Isolation, characterization, and culture of human spermatogonia

Differential marker protein expression specifies rarefaction zone-containing human Adark spermatogonia

It is unclear whether the distinct nuclear morphologies of human Adark(Ad) and Apale(Ap) spermatogonia are manifestations of different stages of germ cell development or phases of the mitotic cycle, or whether they may reflect still unknown molecular differences. According to the classical description by Clermont, human dark type A spermatogonium (Ad) may contain one, sometimes two or three nuclear ‘vacuolar spaces’ representing chromatin rarefaction zones. These structures were readily discerned in paraffin sections of human testis tissue during immunohistochemical and immunofluorescence analyses and thus represented robust morphological markers for our study. While a majority of the marker proteins tested did not discriminate between spermatogonia with and without chromatin rarefaction zones, doublesex- and mab-3-related transcription factor (DMRT1), tyrosine kinase receptor c-Kit/CD117 (KIT) and proliferation-associated antigen Ki-67 (KI-67) appeared to be restricted to subtypes which lacked the rarefaction zones. Conversely, exosome component 10 (EXOSC10) was found to accumulate within the rarefaction zones, which points to a possible role of this nuclear domain in RNA processing.

Isolation of human male germ-line stem cells using enzymatic digestion and magnetic-activated cell sorting

Mammalian spermatogenesis is a process whereby male germ-line stem cells (spermatogonial stem cells) divide and differentiate into sperm. Although a great deal of progress has been made in the isolation and characterization of spermatogonial stem cells (SSCs) in rodents, little is known about human SSCs. We have recently isolated human G protein-coupled receptor 125 (GPR125)-positive spermatogonia and GDNF family receptor alpha 1 (GFRA1)-positive spermatogonia using a 2-step enzymatic digestion and magnetic-activated cell sorting (MACS) from adult human testes. Cell purities of isolated human GPR125- and GFRA1-positive spermatogonia after MACS are greater than 95%, and cell viability is over 96%. The isolated GPR125- and GFRA1-positive spermatogonia coexpress GPR125, integrin, alpha 6 (ITGA6), THY1 (also known as CD90), GFRA1, and ubiquitin carboxyl-terminal esterase L1 (UCHL1), markers for rodent or pig SSCs/progenitors, suggesting that GPR125- and GFRA1-positive spermatogonia are phenotypically the SSCs in human testis. Human GPR125-positive spermatogonia can be cultured for 2 weeks with a remarkable increase in cell number. Immunocytochemistry further reveals that GPR125-positive spermatogonia can be maintained in an undifferentiated state in vitro. Collectively, the methods using enzymatic digestion and MACS can efficiently isolate and purify SSCs from adult human testis with consistent and high quality. The ability of isolating and characterizing human SSCs could provide a population of stem cells with high purity for mechanistic studies on human SSC self-renewal and differentiation as well as potential applications of human SSCs in regenerative medicine.

Human spermatagonial stem cells: a novel therapeutic hope for cardiac regeneration and repair?

Although the identification and characterization of human spermatogonial stem cells was reported nearly 50 years ago, great progress has been made only in the last few years. Spermatogonial stem cells attract a great deal of researchers’ attention because of their unique characteristics, including the ability to be converted spontaneously into pluripotent germline stem cells with embryonic stem cell-like properties. Pluripotent stem cells are able to differentiate into any desired cell type in the body; therefore, they are the most promising cell source for organ regeneration. The advantages of pluripotent germline stem cells over other stem cells are that they maintain a high degree of DNA integrity and can resolve some ethical and immunological problems related to human embryonic stem cells. In this article we address the origin, characteristics and pluripotency of spermatogonial stem cells. Their contribution to stem cell-based organ regeneration therapy with special emphasis on cardiac regeneration and repair in the future is also discussed.

Complete meiosis from human induced pluripotent stem cells

Gamete failure-derived infertility affects millions of people worldwide; for many patients, gamete donation by unrelated donors is the only available treatment. Embryonic stem cells (ESCs) can differentiate in vitro into germ-like cells, but they are genetically unrelated to the patient. Using an in vitro protocol that aims at recapitulating development, we have achieved, for the first time, complete differentiation of human induced pluripotent stem cells (hiPSCs) to postmeiotic cells. Unlike previous reports using human ESCs, postmeiotic cells arose without the over-expression of germline related transcription factors. Moreover, we consistently obtained haploid cells from hiPSCs of different origin (keratinocytes and cord blood), produced with a different number of transcription factors, and of both genetic sexes, suggesting the independence of our approach from the epigenetic memory of the reprogrammed somatic cells. Our work brings us closer to the production of personalized human gametes in vitro.

Isolation and characterization of human spermatogonial stem cells

BACKGROUND

To isolate and characterization of human spermatogonial stem cells from stem spermatogonium.

METHODS

The disassociation of spermatogonial stem cells (SSCs) were performed using enzymatic digestion of type I collagenase and trypsin. The SSCs were isolated by using Percoll density gradient centrifugation, followed by differential surface-attachment method. Octamer-4(OCT4)-positive SSC cells were further identified using immunofluorescence staining and flow cytometry technques. The purity of the human SSCs was also determined, and a co-culture system for SSCs and Sertoli cells was established.

RESULTS

The cell viability was 91.07% for the suspension of human spermatogonial stem cells dissociated using a two-step enzymatic digestion process. The cells isolated from Percoll density gradient coupled with differential surface-attachement purification were OCT4 positive, indicating the cells were human spermatogonial stem cells. The purity of isolated human spermatogonial stem cells was 86.7% as assessed by flow cytometry. The isolated SSCs were shown to form stable human spermatogonial stem cell colonies on the feeder layer of the Sertoli cells.

CONCLUSIONS

The two-step enzyme digestion (by type I collagenase and trypsin) process is an economical, simple and reproducible technique for isolating human spermatogonial stem cells. With little contamination and less cell damage, this method facilitates isolated human spermatogonial stem cells to form a stable cell colony on the supporting cell layer.

Separating spermatogonia from cancer cells in contaminated prepubertal primate testis cell suspensions

BACKGROUND

Chemotherapy and radiation treatments for cancer and other diseases can cause male infertility. There are currently no options to preserve the fertility of prepubertal boys who are not yet making sperm. Cryopreservation of spermatogonial stem cells (SSCs, obtained via testicular biopsy) followed by autologous transplantation back into the testes at a later date may restore fertility in these patients. However, this approach carries an inherent risk of reintroducing cancer.

METHODS

To address this aspect of SSC transplantation safety, prepubertal non-human primate testis cell suspensions were inoculated with MOLT4 T-lymphoblastic leukemia cells and subsequently sorted for cell surface markers CD90 (THY-1) and CD45.

RESULTS

Cancer cells segregated to the CD90-/CD45+ fraction and produced tumors in nude mice. Nearly all sorted DEAD box polypeptide 4-positive (VASA+) spermatogonia segregated to the CD90+/CD45- fraction. In a preliminary experiment, a purity check of the sorted putative stem cell fraction (CD90+/CD45-) revealed 0.1% contamination with cancer cells, which was sufficient to produce tumors in nude mice. We hypothesized that the contamination resulted from mis-sorting due to cell clumping and employed singlet discrimination (SD) in four subsequent experiments. Purity checks revealed no cancer cell contamination in the CD90+/CD45- fraction from three of the four SD replicates and these fractions produced no tumors when transplanted into nude mouse testes.

CONCLUSIONS

We conclude that spermatogonia can be separated from contaminating malignant cells by fluorescence-activated cell sorting prior to SSC transplantation and that post-sorting purity checks are required to confirm elimination of malignant cells.

Reduced numbers of Sertoli, germ, and spermatogonial stem cells in impaired spermatogenesis

A key step in the investigation of male infertility is the appropriate classification of impaired spermatogenesis. In this study, we precisely identified Sertoli and distinct germ-cell types in the rat, the mouse, and in the human testis. As a proof of principle, we studied testis biopsy samples from azoospermic patients with defined spermatogenic defects. Remarkably, we found that already the numbers of Sertoli cells, spermatogonia and a subset of spermatogonia including stem cells are significantly reduced in patients with maturation arrest at the level of primary spermatocytes (n=33) compared with patients with histologically normal spermatogenesis (n=33). In patients with hypospermatogenesis (n=44) a significant reduction of spermatogonial cell numbers was observed. The numbers of primary and diplotene spermatocytes were reduced by 84%. However, the strongest reduction (96%) was revealed in the numbers of spermatids in patients with maturation arrest. In contrast, patients with hypospermatogenesis showed only modestly reduced numbers of spermatocytes and spermatids compared with normal spermatogenesis. No correlation was found with age or obstruction. For a detailed analysis of the patients, we distinguished between 'pool of founder cells'-related deficiencies (reduced numbers of Sertoli cells, spermatogonia, and spermatogonial stem cells) and 'meiotic' deficiencies (reduced numbers of spermatocytes, meiotic divisions, and spermatids). Interestingly, patients with maturation arrest showed meiotic deficiencies (36%), while the majority additionally demonstrated deficiencies in the founder pool (58%). In contrast, patients with normal spermatogenesis most often had no deficiencies at all (45%) or founder pool-related deficiencies (33%) but an apparently normal meiosis. This is the first report showing that many infertile patients face besides meiotic defects the problem of reduced numbers of Sertoli cells, spermatogonia, and spermatogonial stem cells.

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.

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.

Spermatogonial stem cells and in vitro spermatogenesis

Spermatogonial stem cells (SSCs) provide the basis for the life-long production of enormous numbers of sperm. The nature of these mysterious cells is being clarified. Although they were regarded to be mostly dormant, dividing rarely and remaining static in a niche, their rather dynamic behavior in the seminiferous tubules has been disclosed. The territories of each colony of SSCs can also quickly change in size. The development of a culture method for SSCs also shed light on their stable, but at the same time, fragile characteristics. In addition, an in vitro system for spermatogenesis was developed which can produce functional sperm from SSCs. These new developments will contribute to reproductive medicine.

Glial cell line-derived neurotrophic factor and endothelial cells promote self-renewal of rabbit germ cells with spermatogonial stem cell properties

Previous studies suggest that exogenous factors crucial for spermatogonial stem cell (SSC) self-renewal are conserved among several mammalian species. Since glial cell line-derived neurotrophic factor (GDNF) and fibroblast growth factor 2 (FGF2) are critical for rodent SSC self-renewal, we hypothesized that they might promote self-renewal of nonrodent SSCs. Therefore, we cultured testicular germ cells from prepubertal rabbits in the presence of GDNF and FGF2 and found they proliferated indefinitely as cellular clumps that displayed characteristics previously identified for rodent SSCs. The rabbit germ cells could not be maintained on mouse embryonic fibroblast (STO) feeders that support rodent SSC self-renewal in vitro but were rather supported on mouse yolk sac-derived endothelial cell (C166) feeder layers. Proliferation of rabbit germ cells was dependent on GDNF. Of critical importance was that clump-forming rabbit germ cells colonized seminiferous tubules of immunodeficient mice, proliferated for at least 6 mo, while retaining an SSC phenotype in the testes of recipient mice, indicating that they were rabbit SSCs. This study demonstrates that GDNF is a mitogenic factor promoting self-renewal that is conserved between rodent and rabbit SSCs; with an evolutionary separation of ∼ 60 million years. These findings provide a foundation to study the mechanisms governing SSC self-renewal in nonrodent species.

Isolation of enriched carp spermatogonial stem cells from Labeo rohita testis for in vitro propagation

The in vitro culture system for spermatogonial stem cells (SSCs) is a powerful tool for exploring molecular mechanisms of male gametogenesis and gene manipulation. Very little information is available for fish SSC biology. Our aim was to isolate highly pure SSCs from the testis of commercially important farmed carp, Labeo rohita. The minced testis of L. rohita was dissociated with collagenase. Dissociated cells purified by two-step Ficoll gradient centrifugation followed by magnetic activated cell sorting (MACS) using Thy1.2 (CD90.2) antibody dramatically heightened recovery rate for spermatogonial cells. The purified cells were cultured in vitro conditions for more than two months in L-15 media containing 10% fetal bovine serum (FBS), 1% carp serum, and other nutrients. The proliferative cells were dividing as validated by 5-bromo-2'-deoxyuridine (BrdU) incorporation assay and formed colonies/clumps with the typical characteristics of SSCs A majority of enriched cell population represented a Vasa(+), Pou5f1/pou5f1(+), Ssea-1(+), Tra-1-81(+), plzf(+), Gfrα1/gfrα1(-), and c-Kit/c-kit(-) as detected by immunocytochemical and/or quantitative real-time polymerase chain reaction (RT-PCR) analyses. Thus, Thy1(+) SSCs were enriched with greater efficiency from the mixed population of testicular cells of L. rohita. A population of enriched spermatogonial cells could be cultured in an undifferentiated state. The isolated SSCs could provide avenue for undertaking research on basic and applied reproductive biology.

Potential applications of germline cell-derived pluripotent stem cells in organ regeneration

Impressive progress has been made since the turn of the century in the field of stem cells. Different types of stem cells have now been isolated from different types of tissues. Pluripotent stem cells are the most promising cell source for organ regeneration. One such cell type is the germline cell-derived pluripotent cell, which is derived from adult spermatogonial stem cells. The germline cell-derived pluripotent stem cells have been obtained from both human and mouse and, importantly, are adult stem cells with embryonic stem cell-like properties that do not require specific manipulations for pluripotency acquisition, hence bypassing problems related to induced pluripotent stem cells and embryonic stem cells. The germline cell-derived pluripotent stem cells have been induced to differentiate into cells deriving from the three germ layers and shown to be functional in vitro. This review will discuss the plasticity of the germline cell-derived pluripotent stem cells and their potential applications in human organ regeneration, with special emphasis on liver regeneration. Potential problems related to their use are also highlighted.

Alternative sources of pluripotent stem cells: ethical and scientific issues revisited

Stem cell researchers in the United States continue to face an uncertain future, because of the changing federal guidelines governing this research, the restrictive patent situation surrounding the generation of new human embryonic stem cell lines, and the ethical divide over the use of embryos for research. In this commentary, we describe how recent advances in the derivation of induced pluripotent stem cells and the isolation of germ-line-derived pluripotent stem cells resolve a number of these uncertainties. The availability of patient-matched, pluripotent stem cells that can be obtained by ethically acceptable means provides important advantages for stem cell researchers, by both avoiding protracted ethical debates and giving U.S. researchers full access to federal funding. Thus, ethically uncompromised stem cells, such as those derived by direct reprogramming or from germ-cell precursors, are likely to yield important advances in stem cell research and move the field rapidly toward clinical applications.

Long-term Culture of Human SSEA-4 Positive Spermatogonial Stem Cells (SSCs)

Recently we and two other groups have shown that human spermatogonial stem cells (SSCs) have the potential to become pluripotent in vitro in defined culture conditions and to differentiate into cells of the three embryonic germ layers. This discovery could open new avenues for autologous cell-based therapy in degenerative diseases, bypassing the ethical and immunological problems related to the human embryonic stem cells. In addition, human SSCs could be used to treat infertility in cancer survival children. However, in order to reprogram SSCs into pluripotency, or to preserve them for repopulation of infertile testes, the first and limiting step is to have access to a highly purified human SSC population that could be multiplied and efficiently cultured in vitro maintaining their molecular and cellular characteristics. Although various studies have attempted to identify molecular markers of human SSCs, to date there is still limited information related to the specific markers that could be used for their isolation and optimized purification that allows long-term in vitro culture of isolated human SSCs. Here using SSEA-4 as an optimal marker for isolation of a subpopulation of SSCs, we show that SSEA-4 positive cells express the highest level of SSC genes compared to other subpopulations isolated with different markers, and can be maintained in culture for over 14 passages which we were unable to obtain with other SSCs markers including GPR125 and ITGA6. In addition, we have established a new technology for cell sorting and long-term culture of human SSC-SSEA-4 positive cells that maximizes the purity and viability of the sorted cells. Our findings are crucial and could be used for the most efficient isolation, purification and long-term culture of SSCs for clinical applications in regenerative medicine, or for preparation of human SSCs for autologous treatment of infertility in cancer survival children.

Newer insights into premeiotic development of germ cells in adult human testis using Oct-4 as a stem cell marker

The transcription factor octamer-binding transforming factor 4 (Oct-4) is central to the gene regulatory network responsible for self-renewal, pluripotency, and lineage commitment in embryonic stem (ES) cells and induced pluripotent stem cells (PSCs). This study was undertaken to evaluate differential localization and expression of two major transcripts of Oct-4, viz. Oct-4A and Oct-4B, in adult human testis. A novel population of 5- to 10-μm PSCs with nuclear Oct-4A was identified by ISH and immunolocalization studies. Besides Oct-4, other pluripotent markers like Nanog and TERT were also detected by RT-PCR. A(dark) spermatogonial stem cells (SSCs) were visualized in pairs and chains undergoing clonal expansion and stained positive for cytoplasmic Oct-4B. Quantitative PCR and Western blotting revealed both the transcripts, with higher expression of Oct-4B. It is proposed that PSCs undergo asymmetric cell division and give rise to A(dark) SSCs, which proliferate and initiate lineage-specific differentiation. The darkly stained nuclei in A(dark) SSCs may represent extensive nuclear reprogramming by epigenetic changes when a PSC becomes committed. Oct-4B eventually disappeared in mature germ cells, viz. spermatocytes, spermatids, and sperm. Besides maintaining normal testicular homeostasis, PSCs may also be implicated in germ cell tumors and ES-like colonies that have recently been derived from adult human testicular tissue.

Characterization and functionality of proliferative human Sertoli cells

It has long been thought that mammalian Sertoli cells are terminally differentiated and nondividing postpuberty. For most previous in vitro studies immature rodent testes have been the source of Sertoli cells and these have shown little proliferative ability when cultured. We have isolated and characterized Sertoli cells from human cadaveric testes from seven donors ranging from 12 to 36 years of age. The cells proliferated readily in vitro under the optimized conditions used with a doubling time of approximately 4 days. Nuclear 5-ethynyl-2'-deoxyuridine (EdU) incorporation confirmed that dividing cells represented the majority of the population. Classical Sertoli cell ultrastructural features, lipid droplet accumulation, and immunoexpression of GATA-4, Sox9, and the FSH receptor (FSHr) were observed by electron and fluorescence microscopy, respectively. Flow cytometry revealed the expression of GATA-4 and Sox9 by more than 99% of the cells, and abundant expression of a number of markers indicative of multipotent mesenchymal cells. Low detection of endogenous alkaline phosphatase activity after passaging showed that few peritubular myoid cells were present. GATA-4 and SOX9 expression were confirmed by reverse transcription polymerase chain reaction (RT-PCR), along with expression of stem cell factor (SCF), glial cell line-derived neurotrophic factor (GDNF), and bone morphogenic protein 4 (BMP4). Tight junctions were formed by Sertoli cells plated on transwell inserts coated with fibronectin as revealed by increased transepithelial electrical resistance (TER) and polarized secretion of the immunoregulatory protein, galectin-1. These primary Sertoli cell populations could be expanded dramatically in vitro and could be cryopreserved. The results show that functional human Sertoli cells can be propagated in vitro from testicular cells isolated from adult testis. The proliferative human Sertoli cells should have important applications in studying infertility, reproductive toxicology, testicular cancer, and spermatogenesis, and due to their unique biological properties potentially could be useful in cell therapy.

Cloning of cDNA and prediction of peptide structure of Plzf expressed in the spermatogonial cells of Labeo rohita

The promyelocytic leukemia zinc finger (Plzf) gene containing an evolutionary conserved BTB (bric-a-brac/tramtrack/broad complex) domain plays a key role in self-renewal of mammalian spermatogonial stem cells (SSCs) via recruiting transcriptional co-repressors. Little is known about the function of Plzf in vertebrate, especially in fish species. To gain better understanding of its role in fishes, we have cloned Plzf from the testis of Labeo rohita (rohu), a commercially important freshwater carp. The full-length cDNA contains an open reading frame (ORF) of 2004bp translatable to 667 amino acids (aa) containing a conserved N-terminal BTB domain and C-terminal C(2)H(2)-zinc finger motifs. L. rohita Plzf, which is phylogenetically related to Danio rerio counterpart, abundantly expressed in spermatogonial stem cells (SSCs). A three-dimensional (3D) model of BTB domain of Plzf protein was constructed by homology modeling approach. Molecular docking on this 3D structure established a homo-dimer between two BTB domains creating a charged pocket containing conserved aa residues: L33, C34, D35 and R49. Thus, Plzf of SSC is structurally and possibly functionally conserved. The conserved aa residues in the cleft resulting from Plzf BTB self-association are likely to be the binding platform for interaction with recruited co-repressor peptides. The identified Plzf could be the first step towards exploring its role in rohu SSC behavior.

Cadmium suppresses the proliferation of piglet Sertoli cells and causes their DNA damage, cell apoptosis and aberrant ultrastructure

OBJECTIVE

Very little information is known about the toxic effects of cadmium on somatic cells in mammalian testis. The objective of this study is to explore the toxicity of cadmium on piglet Sertoli cells.

METHODS

Sertoli cells were isolated from piglet testes using a two-step enzyme digestion and followed by differential plating. Piglet Sertoli cells were identified by oil red O staining and Fas ligand (FasL) expression as assayed by immunocytochemistry and expression of transferrin and androgen binding protein by RT-PCR. Sertoli cells were cultured in DMEM/F12 supplemented with 10% fetal calf serum in the absence or presence of various concentrations of cadmium chloride, or treatment with p38 MAPK inhibitor SB202190 and with cadmium chloride exposure. Apoptotic cells in seminiferous tubules of piglets were also performed using TUNEL assay in vivo.

RESULTS

Cadmium chloride inhibited the proliferation of Piglet Sertoli cells as shown by MTT assay, and it increased malondialdehyde (MDA) but reduced superoxide dismutase (SOD) and Glutathione peroxidase (GSH-Px) activity. Inhibitor SB202190 alleviated the proliferation inhibition of cadmium on piglet Sertoli cells. Comet assay revealed that cadmium chloride caused DNA damage of Piglet Sertoli cells and resulted in cell apoptosis as assayed by flow cytometry. The in vivo study confirmed that cadmium induced cell apoptosis in seminiferous tubules of piglets. Transmission electronic microscopy showed abnormal and apoptotic ultrastructure in Piglet Sertoli cells treated with cadmium chloride compared to the control.

CONCLUSION

cadmium has obvious adverse effects on the proliferation of piglet Sertoli cells and causes their DNA damage, cell apoptosis, and aberrant morphology. This study thus offers novel insights into the toxicology of cadmium on male reproduction.

Undifferentiated primate spermatogonia and their endocrine control

The biology of spermatogonial stem cells is currently an area of intensive research and contemporary studies in primates are emerging. Quantitative regulation of sperm output by the primate testis seems to be exerted primarily on the transition from undifferentiated to differentiating spermatogonia. This review examines recent advances in our understanding of the mechanisms governing spermatogonial renewal and early differentiation in male primates, with a focus on the monkey. Emerging revisions to the classic view of dark and pale type A spermatogonia as reserve and renewing spermatogonial stem cells, respectively, are critically evaluated and essential features of endocrine control of undifferentiated spermatogonia throughout postnatal primate development are discussed. Obstacles in gaining a more complete understanding of primate spermatogonia are also identified.

Spermatogonial stem cells, in vivo transdifferentiation and human regenerative medicine

IMPORTANCE OF THE FIELD

Embryonic stem (ES) cells have potential for use in regenerative medicine, but use of these cells is hindered by moral, legal and ethical issues. Induced pluripotent cells have promise in regenerative medicine. However, since generation of these cells involves genetic manipulation, it also faces significant hurdles before clinical use. This review discusses spermatogonial stem cells (SSCs) as a potential alternative source of pluripotent cells for use in human regenerative medicine.

AREAS COVERED IN THE REVIEW

The potential of SSCs to give rise to a wide range of other cell types either directly, when recombined with instructive inducers, or indirectly, after being converted to ES-like cells. Current understanding of the differentiation potential of murine SSCs and recent progress in isolating and culturing human SSCs and demonstrating their properties is also discussed.

WHAT THE READER WILL GAIN

Insight into the plasticity of SSCs and the unique properties of these cells for regenerative applications, the limitations of SSCs for stem-cell-based therapy and the potential alternatives available.

TAKE HOME MESSAGE

If methodologies for isolation and conversion of adult human SSCs directly into other cell types can be effectively developed, SSCs could represent an important alternate source of pluripotent cells that can be used in human tissue repair and/or regeneration.

Gynostemma pentaphyllum protects mouse male germ cells against apoptosis caused by zearalenone via Bax and Bcl-2 regulation

The objective of this study was to explore the effects of Gynostemma pentaphyllum on Zearalenone-induced apoptosis in mouse male germ cells. Fifty Kunming male mice at 25-days-old were classified into five groups: group A was the control (10% ethanol, 0.5 ml/day); group B with 10 microg Zearalenone/day; group C with 10 microg Zearalenone and 50 mg/kg/day Gynostemma pentaphyllum; group D with 10 microg Zearalenone and 100 mg/kg/day Gynostemma pentaphyllum; and group E with 10 microg Zearalenone and 200 mg/kg/day Gynostemma pentaphyllum. It was found that Gynostemma pentaphyllum has a marked effect on protecting male germ cells against Zearalenone-induced apoptosis, as evidenced by a reduced apoptosis rate of male germ cells and Bax expression as well as an enhancement of Bcl-2 expression in Gynostemma pentaphyllum-treated groups compared to the control. In addition, Gynostemma pentaphyllum remarkably improved pathologic changes of testicular tissue, reduced the content of malondialdehyde (MDA), and increased the activity of superoxide dismutase (SOD) caused by Zearalenone. Taken together, these results suggest that Gynostemma pentaphyllum protects against toxicity caused by Zearalenone through anti-oxidation and anti-apoptosis via the regulation of Bax and Bcl-2 expression.

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.

Prepubertal human spermatogonia and mouse gonocytes share conserved gene expression of germline stem cell regulatory molecules

In the human testis, beginning at approximately 2 months of age, gonocytes are replaced by adult dark (Ad) and pale (Ap) spermatogonia that make up the spermatogonial stem cell (SSC) pool. In mice, the SSC pool arises from gonocytes approximately 6 days after birth. During puberty in both species, complete spermatogenesis is established by cells that differentiate from SSCs. Essentially pure populations of prepubertal human spermatogonia and mouse gonocytes were selected from testis biopsies and validated by confirming the presence of specific marker proteins in cells. Stem cell potential of germ cells was demonstrated by transplantation to mouse testes, following which the cells migrated to the basement membrane of the seminiferous tubule and were maintained similar to SSCs. Differential gene expression profiles generated between germ cells and testis somatic cells demonstrated that expression of genes previously identified as SSC and spermatogonial-specific markers (e.g., zinc-finger and BTB-domain containing 16, ZBTB16) was greatly elevated in both human spermatogonia and mouse gonocytes compared to somatic cells. Several genes were expressed at significantly higher levels in germ cells of both species. Most importantly, genes known to be essential for mouse SSC self-renewal (e.g., Ret proto-oncogene, Ret; GDNF-family receptor alpha1, Gfr alpha1; and B-cell CLL/lymphoma 6, member B, Bcl6b) were more highly expressed in both prepubertal human spermatogonia and mouse gonocytes than in somatic cells. The results indicate remarkable conservation of gene expression, notably for self-renewal genes, in these prepubertal germline cells between two species that diverged phylogenetically approximately 75 million years ago.