Generation and in vitro differentiation of a spermatogonial cell line

In vitro spermatogenesis: A long journey to get tails

The generation of functional sperm in vitro has been a goal for almost a century. Until recently, researchers have only succeeded in reproducing the early steps of spermatogenesis. This is not surprising given that spermatogenesis is a complicated process that requires the coordinated efforts of germ cells and several somatic cells within the tubular structure of the testis. Finally—last year—Sato et al. reported the successful in vitro production of functional sperm, thereby potentially opening up a new era of reproductive biology. Here, we summarize the history of research directed toward reproducing steps of spermatogenesis in vitro, detail the seminal findings of Sato et al., and suggest ways that their approach can be applied toward clinical applications and addressing fundamental questions about the underlying mechanism of spermatogenesis.

Generation of haploid spermatids with fertilization and development capacity from human spermatogonial stem cells of cryptorchid patients

Generation of functional spermatids from azoospermia patients is of unusual significance in the treatment of male infertility. Here, we report an efficient approach to obtain human functional spermatids from cryptorchid patients. Spermatogonia remained whereas meiotic germ cells were rare in cryptorchid patients. Expression of numerous markers for meiotic and postmeiotic male germ cells was enhanced in human spermatogonial stem cells (SSCs) of cryptorchidism patients by retinoic acid (RA) and stem cell factor (SCF) treatment. Meiotic spreads and DNA content assays revealed that RA and SCF induced a remarkable increase of SCP3-, MLH1-, and CREST-positive cells and haploid cells. Single-cell RNA sequencing analysis reflected distinct global gene profiles in embryos derived from round spermatids and nuclei of somatic cells. Significantly, haploid spermatids generated from human SSCs of cryptorchid patients possessed fertilization and development capacity. This study thus provides an invaluable source of autologous male gametes for treating male infertility in azoospermia patients.

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Spermatogonia remain whereas meiotic male germ cells are rare in cryptorchid patients

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Human SSCs of cryptorchid patients differentiate into phenotypic haploid spermatids

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Round spermatids derived from human SSCs have fertilization and development capacity

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Distinct gene profiles exist in embryos from round spermatid and somatic cell nuclei

Perspectives of germ cell development in vitro in mammals

Pluripotent stem cells, such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) are able to differentiate into all cell lineages of the embryo proper, including germ cells. This pluripotent property has a huge impact on the fields of regenerative medicine, developmental biology and reproductive engineering. Establishing the germ cell lineage from ESCs/iPSCs is the key biological subject, since it would contribute not only to dissection of the biological processes of germ cell development but also to production of unlimited numbers of functional gametes in vitro. Toward this goal, we recently established a culture system that induces functional mouse primordial germ cells (PGCs), precursors of all germ cells, from mouse ESCs/iPSCs. The successful in vitro production of PGCs arose from the study of pluripotent cell state, the signals inducing PGCs and the technology of transplantation. However, there are many obstacles to be overcome for the robust generation of mature gametes or for application of the culture system to other species, including humans and livestock. In this review, we discuss the requirements for a culture system to generate the germ cell lineage from ESCs/iPSCs.

Generation of male differentiated germ cells from various types of stem cells

Infertility is a major and largely incurable disease caused by disruption and loss of germ cells. It affects 10-15% of couples, and male factor accounts for half of the cases. To obtain human male germ cells 'especially functional spermatids' is essential for treating male infertility. Currently, much progress has been made on generating male germ cells, including spermatogonia, spermatocytes, and spermatids, from various types of stem cells. These germ cells can also be used in investigation of the pathology of male infertility. In this review, we focused on advances on obtaining male differentiated germ cells from different kinds of stem cells, with an emphasis on the embryonic stem (ES) cells, the induced pluripotent stem (iPS) cells, and spermatogonial stem cells (SSCs). We illustrated the generation of male differentiated germ cells from ES cells, iPS cells and SSCs, and we summarized the phenotype for these stem cells, spermatocytes and spermatids. Moreover, we address the differentiation potentials of ES cells, iPS cells and SSCs. We also highlight the advantages, disadvantages and concerns on derivation of the differentiated male germ cells from several types of stem cells. The ability of generating mature and functional male gametes from stem cells could enable us to understand the precise etiology of male infertility and offer an invaluable source of autologous male gametes for treating male infertility of azoospermia patients.

[In vitro spermatogenesis… new horizon to restore fertility?]

The survival of the young boy after cancer has considerably progressed in recent years due to the efficiency of chemo/radiotherapy against the tumor cells. However, this treatment causes adverse effects on healthy tissues, including fertility. Freezing testicular tissue before highly gonadotoxic treatment is a prerequisite for preserving fertility in prepubertal boys that do not produce sperm yet. But which strategy proposes to restore fertility from frozen-thawed testicular tissue? One potential solution would be to consider an in vitro maturation of spermatogonial stem cells. In this article we trace the chronological development of in vitro spermatogenesis that resulted in mouse sperm production in vitro and give an overview of new challenges for the future.

Effects of GDNF and LIF on mouse spermatogonial stem cells proliferation in vitro

Spermatogonial stem cells (SSCs) are the only type of cells that transmit genes to the subsequent generations. The proliferation, cultivation and identification of SSCs in vitro are critical to understanding of male infertility, genetic resources and conservation of endangered species. To investigate the effects of glial cell-derived neurotrophic factor (GDNF) and leukemia inhibitory factor (LIF) on the proliferation of mouse SSCs in vitro, supplement of GDNF and/or LIF were designed to culture SSCs. The testes of 6-8 d mouse were harvested and digested by two-step enzyme digestion method. The SSCs and Sertoli cells were separated by differential plating. Then the SSCs were identified by alkaline phosphatase staining, RT-PCR and indirect immunofluorescence cell analysis. The cellular proliferation capacity was measured by methyl thiazolyl tetrazolium assay. The results showed that addition of 20 and 40 ng/ml of GDNF could strongly promote growth of mouse SSCs (p < 0.05). There was no significant difference between LIF treatment groups and the control group in promoting proliferation of the mouse SSCs (p > 0.05). However, the combination of 20 ng/ml GDNF and 1,000 U/ml LIF could significantly enhance the invitro proliferation of mouse SSCs (p < 0.05), and the OD490 value was 0.696 at day 5 of culture when the density of SSCs was 5-10 × 10(4) cells/ml.

Differentiation of spermatogonial stem cell-like cells from murine testicular tissue into haploid male germ cells in vitro

In vitro differentiation of spermatogonial stem cells (SSCs) promotes the understanding of the mechanism of spermatogenesis. The purpose of this study was to isolate spermatogonial stem cell-like cells from murine testicular tissue, which then were induced into haploid germ cells by retinoic acid (RA). The spermatogonial stem cell-like cells were purified and enriched by a two-step plating method based on different adherence velocities of SSCs and somatic cells. Cell colonies were present after culture in M1-medium for 3 days. Through alkaline phosphatase, RT-PCR and indirect immunofluorescence cell analysis, cell colonies were shown to be SSCs. Subsequently, cell colonies of SSCs were cultured in M2-medium containing RA for 2 days. Then the cell colonies of SSCs were again cultured in M1-medium for 6-8 days, RT-PCR and indirect immunofluorescence cell analysis were chosen to detect haploid male germ cells. It could be demonstrated that 10(-7) mol l(-1) of RA effectively induced the SSCs into haploid male germ cells in vitro.

Effects of testicular interstitial fluid on the proliferation of the mouse spermatogonial stem cells in vitro

Spermatogenesis is a process in adult male mammals supported by spermatogonial stem cells (SSCs). The cultivation of SSCs has potential value, for example for the treatment of male infertility or spermatogonial transplantation. Testicular interstitial fluid was added to culture medium to a final concentration of 5, 10, 20, 30 or 40%, in order to investigate its effects on proliferation of mouse SSCs in vitro, Alkaline phosphatase (AKP) assay, reverse transcription polymerase chain reaction (RT-PCR) analysis and indirect immunofluorescence of cells were performed to identify SSCs, and the proliferation rate and diameters of the SSCs colonies were measured. The results showed that the optimal addition of testicular interstitial fluid to culture medium was 30%. When medium supplemented with 30% testicular interstitial fluid was used to culture mouse SSCs, the optimum proliferation rate and diameter of the cell colonies were 72.53% and 249 μm, respectively, after 8 days in culture, values that were significant higher than those found for other groups (P < 0.05). In conclusion, proliferation of mouse SSCs could be promoted significantly by supplementation of the culture medium with 30% testicular interstitial fluid. More research is needed to evaluate and understand the precise physiological role of testicular interstitial fluid during cultivation of SSCs.

Sertoli cells from non-obstructive azoospermia and obstructive azoospermia patients show distinct morphology, Raman spectrum and biochemical phenotype

STUDY QUESTION

Are there differences in the morphology, spectrum and biochemical phenotype between Sertoli cells from non-obstructive azoospermia (NOA) patients and those from obstructive azoospermia (OA) patients with normal spermatogenesis?

SUMMARY ANSWER

Sertoli cells from NOA patients are distinct from those from OA patients in terms of morphological features, Raman spectrum and phenotype including the expression of genes and proteins (e.g. SCF, BMP4 and GDNF).

WHAT IS KNOWN ALREADY

NOA affects 10% of infertile men and has been diagnosed in 60% of azoospermic men. In contrast with OA patients who have normal spermatogenesis, NOA patients have an impaired spermatogenesis.

STUDY DESIGN, SIZE AND DURATION

This case-control study included 100 NOA patients (as cases) and 100 OA patients with normal spermatogenesis (as controls). The study was performed between January 2012 and January 2013.

PARTICIPANTS/MATERIALS, SETTING AND METHODS

Karyotype analysis was performed to check the chromosome content and multiplex PCR was carried out to determine the expression of numerous Y chromosome genes in NOA patients. Human Sertoli cells were then isolated from the testes of NOA and OA patients by two-step enzymatic digestion and differential plating. Transmission electron microscopy was used to determine the ultrastructure of the Sertoli cells and real-time Raman microspectroscopy was used to assess their spectrum. We further compared the two groups of patients for expression of SCF, GDNF and BMP4 in Sertoli cells, using RT-PCR, microarray analysis, immunofluorescence, immunohistochemistry and Western blots.

MAIN RESULTS AND THE ROLE OF CHANCE

NOA patients had normal chromosome karyotypes and Y chromosome microdeletions were excluded. In morphology, Sertoli cells isolated from NOA patients had a series of abnormal ultrastructural features compared with the control Sertoli cells: (i) existence of small and spindle-shaped nuclei, (ii) smaller diameter, (iii) deficient nucleolus or endoplasmic reticulum and (iv) more vacuoles. Spectral intensities in Sertoli cells of NOA patients were distinct at four typical Raman peaks compared with the control Sertoli cells. In phenotype, SCF, BMP4 and GDNF transcripts and proteins were significantly lower in Sertoli cells of NOA patients than in the control Sertoli cells.

LIMITATIONS AND REASONS FOR CAUTION

The Sertoli cells of NOA patients were not compared with Sertoli cells of normal fertile men due to the fact that it is hard to obtain adult testes from normal donors.

WIDER IMPLICATIONS OF THE FINDINGS

This study provides novel insights into understanding the underlying causes for NOA and might offer a basis for developing new therapeutic strategies for patients with NOA.

In vitro spermatogenesis using an organ culture technique

Research on in vitro spermatogenesis has a long history and remained to be an unaccomplished task until very recently. In 2010, we succeeded in producing murine sperm from primitive spermatogonia using an organ culture method. The fertility of the sperm or haploid spermatids was demonstrated by microinsemination. This organ culture technique uses the classical air-liquid interphase method and is based on conditions extensively examined by Steinbergers in 1960s. Among adaptations in the new culture system, application of serum-free media was the most important. The system is very simple and easy to follow.

Biotechnological approaches to the treatment of aspermatogenic men

Aspermatogenesis is a severe impairment of spermatogenesis in which germ cells are completely lacking or present in an immature form, which results in sterility in approximately 25% of patients. Because assisted reproduction techniques require mature germ cells, biotechnology is a valuable tool for rescuing fertility while maintaining biological fatherhood. However, this process involves, for instance, the differentiation of preexisting immature germ cells or the production/derivation of sperm from somatic cells. This review critically addresses four potential techniques: sperm derivation in vitro, germ stem cell transplantation, xenologous systems, and haploidization. Sperm derivation in vitro is already feasible in fish and mammals through organ culture or 3D systems, and it is very useful in conditions of germ cell arrest or in type II Sertoli-cell-only syndrome. Patients afflicted by type I Sertoli-cell-only syndrome could also benefit from gamete derivation from induced pluripotent stem cells of somatic origin, and human haploid-like cells have already been obtained by using this novel methodology. In the absence of alternative strategies to generate sperm in vitro, in germ cells transplantation fertility is restored by placing donor cells in the recipient germ-cell-free seminiferous epithelium, which has proven effective in conditions of spermatogonial arrest. Grafting also provides an approach for ex-vivo generation of mature sperm, particularly using prepubertal testis tissue. Although less feasible, haploidization is an option for creating gametes based on biological cloning technology. In conclusion, the aforementioned promising techniques remain largely experimental and still require extensive research, which should address, among other concerns, ethical and biosafety issues, such as gamete epigenetic status, ploidy, and chromatin integrity.

Clinical review: Present and future prospects of male fertility preservation for children and adolescents

CONTEXT

Rapid progress in fertility preservation strategies has led to the investigation of ways in which fertile gametes could be generated from cryopreserved immature testicular tissue. Childhood cancer patients remain the major group that can benefit from these techniques. Other potential candidates include patients undergoing gonadectomy and patients with Klinefelter's syndrome and cryptorchid testes. This review aims to present an overview of the current state of knowledge in experimental germ cell transplantation, testicular tissue transplantation, and germ cell culture as fertility preservation methods for males.

METHODOLOGY

We included English articles published in PubMed as well as personal files with the focus on studies including human or nonhuman material.

MAIN FINDINGS

Germ cell and testicular tissue transplantation demonstrate clinical options to mature germ cells from immature primate testicular tissue. The most promising approach involves autologous grafting of immature testicular tissue, whereas germ cell maturation in vitro provides the best strategies to overcome problems of cancer contamination in cryopreserved testicular tissue. Three-dimensional and organ culture systems offer the possibility to differentiate immature male germ cells up to the stage of elongated spermatids. Further characterization of early germ cell development in humans is needed to modify these systems for clinical use.

Male reproductive health after childhood, adolescent, and young adult cancers: a report from the Children's Oncology Group

The majority of children, adolescents, and young adults diagnosed with cancer will become long-term survivors. Although cancer therapy is associated with many adverse effects, one of the primary concerns of young male cancer survivors is reproductive health. Future fertility is often the focus of concern; however, it must be recognized that all aspects of male health, including pubertal development, testosterone production, and sexual function, can be impaired by cancer therapy. Although pretreatment strategies to preserve reproductive health have been beneficial to some male patients, many survivors remain at risk for long-term reproductive complications. Understanding risk factors and monitoring the reproductive health of young male survivors are important aspects of follow-up care. The Children's Oncology Group Long-Term Follow-Up Guidelines for Survivors of Childhood, Adolescent, and Young Adult Cancer (COG-LTFU Guidelines) were created by the COG to provide recommendations for follow-up care of survivors at risk for long-term complications. The male health task force of the COG-LTFU Guidelines, composed of pediatric oncologists, endocrinologists, nurse practitioners, a urologist, and a radiation oncologist, is responsible for updating the COG-LTFU Guidelines every 2 years based on literature review and expert consensus. This review summarizes current task force recommendations for the assessment and management of male reproductive complications after treatment for childhood, adolescent, and young adult cancers. Issues related to male health that are being investigated, but currently not included in the COG-LTFU Guidelines, are also discussed. Ongoing investigation will inform future COG-LTFU Guideline recommendations for follow-up care to improve health and quality of life for male survivors.

Direct differentiation of human pluripotent stem cells into haploid spermatogenic cells

Human embryonic stem cells (hESCs) and induced pluripotent stem cells (hiPSCs) have been shown to differentiate into primordial germ cells (PGCs) but not into spermatogonia, haploid spermatocytes, or spermatids. Here, we show that hESCs and hiPSCs differentiate directly into advanced male germ cell lineages, including postmeiotic, spermatid-like cells, in vitro without genetic manipulation. Furthermore, our procedure mirrors spermatogenesis in vivo by differentiating PSCs into UTF1-, PLZF-, and CDH1-positive spermatogonia-like cells; HIWI- and HILI-positive spermatocyte-like cells; and haploid cells expressing acrosin, transition protein 1, and protamine 1 (proteins that are uniquely found in spermatids and/or sperm). These spermatids show uniparental genomic imprints similar to those of human sperm on two loci: H19 and IGF2. These results demonstrate that male PSCs have the ability to differentiate directly into advanced germ cell lineages and may represent a novel strategy for studying spermatogenesis in vitro.

Sperm banking and the cancer patient

The current concepts, recommendations, and principles of sperm banking as it pertains to the comprehensive care of young men of reproductive age with cancer are reviewed. Obstacles to sperm banking are addressed as well as future directions for fertility-preserving technologies. All cancer therapies-chemotherapy, radiation, and surgery-are potential threats to a man's reproductive potential. In addition, cancer itself can impair spermatogenesis. Thus, sperm cryopreservation prior to initiating life-saving cancer treatment offers men and their families the best chance to father biologically related children and should be offered to all men with cancer before treatment. Better patient and provider education, as well as deliberate, coordinated strategies at comprehensive cancer care centers are necessary to make fertility preservation for male cancer patients a priority during pretreatment planning.

Differentiation of murine male germ cells to spermatozoa in a soft agar culture system

Establishment of an in vitro system that allows the development of testicular germ cells to sperm will be valuable for studies of spermatogenesis and future treatments for male infertility. In the present study, we developed in vitro culture conditions using three-dimensional agar culture system (SACS), which has the capacity to induce testicular germ cells to reach the final stages of spermatogenesis, including spermatozoa generation. Seminiferous tubules from testes of 7-day-old mice were enzymatically dissociated, and intratubular cells were cultured in the upper layer of the SACS in RPMI medium supplemented with fetal calf serum (FCS). The lower layer of the SACS contained only RPMI medium supplemented with FCS. Colonies in the upper layer were isolated after 14 and 28 days of culture and were classified according to their size. Immunofluorescence and real-time PCR were used to analyse specific markers expressed in undifferentiated and differentiated spermatogonia (Vasa, Dazl, OCT-4, C-Kit, GFR-α-1, CD9 and α-6-integrin), meiotic cells (LDH, Crem-1 and Boule) and post-meiotic cells (Protamine-1, Acrosin and SP-10). Our results reveal that it is possible to induce mouse testicular pre-meiotic germ cell expansion and induce their differentiation to spermatozoa in SACS. The spermatozoa showed normal morphology and contained acrosomes. Thus, our results demonstrate that SACS could be used as a novel in vitro system for the maturation of pre-meiotic mouse germ cells to post-meiotic stages and morphologically-normal spermatozoa.

An in vitro, short-term culture method for mammalian haploid round spermatids amenable for molecular manipulation

Extensive chromatin remodeling is a characteristic feature of mammalian spermiogenesis. To date, methods for the molecular manipulation of haploid spermatids are not available as there is a lack of a well-established culture system. Biochemical experiments and knockout studies reveal only the final outcome; studying the incremental details of the intricate mechanisms involved is still a challenge. We have established an in vitro culture system for pure haploid round spermatids isolated from rat testes that can be maintained with good viability for up to 72 hr. Changes in cell morphology and flagellar growth were also studied in the cultured spermatids. Further, we have demonstrated that upon treatment of cells with specific histone deacetylase inhibitors, sodium butyrate and trichostatin A, there is an increase in the hyperacetylation status of histone H4, mimicking an important event characteristic of histone replacement process that occurs during later stages of spermiogenesis. We have also tried various methods for introducing DNA and protein into these round spermatids in culture, and report that while DNA transfection is still a challenging task, protein transfection could be achieved using Chariot™ peptide as a transfection reagent. Thus, the method described here sets a stage to study the molecular roles of spermatid-specific proteins and chromatin remodelers in the cellular context.

Retinol (vitamin A) maintains self-renewal of pluripotent male germline stem cells (mGSCs) from adult mouse testis

Studies have shown that male germline stem cells (mGSCs), which are responsible for maintaining spermatogenesis in the male, could be obtained from mouse and human testis. However, the traditional cultural methods were mostly dependent on serum and feeder, and the initial mGSCs were either obtained from neonatal mice or the detailed description of its potency and origin was not provided. Here we reported a novel (retinol (RE) serum-free and feeder-free) system for the successful culture of adult germline stem cells from adult Kunming mice (8-24 weeks) testis. The isolated mGSCs cultured in RE serum-free and feeder-free medium maintained the typical morphology of undifferentiated embryonic stem cells (ESCs), and they proliferated well in RE medium analyzed by proliferation assay, RT-PCR, microarray, and Western blotting. These cells also showed typical properties of ESCs (alkaline phosphatase (AP) positive, expressions of Oct4, Sox2, Nanog, and SSEA1, with the capacity to form teratomas and differentiate into various types of cells within three germ layers). Taken together, we conclude that RE promotes the self-renewal of mGSCs and maintains the pluripotency of mGSCs, the RE serum-free and feeder-free system may be useful for the culture of pluripotent stem cell lines from adult testis tissues, which provides a new resource for tissue engineering and therapy for infertility.

Unexpected role of α-fetoprotein in spermatogenesis

BACKGROUND

Heat shock severely affects sperm production (spermatogenesis) and results in a rapid loss of haploid germ cells, or in other words, sperm formation (spermiogenesis) is inhibited. However, the mechanisms behind the effects of heat shock on spermatogenesis are obscure.

METHODOLOGY/PRINCIPAL FINDINGS

To identify the inhibitory factor of spermiogenesis, experimental cryptorchid (EC) mice were used in this study. Here we show that α-fetoprotein (AFP) is specifically expressed in the testes of EC mice by proteome analysis. AFP was also specifically localized spermatocytes by immunohistochemical analysis and was secreted into the circulation system of EC mice by immunoblot analysis. Since spermatogenesis of an advanced mammal cannot be reproduced with in vitro, we performed the microinjection of AFP into the seminiferous tubules of normal mice to determine whether AFP inhibits spermiogenesis in vivo. AFP was directly responsible for the block in spermiogenesis of normal mice. To investigate whether AFP inhibits cell differentiation in other models, using EC mice we performed a partial hepatectomy (PH) that triggers a rapid regenerative response in the remnant liver tissue. We also found that liver regeneration is inhibited in EC mice with PH. The result suggests that AFP released into the blood of EC mice regulates liver regeneration by inhibiting the cell division of hepatocytes.

CONCLUSIONS/SIGNIFICANCE

AFP is a well-known cancer-specific marker, but AFP has no known function in healthy human beings. Our findings indicate that AFP expressed under EC conditions plays a role as a regulatory factor in spermatogenesis and in hepatic generation.

In vitro production of functional sperm in cultured neonatal mouse testes

Spermatogenesis is one of the most complex and longest processes of sequential cell proliferation and differentiation in the body, taking more than a month from spermatogonial stem cells, through meiosis, to sperm formation. The whole process, therefore, has never been reproduced in vitro in mammals, nor in any other species with a very few exceptions in some particular types of fish. Here we show that neonatal mouse testes which contain only gonocytes or primitive spermatogonia as germ cells can produce spermatids and sperm in vitro with serum-free culture media. Spermatogenesis was maintained over 2 months in tissue fragments positioned at the gas-liquid interphase. The obtained spermatids and sperm resulted in healthy and reproductively competent offspring through microinsemination. In addition, neonatal testis tissues were cryopreserved and, after thawing, showed complete spermatogenesis in vitro. Our organ culture method could be applicable through further refinements to a variety of mammalian species, which will serve as a platform for future clinical application as well as mechanistic understanding of spermatogenesis.

Medaka tert produces multiple variants with differential expression during differentiation in vitro and in vivo

Embryonic stem (ES) cells have immortality for self-renewal and pluripotency. Differentiated human cells undergo replicative senescence. In human, the telomerase reverse transcriptase (Tert), namely the catalytic subunit of telomerase, exhibits differential expression to regulate telomerase activity governing cellular immortality or senescence, and telomerase activity or tert expression is a routine marker of pluripotent ES cells. Here we have identified the medaka tert gene and determined its expression and telomerase activity in vivo and in vitro. We found that the medaka tert locus produces five variants called terta to terte encoding isoforms TertA to TertE. The longest TertA consists of 1090 amino acid residues and displays a maximum of 34% identity to the human TERT and all the signature motifs of the Tert family. TertB to TertE are novel isoforms and have considerable truncation due to alternative splicing. The terta RNA is ubiquitous in embryos, adult tissues and cell lines, and accompanies ubiquitous telomerase activity in vivo and in vitro as revealed by TRAP assays. The tertb RNA was restricted to the testis, absent in embryos before gastrulation and barely detectable in various cell lines The tertc transcript was absent in undifferentiated ES cells but became evident upon ES cell differentiation, in vivo it was barely detectable in early embryos and became evident when embryogenesis proceeds. Therefore, ubiquitous terta expression correlates with ubiquitous telomerase activity in medaka, and expression of other tert variants appears to delineate cell differentiation in vitro and in vivo.

Establishment and characterization of a testicular cell line from the half-smooth tongue sole, Cynoglossus semilaevis

Spermatogenesis within the adult testis is an excellent system for studying stem cell renewal and differentiation, which is under the control of testicular somatic cells. In order to understanding spermatogenesis in the half-smooth tongue sole (Cynoglossus semilaevis) as a marine fish model of aquaculture importance, we established a cell line called CSGC from a juvenile gonad of this organism. CSGC is composed of fibroblast-like cells, retains a diploid karyotype of 42 chromosomes, lacks the heterogametic W chromosome, lacks a female specific marker and expresses the dmrt, a marker for testicular somatic cells. Therefore, CSGC appears to consist of testicular somatic cell cells. We show that this cell line is effective for infection by the turbot reddish body iridovirus and flounder lymphocystis disease virus as evidenced by the appearance of cytopathic effect and virus propagation in the virus-infected cells, and most convincingly, the observation of viral particles by electon microscopy, demonstrateing that CSGC is suitable to study interactions between virus and host cells. As a first fish testicular somatic cell line of the ZZ-ZW genetic sex determination system, CSGC will be a useful tool to study sex-related events and interactions between somatic cells and germ cells during spermatogenesis.

Fish stem cell cultures

Stem cells have the potential for self-renewal and differentiation. First stem cell cultures were derived 30 years ago from early developing mouse embryos. These are pluripotent embryonic stem (ES) cells. Efforts towards ES cell derivation have been attempted in other mammalian and non-mammalian species. Work with stem cell culture in fish started 20 years ago. Laboratory fish species, in particular zebrafish and medaka, have been the focus of research towards stem cell cultures. Medaka is the second organism that generated ES cells and the first that gave rise to a spermatogonial stem cell line capable of test-tube sperm production. Most recently, the first haploid stem cells capable of producing whole animals have also been generated from medaka. ES-like cells have been reported also in zebrafish and several marine species. Attempts for germline transmission of ES cell cultures and gene targeting have been reported in zebrafish. Recent years have witnessed the progress in markers and procedures for ES cell characterization. These include the identification of fish homologs/paralogs of mammalian pluripotency genes and parameters for optimal chimera formation. In addition, fish germ cell cultures and transplantation have attracted considerable interest for germline transmission and surrogate production. Haploid ES cell nuclear transfer has proven in medaka the feasibility of semi-cloning as a novel assisted reproductive technology. In this special issue on "Fish Stem Cells and Nuclear Transfer", we will focus our review on medaka to illustrate the current status and perspective of fish stem cells in research and application. We will also mention semi-cloning as a new development to conventional nuclear transfer.

Anticancer chemotherapeutic agents and testicular dysfunction

The improvement of the survival rates of various cancer patients has resulted in increased focus on the long-term complications of treatment. Most anticancer chemotherapeutic agents are gonadotoxic, and sterility is therefore one of the most common complications for cancer survivors. The degree of gonadal dysfunction induced by anticancer chemotherapeutic agents seems to be drug specific and dose related. Following the development of new chemotherapeutic agents that have high benefit-to-risk ratios, sufficient sperm can be acquired by collection of ejaculated semen after the treatment in relatively many cases, and assisted reproductive techniques enable conceptions with even severe spermatogenesis dysfunction. However, anticancer chemotherapeutic agents have consistently exhibited the potential to induce permanent azoospermia. Cryopreservation of semen, which is currently the only proven successful option for future fertility preservation in male cancer patients, should certainly be recommended before cancer therapy. However, to date, no established effective methods have shown the capability to protect gonadal function from anticancer treatment in prepubertal cancer patients.

Spermatogonial stem cells, infertility and testicular cancer

The spermatogonial stem cells (SSCs) are responsible for the transmission of genetic information from an individual to the next generation. SSCs play critical roles in understanding the basic reproductive biology of gametes and treatments of human infertility. SSCs not only maintain normal spermatogenesis, but also sustain fertility by critically balancing both SSC self-renewal and differentiation. This self-renewal and differentiation in turn is tightly regulated by a combination of intrinsic gene expression within the SSC as well as the extrinsic gene signals from the niche. Increased SSCs self-renewal at the expense of differentiation result in germ cell tumours, on the other hand, higher differentiation at the expense of self-renewal can result in male sterility. Testicular germ cell cancers are the most frequent cancers among young men in industrialized countries. However, understanding the pathogenesis of testis cancer has been difficult because it is formed during foetal development. Recent studies suggest that SSCs can be reprogrammed to become embryonic stem (ES)-like cells to acquire pluripotency. In the present review, we summarize the recent developments in SSCs biology and role of SSC in testicular cancer. We believe that studying the biology of SSCs will not only provide better understanding of stem cell regulation in the testis, but eventually will also be a novel target for male infertility and testicular cancers.

Meiosis in a bottle: new approaches to overcome Mammalian meiocyte study limitations

The study of meiosis is limited because of the intrinsic nature of gametogenesis in mammals. One way to overcome these limitations would be the use of culture systems that would allow meiotic progression in vitro. There have been some attempts to culture mammalian meiocytes in recent years. In this review we will summarize all the efforts to-date in order to culture mammalian sperm and oocyte precursor cells.

Telomerase immortalization of principal cells from mouse collecting duct

Recently, the use of overexpression of telomerase reverse transcriptase (TERT) has led to the generation of immortalized human cell lines. However, this cell immortalization approach has not been reported in well-differentiated mouse cells, such as renal epithelial cells. We sought to establish and then characterize a mouse collecting duct cell line, using ectopic expression of mTERT. Isolated primary cortical collecting duct (CCD) cell lines were transduced with mouse (m)TERT, using a lentiviral vector. mTERT-negative cells did not survive blasticidin selection, whereas mTERT-immortalized cells proliferated in selection media for over 40 subpassages. mTERT messenger RNA and telomerase activity was elevated in these cells, compared with an SV40-immortalized cell line. Flow cytometry with Dolichos biflorus agglutinin was used to select the CCD principal cells, and we designated this cell line mTERT-CCD. Cells were well differentiated and exhibited morphological characteristics typically found in renal epithelial cells, such as tight junction formation, microvilli, and primary cilia. Further characterization using standard immunofluorescence revealed abundant expression of aquaporin-2 and the vasopressin type 2 receptor. mTERT-CCD cells exhibited cAMP-stimulated/benzamil-inhibited whole cell currents. Whole cell patch-clamp currents were also enhanced after a 6-day treatment with aldosterone. In conclusion, we have successfully used mTERT to immortalize mouse collecting duct cells that retain the basic in vivo phenotypic characteristics of collecting duct cells. This technique should be valuable in generating cell lines from genetically engineered mouse models.

Long-term proliferation and characterization of human spermatogonial stem cells obtained from obstructive and non-obstructive azoospermia under exogenous feeder-free culture conditions

OBJECTIVES

The aim of the present study was to improve efficiency of isolation and to optimize proliferative potential of human spermatogonial stem cells (SSCs) obtained from obstructive azoospermic (OA) and non-obstructive azoospermic (NOA) patients, and further, to characterize these cells for potential use in infertility treatment or study of reproductive biology.

MATERIALS AND METHODS

We have applied a cell-sorting method, using collagen and magnetic activated cell separation to overcome obstacles, developing a collection system, and simple long-term proliferation system, that yields large numbers of high-purity SSCs from obstructive OA and NOA patients.

RESULTS

SSCs derived from OA and NOA patients proliferated and maintained their characteristics for more than 12 passages (>6 months) in vitro. Moreover, the population of cells positive for the SSC-specific markers GFRalpha-1 and integrin alpha6, increased to more than 80% at passage 8.

CONCLUSION

These finding may support the idea that in vitro propagation of SSCs could be a useful tool for infertility treatment and study of reproductive biology.

Sex determination in mammalian germ cells: extrinsic versus intrinsic factors

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

Spermatogonial stem cell regulation and spermatogenesis

This article will provide an updated review of spermatogonial stem cells and their role in maintaining the spermatogenic lineage. Experimental tools used to study spermatogonial stem cells (SSCs) will be described, along with research using these tools to enhance our understanding of stem cell biology and spermatogenesis. Increased knowledge about the biology of SSCs improves our capacity to manipulate these cells for practical application. The chapter concludes with a discussion of future directions for fundamental investigation and practical applications of SSCs.

Medaka fish stem cells and their applications

Stem cells are present in developing embryos and adult tissues of multicellular organisms. Owing to their unique features, stem cells provide excellent opportunities for experimental analyses of basic developmental processes such as pluripotency control and cell fate decision and for regenerative medicine by stem cell-based therapy. Stem cell cultures have been best studied in 3 vertebrate organisms. These are the mouse, human and a small laboratory fish called medaka. Specifically, medaka has given rise to the first embryonic stem (ES) cells besides the mouse, the first adult testis-derived male stem cells spermatogonia capable of test-tube sperm production, and most recently, even haploid ES cells capable of producing Holly, a semi-cloned fertile female medaka from a mosaic oocyte created by microinjecting a haploid ES cell nucleus directly into a normal oocyte. These breakthroughs make medaka a favoring vertebrate model for stem cell research, the topic of this review.

Fish germ cells

Fish, like many other animals, have two major cell lineages, namely the germline and soma. The germ-soma separation is one of the earliest events of embryonic development. Germ cells can be specifically labeled and isolated for culture and transplantation, providing tools for reproduction of endangered species in close relatives, such as surrogate production of trout in salmon. Haploid cell cultures, such as medaka haploid embryonic stem cells have recently been obtained, which are capable of mimicking sperm to produce fertile offspring, upon nuclear being directly transferred into normal eggs. Such fish originated from a mosaic oocyte that had a haploid meiotic nucleus and a transplanted haploid mitotic cell culture nucleus. The first semi-cloned fish is Holly. Here we review the current status and future directions of understanding and manipulating fish germ cells in basic research and reproductive technology.

Expression and chromosomal organization of mouse meiotic genes

Microarray technology which enables large scale analysis of gene expression and thus comparison between transcriptomes of different cell types, cells undergoing different treatments or cells at different developmental stages has also been used to study the transcriptome involved with spermatogenesis. Many new germ cell-specific genes were determined, and the resulting genes were classified according to different criteria. However, the biological significance of these classifications and their clustering according to developmental transcriptional patterns during spermatogenesis have not yet been addressed. In this study we utilized mouse testicular transcriptome analysis at five distinct post-natal ages (Days 7, 10, 12, 14, and 17), representing distinct meiotic stages, in an attempt to better understand the biological significance of genes clustered into similar expression patterns during this process. Among 790 sequences that showed an expression level change of twofold or more in any of the five key stages that were monitored, relative to the geometric average of all stages, about 40% peaked and about 30% were specifically suppressed at post-natal day 14 (representing the early pachytene stage of spermatocytes), reflecting tight transcriptional regulation at this stage. We also found that each of the six main transcription clusters that were determined was characterized by statistically significant representation of genes related to specific biological processes. Finally, our results indicated that genes important for meiosis are not randomly distributed along the mouse genome but rather preferentially located on specific chromosomes, suggesting for the first time that chromosomal location might be a regulating factor of meiotic gene expression.

Nodal signaling via an autocrine pathway promotes proliferation of mouse spermatogonial stem/progenitor cells through Smad2/3 and Oct-4 activation

Spermatogenesis is the process that involves the division and differentiation of spermatogonial stem cells into spermatozoa. However, the autocrine molecules and signaling pathways controlling their fate remain unknown. This study was designed to identify novel growth factors and signaling pathways that regulate proliferation, differentiation, and survival of spermatogonial stem/progenitor cells. To this end, we have for the first time explored the expression, function, and signaling pathway of Nodal, a member of the transforming growth factor-beta superfamily, in mouse spermatogonial stem/progenitor cells. We demonstrate that both Nodal and its receptors are present in these cells and in a spermatogonial stem/progenitor cell line (C18-4 cells), whereas Nodal is undetected in Sertoli cells or differentiated germ cells, as assayed by reverse transcription-polymerase chain reaction, Western blots, and immunocytochemistry. Nodal promotes proliferation of spermatogonial stem/progenitor cells and C18-4 cells, whereas Nodal receptor inhibitor SB431542 blocks their propagation as shown by proliferation and bromodeoxyuridine incorporation assays. Nodal knockdown by RNA interference results in a marked increase of cell apoptosis and a reduction of cell division as indicated by terminal deoxynucleotidyl transferase dUTP nick-end labeling and proliferation assays. Conversely, overexpression of Nodal leads to an increase of cell proliferation. Nodal activates Smad2/3 phosphorylation, Oct-4 transcription, cyclin D1, and cyclin E expression, whereas SB431542 completely abolishes their increase. Together, Nodal was identified as the first autocrine signaling molecule that promotes proliferation of mouse spermatogonial stem/progenitor cells via Smad2/3 and Oct-4 activation. This study thus provides novel and important insights into molecular mechanisms regulating proliferation and survival of spermatogonial stem/progenitor cells.

Signaling molecules and pathways regulating the fate of spermatogonial stem cells

Spermatogenesis is the process that involves the division and differentiation of spermatogonial stem cells (SSCs) into mature spermatozoa. SSCs are a subpopulation of type A spermatogonia resting on the basement membrane in the mammalian testis. Self-renewal and differentiation of SSCs are the foundation of normal spermatogenesis, and thus a better understanding of molecular mechanisms and signaling pathways in the SSCs is of paramount importance for the regulation of spermatogenesis and may eventually lead to novel targets for male contraception as well as for gene therapy of male infertility and testicular cancer. Uncovering the molecular mechanisms is also of great interest to a better understanding of SSC aging and for developing novel therapeutic strategies for degenerative diseases in view of the recent work demonstrating the pluripotent potential of the SSC. Progress has recently been made in elucidating the signaling molecules and pathways that determine cell fate decisions of SSCs. In this review, we first address the morphological features, phenotypic characteristics, and the potential of SSCs, and then we focus on the recent advances in defining the key signaling molecules and crucial signaling pathways regulating self-renewal and differentiation of SSCs. The association of aberrant expression of signaling molecules and cascades with abnormal spermatogenesis and testicular cancer are also discussed. Finally, we point out potential future directions to pursue in research on signaling pathways of SSCs.

New horizons for in vitro spermatogenesis? An update on novel three-dimensional culture systems as tools for meiotic and post-meiotic differentiation of testicular germ cells

Culture and differentiation of male germ cells has been performed for various purposes in the past. To date, none of the studies aimed at in vitro spermatogenesis has resulted in a sufficient number of mature gametes. Numerous studies have revealed worthy pieces of information, building up a body of information on conditions that are required to maintain and mature male germ cells in vitro. In this review, we report on previously published and unpublished experiments addressing murine germ cell differentiation in three-dimensional (3D) in vitro culture systems. In a systematic set of experiments, we examined the influence of two different matrices (soft agar and methylcellulose) as well as the need for gonadotrophin support. For the first time, we demonstrate that pre-meiotic male germ cells [revealed by the absence of meiotic marker expression (e.g. Boule)] obtained from immature mice pass through meiosis in vitro. After several weeks of culture, we obtained morphologically normal spermatozoa embedded in the matrix substance. Complete maturation relied on support from somatic testicular cells and the presence of gonadotrophins but appeared independent from the matrix in a 3D culture environment. Further research efforts are required to reveal the applicability of this culture technique for human germ cells and the functionality of the spermatozoa for generating offspring.

Isolation and characterization of pluripotent human spermatogonial stem cell-derived cells

Several reports have documented the derivation of pluripotent cells (multipotent germline stem cells) from spermatogonial stem cells obtained from the adult mouse testis. These spermatogonia-derived stem cells express embryonic stem cell markers and differentiate to the three primary germ layers, as well as the germline. Data indicate that derivation may involve reprogramming of endogenous spermatogonia in culture. Here, we report the derivation of human multipotent germline stem cells (hMGSCs) from a testis biopsy. The cells express distinct markers of pluripotency, form embryoid bodies that contain derivatives of all three germ layers, maintain a normal XY karyotype, are hypomethylated at the H19 locus, and express high levels of telomerase. Teratoma assays indicate the presence of human cells 8 weeks post-transplantation but limited teratoma formation. Thus, these data suggest the potential to derive pluripotent cells from human testis biopsies but indicate a need for novel strategies to optimize hMGSC culture conditions and reprogramming.

Instructing an embryonic stem cell-derived oocyte fate: lessons from endogenous oogenesis

Female reproductive potential is limited in the majority of species due to oocyte depletion. Because functional human oocytes are restricted in number and accessibility, a robust system to differentiate oocytes from stem cells would enable a thorough investigation of the genetic, epigenetic, and environmental factors affecting human oocyte development. Also, the differentiation of functional oocytes from stem cells may permit the success of human somatic cell nuclear transfer for reprogramming studies and for the production of patient-specific embryonic stem cells (ESCs). Thus, ESC-derived oocytes could ultimately help to restore fertility in women. Here, we review endogenous and ESC-derived oocyte development, and we discuss the potential and challenges for differentiating functional oocytes from ESCs.

Production of offspring from a germline stem cell line derived from neonatal ovaries

The idea that females of most mammalian species have lost the capacity for oocyte production at birth has been challenged recently by the finding that juvenile and adult mouse ovaries possess mitotically active germ cells. However, the existence of female germline stem cells (FGSCs) in postnatal mammalian ovaries still remains a controversial issue among reproductive biologists and stem cell researchers. We have now established a neonatal mouse FGSC line, with normal karyotype and high telomerase activity, by immunomagnetic isolation and culture for more than 15 months. FGSCs from adult mice were isolated and cultured for more than 6 months. These FGSCs were infected with GFP virus and transplanted into ovaries of infertile mice. Transplanted cells underwent oogenesis and the mice produced offspring that had the GFP transgene. These findings contribute to basic research into oogenesis and stem cell self-renewal and open up new possibilities for use of FGSCs in biotechnology and medicine.

Generation of mice by transplantation of an adult spermatogonial cell line after cryopreservation

OBJECTIVES

The key to fertility in adult males is production of mature spermatogenic cells. Spermatogonial stem cells (SSC) have the dual capacity of self-renewal and of differentiation into mature sperm. SSC transplantation may provide potential treatment for specific male infertilities. However, until now, there has been no evidence of offspring produced by transplantation of adult SSC line cells in humans or other mammals.

MATERIALS AND METHODS

A new line of SSCs from adult C57BL/6 mouse was established by using magnetic-activated cell sorting. The cell line was characterized by immunocytochemistry, karyotype analysis and telomeric repeat amplification protocol (TRAP) telomerase activity assay. Spermatogenic function was examined by allograft into germ cell-ablated recipient mice.

RESULTS

For more than 14 months with more than 65 maintenance passages, the cell line showed a normal karyotype (40, XY) and high telomerase activity. It represented a Thy-1+, Oct4+, SSEA-1-, c-kit- (99 +/- 1%) cell subpopulation. We cryopreserved these SSCs and successfully produced normal offspring after transplanting them into testes of busulphan-sterilized mice.

CONCLUSIONS

We established and long-term maintained an adult SSC line with normal spermatogenic function, without the need of genetic modification; thus, this study provides a model system for basic research and clinical application.