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

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

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

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

BACKGROUND

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

SUMMARY

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

KEY MESSAGE

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

Classical cadherins in the testis: how are they regulated?

Cadherins (CDH) are crucial intercellular adhesion molecules, contributing to morphogenesis and creating tissue barriers by regulating cells' movement, clustering and differentiation. In the testis, classical cadherins such as CDH1, CDH2 and CDH3 are critical to gonadogenesis by promoting the migration and the subsequent clustering of primordial germ cells with somatic cells. While CDH2 is present in both Sertoli and germ cells in rodents, CDH1 is primarily detected in undifferentiated spermatogonia. As for CDH3, its expression is mainly found in germ and pre-Sertoli cells in developing gonads until the establishment of the blood-testis barrier (BTB). This barrier is made of Sertoli cells forming intercellular junctional complexes. The restructuring of the BTB allows the movement of early spermatocytes toward the apical compartment as they differentiate during a process called spermatogenesis. CDH2 is among many junctional proteins participating in this process and is regulated by several pathways. While cytokines promote the disassembly of the BTB by enhancing junctional protein endocytosis for degradation, testosterone facilitates the assembly of the BTB by increasing the recycling of endocytosed junctional proteins. Mitogen-activated protein kinases (MAPKs) are also mediators of the BTB kinetics in many chemically induced damages in the testis. In addition to regulating Sertoli cell functions, follicle stimulating hormone can also regulate the expression of CDH2. In this review, we discuss the current knowledge on regulatory mechanisms of cadherin localisation and expression in the testis.

Gastruloid-derived primordial germ cell-like cells develop dynamically within integrated tissues

Primordial germ cells (PGCs) are the early embryonic precursors of gametes - sperm and egg cells. PGC-like cells (PGCLCs) can currently be derived in vitro from pluripotent cells exposed to signalling cocktails and aggregated into large embryonic bodies, but these do not recapitulate the native embryonic environment during PGC formation. Here, we show that mouse gastruloids, a three-dimensional in vitro model of gastrulation, contain a population of gastruloid-derived PGCLCs (Gld-PGCLCs) that resemble early PGCs in vivo. Importantly, the conserved organisation of mouse gastruloids leads to coordinated spatial and temporal localisation of Gld-PGCLCs relative to surrounding somatic cells, even in the absence of specific exogenous PGC-specific signalling or extra-embryonic tissues. In gastruloids, self-organised interactions between cells and tissues, including the endodermal epithelium, enables the specification and subsequent maturation of a pool of Gld-PGCLCs. As such, mouse gastruloids represent a new source of PGCLCs in vitro and, owing to their inherent co-development, serve as a novel model to study the dynamics of PGC development within integrated tissue environments.

Factors within the Developing Embryo and Ovarian Microenvironment That Influence Primordial Germ Cell Fate

BACKGROUND

Primordial germ cell (PGC) fate is dictated by the designation, taxis, and influence of the surrounding embryonic somatic cells. Whereas gonadal sex determination results from a balance of factors within the tissue microenvironment.

SUMMARY

Our understanding of mammalian ovary development is formed in large part from developmental time courses established using murine models. Genomic tools where genes implicated in the PGC designation or gonadal sex determination have been modulated through complete or conditional knockouts in vivo, and studies in in situ models with inhibitors or cultures that alter the native gonadal environment have pieced together the interplay of pioneering transcription factors, co-regulators and chromosomes critical for the progression of PGCs to oocytes. Tools such as pluripotent stem cell derivation, genomic modifications, and aggregate differentiation cultures have yielded some insight into the human condition. Additional understanding of sex determination, both gonadal and anatomical, may be inferred from phenotypes that arise from de novo or inherited gene variants in humans who have differences in sex development.

KEY MESSAGES

This review highlights major factors critical for PGC specification and migration, and in ovarian gonad specification by reviewing seminal murine models. These pathways are compared to what is known about the human condition from expression profiles of fetal gonadal tissue, use of human pluripotent stem cells, or disorders resulting from disease variants. Many of these pathways are challenging to decipher in human tissues. However, the impact of new single-cell technologies and whole-genome sequencing to reveal disease variants of idiopathic reproductive tract phenotypes will help elucidate the mechanisms involved in human ovary development.

The transcription factors Junb and Fosl2 cooperate to regulate Cdh3 expression in 15P-1 Sertoli cells

In Sertoli cells of the testis, cadherins (Cdh) are important cell-to-cell interaction proteins and contribute to the formation of the blood-testis barrier being essential for germ cells' protection. P-cadherin or Cdh3 is only expressed in Sertoli cells from embryonic to prepubertal development. Interestingly, the expression profile of Cdh3 correlates with that of activating protein-1 (AP-1) transcription factors during Sertoli cells development. To assess their potential implications in the regulation of Cdh3, different AP-1 transcription factors were overexpressed in 15P-1 Sertoli cells. We found that the overexpressions of Junb and Fosl2 activated Cdh3 promoter. ChIP-qPCR assay and luciferase reporter assay with 5' promoter deletions and site-directed mutagenesis confirmed the recruitment of Junb and Fosl2 to an AP-1 regulatory element at -47 bp in the proximal region of Cdh3 promoter in 15P-1 cells. These findings were further supported by histone modification markers and chromatin accessibility surrounding Cdh3 promoter in mouse testis. Moreover, the knockdowns of Junb and/or Fosl2 by siRNA decreased Cdh3 protein levels. Taken together, these data suggest that in 15P-1 Sertoli cells, the AP-1 family members Junb and Fosl2 are responsible for the regulation of Cdh3 expression, which requires the recruitment of both factors to the proximal region of the Cdh3 promoter.

Germline stem cells in human

The germline cells are essential for the propagation of human beings, thus essential for the survival of mankind. The germline stem cells, as a unique cell type, generate various states of germ stem cells and then differentiate into specialized cells, spermatozoa and ova, for producing offspring, while self-renew to generate more stem cells. Abnormal development of germline stem cells often causes severe diseases in humans, including infertility and cancer. Primordial germ cells (PGCs) first emerge during early embryonic development, migrate into the gentile ridge, and then join in the formation of gonads. In males, they differentiate into spermatogonial stem cells, which give rise to spermatozoa via meiosis from the onset of puberty, while in females, the female germline stem cells (FGSCs) retain stemness in the ovary and initiate meiosis to generate oocytes. Primordial germ cell-like cells (PGCLCs) can be induced in vitro from embryonic stem cells or induced pluripotent stem cells. In this review, we focus on current advances in these embryonic and adult germline stem cells, and the induced PGCLCs in humans, provide an overview of molecular mechanisms underlying the development and differentiation of the germline stem cells and outline their physiological functions, pathological implications, and clinical applications.

SETDB1 Regulates Porcine Spermatogonial Adhesion and Proliferation through Modulating MMP3/10 Transcription

The transition from gonocytes into spermatogonia takes place during the homing process. A subpopulation of undifferentiated spermatogonia in niche then shifts to spermatogonial stem cells (SSCs), accompanied by the self-renewal ability to maintain life-long fertility in males. Enormous changes in cell morphology, gene expression, and epigenetic features have been reported during spermatogenesis. However, little is known about the difference of these features in SSCs during aging. Here, we examined the dynamics of SET domain bifurcated 1 (SETDB1) expression in porcine testes. SETDB1 was expressed in postnatal undifferentiated spermatogonia, while gradually disappeared after being packed within the basal compartment of seminiferous tubules. In addition, the cell-adhesion ability, proliferative activity, and trimethylation of the histone H3 lysine 9 (H3K9me3) level were significantly altered in SETDB1-deficient porcine SSCs. Moreover, the matrix metalloproteinases 3/10 (MMP3/10) was upregulated at both mRNA and protein levels. These results illustrate the significance of SETDB1 in modulating early male germ cell development.

OUP accepted manuscript

The pathways of gametogenesis encompass elaborate cellular specialization accompanied by precise partitioning of the genome content in order to produce fully matured spermatozoa and oocytes. Transcription factors are an important class of molecules that function in gametogenesis to regulate intrinsic gene expression programs, play essential roles in specifying (or determining) germ cell fate and assist in guiding full maturation of germ cells and maintenance of their populations. Moreover, in order to reinforce or redirect cell fate in vitro, it is transcription factors that are most frequently induced, over-expressed or activated. Many reviews have focused on the molecular development and genetics of gametogenesis, in vivo and in vitro, in model organisms and in humans, including several recent comprehensive reviews: here, we focus specifically on the role of transcription factors. Recent advances in stem cell biology and multi-omic studies have enabled deeper investigation into the unique transcriptional mechanisms of human reproductive development. Moreover, as methods continually improve, in vitro differentiation of germ cells can provide the platform for robust gain- and loss-of-function genetic analyses. These analyses are delineating unique and shared human germ cell transcriptional network components that, together with somatic lineage specifiers and pluripotency transcription factors, function in transitions from pluripotent stem cells to gametes. This grand theme review offers additional insight into human infertility and reproductive disorders that are linked predominantly to defects in the transcription factor networks and thus may potentially contribute to the development of novel treatments for infertility.

Tissue and cell interactions in mammalian PGC development

Primordial germ cells (PGCs) form early in embryo development and are crucial precursors to functioning gamete cells. Considerable research has focussed on identifying the transcriptional characteristics and signalling pathway requirements that confer PGC specification and development, enabling the derivation of PGC-like cells (PGCLCs) in vitro using specific signalling cocktails. However, full maturation to germ cells still relies on co-culture with supporting cell types, implicating an additional requirement for cellular- and tissue-level regulation. Here, we discuss the experimental evidence that highlights the nature of intercellular interactions between PGCs and neighbouring cell populations during mouse PGC development. We posit that the role that tissue interactions play on PGCs is not limited solely to signalling-based induction but extends to coordination of development by robust regulation of the proportions and position of the cells and tissues within the embryo, which is crucial for functional germ cell maturation. Such tissue co-development provides a dynamic, contextual niche for PGC development. We argue that there is evidence for a clear role for inter-tissue dependence of mouse PGCs, with potential implications for generating mammalian PGCLCs in vitro.

Proper Balance of Small GTPase rab10 Is Critical for PGC Migration in Zebrafish

MicroRNAs (miRNAs) play important roles in post-transcriptional repression in nearly every biological process including germ cell development. Previously, we have identified a zebrafish germ plasm-specific miRNA miR-202-5p, which regulates PGC migration through targeting cdc42se1 to protect cdc42 expression. However, knockdown of cdc42se1 could not significantly rescue PGC migration in maternal miR-202 mutant (MmiR-202) embryos, indicating that there are other target genes of miR-202-5p required for the regulation of PGC migration. Herein, we revealed the transcriptional profiles of wild type and MmiR-202 PGCs and obtained 129 differentially expressed genes (DEGs), of which 42 DEGs were enriched cell migration-related signaling pathways. From these DEGs, we identified two novel miR-202-5p target genes prdm12b and rab10. Furthermore, we found that disruption of rab10 expression led to significantly migratory defects of PGC by overexpression of rab10 siRNA, or WT, inactive as well as active forms of rab10 mRNA, and WT rab10 overexpression mediated migratory defects could be partially but significantly rescued by overexpression of miR-202-5p, demonstrating that rab10 is an important factor involved miR-202-5p mediated regulation of PGC migration. Taken together, the present results provide significant information for understanding the molecular mechanism by which miR-202-5p regulates PGC migration in zebrafish.

Zebrafish Primordial Germ Cell Migration

Similar to many other organisms, zebrafish primordial germ cells (PGCs) are specified at a location distinct from that of gonadal somatic cells. Guided by chemotactic cues, PGCs migrate through embryonic tissues toward the region where the gonad develops. In this process, PGCs employ a bleb-driven amoeboid migration mode, characterized by low adhesion and high actomyosin contractility, a strategy used by other migrating cells, such as leukocytes and certain types of cancer cells. The mechanisms underlying the motility and the directed migration of PGCs should be robust to ensure arrival at the target, thereby contributing to the fertility of the organism. These features make PGCs an excellent model for studying guided single-cell migration in vivo. In this review, we present recent findings regarding the establishment and maintenance of cell polarity that are essential for motility and discuss the mechanisms by which cell polarization and directed migration are controlled by chemical and physical cues.

The Central Role of Cadherins in Gonad Development, Reproduction, and Fertility

Cadherins are a group of membrane proteins responsible for cell adhesion. They are crucial for cell sorting and recognition during the morphogenesis, but they also play many other roles such as assuring tissue integrity and resistance to stretching, mechanotransduction, cell signaling, regulation of cell proliferation, apoptosis, survival, carcinogenesis, etc. Within the cadherin superfamily, E- and N-cadherin have been especially well studied. They are involved in many aspects of sexual development and reproduction, such as germline development and gametogenesis, gonad development and functioning, and fertilization. E-cadherin is expressed in the primordial germ cells (PGCs) and also participates in PGC migration to the developing gonads where they become enclosed by the N-cadherin-expressing somatic cells. The differential expression of cadherins is also responsible for the establishment of the testis or ovary structure. In the adult testes, N-cadherin is responsible for the integrity of the seminiferous epithelium, regulation of sperm production, and the establishment of the blood-testis barrier. Sex hormones regulate the expression and turnover of N-cadherin influencing the course of spermatogenesis. In the adult ovaries, E- and N-cadherin assure the integrity of ovarian follicles and the formation of corpora lutea. Cadherins are expressed in the mature gametes and facilitate the capacitation of sperm in the female reproductive tract and gamete contact during fertilization. The germ cells and accompanying somatic cells express a series of different cadherins; however, their role in gonads and reproduction is still unknown. In this review, we show what is known and unknown about the role of cadherins in the germline and gonad development, and we suggest topics for future research.

Presence of the matrix metalloproteinases during the migration of the primordial germ cells in zebrafish gonadal ridge

In vertebrates, the primordial germ cells (PGCs) differentiate from extragonadal regions, migrating to gonadal ridge during the embryonic development. However, recent studies in mammals indicate that the PGCs originate from the epiblast and subsequently migrate into the yolk sac. Cell and molecular bases involved in routes during the migration of these cells are still not well understood. Thus, in an attempt to evaluate the participation of matrix metalloproteinases (MMPs) during the gonadal primordium formation in Danio rerio (zebrafish), the route of migration of PGCs was analyzed. In zebrafish, during the migration of the PGCs to the forming gonad, they bind by cytoplasmic processes to the extracellular matrix and migrate through amoeboid movements until they reach the gonadal ridge. During the epiboly, MMPs were not detected. However, after organogenesis, three MMP types were expressed in the somatic cells that were located ahead of the PGCs in the migration route. This expression was maintained throughout the mesentery and was not detected in the PGCs. Upon reaching the gonadal ridge, the PGCs and somatic cells express MMPs and epithelium begins to be formed. After the formation of the basement membrane, the germinal epithelium is delineated by the somatic cells, which remodeling the extracellular matrix. So, a PGC organization occurs through the tissue, forming the gonadal primordium. Concomitantly, granulocytes expressing different MMPs are present. This data in exposing the role of MMPs during the PGC migration to the forming gonad, may point a new way in understanding the reproductive biology of the vertebrates in general.

Collective Cell Sorting Requires Contractile Cortical Waves in Germline Cells

Encapsulation of germline cells by layers of somatic cells forms the basic unit of female reproduction called primordial follicles in mammals and egg chambers in Drosophila. How germline and somatic tissues are coordinated for the morphogenesis of each separated unit remains poorly understood. Here, using improved live imaging of Drosophila ovaries, we uncovered periodic actomyosin waves at the cortex of germ cells. These contractile waves are associated with pressure release blebs, which project from germ cells into somatic cells. We demonstrate that these cortical activities, together with cadherin-based adhesion, are required to sort each germline cyst as one collective unit. Genetic perturbations of cortical contractility, bleb protrusion, or adhesion between germline and somatic cells induced encapsulation defects resulting from failures to encapsulate any germ cells, or the inclusion of too many germ cells per egg chamber, or even the mechanical split of germline cysts. Live-imaging experiments revealed that reducing contractility or adhesion in the germline reduced the stiffness of germline cysts and their proper anchoring to the somatic cells. Germline cysts can then be squeezed and passively pushed by constricting surrounding somatic cells, resulting in cyst splitting and cyst collisions during encapsulation. Increasing germline cysts activity or blocking somatic cell constriction movements can reveal active forward migration of germline cysts. Our results show that germ cells play an active role in physical coupling with somatic cells to produce the female gamete.

Effects of HSP90 inhibition on primordial germ cells migration: A study in the gonad of the chick embryo

INTRODUCTION

Primordial Germ Cells (PGCs) differentiate into spermatozoa or oocytes. They appear early during embryonic development before migrating to the gonadal ridges. Because of their long migration, PGCs have been proposed as a valuable model to study long distance cell migration. Some species also present a vascular phase in the migration of the germline and could therefore be compared to metastatic migration. HSP90 is a heat shock protein involved in the stabilization of several client-proteins, including oncoproteins. HSP90 inhibition has been proved to decrease PGCs migration in mouse and zebrafish.

MATERIAL AND METHODS

We investigated the effect of geldanamycin on PGCs migration in a species with a vascular phase, the chicken. Geldanamycin was injected in the egg at 48h of incubation, PGC's were detected in blood using of blood smears, and in the embryo by immunohistochemistry using anti-HSP90 antibody.

RESULTS

The effects of the treatment were similar to those observed in mouse and zebrafish. We show the presence of ectopic germs cells in the vasculature and in the dorsal mesentery, and some deformities of the gonads.

CONCLUSION

Inhibition of HSP90 decreases the migration of PGCs and proposed the migration of PGCs in the chick embryo as an interesting model to study metastatic invasion.

Germ cell migration-Evolutionary issues and current understanding

In many organisms, primordial germ cells (PGCs) are specified at a different location than where the gonad forms, meaning that PGCs must migrate toward the gonad within the early developing embryo. Following species-specific paths, PGCs can be passively carried by surrounding tissues and also perform active migration. When PGCs actively migrate through and along a variety of embryonic structures in different organisms, they adopt an ancestral robust migration mode termed "amoeboid motility", which allows cells to migrate within diverse environments. In this review, we discuss the possible significance of the PGC migration process in facilitating the evolution of animal body shape. In addition, we summarize the latest findings relevant for the molecular and cellular mechanisms controlling the movement and the directed migration of PGCs in different species.

Testis Development

Production of sperm and androgens is the main function of the testis. This depends on normal development of both testicular somatic cells and germ cells. A genetic program initiated from the Y chromosome gene sex-determining region Y (SRY) directs somatic cell specification to Sertoli cells that orchestrate further development. They first guide fetal germ cell differentiation toward spermatogenic destiny and then take care of the full service to spermatogenic cells during spermatogenesis. The number of Sertoli cells sets the limits of sperm production. Leydig cells secrete androgens that determine masculine development. Testis development does not depend on germ cells; that is, testicular somatic cells also develop in the absence of germ cells, and the testis can produce testosterone normally to induce full masculinization in these men. In contrast, spermatogenic cell development is totally dependent on somatic cells. We herein review germ cell differentiation from primordial germ cells to spermatogonia and development of the supporting somatic cells. Testicular descent to scrota is necessary for normal spermatogenesis, and cryptorchidism is the most common male birth defect. This is a mild form of a disorder of sex differentiation. Multiple genetic reasons for more severe forms of disorders of sex differentiation have been revealed during the last decades, and these are described along with the description of molecular regulation of testis development.

Mammalian germ cell migration during development, growth, and homeostasis

BACKGROUND

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

METHODS

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

MAIN FINDINGS

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

CONCLUSION

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

The Role of Epithelial and Vascular-Endothelial Cadherin in the Differentiation and Maintance of Tissue Integrity

The present review has focused on the cell adhesion molecules from the cadherin superfamily, in particular on E- and VE-cadherin. In general, cadherins are a large group of cell adhesion molecules located at intercellular junctions called adherent junctions. They play an important role in embryogenesis and morphogenesis in animals and humans due to their adhesive and cell-signalling functions. Disturbances of the expression or function of cadherins and their associated proteins called catenins are crucial for the initiation and development of many pathological states. E-cadherin is an epithelium-specific cadherin that is required for the development and maintenance of the normal function of all epithelial cells in tissues. The loss or down-regulation of E-cadherin is a key event in the process of tumour invasion and metastasis. The assessment of E-cadherin immunoreactivity may be a useful prognostic marker in some cancers, complementary to the established prognostic factors. VE-cadherin is an endothelium-specific cadherin, which plays a relevant role in vascular homeostasis. It has been demonstrated that VE-cadherin is required for normal vasculogenesis, angiogenesis, and for the maintenance of vascular integrity. Disruption of VE-cadherin-catenin complexes by some inflammatory agents such as thrombin, by inflammatory cells, or shear stress is accompanied by an increase in vascular permeabilityin vivoandin vitro.

Characterization of migratory primordial germ cells in the aorta-gonad-mesonephros of a 4.5-week-old human embryo: a toolbox to evaluate in vitro early gametogenesis

STUDY QUESTION

Which set of antibodies can be used to identify migratory and early post-migratory human primordial germ cells (hPGCs)?

STUDY FINDING

We validated the specificity of 33 antibodies for 31 markers, including POU5F1, NANOG, PRDM1 and TFAP2C as specific markers of hPGCs at 4.5 weeks of development of Carnegie stage (CS12–13), whereas KIT and SOX17 also marked the intra-aortic hematopoietic stem cell cluster in the aorta-gonad-mesonephros (AGM).

WHAT IS KNOWN ALREADY

The dynamics of gene expression during germ cell development in mice is well characterized and this knowledge has proved crucial to allow the development of protocols for the in vitro derivation of functional gametes. Although there is a great interest in generating human gametes in vitro, it is still unclear which markers are expressed during the early stages of hPGC development and many studies use markers described in mouse to benchmark differentiation of human PGC-like cells (hPGCLCs). Early post-implantation development differs significantly between mice and humans, and so some germ cells markers, including SOX2, SOX17, IFITM3 and ITGA6 may not identify mPGCs and hPGCs equally well.

STUDY DESIGN, SIZE, DURATION

This immunofluorescence study investigated the expression of putative hPGC markers in the caudal part of a single human embryo at 4.5 weeks of development.

PARTICIPANTS/MATERIALS, SETTING, METHODS

We have investigated by immunofluorescence the expression of a set of 33 antibodies for 31 markers, including pluripotency, germ cell, adhesion, migration, surface, mesenchymal and epigenetic markers on paraffin sections of the caudal part, including the AGM region, of a single human embryo (CS12–13). The human material used was anonymously donated with informed consent from elective abortions without medical indication.

MAIN RESULTS AND THE ROLE OF CHANCE

We observed germ cell specific expression of NANOG, TFAP2C and PRDM1 in POU5F1+ hPGCs in the AGM. The epigenetic markers H3K27me3 and 5mC were sufficient to distinguish hPGCs from the surrounding somatic cells. Some mPGC-markers were not detected in hPGCs, but marked other tissues; whereas other markers, such as ALPL, SOX17, KIT, TUBB3, ITGA6 marked both POU5F1+ hPGCs and other cells in the AGM. We used a combination of multiple markers, immunostaining different cellular compartments when feasible, to decrease the chance of misidentifying hPGCs.

LARGE SCALE DATA

Non-applicable.

LIMITATIONS REASONS FOR CAUTION

Material to study early human development is unique and very rare thus restricting the sample size. We have used a combination of antibodies limited by the number of paraffin sections available.

WIDER IMPLICATIONS OF THE FINDINGS

Most of our knowledge on early gametogenesis has been obtained from model organisms such as mice and is extrapolated to humans. However, since there is a dedicated effort to produce human artificial gametes in vitro, it is of great importance to determine the expression and specificity of human-specific germ cell markers. We provide a systematic analysis of the expression of 31 different markers in paraffin sections of a CS12–13 embryo. Our results will help to set up a toolbox of markers to evaluate protocols to induce hPGCLCs in vitro.

STUDY FUNDING AND COMPETING INTEREST(S)

M.G.F. was funded by Fundação para a Ciência e Tecnologia (FCT) [SFRH/BD/78689/2011] and S.M.C.S.L. was funded by the Interuniversity Attraction Poles (IAP, P7/07) and the European Research Council Consolidator (ERC-CoG-725722-OVOGROWTH). The authors declare no conflict of interest.

Sexually dimorphic germ cell identity in mammals

Germ cells are the stem cells of the species. Thus, it is critical that we have a good understanding of how they are specified, how the somatic cells instruct and support them, how they commit to one or other sex, and how they ultimately develop into functional gametes. Here, we focus on specifics of how sexual fate is determined during fetal life. Because the majority of relevant experimental work has been done using the mouse model, we focus on that species. We review evidence regarding the identity of instructive signals from the somatic cells, and the molecular responses that occur in germ cells in response to those extrinsic signals. In this way we aim to clarify progress to date regarding the mechanisms underlying the mitotic to meiosis switch in germ cells of the fetal ovary, and those involved in adopting and securing male fate in germ cells of the fetal testis.

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

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

Cell-based computational model of early ovarian development in mice

Despite its importance to reproduction, certain mechanisms of early ovarian development remain a mystery. To improve our understanding, we constructed the first cell-based computational model of ovarian development in mice that is divided into two phases: Phase I spans embryonic day 5.5 (E5.5) to E12.5; and Phase II spans E12.5 to postnatal day 2. We used the model to investigate four mechanisms: in Phase I, (i) whether primordial germ cells (PGCs) undergo mitosis during migration; and (ii) if the mechanism for secretion of KIT ligand from the hindgut resembles inductive cell-cell signaling or is secreted in a static manner; and in Phase II, (iii) that changes in cellular adhesion produce germ cell nest breakdown; and (iv) whether localization of primordial follicles in the cortex of the ovary is due to proliferation of granulosa cells. We found that the combination of the first three hypotheses produced results that aligned with experimental images and PGC abundance data. Results from the fourth hypothesis did not match experimental images, which suggests that more detailed processes are involved in follicle localization. Phase I and Phase II of the model reproduce experimentally observed cell counts and morphology well. A sensitivity analysis identified contact energies, mitotic rates, KIT chemotaxis strength, and diffusion rate in Phase I and oocyte death rate in Phase II as parameters with the greatest impact on model predictions. The results demonstrate that the computational model can be used to understand unknown mechanisms, generate new hypotheses, and serve as an educational tool.

Cell adhesion molecules expression pattern indicates that somatic cells arbitrate gonadal sex of differentiating bipotential fetal mouse gonad

Unlike other organ anlagens, the primordial gonad is sexually bipotential in all animals. In mouse, the bipotential gonad differentiates into testis or ovary depending on the genetic sex (XY or XX) of the fetus. During gonad development cells segregate, depending on genetic sex, into distinct compartments: testis cords and interstitium form in XY gonad, and germ cell cysts and stroma in XX gonad. However, our knowledge of mechanisms governing gonadal sex differentiation remains very vague. Because it is known that adhesion molecules (CAMs) play a key role in organogenesis, we suspected that diversified expression of CAMs should also play a crucial role in gonad development. Using microarray analysis we identified 129 CAMs and factors regulating cell adhesion during sexual differentiation of mouse gonad. To identify genes expressed differentially in three cell lines in XY and XX gonads: i) supporting (Sertoli or follicular cells), ii) interstitial or stromal cells, and iii) germ cells, we used transgenic mice expressing EGFP reporter gene and FACS cell sorting. Although a large number of CAMs expressed ubiquitously, expression of certain genes was cell line- and genetic sex-specific. The sets of CAMs differentially expressed in supporting versus interstitial/stromal cells may be responsible for segregation of these two cell lines during gonadal development. There was also a significant difference in CAMs expression pattern between XY supporting (Sertoli) and XX supporting (follicular) cells but not between XY and XX germ cells. This indicates that differential CAMs expression pattern in the somatic cells but not in the germ line arbitrates structural organization of gonadal anlagen into testis or ovary.

Mechanisms controlling germline cyst breakdown and primordial follicle formation

In fetal females, oogonia proliferate immediately after sex determination. The progress of mitosis in oogonia proceeds so rapidly that the incompletely divided cytoplasm of the sister cells forms cysts. The oogonia will then initiate meiosis and arrest at the diplotene stage of meiosis I, becoming oocytes. Within each germline cyst, oocytes with Balbiani bodies will survive after cyst breakdown (CBD). After CBD, each oocyte is enclosed by pre-granulosa cells to form a primordial follicle (PF). Notably, the PF pool formed perinatally will be the sole lifelong oocyte source of a female. Thus, elucidating the mechanisms of CBD and PF formation is not only meaningful for solving mysteries related to ovarian development but also contributes to the preservation of reproduction. However, the mechanisms that regulate these phenomena are largely unknown. This review summarizes the progress of cellular and molecular research on these processes in mice and humans.

Extragonadal oocytes residing in the mouse ovarian hilum contribute to fertility

The observation of pups born from recipient and donor mice after ovariectomy followed by ovarian transplant poses the interesting possibility of an extraovarian source of oocytes. However, whether mammalian adult oocytes reside in extragonadal tissues remains elusive. Using transgenic fluorescent reporter mice and transplantation surgeries, we demonstrate the presence of both donor- and recipient-derived corpora lutea and recovery of both donor- and recipient-derived offspring from ovariectomized mice after transplantation of donor ovaries. A potential region for extraovarian oocytes is the hilum, a ligament-like structure between the ovary and the reproductive tract. Immunofluorescent confocal microscopy of mouse ovaries and reproductive tracts revealed that a population of primordial follicles resides outside the ovary within the hilum. Ovariectomy-only controls confirmed that oocytes remain in the recipient hilum after surgery. These results provide evidence that the hilum is a reserve source of follicles, which likely return to the ovary for maturation and ovulation. By identifying a new follicle reservoir, our study addresses a long-standing question in reproductive biology and contributes to new conceptual knowledge about ovarian function and fertility.

Finding their way: themes in germ cell migration

Embryonic germ cell migration is a vital component of the germline lifecycle. The translocation of germ cells from the place of origin to the developing somatic gonad involves several processes including passive movements with underlying tissues, transepithelial migration, cell adhesion dynamics, the establishment of environmental guidance cues and the ability to sustain directed migration. How germ cells accomplish these feats in established model organisms will be discussed in this review, with a focus on recent discoveries and themes conserved across species.

Msx1 and Msx2 function together in the regulation of primordial germ cell migration in the mouse

Primordial germ cells (PGCs) are a highly migratory cell population that gives rise to eggs and sperm. Much is known about PGC specification, but less about the processes that control PGC migration. In this study, we document a deficiency in PGC development in embryos carrying global homozygous null mutations in Msx1 and Msx2, both immediate downstream effectors of Bmp signaling pathway. We show that Msx1^−/−;Msx2^−/− mutant embryos have defects in PGC migration as well as a reduced number of PGCs. These phenotypes are also evident in a Mesp1-Cre-mediated mesoderm-specific mutant line of Msx1 and Msx2. Since PGCs are not marked in Mesp1-lineage tracing, our results suggest that Msx1 and Msx2 function cell non-autonomously in directing PGC migration. Consistent with this hypothesis, we noted an upregulation of fibronectin, well known as a mediator of cell migration, in tissues through which PGCs migrate. We also noted a reduction in the expression of Wnt5a and an increase in the expression in Bmp4 in such tissues in Msx1^−/−;Msx2^−/− mutants, both known effectors of PGC development.

Discrete somatic niches coordinate proliferation and migration of primordial germ cells via Wnt signaling

Inheritance depends on the expansion of a small number of primordial germ cells (PGCs) in the early embryo. Proliferation of mammalian PGCs is concurrent with their movement through changing microenvironments; however, mechanisms coordinating these conflicting processes remain unclear. Here, we find that PGC proliferation varies by location rather than embryonic age. Ror2 and Wnt5a mutants with mislocalized PGCs corroborate the microenvironmental regulation of the cell cycle, except in the hindgut, where Wnt5a is highly expressed. Molecular and genetic evidence suggests that Wnt5a acts via Ror2 to suppress β-catenin-dependent Wnt signaling in PGCs and limit their proliferation in specific locations, which we validate by overactivating β-catenin in PGCs. Our results suggest that the balance between expansion and movement of migratory PGCs is fine-tuned in different niches by the opposing β-catenin-dependent and Ror2-mediated pathways through Wnt5a This could serve as a selective mechanism to favor early and efficient migrators with clonal dominance in the ensuing germ cell pool while penalizing stragglers.

From primordial germ cells to primordial follicles: a review and visual representation of early ovarian development in mice

Background

Normal development of reproductive organs is crucial for successful reproduction. In mice the early ovarian developmental process occurs during the embryonic and postnatal period and is regulated through a series of molecular signaling events. Early ovarian development in mice is a seventeen-day process that begins with the rise of six primordial germ cells on embryonic day five (E5) and ends with the formation of primordial follicles on postnatal day two (P2).

Results

We reviewed the current literature and created a visual representation of early ovarian development that depicts the important molecular events and associated phenotypic outcomes based on primary data. The visual representation shows the timeline of key signaling interactions and regulation of protein expression in different cells involved in ovarian development. The major developmental events were divided into five phases: 1) origin of germ cells and maintenance of pluripotency; 2) primordial germ cell migration; 3) sex differentiation; 4) formation of germ cell nests; and 5) germ cell nest breakdown and primordial follicle formation.

Conclusions

This review and visual representation provide a summary of the current scientific understanding of the key regulation and signaling during ovarian development and highlights areas needing further study. The visual representation can be used as an educational resource to link molecular events with phenotypic outcomes; serves as a tool to generate new hypotheses and predictions of adverse reproductive outcomes due to perturbations at the molecular and cellular levels; and provides a comprehendible foundation for computational model development and hypothesis testing.

Methods for the Study of Gonadal Development

Current knowledge on gonadal development and sex determination is the product of many decades of research involving a variety of scientific methods from different biological disciplines such as histology, genetics, biochemistry, and molecular biology. The earliest embryological investigations, followed by the invention of microscopy and staining methods, were based on histological examinations. The most robust development of histological staining techniques occurred in the second half of the nineteenth century and resulted in structural descriptions of gonadogenesis. These first studies on gonadal development were conducted on domesticated animals; however, currently the mouse is the most extensively studied species. The next key point in the study of gonadogenesis was the advancement of methods allowing for the in vitro culture of fetal gonads. For instance, this led to the description of the origin of cell lines forming the gonads. Protein detection using antibodies and immunolabeling methods and the use of reporter genes were also invaluable for developmental studies, enabling the visualization of the formation of gonadal structure. Recently, genetic and molecular biology techniques, especially gene expression analysis, have revolutionized studies on gonadogenesis and have provided insight into the molecular mechanisms that govern this process. The successive invention of new methods is reflected in the progress of research on gonadal development.

FGF, Insulin, and SMAD Signaling Cooperate for Avian Primordial Germ Cell Self-Renewal

Precise self-renewal of the germ cell lineage is fundamental to fertility and reproductive success. The early precursors for the germ lineage, primordial germ cells (PGCs), survive and proliferate in several embryonic locations during their migration to the embryonic gonad. By elucidating the active signaling pathways in migratory PGCs in vivo, we were able to create culture conditions that recapitulate this embryonic germ cell environment. In defined medium conditions without feeder cells, the growth factors FGF2, insulin, and Activin A, signaling through their cognate-signaling pathways, were sufficient for self-renewal of germline-competent PGCs. Forced expression of constitutively active MEK1, AKT, and SMAD3 proteins could replace their respective upstream growth factors. Unexpectedly, we found that BMP4 could replace Activin A in non-clonal growth conditions. These defined medium conditions identify the key molecular pathways required for PGC self-renewal and will facilitate efforts in biobanking of chicken genetic resources and genome editing.

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Avian primordial germ cell self-renewal is dependent on FGF2, insulin, and Activin A molecules

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BMP4 can replace Activin A in non-clonal growth conditions

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Defined culture medium conditions will facilitate studies of germ cell self-renewal in other vertebrate species

Primordial Germ Cells: Current Knowledge and Perspectives

Infertility is a condition that occurs very frequently and understanding what defines normal fertility is crucial to helping patients. Causes of infertility are numerous and the treatment often does not lead to desired pregnancy especially when there is a lack of functional gametes. In humans, the primordial germ cell (PGC) is the primary undifferentiated stem cell type that will differentiate towards gametes: spermatozoa or oocytes. With the development of stem cell biology and differentiation protocols, PGC can be obtained from pluripotent stem cells providing a new therapeutic possibility to treat infertile couples. Recent studies demonstrated that viable mouse pups could be obtained from in vitro differentiated stem cells suggesting that translation of these results to human is closer. Therefore, the aim of this review is to summarize current knowledge about PGC indicating the perspective of their use in both research and medical application for the treatment of infertility.

Heparan sulfate glycosaminoglycans modulate migration and survival in zebrafish primordial germ cells

Early in embryonic development, primordial germ cells (PGCs) are specified and migrate from the site of their origin to where the gonad develops, following a specific route. Heparan sulfate glycosaminoglycans (HS-GAGs) are ubiquitous in extracellular matrix and the cell surface and have long been speculated to play a role during the migration of PGCs. In line with this speculation, whole-mount immunohistochemistry revealed the existence of HS-GAGs in the vicinity of migrating PGCs in early zebrafish embryos. To examine the roles of HS-GAGs during PGC migration, zebrafish heparanase 1 (hpse1), which degrades HS-GAGs, was cloned and overexpressed specifically in PGCs. The guidance signal for the migration of PGCs was disrupted with the overexpression of hpse1, as cluster formation and marginal localization at the blastoderm were significantly perturbed at 6 hours postfertilization. Furthermore, the number of PGCs was significantly decreased with the lack of vicinal HS-GAGs, as observed in the whole-mount in situ hybridization and quantitative PCR of the PGC marker gene vasa. Terminal deoxynucleotidyl transferase dUTP nick-end labeling indicated significantly increased apoptosis in PGCs overexpressing hpse1, suggesting that HS-GAGs contribute to the maintenance of PGC survival. In conclusion, HS-GAGs play multifaceted roles in PGCs during migration and are required both for guidance signals and multiplication of PGCs.

Spermatogonial stem cell self-renewal and development

Spermatogenesis originates from spermatogonial stem cells (SSCs). Development of the spermatogonial transplantation technique in 1994 provided the first functional assay to characterize SSCs. In 2000, glial cell line-derived neurotrophic factor was identified as a SSC self-renewal factor. This discovery not only provided a clue to understand SSC self-renewing mechanisms but also made it possible to derive germline stem (GS) cell cultures in 2003. In vitro culture of GS cells demonstrated their potential pluripotency and their utility in germline modification. However, in vivo SSC analyses have challenged the traditional concept of SSC self-renewal and have revealed their relationship with the microenvironment. An improved understanding of SSC self-renewal through functional assays promises to uncover fundamental principles of stem cell biology and will enable us to use these cells for applications in animal transgenesis and medicine.

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.

Molecular characteristics of malignant ovarian germ cell tumors and comparison with testicular counterparts: implications for pathogenesis

This review focuses on the molecular characteristics and development of rare malignant ovarian germ cell tumors (mOGCTs). We provide an overview of the genomic aberrations assessed by ploidy, cytogenetic banding, and comparative genomic hybridization. We summarize and discuss the transcriptome profiles of mRNA and microRNA (miRNA), and biomarkers (DNA methylation, gene mutation, individual protein expression) for each mOGCT histological subtype. Parallels between the origin of mOGCT and their male counterpart testicular GCT (TGCT) are discussed from the perspective of germ cell development, endocrinological influences, and pathogenesis, as is the GCT origin in patients with disorders of sex development. Integrated molecular profiles of the 3 main histological subtypes, dysgerminoma (DG), yolk sac tumor (YST), and immature teratoma (IT), are presented. DGs show genomic aberrations comparable to TGCT. In contrast, the genome profiles of YST and IT are different both from each other and from DG/TGCT. Differences between DG and YST are underlined by their miRNA/mRNA expression patterns, suggesting preferential involvement of the WNT/β-catenin and TGF-β/bone morphogenetic protein signaling pathways among YSTs. Characteristic protein expression patterns are observed in DG, YST and IT. We propose that mOGCT develop through different developmental pathways, including one that is likely shared with TGCT and involves insufficient sexual differentiation of the germ cell niche. The molecular features of the mOGCTs underline their similarity to pluripotent precursor cells (primordial germ cells, PGCs) and other stem cells. This similarity combined with the process of ovary development, explain why mOGCTs present so early in life, and with greater histological complexity, than most somatic solid tumors.

Mouse primordial germ cells produce cysts that partially fragment prior to meiosis

Mammalian germ cells divide mitotically and form nests of associated cells just prior to entering meiosis. At least some nests contain germline cysts that arise by synchronous, incomplete mitotic divisions, but others may form by aggregation. To systematically investigate early murine germ cell development, we lineage marked the progeny of individual, newly arrived primordial germ cells in the E10.5 gonad. All the marked germ cells initially develop into clones containing two, four or eight cells, indicating cyst formation. Surprisingly, growing cysts in both sexes partially fragment into smaller cysts prior to completion and associate with cysts from unrelated progenitors. At the time divisions cease, female clones comprise five cysts on average that eventually give rise to about six primordial follicles. Male cyst cells break apart and probably become spermatogonial stem cells. Thus, cysts are invariant units of mouse germ cell development and cyst fragmentation provides insight into the amplification of spermatogonial stem cells and the origin of primordial follicles.

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.

Developmental regulation of locomotive activity in Xenopus primordial germ cells

Primordial germ cells (PGCs) arise in the early embryo and migrate toward the future gonad through species-specific pathways. They are assumed to change their migration properties dependent on their own genetic program and/or environmental cues, though information concerning the developmental change in PGC motility is limited. First, we re-examined the distribution of PGCs in the endodermal region of Xenopus embryos at various stages by using an antibody against Xenopus Daz-like protein, and found four stages of migration, namely clustering, dispersing, directionally migrating and re-aggregating. Next, we isolated living PGCs at each stage and directly examined their morphology and locomotive activity in cell cultures. PGCs at the clustering stage were round in shape with small blebs and showed little motility. PGCs in both the dispersing and the directionally migrating stages alternated between the locomotive phase with an elongated morphology and the pausing phase with a rugged morphology. The locomotive activity of the elongated PGCs was accompanied by the persistent formation of a large bleb at the leading front. The duration of the locomotive phase was shortened gradually with the transition from the dispersing stage to the directionally migrating stage. At the re-aggregating stage, PGCs became round in shape and showed no motility. Thus, we directly showed that the locomotive activity of PGCs changes dynamically depending upon the migrating stage. We also showed that the locomotion and blebbing of the PGCs required F-actin, myosin II activity and RhoA/Rho-associated protein kinase (ROCK) signaling.

Membrane β-catenin and adherens junctions in early gonadal patterning

BACKGROUND

Mechanisms involved in early patterning of the mammalian gonad as it develops from a bipotential state into a testis or an ovary are as yet not well understood. Sex-specific vascularization is essential in this process, but more specific mechanisms required to, for example, establish interstitial vs. cord compartments in the testis or ovigerous cords in the ovary have not been reported. Adherens junctions (AJs) are known for their roles in morphogenesis; we, therefore, examined expression of AJ components including β-catenin, p120 catenin, and cadherins for possible involvement in sex-specific patterning of the gonad.

RESULTS

We show that, at the time of early gonadal sex differentiation, membrane-associated β-catenin and p120 catenin colocalize with cell-specific cadherins in both sex-nonspecific and sex-specific patterns. These expression patterns are consistent with an influence of AJs in overall patterning of the testis vs. ovary through known AJ mechanisms of cell-cell adhesion, cell sorting, and boundary formation.

CONCLUSIONS

Together these complex and dynamic patterns of AJ component expression precisely mirror patterning of tissues during gonadogenesis and raise the possibility that AJs are essential effectors of patterning within the developing testis and ovary.

The molecular genetics of ovarian differentiation in the avian model

In birds as in mammals, sex is determined at fertilization by the inheritance of sex chromosomes. However, sexual differentiation - development of a male or female phenotype - occurs during embryonic development. Sex differentiation requires the induction of sex-specific developmental pathways in the gonads, resulting in the formation of ovaries or testes. Birds utilize a different sex chromosome system to that of mammals, where females are the heterogametic sex (carrying Z and W chromosomes), while males are homogametic (carrying 2 Z chromosomes). Therefore, while some genes essential for testis and ovarian development are conserved, important differences also exist. Namely, the key mammalian male-determining factor SRY does not exist in birds, and another transcription factor, DMRT1, plays a central role in testis development. In contrast to our understanding of testis development, ovarian differentiation is less well-characterized. Given the presence of a female-specific chromosome, studies in chicken will provide insight into the induction and function of female-specific gonadal pathways. In this review, we discuss sexual differentiation in chicken embryos, with emphasis on ovarian development. We highlight genes that may play a conserved role in this process, and discuss how interaction between ovarian pathways may be regulated.

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.

Mouse germ cell clusters form by aggregation as well as clonal divisions

After their arrival in the fetal gonad, mammalian germ cells express E-cadherin and are found in large clusters, similar to germ cell cysts in Drosophila. In Drosophila, germ cells in cysts are connected by ring canals. Several molecular components of intercellular bridges in mammalian cells have been identified, including TEX14, a protein required for the stabilization of intercellular bridges, and several associated proteins that are components of the cytokinesis complex. This has led to the hypothesis that germ cell clusters in the mammalian gonad arise through incomplete cell divisions. We tested this hypothesis by generating chimeras between GFP-positive and GFP-negative mice. We show that germ cell clusters in the fetal gonad arise through aggregation as well as cell division. Intercellular bridges, however, are likely restricted to cells of the same genotype.

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.