Time-lapse analysis of living mouse germ cell migration

Control of mammalian germ cell entry into meiosis

Germ cells are unique in undergoing meiosis to generate oocytes and sperm. In mammals, meiosis onset is before birth in females, or at puberty in males, and recent studies have uncovered several regulatory steps involved in initiating meiosis in each sex. Evidence suggests that retinoic acid (RA) induces expression of the critical pre-meiosis gene Stra8 in germ cells of the fetal ovary, pubertal testis and adult testis. In the fetal testis, CYP26B1 degrades RA, while FGF9 further antagonises RA signalling to suppress meiosis. Failsafe mechanisms involving Nanos2 may further suppress meiosis in the fetal testis. Here, we draw together the growing knowledge relating to these meiotic control mechanisms, and present evidence that they are co-ordinately regulated and that additional factors remain to be identified. Understanding this regulatory network will illuminate not only how the foundations of mammalian reproduction are laid, but also how mis-regulation of these steps can result in infertility or germline tumours.

Epigenetic reprogramming: preparing the epigenome for the next generation

Epigenetic reprogramming of germ cells involves the genome-wide erasure and subsequent re-establishment of DNA methylation, along with reprogramming of histone modification profiles and the eventual incorporation of histone variants. These linked processes appear to be key for the establishment of the correct epigenetic regulation of this cell lineage. Mouse studies indicate that DNA demethylation may be initiated at E (embryonic day) 8 with rapid and substantial erasure occurring between E11.5 and E12.5. This is accompanied by a reduction in H3K9 dimethylation and an increase in H3K27 trimethylation. DNA remethylation subsequently occurs in late gestation in male germ cells and postnatally in female germ cells. This reprogramming occurs throughout the genome, with the exception of specific sequences. The conservation of this process across species remains largely undetermined, and, with recent discoveries of new DNA modifications, there is still much to be explored.

Investigating the role of tbx4 in the female germline in mice

Normal development of germ cells is essential for fertility and mammalian reproduction. Although abnormal development of oocytes or follicles may lead to primary ovarian insufficiency (POI), a disorder that causes infertility in 1% of women less than 40 yr of age, the genes and signaling pathways activated in POI are not as yet fully elucidated. Tbx4, a member of the T-box family of transcription factors, is expressed in embryonic germ cells and postnatal oocytes at all stages of folliculogenesis. To investigate the requirement for Tbx4 in the germline, we analyzed germ cell development in the absence of Tbx4. We show that primordial germ cells (PGCs) are reduced in Tbx4 homozygous null (Tbx4(-/-)) embryos at Embryonic Day (E) 10.0. Tbx4(-/-) embryos die by E10.5; to study later time points in vitro, a tamoxifen-inducible estrogen receptor Cre recombinase was used to delete Tbx4 conditional mutant alleles. In addition, Gdf9cre and Zp3cre, two oocyte-specific Cre recombinases, were used to delete Tbx4 from postnatal primordial and primary follicles, respectively. We show that in vitro differentiation of the gonad into morphologically distinct testes and ovaries occurs normally starting at E11.5 when Tbx4 is deleted. In Gdf9cre; Tbx4(fl/-) and Zp3cre; Tbx4(fl/-) adult females, primordial, primary, secondary, and antral follicles form, ovulation occurs, corpus luteum formation is normal, and the mice are fertile without any evidence of diminished ovarian reserve. Although postnatal deletion of Tbx4 in oocytes does not obviously impair fertility, it is possible that the reduction in PGCs observed in Tbx4 homozygous null mutant embryos could affect long-term fertility in adults.

Defining the neighborhoods that escort the oocyte through its early life events and into a functional follicle

The ovary functions to chaperone the most precious cargo for female individuals, the oocyte, thereby allowing the passage of genetic material to subsequent generations. Within the ovary, single oocytes are surrounded by a legion of granulosa cells inside each follicle. These two cell types depend upon one another to support follicle formation and oocyte survival. The infrastructure and events that work together to ultimately form these functional follicles within the ovary are unprecedented, given that the oocyte originates as a cell like all other neighboring cells within the embryo prior to gastrulation. This review discusses the journey of the germ cell in the context of the developing female mouse embryo, with a focus on specific signaling events and cell-cell interactions that escort the primordial germ cell as it is specified into the germ cell fate, migrates through the hindgut into the gonad, differentiates into an oocyte, and culminates upon formation of the primordial and then primary follicle.

Building the mammalian testis: origins, differentiation, and assembly of the component cell populations

Development of testes in the mammalian embryo requires the formation and assembly of several cell types that allow these organs to achieve their roles in male reproduction and endocrine regulation. Testis development is unusual in that several cell types such as Sertoli, Leydig, and spermatogonial cells arise from bipotential precursors present in the precursor tissue, the genital ridge. These cell types do not differentiate independently but depend on signals from Sertoli cells that differentiate under the influence of transcription factors SRY and SOX9. While these steps are becoming better understood, the origins and roles of many testicular cell types and structures-including peritubular myoid cells, the tunica albuginea, the arterial and venous blood vasculature, lymphatic vessels, macrophages, and nerve cells-have remained unclear. This review synthesizes current knowledge of how the architecture of the testis unfolds and highlights the questions that remain to be explored, thus providing a roadmap for future studies that may help illuminate the causes of XY disorders of sex development, infertility, and testicular cancers.

Differentiation of newborn mouse skin derived stem cells into germ-like cells in vitro

Studying germ cell formation and differentiation has traditionally been very difficult due to low cell numbers and their location deep within developing embryos. The availability of a "closed" in vitro based system could prove invaluable for our understanding of gametogenesis. The formation of oocyte-like cells (OLCs) from somatic stem cells, isolated from newborn mouse skin, has been demonstrated and can be visualized in this video protocol. The resulting OLCs express various markers consistent with oocytes such as Oct4 , Vasa , Bmp15, and Scp3. However, they remain unable to undergo maturation or fertilization due to a failure to complete meiosis. This protocol will provide a system that is useful for studying the early stage formation and differentiation of germ cells into more mature gametes. During early differentiation the number of cells expressing Oct4 (potential germ-like cells) reaches ~5%, however currently the formation of OLCs remains relatively inefficient. The protocol is relatively straight forward though special care should be taken to ensure the starting cell population is healthy and at an early passage.

Gata4 is required for formation of the genital ridge in mice

In mammals, both testis and ovary arise from a sexually undifferentiated precursor, the genital ridge, which first appears during mid-gestation as a thickening of the coelomic epithelium on the ventromedial surface of the mesonephros. At least four genes (Lhx9, Sf1, Wt1, and Emx2) have been demonstrated to be required for subsequent growth and maintenance of the genital ridge. However, no gene has been shown to be required for the initial thickening of the coelomic epithelium during genital ridge formation. We report that the transcription factor GATA4 is expressed in the coelomic epithelium of the genital ridge, progressing in an anterior-to-posterior (A-P) direction, immediately preceding an A-P wave of epithelial thickening. Mouse embryos conditionally deficient in Gata4 show no signs of gonadal initiation, as their coelomic epithelium remains a morphologically undifferentiated monolayer. The failure of genital ridge formation in Gata4-deficient embryos is corroborated by the absence of the early gonadal markers LHX9 and SF1. Our data indicate that GATA4 is required to initiate formation of the genital ridge in both XX and XY fetuses, prior to its previously reported role in testicular differentiation of the XY gonad.

During mammalian fetal development, the precursor of the testis or ovary first appears as a simple thickening, in a specific region, of the epithelial cell layer that lines the body cavity. The resulting structure is called the genital ridge, which then differentiates into either testis or ovary, depending on whether the sex chromosome constitution is XY or XX. A handful of genes, including Lhx9, Sf1, Wt1, and Emx2, are required to sustain the growth of the genital ridge. However, mice with mutations in any of these genes still undergo the initial step of epithelial thickening, suggesting that an additional step (or factor) is required to initiate genital ridge formation. We found that the evolutionarily conserved transcription factor GATA4 is expressed in the epithelium of the genital ridge before initial thickening. We produced a mouse with a Gata4 mutation in this tissue and demonstrated that the initial thickening does not take place; the mutant embryos fail to initiate gonad development. In support of this observation, the Gata4 mutant does not express the early gonadal markers LHX9 and SF1. These findings indicate that a genetically discrete, Gata4-dependent initiation step precedes the previously known processes that result in formation of testes and ovaries.

A role for hedgehog signaling in the differentiation of the insertion site of the patellar tendon in the mouse

Tendons are typically composed of two histologically different regions: the midsubstance and insertion site. We previously showed that Gli1, a downstream effector of the hedgehog (Hh) signaling pathway, is expressed only in the insertion site of the mouse patellar tendon during its differentiation. To test for a functional role of Hh signaling, we targeted the Smoothened (Smo) gene in vivo using a Cre/Lox system. Constitutive activation of the Hh pathway in the mid-substance caused molecular markers of the insertion site, e.g. type II collagen, to be ectopically expressed or up-regulated in the midsubstance. This was confirmed using a novel organ culture method in vitro. Conversely, when Smo was excised in the scleraxis-positive cell population, the development of the fibrocartilaginous insertion site was affected. Whole transcriptome analysis revealed that the expression of genes involved in chondrogenesis and mineralization was down-regulated in the insertion site, and expression of insertion site markers was decreased. Biomechanical testing of murine adult patellar tendon, which developed in the absence of Hh signaling, showed impairment of tendon structural properties (lower linear stiffness and greater displacement) and material properties (greater strain), although the linear modulus of the mutant group was not significantly lower than controls. These studies provide new insights into the role of Hh signaling during tendon development.

Tet family of 5-methylcytosine dioxygenases in mammalian development

Methylation of cytosines is a major epigenetic modification in mammalian genomes. The levels and patterns of DNA methylation are the results of the opposing actions of methylating and demethylating machineries. Over the past two decades, great progress has been made in elucidating the methylating machinery including the identification and functional characterization of the DNA methyltransferases (Dnmts). However, the mechanisms of demethylation and the major players involved had been elusive. A major breakthrough came in 2009, when the ten-eleven translocation (Tet) family of proteins was discovered as 5-methylcytosine (5mC) dioxygenases that convert 5mC to 5-hydroxymethylcytosine (5hmC). Studies in the past several years have established that 5hmC serves as an intermediate in DNA demethylation and that Tet proteins have important roles in epigenetic reprogramming in early embryos and primordial germ cells. In this review, we discuss recent advances in this exciting field, focusing on the role of Tet proteins in mammalian development.

Embryonic stem cell-derived granulosa cells participate in ovarian follicle formation in vitro and in vivo

Differentiating embryonic stem cells (ESCs) can form ovarian follicle-like structures in vitro, consisting of an oocyte-like cell surrounded by somatic cells capable of steroidogenesis. Using a dual-fluorescence reporter system in which mouse ESCs express green fluorescent protein (GFP) under the control of a germ cell-specific Pou5f1 gene promoter and red fluorescent protein (Discosoma sp red [DsRed]) driven by the granulosa cell-specific Forkhead box L2 (Foxl2) gene promoter, we first confirmed in vitro formation of follicle-like structures containing GFP-positive cells surrounded by DsRed-positive cells. Isolated DsRed-positive cells specified from ECSs exhibited a gene expression profile consistent with granulosa cells, as revealed by the detection of messenger RNAs (mRNAs) for Foxl2, follistatin (Fst), anti-Müllerian hormone (Amh), and follicle-stimulating hormone receptor (Fshr) as well as by production of both progesterone and estradiol. In addition, treatment of isolated DsRed-expressing cells with follicle-stimulating hormone (FSH) significantly increased estradiol production over basal levels, confirming the presence of functional FSH receptors in these cells. Last, ESC-derived DsRed-positive cells injected into neonatal mouse ovaries became incorporated within the granulosa cell layer of immature follicles. These studies demonstrate that Foxl2-expressing ovarian somatic cells derived in vitro from differentiating ESCs express granulosa cell markers, actively associate with germ cells in vitro, synthesize steroids, respond to FSH, and participate in folliculogenesis in vivo.

Embryonic stem cell-derived granulosa cells participate in ovarian follicle formation in vitro and in vivo

Differentiating embryonic stem cells (ESCs) can form ovarian follicle-like structures in vitro, consisting of an oocyte-like cell surrounded by somatic cells capable of steroidogenesis. Using a dual-fluorescence reporter system in which mouse ESCs express green fluorescent protein (GFP) under the control of a germ cell-specific Pou5f1 gene promoter and red fluorescent protein (Discosoma sp red [DsRed]) driven by the granulosa cell-specific Forkhead box L2 (Foxl2) gene promoter, we first confirmed in vitro formation of follicle-like structures containing GFP-positive cells surrounded by DsRed-positive cells. Isolated DsRed-positive cells specified from ECSs exhibited a gene expression profile consistent with granulosa cells, as revealed by the detection of messenger RNAs (mRNAs) for Foxl2, follistatin (Fst), anti-Müllerian hormone (Amh), and follicle-stimulating hormone receptor (Fshr) as well as by production of both progesterone and estradiol. In addition, treatment of isolated DsRed-expressing cells with follicle-stimulating hormone (FSH) significantly increased estradiol production over basal levels, confirming the presence of functional FSH receptors in these cells. Last, ESC-derived DsRed-positive cells injected into neonatal mouse ovaries became incorporated within the granulosa cell layer of immature follicles. These studies demonstrate that Foxl2-expressing ovarian somatic cells derived in vitro from differentiating ESCs express granulosa cell markers, actively associate with germ cells in vitro, synthesize steroids, respond to FSH, and participate in folliculogenesis in vivo.

Disruption of genital ridge development causes aberrant primordial germ cell proliferation but does not affect their directional migration

Background

The directional migration and the following development of primordial germ cells (PGCs) during gonad formation are key steps for germline development. It has been proposed that the interaction between germ cells and genital ridge (GR) somatic cells plays essential roles in this process. However, the in vivo functional requirements of GR somatic cells in germ cell development are largely unknown.

Results

Wt1 mutation (Wt1^R394W/R394W) results in GR agenesis through mitotic arrest of coelomic epitheliums. In this study, we employed the GR-deficient mouse model, Wt1^R394W/R394W, to investigate the roles of GR somatic cells in PGC migration and proliferation. We found that the number of PGCs was dramatically reduced in GR-deficient embryos at embryonic day (E) 11.5 and E12.5 due to decreased proliferation of PGCs, involving low levels of BMP signaling. In contrast, the germ cells in Wt1^R394W/R394W embryos were still mitotically active at E13.5, while all the germ cells in control embryos underwent mitotic arrest at this stage. Strikingly, the directional migration of PGCs was not affected by the absence of GR somatic cells. Most of the PGCs reached the mesenchyme under the coelomic epithelium at E10.5 and no ectopic PGCs were noted in GR-deficient embryos. However, the precise positioning of PGCs was disrupted.

Conclusions

Our work provides in vivo evidence that the proliferation of germ cells is precisely regulated by GR somatic cells during different stages of gonad development. GR somatic cells are probably dispensable for the directional migration of PGCs, but they are required for precise positioning of PGCs at the final step of migration.

Generation of a germ cell-specific mouse transgenic CHERRY reporter, Sohlh1-mCherryFlag

Visualization of differentiating germ cells is critical to understanding the formation of primordial follicles in the ovary, and the commitment of spermatogonial stem cells to differentiation. We engineered and generated a BAC transgenic mouse line, Sohlh1-mCherryFlag (S1CF), under the direction of the native Sohlh1 promoter. Sohlh1 is a germ cell-specific gene that encodes the basic helix-loop-helix (bHLH) transcriptional regulator that is essential in oogenesis and spermatogenesis. Sohlh1 expression is unique, and is limited to perinatal and early follicle oocytes and differentiating spermatogonia. The Sohlh1-mCherryFlag transgene was engineered to fuse SOHLH1 to the red fluorescent protein CHERRY with 3-tandem-FLAG tags. S1CF animals fluoresce specifically in the oocytes of perinatal ovaries and small follicles in adult ovaries, as well as in spermatogonia, a pattern that is similar to endogenous SOHLH1. Moreover, S1CF rescued germ cell loss and infertility in both male and female Sohlh1(-/-) animals. The FLAG-tag on S1CF was effective for immunostaining and immunoprecipitation. The Sohlh1-mCherryFlag transgenic mouse provides a unique model to study early germ cell differentiation, as well as in vivo imaging and purification of differentiating germ cells.

Deleted in azoospermia-like enhances in vitro derived porcine germ cell formation and meiosis

Evidence supporting that deleted in azoospermia-like (DAZL) plays a key role during gametogenesis and meiosis continues to emerge. Our study aimed to determine whether overexpression of DAZL using a lentiviral approach in a somatic stem cell to germ cell in vitro differentiation culture could enhance the formation of primordial germ cell-like cells (PLCs) and oocyte-like cells (OLCs). Introduction of DAZL at the beginning of induced differentiation significantly increased the formation of Fragilis-positive PLCs, which was independent of mitotic proliferation. In addition, mRNA levels of the germ cell markers Oct4, Stella, and Vasa were also higher in the DAZL-transduced group and suppressed when DAZL was knocked down using small interference RNA. At later stages of differentiation, the expression of several genes associated with meiosis, including Scp3, Dmc1, Rec8, and Stra8, was determined to be significantly higher when DAZL was overexpressed, which was abrogated by its knockdown. Exogenous introduction of DAZL also increased the protein levels of SCP3 and VASA, which again was reversed by its knockdown. Although not a common phenomenon in the in vitro differentiation system, the percentage of SCP3-positive cells displaying meiotic chromosome patterns in the DAZL-transduced group was higher than in the control, as was the overall percentage of OLCs that were generated. The introduction of factors such as DAZL into a stem cell-to-germ cell differentiation culture may provide an opportunity to better understand the key genes and their interactions during gametogenesis, also providing a means to enhance the generation of germ cells in vitro.

Primordial germ cells in mice

Germ cell development creates totipotency through genetic as well as epigenetic regulation of the genome function. Primordial germ cells (PGCs) are the first germ cell population established during development and are immediate precursors for both the oocytes and spermatogonia. We here summarize recent findings regarding the mechanism of PGC development in mice. We focus on the transcriptional and signaling mechanism for PGC specification, potential pluripotency, and epigenetic reprogramming in PGCs and strategies for the reconstitution of germ cell development using pluripotent stem cells in culture. Continued studies on germ cell development may lead to the generation of totipotency in vitro, which should have a profound influence on biological science as well as on medicine.

Early gonadogenesis in mammals: significance of long and narrow gonadal structure

In mammalian embryogenesis, the gonadal primordium arises from the thickening of the coelomic epithelium, which results in a pair of extremely long and narrow gonadal structures along the anteroposterior axis. These gonadal structures are conserved in various mammalian species, suggesting a great advantage in properly receiving migrating primordial germ cells (PGCs) that are widely scattered throughout the hindgut tube. Soon after the PGCs settle, the bipotential gonads undergo sex determination into testes or ovaries by the sex-determining gene, Sry, which is expressed in supporting cell precursors in a center-to-pole manner. Such a long, narrow gonadal structure bestows a considerable time lag on Sry expression between the center and pole regions, but testiculogenesis with cord formation and Leydig cell differentiation occurs synchronously throughout the whole organ. This synchronous testiculogenesis could be explained by a positive-feedback mechanism between SOX9 (another SRY-related transcription factor) and FGF9 downstream of Sry. FGF signals are likely secreted from the center region, rapidly diffuse into the poles, and then induce the establishment of SOX9 expression in Sertoli cells in the pole domains. This work focuses on recent knowledge of the molecular and cellular events of PGC migration, gonadogenesis, and testiculogenesis, and their biological significance in mammalian embryogenesis.

Genetics of germ cell development

The germ line represents a continuous cellular link between generations and between species, but the germ cells themselves develop in a specialized, organism-specific context. The model organisms Caenorhabditis elegans, Drosophila melanogaster and the mouse display striking similarities, as well as major differences, in the means by which they control germ cell development. Recent developments in genetic technologies allow a more detailed comparison of the germ cells of these three organisms than has previously been possible, shedding light not only on universal aspects of germline regulation, but also on the control of the pluripotent state in vivo and on the earliest steps of embryogenesis. Here, we highlight themes from the comparison of these three alternative strategies for navigating the fundamental cycle of sexual reproduction.

Reprogramming and the mammalian germline: the Weismann barrier revisited

The germline represents a unique cell type that can transmit genetic material to the next generation. During early embryonic development, somatic cells give rise to a small population of cells known as germ cells, which eventually differentiate into mature gametes. Germ cells undergo a process of removing and resetting relevant epigenetic information, mainly by DNA demethylation. This extensive epigenetic reprogramming leads to the conversion of germ cells into immortal cells that can pass on the genome to the next generation. In the absence of germline-specific reprogramming, germ cells would preserve the old, parental epigenetic memory, which would prevent the transfer of heritable information to the offspring. On the contrary, somatic cells cannot reset epigenetic information by preserving the full methylation pattern on imprinting genes. In this review, we focus on the capacity of germ cells and somatic cells (soma) to transfer genetic information to the next generation, and thus revisit the Weismann theory of heredity.

Growth of mouse oocytes to maturity from premeiotic germ cells in vitro

In the present study, we established an in vitro culture system suitable for generating fertilizable oocytes from premeiotic mouse female germ cells. These results were achieved after first establishing an in vitro culture system allowing immature oocytes from 12–14 day- old mice to reach meiotic maturation through culture onto preantral granulosa cell (PAGC) monolayers in the presence of Activin A (ActA). To generate mature oocytes from premeiotic germ cells, pieces of ovaries from 12.5 days post coitum (dpc) embryos were cultured in medium supplemented with ActA for 28 days and the oocytes formed within the explants were isolated and cocultured onto PAGC monolayers in the presence of ActA for 6–7 days. The oocytes were then subjected to a final meiotic maturation assay to evaluate their capability to undergo germinal vesicle break down (GVBD) and reach the metaphase II (MII) stage. We found that during the first 28 days of culture, a significant number of oocytes within the ovarian explants reached nearly full growth and formed preantral follicle-like structures with the surrounding somatic cells. GSH level and Cx37 expression in the oocytes within the explants were indicative of proper developmental conditions. Moreover, the imprinting of Igf2r and Peg3 genes in these oocytes was correctly established. Further culture onto PAGCs in the presence of ActA allowed about 16% of the oocytes to undergo GVBD, among which 17% reached the MII stage during the final 16–18 hr maturation culture. These MII oocytes showed normal spindle and chromosome assembly and a correct ERK1/2 activity. About 35% of the in vitro matured oocytes were fertilized and 53.44% of them were able to reach the 2-cell stage. Finally, around 7% of the 2-cell embryos developed to the morula/blastocyst stage.

Molecular control of rodent spermatogenesis

Spermatogenesis is a complex developmental process that ultimately generates mature spermatozoa. This process involves a phase of proliferative expansion, meiosis, and cytodifferentiation. Mouse models have been widely used to study spermatogenesis and have revealed many genes and molecular mechanisms that are crucial in this process. Although meiosis is generally considered as the most crucial phase of spermatogenesis, mouse models have shown that pre-meiotic and post-meiotic phases are equally important. Using knowledge generated from mouse models and in vitro studies, the current review provides an overview of the molecular control of rodent spermatogenesis. Finally, we briefly relate this knowledge to fertility problems in humans and discuss implications for future research. This article is part of a Special Issue entitled: Molecular Genetics of Human Reproductive Failure.

Membrane-bound steel factor maintains a high local concentration for mouse primordial germ cell motility, and defines the region of their migration

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

Mir-290-295 deficiency in mice results in partially penetrant embryonic lethality and germ cell defects

Mir-290 through mir-295 (mir-290-295) is a mammalian-specific microRNA (miRNA) cluster that, in mice, is expressed specifically in early embryos and embryonic germ cells. Here, we show that mir-290-295 plays important roles in embryonic development as indicated by the partially penetrant lethality of mutant embryos. In addition, we show that in surviving mir-290-295-deficient embryos, female but not male fertility is compromised. This impairment in fertility arises from a defect in migrating primordial germ cells and occurs equally in male and female mutant animals. Male mir-290-295(-/-) mice, due to the extended proliferative lifespan of their germ cells, are able to recover from this initial germ cell loss and are fertile. Female mir-290-295(-/-) mice are unable to recover and are sterile, due to premature ovarian failure.

Spontaneous metastasis in mouse models of testicular germ-cell tumours

Testicular germ-cell tumours (TGCTs) are the most common cancer in young men; the incidence is increasing worldwide and they have an unusually high rate of metastasis. Despite significant work on TGCTs and their metastases in humans, absence of a mouse model of spontaneous metastasis has greatly limited our understanding of the mechanisms by which metastatic potential is acquired and on their modes of dissemination. We report a new model of spontaneous TGCT metastasis in the 129 family of mice and provide evidence that these are true metastases derived directly from primary testicular cancers rather than independently from ectopic stem cells. These putative metastases (pMETs) occur at similar frequencies among TGCT-affected males in six genetically distinct TGCT-susceptible strains and were largely found in anatomical sites that are consistent with patterns of TGCT metastasis in humans. Various lines of evidence support their pluripotency and germ-cell origin, including presence of multiple endodermal, mesodermal and ectodermal derivatives as well as cells showing OCT4 and SSEA-1 pluripotency markers. In addition, pMETs were never found in males that did not have a TGCT, suggesting that metastases are derived from primary tumours. Finally, pMETS and primary TGCTs shared several DNA copy number variants suggesting a common cellular and developmental origin. Together, these results provide the first evidence for spontaneous TGCT metastasis in mice and show that these metastases originate from primary TGCTs rather than independently from ectopic stem cells.

In vivo germ line stem cell migration: a mouse model

A stem cell niche is a specialized tissue environment that controls the proliferation and differentiation of its resident stem cells. The functions of these structures have been well characterized in adult organisms. In particular, the bone marrow stem cell niche in mammals has been amenable to analysis because of the ability of transplanted hematopoietic cells to home and to recolonize the bone marrow of an irradiated host. Despite clues from adult models, it remains unclear how stem cells become partitioned into appropriate niches during embryonic development. To examine the earliest steps in niche formation, we created an organ culture system to observe the development of primordial germ cells (PGCs), a migratory stem cell population that will eventually give rise to the gametes. Using this assay, we can watch PGCs as they migrate to colonize the developing gonads and can introduce growth factor agonists or antagonists to test the function of proteins that regulate this process. This provides an unprecedented opportunity to identify the cellular and molecular interactions required for the formation of the germ cell niche.

Regulation of male germ cell cycle arrest and differentiation by DND1 is modulated by genetic background

Human germ cell tumors show a strong sensitivity to genetic background similar to Dnd1(Ter/Ter) mutant mice, where testicular teratomas arise only on the 129/SvJ genetic background. The introduction of the Bax mutation onto mixed background Dnd1(Ter/Ter) mutants, where teratomas do not typically develop, resulted in a high incidence of teratomas. However, when Dnd1(Ter/Ter); Bax(-/-) double mutants were backcrossed to C57BL/6J, no tumors arose. Dnd1(Ter/Ter) germ cells show a strong downregulation of male differentiation genes including Nanos2. In susceptible strains, where teratomas initiate around E15.5-E17.5, many mutant germ cells fail to enter mitotic arrest in G0 and do not downregulate the pluripotency markers NANOG, SOX2 and OCT4. We show that DND1 directly binds a group of transcripts that encode negative regulators of the cell cycle, including p27(Kip1) and p21(Cip)(1). P27(Kip1) and P21(Cip1) protein are both significantly decreased in Dnd1(Ter/Ter) germ cells on all strain backgrounds tested, strongly suggesting that DND1 regulates mitotic arrest in male germ cells through translational regulation of cell cycle genes. Nonetheless, in C57BL/6J mutants, germ cells arrest prior to M-phase of the cell cycle and downregulate NANOG, SOX2 and OCT4. Consistent with their ability to rescue cell cycle arrest, C57BL/6J germ cells overexpress negative regulators of the cell cycle relative to 129/SvJ. This work suggests that reprogramming of pluripotency in germ cells and prevention of tumor formation requires cell cycle arrest, and that differences in the balance of cell cycle regulators between 129/SvJ and C57BL/6 might underlie differences in tumor susceptibility.

Zebrafish models of germ cell tumor

Germ cell tumors are neoplasms arising from pluripotent germ cells. In humans, these tumors occur in infants, children and young adults. The tumors display a wide range of histologic differentiation states which exhibit different clinical behaviors. Information about the molecular basis of germ cell tumors, and representative animal models of these neoplasms, are lacking. Germline development in zebrafish and humans is broadly conserved, making the fish a useful model to probe the connections between germ cell development and tumorigenesis. Here, we provide an overview of germline development and a brief review of germ cell tumor biology in humans and zebrafish. We also outline some methods for studying the zebrafish germline.

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.

Epigenetic profiles in primordial germ cells: global modulation and fine tuning of the epigenome for acquisition of totipotency

Germ cells, after fate determination as primordial germ cells (PGCs) in early embryos, undergo various unique changes in epigenetic status during their development, and these changes differ from the epigenetic changes occurring in any other somatic cells. For example, PGCs undergo demethylation of DNA and change histone modification states on a genome-wide scale. Although the full physiological significance of these epigenetic alterations is still unclear, we can now discuss some of their mechanisms due to recent experimental evidence demonstrating the expression of candidate molecules involved in the processes of epigenetic change. On the other hand, DNA demethylation associated with PGC-specific gene expression, reprogramming of imprinted genes and regulation of retrotransposons in PGCs differentially occur from the genome-wide DNA demethylation. The tendency of epigenetic changes to appear on the whole genome, as well as more precise changes in the epigenetic status of particular parts of the genome, may play important roles in establishing the properties of PGCs required for acquiring totipotency.

Primordial germ cell proliferation is impaired in Fused Toes mutant embryos

Over the first 4 days of their life, primordial germ cells invade the endoderm, migrate into and through the developing hindgut, and traverse to the genital ridge where they cluster and ultimately inhabit the nascent gonad. Specific signal-receptor combinations between primordial germ cells and their immediate environment establish successful migration and colonization. Here we demonstrate that disruption of a cluster of six genes on murine chromosome 8, as exemplified by the Fused Toes (Ft) mutant mouse model, results in severely decreased numbers of primordial germ cells within the early gonad. Primordial germ cell migration appeared normal within Ft mutant embryos; however, germ cell counts progressively decreased during this time. Although no difference in apoptosis was detected, we report a critical decrease in primordial germ cell proliferation by E12.5. The six genes within the Ft locus include the IrxB cluster (Irx3, -5, -6), Fts, Ftm, and Fto, of which only Ftm, Fto, and Fts are expressed in primordial germ cells of the early gonad. From these studies, we have discovered that the Ft locus on mouse chromosome 8 is associated with cell cycle deficits within the primordial germ cell population that initiates just before translocation into the genital ridge.

Identification of genes specific to mouse primordial germ cells through dynamic global gene expression

Molecular mechanisms underlying the commitment of cells to the germ cell lineage during mammalian embryogenesis remain poorly understood due to the limited availability of cellular materials to conduct in vitro analyses. Although primordial germ cells (PGCs)--precursors to germ cells--have been generated from embryonic stem cells (ESCs)--pluripotent stem cells derived from the inner cell mass of the blastocyst of the early embryo in vitro-the simultaneous expression of cell surface receptors and transcription factors complicates the detection of PGCs. To date, only a few genes that mark the onset of germ cell commitment in the epiblast--the outer layer of cells of the embryo--including tissue non-specific alkaline phosphatase (TNAP), Blimp1, Stella and Fragilis--have been used with some success to detect PGC formation in in vitro model systems. Here, we identified 11 genes (three of which are novel) that are specifically expressed in male and female fetal germ cells, both in vivo and in vitro, but are not expressed in ESCs. Expression of these genes allows us to distinguish committed germ cells from undifferentiated pluripotent cell populations, a prerequisite for the successful derivation of germ cells and gametes in vitro.

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.

Sex determination in mammalian germ cells: extrinsic versus intrinsic factors

Mammalian germ cells do not determine their sexual fate based on their XX or XY chromosomal constitution. Instead, sexual fate is dependent on the gonadal environment in which they develop. In a fetal testis, germ cells commit to the spermatogenic programme of development during fetal life, although they do not enter meiosis until puberty. In a fetal ovary, germ cells commit to oogenesis by entering prophase of meiosis I. Although it was believed previously that germ cells are pre-programmed to enter meiosis unless they are actively prevented from doing so, recent results indicate that meiosis is triggered by a signaling molecule, retinoic acid (RA). Meiosis is avoided in the fetal testis because a male-specifically expressed enzyme actively degrades RA during the critical time period. Additional extrinsic factors are likely to influence sexual fate of the germ cells, and in particular, we postulate that an additional male-specific fate-determining factor or factors is involved. The full complement of intrinsic factors that underlie the competence of gonadal germ cells to respond to RA and other extrinsic factors is yet to be defined.

Effects of 6-bromoindirubin-3′-oxime on the maintenance of pluripotency of porcine embryonic germ cells in combination with stem cell factor, leukemia inhibitory factor and fibroblast growth factor

6-Bromoindirubin-3′-oxime (BIO), which is one of the glycogen synthase kinase 3 inhibitors and a key regulator of numerous signaling pathways, was reported to be capable of maintaining the pluripotency of human and mouse embryonic stem cells. Presently, it is unknown whether BIO can influence the derivation of porcine embryonic germ (EG) cells. In this study, porcine primordial germ cells (PGCs) were isolated from gonads of 24- and 28-day embryos, and were then treated with BIO either individually or in combination with other cytokines (stem cell factor (SCF), leukemia inhibitory factor (LIF), and fibroblast growth factor (FGF); abbreviated as ‘3F’), and the effects of the treatment on the proliferation ability of porcine PGCs at early stage were examined using 5-bromo-2-deoxyuridine (Brdu) immunostaining assay. After continuous culture, the effects on the efficiency of porcine undifferentiated EG cells in the third passage and differentiated EG cells from embryoid bodies were examined as well. The results obtained through the observation of the Brdu-labeled PGCs indicated that BIO in combination with 3F resulted in a significant increase in the mitosis index, and also indicated that the BIO in combination with 3F had a higher efficiency in promoting the formation of porcine EG colony derived from porcine day 24 PGCs than BIO used either individually or in combination with LIF. In addition, BIO in combination with 3F exhibited the apparent anti-differentiation activity by reversing the differentiated EG cells to the undifferentiated status. Our results demonstrate that BIO in combination with SCF, LIF, and FGF could significantly contribute to the establishment of a porcine EG cell colony and maintain the undifferentiated status.

Molecular signature of adult bone marrow-purified very small embryonic-like stem cells supports their developmental epiblast/germ line origin

We postulated that Oct4(+)SSEA-1(+)Sca-1(+)Lin(-)CD45(-) very small embryonic-like stem cells (VSELs) isolated from adult bone marrow (BM) could be a reserve population for tissue-committed stem cells. The aim of this study was to elucidate the developmental origin of these cells. We report that during embryogenesis, VSELs are enriched in embryonic day (E)12.5 murine fetal livers (FLs) and subsequently follow the developmental route of hematopoietic stem cells (H)SCs to colonize BM. Molecular analysis of purified VSELs revealed that both FL-derived VSELs and their adult BM-derived counterparts express: (i) several epiblast/primordial germ cell (PGC) markers; (ii) migrating PGC-like epigenetic reprogramming profiles of Oct4, Nanog and Stella loci; as well as (iii) a unique pattern of genomic imprinting. Thus, these data suggest that VSELs may originate from epiblast/migrating PGC-like cells and, in spite of the expression of pluripotent stem cell markers, changes in the epigenetic signature of imprinted genes keep these cells quiescent in adult tissues and prevent them from teratoma formation.

BMP signaling controls formation of a primordial germ cell niche within the early genital ridges

Stem cells are necessary to maintain tissue homeostasis and the microenvironment (a.k.a. the niche) surrounding these cells controls their ability to self-renew or differentiate. For many stem cell populations it remains unclear precisely what cells and signals comprise a niche. Here we identify a possible PGC niche in the mouse genital ridges. Conditional ablation of Bmpr1a was used to demonstrate that BMP signaling is required for PGC survival and migration as these cells colonize the genital ridges. Reduced BMP signaling within the genital ridges led to increased somatic cell death within the mesonephric mesenchyme. Loss of these supporting cells correlated with decreased levels of the mesonephric marker, Pax2, as well as a reduction in genes expressed in the coelomic epithelium including the putative PGC chemo-attractants Kitl and Sdf1a. We propose that BMP signaling promotes mesonephric cell survival within the genital ridges and that these cells support correct development of the coelomic epithelium, the target of PGC migration. Loss of BMP signaling leads to the loss of the PGC target resulting in reduced PGC numbers and disrupted PGC migration.

Mediastinal germ cell tumors with an angiosarcomatous component: a report of 12 cases

The development of an angiosarcomatous component in germ cell tumors is rare. Here we studied 12 cases of mediastinal germ cell tumors with an angiosarcomatous component. All patients were men with a mean age of 34 years (range, 24-49 years). No patient had a documented testicular germ cell tumor. The mean size of mediastinal tumors was 12.9 cm (range, 5.5-16.0 cm). Grossly, the tumors were cystic with variegated hemorrhagic, mucinous, and fleshy solid areas. Microscopically, all tumors were composed of germ cell tumor. The most common germ cell tumor component was teratoma (n = 10); and other germ cell tumor components included seminoma (n = 3), yolk sac tumor (n = 3), embryonal carcinoma (n = 2), and choriocarcinoma (n = 1). The angiosarcomatous component was present in primary mediastinal tumors (n = 6), metastasis (n = 3), or both primary mediastinal tumor and metastasis (n = 3). The angiosarcomatous component accounted for an average of 30% (range, 5%-95%) of the primary mediastinal tumor. In addition, other non-germ cell components, including rhabdomyosarcoma (n = 3), leiomyosarcoma (n = 1), and poorly differentiated carcinoma (n = 1), were also present in the tumors. Of the 10 patients with follow-up available, all patients developed metastasis (n = 8) or local recurrence (n = 2); 7 died of disease at a mean of 33 months (range, 21-75 months), and 3 patients were alive at a mean of 75 months (range, 5-120 months). Our findings suggest that the presence of an angiosarcomatous component in mediastinal germ cell tumor, even in a small amount, is associated with a poor clinical outcome.

Building pathways for ovary organogenesis in the mouse embryo

Despite its significant role in oocyte generation and hormone production in adulthood, the ovary, with regard to its formation, has received little attention compared to its male counterpart, the testis. With the exception of germ cells, which undergo a female-specific pattern of meiosis, morphological changes in the fetal ovary are subtle. Over the past 40 years, a number of hypotheses have been proposed for the organogenesis of the mammalian ovary. It was not until the turn of the millennium, thanks to the advancement of genetic and genomic approaches, that pathways for ovary organogenesis that consist of positive and negative regulators have started to emerge. Through the action of secreted factors (R-spondin1, WNT4, and follistatin) and transcription regulators (beta-catenin and FOXL2), the developmental fate of the somatic cells is directed toward ovarian, while testicular components are suppressed. In this chapter, we review the history of studying ovary organogenesis in mammals and present the most recent discoveries using the mouse as the model organism.

Mechanisms guiding primordial germ cell migration: strategies from different organisms

The regulated migration of cells is essential for development and tissue homeostasis, and aberrant cell migration can lead to an impaired immune response and the progression of cancer. Primordial germ cells (PGCs), precursors to sperm and eggs, have to migrate across the embryo to reach somatic gonadal precursors, where they carry out their function. Studies of model organisms have revealed that, despite important differences, several features of PGC migration are conserved. PGCs require an intrinsic motility programme and external guidance cues to survive and successfully migrate. Proper guidance involves both attractive and repulsive cues and is mediated by protein and lipid signalling.

Imaging mouse embryonic development

For the past three decades, methods for culturing mouse embryos ex vivo have been optimized in order to improve embryo viability and physiology throughout critical stages of embryogenesis. Combining advances made in the production of transgenic animals and in the development of different varieties of fluorescent proteins (FPs), time-lapse imaging is becoming more and more popular in the analysis of dynamic events during mouse development. Targeting FPs to specific cell types or subcellular compartments has enabled researchers to study cell proliferation, apoptosis, migration, and changes in cell morphology in living mouse embryos in real time. Here we provide a guide for time-lapse imaging of early stages of mouse embryo development.