Constitutive WNT/CTNNB1 activation triggers spermatogonial stem cell proliferation and germ cell depletion

Effects of INSL3 and WNT2B gene polymorphisms on seasonal reproductive traits of Xinjiang Qira black sheep, Kazakh sheep and Duolang sheep

The purpose of this study was to investigate the polymorphism and genetic correlation of INSL3 and WNT2B genes with seasonal estrus and litter size in three different Xinjiang sheep breeds. The genetic diversity of INSL3 and WNT2B genes were analyzed, and their association with litter size and estrous traits were analyzed. The results showed that two SNPs (SNP1, SNP2) were detected in INSL3 gene and there were three genotypes in SNP2 (INSL3 (A100T)), named of AA, AT and TT, A was the dominant allele. Additionally, five SNPs (SNP3, SNP4, SNP5, SNP6, SNP7) were detected in the WNT2B gene and there were three genotypes in SNP4 (WNT2B (G126T)), named GG, GT and TT, G was dominant allele. SNP2 was in Hardy-Weinberg equilibrium in three sheep breeds (P > 0.05). SNP4 was deviated from Hardy-Weinberg equilibrium in three sheep breeds (P < 0.05). Further, AT genotype of SNP2 (INSL3 (A100T)) could significantly affect the estrus trait in Duolang sheep and Qira black sheep, and related to the litter size in Duolang sheep. The WNT2B significantly affected the estrus and litter size of Duolang sheep and Qira black sheep. INSL3 (A100T) and WNT2B (G126T) may be potential molecular markers for controlling seasonal reproductive trait in sheep.

Isoform-resolution single-cell RNA sequencing reveals the transcriptional panorama of adult Baoshan pig testis cells

BACKGROUND

As the primary organ of the male reproductive system, the testis facilitates spermatogenesis and androgen secretion. Due to the complexity of spermatogenesis, elucidating cellular heterogeneity and gene expression dynamics within the porcine testis is critical for advancing reproductive biology. Nevertheless, the cellular composition and regulatory mechanisms of porcine testes remain insufficiently characterized. In this study, we applied integrated long-read (Nanopore) and short-read (Illumina) scRNA-seq to Baoshan pig testes, establishing a comprehensive transcriptional profile to delineate cellular heterogeneity and molecular regulation.

RESULTS

Through systematic analysis of testicular architecture and the temporal progression of spermatogenesis, we characterized 11,520 single cells and 23,402 genes, delineating germ cell developmental stages: proliferative-phase spermatogonia (SPG), early-stage spermatocytes (Early SPC) and late-stage spermatocytes (Late SPC) during meiosis, and spermiogenic-phase round spermatids (RS) followed by elongating/elongated spermatids (ES), culminating in mature spermatozoa (Sperm). We further identified nine distinct testicular cell types, with germ cells spanning all developmental stages and somatic components comprising Sertoli cells, macrophages, and peritubular myoid cells as microenvironmental constituents, revealing the cellular heterogeneity of testicular tissue and dynamic characteristics of spermatogenesis. We obtained the dynamic expression changes of 16 vital marker genes during spermatogenesis and performed immunofluorescence validation on 7 marker genes. Gene ontology analysis revealed that germ cells at various stages were involved in specific biological processes, while cell communication networks highlighted eight pivotal signaling pathways, including MIF, NRG, WNT, VEGF, BMP, CCL, PARs, and ENHO pathways. Long-read sequencing further captured the full integrity and diversity of RNA transcripts, identifying 60% of the novel annotated isoforms and revealing that FSM isoforms exhibited longer transcript lengths, longer coding sequences, longer open reading frames, and a great number of exons, suggesting the complexity of isoforms within the testicular microenvironment.

CONCLUSIONS

Our results provide insight into the cellular heterogeneity, intercellular communication, and gene expression/transcript diversity in porcine testes, and offer a valuable resource for understanding the molecular mechanisms of porcine spermatogenesis.

3D Bioprinted Coaxial Testis Model Using Human Induced Pluripotent Stem Cells:A Step Toward Bicompartmental Cytoarchitecture and Functionalization

Fertility preservation following pediatric cancer therapy programs has become a major avenue of infertility research. In vitro spermatogenesis (IVS) aims to generate sperm from banked prepubertal testicular tissues in a lab setting using specialized culture conditions. While successful using rodent tissues, progress with human tissues is limited by the scarcity of human prepubertal testicular tissues for research. This study posits that human induced pluripotent stem cells (hiPSCs) can model human prepubertal testicular tissue to facilitate the development of human IVS conditions. Testicular cells derived from hiPSCs are characterized for phenotype markers and profiled transcriptionally. HiPSC-derived testicular cells are bioprinted into core-shell constructs representative of testis cytoarchitecture and found to capture functional aspects of prepubertal testicular tissues within 7 days under xeno-free conditions. Moreover, hiPSC-derived Sertoli cells illustrate the capacity to mature under pubertal-like conditions. The utility of the model is tested by comparing 2 methods of supplementing retinoic acid (RA), the vitamin responsible for inducing spermatogenesis. The model reveals a significant gain in activity under microsphere-released RA compared to RA medium supplementation, indicating that the fragility of free RA in vitro may be a contributing factor to the molecular dysfunction observed in human IVS studies to date.

Evolutionary innovations in germline biology of placental mammals identified by transcriptomics of first-wave spermatogenesis in opossum

Mammalian spermatogenesis is a highly stereotyped and conserved developmental process that is essential for fitness. At the same time, gene expression in spermatogenic cells is rapidly evolving. This combination of features has been suggested to drive rapid fixation of new gene expression patterns. Using a high-resolution dataset comprising bulk and single-cell data from juvenile and adult testes of the opossum Monodelphis domestica, a model marsupial, we define the developmental timing of the spermatogenic first wave in opossum and delineate conserved and divergent gene expression programs across the placental-marsupial split by comparison to equivalent data from mouse, a model placental mammal. Epigenomic data confirmed divergent regulation at the level of transcription, and comparison to data from four additional amniote species identified hundreds of genes with evidence of rapid fixation of expression. This gene set encompasses known and previously undescribed regulators of spermatogenic development.

Pluripotency factor Tex10 finetunes Wnt signaling for spermatogenesis and primordial germ cell development

Testis-specific transcript 10 (Tex10) is highly expressed in the testis, embryonic stem cells (ESCs), and primordial germ cells (PGCs). We previously generated a Tex10 knockout mouse model demonstrating its critical roles in ESC pluripotency and preimplantation development. Here, using conditional knockout mice and dTAG-degron ESCs, we show Tex10 is required for spermatogenesis and ESC-to-PGCLC differentiation. Specifically, Tex10-null spermatocytes arrest at metaphase I, compromising round spermatid formation. Tex10 depletion and overexpression compromise and enhance ESC-to-PGCLC differentiation, respectively. Mechanistically, bulk and single-cell RNA sequencing reveals that Tex10 depletion downregulates genes involved in pluripotency, PGC development, and spermatogenesis while upregulating genes promoting somatic programs. Chromatin occupancy study reveals that Tex10 binds to H3K4me3-marked promoters of Psmd3 and Psmd7, negative regulators of Wnt signaling, and activates their expression, thereby restraining Wnt signaling. Our study identifies Tex10 as a previously unappreciated factor in spermatogenesis and PGC development, offering potential therapeutic insights for treating male infertility.

Circular RNAs: novel noncoding players in male infertility

Infertility is a global problem being associated with emotional and financial burden. Recent studies have shown contribution of a group of non-coding RNAs, namely circular RNAs (circRNAs) to the etiology of some infertility conditions. CircRNA are transcribed from exons and form a circular RNA molecule, being abundant in eukaryotes. Traditionally classified as non-coding RNA, these transcripts are endogenously produced through either non-canonical back-splicing or linear splicing, typically produced from precursor messenger ribonucleic acid (pre-mRNA). While during the canonical splicing process the 3' end of the exon is joined to the 5' end of the succeeding exon to form linear mRNA, during backsplicing, the 3' end to the 5' end of the same exon is joined to make a circular molecule. circRNAs are involved in the regulation of several aspects of spermatogenesis. They appear to influence how stem germ cells grow and divide during the sperm production process. Malfunctions in circRNA activity could contribute to male infertility issues stemming from abnormalities in spermatogenesis. In the current review, we highlight the exciting potential of circRNAs as key players in the male fertility.

Deficiency in the Rab25 gene leads to a decline in male fertility and testicular injury: Impact on the regulation of germ cell proliferation and apoptosis

BACKGROUND

Rab25 is a member of the Rab family, functioning as a regulatory molecule in intracellular transport. Although its involvement in cellular functions and disease development is well-established, its precise roles in male reproductive physiology remain elusive.

METHODS

To explore the specific roles of Rab25 in testicular development and spermatogenesis, we established the Rab25-/- mouse model and Rab25 knockdown germ cell line (GC-2). We compared the fertility, sperm analysis, and testicular tissues between Rab25-/- and wild-type male mice. To delve deeper into potential mechanisms, we employed immunohistochemistry, TUNEL assay, Western Blotting, CCK-8 assay, etc. to evaluate cell proliferation and apoptosis in testicular tissues and GC-2 cells.

RESULTS

Our findings indicated that Rab25 was expressed in germ cells and Leydig cells in the testes. Although the weight of Rab25-/- mice testes exhibited no significant changes, fertility was compromised, with a decrease in sperm quantity and reduced motility. HE staining revealed a disorganized arrangement of germ cells and vacuolization. Additionally, chromatin marginalization and nuclear pyknosis were observed in the Rab25-/- mice. In both Rab25-/- mice testes and Rab25 knockdown GC-2 cells, we found that germ cell proliferation was reduced, while apoptosis was increased.

CONCLUSIONS

In conclusion, our study proposes that Rab25 plays a vital role in spermatogenesis by regulating the proliferation and apoptosis of germ cells.

Clinical and gonadal transcriptome analysis of 38,XX disorder of sex development pigs†

Pigs serve as a robust animal model for the study of human diseases, notably in the context of disorders of sex development (DSD). This study aims to investigate the phenotypic characteristics and molecular mechanisms underlying the reproductive and developmental abnormalities of 38,XX ovotestis-DSD (OT-DSD) and 38,XX testis-DSD (T-DSD) in pigs. Clinical and transcriptome sequencing analyses were performed on DSD and normal female pigs. Cytogenetic and SRY analyses confirmed that OT/T-DSD pigs exhibited a 38,XX karyotype and lacked the SRY gene. The DSD pigs had higher levels of follicle-stimulating hormone, luteinizing hormone, and progesterone, but lower testosterone levels when compared with normal male pigs. The reproductive organs of OT/T-DSD pigs exhibit abnormal development, displaying both male and female characteristics, with an absence of germ cells in the seminiferous tubules. Sex determination and development-related differentially expressed genes shared between DSD pigs were identified in the gonads, including WT1, DKK1, CTNNB1, WTN9B, SHOC, PTPN11, NRG1, and NXK3-1. DKK1 is proposed as a candidate gene for investigating the regulatory mechanisms underlying gonadal phenotypic differences between OT-DSD and T-DSD pigs. Consequently, our findings provide insights into the molecular pathogenesis of DSD pigs and present an animal model for studying into DSD in humans.

Transcriptome Analysis of Key Genes Involved in the Initiation of Spermatogonial Stem Cell Differentiation

PURPOSE

The purpose of this study was to screen the genes and pathways that are involved in spermatogonia stem cell (SSC) differentiation regulation during the transition from Aundiff to A1. Methods: RNA sequencing was performed to screen differentially expressed genes at 1 d and 2 d after SSC differentiation culture. KEGG pathway enrichment and GO function analysis were performed to reveal the genes and pathways related to the initiation of early SSC differentiation.

RESULTS

The GO analysis showed that Rpl21, which regulates cell differentiation initiation, significantly increased after 1 day of SSC differentiation. The expressions of Fn1, Cd9, Fgf2, Itgb1, Epha2, Ctgf, Cttn, Timp2 and Fgfr1, which are related to promoting differentiation, were up-regulated after 2 days of SSC differentiation. The analysis of the KEGG pathway revealed that RNA transport is the most enriched pathway 1 day after SSC differentiation. Hspa2, which promotes the differentiation of male reproductive cells, and Cdkn2a, which participates in the cell cycle, were significantly up-regulated. The p53 pathway and MAPK pathway were the most enriched pathways 2 days after SSC differentiation. Cdkn1a, Hmga2, Thbs1 and Cdkn2a, microRNAs that promote cell differentiation, were also significantly up-regulated.

CONCLUSIONS

RNA transport, the MAPK pathway and the p53 pathway may play vital roles in early SSC differentiation, and Rpl21, Fn1, Cd9, Fgf2, Itgb1, Epha2, Ctgf, Cttn, Timp2, Fgfr1, Hspa2, Cdkn2a, Cdkn1a, Hmga2 and Thbs1 are involved in the initiation of SSC differentiation. The findings of this study provide a reference for further revelations of the regulatory mechanism of SSC differentiation.

A rapid and stable spontaneous reprogramming system of Spermatogonial stem cells to Pluripotent State

BACKGROUND

The scarcity of pluripotent stem cells poses a major challenge to the clinical application, given ethical and biosafety considerations. While germline stem cells commit to gamete differentiation throughout life, studies demonstrated the spontaneous acquisition of pluripotency by spermatogonial stem cells (SSCs) from neonatal testes at a low frequency (1 in 1.5 × 107). Notably, this process occurs without exogenous oncogenes or chemical supplementation. However, while knockout of the p53 gene accelerates the transformation of SSCs, it also increases risk and hampers their clinical use.

RESULTS

We report a transformation system that efficiently and stably convert SSCs into pluripotent stem cells around 10 passages with the morphology similar to that of epiblast stem cells, which convert to embryonic stem (ES) cell-like colonies after change with ES medium. Epidermal growth factor (EGF), leukemia inhibitory factor (LIF) and fresh mouse embryonic fibroblast feeder (MEF) are essential for transformation, and addition of 2i (CHIR99021 and PD0325901) further enhanced the pluripotency. Transcriptome analysis revealed that EGF activated the RAS signaling pathway and inhibited p38 to initiate transformation, and synergically cooperated with LIF to promote the transformation.

CONCLUSION

This system established an efficient and safe resource of pluripotent cells from autologous germline, and provide new avenues for regenerative medicine and animal cloning.

iTRAQ-based proteomic analysis provides novel insight into the postnatal testicular development of Hu sheep

Testicular development is an intricate and coordinated process in which thousands of proteins are involved in the regulation of somatic cells development and spermatogenesis. However, knowledge about the proteomic changes during postnatal testicular development in Hu sheep is still elusive. The study was conducted to characterize the protein profiles at four key stages during postnatal testicular development, including infant (0-month-old, M0), puberty (3-month-old, M3), sexual maturity (6-month-old, M6) and body maturity (12-month-old, M12), and between the large- and small-testis groups at 6 months in Hu sheep. Consequently, 5252 proteins were identified using isobaric tags for relative and absolute quantification (iTRAQ) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods, and 465, 1261, 231 and 1080 differentially abundant proteins (DAPs) were found between M0_vs_M3, M3_vs_M6L, M6L_vs_M12, and M6L_vs_M6S, respectively. GO and KEGG analysis revealed that the majority of DAPs were involved in cellular process, metabolic process and immune system-related pathways. Furthermore, a protein-protein interaction network was constructed using 86 fertility-related DAPs, and five proteins with the highest degree were represented as hub proteins, including CTNNB1, ADAM2, ACR, HSPA2 and GRB2. This study provided new insights into the regulation mechanisms of postnatal testicular development and identified several potential biomarkers for selecting the high-fertility rams. SIGNIFICANCE OF THE STUDY: Testicular development is an intricate developmental process in which thousands of proteins are involved in regulating the somatic cells development and spermatogenesis. However, knowledge about the proteome changes during postnatal testicular development in Hu sheep is still elusive. This study provides comprehensive insights into the dynamic changes in the sheep testis proteome during postnatal testicular development. Additionally, testis size is positively correlated with semen quality and ejaculation volume, also for the merits of easy measurement, high heritability and selection efficiency, is an important indicator to select candidate rams with high fertility. The functional analyses of the acquired candidate proteins may help us gain a better understanding of the molecular regulatory mechanisms of testicular development.

The pluripotency factor Tex10 finetunes Wnt signaling for PGC and male germline development

Testis-specific transcript 10 (Tex10) is a critical factor for pluripotent stem cell maintenance and preimplantation development. Here, we dissect its late developmental roles in primordial germ cell (PGC) specification and spermatogenesis using cellular and animal models. We discover that Tex10 binds the Wnt negative regulator genes, marked by H3K4me3, at the PGC-like cell (PGCLC) stage in restraining Wnt signaling. Depletion and overexpression of Tex10 hyperactivate and attenuate the Wnt signaling, resulting in compromised and enhanced PGCLC specification efficiency, respectively. Using the Tex10 conditional knockout mouse models combined with single-cell RNA sequencing, we further uncover critical roles of Tex10 in spermatogenesis with Tex10 loss causing reduced sperm number and motility associated with compromised round spermatid formation. Notably, defective spermatogenesis in Tex10 knockout mice correlates with aberrant Wnt signaling upregulation. Therefore, our study establishes Tex10 as a previously unappreciated player in PGC specification and male germline development by fine-tuning Wnt signaling.

Transcriptome sequencing reveals the effects of circRNA on testicular development and spermatogenesis in Qianbei Ma goats

Circular RNAs (circRNAs) play an important role in regulating the mammalian reproductive system, especially testicular development and spermatogenesis. However, their functions in testicular development and spermatogenesis in the Qianbei Ma goat, the Guizhou endemic breed are still unclear. In this study, tissue sectioning and circRNAs transcriptome analysis were conducted to compare the changes of morphology and circular RNAs gene expression profile at four different developmental stages (0Y, 0-month-old; 6Y, 6-month-old; 12Y, 12-month-old; 18Y, 18-month-old). The results showed that the circumferences and area of the seminiferous tubule gradually increased with age, and the lumen of the seminiferous tubule in the testis differentiated significantly. 12,784 circRNAs were detected from testicular tissues at four different developmental stages by RNA sequencing, and 8,140 DEcircRNAs (differentially expressed circRNAs) were found in 0Y vs. 6Y, 6Y vs. 12Y, 12Y vs. 18Y and 0Y vs. 18Y, 0Y vs. 12Y, 6Y vs. 18Y Functional enrichment analysis of the source genes showed that they were mainly enriched in testicular development and spermatogenesis. In addition, the miRNAs and mRNAs associated with DECircRNAs in 6 control groups were predicted by bioinformatics, and 81 highly expressed DECircRNAs and their associated miRNAs and mRNAs were selected to construct the ceRNA network. Through functional enrichment analysis of the target genes of circRNAs in the network, some candidate circRNAs related to testicular development and spermatogenesis were obtained. Such as circRNA_07172, circRNA_04859, circRNA_07832, circRNA_00032 and circRNA_07510. These results will help to reveal the mechanism of circRNAs in testicular development and spermatogenesis, and also provide some guidance for goat reproduction.

A homozygous PIWIL2 frameshift variant affects the formation and maintenance of human-induced pluripotent stem cell-derived spermatogonial stem cells and causes Sertoli cell-only syndrome

Background

The most serious condition of male infertility is complete Sertoli cell-only syndrome (SCOS), which refers to the lack of all spermatogenic cells in the testes. The genetic cause of SCOS remains to be explored. We aimed to investigate the genetic cause of SCOS and assess the effects of the identified causative variant on human male germ cells.

Methods

Whole-exome sequencing was performed to identify potentially pathogenic variants in a man with complete SCOS, and Sanger sequencing was performed to verify the causative variant in this man and his father and brother. The pathogenic mechanisms of the causative variant were investigated by in vitro differentiation of human-induced pluripotent stem cells (hiPSCs) into germ cell-like cells.

Results

The homozygous loss-of-function (LoF) variant p.His244ArgfsTer31 (c.731_732delAT) in PIWIL2 was identified as the causative variant in the man with complete SCOS, and the same variant in heterozygosis was confirmed in his father and brother. This variant resulted in a truncated PIWIL2 protein lacking all functional domains, and no PIWIL2 expression was detected in the patient’s testes. The patient and PIWIL2^−/− hiPSCs could be differentiated into primordial germ cell-like cells and spermatogonial stem cell-like cells (SSCLCs) in vitro, but the formation and maintenance of SSCLCs were severely impaired. RNA-seq analyses suggested the inactivation of the Wnt signaling pathway in the process of SSCLC induction in the PIWIL2^−/− group, which was validated in the patient group by RT-qPCR. The Wnt signaling pathway inhibitor hindered the formation and maintenance of SSCLCs during the differentiation of normal hiPSCs.

Conclusions

Our study revealed the pivotal role of PIWIL2 in the formation and maintenance of human spermatogonial stem cells. We provided clinical and functional evidence that the LoF variant in PIWIL2 is a genetic cause of SCOS, which supported the potential role of PIWIL2 in genetic diagnosis. Furthermore, our results highlighted the applicability of in vitro differentiation models to function validation experiments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03175-6.

E-cadherin maintains the undifferentiated state of mouse spermatogonial progenitor cells via β-catenin

Background

Cadherins play a pivotal role in facilitating intercellular interactions between spermatogonial progenitor cells (SPCs) and their surrounding microenvironment. Specifically, E-cadherin serves as a cellular marker of SPCs in many species. Depletion of E-cadherin in mouse SPCs showed no obvious effect on SPCs homing and spermatogenesis.

Results

Here, we investigated the regulatory role of E-cadherin in regulating SPCs fate. Specific deletion of E-cadherin in germ cells was shown to promote SPCs differentiation, evidencing by reduced PLZF⁺ population and increased c-Kit⁺ population in mouse testes. E-cadherin loss down-regulated the expression level of β-catenin, leading to the reduced β-catenin in nuclear localization for transcriptional activity. Remarkably, increasing expression level of Cadherin-22 (CDH22) appeared specifically after E-cadherin deletion, indicating CDH22 played a synergistic effect with E-cadherin in SPCs. By searching for the binding partners of β-catenin, Lymphoid enhancer-binding factor 1 (LEF1), T-cell factor (TCF3), histone deacetylase 4 (HDAC4) and signal transducer and activator 3 (STAT3) were identified as suppressors of SPCs differentiation by regulating acetylation of differentiation genes with PLZF.

Conclusions

Two surface markers of SPCs, E-cadherin and Cadherin-22, synergically maintain the undifferentiation of SPCs via the pivotal intermediate molecule β-catenin. LEF1, TCF3, STAT3 and HDAC4 were identified as co-regulatory factors of β-catenin in regulation of SPC fate. These observations revealed a novel regulatory pattern of cadherins on SPCs fate.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00880-w.

Early Gonadal Development and Sex Determination in Mammal

Sex determination is crucial for the transmission of genetic information through generations. In mammal, this process is primarily regulated by an antagonistic network of sex-related genes beginning in embryonic development and continuing throughout life. Nonetheless, abnormal expression of these sex-related genes will lead to reproductive organ and germline abnormalities, resulting in disorders of sex development (DSD) and infertility. On the other hand, it is possible to predetermine the sex of animal offspring by artificially regulating sex-related gene expression, a recent research hotspot. In this paper, we reviewed recent research that has improved our understanding of the mechanisms underlying the development of the gonad and primordial germ cells (PGCs), progenitors of the germline, to provide new directions for the treatment of DSD and infertility, both of which involve manipulating the sex ratio of livestock offspring.

Identification and validation of a tumor mutation burden-related signature combined with immune microenvironment infiltration in adrenocortical carcinoma

Tumor mutation burden (TMB), an emerging molecular determinant, is accompanied by microsatellite instability and immune infiltrates in various malignancies. However, whether TMB is related to the prognosis or immune responsiveness of adrenocortical carcinoma (ACC) remains to be elucidated. This paper aims to investigate the impact of TMB on the prognosis and immune microenvironment infiltration in ACC. The somatic mutation data, gene expression profile, and corresponding clinicopathological information were retrieved from TCGA. The mutation landscape was summarized and visualized with the waterfall diagram. The ACC patients were divided into low and high TMB groups based on the median TMB value and differentially expressed genes (DEGs) between the two groups were identified. Diverse functional analyses were conducted to determine the functionality of the DEGs. The immune cell infiltration signatures were evaluated based on multiple algorithms. Eventually, a TMB Prognostic Signature (TMBPS) was established and its predictive accuracy for ACC was evaluated. Single nucleotide polymorphism and C > T were found to be more common than other missense mutations. In addition, lower TMB levels indicated improved survival outcomes and were correlated with younger age and earlier clinical stage. Functional analysis suggested that DEGs were primarily related to the cell cycle, DNA replication, and cancer progression. Additionally, significant differences in infiltration levels of activated CD4+ T cells, naive B cells, and activated NK cells were observed in two TMB groups. We also found that patients with higher TMBPS showed worse survival outcomes, which was validated in the Gene Expression Omnibus database. Our study systematically analyzed the mutation and identified a TMBPS combined with immune microenvironment infiltration in ACC. It is expected that this paper can promote the development of ACC treatment strategies.

Retinoblastoma-E2F Transcription Factor Interplay Is Essential for Testicular Development and Male Fertility

The retinoblastoma (RB) protein family members (pRB, p107 and p130) are key regulators of cell cycle progression, but also play crucial roles in apoptosis, and stem cell self-renewal and differentiation. RB proteins exert their effects through binding to E2F transcription factors, which are essential developmental and physiological regulators of tissue and organ homeostasis. According to the canonical view, phosphorylation of RB results in release of E2Fs and induction of genes needed for progress of the cell cycle. However, there are eight members in the E2F transcription factor family with both activator (E2F1-3a) and repressor (E2F3b-E2F8) roles, highlighting the functional diversity of RB-E2F pathway. In this review article we summarize the data showing that RB-E2F interaction is a key cell-autonomous mechanism responsible for establishment and maintenance of lifelong male fertility. We also review the expression pattern of RB proteins and E2F transcription factors in the testis and male germ cells. The available evidence supports that RB and E2F family members are widely and dynamically expressed in the testis, and they are known to have versatile roles during spermatogenesis. Knowledge of the function and significance of RB-E2F interplay for testicular development and spermatogenesis comes primarily from gene knock-out (KO) studies. Several studies conducted in Sertoli cell-specific pRB-KO mice have demonstrated that pRB-mediated inhibition of E2F3 is essential for Sertoli cell functional maturation and cell cycle exit, highlighting that RB-E2F interaction in Sertoli cells is paramount to male fertility. Similarly, ablation of either pRB or E2F1 in the germline results in progressive testicular atrophy due to germline stem cell (GSC) depletion, emphasizing the importance of proper RB-E2F interplay for germline maintenance and lifelong sperm production. In summary, while balanced RB-E2F interplay is essential for cell-autonomous maintenance of GSCs and, the pRB-E2F3 system in Sertoli cells is critical for providing GSC niche thus laying the basis for spermatogenesis.

The Proliferation of Pre-Pubertal Porcine Spermatogonia in Stirred Suspension Bioreactors Is Partially Mediated by the Wnt/β-Catenin Pathway

Male survivors of childhood cancer are at risk of suffering from infertility in adulthood because of gonadotoxic chemotherapies. For adult men, sperm collection and preservation are routine procedures prior to treatment; however, this is not an option for pre-pubertal children. From young boys, a small biopsy may be taken before chemotherapy, and spermatogonia may be propagated in vitro for future transplantation to restore fertility. A robust system that allows for scalable expansion of spermatogonia within a controlled environment is therefore required. Stirred suspension culture has been applied to different types of stem cells but has so far not been explored for spermatogonia. Here, we report that pre-pubertal porcine spermatogonia proliferate more in bioreactor suspension culture, compared with static culture. Interestingly, oxygen tension provides an avenue to modulate spermatogonia status, with culture under 10% oxygen retaining a more undifferentiated state and reducing proliferation in comparison with the conventional approach of culturing under ambient oxygen levels. Spermatogonia grown in bioreactors upregulate the Wnt/ β-catenin pathway, which, along with enhanced gas and nutrient exchange observed in bioreactor culture, may synergistically account for higher spermatogonia proliferation. Therefore, stirred suspension bioreactors provide novel platforms to culture spermatogonia in a scalable manner and with minimal handling.

Protein kinase A drives paracrine crisis and WNT4-dependent testis tumor in Carney complex

Large-cell calcifying Sertoli cell tumors (LCCSCTs) are among the most frequent lesions occurring in male Carney complex (CNC) patients. Although they constitute a key diagnostic criterion for this rare multiple neoplasia syndrome resulting from inactivating mutations of the tumor suppressor PRKAR1A, leading to unrepressed PKA activity, LCCSCT pathogenesis and origin remain elusive. Mouse models targeting Prkar1a inactivation in all somatic populations or separately in each cell type were generated to decipher the molecular and paracrine networks involved in the induction of CNC testis lesions. We demonstrate that the Prkar1a mutation was required in both stromal and Sertoli cells for the occurrence of LCCSCTs. Integrative analyses comparing transcriptomic, immunohistological data and phenotype of mutant mouse combinations led to the understanding of human LCCSCT pathogenesis and demonstrated PKA-induced paracrine molecular circuits in which the aberrant WNT4 signal production is a limiting step in shaping intratubular lesions and tumor expansion both in a mouse model and in human CNC testes.

Current scenario and challenges ahead in application of spermatogonial stem cell technology in livestock

PURPOSE

Spermatogonial stem cells (SSCs) are the source for the mature male gamete. SSC technology in humans is mainly focusing on preserving fertility in cancer patients. Whereas in livestock, it is used for mining the factors associated with male fertility. The review discusses the present status of SSC biology, methodologies developed for in vitro culture, and challenges ahead in establishing SSC technology for the propagation of superior germplasm with special reference to livestock.

METHOD

Published literatures from PubMed and Google Scholar on topics of SSCs isolation, purification, characterization, short and long-term culture of SSCs, stemness maintenance, epigenetic modifications of SSCs, growth factors, and SSC cryopreservation and transplantation were used for the study.

RESULT

The fine-tuning of SSC isolation and culture conditions with special reference to feeder cells, growth factors, and additives need to be refined for livestock. An insight into the molecular mechanisms involved in maintaining stemness and proliferation of SSCs could facilitate the dissemination of superior germplasm through transplantation and transgenesis. The epigenetic influence on the composition and expression of the biomolecules during in vitro differentiation of cultured cells is essential for sustaining fertility. The development of surrogate males through gene-editing will be historic achievement for the foothold of the SSCs technology.

CONCLUSION

Detailed studies on the species-specific factors regulating the stemness and differentiation of the SSCs are required for the development of a long-term culture system and in vitro spermatogenesis in livestock. Epigenetic changes in the SSCs during in vitro culture have to be elucidated for the successful application of SSCs for improving the productivity of the animals.

Arrest of WNT/β-catenin signaling enables the transition from pluripotent to differentiated germ cells in mouse ovaries

Germ cells form the basis for sexual reproduction by producing gametes. In ovaries, primordial germ cells exit the cell cycle and the pluripotency-associated state, differentiate into oogonia, and initiate meiosis. Despite the importance of germ cell differentiation for sexual reproduction, signaling pathways regulating their fate remain largely unknown. Here, we show in mouse embryonic ovaries that germ cell-intrinsic β-catenin activity maintains pluripotency and that its repression is essential to allow differentiation and meiosis entry in a timely manner. Accordingly, in β-catenin loss-of-function and gain-of-function mouse models, the germ cells precociously enter meiosis or remain in the pluripotent state, respectively. We further show that interaction of β-catenin and the pluripotent-associated factor POU5F1 in the nucleus is associated with germ cell pluripotency. The exit of this complex from the nucleus correlates with germ cell differentiation, a process promoted by the up-regulation of Znrf3, a negative regulator of WNT/β-catenin signaling. Together, these data identify the molecular basis of the transition from primordial germ cells to oogonia and demonstrate that β-catenin is a central gatekeeper in ovarian differentiation and gametogenesis.

Constitutive activation of CTNNB1 results in a loss of spermatogonial stem cell activity in mice

Spermatogenesis requires that a careful balance be maintained between the self-renewal of spermatogonial stem cells (SSCs) and their commitment to the developmental pathway through which they will differentiate into spermatozoa. Recently, a series of studies employing various in vivo and in vitro models have suggested a role of the wingless-related MMTV integration site gene family/beta-catenin (WNT/CTNNB1) pathway in determining the fate of SSCs. However, conflicting data have suggested that CTNNB1 signaling may either promote SSC self-renewal or differentiation. Here, we studied the effects of sustained CTNNB1 signaling in SSCs using the Ctnnb1^tm1Mmt/+; Ddx4-Cre^Tr/+ (ΔCtnnb1) mouse model, in which a stabilized form of CTNNB1 is expressed in all germ cells. ΔCtnnb1 mice were found to have reduced testis weights and partial germ cell loss by 4 months of age. Germ cell transplantation assays showed a 49% reduction in total functional SSC numbers in 8 month-old transgenic mice. In vitro, Thy1-positive undifferentiated spermatogonia from ΔCtnnb1 mice formed 57% fewer clusters, which was associated with decreased cell proliferation. A reduction in mRNA levels of genes associated with SSC maintenance (Bcl6b, Gfra1, Plzf) and increased levels for markers associated with progenitor and differentiating spermatogonia (Kit, Rarg, Sohlh1) were detected in these cluster cells. Furthermore, RNAseq performed on these clusters revealed a network of more than 900 genes regulated by CTNNB1, indicating that CTNNB1 is an important regulator of spermatogonial fate. Together, our data support the notion that CTNNB1 signaling promotes the transition of SSCs to undifferentiated progenitor spermatogonia at the expense of their self-renewal.

POSTN Promotes the Proliferation of Spermatogonial Cells by Activating the Wnt/β-Catenin Signaling Pathway

The self-renewal of spermatogonial cells (SCs) provides the foundation for life-long spermatogenesis. To date, only a few growth factors have been used for the culture of SCs in vitro, and how to enhance proliferation capacity of SCs in vitro needs further research. This study aimed to explore the effects of periostin (POSTN) on the proliferation of human SCs. GC-1 spg cells were cultured in a medium with POSTN, cell proliferation was evaluated by MTS analysis and EdU assay, and the Wnt/β-catenin signaling pathway was examined. Thereafter, the proliferations of human SC were detected using immunofluorescence and RT-PCR. In this study, we found that CM secreted by human amniotic mesenchymal stem cells (hAMSCs) could enhance the proliferation capacity of mouse GC-1 spg cells. Label-free mass spectrometry and ELISA analysis demonstrated that high level of POSTN was secreted by hAMSCs. MTS and EdU staining showed that POSTN increased GC-1 spg cell proliferation, whereas CM from POSTN-silenced hAMSCs suppressed cell proliferation capacity. Then POSTN was found to activate the Wnt/β-catenin signaling pathway to regulate the proliferation of GC-1 spg cells. XAV-939, a Wnt/β-catenin inhibitor, partially reversed the effects of POSTN on GC-1 spg cell proliferation. We then analyzed human SCs and found that POSTN promoted human SC proliferation in vitro. These findings provide insights regarding the role of POSTN in regulating SC proliferation via the Wnt/β-catenin signaling pathway and suggest that POSTN may serve as a cytokine for male infertility therapy.

SPATS1 (spermatogenesis-associated, serine-rich 1) is not essential for spermatogenesis and fertility in mouse

SPATS1 (spermatogenesis-associated, serine-rich 1) is an evolutionarily conserved, testis-specific protein that is differentially expressed during rat male meiotic prophase. Some reports have suggested a link between SPATS1 underexpression/mutation and human pathologies such as male infertility and testicular cancer. Given the absence of functional studies, we generated a Spats1 loss-of-function mouse model using CRISPR/Cas9 technology. The phenotypic analysis showed no overt phenotype in Spats1^-/- mice, with both males and females being fertile. Flow cytometry and histological analyses did not show differences in the testicular content and histology between WT and knockout mice. Moreover, no significant differences in sperm concentration, motility, and morphology, were observed between WT and KO mice. These results were obtained both for young adults and for aged animals. Besides, although an involvement of SPATS1 in the Wnt signaling pathway has been suggested, we did not detect changes in the expression levels of typical Wnt pathway-target genes in mutant individuals. Thus, albeit Spats1 alteration might be a risk factor for male testicular health, we hereby show that this gene is not individually essential for male fertility and spermatogenesis in mouse.

WNT signalling in the normal human adult testis and in male germ cell neoplasms

STUDY QUESTION

Is WNT signalling functional in normal and/or neoplastic human male germ cells?

SUMMARY ANSWER

Regulated WNT signalling component synthesis in human testes indicates that WNT pathway function changes during normal spermatogenesis and is active in testicular germ cell tumours (TGCTs), and that WNT pathway blockade may restrict seminoma growth and migration.

WHAT IS KNOWN ALREADY

Regulated WNT signalling governs many developmental processes, including those affecting male fertility during early germ cell development at embryonic and adult (spermatogonial) ages in mice. In addition, although many cancers arise from WNT signalling alterations, the functional relevance and WNT pathway components in TGCT, including germ cell neoplasia in situ (GCNIS), are unknown.

STUDY DESIGN, SIZE, DURATION

The cellular distribution of transcripts and proteins in WNT signalling pathways was assessed in fixed human testis sections with normal spermatogenesis, GCNIS and seminoma (2-16 individuals per condition). Short-term (1-7 h) ligand activation and long-term (1-5 days) functional outcomes were examined using the well-characterised seminoma cell line, TCam-2. Pathway inhibition used siRNA or chemical exposures over 5 days to assess survival and migration.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The cellular localisation of WNT signalling components was determined using in situ hybridisation and immunohistochemistry on Bouin's- and formalin-fixed human testis sections with complete spermatogenesis or germ cell neoplasia, and was also assessed in TCam-2 cells. Pathway function tests included exposure of TCam-2 cells to ligands, small molecules and siRNAs. Outcomes were measured by monitoring beta-catenin (CTNNB1) intracellular localisation, cell counting and gap closure measurements.

MAIN RESULTS AND THE ROLE OF CHANCE

Detection of nuclear-localised beta-catenin (CTNNB1), and key WNT signalling components (including WNT3A, AXIN2, TCF7L1 and TCF7L2) indicate dynamic and cell-specific pathway activity in the adult human testis. Their presence in germ cell neoplasia and functional analyses in TCam-2 cells indicate roles for active canonical WNT signalling in TGCT relating to viability and migration. All data were analysed to determine statistical significance.

LARGE SCALE DATA

No large-scale datasets were generated in this study.

LIMITATIONS, REASONS FOR CAUTION

As TGCTs are rare and morphologically heterogeneous, functional studies in primary cancer cells were not performed. Functional analysis was performed with the only well-characterised, widely accepted seminoma-derived cell line.

WIDER IMPLICATIONS OF THE FINDINGS

This study demonstrated the potential sites and involvement of the WNT pathway in human spermatogenesis, revealing similarities with murine testis that suggest the potential for functional conservation during normal spermatogenesis. Evidence that inhibition of canonical WNT signalling leads to loss of viability and migratory activity in seminoma cells suggests that potential treatments using small molecule or siRNA inhibitors may be suitable for patients with metastatic TGCTs.

STUDY FUNDING AND COMPETING INTEREST(S)

This study was funded by National Health and Medical Research Council of Australia (Project ID 1011340 to K.L.L. and H.E.A., and Fellowship ID 1079646 to K.L.L.) and supported by the Victorian Government's Operational Infrastructure Support Program. None of the authors have any competing interests.

BMP4 activates the Wnt-Lin28A-Blimp1-Wnt pathway to promote primordial germ cell formation via altering H3K4me2

The unique developmental characteristics of chicken primordial germ cells (PGCs) enable them to be used in recovery of endangered bird species, gene editing and the generation of transgenic birds, but the limited number of PGCs greatly limits their application. Studies have shown that the formation of mammalian PGCs is induced by BMP4 signal, but the mechanism underlying chicken PGC formation has not been determined. Here, we confirmed that Wnt signaling activated via BMP4 activates transcription of Lin28A by inducing β-catenin to compete with LSD1 for binding to TCF7L2, causing LSD1 to dissociate from the Lin28A promoter and enhancing H3K4me2 methylation in this region. Lin28A promotes PGC formation by inhibiting gga-let7a-3p maturation to initiate Blimp1 expression. Interestingly, expression of Blimp1 helped sustain Wnt5A expression by preventing LSD1 binding to the Wnt5A promoter. We thus elucidated a positive feedback pathway involving Wnt-Lin28A-Blimp1-Wnt that ensures PGC formation. In summary, our data provide new insight into the development of PGCs in chickens.

The role of Wnt signaling in male reproductive physiology and pathology

Accumulating evidence has shown that Wnt signaling is deeply involved in male reproductive physiology, and malfunction of the signal path can cause pathological changes in genital organs and sperm cells. These abnormalities are diverse in manifestation and have been constantly found in the knockout models of Wnt studies. Nevertheless, most of the research solely focused on a certain factor in the Wnt pathway, and there are few reports on the overall relation between Wnt signals and male reproductive physiology. In our review, Wnt findings relating to the reproductive system were sought and summarized in terms of Wnt ligands, Wnt receptors, Wnt intracellular signals and Wnt regulators. By sorting out and integrating relevant functions, as well as underlining the controversies among different reports, our review aims to offer an overview of Wnt signaling in male reproductive physiology and pathology for further mechanistic studies.

Mouse Spermatogenesis Reflects the Unity and Diversity of Tissue Stem Cell Niche Systems

Mouse spermatogenesis is supported by spermatogenic stem cells (SSCs). SSCs maintain their pool while migrating over an open (or facultative) niche microenvironment of testicular seminiferous tubules, where ligands that support self-renewal are likely distributed widely. This contrasts with the classic picture of closed (or definitive) niches in which stem cells are gathered and the ligands are highly localized. Some of the key properties observed in the dynamics of SSCs in the testicular niche in vivo, which show the flexible and stochastic (probabilistic) fate behaviors, are found to be generic for a wide range of, if not all, tissue stem cells. SSCs also show properties characteristic of an open niche-supported system, such as high motility. Motivated by the properties of SSCs, in this review, I will reconsider the potential unity and diversity of tissue stem cell systems, with an emphasis on the varying degrees of ligand distribution and stem cell motility.

KIF3A regulates the Wnt/β-catenin pathway via transporting β-catenin during spermatogenesis in Eriocheir sinensis

The Wnt/β-catenin pathway participates in many important physiological events such as cell proliferation and differentiation in the male reproductive system. We found that Kinesin-2 motor KIF3A is highly expressed during spermatogenesis in Eriocheir sinensis; it may potentially promote the intracellular transport of cargoes in this process. However, only a few studies have focused on the relationship between KIF3A and the Wnt/β-catenin pathway in the male reproductive system of decapod crustaceans. In this study, we cloned and characterized the CDS of β-catenin in E. sinensis for the first time. Fluorescence in situ hybridization and immunofluorescence results showed the colocalization of Es-KIF3A and Es-β-catenin at the mRNA and the protein level respectively. To further explore the regulatory function of Es-KIF3A to the Wnt/β-catenin pathway, the es-kif3a was knocked down by double-stranded RNA (dsRNA) in vivo and in primary cultured cells in testes of E. sinensis. Results showed that the expression of es-β-catenin and es-dvl were decreased in the es-kif3a knockdown group. The protein expression level of Es-β-catenin was also reduced and the location of Es-β-catenin was changed from nucleus to cytoplasm in the late stage of spermatogenesis when es-kif3a was knocked down. Besides, the co-IP result demonstrated that Es-KIF3A could bind with Es-β-catenin. In summary, this study indicates that Es-KIF3A can positively regulate the Wnt/β-catenin pathway during spermatogenesis and Es-KIF3A can bind with Es-β-catenin to facilitate the nuclear translocation of Es-β-catenin.

Stem Cells, Self-Renewal, and Lineage Commitment in the Endocrine System

The endocrine system coordinates a wide array of body functions mainly through secretion of hormones and their actions on target tissues. Over the last decades, a collective effort between developmental biologists, geneticists, and stem cell biologists has generated a wealth of knowledge related to the contribution of stem/progenitor cells to both organogenesis and self-renewal of endocrine organs. This review provides an up-to-date and comprehensive overview of the role of tissue stem cells in the development and self-renewal of endocrine organs. Pathways governing crucial steps in both development and stemness maintenance, and that are known to be frequently altered in a wide array of endocrine disorders, including cancer, are also described. Crucially, this plethora of information is being channeled into the development of potential new cell-based treatment modalities for endocrine-related illnesses, some of which have made it through clinical trials.

Integrated analysis of proteomics and metabolomics reveals the potential sex determination mechanism in Odontobutis potamophila

Odontobutis potamophila is a valuable species for aquaculture in China, which shows asexually dimorphic growth pattern. In this study, the integrated proteomics and metabolomics were used to analyze the sex determination mechanism. A total of 2781 significantly different regulated proteins were identified by proteomics and 2693 significantly different expressed metabolites were identified by metabolomics. Among them, 2560 proteins and 1701 metabolites were significantly up-regulated in testes, whereas 221 proteins and 992 metabolites were significantly up-regulated in ovaries. Venn diagram analysis showed 513 proteins were differentially regulated at both protein and metabolite levels. Correlation analysis of differentially-regulated proteins and metabolites were identified by Gene Ontology annotation and Kyoto Encyclopedia of Genes and Genomes pathway analysis. The results showed lipid metabolism plays an important role in sex determination. The metabolites decanoyl-CoA, leukotriene, 3-dehydrosphinganine, and arachidonate were the biomarkers in testes, whereas estrone and taurocholate were the biomarkers in ovaries. Interaction networks of the significant differentially co-regulated proteins and metabolites in the process of lipid metabolism showed arachidonic acid metabolism and steroid hormone biosynthesis were the most important pathways in sex determination. The findings of this study provide valuable information for selective breeding of O. potamophila. SIGNIFICANCE OF THE STUDY: The male O. potamophila grows substantially larger and at a quicker rate than the female. Thus, males have greater economic value than females. However, limited research was done to analyze the sex determination mechanism of O. potamophila, which seriously hindered the development of whole-male O. potamophila breeding. In this study, four key proteins (Ctnnb1, Piwil1, Hsd17b1, and Dnali1), six most important biomarkers (decanoyl-CoA, leukotriene, 3-dehydrosphinganine, arachidonate, estrone, and taurocholate) and two key pathways (arachidonic acid metabolism and steroid hormone biosynthesis) in sex determination of O. potamophila were found by integrated application of iTRAQ and LC-MS techniques. The results give valuable information for molecular breeding of O. potamophila in aquaculture.

Testis Development

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

Testicular organoids to study cell-cell interactions in the mammalian testis

BACKGROUND

Over the last ten years, three-dimensional organoid culture has garnered renewed interest, as organoids generated from primary cells or stem cells with cell associations and functions similar to organs in vivo can be a powerful tool to study tissue-specific cell-cell interactions in vitro. Very recently, a few interesting approaches have been put forth for generating testicular organoids for studying the germ cell niche microenvironment.

AIM

To review different model systems that have been employed to study germ cell biology and testicular cell-cell interactions and discuss how the organoid approach can address some of the shortcomings of those systems.

RESULTS AND CONCLUSION

Testicular organoids that bear architectural and functional similarities to their in vivo counterparts are a powerful model system to study cell-cell interactions in the germ cell niche. Organoids enable studying samples in humans and other large animals where in vivo experiments are not possible, allow modeling of testicular disease and malignancies and may provide a platform to design more precise therapeutic interventions.

Three-dimensional testicular organoids as novel in vitro models of testicular biology and toxicology

Organoids are three dimensional structures consisting of multiple cell types that recapitulate the cellular architecture and functionality of native organs. Over the last decade, the advent of organoid research has opened up many avenues for basic and translational studies. Following suit of other disciplines, research groups working in the field of male reproductive biology have started establishing and characterizing testicular organoids. The three-dimensional architectural and functional similarities of organoids to their tissue of origin facilitate study of complex cell interactions, tissue development and establishment of representative, scalable models for drug and toxicity screening. In this review, we discuss the current state of testicular organoid research, their advantages over conventional monolayer culture and their potential applications in the field of reproductive biology and toxicology.

Open niche regulation of mouse spermatogenic stem cells

In mammalian testes, robust stem cell functions ensure the continual production of sperm. In testicular seminiferous tubules, spermatogenic stem cells (SSCs) are highly motile and are interspersed between their differentiating progeny, while undergoing self-renewal and differentiation. In such an "open niche" microenvironment, some SSCs proliferate, while others exit the stem cell compartment through differentiation; therefore, self-renewal and differentiation are perfectly balanced at the population (or tissue) level, a dynamics termed "population asymmetry." This is in stark contrast to the classical perception of tissue stem cells being cells that are clustered in a specialized "closed niche" region and that invariantly undergo asymmetric divisions. However, despite its importance, how the self-renewal and differentiation of SSCs are balanced in an open niche environment is poorly understood. Recent studies have thrown light on the key mechanism that enables SSCs to follow heterogeneous fates, although they are equally exposed to signaling molecules controlling self-renewal and differentiation. In particular, SSCs show heterogeneous susceptibilities to differentiation-promoting signals such as Wnt and retinoic acid. Heterogeneous susceptibility to the ubiquitously distributed fate-controlling extracellular signal might be a key generic mechanism for the heterogeneous fate of tissue stem cells in open niche microenvironments.