Fine structural observations on the origin and associations of primordial germ cells of the mouse

The journey of a generation: advances and promises in the study of primordial germ cell migration

The germline provides the genetic and non-genetic information that passes from one generation to the next. Given this important role in species propagation, egg and sperm precursors, called primordial germ cells (PGCs), are one of the first cell types specified during embryogenesis. In fact, PGCs form well before the bipotential somatic gonad is specified. This common feature of germline development necessitates that PGCs migrate through many tissues to reach the somatic gonad. During their journey, PGCs must respond to select environmental cues while ignoring others in a dynamically developing embryo. The complex multi-tissue, combinatorial nature of PGC migration is an excellent model for understanding how cells navigate complex environments in vivo. Here, we discuss recent findings on the migratory path, the somatic cells that shepherd PGCs, the guidance cues somatic cells provide, and the PGC response to these cues to reach the gonad and establish the germline pool for future generations. We end by discussing the fate of wayward PGCs that fail to reach the gonad in diverse species. Collectively, this field is poised to yield important insights into emerging reproductive technologies.

Using embryo models to understand the development and progression of embryonic lineages: a focus on primordial germ cell development

BACKGROUND

Recapitulating mammalian cell type differentiation in vitro promises to improve our understanding of how these processes happen in vivo, while bringing additional prospects for biomedical applications. The establishment of stem cell-derived embryo models and embryonic organoids, which have experienced explosive growth over the last few years, open new avenues for research due to their scale, reproducibility, and accessibility. Embryo models mimic various developmental stages, exhibit different degrees of complexity, and can be established across species. Since embryo models exhibit multiple lineages organised spatially and temporally, they are likely to provide cellular niches that, to some degree, recapitulate the embryonic setting and enable "co-development" between cell types and neighbouring populations. One example where this is already apparent is in the case of primordial germ cell-like cells (PGCLCs).

SUMMARY

While directed differentiation protocols enable the efficient generation of high PGCLC numbers, embryo models provide an attractive alternative as they enable the study of interactions of PGCLCs with neighbouring cells, alongside the regulatory molecular and biophysical mechanisms of PGC competency. Additionally, some embryo models can recapitulate post-specification stages of PGC development (including migration or gametogenesis), mimicking the inductive signals pushing PGCLCs to mature and differentiate, and enabling the study of PGCLC development across stages. Therefore, in vitro models may allow us to address questions of cell type differentiation, and PGC development specifically, that have hitherto been out of reach with existing systems.

KEY MESSAGE

This review evaluates the current advances in stem cell-based embryo models, with a focus on their potential to model cell type-specific differentiation in general, and in particular to address open questions in PGC development and gametogenesis.

Mitochondrial regulation during male germ cell development

Mitochondria tailor their morphology to execute their specialized functions in different cell types and/or different environments. During spermatogenesis, mitochondria undergo continuous morphological and distributional changes with germ cell development. Deficiencies in these processes lead to mitochondrial dysfunction and abnormal spermatogenesis, thereby causing male infertility. In recent years, mitochondria have attracted considerable attention because of their unique role in the regulation of piRNA biogenesis in male germ cells. In this review, we describe the varied characters of mitochondria and focus on key mitochondrial factors that play pivotal roles in the regulation of spermatogenesis, from primordial germ cells to spermatozoa, especially concerning metabolic shift, stemness and reprogramming, mitochondrial transformation and rearrangement, and mitochondrial defects in human sperm. Further, we discuss the molecular mechanisms underlying these processes.

Biology of primordial germ cells in vertebrates with emphasis in urodeles amphibians

Primordial germ cells (PGCs) are highly specialized cells that play a relevant role in the maintenance and evolution of the species, since they create new combinations of genetic information between the organisms. Amphibians are a class of amniote vertebrates that are divided into three subclasses, the anurans (frogs and toads), the urodeles (salamanders and newts), and the gymnophiones (caecilians). The study of PGCs in amphibians has been addressed in more detail in anurans while little is known about the biology of this cell lineage in urodeles. Studies in some urodeles species have suggested that PGCs are of mesodermal origin, specifying in the lateral plate mesoderm at the late gastrula stage. With classical experiments it shown that, there is an induction of mesoderm, therefore most likely urodeles PGCs develop from unspecialized mesodermal tissue that responds to extracellular signals. However, some fundamental biological processes of PGCs such as the analysis of their specification, arrival, and colonization to the gonads, and their maintenance and differentiation into mature and fertile gametes remain to be elucidated. Therefore, knowledge about the biology of PGCs is of great importance to ensure the perpetuation of urodeles amphibians, as some species are in danger of becoming extinct.

Extensive nuclear gyration and pervasive non-genic transcription during primordial germ cell development in zebrafish

Primordial germ cells (PGCs) are the precursors of germ cells, which migrate to the genital ridge during early development. Relatively little is known about PGCs after their migration. We studied this post-migratory stage using microscopy and sequencing techniques, and found that many PGC-specific genes, including genes known to induce PGC fate in the mouse, are only activated several days after migration. At this same time point, PGC nuclei become extremely gyrated, displaying general broad opening of chromatin and high levels of intergenic transcription. This is accompanied by changes in nuage morphology, expression of large loci (PGC-expressed non-coding RNA loci, PERLs) that are enriched for retro-transposons and piRNAs, and a rise in piRNA biogenesis signatures. Interestingly, no nuclear Piwi protein could be detected at any time point, indicating that the zebrafish piRNA pathway is fully cytoplasmic. Our data show that the post-migratory stage of zebrafish PGCs holds many cues to both germ cell fate establishment and piRNA pathway activation.

Derivation of oocyte-like cells from putative embryonic stem cells and parthenogenetically activated into blastocysts in goat

Germ cells are responsible for the propagation of live animals from generation to generation, but to surprise, a steep increase in infertile problems among livestock poses great threat for economic development of human race. An alternative and robust approach is essential to combat these ailments. Here, we demonstrate that goat putative embryonic stem cells (ESCs) were successfully in vitro differentiated into primordial germ cells and oocyte-like cells using bone morphogenetic protein-4 (BMP-4) and trans-retinoic acid (RA). Oocyte-like cells having distinct zonapellucida recruited adjacent somatic cells in differentiating culture to form cumulus-oocyte complexes (COCs). The putative COCs were found to express the zonapellucida specific (ZP1 and ZP2) and oocyte-specific markers. Primordial germ cell-specific markers VASA, DAZL, STELLA, and PUM1 were detected at protein and mRNA level. In addition to that, the surface architecture of these putative COCs was thoroughly visualized by the scanning electron microscope. The putative COCs were further parthenogenetically activated to develop into healthy morula, blastocysts and hatched blastocyst stage like embryos. Our findings may contribute to the fundamental understanding of mammalian germ cell biology and may provide clinical insights regarding infertility ailments.

Meiosis occurs normally in the fetal ovary of mice lacking all retinoic acid receptors

Gametes are generated through a specialized cell differentiation process, meiosis, which, in ovaries of most mammals, is initiated during fetal life. All-trans retinoic acid (ATRA) is considered as the molecular signal triggering meiosis initiation. In the present study, we analyzed female fetuses ubiquitously lacking all ATRA nuclear receptors (RAR), obtained through a tamoxifen-inducible cre recombinase-mediated gene targeting approach. Unexpectedly, mutant oocytes robustly expressed meiotic genes, including the meiotic gatekeeper STRA8. In addition, ovaries from mutant fetuses grafted into adult recipient females yielded offspring bearing null alleles for all Rar genes. Thus, our results show that RAR are fully dispensable for meiotic initiation, as well as for the production of functional oocytes. Assuming that the effects of ATRA all rely on RAR, our study goes against the current model according to which meiosis is triggered by endogenous ATRA in the developing ovary. It therefore revives the search for the meiosis-inducing substance.

Evolving Role of RING1 and YY1 Binding Protein in the Regulation of Germ-Cell-Specific Transcription

Separation of germline cells from somatic lineages is one of the earliest decisions of embryogenesis. Genes expressed in germline cells include apoptotic and meiotic factors, which are not transcribed in the soma normally, but a number of testis-specific genes are active in numerous cancer types. During germ cell development, germ-cell-specific genes can be regulated by specific transcription factors, retinoic acid signaling and multimeric protein complexes. Non-canonical polycomb repressive complexes, like ncPRC1.6, play a critical role in the regulation of the activity of germ-cell-specific genes. RING1 and YY1 binding protein (RYBP) is one of the core members of the ncPRC1.6. Surprisingly, the role of Rybp in germ cell differentiation has not been defined yet. This review is focusing on the possible role of Rybp in this process. By analyzing whole-genome transcriptome alterations of the Rybp^-/- embryonic stem (ES) cells and correlating this data with experimentally identified binding sites of ncPRC1.6 subunits and retinoic acid receptors in ES cells, we propose a model how germ-cell-specific transcription can be governed by an RYBP centered regulatory network, underlining the possible role of RYBP in germ cell differentiation and tumorigenesis.

Mammalian germ cell migration during development, growth, and homeostasis

BACKGROUND

Germ cells represent one of the typical cell types that moves over a long period of time and large distance within the animal body. To continue its life cycle, germ cells must migrate to spatially distinct locations for proper development. Defects in such migration processes can result in infertility. Thus, for more than a century, the principles of germ cell migration have been a focus of interest in the field of reproductive biology.

METHODS

Based on published reports (mainly from rodents), investigations of germ cell migration before releasing from the body, including primordial germ cells (PGCs), gonocytes, spermatogonia, and immature spermatozoon, were summarized.

MAIN FINDINGS

Germ cells migrate with various patterns, with each migration step regulated by distinct mechanisms. During development, PGCs actively and passively migrate from the extraembryonic region toward genital ridges through the hindgut epithelium. After sex determination, male germline cells migrate heterogeneously in a developmental stage-dependent manner within the testis.

CONCLUSION

During migration, there are multiple gates that disallow germ cells from re-entering the proper developmental pathway after wandering off the original migration path. The presence of gates may ensure the robustness of germ cell development during development, growth, and homeostasis.

Targeted DamID reveals differential binding of mammalian pluripotency factors

The precise control of gene expression by transcription factor networks is crucial to organismal development. The predominant approach for mapping transcription factor-chromatin interactions has been chromatin immunoprecipitation (ChIP). However, ChIP requires a large number of homogeneous cells and antisera with high specificity. A second approach, DamID, has the drawback that high levels of Dam methylase are toxic. Here, we modify our targeted DamID approach (TaDa) to enable cell type-specific expression in mammalian systems, generating an inducible system (mammalian TaDa or MaTaDa) to identify genome-wide protein/DNA interactions in 100 to 1000 times fewer cells than ChIP-based approaches. We mapped the binding sites of two key pluripotency factors, OCT4 and PRDM14, in mouse embryonic stem cells, epiblast-like cells and primordial germ cell-like cells (PGCLCs). PGCLCs are an important system for elucidating primordial germ cell development in mice. We monitored PRDM14 binding during the specification of PGCLCs, identifying direct targets of PRDM14 that are key to understanding its crucial role in PGCLC development. We show that MaTaDa is a sensitive and accurate method for assessing cell type-specific transcription factor binding in limited numbers of cells.

"A narrow bridge home": The dorsal mesentery in primordial germ cell migration

Specification of primordial germ cells (PGCs) in all vertebrates takes place in extragonadal sites. This requires migration of PGCs through embryonic tissues towards the genital ridges by both passive and active types of migration. Commonly, colonization in the genital ridges follows migration of the PGCs along the thin tissue of the dorsal mesentery. Here we review the anatomy of the dorsal mesentery, the role it plays in migration of PGCs, and the interactions of PGCs with different cell types, extracellular matrix and signaling pathways that are all essential for attraction and orientation of PGCs along the dorsal mesentery towards the gonad anlage.

Effects of gestational exposure to di-n-butyl phthalate and mineral oil on testis development of the Mongolian gerbil

Phthalate esters are endocrine disrupters that can affect the development of the testis in a species-specific manner. However, their interference in the male gonads of the Mongolian gerbil is unknown. The aim of the present study was to evaluate whether gestational exposure to di-n-butyl phthalate (DBP) interferes with the development of the gerbil testis during the first six weeks of life. Males were evaluated at 1, 7, 14, 28, 35 and 42 days of age in an untreated (control) group or groups exposed from 8 to 23 days gestation to DBP (100 mg kg−1 day−1 in mineral oil) or vehicle by maternal gavage. DBP exposure impaired cell proliferation within the seminiferous cords at birth, but increased proliferation at the end of the first week, when higher testosterone concentrations were observed. The vehicle (mineral oil) reduced the total number of gonocytes and attenuated the decrease in testosterone concentrations at 7 days. The vehicle also altered gonocyte relocation at 14 days and increased oestrogen concentrations at 28 days by approximately 112%. In summary, both DBP and oil interfered in gonadal development and testosterone plasma concentrations in the first week of postnatal life. However, the changes observed at the beginning of puberty were not seen after exposure to DBP, indicating a more harmful effect of mineral oil in this period.

A pilgrim's progress: Seeking meaning in primordial germ cell migration

Comparative studies of primordial germ cell (PGC) development across organisms in many phyla reveal surprising diversity in the route of migration, timing and underlying molecular mechanisms, suggesting that the process of migration itself is conserved. However, beyond the perfunctory transport of cellular precursors to their later arising home of the gonads, does PGC migration serve a function? Here we propose that the process of migration plays an additional role in quality control, by eliminating PGCs incapable of completing migration as well as through mechanisms that favor PGCs capable of responding appropriately to migration cues. Focusing on PGCs in mice, we explore evidence for a selective capacity of migration, considering the tandem regulation of proliferation and migration, cell-intrinsic and extrinsic control, the potential for tumors derived from failed PGC migrants, the potential mechanisms by which migratory PGCs vary in their cellular behaviors, and corresponding effects on development. We discuss the implications of a selective role of PGC migration for in vitro gametogenesis.

PRDM1/BLIMP1 is widely distributed to the nascent fetal-placental interface in the mouse gastrula

BACKGROUND

PRDM1 is a transcriptional repressor that contributes to primordial germ cell (PGC) development. During early gastrulation, epiblast-derived PRDM1 is thought to be restricted to a lineage-segregated germ line in the allantois. However, given recent findings that PGCs overlap an allantoic progenitor pool that contributes widely to the fetal-umbilical interface, posterior PRDM1 may also contribute to soma.

RESULTS

Within the posterior mouse gastrula (early streak, 12-s stages, embryonic days ∼6.75-9.0), PRDM1 localized to all tissues containing putative PGCs; however, PRDM1 was also found in all three primary germ layers, their derivatives, and two presumptive growth centers, the allantoic core domain and ventral ectodermal ridge. While PRDM1 and STELLA colocalized predominantly within the hindgut, where putative PGCs reside, other colocalizing cells were found in non-PGC sites. Additional PRDM1 and STELLA cells were found independent of each other throughout the posterior region, including the hindgut. The Prdm1-Cre-driven reporter supported PRDM1 localization in the majority of sites; however, some Prdm1 descendants were found in sites independent of PRDM1 protein, including allantoic mesothelium and hindgut endoderm.

CONCLUSIONS

Posterior PRDM1 contributes more broadly to the developing fetal-maternal connection than previously recognized, and PRDM1 and STELLA, while overlapping in putative PGCs, also co-localize in several other tissues. Developmental Dynamics 246:50-71, 2017. © 2016 Wiley Periodicals, Inc.

Mouse primordial germ cells: a reappraisal

Current dogma is that mouse primordial germ cells (PGCs) segregate within the allantois, or source of the umbilical cord, and translocate to the gonads, differentiating there into sperm and eggs. In light of emerging data on the posterior embryonic-extraembryonic interface, and the poorly studied but vital fetal-umbilical connection, we have reviewed the past century of experiments on mammalian PGCs and their relation to the allantois. We demonstrate that, despite best efforts and valuable data on the pluripotent state, what is and is not a PGC in vivo is obscure. Furthermore, sufficient experimental evidence has yet to be provided either for an extragonadal origin of mammalian PGCs or for their segregation within the posterior region. Rather, most evidence points to an alternative hypothesis that PGCs in the mouse allantois are part of a stem/progenitor cell pool that exhibits all known PGC "markers" and that builds/reinforces the fetal-umbilical interface, common to amniotes. We conclude by suggesting experiments to distinguish the mammalian germ line from the soma.

Widespread but tissue-specific patterns of interferon-induced transmembrane protein 3 (IFITM3, FRAGILIS, MIL-1) in the mouse gastrula

Interferon-induced transmembrane protein 3 (IFITM3; FRAGILIS; MIL-1) is part of a larger family of important small interferon-induced transmembrane genes and proteins involved in early development, cell adhesion, and cell proliferation, and which also play a major role in response to bacterial and viral infections and, more recently, in pronounced malignancies. IFITM3, together with tissue-nonspecific alkaline phosphatase (TNAP), PRDM1, and STELLA, has been claimed to be a hallmark of segregated primordial germ cells (PGCs) (Saitou et al., 2002). However, whether IFITM3, like STELLA, is part of a broader stem/progenitor pool that builds the posterior region of the mouse conceptus (Mikedis and Downs, 2012) is obscure. To discover the whereabouts of IFITM3 during mouse gastrulation (~E6.5-9.0), systematic immunohistochemical analysis was carried out at closely spaced 2-4-h intervals. Results revealed diverse, yet consistent, profiles of IFITM3 localization throughout the gastrula. Within the putative PGC trajectory and surrounding posterior tissues, IFITM3 localized as a large cytoplasmic spot with or without staining in the plasma membrane. IFITM3, like STELLA, was also found in the ventral ectodermal ridge (VER), a posterior progenitor pool that builds the tailbud. The large cytoplasmic spot with plasma membrane staining was exclusive to the posterior region; the visceral yolk sac, non-posterior tissues, and epithelial tissues exhibited spots of IFITM3 without cell surface staining. Colocalization of the intracellular IFITM3 spot with the endoplasmic reticulum, Golgi apparatus, or endolysosomes was not observed. That relatively high levels of IFITM3 were found throughout the posterior primitive streak and its derivatives is consistent with evidence that IFITM3, like STELLA, is part of a larger stem/progenitor cell pool at the posterior end of the primitive streak that forms the base of the allantois and builds the fetal-umbilical connection, thus further obfuscating practical phenotypic distinctions between so-called PGCs and surrounding soma.

RNA granules in germ cells

"Germ granules" are cytoplasmic, nonmembrane-bound organelles unique to germline. Germ granules share components with the P bodies and stress granules of somatic cells, but also contain proteins and RNAs uniquely required for germ cell development. In this review, we focus on recent advances in our understanding of germ granule assembly, dynamics, and function. One hypothesis is that germ granules operate as hubs for the posttranscriptional control of gene expression, a function at the core of the germ cell differentiation program.

The many faces of RNAi

Small non-coding RNAs, through association with Argonaute protein family members, have a variety of functions during the development of an organism. Although there is increased mechanistic understanding of the RNA interference (RNAi) pathways surrounding these small RNAs, how their effects are modulated by subcellular compartmentalization and cross-pathway functional interactions is only beginning to be explored. This review examines the current understanding of these aspects of RNAi pathways and the biological functions of these pathways.

Cell cycle control of germ cell differentiation

The germ cell lineage is our lifelong reservoir of reproductive stem cells and our mechanism for transmitting genes to future generations. These highly specialised cells are specified early during development and then migrate to the embryonic gonads where sex differentiation occurs. Germ cell sex differentiation is directed by the somatic gonadal environment and is characterised by two distinct cell cycle states that are maintained until after birth. In the mouse, XY germ cells in a testis cease mitotic proliferation and enter G(1)/G(0) arrest from 12.5 dpc, while XX germ cells in an ovary enter prophase I of meiosis from 13.5 dpc. This chapter discusses the factors known to control proliferation and survival of germ cells during their journey of specification to sex differentiation during development.

Mouse germ cell development: from specification to sex determination

Primordial germ cells (PGCs) are embryonic progenitors for the gametes. In the gastrulating mouse embryo, a small group of cells begin expressing a unique set of genes and so commit to the germline. Over the next 3-5 days, these PGCs migrate anteriorly and increase rapidly in number via mitotic division before colonizing the newly formed gonads. PGCs then express a different set of unique genes, their inherited epigenetic imprint is erased and an individual methylation imprint is established, and for female PGCs, the silent X chromosome is reactivated. At this point, germ cells (GCs) commit to either a female or male sexual lineage, denoted by meiosis entry and mitotic arrest, respectively. This developmental program is determined by cues emanating from the somatic environment.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.

Expression and localization of the novel and highly conserved gametocyte-specific factor 1 during oogenesis and spermatogenesis

OBJECTIVE

To determine the onset of gametocyte-specific factor 1 (Gtsf1) expression in embryogenesis and its relation to Nobox; and to determine its localization during gonadal development and gametocyte maturation.

DESIGN

Developmental animal study.

SETTING

University reproductive biology laboratory.

ANIMAL(S)

Mice ranging in age from embryonic day 12.5 to 8 weeks.

INTERVENTION(S)

Polymerase chain reaction and quantitative polymerase chain reaction were performed to determine the onset of and relative messenger RNA expression. Western blot was performed to confirm protein expression and antibody specificity. In situ hybridization and immunohistochemistry were used determine localization of expression.

MAIN OUTCOME MEASURE(S)

Gtsf1 messenger RNA expression levels during embryogenesis through adulthood in wild-type mice and in newborn Nobox knockout mice; GTSF1 expression and localization in postnatal mice.

RESULT(S)

Gtsf1 functions downstream of Nobox and is highly expressed in embryonic male and female gonads, localizing to germ cells throughout development. GTSF1 expression is confined to the cytoplasm in all stages of postnatal oocyte maturation and to prespermatogonia during early postnatal testicular development.

CONCLUSION(S)

The expression pattern of Gtsf1 and its high conservation suggests that it may play an important role in germ cell development. Further characterization of Gtsf1 may elucidate mechanisms involved in premature ovarian failure.

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.

Normal germ line establishment in mice carrying a deletion of the Ifitm/Fragilis gene family cluster

The family of interferon-inducible transmembrane proteins (Ifitm) consists of five highly sequence-related cell surface proteins, which are implicated in diverse cellular processes. Ifitm genes are conserved, widely expressed, and characteristically found in genomic clusters, such as the 67-kb Ifitm family locus on mouse chromosome 7. Recently, Ifitm1 and Ifitm3 have been suggested to mediate migration of early primordial germ cells (PGCs), a process that is little understood. To investigate Ifitm function during germ cell development, we used targeted chromosome engineering to generate mutants which either lack the entire Ifitm locus or carry a disrupted Ifitm3 gene only. Here we show that the mutations have no detectable effects on development of the germ line or on the generation of live young. Hence, contrary to previous reports, Ifitm genes are not essential for PGC migration. The Ifitm family is a striking example of a conserved gene cluster which appears to be functionally redundant during development.

Characterization and distribution of hyaluronan and the proteoglycans decorin, biglycan and perlecan in the developing embryonic mouse gonad

The morphogenesis of tissues and organs requires dynamic changes in cells and in extracellular matrix components. It is known that various extracellular matrix molecules are of fundamental importance for gonad differentiation and growth. In the adult testis, the extracellular matrix represents an important component of the interstitium, participating in the transport of biologically active substances needed for the communication between different cellular components, as well as for the regulation of spermatogenesis and hormone production. The present study was designed in order to identify the proteoglycans biglycan, decorin and perlecan, as well as the glycosaminoglycan hyaluronan, during testis development in mouse embryos. Our data profile the chronology of testis differentiation, as well as the distribution of these extracellular matrix components during testis development in mice. We show that these extracellular matrix molecules are present early in the development of the gonads, suggesting that they play a role in gonad development. In addition, we found no decorin in the testicular cords. Furthermore, of the proteoglycans analysed, only biglycan was seen surrounding immature Sertoli cells and Leydig cell precursors in the testicular cords. This indicates that specific sets of extracellular matrix molecules are required in the various compartments of the developing gonad.

(Re)defining stem cells

Stem-cell nomenclature is in a muddle! So-called stem cells may be self-renewing or emergent, oligopotent (uni- and multipotent) or pluri- and totipotent, cells with perpetual embryonic features or cells that have changed irreversibly. Ambiguity probably seeped into stem cells from common usage, flukes in biology's history beginning with Weismann's divide between germ and soma and Haeckel's biogenic law and ending with contemporary issues over the therapeutic efficacy of adult versus embryonic cells. Confusion centers on tissue dynamics, whether stem cells are properly members of emerging or steady-state populations. Clarity might yet be achieved by codifying differences between cells in emergent populations, including embryonic stem and embryonic germ (ES and EG) cells in tissue culture as opposed to self-renewing (SR) cells in steady-state populations.

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.

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.

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.

Erasure of methylation imprinting of Igf2r during mouse primordial germ-cell development

During germ cell differentiation in mice, the genome undergoes specific epigenetic modifications. These include demethylation of imprinted genes and subsequent establishment of parental allele-specific methylation. The mouse Igf2r gene is an imprinted gene that shows maternal-specific expression. Maternal-specific methylation of differentially methylated region 2 (DMR2) of this gene may be necessary for its maternal-specific expression. Before the allele-specific methylation is established, DMR2 is demethylated in both male and female primordial germ cells (PGCs) by 13.5 days post coitum (dpc), indicating that the demethylation of this region occurs earlier in PGC development. The timing of the demethylation has been, however, unknown. In this study, we attempted to determine the timing of methylation erasure of Igf2r DMR2 in developing PGCs, using transgenic mice expressing green fluorescent protein specifically in the germ line. We purified migrating PGCs from the transgenic mice and examined the methylation status of DMR2. The results show that some CpG sites within DMR2 start demethylation at 9.5 dpc in some migrating PGCs, before the cells colonize genital ridges, and the progression of demethylation is rapid after colonization of the genital ridges. To examine whether the gonadal environment is involved in demethylation, we analyzed the methylation of DMR2 after culturing migrating PGCs in the absence of a gonadal environment. These culture experiments support the idea that a gonadal environment is not required for demethylation of the region in at least a fraction of PGCs.

Genetic control of early folliculogenesis in mice

Perinatally, granulosa cells encase individual oocytes within the ovary to form primordial follicles. The initial stages of folliculogenesis are independent of gonadotropins and involve cell-autonomous and non-cell-autonomous factors. Although still poorly understood at a molecular level, successful follicle formation and initiation of follicle growth must involve genetic networks both in germ and in somatic cells. Mouse models offer useful windows into these essential processes. By investigating phenotypes of mouse lines lacking specific gene products, genetic hierarchies that regulate the initial stages of folliculogenesis are being elucidated. These investigations will provide insight into the regulation of mammalian fertility.

Time-lapse analysis of living mouse germ cell migration

In mouse embryos, the primordial germ cells arise during gastrulation prior to, and distant from, the prospective gonads. Observations of PGCs in culture, and in fixed sections, have suggested, but not proved, that they migrate to the gonad by a process of active migration. The opaque nature of the early mouse embryo has precluded direct observation. Using confocal microscopy, we have filmed living PGCs expressing eGFP in tissue slices from mouse embryos at different stages of development. We find four clearly distinct phases of PGC migration. First, until E9.0-E9.5, PGCs are already highly motile, but do not leave the gut. Second, in the E9.0-E9.5 period, before the mesentery forms, PGCs very rapidly exit the gut, but do not migrate towards the genital ridges. Third, during the E10.0-E10.5 period, PGCs migrate directionally from the dorsal body wall into the genital ridges. In contrast to the prevailing model of germ cell migration, very few, if any, PGCs found in the gut mesentery at E10.5 migrate into the genital ridges. Finally, at E11.5, PGCs are slowing and the direction of movement is dependent on the sex of the embryo. This allows, for the first time, a formal description of the events of PGC migration in the mouse.

Eggs in the balance

Polycyclic aromatic hydrocarbons (PAH), found in cigarette smoke and air pollution, interact with the aryl hydrocarbon receptor (Ahr) to cause reproductive defects. Mice lacking either Ahr or the pro-apoptotic protein Bax have an increased number of primordial follicles, and these mutant oocytes are resistant to PAH toxicity. A new study shows that the Bax promoter contains two core Ahr response elements, which are required for PAH stimulation of Bax promoter activity in oocytes. Thus, the toxic effects of PAH in oocytes are mediated directly by Ahr induction of the Bax pathway.

Moving towards the next generation

In most organisms, primordial germ cells are set aside from the cells of the body early in development. To form an embryonic gonad, germ cells often have to migrate along complex routes through and along diverse tissues until they reach the somatic part of the gonad. Recent advances have been made in the genetic analysis of these early stages of germ line development. Here we review findings from Drosophila, zebrafish, and mouse; each organism provides unique insight into the mechanisms that determine germ cell fate and the cues that may guide their migration.

Effects of protease inhibitors and antioxidants on In vitro survival of porcine primordial germ cells

One of the problems associated with in vitro culture of primordial germ cells (PGCs) is the large loss of cells during the initial period of culture. This study characterized the initial loss and determined the effectiveness of two classes of apoptosis inhibitors, protease inhibitors, and antioxidants on the ability of porcine PGCs to survive in culture. Results from electron microscopic analysis and in situ DNA fragmentation assay indicated that porcine PGCs rapidly undergo apoptosis when placed in culture. Additionally, alpha(2)-macroglobulin, a protease inhibitor and cytokine carrier, and N:-acetylcysteine, an antioxidant, increased the survival of PGCs in vitro. While other protease inhibitors tested did not affect survival of PGCs, all antioxidants tested improved survival of PGCs (P: < 0.05). Further results indicated that the beneficial effect of the antioxidants was critical only during the initial period of culture. Finally, it was determined that in short-term culture, in the absence of feeder layers, antioxidants could partially replace the effect(s) of growth factors and reduce apoptosis. Collectively, these results indicate that the addition of alpha(2)-macroglobulin and antioxidants can increase the number of PGCs in vitro by suppressing apoptosis.

Gpbox (Psx2), a homeobox gene preferentially expressed in female germ cells at the onset of sexual dimorphism in mice

XX gonads differentiate into ovaries, a morphologic event evident by embryonic day 13.5 (E13.5) in mice. To identify early markers of oogenesis, sex-specific urogenital ridge cDNA libraries were constructed from E12-13 embryos. After mass excision and isolation of plasmid DNA, approximately 4800 expressed sequence tags were determined and compared to existing databases. Few cDNAs were specifically expressed in the urogenital ridge, but one, designated GPBOX, encodes a 227-amino-acid homeobox protein that is first expressed at E10.5 in the embryo as well as in the extraembryonic tissues. The Gpbox gene is single copy in the mouse genome and is located on the X chromosome in close proximity to two other homeobox genes, Pem and Psx1. Within the embryo, its expression is limited to the gonad, and transcripts are not detected in adult tissues. Although comparable levels are initially present in both sexes, GPBOX transcripts accumulate faster in female germ cells and peak at E12.5 when they are present in fivefold greater abundance than in males. The persistence of GPBOX transcripts in female germ cells until E15.5 and their virtual disappearance in males by E13.5 suggest that Gpbox may play a role in mammalian oogenesis.

Bcl-x and Bax regulate mouse primordial germ cell survival and apoptosis during embryogenesis

Restricted germ cell loss through apoptosis is initiated in the fetal gonad around embryonic day 13.5 (E13.5) as part of normal germ cell development. The mechanism of this germ cell attrition is unknown. We show that Bcl-x plays a crucial role in maintaining the survival of mouse germ cells during gonadogenesis. A bcl-x hypomorphic mouse was generated through the introduction of a neomycin (neo) gene into the promoter of the bcl-x gene by homologous recombination. Mice that contained two copies of the hypomorphic allele had severe reproductive defects attributed to compromised germ cell development. Males with two mutant alleles lacked spermatogonia and were sterile; females showed a severely reduced population of primordial and primary follicles and exhibited greatly impaired fertility. Primordial germ cells (PGCs) in bcl-x hypomorph mice migrated to the genital ridge by E12.5 but were depleted by E15.5, a time when Bcl-x and Bax were present. Two additional bcl-x transcripts were identified in fetal germ cells more than 300 bp upstream of previously reported start sites. Insertion of a neo cassette led to a down-regulation of the bcl-x gene at E12.5 in the hypomorph. Bax was detected by immunohistochemistry in germ cells from bcl-x hypomorph and control testes at E12.5 and E13.5. Bcl-x function was restored, and animals of both genders were fertile after removal of the neo selection cassette using Cre-mediated recombination. Alternatively, the loss of Bcl-x function in the hypomorph was corrected by the deletion of both copies of the bax gene, resulting in a restoration of germ cell survival. These findings demonstrate that the balance of Bcl-x and Bax control PGC survival and apoptosis.

In vitro studies on PGC or PGC-like cells in cultured yolk sac cells and embryonic stem cells of the mouse

The present study aims: 1) to determine those conditions which promote the proliferation of primordial germ cells (PGCs) of the migratory phase in the yolk sac; and 2) to examine the effects of yolk sac cells as a feeder layer under the conditions mentioned above upon the embryonic stem (ES) cells (R1) with high potential for entering the germ line in vivo in chimeras. In murine yolk sac cells obtained on Day 10.5-11.5 of pregnancy and cultured in a modified Dulbecco's modified Eagle's medium (DMEM-plus/20: the postfix represents the concentration of FBS added in percentage), many cells exhibited strong immunoreactivities to the monoclonal antibodies 4C9 and 2C9 which are known to react with PGC specifically. Both the 4C9- and the 2C9-positive cells were sensitive to the treatment with busulfan added in vitro, supporting the supposition that they were PGCs. The respective numbers of the 4C9- and the 2C9-positive cells increased approximately 4 and 12 times when they were cultured in DMEM-plus/20 fortified with SCF, LIF, bFGF and TNF-alpha (DMEM-NT/20). When the R1 cells were cultured in the yolk sac-conditioned DMEM-NT/20 medium on the laminin substratum, the entire colonies were faintly stained with 4C9 but not with 2C9. At times solitary ES cells migrated out from the colonies, and reacted strongly with 4C9. When yolk sac cells and R1 cells were cultured on the two sides of a collagen-coated membrane, the yolk sac cells being feeder cells, some R1 cell colonies were intensely stained as a whole with either the 4C9 or the 2C9 antibody, suggesting that these colonies might be composed of cells clonally derived from stem cells which either had been destined to become the germ line cells or had already acquired cellular characteristics close to PGCs. It was tentatively concluded that the R1 cell population contained, as judged from the immunoreactivities, germ-cell-like cells, and that the yolk sac cells and/or their secretory products might facilitate the proliferation of, or the conversion of R1 cells to, the germ-cell-like cells.

The onset of germ cell migration in the mouse embryo

Mouse primordial germ cells (PGCs) are specified between embryonic day 6.5 (E6.5) and E7.5, when they have been visualized as an alkaline phosphatase-positive (AP+) cell population in the developing allantois. By E8.5, they are embedded in the hind-gut epithelium. Previous experiments have suggested different sites for PGCs' origin, and it is unclear how they reach the gut epithelium. We have used transgenic mice expressing GFP under a truncated Oct4 promoter to visualize living PGCs. We find GFP+/AP+ cells in the posterior end of the primitive streak as a dispersed population of cells actively migrating into the allantois, and directly into the adjacent embryonic endoderm. Time-lapse analysis shows these cells to be actively migratory from the time they exit the primitive streak.

Characterization of zebrafish primordial germ cells: morphology and early distribution of vasa RNA

Research into germ line development is of conceptual and biotechnologic importance. In this study, we used morphology at the level of light and electron microscope to characterize the primordial germ cells (PGCs) of the zebrafish throughout embryonic and larval development. The study was complemented by the detailed analysis of mRNA expression of a putative germ line marker vasa. By morphology alone PGCs were identified at the earliest at the 5-somite stage in the peripheral endoderm in contact with the yolk syncytial layer. Subsequently, they move from lateral to medial positions into the median mesoderm and from there by means of the dorsal mesentery into the gonadal anlage at day 5 postfertilization (pf), to establish gonads with mesenchymal cells by day 9 pf. Ultrastructural analysis of the 4-day-old zebrafish larvae demonstrates the presence of the germ line-specific structures, nuage, and annulate lamellae. vasa RNA-positive cells can be followed during zebrafish embryogenesis from the 32-cell stage onward (Yoon et al., 1997). Upon completion of gastrulation, the RNA is exclusively present in the cells of the hypoblast, which as a consequence of convergence and extension movements first arrange themselves in a V-shaped string-like conformation to end up, by late somitogenesis, as a string of cells on each side of the midline. We show that the localization of maternal vasa RNA in the ovary changes from cytoplasmic, in the previtellogenic oocytes, to cortical in the vitellogenic oocytes, to concentrate at the boundary of the yolk and cytoplasm in the one cell stage zygote. These results demonstrate that the cortical vasa RNA localization precedes its cleavage furrow-associated localization in the embryos and is presumably cytoskeleton dependent. vasa RNA localization changes from asymmetric subcellular at the sphere stage, to become entirely cytoplasmic at the dome stage. These data suggest a close resemblance in modes of segregation of the germ plasma in the frog and vasa mRNA in the fish during cleavage stages. Based on the significantly larger size and the stereotype and similar position of morphologically distinct cells, presumed to be PGCs, and their vasa RNA-positive counterparts, we conclude that vasa RNA-positive cells are the PGCs and vasa RNA represents a definitive germ line marker in the fish. Dev Dyn 1999;216:153-167.

Distribution and space-time relationship of proteoglycans in the extracellular matrix of the migratory pathway of primordial germ cells in mouse embryos

In this paper we present an in situ ultrastructural cytochemical study on the distribution and spatial-temporal expression of proteoglycans (PGs) in the extracellular matrix of the migratory pathway of mouse primordial germ cells (PGCs) during the different phases of migration, by the use of the cationic dye ruthenium hexammine trichloride (RHT). Embryos of 9, 10, 11 and 12 days of development were used. The treatment with RHT revealed PGs as electron dense layers, granules, and filaments. Whereas granules prevailed in the extracellular spaces of the migratory route during the whole migratory process, the amount of filamentous structures increased during the migration phase of PGCs. At the end of the migratory process the surface of the PGCs lost its reaction by RHT. There were differences in the size of the granules of PGs at the initial migratory period (9-day-old embryos) as compared with the other days of gestation. There was a strong reaction for PGs in the extracellular spaces, expressed as a meshwork of granules interconnected by filaments, as well as reaction on the basement membranes during the peak of the PGCs migration in 10-day-old embryos. These results support the hypothesis that these molecules may have an important role in the migration of PGCs, although the precise mechanism involved in this process is not yet clear.