Ultrastructure of germ cell development in the human fetal testis

The fate of germ cells in cryptorchid testis

Cryptorchidism in males constitutes a notable risk factor for both infertility and testicular cancer. Infertility in adulthood is closely linked to the germ cell status in childhood. Furthermore, the significance of germ cell status is important as more than 95% of all reported testicular malignancies are germ cell tumors. The review aims to elucidate the pathogenesis of germ cells in cryptorchid testes concerning their association with infertility and testicular malignancies. Impaired germ cell numbers are evident in cryptorchid testes even during antenatal and neonatal stages. In cryptorchidism there is a rapid decline in germ cell number within the first year of life, partially attributed to physiologic gonocyte apoptosis. Additionally, germ cells fail to differentiate normally during mini-puberty leading to reduced germ cell proliferation and delayed clearance of gonocytes from the seminiferous epithelium. Absence of germ cells in testicular biopsies occurs already 10 months of age and germ cell deterioration progressively worsens with approximately 50% of persisting cryptorchid testes lacking germ cells during puberty. The deficient germ cell maturation and proliferation leads to later infertility. Elevated temperature in the cryptorchid testes and also hormonal deficiency contribute to this phenomenon. Germ cell neoplasia in situ (GCNIS) originating during fetal development may manifest in rare cases associated with disorders of sexual development, chromosomal abnormalities in boys, specific syndromes, and teratomas that include cryptorchidism. In adults, the presence of GCNIS predominantly represents a new histology pattern before invasive germ cell cancer is demonstrated and is neither congenital nor related to abnormal gonocyte transformation.

Making a good egg: human oocyte health, aging, and in vitro development

Mammalian eggs (oocytes) are formed during fetal life and establish associations with somatic cells to form primordial follicles that create a store of germ cells (the primordial pool). The size of this pool is influenced by key events during the formation of germ cells and by factors that influence the subsequent activation of follicle growth. These regulatory pathways must ensure that the reserve of oocytes within primordial follicles in humans lasts for up to 50 years, yet only approximately 0.1% will ever be ovulated with the rest undergoing degeneration. This review outlines the mechanisms and regulatory pathways that govern the processes of oocyte and follicle formation and later growth, within the ovarian stroma, through to ovulation with particular reference to human oocytes/follicles. In addition, the effects of aging on female reproductive capacity through changes in oocyte number and quality are emphasized, with both the cellular mechanisms and clinical implications discussed. Finally, the details of current developments in culture systems that support all stages of follicle growth to generate mature oocytes in vitro and emerging prospects for making new oocytes from stem cells are outlined.

Comparing the adult and pre-pubertal testis: Metabolic transitions and the change in the spermatogonial stem cell metabolic microenvironment

BACKGROUND

Survivors of childhood cancer often suffer from infertility. While sperm cryopreservation is not feasible before puberty, the patient's own spermatogonial stem cells could serve as a germ cell reservoir, enabling these patients to father their own children in adulthood through the isolation, in vitro expansion, and subsequent transplantation of spermatogonial stem cells. However, this approach requires large numbers of stem cells, and methods for successfully propagating spermatogonial stem cells in the laboratory are yet to be established for higher mammals and humans. The improvement of spermatogonial stem cell culture requires deeper understanding of their metabolic requirements and the mechanisms that regulate metabolic homeostasis.

AIM

This review gives a summary on our knowledge of spermatogonial stem cell metabolism during maintenance and differentiation and highlights the potential influence of Sertoli cell and stem cell niche maturation on spermatogonial stem cell metabolic requirements during development.

RESULTS AND CONCLUSIONS

Fetal human spermatogonial stem cell precursors, or gonocytes, migrate into the seminiferous cords and supposedly mature to adult stem cells within the first year of human development. However, the spermatogonial stem cell niche does not fully differentiate until puberty, when Sertoli cells dramatically rearrange the architecture and microenvironment within the seminiferous epithelium. Consequently, pre-pubertal and adult spermatogonial stem cells experience two distinct niche environments potentially affecting spermatogonial stem cell metabolism and maturation. Indeed, the metabolic requirements of mouse primordial germ cells and pig gonocytes are distinct from their adult counterparts, and novel single-cell RNA sequencing analysis of human and porcine spermatogonial stem cells during development confirms this metabolic transition. Knowledge of the metabolic requirements and their changes and regulation during spermatogonial stem cell maturation is necessary to implement laboratory-based techniques and enable clinical use of spermatogonial stem cells. Based on the advancement in our understanding of germline metabolism circuits and maturation events of niche cells within the testis, we propose a new definition of spermatogonial stem cell maturation and its amendment in the light of metabolic change.

Diverse Roles and Targets of miRNA in the Pathogenesis of Testicular Germ Cell Tumour

Testicular germ cell tumour (TGCT) is the most common cancer type among young adults in many parts of the world. Although the pathogenesis of TGCT is not well understood, the involvement of heritable components is evident, and the risk is polygenic. Genome-wide association studies have so far found 78 susceptibility loci for TGCT, and many of the loci are in non-coding regions indicating the involvement of non-coding RNAs in TGCT pathogenesis. MicroRNAs (miRNAs), a class of non-coding RNAs, have emerged as important gene regulators at the post-transcriptional level. They are crucial in controlling many cellular processes, such as proliferation, differentiation, and apoptosis, and an aberrant miRNA expression may contribute to the pathogenesis of several cancers, including TGCT. In support of this notion, several studies reported differential expression of miRNAs in TGCTs. We previously demonstrated that miRNAs were the most common group of small non-coding RNAs in TGCTs, and several functional studies of miRNAs in TGCTs suggest that they may act as either oncogene or tumour suppressors. Moreover, individual miRNA targets and downstream pathways in the context of TGCT development have been explored. In this review, we will focus on the diverse roles and targets of miRNAs in TGCT pathogenesis.

Live-cell imaging and ultrastructural analysis reveal remarkable features of cultured porcine gonocytes

Gonocytes in the neonatal testis have male germline stem cell potential. The objective of the present study was to examine the behavior and ultrastructure of gonocytes in culture. Neonatal porcine testis cells were cultured for 4 weeks and underwent live-cell imaging to explore real-time interactions among cultured cells. This included imaging every 1 h from day 0 to day 3, every 2 h from day 4 to day 7, and every 1 h for 24 h at days 14, 21, and 28. Samples also underwent scanning electron microscopy, transmission electron microscopy, morphometric evaluations, immunofluorescence, and RT-PCR. Live-cell imaging revealed an active amoeboid-like movement of gonocytes, assisted by the formation of extensive cytoplasmic projections, which, using scanning electron microscopy, were categorized into spike-like filopodia, leaf-like lamellipodia, membrane ruffles, and cytoplasmic blebs. In the first week of culture, gonocytes formed loose attachments on top of a somatic cell monolayer and, in week 2, formed grape-like clusters, which, over time, grew in cell number. Starting at week 3 of culture, some of the gonocyte clusters transformed into large multinucleated embryoid body-like colonies (EBLCs) that expressed both gonocyte- and pluripotent-specific markers. The number and diameter of individual gonocytes, the number and density of organelles within gonocytes, as well as the number and diameter of the EBLCs increased over time (P < 0.05). In conclusion, cultured porcine gonocytes displayed extensive migratory behavior facilitated by their various cytoplasmic projections, propagated, and transformed into EBLCs that increased in size and complexity over time.

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.

On the Origin of Testicular Germ Cell Tumors: From Gonocytes to Testicular Cancer

Human primordial germ cells (PGCs) have been described in the yolk sac wall around the beginning of the third week. From week 4 to 5, they migrate under control of SCF/c-KIT signaling pathway to the genital ridge, where they become gonocytes. PGCs and gonocytes express classic pluripotency markers, such as KIT, NANOG, and OCT3/4 that, during spermatogonia differentiation, are gradually suppressed, and substituted by the expression of some germ cell specific genes, such as VASA, SOX17, and TSPY. These genes, during normal development of germ cells, are tightly regulated by epigenetic modification, in terms of microRNA expression and DNA methylation. In adolescents and young adults, testicular germ cell tumors (TGCT) have a common precursor, the germ cell neoplasia in situ (GCNIS); the hypothesis of their origin from PGCs or gonocytes, whose maturation is altered, is widely accepted. The origin of TGCT, probably starting at early stages of embryogenesis, seems to be a part of the Testicular Dysgenesis Syndrome (TDS) where some early PGC/gonocytes, for still unclear reasons, are blocked in their differentiation, retaining their early marker profile. In this paper, current knowledge on the combination of epidemiological and genomic factors, involved in the development of testicular germ cell tumors, is reviewed.

Di-n-Butyl Phthalate Induces Multinucleated Germ Cells in the Rat Fetal Testis Through a Nonproliferative Mechanism

In utero exposure to some phthalate esters adversely affects the development of the rat seminiferous cord, causing germ cell loss and increasing the number of multinucleated germ cells (MNGs). To understand the timing of MNG formation and determine whether it requires nuclear division, timed pregnant Sprague Dawley rats were exposed to 500 mg/kg di-n-butyl phthalate (DBP) or corn oil vehicle by oral gavage on Gestational Day (GD) 17 or 18 (0 h) and euthanized after 2, 4, 6, or 24 h or given a second dose at 24 h and euthanized 48 h after the initial dose. Dams were simultaneously exposed to 0.3 M 5-bromo-2'-deoxycitidine (BrdC; converted to 5-bromo-2'-deoxyuridylate [BrdU] in vivo) through a subcutaneous micro-osmotic pump implanted at -2 h. In the testes of male fetuses, DBP induced MNGs significantly beginning at 4-6 h and dramatically by 24 h when exposure began on GD 18 but not GD 17. Seminiferous cord diameter was significantly elevated in testes of rats treated with DBP at 24 and 48 h, and cell death, measured by TUNEL assay, was significantly elevated by DBP only at 48 h, when treatment began on GD 18. TUNEL-labeled MNGs were rare. Overall BrdU labeling rate in the testis was unaffected by DBP. Only one of 606 MNGs in BrdU-labeled sections had a strongly positive nucleus, confirming a nonproliferative mechanism of MNG formation, which is a degenerative process with the potential to adversely affect testis development.

Male germ cell development in humans

BACKGROUND

Germ cells are unique cells that possess the ability to transmit genetic information between generations. Detailed knowledge about the molecular and cellular mechanisms determining the fate of human male germ cells still remains sparse. This is partially due to ethical issues limiting the access to research material. Therefore, the mechanisms of proliferation, differentiation and apoptosis of human male germ cells still remain challenging study objectives.

METHODS

This review focuses on using English articles accessible in PubMed as well as personal files on the current knowledge of the molecular and cellular mechanisms connected with human testicular germ cell development, maturation failure and the possibility of fertility preservation in patients in whom there is a risk of gonadal failure. However, since rodents, particularly mice, offer the possibility of studying germ cell development by use of genetic modification techniques, some studies using animal models are also discussed.

CONCLUSION

This mini review focuses on the current knowledge about male germ cells. However, the reader is referred to two previous mini reviews focusing on testicular somatic cells, i.e. on Sertoli cells and Leydig cells.

Squash Preparation Studies of Germ Cells in Human Fetal Testes

Testes from human fetuses 21 to 41 weeks of gestation were investigated using a modified squash technique. At all of the gestational ages examined, 90 to 95% of the germ cells were in interphase. Preleptotene germ cells, which exhibited densely staining condensed clumps of chromosomes, were found in all samples studied and constituted 3 to 9% of the germ cells observed. In appearance they resembled preleptotene figures seen in the fetal ovary. Occasionally, germ cells containing slender chromosomes with a beaded appearance, but without prominent condensation, were encountered. These cells were interpreted as preleptotene germ cells in the early stages of degeneration. Germ cells in other meiotic stages were not observed in this study.

Germ cell intercellular bridges

Stable intercellular bridges are a conserved feature of gametogenesis in multicellular animals observed more than 100 years ago, but their function was unknown. Many of the components necessary for this structure have been identified through the study of cytokinesis in Drosophila; however, mammalian intercellular bridges have distinct properties from those of insects. Mammalian germ cell intercellular bridges are composed of general cytokinesis components with additional germ cell-specific factors including TEX14. TEX14 is an inactive kinase essential for the maintenance of stable intercellular bridges in gametes of both sexes but whose loss specifically impairs male meiosis. TEX14 acts to impede the terminal steps of abscission by competing for essential component CEP55, blocking its interaction in nongerm cells with ALIX and TSG101. Additionally, TEX14-interacting protein RBM44, whose localization in stabile intercellular bridges is limited to pachytene and secondary spermatocytes, may participate in processes such as RNA transport but is nonessential to the maintenance of intercellular bridge stability.

Structure and functions of stable intercellular bridges formed by incomplete cytokinesis during development

Cytokinesis, the final step of cell division, normally proceeds to completion in living organisms, so that daughter cells physically separate by abscission. In certain tissues and developmental stages, on the other hand, the cytokinesis process is incomplete, giving rise to cells interconnected in syncytia by stable intercellular bridges. This evolutionarily conserved physiological process occurs in the female and male germline in species ranging from insects to humans, and has also been observed in some somatic tissues in invertebrates. Stable intercellular bridges have fascinated cell biologists ever since they were first described more than 50 years ago, and even though substantial progress has been made concerning their ultrastructure and molecular composition, much remains to be understood about their biological functions. Another major question is by which mechanisms complete versus incomplete cytokinesis is determined. In this mini-review we will try to give an overview of the current knowledge about the structure, composition and functions of stable intercellular bridges, and discuss recent insights into the molecular control of the incomplete cytokinesis process.

Expression of P-450 aromatase, estrogen receptor α and β, and α-inhibin in the fetal baboon testis after estrogen suppression during the second half of gestation

Expression of the molecules that modulate the synthesis and action of estrogen in, or reflect function of, Sertoli cells was determined in the fetal testis of baboons in which estrogen levels were suppressed in the second half of gestation to determine whether this may account for the previously reported alteration in fetal testis germ cell development. P-450 aromatase, estrogen receptor (ER) β, and α-inhibin protein assessed by immunocytochemistry was abundantly expressed in Sertoli cells of the fetal baboon testis, but unaltered in baboons in which estrogen levels were suppressed by letrozole administration. Moreover, P-450 aromatase and ERα and β mRNA levels, assessed by real-time RT-PCR, were similar in germ/Sertoli cells and interstitial cells isolated from the fetal testis of untreated and letrozole-treated baboons. These results indicate that expression of the proteins that modulate the formation and action of estrogen in, and function of, Sertoli cells is not responsible for the changes in germ cell development in the fetal testis of estrogen-deprived baboons.

Testicular germline stem cells

Stem cells have the ability to both differentiate into other mature cell types and maintain an undifferentiated state by self-renewal. These unique properties form the basis for stem cell use in organ replacement and tissue regeneration in clinical medicine. Currently, embryonic stem cells are the best-studied stem cell type. However alternative stem cells such as induced pluripotent stem cells and other adult stem cells are also being actively investigated for their potential for cell-based therapy. Among adult stem cells, emerging research has focused on evaluating the pluripotency potential of testis stem cells. To date, stem cells with embryonic-like potential have been created from adult testis germ cells. These cells could provide patient-specific, non-embryo-derived stem cells for men in the future.

National Academy of Clinical Biochemistry Laboratory Medicine Practice Guidelines for Use of Tumor Markers in Testicular, Prostate, Colorectal, Breast, and Ovarian Cancers

Background: Updated National Academy of Clinical Biochemistry (NACB) Laboratory Medicine Practice Guidelines for the use of tumor markers in the clinic have been developed.

Methods: Published reports relevant to use of tumor markers for 5 cancer sites—testicular, prostate, colorectal, breast, and ovarian—were critically reviewed.

Results: For testicular cancer, α-fetoprotein, human chorionic gonadotropin, and lactate dehydrogenase are recommended for diagnosis/case finding, staging, prognosis determination, recurrence detection, and therapy monitoring. α-Fetoprotein is also recommended for differential diagnosis of nonseminomatous and seminomatous germ cell tumors. Prostate-specific antigen (PSA) is not recommended for prostate cancer screening, but may be used for detecting disease recurrence and monitoring therapy. Free PSA measurement data are useful for distinguishing malignant from benign prostatic disease when total PSA is &lt;10 μg/L. In colorectal cancer, carcinoembryonic antigen is recommended (with some caveats) for prognosis determination, postoperative surveillance, and therapy monitoring in advanced disease. Fecal occult blood testing may be used for screening asymptomatic adults 50 years or older. For breast cancer, estrogen and progesterone receptors are mandatory for predicting response to hormone therapy, human epidermal growth factor receptor-2 measurement is mandatory for predicting response to trastuzumab, and urokinase plasminogen activator/plasminogen activator inhibitor 1 may be used for determining prognosis in lymph node–negative patients. CA15-3/BR27–29 or carcinoembryonic antigen may be used for therapy monitoring in advanced disease. CA125 is recommended (with transvaginal ultrasound) for early detection of ovarian cancer in women at high risk for this disease. CA125 is also recommended for differential diagnosis of suspicious pelvic masses in postmenopausal women, as well as for detection of recurrence, monitoring of therapy, and determination of prognosis in women with ovarian cancer.

Conclusions: Implementation of these recommendations should encourage optimal use of tumor markers.

Mouse TEX14 is required for embryonic germ cell intercellular bridges but not female fertility

A conserved feature of germ cell cytokinesis is the formation of stable intercellular bridges between daughter cells. These intercellular bridges are seen in diverse species from Drosophila melanogaster to Homo sapiens and have been shown to have roles in communication of large numbers of germ cells. In testis expressed gene 14 (Tex14) knockout mice, intercellular bridges do not form during spermatogenesis, and male mice are sterile, demonstrating an essential role for intercellular bridges in postnatal spermatogenesis in mammals. Intercellular bridges also form between dividing germ cells in both male and female embryos. However, little is known about the formation or role of the embryonic intercellular bridges in mammals. In females, embryonic intercellular bridges have been proposed to have a role in development of the presumptive oocyte. Herein, we show that TEX14 is an essential component of male and female embryonic intercellular bridges. In addition, we demonstrate that mitotic kinesin-like protein 1 (MKLP1, official symbol KIF23), which we have discovered is a component of intercellular bridges during spermatogenesis, is also a component of male and female embryonic intercellular bridges. Germ cell intercellular bridges are readily identified by KIF23 immunofluorescence between the gonocytes and oogonia of control mice but are absent between germ cells of Tex14-null mice. Furthermore, by electron microscopy, intercellular bridges are present in all control newborn ovaries but are absent in the Tex14 knockout ovaries. Despite the absence of embryonic intercellular bridges in the Tex14-null mice, male mice initiate spermatogenesis, and female mice are fertile. Although fewer oocytes were present in Tex14-null neonatal ovaries, folliculogenesis was still active at 1 yr of age. Thus, while TEX14 and intercellular bridges have an essential role in postnatal spermatogenesis, they are not required in the embryo.

Historical survey on chromatoid body research

The chromatoid body (CB) is a male reproductive cell-specific organelle that appears in spermatocytes and spermatids. The cytoplasmic granule corresponding to the CB was first discovered some 130 years ago by von Brunn in 1876. Thirty years later the German term "chromatoide Körper" (chromatoid body) was introduced to describe this granule and is still used today. In this review, first, the results obtained by light microscopic studies on the CB for the first 60 years are examined. Next, many findings revealed by electron microscopic studies are reviewed. Finally, recent molecular cell biological studies concerning the CB are discussed. The conclusion obtained by exploring the papers on CB published during the past 130 years is that many of the modern molecular cell biological studies are undoubtedly based on information accumulated by vast amounts of early studies.

Expression of the glucocorticoid receptor and 11 beta-hydroxysteroid dehydrogenase 2 in pig testes cells along fetal development

The glucocorticoid (GC)-cortisol receptor (GCR)-11 beta-hydroxysteroid dehydrogenase 2 (11 beta-HSD2) system is involved in the regulation of Leydig cell function and spermatogenesis in mature animals. Herein, we describe the expression of the GCR and 11 beta-HSD2 and the occurrence of apoptosis during fetal development. Male fetuses were collected from Weeks 6, 10, 13, and 15 of pregnancy and from neonates. The testes were used for the immunocytochemical staining of GCR, 11 beta-HSD2 and for terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick end-labelling (TUNEL) staining of apoptosis. Apoptosis did not occur in any Leydig cells, but approximately 30% expressed GCR and 11 beta-HSD2. The number of GCR-positive cells was similar at all stages, but the number of 11 beta-HSD2-positive cells tended to be higher at Weeks 6 and 15. Steroid synthesis was also higher compared with Weeks 10 and 13. Apoptosis occurred in only a few germ cells. Nearly all germ cells were GCR positive at Weeks 10 and 13, when 11 beta-HSD2 was also increased. The total number of 11 beta-HSD2-positive germ cells was approximately 30%. Thus, elevated GCR expression coincided with the differentiation of gonocytes to spermatogonia and their migration to the basal lamina.

Fibroblast-like cells derived from the gonadal ridges and dorsal mesenteries of human embryos as feeder cells for the culture of human embryonic germ cells

The establishment of optimal hEG culture systems is a major challenge for the field of embryonic germ cell research. It is important to find appropriate feeder cells to support the growth of hEG. The clinical application of human embryonic germ cells cultured on mouse-derived feeder cells is restricted, since human embryonic germ cells cultured on mouse-derived feeder cells are at risk of contamination by heterogeneous proteins or pathogens. In order to avoid this limitation, we have isolated and cultured three human embryonic fibroblast-like cell lines derived from the gonadal ridges and dorsal mesenteries of 5- to 10-week old embryos. These cells expressed basic fibroblast growth factor and leukemia inhibitory factor, both essential for the growth of human embryonic germ cells. We then used the mitomycin-inactivited human embryonic fibroblast-like cells as feeder cells to culture human embryonic germ cells derived from the gonadal ridges and dorsal mesenteries of 5- to 10-week old embryos. Of 21 human primordial germ cell cultures initiated, seven were continuously grown and split for 10 passages with normal and stable human karyotypes. These cells expressed markers characteristic of pluripotent stem cells, including alkaline phosphatase, stage-specific embryonic antigens (SSEA)-1, SSEA-3, SSEA-4, tumor related antigens (TRA)-1-60, TRA-1-81, and the POU transcription factor Octamer-4 (Oct-4). Moreover, the cells possessed the capacity to differentiate into all three primary germ layers (ectoderm, mesoderm, and endoderm). Therefore, we have successfully used the human embryonic fibroblast-like cells derived from gonadal ridges and dorsal mesenteries as feeder cells to culture proliferative, undifferentiated and pluripotent human embryonic germ cells.

Sex-specific windows for high mRNA expression of DNA methyltransferases 1 and 3A and methyl-CpG-binding domain proteins 2 and 4 in human fetal gonads

DNA methyltransferases (DNMTs) and 5-methyl-CpG-binding domain proteins (MBDs) are involved in the acquisition of parent-specific epigenetic modifications in human male and female germ cells. Reverse Northern blot analyses demonstrated sex-specific differences in mRNA expression for the maintenance DNMT1 and the de novo DNMT3A in developing testis and ovary. In fetal testis DNMT1 and DNMT3A expression peaked in mitotically arrested spermatogonia around 21 weeks gestation. In fetal ovary transcriptional upregulation of DNMT1 and DNMT3A occurred during a very brief period at 16 weeks gestation, when the oocytes proceeded through meiotic prophase. Fetal gonads showed several fold higher DNMT3A expression levels than fetal brain and adult tissues. The most abundant DNMT3A isoform in fetal testis and ovary was DNMT3A2, whereas in all other analyzed tissues DNMT3A1 predominated. The catalytically inactive DNMT3A3 isoform was also present at relatively high levels in developing gonads and may perform a regulatory function(s). In both male and female fetal gonads expression of genes for MBD2 and MBD4, which may be implicated in chromatin remodeling of methylated genomic DNA sequences, was tightly linked to DNMT expression. We propose that the sex-specific time windows for concomitant upregulation of DNMT1, DNMT3A, MBD2, and MBD4 are associated with prenatal remethylation of the human male and female germ line.

The human X chromosome is enriched for germline genes expressed in premeiotic germ cells of both sexes

The role of X-chromosomal genes in spermatogenesis has been subject to a number of studies in different organisms. Recently, it was proposed that the X chromosome has a predominant role in premeiotic stages of mammalian spermatogenesis. We analyzed the expression of a representative set of 17 X-linked and 48 autosomal germline-restricted genes in different stages of human germ cell development. In accordance with data from other species, we show that the human X chromosome is indeed significantly enriched for genes activated in premeiotic stages of spermatogenesis. In contrast to recent studies, however, we found that expression of these genes is not restricted to spermatogenesis, but is activated in oogenesis as well. Furthermore, we show that activation of this subset of genes merely depends on demethylation of their promoter regions. Moreover, our data suggest that genes activated in premeiotic stages of gametogenesis are sex-indifferent and are regulated by DNA methylation. Gene activation patterns involved in spermatocyte-specific differentiation, in contrast, appear to be initiated not before entry into meiosis and underlie a more complex regulation, presumably involving specific transcription factors and/or chromatin remodeling mechanisms.

Genetic analysis of chromosome pairing, recombination, and cell cycle control during first meiotic prophase in mammals

Meiosis is a double-division process that is preceded by only one DNA replication event to produce haploid gametes. The defining event in meiosis is prophase I, during which chromosome pairs locate each other, become physically connected, and exchange genetic information. Although many aspects of this process have been elucidated in lower organisms, there has been scant information available until now about the process in mammals. Recent advances in genetic analysis, especially in mice and humans, have revealed many genes that play essential roles in meiosis in mammals. These include cell cycle-regulatory proteins that couple the exit from the premeiotic DNA synthesis to the progression through prophase I, the chromosome structural proteins involved in synapsis, and the repair and recombination proteins that process the recombination events. Failure to adequately repair the DNA damage caused by recombination triggers meiotic checkpoints that result in ablation of the germ cells by apoptosis. These analyses have revealed surprising sexual dimorphism in the requirements of different gene products and a much less stringent checkpoint regulation in females. This may provide an explanation for the 10-fold increase in meiotic errors in females compared with males. This review provides a comprehensive analysis of the use of genetic manipulation, particularly in mice, but also of the analysis of mutations in humans, to elucidate the mechanisms that are required for traverse through prophase I.

Pluripotent Stem Cells from Germ Cells

To date, stem cells have been derived from three sources of germ cells. These include embryonic germ cells (EGCs), embryonal carcinoma cells (ECCs), and multipotent germ line stem cells (GSCs). EGCs are derived from primordial germ cells that arise in the late embryonic and early fetal period of development. ECCs are derived from adult testicular tumors whereas GSCs have been derived by culturing spermatogonial stem cells from mouse neonates and adults. For each of these lines, their pluripotency has been demonstrated by their ability to differentiate into cell types derived from the three germ layers in vitro and in vivo and in chimeric animals, including germ line transmission. These germ line-derived stem cells have been generated from many species including human, mice, porcine, and chicken albeit with only slight modifications. This chapter describes general considerations regarding critical aspects of their derivation compared with their counterpart, embryonic stem cells (ESCs). Detailed protocols for EGC derivation and maintenance from human and mouse primordial germ cells (PGCs) will be presented.

Involvement of adenomatous polyposis coli (APC) gene in testicular yolk sac tumor of infants

The pathogenesis of testicular yolk sac tumor (YST) of infants is still unclear. Infantile YSTs rarely show isochromosome 12p or aneuploidy, which are common in adult germ cell tumors. On the other hand, recent epigenetic studies suggest the involvement of some tumor suppressor genes, including the adenomatous polyposis coli (APC) gene. In the present study, we examined 10 infantile pure YSTs for mutation, allelic loss, promoter methylation, and protein expression status of the APC gene to evaluate whether the APC gene plays a significant role in the pathogenesis of infantile YSTs. Loss of heterozygosity at 5q21, where the APC gene is localized, was detected in at least 3 (30%) of the 9 YSTs examined. None of the 10 YSTs showed mutations. Promoter methylation was detected in 7 (70%) of the 10 YSTs; among 7 YSTs showing methylation, 3 YSTs also harbored loss of heterozygosity at 5q21. Immunohistochemically, 8 infantile YSTs did not express the APC protein, whereas 2 YSTs without showing APC methylation, as well as germ cells of normal infantile testes, expressed this protein in the cytoplasm. These data indicate that inactivation of the APC gene, by allelic loss and/or promoter methylation, is related to the occurrence of infantile YSTs.

Fetal gene transfer using lentiviral vectors and the potential for germ cell transduction in rhesus monkeys (Macaca mulatta)

Genetic modification of germ cells in somatic gene therapy protocols is a concern, particularly with fetal approaches. This study focused on the potential for germ cell gene transfer post-fetal gene delivery using a human immunodeficiency virus type 1 (HIV-1)-derived lentiviral vector pseudotyped with the vesicular stomatitis virus-glycoprotein (VSV-G). Rhesus monkey fetuses (n = 47) were administered vector supernatant (10(7) infectious particles per fetus) via the intraperitoneal (IP), intrapulmonary (Ipu), or intracardiac routes (Ica) under ultrasound guidance. Tissue harvests were performed near term or 3 months postnatal age, and genomic DNA obtained to analyze for vector sequences from collected sections of gonads and gonadal cells obtained by laser capture microdissection (germ cells, stroma, epithelium). Results indicated no evidence of germ cell gene transfer in fetuses or infants with Ipu or Ica routes of administration. However, evidence of the transgene (1.33 +/- 0.78 enhanced green fluorescent protein [EGFP] copies per copy epsilon-globin) was found in females, but not males, when using the IP administration approach (p < 0.05). The highest EGFP copies were detected on the surface epithelium (p < 0.05). The results of these studies suggest that the HIV-1-derived lentiviral vector pseudotyped with VSV-G may transduce a subpopulation of gonadal cells in female fetuses with IP administration, whereas no evidence of gene transfer was shown to occur in males or with organ-targeting approaches.

Frequent nonrandom activation of germ-line genes in human cancer

The growing class of cancer/germ-line genes is characterized by a unique expression pattern with transcription restricted to germ cells and cancer cells. It is not known which fraction of germ-line genes is ectopically activated in tumor cells and whether this fraction displays common features as compared with strictly germ-line genes remaining silent in cancer. Using an unbiased genome-wide scanning approach, representative samples of both cancer/germ-line genes as well as strictly germ-line-specific genes were determined. Comparative analysis disclosed highly significant diametric characteristics for these two categories of genes with regard to sex specificity, developmental stage of physiological expression during gametogenesis, chromosomal localization, and epigenetic regulation of expression. Our findings provide class predictors for germ cell-specific gene activation in cancer. The identification of highly congruent expression patterns in cancer and in DNA methyltransferase-deficient cells suggests an underlying common epigenetic mechanism for activation of germ-line genes in cancer.

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.

Intercellular bridges between germ cells in the immature golden hamster testis: evidence for clonal and non-clonal mode of proliferation

Intercellular bridges of prespermatogonia and of the first A-spermatogonia in the maturing testes of newborn to 17-day-old golden hamsters have been studied by electron microscopy. Incomplete cytokinesis of dividing M- and T2-prespermatogonia and A-spermatogonia produces these bridges, which undergo different developmental fates. Bridges of the first A-spermatogonia are stable beyond subsequent mitoses of these cells; this gradually leads to the formation of bridge-connected groups of synchronously developing germ cells. Thus, the clonal mode of male germ cell proliferation is already established in this period of testis maturation. During mitoses, pre-existing bridges reversibly develop structural modifications, i.e., considerable elongation and formation of a bridge-partitioning complex. In contrast, intercellular bridges of prespermatogonia are mostly severed and become lost during subsequent mitoses of the cells involved; this results in separation of the germ cells and represents a mainly non-clonal mode of M- and T2-prespermatogonial proliferation. Here, too, pre-existing bridges elongate and develop the bridge-partitioning complex during subsequent mitoses of the joined cells, but this is superposed and interrupted by the simultaneous process of disconnection of the bridges.

Immunohistochemical and ultrastructural study of Sertoli cells in androgen insensitivity

The ultrastructure of a perfusion-fixed gonad in a case of androgen insensitivity was studied using thin sections and freeze-fracture replicas. The distribution and arrangement of intermediate filaments in Sertoli cells was visualized immunohistochemically using an antibody against vimentin. Leydig cells lacked Reinke crystals, but contained all of the cytoplasmic organelles involved in steroid synthesis and additionally several lysosomes. The basement membrane and the basal lamina of the testicular tubules were considerably thickened. The testicular tubules consisted of gonocytes and Sertoli cells which had an immature nuclear structure, incomplete development of intercellular junctions and a primitive distribution pattern of intermediate cytoplasmic filaments. The previously reported differences in electron density of Sertoli cell cytoplasm are a non-specific feature without significance to Sertoli cell maturation.

Foetal meiosis in the testis of the rodent Octodon degus

Different stages of meiotic prophase have been studied in foetal testes of the rodent, Octodon degus, using the light and electron microscope. Special attention was focused on the ultrastructural morphology of these meiotic cells in comparison to pre-spermatogonia of foetal testes and meiotic spermatocytes of the adult male testis. Meiosis occurs in only a few cells located among fibroblasts of the tunica albuginea or in the region of the gonadal blastema. The foetal meiotic process resembles adult meiosis in its ultrastructural characteristics; typical pachytene synaptonemal complexes and leptotene or diplotene axial elements appear associated to the chromatin. This process occurs at the same foetal age that meiosis commences in the ovary, thus reinforcing the idea that both meiosis-inducing and meiosis-preventing substances are secreted in both sexes. The intra-or extracordonal localization of the germ cells would be an important factor in determining the cells' response to these substances.

Degeneration of mouse oocytes in response to polycyclic aromatic hydrocarbons

Destruction of mouse oocytes in primordial and small primary follicles in response to treatment with 3-methylcholanthrene (MC) was studied at the ultrastructural level. Four-week old C57Bl/6N (B6) strain mice received a single injection of 80 mg/Kg MC in corn oil intraperitoneally. Controls received only corn oil. Ovaries from animals were prepared for light and electron microscopic examination at specified intervals after treatment. The number of primordial follicles remained constant in control animals. In contrast, their number decreased significantly (P less than 0.01) by three days, and they were depleted by seven days after MC treatment. Subtle degenerative modifications were noted in the ooplasm of primordial follicles two days after treatment. These changes consisted of vesiculation of mitochondrial cristae, increased electron density of the mitochondrial matrix, myelin structures in lipid droplets and in mitochondria. More advanced stages of degeneration of primordial follicles were characterized by further vesiculation or disappearance of mitochondrial cristae, chromatin clumping, and increased density of the ooplasm. Small primary follicles had undergone similar initial degeneration as primordial follicles. In more advanced stages of degeneration nuclear and cytoplasmic contents condensed, endoplasmic reticulum, Golgi complex and mitochondria swelled, small vesicles and multivesicular bodies appeared. In the most advanced stages of degeneration of small primary follicles it appeared that small portions of the oocyte were engulfed by the surrounding follicular cells. It is concluded that exposure of B6 mice to a single dose of MC results in atresia of oocytes in primordial and small primary follicles. Ultrastructurally, these degenerating oocytes of treated mice looked much like the spontaneously atretic oocytes in untreated animals.

Fine structure of fetal human testis and epididymis

Observations on the structure of the human fetal testis and epididymis at 16 weeks' gestation have been made with the light and electron microscope. The fetal testis is organized into solid cords surrounded by a well-defined cellular investment. The basement membrane is multilaminated with a highly redundant basal lamina. The germ cells rest on thin processes from adjacent Sertoli cells. Intercellular bridges between centrally located germ cells have been observed. The epididymis is remarkably well developed. The tall pseudostratified epithelium lines a discrete duct with a patent lumen. Stereocilia and cilia are seen on apical surface of the principal cells.

Occurrence of nuage in fetal human germ cells

Germ cells of human fetuses aged 7-13 weeks were studied. Dense fibrous material called nuage was found in the cytoplasm of human fetal germ cells. The nuage increased in frequency in the germ cells during the interval studied. In oogonia the nuage occurred much more frequently than in spermatogonia and the frequency of nuage in oogonia increased earlier than in spermatogonia. The number of nuage was continuously at a higher level in oogonia than in spermatogonia. It is suggested that these findings support the theory that the nuage may act as a germ cell determinant.

Cell to cell relationships in the seminiferous epithelium in the mouse embryo

Germ cells and Sertoli cells in embryonic mouse testes (day 14 to 20 of gestation) were examined by sectioning and freeze-fracture. Intercellular cytoplasmic bridges between the germ cells are observed in day 14 and older embryos. Membrane specializations with dense fuzzy material similar to the so-called "desmosome-like structures" are found between Sertoli cells and germ cells. A cell contact area with dense opposed membranes is also found between adjacent germ cells. Asymmetrical dense fuzzy lining of both Sertoli and germ cell membranes is noted. Pinocytotic pits or caveolae are frequently found in the Sertoli cell membrane. Between adjacent Sertoli cells, gap junctions of various sizes and focal meshworks of the occluding junctions are found. Most of the occluding junctional particles are located in the center of the grooves in the E face, and are similar to those in postnatal and adult Sertoli cell junctions. In addition, on both fractured faces there are ridges and grooves devoid of particles which are continuous with occluding junctions with particles, suggesting an initial stage in the formation of occluding junctions of the Sertoli cells. Particles gathered at the site of desmosome-like structures are present on the P face of the Sertoli cell.

Ultrastructure of gonadoblastoma and disgerminoma (seminoma) in a patient with XY gonadal dysgenesis

The gross and fine morphology of gonadoblastoma and disgerminoma in a young phenotypic female with male karyotype and gonadal dysgenesis is described. Both the rudimentary gonads contain gonadoblastomas on the left side in association with a disgerminoma. In the light microscope the gonadoblastomas show peculiar hyaline masses, surrounded by Sertoli-like cells. In some areas a direct continuation between hyaline masses and basement membranes of round nests or large anomalous seminiferous tubules is obvious. At the ultrastructural level the hyaline masses consist of densely packed, largely concentric membranes closely similar to basement membranes or basal laminae of seminiferous tubules. These observations and the finding of transitional zones with gonadoblastoma elements in enlarged tubular structures suggest that gonadoblastomas originate inside tubules of dysgenetic testes. The peculiar hyaline masses with their concentric membranes might reflect hyperplasia and death of the Sertoli cells, each new generation of cells producing a basement membrane outside the preceding one.

Spermatogenic cells of the prepuberal mouse. Isolation and morphological characterization

A procedure is described which permits the isolation from the prepuberal mouse testis of highly purified populations of primitive type A spermatogonia, type A spermatogonia, type B spermatogonia, preleptotene primary spermatocytes, leptotene and zygotene primary spermatocytes, pachytene primary spermatocytes and Sertoli cells. The successful isolation of these prepuberal cell types was accomplished by: (a) defining distinctive morphological characteristics of the cells, (b) determining the temporal appearance of spermatogenic cells during prepuberal development, (c) isolating purified seminiferous cords, after dissociation of the testis with collagenase, (d) separating the trypsin-dispersed seminiferous cells by sedimentation velocity at unit gravity, and (e) assessing the identity and purity of the isolated cell types by microscopy. The seminiferous epithelium from day 6 animals contains only primitive type A spermatogonia and Sertoli cells. Type A and type B spermatogonia are present by day 8. At day 10, meiotic prophase is initiated, with the germ cells reaching the early and late pachytene stages by 14 and 18, respectively. Secondary spermatocytes and haploid spermatids appear throughout this developmental period. The purity and optimum day for the recovery of specific cell types are as follows: day 6, Sertoli cells (purity>99 percent) and primitive type A spermatogonia (90 percent); day 8, type A spermatogonia (91 percent) and type B spermatogonia (76 percent); day 18, preleptotene spermatocytes (93 percent), leptotene/zygotene spermatocytes (52 percent), and pachytene spermatocytes (89 percent), leptotene/zygotene spermatocytes (52 percent), and pachytene spermatocytes (89 percent).

Alternative explanations of the differing behaviour of ovarian and testicular teratomas

There are several aspects of the biology of teratomas which may be important in the aetiology of malignant versus non-malignant forms. Firstly, the ages when malignant teratomas form in the ovary (children) and testis (adult) follow by a similar span of years the time of oogenesis (fetus) and spermatogenesis (onset at puberty) and, specifically, follow by less than a decade the periods of active mitosis and meiosis during which interconnection of germ cells by intercellular bridges occurs. Secondly, ovarian teratomas may be viewed as parthenogenically activated oocytes, while testicular teratocarcinomas may represent post-meiotic fusion events more comparable to fertilisation; the malignant potential in the latter case could relate to fundamental differences in male and femal germ plasm. Thirdly,, the fact that malignant teratomas are easily produced by transplanting early embryos to extrauterine sites, while there is loss of malignancy with differentiation within a tumour or when malignant cells are injected into pre-implantation embryos and contribute cells to a variety of normal tissues, suggests that the malignant potential of teratomas has to do with gene control rather than mutational events.

Ultrastructure of developing germ cells in the fetal human testis

Electron microscopic studies of the testis were performed on 12 human embryos and fetuses between 9 and 30 weeks post conceptionem. According to their ultrastructural features, the fetal germ cells could be divided into the following three stages of differentiation: 1) gonocytes, 2) intermediate cells, and 3) fetal spermatogonia. Sertoli cells were present among the germ cells in all the testes studied. The gonocytes showed the highest nucleo-cytoplasmic ratio. Their round nucleus contained a centrally located, prominent nucleolus. The cytoplasm displayed a well developed Golgi apparatus, lipid droplets and parallel arrays of short cisternae of the rough surfaced endoplasmic reticulum (rER). Microfilaments were numerous, particularly just beneath the cell membrane. The intermediate cells were found to extend several cytoplasmic processes and to contain a moderate number of long, branched and/or widened rER cisterna which were frequently connected to the perinuclear cisterna. Intermediate cells were often connected to one another by intercellular cytoplasmic bridges. The fetal spermatogonia also displayed cytoplasmic bridges. These cells showed the lowest nucleo-cytoplasmic ratio and more condensed nuclear chromatin. The mitochondria were situated close to the nucleus. Many of them were connected by a cementing substance. Lipid droplets and rER cisternae were rare in these cells. Infoldings of the inner nuclear membrane were often present in the gonocytes and in the intermediate cells, but were rarely observed in the fetal spermatogonia. Glycogen particles, polyribosomes, and chromatoid bodies ("nuage") were present in all the three germ cell types. With the maturation of the fetus, the number of gonocytes was found to decrease, whereas the number of fetal spermatogonia increased. The Sertoli cells also changed their ultrastructure, showing an increase in the number of rER cisternae, as well as of microfilaments, lipid droplets, and secondary lysosomes.