge of 3.5-day chicken embryos and primary culture was performed with 5% fetal calf serum (FCS)-supplemented medium 199. After culture for 24 h, PGCs were subcultured on chicken embryonic fibroblast feeder (CEF) and the cells were characterized by histochemical stainings of alkaline phosphatase (ALP) and periodic acid-Schiff (PAS) reagent as well as immunocytochemical stainings of c-kit and stage-specific embryonic antigen-1 (SSEA-I). In addition, cells were challenged with adenylate cyclase activator forskolin (FRSK) and PKC activator phorbol-12-myristate-13-acetate (PMA) alone or in combinations with PKA inhibitor H(89) and PKC inhibitor H(7), respectively. Results showed that subcultured PGCs on CEF displayed positive histochemical and immunocytochemical stainings for ALP, PAS, c-kit and SSEA-I and manifested intensive proliferating activity by colony formation. Downstream activation of PKA by FRSK (10(-7) to 10(-5)M) significantly promoted the proliferation of PGCs by increasing colony number (ALP-stained) in a dose-dependant manner. PMA (10(-8)M) also increased PGC colony number (P<0.05). However, the proliferating effects elicited by FRSK or PMA could be inhibited by the respective protein kinase inhibitor H(89) or H(7). Therefore, the above results suggest that activation of intracellular protein kinases A and C by external factors may promote proliferation of cultured PGCs and PKA represents the most likely mediator of PGC proliferation in embryonic chickens.

Pluripotent stem cells and their niches

The ability of stem cells to self-renew and to replace mature cells is fundamental to ontogeny and tissue regeneration. Stem cells of the adult organism can be categorized as mono-, bi-, or multipotent, based on the number of mature cell types to which they can give rise. In contrast, pluripotent stem cells of the early embryo have the ability to form every cell type of the adult body. Permanent lines of pluripotent stem cells have been derived from preimplantation embryos (embryonic stem cells), fetal primordial germ cells (embryonic germ cells), and malignant teratocarcinomas (embryonal carcinoma cells). Cultured pluripotent stem cells can easily be manipulated genetically, and they can be matured into adult-type stem cells and terminally differentiated cell types in vitro, thereby, providing powerful model systems for the study of mammalian embryogenesis and disease processes. In addition, human embryonic stem cell lines hold great promise for the development of novel regenerative therapies. To fully utilize the potential of these cells, we must first understand the mechanisms that control pluripotent stem cell fate and function. In recent decades, the microenvironment or niche has emerged as particularly critical for stem cell regulation. In this article, we review how pluripotent stem cell signal transduction mechanisms and transcription factor circuitries integrate information provided by the microenvironment. In addition, we consider the potential existence and location of adult pluripotent stem cell niches, based on the notion that a revealing feature indicating the presence of stem cells in a given tissue is the occurrence of tumors whose characteristics reflect the normal developmental potential of the cognate stem cells.

Intermolecular interactions of homologs of germ plasm components in mammalian germ cells

In some species such as flies, worms, frogs and fish, the key to forming and maintaining early germ cell populations is the assembly of germ plasm, microscopically distinct egg cytoplasm that is rich in RNAs, RNA-binding proteins and ribosomes. Cells which inherit germ plasm are destined for the germ cell lineage. In contrast, in mammals, germ cells are formed and maintained later in development as a result of inductive signaling from one embryonic cell type to another. Research advances, using complementary approaches, including identification of key signaling factors that act during the initial stages of germ cell development, differentiation of germ cells in vitro from mouse and human embryonic stem cells and the demonstration that homologs of germ plasm components are conserved in mammals, have shed light on key elements in the early development of mammalian germ cells. Here, we use FRET (Fluorescence Resonance Energy Transfer) to demonstrate that living mammalian germ cells possess specific RNA/protein complexes that contain germ plasm homologs, beginning in the earliest stages of development examined. Moreover, we demonstrate that, although both human and mouse germ cells and embryonic stem cells express the same proteins, germ cell-specific protein/protein interactions distinguish germ cells from precursor embryonic stem cells in vitro; interactions also determine sub-cellular localization of complex components. Finally, we suggest that assembly of similar protein complexes may be central to differentiation of diverse cell lineages and provide useful diagnostic tools for isolation of specific cell types from the assorted types differentiated from embryonic stem cells.

Gene expression dynamics during germline specification in mice identified by quantitative single-cell gene expression profiling

Germ cell fate in mice is induced in proximal epiblast cells at Embryonic Day (E) 6.5 by signaling molecules. Prdm1(also known as Blimp1)-positive lineage-restricted precursors of primordial germ cells (PGCs) initiate the formation of a cluster that differentiates into Dppa3 (also known as stella)-positive PGCs from around E7.0 onwards in the extra-embryonic mesoderm. Around E7.5, these PGCs begin migrating towards the definitive endoderm, with concomitant extensive epigenetic reprogramming. To gain a more precise insight into the mechanism of PGC specification and its subsequent development, we exploited quantitative, single-cell, gene expression profiling to explore gene expression dynamics during the 36 h of PGC differentiation from E6.75 to E8.25, in comparison with the corresponding profiles of somatic neighbors. This analysis revealed that the transitions from Prdm1-positive PGC precursors to Dppa3-positive PGCs and to more advanced migrating PGCs involve a highly dynamic, stage-dependent transcriptional orchestration that begins with the regaining of the pluripotency-associated gene network, followed by stepwise activation of PGC-specific genes, differential repression of the somatic mesodermal program, as well as potential modulations of signal transduction capacities and unique control of epigenetic regulators. The information presented here regarding the cascade of events involved in PGC development should serve as a basis for detailed functional analyses of the gene products associated with this process, as well as for appropriate reconstitution of PGCs and their descendant cells in culture.

FGF9 promotes survival of germ cells in the fetal testis

In addition to its role in somatic cell development in the testis, our data have revealed a role for Fgf9 in XY germ cell survival. In Fgf9-null mice, germ cells in the XY gonad decline in numbers after 11.5 days post coitum (dpc), while germ cell numbers in XX gonads are unaffected. We present evidence that germ cells resident in the XY gonad become dependent on FGF9 signaling between 10.5 dpc and 11.5 dpc, and that FGF9 directly promotes XY gonocyte survival after 11.5 dpc, independently from Sertoli cell differentiation. Furthermore, XY Fgf9-null gonads undergo true male-to-female sex reversal as they initiate but fail to maintain the male pathway and subsequently express markers of ovarian differentiation(Fst and Bmp2). By 14.5 dpc, these gonads contain germ cells that enter meiosis synchronously with ovarian gonocytes. FGF9 is necessary for 11.5 dpc XY gonocyte survival and is the earliest reported factor with a sex-specific role in regulating germ cell survival.

Confinement and clearance of OCT4 in the porcine embryo at stereomicroscopically defined stages around gastrulation

In the areas of developmental biology and embryonic stem cell research, reliable molecular markers of pluripotency and early lineage commitment are sparse in large animal species. In this study, we present morphological and immunohistochemical findings on the porcine embryo in the period around gastrulation, days 8-17 postinsemination, introducing a stereomicroscopical staging system in this species. In embryos at the expanding hatched blastocyst stage, OCT4 is confined to the inner cell mass. Following detachment of the hypoblast, and formation of the embryonic disk, this marker of pluripotency was selectively observed in the epiblast. A prominent crescent-shaped thickening at the posterior region of the embryonic disk marked the first polarization within this structure reflecting incipient cell ingression. Following differentiation of the epiblast, clearance of OCT4 from the three germ layers was observed at defined stages, suggesting correlations to lineage specification. In the endoderm, clearance of OCT4 was apparent from early during its formation at the primitive streak stage. The endoderm harbored progenitors of the "fourth germ layer," the primordial germ cells (PGCs), the only cells maintaining expression of OCT4 at the end of gastrulation. In the ectodermal and mesodermal cell lineages, OCT4 became undetectable at the neural groove and somite stage, respectively. As in the mouse, PGCs showed onset of c-kit expression when located in extraembryonal compartments. They appeared to follow the endoderm during extraembryonal allocation and the mesoderm on return to the genital ridge.

Migration of human and mouse primordial germ cells and colonization of the developing ovary: An ultrastructural and cytochemical study

This review is an account of the origin and migratory events of primordial germ cells until their settlement in the gonad before sexual differentiation in the human as well as mice. In this context, the morphodynamic characteristics of the migration of the primordial germ cells, the macromolecular characteristics of the extracellular matrix of the migratory pathway, and the factors involved in the germ cell guidance have been analyzed and discussed in the light of recent advances in this field, by means of immunocytochemical procedures. The events prior to gonadal morphogenesis and the origin of the somatic cell content of the human gonadal primordium have been also analyzed. In particular, evidences are presented showing that cells derived from the coelomic epithelium and mesenchyme are at the origin of the somatic components of the gonadal primordium, and that a mesonephric cell contribution to the generation of somatic cell components of the genital ridge in humans should be discarded due to the morphological stability of the different nephric structures during the period preceding the sexual differentiation of the gonad.

Germline recruitment in mice: a genetic program for epigenetic reprogramming

Germ cells provide an enduring link between generations and therefore must possess the fundamental ability of reprogramming their genome to generate a totipotent state. We wish to understand the molecular basis of the unique properties of the mammalian germ line. Recently we identified Blimp1, a potent transcriptional repressor of a histone methyltransferase subfamily, as a critical determinant of the germ cell lineage in mice. Surprisingly, Blimp1 expression marks the origin of the germ line in proximal epiblast cells in pregastrulation embryos, substantially earlier than previously thought. Furthermore, we showed that established primordial germ cells undergo extensive erasure of genome-wide histone H3 lysine 9 dimethylation (H3K9me2) and DNA methylation, two major repressive epigenetic modifications, and instead acquire high levels of H3-K27 trimethylation (H3K27me3) in their migration period. We suggest that germline specification is a genetic system for the orderly reprogramming of the cells' epigenome toward a totipotent state, with reacquisition of totipotency-associated transcription factors and continued Blimp1 expression preventing their reversion to an explicit pluripotent state or somatic differentiation.

Male and female germline specific expression of an EGFP reporter gene in a unique strain of transgenic rats

A rat line was generated in which genomic integration of a ROSA-EGFP transgene resulted in exclusive expression of EGFP in the germ cells of both sexes. EGFP expression was uniform and robust in cleavage stage embryos beginning at the late 2-cell stage and continuing through blastocyst development where expression became restricted to cells of the inner cell mass. Subsequent analysis showed high EGFP expression exclusively in primordial, embryonic, and adult germ cells. This unique expression pattern makes this EGFP marked locus the first molecular marker of the germline lineage in both sexes in mammals. FISH was used to localize the transgene insertion to chromosome 11q11-q12, proximal to Grik1 and near Ncam2. Analysis of the region did not identify known germ cell-specific genes but did identify 19 ESTs or transcribed loci present in testes, ovary, or pre-implantation libraries from mice or rats. To assess the utility of the transgenic line for germ cell transplantation studies, non-selected, freshly isolated seminiferous tubule cells were transferred to the testis of recipient males. The donor cell population colonized the testis at a surprisingly high efficiency within 30 days following transfer. Since EGFP is a vital marker, the colonization process can be followed in vivo and the extent of colonization quantified. The unique germ cell specific expression of EGFP makes this line of transgenic rats an excellent novel tool to study germ cell origin, development, and differentiation, and to assess the plasticity of adult somatic stem cells to become male germ cells.

Altered primordial germ cell migration in the absence of transforming growth factor beta signaling via ALK5

Transforming growth factor beta (TGFbeta) inhibits proliferation and promotes the migration of primordial germ cells (PGCs) towards explants of gonadal ridges in vitro. However, its effects in vivo are still unclear. Here, we analyzed the behavior of PGCs in embryos lacking TGFbeta signaling via the type I receptor ALK5. TGFbeta in vivo was neither a chemoattractant for PGCs, nor did it affect their proliferation during migration towards the gonadal ridges up to embryonic day (E)10. Unexpectedly, the absence of TGFbeta signaling in fact resulted in significant facilitation of PGC migration out of the hindgut, due to the reduced deposition of collagen type I surrounding the gut of Alk5-deficient mutant embryos. Migratory PGCs adhere strongly to collagen; therefore, reduced collagen type I along the gut may result in reduced adhesion, facilitating migration into the dorsal mesenterium and gonadal ridges. Our results provide new evidence for the role of TGFbeta signaling in migration of PGCs in vivo distinct from that described previously.

How is the mouse germ-cell lineage established?

Six cells have been detected in the early mouse embryo that express the transcriptional repressor Blimp1--as also do all the 40 or so cells that constitute the founder germ cell pool a day later. Are these half-dozen cells the ancestors of the entire mouse germ cell lineage?

In Vitro Generation of Germ Cells: New Techniques to Solve Current Issues

Primordial germ cells comprise a privileged cellular class within the embryo charged with the elite task of maintaining species longevity. While in lower organisms germ-cell fate is determined by the allocation of germ plasm, mammalian germ-line differentiation requires extracellular signals that converge upon the proximal epiblast. Studies using mutant mice or explanted embryos have identified some of the factors controlling primordial germ-cell specification, such as members of the BMP family, but considerable gaps still exist in our understanding of the complete signaling network. Comprehensive investigations of mammalian germ-line specification have been hampered by the inaccessibility of this cell population in the early embryo. Recently, however, several labs including our own have derived primordial germ cells from embryonic stem cells in vitro, thus providing a powerful new technique for the study of germ cells. In this review the different methods used for the in vitro generation of germ cells and how these techniques may be improved and applied to further advance our knowledge of germ-cell biology are discussed.

Effects of growth factors and feeder cells on porcine primordial germ cells in vitro

As embryonic stem (ES) cells are not available in swine, embryonic germ (EG) cells derived from primordial germ cells (PGCs) are an alternate source of pluripotent embryonic cells for genetic modification through homologous recombination. Although morphological and biochemical characteristics are similar between ES and EG cells, culture conditions are quite different. To optimize the culture condition for the establishment of porcine EG cells, porcine PGCs were cultured in vitro with various combinations of growth factors (leukemia inhibitory factor [LIF], stem cell factor [SCF], and basic fibroblast growth factor [bFGF]) and on different kinds of feeder cells (STO, TM(4), Sl/Sl(4) m220, porcine embryonic fibroblasts, and COS-7 cells). Optimal results were obtained when all three growth factors (LIF, SCF, and bFGF) were present in the media. Also, feeder cells expressing membrane-bound SCF are required for survival and establishment of porcine EG cells. Therefore, a combination of growth factors and proper feeder cells are critical for the establishment of undifferentiated porcine EG cells.

Transcriptional regulation of early oogenesis: in search of masters

Transcription factors in the germline play important roles in ovary formation and folliculogenesis, and control both oocyte development and somatic cell function. Factor in the germline (Figla) and newborn ovary homeobox gene (Nobox) represent a growing number of oocyte-specific transcription factors that regulate genes unique to oocytes. Studies on oocyte-specific transcription factors are important in understanding the genetic pathways essential for oogenesis, pluripotency, and embryonic development. Likely, these genes regulate reproductive life span and represent candidate genes for reproductive disorders, such as premature ovarian failure, and infertility. Therefore, oocyte-specific transcription factors, and oocyte-specific genes regulated by such factors, are attractive tissue-specific pharmacological targets to regulate human fertility.

TGFβ signalling in the development of ovarian function

Ovarian development begins back in the embryo with the formation of primordial germ cells and their subsequent migration and colonisation of the genital ridges. Once the ovary has been defined structurally, the primordial germ cells transform into oocytes and become housed in structures called follicles (in this case, primordial follicles), a procedure that, in most mammals, occurs either shortly before or during the first few days after birth. The growth and differentiation of follicles from the primordial population is termed folliculogenesis. Primordial follicles give rise to primary follicles that transform into preantral follicles, then antral follicles (secondary follicles) and, finally (preovulatory) Graafian follicles (tertiary follicles) in a co-ordinated series of transitions regulated by hormones and local intraovarian factors. Members of the transforming growth factor-beta (TGFbeta) superfamily have been shown to play important roles in this developmental process starting with the specification of primordial germ cells by the bone morphogenetic proteins through to the recruitment of primordial follicles by anti-Mullerian hormone and, potentially, growth and differentiation factor-9 (GDF9) and, finally, their transformation into preantral and antral follicles in response to activin and TGF-beta. Developmental and mutant mouse models have been used to show the importance of this family of growth factors in establishing the first wave of folliculogenesis.

A germ cell origin of embryonic stem cells?

Because embryonic stem (ES) cells are generally derived by the culture of inner cell mass (ICM) cells, they are often assumed to be the equivalent of ICM cells. However, various evidence indicates that ICM cells transition to a different cell type during ES-cell derivation. Historically, ES cells have been believed to most closely resemble pluripotent primitive ectoderm cells derived directly from the ICM. However, differences between ES cells and primitive ectoderm cells have caused developmental biologists to question whether ES cells really have an in vivo equivalent, or whether their properties merely reflect their tissue culture environment. Here, we review recent evidence that the closest in vivo equivalent of an ES cell is an early germ cell.

BMP signaling mediated by ALK2 in the visceral endoderm is necessary for the generation of primordial germ cells in the mouse embryo

Deletion of various bone morphogenetic proteins (BMPs) and their downstream Smads in mice have clearly shown that BMP signaling is essential for the formation of primordial germ cells (PGCs). However, the molecular mechanism through which this takes place is still unclear. Here, we demonstrate that BMP4 produced in the extraembryonic ectoderm signals through ALK2, a type I BMP receptor, in the visceral endoderm (VE) to induce formation of PGCs from the epiblast. Firstly, embryonic day 5.5-6.0 (E5.5-E6.0) embryos cultured on fibronectin formed PGCs in the presence of VE, but not in its absence. Secondly, Alk2-deficient embryos completely lacked PGCs and the heterozygotes had reduced numbers, resembling Bmp4-deficient phenotypes. Thirdly, expression of constitutively active ALK2 in the VE, but not in the epiblast, was sufficient to rescue the PGC phenotype in Bmp4-deficient embryos. In addition, we show that the requirement for the VE at E5.5-E6.0 can be replaced by culturing embryos stripped of VE on STO cells, indicating that STO cells provide or transduce signals necessary for PGC formation that are normally transmitted by the VE. We propose a model in which direct signaling to proximal epiblast is supplemented by an obligatory indirect BMP-dependent signal via the VE.

Cited1 is required in trophoblasts for placental development and for embryo growth and survival

Cited1 is a transcriptional cofactor that interacts with Smad4, estrogen receptors alpha and beta, TFAP2, and CBP/p300. It is expressed in a restricted manner in the embryo as well as in extraembryonic tissues during embryonic development. In this study we report the engineering of a loss-of-function Cited1 mutation in the mouse. Cited1 null mutants show growth restriction at 18.5 days postcoitum, and most of them die shortly after birth. Half the heterozygous females, i.e., those that carry a paternally inherited wild-type Cited1 allele, are similarly affected. Cited1 is normally expressed in trophectoderm-derived cells of the placenta; however, in these heterozygous females, Cited1 is not expressed in these cells. This occurs because Cited1 is located on the X chromosome, and thus the wild-type Cited1 allele is not expressed because the paternal X chromosome is preferentially inactivated. Loss of Cited1 resulted in abnormal placental development. In mutants, the spongiotrophoblast layer is irregular in shape and enlarged while the labyrinthine layer is reduced in size. In addition, the blood spaces within the labyrinthine layer are disrupted; the maternal sinusoids are considerably larger in mutants, leading to a reduction in the surface area available for nutrient exchange. We conclude that Cited1 is required in trophoblasts for normal placental development and subsequently for embryo viability.

Commitment of fetal male germ cells to spermatogonial stem cells during mouse embryonic development

The continuous production of mammalian sperm is maintained by the proliferation and differentiation of spermatogonial stem cells that originate from primordial germ cells (PGCs) in the early embryo. Although spermatogonial stem cells arise from PGCs, it is not clear whether fetal male germ cells function as spermatogonial stem cells able to produce functional sperm. In the present study, we examined the timing and mechanisms of the commitment of fetal germ cells to differentiate into spermatogonial stem cells by transplantation techniques. Transplantation of fetal germ cells into the seminiferous tubules of adult testis showed that donor germ cells, at 14.5 days postcoitum (dpc), were able to initiate spermatogenesis in the adult recipient seminiferous tubules, whereas no germ cell differentiation was observed in the transplantation of 12.5-dpc germ cells. These results indicate that the commitment of fetal germ cells to differentiate into spermatogonial stem cells initiates between embryonic days 12.5 and 14.5. Furthermore, the results suggest the importance of the interaction between germ cells and somatic cells in the determination of fetal germ cell differentiation into spermatogonial stem cells, as normal spermatogenesis was observed when a 12.5-dpc whole gonad was transplanted into adult recipient testis. In addition, sperm obtained from the 12.5- dpc male gonadal explant had the ability to develop normally if injected into the cytoplasm of oocytes, indicating that normal development of fetal germ cells in fetal gonadal explant occurred in the adult testicular environment.

Mechanisms of germ cell specification across the metazoans: epigenesis and preformation

Germ cells play a unique role in gamete production, heredity and evolution. Therefore, to understand the mechanisms that specify germ cells is a central challenge in developmental and evolutionary biology. Data from model organisms show that germ cells can be specified either by maternally inherited determinants (preformation) or by inductive signals (epigenesis). Here we review existing data on 28 metazoan phyla, which indicate that although preformation is seen in most model organisms, it is actually the less prevalent mode of germ cell specification, and that epigenetic germ cell specification may be ancestral to the Metazoa.

Investigation into a role for the primitive streak in development of the murine allantois

Despite its importance as the source of one of three major vascular systems in the mammalian conceptus, little is known about the murine allantois, which will become the umbilical cord of the chorio-allantoic placenta. During gastrulation, the allantois grows into the exocoelomic cavity as a mesodermal extension of the posterior primitive streak. On the basis of morphology, gene expression and/or function, three cell types have been identified in the allantois: an outer layer of mesothelial cells, whose distal portion will become transformed into chorio-adhesive cells, and endothelial cells within the core. Formation of endothelium and chorio-adhesive cells begins in the distal region of the allantois, farthest from the streak. Over time, endothelium spreads to the proximal allantoic region, whilst the distal outer layer of presumptive mesothelium gradually acquires vascular cell adhesion molecule (VCAM1) and mediates chorio-allantoic union. Intriguingly, the VCAM1 domain does not extend into the proximal allantoic region. How these three allantoic cell types are established is not known, although contact with the chorion has been discounted. In this study, we have investigated how the allantois differentiates, with the goal of discriminating between extrinsic mechanisms involving the primitive streak and an intrinsic role for the allantois itself. Exploiting previous observations that the streak contributes mesoderm to the allantois throughout the latter's early development, microsurgery was used to remove allantoises at ten developmental stages. Subsequent whole embryo culture of operated conceptuses resulted in the formation of regenerated allantoises at all time points. Aside from being generally shorter than normal, none of the regenerates exhibited abnormal differentiation or inappropriate cell relationships. Rather, all of them resembled intact allantoises by morphological, molecular and functional criteria. Moreover, fate mapping adjacent yolk sac and amniotic mesoderm revealed that these tissues and their associated bone morphogenetic protein 4 (BMP4) did not contribute to restoration of allantoic outgrowth and differentiation during allantoic regeneration. Thus, on the basis of these observations, we conclude that specification of allantoic endothelium, mesothelium and chorio-adhesive cells does not occur by a streak-related mechanism during the time that proximal epiblast travels through it and is transformed into allantoic mesoderm. Rather, all three cell-types are established by mechanisms intrinsic to the allantois, and possibly include roles for cell age and cell position. However, although chorio-adhesive cells were not specified within the streak, we discovered that the streak nonetheless plays a role in establishing VCAM1's expression domain, which typically began and was thereafter maintained at a defined distance from the primitive streak. When allantoises were removed from contact with the streak, normally VCAM1-negative proximal allantoic regions acquired VCAM1. These results suggested that the streak suppresses formation of chorio-adhesive cells in allantoic mesoderm closest to it. Together with previous results, findings presented here suggest a model of differentiation of allantoic mesoderm that invokes intrinsic and extrinsic mechanisms, all of which appear to be activated once the allantoic bud has formed.

Specification of germ cell fate in mice

An early fundamental event during development is the segregation of germ cells from somatic cells. In many organisms, this is accomplished by the inheritance of preformed germ plasm, which apparently imposes transcriptional repression to prevent somatic cell fate. However, in mammals, pluripotent epiblast cells acquire germ cell fate in response to signalling molecules. We have used single cell analysis to study how epiblast cells acquire germ cell competence and undergo specification. Germ cell competent cells express Fragilis and initially progress towards a somatic mesodermal fate. However, a subset of these cells, the future primordial germ cells (PGCs), then shows rapid upregulation of Fragilis with concomitant transcriptional repression of a number of genes, including Hox and Smad genes. This repression may be a key event associated with germ cell specification. Furthermore, PGCs express Stella and other genes, such as Oct-4 that are associated with pluripotency. While these molecules are also detected in mature oocytes as maternally inherited factors, their early role is to regulate development and maintain pluripotency, and they do not serve the role of classical germline determinants.