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

Morphodynamics of the follicular-luteal complex during early ovarian development and reproductive life

Female reproductive activity depends upon cyclic morphofunctional changes of the ovarian tissue during the female's fertile period, but the primum movens of an active gonadal rearrangement can be found from early phases of embryo development. To offer a basic account of the main steps of ovarian dynamics, we review the morphofunctional behavior of the follicular-luteal complex in an integrated study using light microscopy and transmission and scanning electron microscopy as well as through the use of numerous drawings. Particular emphasis is given to some reproductive aspects including (1) germ-somatic cell relationships and onset of folliculogenesis during early gonadal development; (2) follicular development and oocyte-follicle cell associations through adult folliculogenesis, finally leading to ovulation; (3) morphodynamics of corpus luteum formation, development, and regression, and (4) degenerative processes involving germ and somatic cells. The results reported, many of which originated in our laboratory, arise from some experiments on laboratory mammals but mostly from a large selection of human specimens. The data obtained are integrated and correlated with classic reports as well as with current views. Crucial biochemical, histophysiological, and clinical aspects are also emphasized.

Peri-gastrulation development of the dasyurid marsupial Sminthopsis macroura (stripe-faced dunnart) in vitro and evidence for patterning of the epiblast prior to gastrulation

Marsupials are potentially excellent models for the study of gastrulation because of their superficial embryonic area (EA), post-gastrulation implantation and their potential to provide information about the evolution of gastrulation. Very few studies have examined this developmental period in marsupials. Using an established developmental timetable, peri-gastrula stage Sminthopsis macroura blastocysts were collected and described in detail by observations on live blastocysts and by the use of histological and immunohistochemical techniques on fixed blastocysts. Gastrulation in S. macroura shares several aspects common to that of both eutherian mammals and birds, but in terms of tissue arrangement and conceptus form, is more similar to the chick than other mammals. Two methods of culturing EA explants flat without their shell were devised. The techniques will markedly increase the number of possible experimental manipulations, which previously were limited by the presence of blastocyst investments. Exposure of fractions of explants of round, morphologically uniform pre-gastrula stage EA to growth factors or signaling molecules implicated in vertebrate gastrulation suggests that like the chick and mouse, the marsupial epiblast is patterned prior to gastrulation. Of all factors tested, basic fibroblast growth factor (bFGF) had the most prominent effect, promoting cell differentiation, and possible mesoderm formation. Data from explant culture suggests that similar to the chick and mouse, limited specification precedes the onset of gastrulation.

Knock-in of integrin beta 1D affects primary but not secondary myogenesis in mice

Integrins are extracellular matrix receptors composed of alpha and beta subunits involved in cell adhesion, migration and signal transduction. The beta1 subunit has two isoforms, beta 1A ubiquitously expressed and beta 1D restricted to striated muscle. They are not functionally equivalent. Replacement of beta 1A by beta 1D (beta 1D knock-in) in the mouse leads to midgestation lethality on a 50% Ola/50% FVB background [Baudoin, C., Goumans, M. J., Mummery, C. and Sonnenberg, A. (1998). Genes Dev. 12, 1202-1216]. We crossed the beta 1D knock-in line into a less penetrant genetic background. This led to an attenuation of the midgestation lethality and revealed a second period of lethality around birth. Midgestation death was apparently not caused by failure in cell migration, but rather by abnormal placentation. The beta 1D knock-in embryos that survived midgestation developed until birth, but exhibited severely reduced skeletal muscle mass. Quantification of myotube numbers showed that substitution of beta 1A with beta 1D impairs primary myogenesis with no direct effect on secondary myogenesis. Furthermore, long-term primary myotube survival was affected in beta 1D knock-in embryos. Finally, overexpression of beta 1D in C2C12 cells impaired myotube formation while overexpression of beta 1A primarily affected myotube maturation. Together these results demonstrate for the first time distinct roles for beta1 integrins in primary versus secondary myogenesis and that the beta 1A and beta 1D variants are not functionally equivalent in this process.

Germ cell apoptosis and its molecular trigger in mouse testes

Germ cell apoptosis is very common during various stages of mammalian testicular development. However, our understanding of the mechanisms underlying male germ cell apoptosis is still limited. This review firstly covers the general features of germ cell death in normal testes of fetal, neonatal, and adult mice from electron microscopy (EM) and terminal dUTP nick-end labeling (TUNEL) staining. The issue of whether the Fas and Fas ligand (FasL) system and/or the Bax and Bcl-2 system is involved in the induction of germ cell apoptosis in normal and damaged testes will then be addressed, including a special consideration of the ischemia-reperfusion model, the endocrine disruptor-treated model, and others. Finally, this review will propose that the process of normal spermatogenesis seems skillfull in taking advantage of apoptotic processes of germ cells and that different molecular pathways may be triggered to induce male germ cell apoptosis, depending upon the physiological and pathological states of germ cells.

Functional ablation of the mouse Ldb1 gene results in severe patterning defects during gastrulation

The LIM domain-binding protein 1 (Ldb1) is found in multi-protein complexes containing various combinations of LIM-homeodomain, LIM-only, bHLH, GATA and Otx transcription factors. These proteins exert key functions during embryogenesis. Here we show that targeted deletion of the Ldb1 gene in mice results in a pleiotropic phenotype. There is no heart anlage and head structures are truncated anterior to the hindbrain. In about 40% of the mutants, posterior axis duplication is observed. There are also severe defects in mesoderm-derived extraembryonic structures, including the allantois, blood islands of the yolk sack, primordial germ cells and the amnion. Abnormal organizer gene expression during gastrulation may account for the observed axis defects in Ldb1 mutant embryos. The expression of several Wnt inhibitors is curtailed in the mutant, suggesting that Wnt pathways may be involved in axial patterning regulated by Ldb1.

Genes associated with the development of the male germ line

The development of the mammalian germ line has been well studied, from the designation of primordial germ cells and their migration in the embryo to their progression through gametogenesis. The pattern of germ cell development, as established through classical studies, is now being overlaid with molecular, genetic and epigenetic data. Eventually, proteonomics will lead to a deeper understanding of the function of these genes. Through knowledge of germ cell gene expression patterns, it is now possible to develop transgenic molecular tools for the isolation of germ cells at different stages of development. By linking stage-specific germ cell promoter regions to the green fluorescent protein (GFP) reporter gene it is possible to tag these cells genetically for histological identification and cell sorting. Our long-term goal is to develop male germ cells as stem cells for therapeutic purposes. It is hoped that this goal will be achieved by purifying germ cells at different stages in development and gaining a deeper understanding of them by studying their gene expression patterns, potency and plasticity, both in vivo and in vitro.

Male germ line stem cells: from cell biology to cell therapy

Research using stem cells has several applications in basic biology and clinical medicine. Recent advances in the establishment of male germ line stem cells provided researchers with the ability to identify, isolate, maintain, expand and differentiate the spermatogonia, the primitive male germ cells, as cell lines under in vitro conditions. The ability to culture and manipulate stem cell lines from male germ cells has gradually facilitated research into spermatogenesis and male infertility, to an extent beyond that facilitated by the use of somatic stem cells. After the introduction of exogenous genes, the spermatogonial cells can be transplanted into the seminiferous tubules of recipients, where the transplanted cells can contribute to the offspring. The present review concentrates on the origin, life cycle and establishment of stem cell lines from male germ cells, as well as the current status of transplantation techniques and the application of spermatogonial stem cell lines.

A molecular programme for the specification of germ cell fate in mice

Germ cell fate in mice is induced in proximal epiblast cells by the extra-embryonic ectoderm, and is not acquired through the inheritance of any preformed germ plasm. To determine precisely how germ cells are specified, we performed a genetic screen between single nascent germ cells and their somatic neighbours that share common ancestry. Here we show that fragilis, an interferon-inducible transmembrane protein, marks the onset of germ cell competence, and we propose that through homotypic association, it demarcates germ cells from somatic neighbours. Using single-cell gene expression profiles, we also show that only those cells with the highest expression of fragilis subsequently express stella, a gene that we detected exclusively in lineage-restricted germ cells. The stella positive nascent germ cells exhibit repression of homeobox genes, which may explain their escape from a somatic cell fate and the retention of pluripotency.

Temporal and spatial localization patterns of Gata4 during porcine gonadogenesis

The zinc finger transcription factor Gata4, is associated with gonadal development in many species. The present study characterizes temporal and spatial localization of Gata4 throughout gonadogenesis in porcine embryos. Immunohistochemical studies illustrated that Gata4 protein is present in the coelomic epithelium prior to histological differentiation of the nascent bipotential gonad, marking the future site of both XX and XY porcine gonads. Many somatic cells of both XX and XY bipotential gonads continue to retain Gata4 immunoreactivity throughout sexual differentiation and subsequent gonadal development. Testicular cords were evident by 26 days postcoitum. Gata4 was present in Sertoli cells, identified by virtue of coexpression with Müllerian inhibiting substance and also interstitial cells including Leydig cells throughout fetal and postnatal life. Many somatic cells of the differentiating ovary including follicular cells also contained Gata4 protein throughout fetal and postnatal life. Gata4 was not present in germ cells, endothelial cells, or other undifferentiated mesenchymal cells of both XX and XY gonads. A population of Gata4-positive cells in the dorsal mesentery was continuous with the coelomic epithelium of the gonad. This localization pattern led to the hypothesis that a subpopulation of somatic cells in the dorsal mesentery moves toward the gonad. An in vitro cell migration assay demonstrated that Gata4-positive cells preferentially migrate toward explanted gonadal tissue, and morphological features of the developing gonad supported this hypothesis. This study illustrates that Gata4 is a very early marker for gonad formation, highlights species differences in temporal and spatial localization patterns, and suggests a potential role for Gata4 in the development of both XX and XY porcine gonads. Further, we suggest that mesenchymal cells of the dorsal mesentery may provide a source of somatic cells that migrate and incorporate into the gonad and contribute to various somatic cell lineages. Overall, the spatial and temporal localization patterns of Gata4 during porcine gonadogenesis implies a much earlier and wider role for Gata4 than previously reported in other species.

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.

Smad5 is required for mouse primordial germ cell development

Smad5, together with Smad1 and Smad8, have been implicated as downstream signal mediators for several bone morphogenetic proteins (BMPs). Recent studies have shown that primordial germ cells (PGCs) are absent or greatly reduced in Bmp4 or Bmp8b mutant mice. To define the role of Smad5 in PGC development, we examined PGC number in Smad5 mutant mice by Oct4 whole-mount in situ hybridization and alkaline phosphatase staining. We found ectopic PGC-like cells in the amnion of some Smad5 mutant mice, however, the total number of PGCs was greatly reduced or completely absent in Smad5 mutant embryos, similar to Bmp4 or Bmp8b mutant embryos. Therefore, Smad5 is an important factor involved in PGC generation and localization.

Early events in the mammalian germ line

Germ cells represent the genetic and cellular link between generations, as well as the transmitters of inherited diseases. Despite their central importance, not much is known about the molecular mechanisms whereby a germ cell lineage becomes set aside during development, or how the germ cells, once formed, migrate to the gonads and combine with somatic cells to make a gonad. This article provides a brief review of current knowledge on these issues, with particular focus on the mammalian germ line.

Stage-specific tissue and cell interactions play key roles in mouse germ cell specification

Primordial germ cells (PGCs) in mice have been recognized histologically as alkaline phosphatase (AP) activity-positive cells at 7.2 days post coitum (dpc) in the extra-embryonic mesoderm. However, mechanisms regulating PGC formation are unknown, and an appropriate in vitro system to study the mechanisms has not been established. Therefore, we have developed a primary culture of explanted embryos at pre- and early-streak stages, and have studied roles of cell and/or tissue interactions in PGC formation. The emergence of PGCs from 5.5 dpc epiblasts was observed only when they were co-cultured with extra-embryonic ectoderm, which may induce the conditions required for PGC formation within epiblasts. From 6.0 dpc onwards, PGCs emerged from whole epiblasts as did a fragment of proximal epiblast that corresponds to the area containing presumptive PGC precursors without neighboring extra-embryonic ectoderm and visceral endoderm. Dissociated epiblasts at these stages, however, did not give rise to PGCs, indicating that interactions among a cluster of a specific number of proximal epiblast cells is needed for PGC differentiation. In contrast, we observed that dissociated epiblast cells from a 6.5-b (6.5+15-16 hours) to 6.75 dpc embryo that had undergone gastrulation gave rise to PGCs. Our results demonstrate that stage-dependent tissue and cell interactions play key roles in PGC determination.

Autonomous transition into meiosis of mouse fetal germ cells in vitro and its inhibition by gp130-mediated signaling

Mouse primordial germ cells (PGCs) arrive at the urogenital ridge (UGR) at around 10.5 days postcoitum (dpc). They proliferate until around 13.5 dpc, then enter into meiosis in the female or become mitotically arrested in the male gonads. In this study, meiotic transition of mouse PGCs was examined in vitro. Female PGCs obtained from UGRs or genital ridges at 10.5-11.5 dpc began to express meiosis-specific genes, Scp3 and Dmc1, after dissociation and cultivation on feeder cells for several days. Meiotic transition into the leptotene stage was confirmed by the formation of axial cores. Male PGCs at 10.5-11.5 dpc and migratory PGCs obtained from mesenteries at 10.5 dpc also expressed Scp3 and formed axial cores after several days of culture, supporting the hypothesis that PGCs are capable of entering meiosis before arriving at the UGR. gp130-mediated signaling, known to promote survival/growth of PGCs and also to inhibit the differentiation of embryonic stem cells, suppressed the expression of Scp3 in PGCs and inhibited the following formation of axial cores in vitro. This novel activity of gp130-mediated signaling may provide some clues for the understanding of pluripotency of mammalian germ-line cells and/or the sex differentiation of fetal germ cells.

In vitro germ cell models for the detection of fertility impairment

Pluripotent embryonic carcinoma cells and pluripotent embryonic stem cells established from undifferentiated cells of an early mouse embryo were investigated for induction of proliferation inhibition, sister chromatid exchanges (SCE) and single-strand breaks by treatment with various germ cell mutagens. The comparison of malignant cells with nonmalignant cells showed an increased sensitivity of nonmalignant cells independent of their state of differentiation. Mitomycin C (MMC) inhibited the proliferation of nonmalignant cells at a concentration of 10(-6) M but did not affect growth of the teratocarcinoma cell line P19. There were no differences between the investigated cell lines at a lower MMC concentration. At the concentration of 10(-6) M MMC the sister chromatid exchanges of P19 were enhanced up to 41 SCE per metaphase. Testing of another germ cell mutagen, ethylnitrosourea (ENU), gave similar results: a decreasing generation time of nonmalignant cell lines after treatment with 1 mM ENU and no effect on the teratocarcinoma cells. This concentration also induced a high number of SCE. Single-strand breaks could be produced by exposure to methanmethylsulphonate (MMS). 56.3% of embryonic stem cell DNA was passing through the filter after MMS treatment. In contrast to the embryonic stem cells, only 35.6% of teratocarcinoma DNA was affected.

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.

Spermatogonial stem cell enrichment by multiparameter selection of mouse testis cells

The spermatogonial stem cell initiates and maintains spermatogenesis in the testis. To perform this role, the stem cell must self replicate as well as produce daughter cells that can expand and differentiate to form spermatozoa. Despite the central importance of the spermatogonial stem cell to male reproduction, little is known about its morphological or biochemical characteristics. This results, in part, from the fact that spermatogonial stem cells are an extremely rare cell population in the testis, and techniques for their enrichment are just beginning to be established. In this investigation, we used a multiparameter selection strategy, combining the in vivo cryptorchid testis model with in vitro fluorescence-activated cell sorting analysis. Cryptorchid testis cells were fractionated by fluorescence-activated cell sorting analysis based on light-scattering properties and expression of the cell surface molecules alpha6-integrin, alphav-integrin, and the c-kit receptor. Two important observations emerged from these analyses. First, spermatogonial stem cells from the adult cryptorchid testis express little or no c-kit. Second, the most effective enrichment strategy, in this study, selected cells with low side scatter light-scattering properties, positive staining for alpha6-integrin, and negative or low alphav-integrin expression, and resulted in a 166-fold enrichment of spermatogonial stem cells. Identification of these characteristics will allow further purification of these valuable cells and facilitate the investigation of molecular mechanisms governing spermatogonial stem cell self renewal and hierarchical differentiation.

Definitive hematopoietic stem cells first develop within the major arterial regions of the mouse embryo

The aorta-gonad-mesonephros (AGM) region is a potent hematopoietic site within the mammalian embryo body, and the first place from which hematopoietic stem cells (HSCs) emerge. Within the complex embryonic vascular, excretory and reproductive tissues of the AGM region, the precise location of HSC development is unknown. To determine where HSCs develop, we subdissected the AGM into aorta and urogenital ridge segments and transplanted the cells into irradiated adult recipients. We demonstrate that HSCs first appear in the dorsal aorta area. Furthermore, we show that vitelline and umbilical arteries contain high frequencies of HSCs coincident with HSC appearance in the AGM. While later in development and after organ explant culture we find HSCs in the urogenital ridges, our results strongly suggest that the major arteries of the embryo are the most important sites from which definitive HSCs first emerge.

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.

The role of cadherins during primordial germ cell migration and early gonad formation in the mouse

Primordial germ cells (PGCs) are the founder cells of the gametes. In mammals, PGCs migrate from the hindgut to the genital ridges, where they coalesce with each other and with somatic cells to form the primary sex cords. We show here that, in both sexes, PGCs express P- and E-cadherins during and after migration, and N-cadherin at post-migratory stages. E-Cadherin is not expressed by PGCs whilst in the hindgut, but is upregulated as they leave. Blocking antibodies against E-, but not P-cadherin cause defective PGC-PGC coalescence, and in some cases, ectopic PGCs.

Establishment of the germ cell lineage in mammals

The germ cell lineage in mice becomes lineage-restricted about 7.2 days postcoitum. Its progenitors have migrated from the proximal region of the epiblast. Cells from the distal region of the epiblast will also give rise to germ cells, if they are transplanted to the proximal region at the appropriate time. Cells in this region are subject to a predisposing signal from the adjacent extra-embryonic ectoderm. It appears that this and other signals determine the emergence of germ cells: unlike in some other organisms, this event is not predetermined.

Germline-specific expression of the Oct-4/green fluorescent protein (GFP) transgene in mice

The Pic-1, Oct-1,2, Unc-86 (POU) transcription factor Oct-4 is specifically expressed in the germ cell line, and a previous study has indicated that the expression of the lacZ gene inserted into an 18 kb genomic fragment encompassing the Oct-4 gene can come close to mimicking the endogenous embryonic expression pattern of Oct-4 in transgenic mice. In the present study transgenic mice expressing green fluorescent protein (GFP) in the germ cell line were generated using the same Oct-4 genomic fragments and the expression pattern was analyzed in detail through all stages of germ cell development. The GFP expressing primordial germ cells were first detected as early as 8.0 days post-coitum (d.p.c.; early head fold stage) at the base of the allantois in living embryos. The GFP expression was thereafter found in both male and female germ cells at all developmental stages except in male germ cells after differentiating into type A spermatogonia in the postnatal testis. There was also a lower level of expression in female germ cells in the prophase of the first meiotic division. These transgenic mice therefore proved to be powerful tools for isolating living germ cells at various developmental stages to study their nature and to isolate new genes.

Differentiation of murine premigratory primordial germ cells in culture

In the mouse embryo, primordial germ cells first appear in the extraembryonic mesoderm and divide rapidly while migrating to the fetal gonad. Shortly after their arrival in the gonad, germ cells sexually differentiate as proliferation ceases. Previous studies have established that primordial germ cells proliferate and migrate in feeder layer culture. To explore cellular regulation of fetal germ cell development, we have used germ cell nuclear antigen 1 (GCNA1), a marker normally expressed only in postmigratory germ cells, to investigate the developmental potency of both pre- and postmigratory cells in this culture system. We found that explanted premigratory germ cells will initiate expression of this marker and are, therefore, capable of undertaking some aspects of gonocyte differentiation without intimate exposure to the fetal gonad. We have also tested whether postmigratory gonocytes are stable in culture. As detected by either alkaline phosphatase or GCNA1, we did not detect long-term survival of either prospermatogonia or oogonia under conditions that support the survival, proliferation, and differentiation of earlier premigratory cells. These observations are consistent with an autonomous cellular mechanism governing the initial stages of gonocyte differentiation, and suggest that differentiation towards gonocytes is accompanied by a change in requirements for cell survival.

Identification of genes expressed in human primordial germ cells at the time of entry of the female germ line into meiosis

In mammals, primordial germ cells (PGCs) are first observed in the extraembryonic mesoderm from where they migrate through the hindgut and its mesentery to the genital ridge to colonize the developing gonads. Soon after reaching the gonads, the female PGCs enter meiosis, while the male PGCs are arrested in mitosis and enter meiosis postnatally. To gain an insight into the molecular events controlling human germ cell development, we determined specific profiles of gene expression using cDNA prepared from PGCs isolated from male and female fetal gonads at 10 weeks gestation, when female PGCs start to enter meiosis. The identity of the isolated PGCs, and the cDNA molecules prepared from them, was confirmed respectively, by alkaline phosphatase staining and by the presence of transcripts of OCT4, a marker gene for PGCs and pluripotent stem cells in mice. Using differential display to compare the profiles of gene expression of male and female germ cells with each other and with that of a whole 10 week old fetus, we have identified eight transcripts differentially expressed in male and/or female germ cells. Among these transcripts, we have identified a member of the olfactory receptor gene family, which contains genes known to be germline-specific in the dog and possibly associated with chemotactic function. Another transcript is common to a previously isolated sequence from the human testis and we have extended this sequence towards the 5' end for partial characterization. The germline-specific sequences also include two novel sequences not represented in the databases. These findings are highly encouraging for the elucidation of the genetic programming of male and female germ line development.

Pluripotent stem cells of the mouse as a potential in vitro model for mammalian germ cells. Sister chromatid exchanges induced by MMC and ENU in undifferentiated cell lines compared to differentiated cell lines

We tried to develop an in-vitro test system which could serve as a model for mammalian germ cells in vivo. Two pluripotent cell types were used, because they express some germ cell specific immunological and biochemical markers: (1) Embryonal carcinoma cells (ECC) of the line P19 had been isolated from a teratocarcinoma of murine primordial germ cells (PGC). (2) Embryonal stem cells (ESC) are obtained from the inner cell mass of mouse blastocysts. Sister chromatid exchanges (SCE) induced by mitomycin C and ethylnitrosourea (ENU) were analysed in the two undifferentiated cell lines, ECC and ESC, to detect differences in their sensitivity compared with differentiated cell lines of the mouse. Neither of the model cell lines have shown a greater sensibility after exposure to MMC and ENU. In contrary, the carcinoma cell line was able to tolerate higher concentrations of these genotoxicants. Therefore, SCE analysis in the ECC and ESC lines used does not provide a suitable model for genotoxicity testing on mammalian germ cells.

Germ cell migration and early development of the gonads in the trisomy 16 mouse--an animal model for Down's syndrome

The aneuploid condition of patients with Down's syndrome (trisomy 21) frequently leads to a sub- or infertility of these individuals. Gonads from adults and fetuses with trisomy 21 demonstrated histologically a remarkable reduction in germ cells. Disorders in the germ cell migration, the early development of the gonads as well as meiotic defects are thought to contribute to this pathomorphology. To gain information about premeiotic defects, investigations on the trisomy 16 mouse, an animal model for Down's syndrome, were carried out. By means of morphometric studies a delay in migration and a reduction in primordial germ cells was evaluated in trisomic mice of embryonic day 11 (E11). At day E13 a generalized growth retardation of the developing gonads was obvious in trisomic animals. Additionally performed electron microscopic examinations revealed signs of germ cell demise in trisomy 16 mice. Thus, the mechanisms of a diminished proliferation capacity, impaired migration and premature death of germ cells represent premeiotic disorders that presumably contribute to the pathomorphology observed in the gonads of individuals with Down's syndrome.

Life cycle of the mammalian germ cell: implication for spontaneous mutation frequencies

A brief history of the developmental life cycle of the mammalian germ cell, from fertilization to gametogenesis in the mature gonad, is presented. The differences between gametogenesis in the mature gonad of males and females are also described with regard to properties that may affect their susceptibilities to mutation. It is emphasized that any historical control background rate of necessity will include mutations that occur in germinal tissue at all stages of development and differentiation, although it is not always possible to determine at what stage of germline development a spontaneous mutation has occurred. Studies of induced mutations suggest that the impact on the molecular level and the distribution of mutations among the F1 and F2 progeny may be partly determined by the stage and sex of germ cells in which spontaneous mutations occur. In summary, historical control rates should only be considered the sum total of mutations that occur during the entire life of the individual and cannot represent the control values of any individual germ cell stage. Nonetheless, it is certainly important and valid to use historical control data for calculating human risk, because the primary use of the estimation of mutant frequencies is to access the potential impact of agents in increasing the genetic load in the human population.

Bmp4 is required for the generation of primordial germ cells in the mouse embryo

In many organisms the allocation of primordial germ cells (PGCs) is determined by the inheritance of maternal factors deposited in the egg. However, in mammals, inductive cell interactions are required around gastrulation to establish the germ line. Here, we show that Bmp4 homozygous null embryos contain no PGCs. They also lack an allantois, an extraembryonic mesodermal tissue derived, like the PGCs, from precursors in the proximal epiblast. Heterozygotes have fewer PGCs than normal, due to a reduction in the size of the founding population and not to an effect on its subsequent expansion. Analysis of beta-galactosidase activity in Bmp4(lacZneo) embryos reveals that prior to gastrulation, Bmp4 is expressed in the extraembryonic ectoderm. Later, Bmp4 is expressed in the extraembryonic mesoderm, but not in PGCs. Chimera analysis indicates that it is the Bmp4 expression in the extraembryonic ectoderm that regulates the formation of allantois and primordial germ cell precursors, and the size of the founding population of PGCs. The initiation of the germ line in the mouse therefore depends on a secreted signal from the previously segregated, extraembryonic, trophectoderm lineage.

Derivation of pluripotent stem cells from cultured human primordial germ cells

Human pluripotent stem cells would be invaluable for in vitro studies of aspects of human embryogenesis. With the goal of establishing pluripotent stem cell lines, gonadal ridges and mesenteries containing primordial germ cells (PGCs, 5-9 weeks postfertilization) were cultured on mouse STO fibroblast feeder layers in the presence of human recombinant leukemia inhibitory factor, human recombinant basic fibroblast growth factor, and forskolin. Initially, single PGCs in culture were visualized by alkaline phosphatase activity staining. Over a period of 7-21 days, PGCs gave rise to large multicellular colonies resembling those of mouse pluripotent stem cells termed embryonic stem and embryonic germ (EG) cells. Throughout the culture period most cells within the colonies continued to be alkaline phosphatase-positive and tested positive against a panel of five immunological markers (SSEA-1, SSEA-3, SSEA-4, TRA-1-60, and TRA-1-81) that have been used routinely to characterize embryonic stem and EG cells. The cultured cells have been continuously passaged and found to be karyotypically normal and stable. Both XX and XY cell cultures have been obtained. Immunohistochemical analysis of embryoid bodies collected from these cultures revealed a wide variety of differentiated cell types, including derivatives of all three embryonic germ layers. Based on their origin and demonstrated properties, these human PGC-derived cultures meet the criteria for pluripotent stem cells and most closely resemble EG cells.

Role of fibroblast growth factors and their receptors in mouse primordial germ cell growth

Primordial germ cells (PGCs) are the embryonic progenitors of mature germ cells. During their proliferative stage, murine PGCs may be transiently cultured on mitotically inactive feeder layers. This culture system has permitted identification of several growth factors active toward PGCs. We and others have previously identified basic fibroblast growth factor (bFGF) as a powerful mitogen in this system. Here we characterize some of the functions of bFGF in PGC culture. Our data demonstrate that fibroblast growth factor (FGF) receptors I and II are present in the developing gonad and are consistent with expression of these receptors by PGCs. Moreover, PGCs can bind radiolabeled bFGF in vitro, demonstrating that the factor can act directly on these cells. While mitotic PGCs of either sex are shown to bind radiolabeled bFGF, oogonia that are undergoing meiotic arrest exhibit reduced bFGF binding, indicating potential developmental regulation of an FGF receptor.

On the persistence of oocyte nuclei from fetus to maturity in the laboratory mouse

Tritiated thymidine injected intraperitoneally into female mice midway through the gestation period was demonstrated by autoradiographic methods to be incorporated into the nuclei of oocytes of female embryos, observed at the pachytene stage of meiosis 2 to 4 days after the injection. The tritium label was also demonstrated in the oocyte nuclei of the daughters of similarly treated females at maturity (6 weeks post partum). It was also found that some follicle cells, likewise labeled with H(3)-thymidine in mid-fetal life, persisted to maturity with few or no intervening mitoses. The observations are presented in support of the prevailing view that individual oocytes which arise from germ cell primordia in fetal stages become the egg cells of the adult female mammal.

Female mouse germ cells form synchronously dividing cysts

Oocytes from many invertebrates initiate development within distinctive cysts of interconnected cells, which are formed through synchronous divisions of a progenitor cell. Recently, processes underlying cyst formation have been extensively characterized at the molecular level in Drosophila. Defects in this process cause sterility in female flies. Early female mouse germ cells are organized as cell clusters as well, but it is uncertain whether these groups are similar to the cysts of invertebrates. We find that mouse germ cells are connected by intercellular bridges in the ovaries of 11.5 to 17.5 days postcoitum embryos; microtubules and organelles have been observed within these bridges. Confocal microscopy shows that cells within mouse clusters divide synchronously and frequently correspond in number to powers of two. Thus, female mouse germ cell clusters exhibit key characteristics of invertebrate germline cysts indicating that the process of germline cyst formation is conserved in the mouse.

Timing of gene expression and oolemma localization of mouse alpha6 and beta1 integrin subunits during oogenesis

The sperm antigen fertilin alpha/beta and the integrin complex alpha6beta1 present on the oolemma are two of the most promising candidates to mediate gamete interaction. During growth, the plasma membrane of both hamster and mouse zona-free oocytes acquires the capacity to fuse with acrosome-reacted sperm when oocytes reach the size of 25-30 microm in diameter, suggesting changes in the membrane molecular composition. The present study has two aims: to determine the timing of (1) gene expression of alpha6 and beta1 integrins and (2) localization of these integrin subunits on the plasma membrane in primordial germ cells and in oocytes during oogenesis. We found that both alpha6 and beta1 genes are expressed in female germ cells during all the stages of development analyzed, from 10.5 to 18.5 d.p. c., during oocyte growth, and in ovulated eggs. The alternatively spliced isoform alpha6B is expressed from 10.5 d.p.c., whereas alpha6A begins to be expressed at 12.5 d.p.c., suggesting a different role for the two variants. In situ immunodetection of alpha6 or beta1 shows a ring of fluorescence on the female germ cell plasma membrane for both integrins at 10.5 d.p.c., then the fluorescent signal becomes undetectable at 12.5 d.p.c. to reappear again, this time with a patchy distribution, at 18.5 d.p.c. This pattern of localization is maintained in oocytes isolated from newborn individuals and only when oocytes during growth reach the size of about 25-30 microm in diameter does the fluorescence become homogenous all around the whole oocyte surface. These data, although not conclusive, support the hypothesis of an involvement of alpha6 and beta1 integrins in sperm-egg fusion.

In vitro survival and proliferation of porcine primordial germ cells

Primordial germ cells (PGC) collected from the genital ridge of Day 25 porcine embryos were cultured on STO feeder cells in medium with or without supplemented growth factors. The effects on porcine PGC proliferation of leukemia inhibitory factor (LIF), LIF + stem cell factor (SCF) or LIF + SCF + basic fibroblast growth factor (bFGF), growth factors shown to be essential for in vitro survival and proliferation of murine PGC, were tested. After histochemical staining, both freshly collected and cultured PGC expressed alkaline phosphatase activity. With or without supplemented growth factors, porcine PGC survived and proliferated in culture for at least 5 d. None of the growth factors tested markedly enhanced in vitro growth of porcine PGC. These results suggest that growth factors provided by either the STO feeder layer or the cultured PGC themselves are sufficient to support in vitro survival and proliferation of porcine PGC. With the support of STO cells, addition of growth factors shown to be essential for the in vitro growth of murine PGC is not required for survival and proliferation of cultured porcine PGC.

The tao of stem cells in the germline

Germline stem cells (GSCs) are the self-renewing population of germ cells that serve as the source for gametogenesis. GSCs exist in diverse forms, from those that undergo strict self-renewing asymmetric divisions in Drosophila to those that maintain their population by balancing between mitosis and differentiation in Caenorhabditis elegans. Most vertebrate spermatogonial GSCs appear to adopt an intermediate strategy. In most animals, GSCs are established during preadult gonadogenesis following the proliferation and migration of embryonic primordial germ cells. GSCs produce numerous gametes throughout the sexually active period of adult life. The establishment and self-renewing division of GSCs are controlled by extracellular signals such as hormones from the hypothalamic-pituitary axis and local interactions between GSCs and their neighboring cells. These extracellular signals may then influence differential gene expression, cell cycle machinery, and cytoskeletal organization of GSCs for their formation and/or divisional asymmetry. In addition, the GSC mechanism is related to that for germline and sex determination. Current knowledge has provided a solid framework for further study of GSCs and stem cells in general.

The cytogenetic theory of the pathogenesis of human adult male germ cell tumors. Review article

Human male germ cell tumors (GCTs) represent a biological paradox because, in order to develop into a pluripotential tumor, a germ cell destined to a path of limited or no proliferation must acquire the potential for unlimited proliferation. In addition, it must acquire the ability to elicit embryonal differentiation patterns without the reciprocal inputs from fertilization and the imprinting-associated genomic changes which are a part of normal embryonal development. Although much speculated about, the genetic mechanisms underlying these properties of male GCTs remain enigmatic. Recent cytogenetic and molecular genetic analyses of these tumors are providing new insights and new testable hypotheses. Based on our recent work, we propose two such hypotheses. One relates to the mechanism of germ cell transformation and germ cell tumor development. We suggest that the invariable 12p amplification noted as early as in carcinoma in situ/intratubular germ cell neoplasia (CIS/ITGCN) lesions leads to deregulated overexpression of cyclin D2, a cell cycle G1/S checkpoint regulator with oncogeneic potential. Such overexpression reinitiates the cell cycle. We visualize this happening during the pachytene stage of meiosis through aberrant recombinational events which lead to 12p amplification. The other hypothesis relates to the origin of primary extragonadal GCTs. By comparing cytogenetic changes in primary mediastinal versus gonadal lesions, we propose that, in contrast to long-standing speculation that primary extra-gonadal tumors arise from embryonally misplaced primordial germ cells, these lesions arise from migration of transformed gonadal germ cells.

Early stages in male germ cell differentiation in the mouse. Review article

Primordial germ cells arise during gastrulation and migrate from the hindgut into the gonad primordium during early organogenesis. In this article, we discuss factors that control migration, proliferation and targeting of the PGCs. In particular we discuss how changes in adhesiveness control germ cell positioning in the gonad, and the molecules involved.

Distribution of putative primordial germ cells in equine embryos

Eighteen equine embryos, 3 each on Days 20, 22, 24, 26, 28 and 30 post ovulation, were collected transcervically by uterine lavage, fixed in 4% paraformaldehyde and embedded in paraffin wax. Ten micron serial sections were stained to determine alkaline phosphatase (AP) activity in the cells. Positive cells were counted and their approximate location determined. The cells were approximately 8 microm in diameter and the entire cell, except the nucleus, stained strongly with many small round areas of intense staining in the cytoplasm. The cells varied from round to elongated in shape and pseudopodia were often present. Thus, they were similar in shape and staining pattern to primordial germ cells described in other species. A total of 2 AP-positive cells were found in the 3 Day 20 embryos and the mean number of AP-positive cells changed (P<0.05) over the succeeding days as follows; Day 20 = 1; Day 22 = 251; Day 24 = 1484; Day 26 = 2385; Day 28 = 3267; Day 30 = 2424. AP-positive cells present in the liver were not included in the calculations. In the Day 22 embryos, 10% of the putative primordial germ cells were found within the vascular system, including the heart chambers, and only 4% were found along the genital ridge. The percentage of cells found in the vascular system decreased from 10% on Day 22 to 1% on Day 30, although not significantly. The percentage of cells found along the genital ridge changed (P<0.05) over gestational age as follows; Day 22 = 4%; Day 24 = 10%; Day 26 = 28%; Day 28 = 28%; Day 30 = 16%. Once the putative primordial germ cells reached the developing gonads they were no longer AP-positive and nor was the gonadal stroma. The rest of the cells were distributed along the dorsal mesentery (range, 14-24%), near the dorsal aorta (16-29%), in the mesonephros (1-3%) and in other areas of the embryo (27-44%). Large numbers were in the cranial portion of the embryo. Although it is likely that the population of AP-positive cells counted included the primordial germ cells, other cells, such as haematopoietic precursor cells, could not be ruled out. The AP reactivity, the appearance of the cells and their migratory pattern through the dorsal mesentery to the gonadal ridge were consistent with descriptions of primordial germ cells in other species. Their distribution throughout the embryo, especially its cranial aspect and their location within, or in close proximity to, blood vessels suggested that the equine embryo is unusual among mammals in that some of its primordial germ cells migrate through the blood.

Germ cell nuclear antigen (GCNA1) expression does not require a gonadal environment or steroidogenic factor 1: examination of GCNA1 in ectopic germ cells and in Ftz-F1 null mice

The germ cell lineage is first recognized as a population of mitotically proliferating primordial germ cells that migrate toward the gonadal ridge. Shortly after arriving at the gonadal ridge, the germ cells begin to initiate a commitment to gamete production in the developing gonad. The mechanisms controlling this transition are poorly understood. We recently reported that a mouse germ cell nuclear antigen 1 (GCNA1) is initially detected in both male and female germ cells as they reach the gonad at 11.5 days postcoitum (dpc). GCNA1 is continually expressed in germ cells through all stages of gametogenesis until the diplotene/dictyate stage of meiosis I. Since GCNA1 expression commences soon after primordial germ cells arrive at the gonadal ridge, we wanted to determine whether the gonadal environment was essential for induction of GCNA1 expression. By examining GCNA1 expression in germ cells that migrate ectopically into the adrenal gland, we determined that both the gonadal and adrenal gland environments allow GCNA1 expression. We also examined GCNA1 expression Ftz-F1 null mice, which were born lacking gonads and adrenal glands. During embryonic development in the Ftz-F1 null mice, the gonad and most germ cells undergo apoptotic degeneration at about 12.5 dpc. While most of the germ cells undergo apoptosis without expressing GCNA1, a few surviving germs cells, especially outside the involuting gonad clearly express GCNA1. Thus, although the Ftz-F1 gene is essential for gonadal and adrenal development, induction of GCNA1 expression in germ cells does not require Ftz-F1 gene products. The finding that germ cell GCNA1 expression is not restricted to the gonadal environment and is not dependent on the Ftz-F1 gene products suggests that GCNA1 expression may be initiated in the germ cell lineage by autonomous means.

A vasa-like gene in zebrafish identifies putative primordial germ cells

The vasa gene is essential for germline formation in Drosophila. Vasa-related genes have been isolated from several organisms including nematode, frog and mammals. In order to gain insight into the early events in vertebrate germline development, zebrafish was chosen as a model. Two zebrafish vasa-related genes were isolated, pl10a and vlg. The pl10a gene was shown to be widely expressed during embryogenesis. The vlg gene and vasa belong to the same subfamily of RNA helicase encoding genes. Putative maternal vlg transcripts were detected shortly after fertilization and from the blastula stage onwards, expression was restricted to migratory cells most likely to be primordial germ cells.