Biological activity of cryopreserved bovine spermatogonial stem cells during in vitro culture

Functional roles of spermatogonial stem cells in spermatogenesis are self-renewing proliferation and production of differentiated daughter progeny. The ability to recapitulate these actions in vitro is important for investigating their biology and inducing genetic modification that could potentially lead to an alternative means of generating transgenic animals. The objective of this study was to evaluate the survival and proliferation of frozen-thawed bovine spermatogonial stem cells in vitro and investigate the effects of exogenous glial cell line-derived neurotrophic factor (GDNF). In order to accomplish this objective we developed a bovine embryonic fibroblast feeder cell line, termed BEF, to serve as feeder cells in a coculture system with bovine germ cells. Bovine spermatogonial stem cell survival and proliferation in vitro were evaluated by xenogeneic transplantation into the seminiferous tubules of immunodeficient mice. Bovine germ cells cocultured for 1 wk resulted in significantly more round cell donor colonies in recipient mouse testes compared to donor cells transplanted just after thawing. Bovine germ cells cocultured for 2 wk had fewer colony-forming cells than the freshly thawed cell suspensions or cells cultured for 1 wk. Characterization of the feeder cell line revealed endogenous expression of Gdnf mRNA and protein. Addition of exogenous GDNF to the culture medium decreased the number of stem cells present at 1 wk of coculture, but enhanced stem cell maintenance at 2 wk compared to cultures without added GDNF. These data indicate that frozen-thawed bovine spermatogonial stem cells survive cryopreservation and can be maintained during coculture with a feeder cell line in which the maintenance is influenced by GDNF.     

Basic features of bovine spermatogonial culture and effects of glial cell line-derived neurotrophic factor

Spermatogonial stem cells (SSC) are a small self-renewing subpopulation of type A spermatogonia, which for the rest are composed of differentiating cells with a very similar morphology. We studied the development of primary co-cultures of prepubertal bovine Sertoli cells and A spermatogonia and the effect of glial cell line-derived neurotropic factor (GDNF) on the numbers and types of spermatogonia, the formation of spermatogonial colonies and the capacity of the cultured SSC to colonize a recipient mouse testis. During the first week of culture many, probably differentiating, A spermatogonia entered apoptosis while others formed pairs and chains of A spermatogonia. After 1 week colonies started to appear that increased in size with time. Numbers of single (A(s)) and paired (A(pr)) spermatogonia were significantly higher in GDNF treated cultures at Days 15 and 25 (P < 0.01 and 0.05, respectively), and the ratio of A(s) to A(pr) and spermatogonial chains (A(al)) was also higher indicating enhanced self-renewal of the SSC. Furthermore, spermatogonial outgrowths in the periphery of the colonies showed a significantly higher number of A spermatogonia with a more primitive morphology under the influence of GDNF (P < 0.05). Spermatogonial stem cell transplantation experiments revealed a 2-fold increase in stem cell activity in GDNF treated spermatogonial cultures (P < 0.01). We conclude that GDNF rather than inducing proliferation, enhances self-renewal and increases survival rates of SSC in the bovine spermatogonial culture system.     

Proliferation and differentiation of bovine type A spermatogonia during long-term culture

The present study was aimed at developing a method for long-term culture of bovine type A spermatogonia. Testes from 5-mo-old calves were used, and pure populations of type A spermatogonia were isolated. Cells were cultured in minimal essential medium (MEM) or KSOM (potassium-rich medium prepared according to the simplex optimization method) and different concentrations of fetal calf serum (FCS) for 2-4 wk at 32 degrees C or 37 degrees C. Culture in MEM resulted in more viable cells and more proliferation than culture in KSOM, and better results were obtained at 37 degrees C than at 32 degrees C. After 1 wk of culture in the absence of serum, only 20% of the cells were alive. However, in the presence of 2.5% FCS, approximately 80% of cells were alive and proliferating. Higher concentrations of FCS only enhanced numbers of somatic cells. In long-term culture, spermatogonia continued to proliferate, and eventually, type A spermatogonial colonies were formed. The majority of colonies consisted mostly of groups of cells connected by intercellular bridges. Most of the cells in these colonies underwent differentiation because they were c-kit positive, and ultimately, cells with morphological and molecular characteristics of spermatocytes and spermatids were formed. Occasionally, large round colonies consisting of single, c-kit-negative, type A spermatogonia (presumably spermatogonial stem cells) were observed. For the first time to our knowledge, a method has been developed to allow proliferation and differentiation of highly purified type A spermatogonia, including spermatogonial stem cells during long-term culture.     

Efficiency of adult mouse spermatogonial stem cell colony formation under several culture conditions

The aim of this study was to compare the in vitro effects of glial cell line-derived neurotrophic factor, stem cell factor, granulocyte macrophage-colony stimulating factor, and co-culture with Sertoli cells on the efficiency of adult mouse spermatogonial stem cells colony formation. For these purpose, both Sertoli and spermatogonial cells were isolated from adult mouse testes. The identity of the cells was confirmed through analysis of alkaline phosphatase activity, immunocytochemistry against OCT-4, c-kit, and vimentin, and also by transplantation of these cells in the recipient testes. The isolated spermatogonial cells were treated either with various concentrations of the above mentioned factors or co-cultured with Sertoli cells for 3 wk. The spermatogonial cells of the resulting colonies were transplanted via rete testis into the mouse testes, which were irradiated with 14 Gy. The results indicated that glial cell line-derived neurotrophic factor is the most appropriate factor for in vitro colonization of adult mice spermatogonial cells compared with other cytokines and growth factors. A short-term co-culture with Sertoli cells showed a significant increase in the number and diameter of the colonies compared with the treated growth factors and the control group. We have also demonstrated that mouse spermatogonial stem cells in the colonies after co-culturing with Sertoli cells could induce spermatogenesis in the recipient testes after transplantation.     

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.     

Expansion of murine spermatogonial stem cells through serial transplantation

Mammalian male germ cells might be generally thought to have infinite proliferative potential based on their life-long production of huge numbers of sperm. However, there has been little substantial evidence that supports this assumption. In the present study, we performed serial transplantation of spermatogonial stem cells to investigate if they expand by self-renewing division following transplantation. The transgenic mouse carrying the Green fluorescent protein gene was used as the donor cell source that facilitated identification and recollection of colonized donor germ cells in the recipient testes. The established colonies of germ cells in the recipient testes were collected and transplanted to new recipients. This serial transplantation of spermatogonial stem cells repopulated the recipient testes, which were successfully performed sequentially up to four times from one recipient to the next. The incubation periods between two sequential transplantations ranged from 55 to 373 days. During these passages, the spermatogonial stem cells showed constant activity to form spermatogenic colonies in the recipient testis. They continued to increase in number for more than a year following transplantation. Colonization efficiency of spermatogonial stem cells was determined to be 4.25% by using Sl/Sl(d) mice as recipients that propagated only undifferentiated type A spermatogonia in their testes. Based on the colonization efficiency, one colony-forming activity was assessed to equate to about 20 spermatogonial stem cells. The spermatogonial stem cells were estimated to expand over 50-fold in 100 days in this experiment.     

Gfra1 silencing in mouse spermatogonial stem cells results in their differentiation via the inactivation of RET tyrosine kinase

Spermatogenesis is the process by which spermatogonial stem cells divide and differentiate into sperm. The role of growth factor receptors in regulating self-renewal and differentiation of spermatogonial stem cells remains largely unclear. This study was designed to examine Gfra1 receptor expression in immature and adult mouse testes and determine the effects of Gfra1 knockdown on the proliferation and differentiation of type A spermatogonia. We demonstrated that GFRA1 was expressed in a subpopulation of spermatogonia in immature and adult mice. Neither Gfra1 mRNA nor GFRA1 protein was detected in pachytene spermatocytes and round spermatids. GFRA1 and POU5F1 (also known as OCT4), a marker for spermatogonial stem cells, were co-expressed in a subpopulation of type A spermatogonia from 6-day-old mice. In addition, the spermatogonia expressing GFRA1 exhibited a potential for proliferation and the ability to form colonies in culture, which is a characteristic of stem cells. RNA interference assays showed that Gfra1 small interfering RNAs (siRNAs) knocked down the expression of Gfra1 mRNA and GFRA1 protein in type A spermatogonia. Notably, the reduction of Gfra1 expression by Gfra1 siRNAs induced a phenotypic differentiation, as evidenced by the elevated expression of KIT, as well as the decreased expression of POU5F1 and proliferating cell nuclear antigen (PCNA). Furthermore, Gfra1 silencing resulted in a decrease in RET phosphorylation. Taken together, these data indicate that Gfra1 is expressed dominantly in mouse spermatogonial stem cells and that Gfra1 knockdown leads to their differentiation via the inactivation of RET tyrosine kinase, suggesting an essential role for Gfra1 in spermatogonial stem cell regulation.     

Retrovirus-mediated modification of male germline stem cells in rats

The ability to isolate, manipulate, and transplant spermatogonial stem cells provides a unique opportunity to modify the germline. We used the rat-to-nude mouse transplantation assay to characterize spermatogonial stem cell activity in rat testes and in culture. Our results indicate that rat spermatogonial stem cells can survive and proliferate in short-term culture, although a net loss of stem cells was observed. Rat spermatogonial stem cells also were susceptible to transduction with a retroviral vector carrying a lacZ reporter transgene. Using a 3-day periodic infection protocol, 0.5% of stem cells originally cultured were transduced and produced transgenic colonies of spermatogenesis in recipient mouse testes. The level of transgenic donor-derived spermatogenesis observed in the rat-to-mouse transplantation was similar to levels that produced transgenic progeny in the mouse-to-mouse transplantation. This work provides a basis for understanding the biology of rat spermatogonial stem cells. Development of an optimal rat recipient testis model and application of these methods for germline modification will enable the production of transgenic rats, potentially valuable tools for evaluating genes and their functions. In addition, these methods may be applicable in other species where existing transgenic methods are inefficient or not available.     

Commitment of fetal male germ cells to spermatogonial stem cells during mouse embryonic development

The continuous production of mammalian sperm is maintained by the proliferation and differentiation of spermatogonial stem cells that originate from primordial germ cells (PGCs) in the early embryo. Although spermatogonial stem cells arise from PGCs, it is not clear whether fetal male germ cells function as spermatogonial stem cells able to produce functional sperm. In the present study, we examined the timing and mechanisms of the commitment of fetal germ cells to differentiate into spermatogonial stem cells by transplantation techniques. Transplantation of fetal germ cells into the seminiferous tubules of adult testis showed that donor germ cells, at 14.5 days postcoitum (dpc), were able to initiate spermatogenesis in the adult recipient seminiferous tubules, whereas no germ cell differentiation was observed in the transplantation of 12.5-dpc germ cells. These results indicate that the commitment of fetal germ cells to differentiate into spermatogonial stem cells initiates between embryonic days 12.5 and 14.5. Furthermore, the results suggest the importance of the interaction between germ cells and somatic cells in the determination of fetal germ cell differentiation into spermatogonial stem cells, as normal spermatogenesis was observed when a 12.5-dpc whole gonad was transplanted into adult recipient testis. In addition, sperm obtained from the 12.5- dpc male gonadal explant had the ability to develop normally if injected into the cytoplasm of oocytes, indicating that normal development of fetal germ cells in fetal gonadal explant occurred in the adult testicular environment.     

Magnetic activated cell sorting allows isolation of spermatogonia from adult primate testes and reveals distinct GFRa1‐positive subpopulations in men

Background Isolation of spermatogonial stem cells (SSCs) could enable in vitro approaches for exploration of spermatogonial physiology and therapeutic approaches for fertility preservation. SSC isolation from adult testes is difficult due to low cell numbers and lacking cell surface markers. Glial cell‐derived neurotrophic factor family receptor alpha‐1 (GFRα1) plays a crucial role for the maintenance of SSCs in rodents and is expressed in monkey spermatogonia. Methods Magnetic activated cell sorting was employed for the enrichment of GFRα1+ spermatogonia from adult primate testes. Results Magnetic activated cell sorting of monkey cells enriched GFRα1+ cells threefold. 11.4% of GFRα1+ cells were recovered. 42.9% of GFRα1+ cells were recovered in sorted fractions of human testicular cells, representing a fivefold enrichment. Interestingly, a high degree of morphological heterogeneity among the GFRα1+ cells from human testes was observed. Conclusions Magnetic activated cell sorting using anti‐GFRα1 antibodies provides an enrichment strategy for spermatogonia from monkey and human testes.     

Functional analysis of stem cells in the adult rat testis

Adult stem cells maintain several self-renewing systems and processes in the body, including the epidermis, hematopoiesis, intestinal epithelium, and spermatogenesis. However, studies on adult stem cells are hampered by their low numbers, lack of information about morphologic or biochemical characteristics, and absence of functional assays, except for hematopoietic and spermatogonial stem cells. We took advantage of the recently developed spermatogonial transplantation technique to analyze germ line stem cells of the rat testis. The results indicate that the stem cell concentration in rat testes is 9.5-fold higher than that in mouse testes, and spermatogenic colonies derived from rat donor testis cells are 2.75 times larger than mouse-derived colonies by 3 mo after transplantation. Therefore, the extent of spermatogenesis from rat stem cells was 26-fold greater than that from mouse stem cells at the time of recipient testis analysis. Attempts to enrich spermatogonial stem cells in rat testis populations using the experimental cryptorchid procedure were not successful, but selection by attachment to laminin-coated plates resulted in 8.5-fold enrichment. Spermatogonial stem cells are unique among adult stem cells because they pass genetic information to the next generation. The high concentration of stem cells in the rat testis and the rapid expansion of spermatogenesis after transplantation will facilitate studies on stem cell biology and the introduction of genetic modifications into the male germ line. The functional differences between spermatogonial stem cells of rat vs. mouse origin after transplantation suggest that the potential of these cells may vary greatly among species.     

Juvenile spermatogonial depletion (jsd): a genetic defect of germ cell proliferation of male mice

Adult C57BL/6J male mice homozygous for the mutant gene, juvenile spermatogonial depletion (jsd/jsd), show azoosper4ia and testes reduced to one-third normal size, but are otherwise phenotypically normal. In contrast, adult jsd/jsd females are fully fertile. This feature facilitated mapping the jsd gene to the centromeric end of chromosome 1; the gene order is jsd-Isocitrate dehydrogenase-1 (Idh-1)-Peptidase-3 (Pep-3). Analysis of testicular histology from jsd/jsd mice aged 3-10 wk revealed that these mutant mice experience one wave of spermatogenesis, but fail to continue mitotic proliferation of type A spermatogonial cells at the basement membrane. As a consequence, histological sections of testes from mutant mice aged 8-52 wk showed tubules populated by modest numbers of Sertoli cells, with only an occasional spermatogonial cell. Some sperm with normal morphology and motility were observed in epididymides of 6.5- but not in 8-wk or older mutants. Treatment with retinol failed to alter the loss of spermatogenesis in jsd/jsd mice. Analyses of serum hormones of jsd/jsd males showed that testosterone levels were normal at all ages--a finding corroborated by normal seminal vesicle and vas deferens weights, whereas serum follicle-stimulating hormone levels were significantly elevated in mutant mice from 4 to 20 wk of age. We hypothesize the jsd/jsd male may be deficient in proliferative signals from Sertoli cells that are needed for spermatogenesis.     

Effect of vascular endothelial growth factor and testis tissue culture on spermatogenesis in bovine ectopic testis tissue xenografts

Bovine ectopic testis tissue grafting is a technique that can be used to study bovine spermatogenesis and for the production of germ cells for a variety of applications. Approximately 10% of seminiferous tubule cross sections in testis grafts contain spermatids, providing a unique tool to investigate what regulates germ cell differentiation. We hypothesized that manipulation of testis tissue grafts would increase the percentage of seminiferous tubule cross sections undergoing complete germ cell differentiation. To test this hypothesis, bovine testis tissue was treated with vascular endothelial growth factor (VEGF) at the time of grafting or explant cultured for 1 wk prior to grafting. For the VEGF experiment, 8-wk donor tissue and graft sites were treated with 1 microg of VEGF in order to increase angiogenesis at the graft site. For the testis tissue culture experiment, 4-wk-old donor testis was cultured for 1 wk prior to grafting to stimulate spermatogonial stem cell proliferation. Testis tissue grafts were removed from the mice 24 wk after grafting. VEGF treatment increased graft weight and the percentage of seminiferous tubule cross sections with elongating spermatids at the time of graft removal. Cultured testis tissue grafts were smaller and had fewer seminiferous tubules per graft. However, there was no difference in the percentage of seminiferous tubule cross sections that contained any germ cell type between groups. These data indicate for the first time that bovine testis tissue can be manipulated to better support germ cell differentiation in grafted tissue.     

Biological activity and enrichment of spermatogonial stem cells in vitamin A-deficient and hyperthermia-exposed testes from mice based on colonization following germ cell transplantation

Spermatogenesis is a complex process in which spermatogonial stem cells divide and subsequently differentiate into spermatozoa. This process requires spermatogonial stem cells to self-renew and provide a continual population of cells for differentiation. Studies on spermatogonial stem cells have been limited due to a lack of unique markers and an inability to detect the presence of these cells. The technique of germ cell transplantation provides a functional assay to identify spermatogonial stem cells in a cell population. We hypothesized that vitamin A-deficient (VAD) and hyperthermically treated testes would provide an enriched in vivo source of spermatogonial stem cells. The first model, hyperthermic treatment, depends on the sensitivity of maturing germ cells to high temperatures. Testes of adult mice were exposed to 43 degrees C for 15 min to eliminate the majority of differentiating germ cells. Treated donor testes were 50% of normal adult testis size and, when transplanted into recipients, resulted in a 5.3- and 19-fold (colonies and area, respectively) increase in colonization efficiency compared to controls. The second model, VAD animals, also lacked differentiating germ cells, and testes weights were 25% of control values. Colonization efficiency of germ cells from VAD testes resulted in a 2.5- and 6.2-fold (colonies and area, respectively) increase in colonization compared to controls. Hyperthermically treated mice represent an enriched source of spermatogonial stem cells. In contrast, the low extent of colonization with germ cells from VAD animals raises important questions regarding the competency of stem cells from this model.     

Characterization of spermatogonial stem cell maturation and differentiation in neonatal mice

Initiation of the first wave of spermatogenesis in the neonatal mouse testis is characterized by the differentiation of a transient population of germ cells called gonocytes found in the center of the seminiferous tubule. The fate of gonocytes depends upon these cells resuming mitosis and developing the capacity to migrate from the center of the seminiferous tubule to the basement membrane. This process begins approximately Day 3 postpartum in the mouse, and by Day 6 postpartum differentiated type A spermatogonia first appear. It is essential for continual spermatogenesis in adults that some gonocytes differentiate into spermatogonial stem cells, which give rise to all differentiating germ cells in the testis, during this neonatal period. The presence of spermatogonial stem cells in a population of cells can be assessed with the use of the spermatogonial stem cell transplantation technique. Using this assay, we found that germ cells from the testis of Day 0-3 mouse pups can colonize recipient testes but do not proliferate and establish donor-derived spermatogenesis. However, germ cells from testes of Day 4-5 postpartum mice colonize recipient testes and generate large areas of donor-derived spermatogenesis. Likewise, germ cells from Day 10, 12, and 28 postpartum animals and adult animals colonize and establish donor-derived spermatogenesis, but a dramatic reduction in the number of colonies and the extent of colonization occurs from germ cell donors Days 12-28 postpartum that continues in adult donors. These results suggest spermatogonial stem cells are not present or not capable of initiating donor-derived spermatogenesis until Days 3-4 postpartum. The analysis of germ cell development during this time frame of development and spermatogonial stem cell transplantation provides a unique system to investigate the establishment of the stem cell niche within the mouse testis.     

A Distinct Expression Pattern of Cyclin K in Mammalian Testes Suggests a Functional Role in Spermatogenesis

Germ cell and embryonic stem cells are inextricably linked in many aspects. Remarkably both can generate all somatic cell types in organisms. Yet the molecular regulation accounting for these similarities is not fully understood. Cyclin K was previously thought to associate with CDK9 to regulate gene expression. However, we and others have recently shown that its cognate interacting partners are CDK12 and CDK13 in mammalian cells. We further demonstrated that cyclin K is essential for embryonic stem cell maintenance. In this study, we examined the expression of cyclin K in various murine and human tissues. We found that cyclin K is highly expressed in mammalian testes in a developmentally regulated manner. During neonatal spermatogenesis, cyclin K is highly expressed in gonocytes and spermatogonial stem cells. In adult testes, cyclin K can be detected in spermatogonial stem cells but is absent in differentiating spermatogonia, spermatids and spermatozoa. Interestingly, the strongest expression of cyclin K is detected in primary spermatocytes. In addition, we found that cyclin K is highly expressed in human testicular cancers. Knockdown of cyclin K in a testicular cancer cell line markedly reduces cell proliferation. Collectively, we suggest that cyclin K may be a novel molecular link between germ cell development, cancer development and embryonic stem cell maintenance.     

Clonal origin of germ cell colonies after spermatogonial transplantation in mice

Spermatogenesis originates from a small number of spermatogonial stem cells that can reinitiate spermatogenesis and produce germ cell colonies following transplantation into infertile recipient testes. Although several previous studies have suggested a single-cell origin of germ cell colonies, only indirect evidence has been presented. In this investigation, we tested the clonal origin hypothesis using a retrovirus, which could specifically mark an individual spermatogonial stem cell. Spermatogonial stem cells were infected in vitro with an enhanced green fluorescence protein-expressing retrovirus and subsequently transplanted into infertile recipient mice. Live haploid germ cells were recovered from individual colonies and were microinjected into eggs to create offspring. In total, 45 offspring were produced from five colonies, and 23 (51%) of the offspring were transgenic. Southern blot analysis indicated that the transgenic offspring from the single colony carried a common integration site, and the integration site was different among the transgenic offspring from different colonies. These results provide evidence that germ cell colonies develop from single spermatogonial stem cells.     

Bcl-2 inhibits apoptosis of spermatogonia and growth of spermatogonial stem cells in a cell-intrinsic manner

The growth, differentiation, and death/survival of spermatogonia are precisely regulated for the proper production of spermatozoa. We have previously shown that Bcl-2 ectopically expressed in spermatogonia caused the inhibition of normal spermatogonial apoptosis and the subsequent failure of differentiation in transgenic mice. In addition, the growth of spermatogonial stem cells seemed to be temporally arrested in the transgenic mice. In the present study, we attempted to examine whether the abnormality of spermatogonia described above was caused by Bcl-2 misexpression in the spermatogonia or by an abnormal spermatogenic environment of the transgenic mice. We transplanted testicular cells of transgenic mice to seminiferous tubules of W/Wv mice in which transplanted normal testicular cells can undergo spermatogenesis. We found that the transplanted spermatogonia of the transgenic mice reproduced a series of abnormal changes including temporal growth arrest of spermatogonial stem cells and abnormal accumulation of spermatogonia in tubules, which were also observed in the testes of the transgenic mice. The results indicated that Bcl-2 inhibited apoptosis of spermatogonia and growth of spermatogonial stem cells in a cell-intrinsic manner. We also cultured testicular cells of transgenic mice and found that the spermatogonia of the transgenic mice were better able to survive than were those of wild-type mice but that their differentiation was not affected. The result suggested that failure of differentiation of the accumulated spermatogonia in the transgenic testes is not due to the abnormality of the bcl-2 misexpressing spermatogonia, but may be caused by extrinsic problems including improper interaction of spermatogonia with supporting cells.     

Characterization of spermatogonial stem cells lacking intercellular bridges and genetic replacement of a mutation in spermatogonial stem cells

Stem cells have a potential of gene therapy for regenerative medicine. Among various stem cells, spermatogonial stem cells have a unique characteristic in which neighboring cells can be connected by intercellular bridges. However, the roles of intercellular bridges for stem cell self-renewal, differentiation, and proliferation remain to be elucidated. Here, we show not only the characteristics of testis-expressed gene 14 (TEX14) null spermatogonial stem cells lacking intercellular bridges but also a trial application of genetic correction of a mutation in spermatogonial stem cells as a model for future gene therapy. In TEX14 null testes, some genes important for undifferentiated spermatogonia as well as some differentiation-related genes were activated. TEX14 null spermatogonial stem cells, surprisingly, could form chain-like structures even though they do not form stable intercellular bridges. TEX14 null spermatogonial stem cells in culture possessed both characteristics of undifferentiated and differentiated spermatogonia. Long-term culture of TEX14 null spermatogonial stem cells could not be established likely secondary to up-regulation of CDK4 inhibitors and down-regulation of cyclin E. These results suggest that intercellular bridges are essential for both maintenance of spermatogonial stem cells and their proliferation. Lastly, a mutation in Tex14(+/-) spermatogonial stem cells was successfully replaced by homologous recombination in vitro. Our study provides a therapeutic potential of spermatogonial stem cells for reproductive medicine if they can be cultured long-term.