Efficient generation of functional haploid spermatids from human germline stem cells by three-dimensional-induced system

Elucidating the Transcriptional States of Spermatogenesis-Joint Analysis of Germline and Supporting Cell, Mice and Human, Normal and Perturbed, Bulk and Single-Cell RNA-Seq

In studying the molecular underpinning of spermatogenesis, we expect to understand the fundamental biological processes better and potentially identify genes that may lead to novel diagnostic and therapeutic strategies toward precision medicine in male infertility. In this review, we emphasized our perspective that the path forward necessitates integrative studies that rely on complementary approaches and types of data. To comprehensively analyze spermatogenesis, this review proposes four axes of integration. First, spanning the analysis of spermatogenesis in the healthy state alongside pathologies. Second, the experimental analysis of model systems (in which we can deploy treatments and perturbations) alongside human data. Third, the phenotype is measured alongside its underlying molecular profiles using known markers augmented with unbiased profiles. Finally, the testicular cells are studied as ecosystems, analyzing the germ cells alongside the states observed in the supporting somatic cells. Recently, the study of spermatogenesis has been advancing using single-cell RNA sequencing, where scientists have uncovered the unique stages of germ cell development in mice, revealing new regulators of spermatogenesis and previously unknown cell subtypes in the testis. An in-depth analysis of meiotic and postmeiotic stages led to the discovery of marker genes for spermatogonia, Sertoli and Leydig cells and further elucidated all the other germline and somatic cells in the testis microenvironment in normal and pathogenic conditions. The outcome of an integrative analysis of spermatogenesis using advanced molecular profiling technologies such as scRNA-seq has already propelled our biological understanding, with additional studies expected to have clinical implications for the study of male fertility. By uncovering new genes and pathways involved in abnormal spermatogenesis, we may gain insights into subfertility or sterility.

In Vitro Generation of Haploid Germ Cells from Human XY and XXY Immature Testes in a 3D Organoid System

Increasing survival rates of children following cancer treatment have resulted in a significant population of adult survivors with the common side effect of infertility. Additionally, the availability of genetic testing has identified Klinefelter syndrome (classic 47,XXY) as the cause of future male infertility for a significant number of prepubertal patients. This study explores new spermatogonia stem cell (SSC)-based fertility therapies to meet the needs of these patients. Testicular cells were isolated from cryopreserved human testes tissue stored from XY and XXY prepubertal patients and propagated in a two-dimensional culture. Cells were then incorporated into a 3D human testicular organoid (HTO) system. During a 3-week culture period, HTOs maintained their structure, viability, and metabolic activity. Cell-specific PCR and flow cytometry markers identified undifferentiated spermatogonia, Sertoli, Leydig, and peritubular cells within the HTOs. Testosterone was produced by the HTOs both with and without hCG stimulation. Upregulation of postmeiotic germ cell markers was detected after 23 days in culture. Fluorescence in situ hybridization (FISH) of chromosomes X, Y, and 18 identified haploid cells in the in vitro differentiated HTOs. Thus, 3D HTOs were successfully generated from isolated immature human testicular cells from both euploid (XY) and Klinefelter (XXY) patients, supporting androgen production and germ cell differentiation in vitro.

Alkylating agents-induced gonadotoxicity in prepubertal males: Insights on the clinical and preclinical front

Rising cure rates in pediatric cancer patients warrants an increased attention toward the long-term consequences of the diagnosis and treatment in survivors. Chemotherapeutic agents can be gonadotoxic, rendering them at risk for infertility post-survival. While semen cryopreservation is an option that can be provided for most (post)pubertal boys before treatment, this is unfortunately not an option prepubertal in age, simply due to the lack of spermatogenesis. Over the last couple of years, studies have thus focused on better understanding the testis niche in response to various chemotherapeutic agents that are commonly administered and their direct and indirect impact on the germ cell populations. These are generally compounds that have a high risk of infertility and have been classified into risk categories in curated fertility guidelines. However, with it comes the lack of evidence and the challenge of using informative models and conditions most reflective of the physiological scenario, in short, the appropriate study designs for clinically relevant outcomes. Besides, the exact mechanism(s) of action for many of these "risk" compounds as well as other agents is unclear. Understanding their behavior and effect on the testis niche will pave the way for incorporating new strategies to ultimately combat infertility. Of the various drug classes, alkylating agents pose the highest risk of gonadotoxicity as per previously established studies as well as risk stratification guidelines. Therefore, this review will summarize the findings in the field of male fertility concerning gonadotoxicity of akylating agents as a result of chemotherapy exposure.

The effect of epididymosomes on the development of frozen-thawed mouse spermatogonial stem cells after culture in a decellularized testicular scaffold and transplantation into azoospermic mice

PURPOSE

This study examined SSC proliferation on an epididymosome-enriched decellularized testicular matrix (DTM) hydrogel and spermatogenesis induction in azoospermic mice.

METHODS

Epididymosomes were extracted and characterized using SEM and western blotting. After cryopreservation, thawed SSCs were cultured in a hydrogel-based three-dimensional (3D) culture containing 10 ng/mL GDNF or 20 µg/mL epididymosomes. SSCs were assessed using the MTT assay, flow cytometry, and qRT-PCR after two weeks of culture. The isolated SSCs were microinjected into the efferent ducts of busulfan-treated mice. DiI-labeled SSCs were followed, and cell homing was assessed after two weeks. After 8 weeks, the testes were evaluated using morphometric studies and immunohistochemistry.

RESULTS

The expression of PLZF, TGF-β, and miR-10b did not increase statistically significantly in the 3D + GDNF and 3D + epididymosome groups compared to the 3D group. Among the groups, the GDNF-treated group exhibited the highest expression of miR-21 (*P < 0.05). Caspase-3 expression was lower in the epididymosome-treated group than in the other groups (***P < 0.001). Compared to the 3D and negative control groups, the 3D + epididymosomes and 3D + GDNF groups showed an increase in spermatogenic cells. Immunohistochemical results confirmed the growth and differentiation of spermatogonial cells into spermatids in the treatment groups.

CONCLUSION

The DTM hydrogel containing 20 µg/mL epididymosomes or 10 ng/mL GDNF is a novel and safe culture system that can support SSC proliferation in vitro to obtain adequate SSCs for transplantation success. It could be a novel therapeutic agent that could recover deregulated SSCs in azoospermic patients.

Use of assisted reproductive technologies (ARTs) to shorten the generational interval in ruminants: current status and perspectives

The challenges posed by climate change and increasing world population are stimulating renewed efforts for improving the sustainability of animal production. To meet such challenges, the contribution of genomic selection approaches, in combination with assisted reproductive technologies (ARTs), to spreading and preserving animal genetics is essential. The largest increase in genetic gain can be achieved by shortening the generation interval. This review provides an overview of the current status and progress of advanced ARTs that could be applied to reduce the generation time in both female and male of domestic ruminants. In females, the use of juvenile in vitro embryo transfer (JIVET) enables to generate offspring after the transfer of in vitro produced embryos derived from oocytes of prepubertal genetically superior donors reducing the generational interval and acceleration genetic gain. The current challenge is increasing in vitro embryo production (IVEP) from prepubertal derived oocytes which is still low and variable. The two main factors limiting IVEP success are the intrinsic quality of prepubertal oocytes and the culture systems for in vitro maturation (IVM). In males, advancements in ARTs are providing new strategies to in vitro propagate spermatogonia and differentiate them into mature sperm or even to recapitulate the whole process of spermatogenesis from embryonic stem cells. Moreover, the successful use of immature cells, such as round spermatids, for intracytoplasmic injection (ROSI) and IVEP could allow to complete the entire process in few months. However, these approaches have been successfully applied to human and mouse whereas only a few studies have been published in ruminants and results are still controversial. This is also dependent on the efficiency of ROSI that is limited by the current isolation and selection protocols of round spermatids. In conclusion, the current efforts for improving these reproductive methodologies could lead toward a significant reduction of the generational interval in livestock animals that could have a considerable impact on agriculture sustainability.

The Evolutionary Route of in vitro Human Spermatogenesis: What is the Next Destination?

Malfunction in spermatogenesis due to genetic diseases, trauma, congenital disorders or gonadotoxic treatments results in infertility in approximately 7% of males. The behavior of spermatogonial stem cells (SSCs) within three-dimensional, multifactorial, and dynamic microenvironment implicates a niche that serves as a repository for fertility, since can serve as a source of mature and functional male germ cells. Current protocols enable reprogramming of mature somatic cells into induced pluripotent stem cells (iPSCs) and their limited differentiation to SSCs within the range of 0-5%. However, the resulting human iPSC-derived haploid spermatogenic germ cell yield in terms of number and functionality is currently insufficient for transfer to infertility clinic as a therapeutic tool. In this article, we reviewed the evolution of experimental culture platforms and introduced a novel iPSCs-based approach for in vitro spermatogenesis based on a niche perspective bearing cellular, chemical, and physical factors that provide the complex arrangement of testicular seminiferous tubules embedded within a vascularized stroma. We believe that bioengineered organoids supported by smart bio-printed tubules and microfluidic organ-on-a-chip systems offer efficient, precise, personalized platforms for autologous pluripotent stem cell sources to undergo the spermatogenetic cycle, presenting a promising tool for infertile male patients with complete testicular aplasia.

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.

In vitro spermatogenesis in artificial testis: current knowledge and clinical implications for male infertility

Men's reproductive health exclusively depends on the appropriate maturation of certain germ cells known as sperm. Certain illnesses, such as Klinefelter syndrome, cryptorchidism, and syndrome of androgen insensitivity or absence of testis maturation in men, resulting in the loss of germ cells and the removal of essential genes on the Y chromosome, can cause non-obstructive azoospermia. According to laboratory research, preserving, proliferating, differentiating, and transplanting spermatogonial stem cells or testicular tissue could be future methods for preserving the fertility of children with cancer and men with azoospermia. Therefore, new advances in stem cell research may lead to promising therapies for treating male infertility. The rate of progression and breakthrough in the area of in vitro spermatogenesis is lower than that of SSC transplantation, but newer methods are also being developed. In this regard, tissue and cell culture, supplements, and 3D scaffolds have opened new horizons in the differentiation of stem cells in vitro, which could improve the outcomes of male infertility. Various 3D methods have been developed to produce cellular aggregates and mimic the organization and function of the testis. The production of an artificial reproductive organ that supports SSCs differentiation will certainly be a main step in male infertility treatment.

Differentiation of human primary testicular cells in the presence of SCF using the organoid culture system

PURPOSE

Development of organoids using human primary testicular cells has remained a challenge due to the complexity of the mammalian testicular cytoarchitecture and culture methods. In this study, we generated testicular organoids derived from human primary testicular cells. Then, we evaluated the effect of stem cell factor (SCF) on cell differentiation and apoptosis in the testicular organoid model.

METHODS

The testicular cells were harvested from the three brain-dead donors. Human spermatogonial stem cells (SSCs) were characterized using immunocytochemistry (ICC), RT-PCR and flow cytometry. Testicular organoids were generated from primary testicular cells by hanging drop culture method and were cultured in three groups: control group, experimental group 1 (treated FSH and retinoic acid (RA)), and experimental group 2 (treated FSH, RA and SCF), for five weeks. We assessed the expression of SCP3 (Synaptonemal Complex Protein 3) as a meiotic gene, PRM2 (Protamine 2) as a post-meiotic marker and apoptotic genes of Bax (BCL2-Associated X Protein) and Bcl-2 (B-cell lymphoma 2), respectively by using RT-qPCR. In addition, we identified the expression of PRM2 by immunohistochemistry (IHC).

RESULTS

Relative expression of SCP3, PRM2 and Bcl-2 were highest in group 2 after five weeks of culture. In contrast, BAX expression level was lower in experimental group 2 in comparison with other groups. IHC analyses indicated the highest expression of PRM2 as a postmeiotic marker in group 2 in comparison to 2D culture and control groups but not find significant differences between experimental group 1 and experimental group 2 groups. Morphological evaluations revealed that organoids are compact spherical structures and in the peripheral region composed of uncharacterized elongated fibroblast-like cells.

CONCLUSION

Our findings revealed that the testicular organoid culture system promote the spermatogonial stem cell (SSC) differentiation, especially in presence of SCF. Developed organoids are capable of recapitulating many important properties of a stem cell niche.

The construction of a testis transcriptional cell atlas from embryo to adult reveals various somatic cells and their molecular roles

BACKGROUND

The testis is a complex organ that undergoes extensive developmental changes from the embryonic stage to adulthood. The development of germ cells, which give rise to spermatozoa, is tightly regulated by the surrounding somatic cells.

METHODS

To better understand the dynamics of these changes, we constructed a transcriptional cell atlas of the testis, integrating single-cell RNA sequencing data from over 26,000 cells across five developmental stages: fetal germ cells, infants, childhood, peri-puberty, and adults. We employed various analytical techniques, including clustering, cell type assignments, identification of differentially expressed genes, pseudotime analysis, weighted gene co-expression network analysis, and evaluation of paracrine cell-cell communication, to comprehensively analyze this transcriptional cell atlas of the testis.

RESULTS

Our analysis revealed remarkable heterogeneity in both somatic and germ cell populations, with the highest diversity observed in Sertoli and Myoid somatic cells, as well as in spermatogonia, spermatocyte, and spermatid germ cells. We also identified key somatic cell genes, including RPL39, RPL10, RPL13A, FTH1, RPS2, and RPL18A, which were highly influential in the weighted gene co-expression network of the testis transcriptional cell atlas and have been previously implicated in male infertility. Additionally, our analysis of paracrine cell-cell communication supported specific ligand-receptor interactions involved in neuroactive, cAMP, and estrogen signaling pathways, which support the crucial role of somatic cells in regulating germ cell development.

CONCLUSIONS

Overall, our transcriptional atlas provides a comprehensive view of the cell-to-cell heterogeneity in the testis and identifies key somatic cell genes and pathways that play a central role in male fertility across developmental stages.

Recent Progress of In Vitro 3D Culture of Male Germ Stem Cells

Male germline stem cells (mGSCs), also known as spermatogonial stem cells (SSCs), are the fundamental seed cells of male animal reproductive physiology. However, environmental influences, drugs, and harmful substances often pose challenges to SSCs, such as population reduction and quality decline. With advancements in bioengineering technology and biomaterial technology, an increasing number of novel cell culture methods and techniques have been employed for studying the proliferation and differentiation of SSCs in vitro. This paper provides a review on recent progress in 3D culture techniques for SSCs in vitro; we summarize the microenvironment of SSCs and spermatocyte development, with a focus on scaffold-based culture methods and 3D printing cell culture techniques for SSCs. Additionally, decellularized testicular matrix (DTM) and other biological substrates are utilized through various combinations and approaches to construct an in vitro culture microenvironment suitable for SSC growth. Finally, we present some perspectives on current research trends and potential opportunities within three areas: the 3D printing niche environment, alternative options to DTM utilization, and advancement of the in vitro SSC culture technology system.

Generation of Haploid Spermatids on Silk Fibroin-Alginate-Laminin-Based Porous 3D Scaffolds

In vitro production of sperm is a desirable idea for fertility preservation in azoospermic men and prepubertal boys suffering from cancer. In this study, a biocompatible porous scaffold based on a triad mixture of silk fibroin (SF), alginate (Alg), and laminin (LM) is developed to facilitate the differentiation of mouse spermatogonia stem cells (SSCs). Following SF extraction, the content is analyzed by SDS-PAGE and stable porous 3D scaffolds are successfully prepared by merely Alg, SF, and a combination of Alg-SF, or Alg-SF-LM through freeze-drying. Then, the biomimetic scaffolds are characterized regarding the structural and biological properties, water absorption capacity, biocompatibility, biodegradability, and mechanical behavior. Neonatal mice testicular cells are seeded on three-dimensional scaffolds and their differentiation efficiency is evaluated using real-time PCR, flow cytometry, immunohistochemistry. Blend matrices showed uniform porous microstructures with interconnected networks, which maintained long-term stability and mechanical properties better than homogenous structures. Molecular analysis of the cells after 21 days of culture showed that the expression of differentiation-related proteins in cells that are developed in composite scaffolds is significantly higher than in other groups. The application of a composite system can lead to the differentiation of SSCs, paving the way for a novel infertility treatment landscape in the future.

Evaluation of Apoptosis-related Genes and Hormone Secretion Profiles Using Three Dimensional Culture System of Human Testicular Organoids

BACKGROUND

In reproductive biology, testicular organoids can be used to treat infertility and to study testicular development and spermatogonial stem cells (SSCs) differentiation. Generating organoid from primary cells is challenging. In this study, testicular organoids were created using human primary testicular cells and evaluated the apoptotic gene expression and hormone secretion profiles of the organoids.

MATERIALS AND METHODS

Primary human testicular cells were isolated using 2-step enzymatic digestion from three brain-dead donors. Immunocytochemistry and flow cytometry analyses were performed to confirm human SSCs. Isolated cells were cultured in three experimental groups: control group (2 dimensional (2D)), group 1 (organoid culture after 2D culture), and group 2 (organoid culture immediately after enzymatic digestion). Testicular organoids were cultured in DMEM/F-12 media supplemented with follicle-stimulating hormone (FSH) and fetal bovine serum (FBS) for four weeks. After 24 hours and four weeks of culture, reverse transcription quantitative real-time PCR (RT-qPCR) was used to investigate the relative expression of apoptotic genes (caspase 3, 9, Bax, and Bcl-2). At 24 hours, two weeks, and four weeks after culture, enzyme-linked immunoassay (ELISA) was used to determine the testosterone and inhibin B concentrations. Light microscopy and toluidine blue staining were also used for morphological analysis.

RESULTS

RT-qPCR results revealed that pro-apoptotic (caspase 3, 9, Bax) gene expression levels were highest in group 2 after 24 h and four weeks of culture. In contrast, the expression level of Bcl-2 (anti-apoptotic) was lower in group 2 compared to other groups. The hormone secretion levels decreased in a time-dependent manner during the cultivation. According to morphological evaluations, testicular organoids are compact, spherical structures with two to three elongated cells organized along their border.

CONCLUSION

Our findings revealed that the testicular organoid culture system maintained hormonal secretory abilities, demonstrating the function of Sertoli and Leydig cells in the absence of testis-specific environments.

GPx3 knockdown inhibits the proliferation and DNA synthesis and enhances the early apoptosis of human spermatogonial stem cells via mediating CXCL10 and cyclin B1

Spermatogenesis is regulated by genetic and epigenetic factors. However, the genes and signaling pathways mediating human spermatogenesis remain largely unknown. Here, we have for the first time explored the expression, function, and mechanism of glutathione peroxidase 3 (GPx3) in controlling the proliferation and apoptosis of human spermatogonial stem cells (SSCs). We found that GPx3 was expressed in human SSCs. Notably, we revealed that GPx3 knockdown resulted in the decrease in the proliferation, DNA synthesis, and cyclin B1 level in human SSC lines, which possessed the phenotypic features of human primary SSCs. Flow cytometry and TUNEL assays showed that GPx3 silencing led to enhancement of early apoptosis of human SSC line. RNA sequencing was utilized to identify CXCL10 as a target of GPx3 in human SSCs, and notably, both double immunostaining and co-immunoprecipitation (co-IP) demonstrated that there was an association between GPx3 and CXCL10 in these cells. CXCL10-shRNA resulted in the reduction in the proliferation and DNA synthesis of human SSC line and an increase in apoptosis of these cells. Taken together, these results implicate that GPx3 regulates the proliferation, DNA synthesis, and early apoptosis of human SSC line via mediating CXCL10 and cyclin B1. This study, thus, offers a novel insight into the molecular mechanism regulating the fate determinations of human SSCs and human spermatogenesis.

OIP5 Interacts with NCK2 to Mediate Human Spermatogonial Stem Cell Self-Renewal and Apoptosis through Cell Cyclins and Cycle Progression and Its Abnormality Is Correlated with Male Infertility

Spermatogonial stem cells (SSCs) have important applications in both reproduction and regenerative medicine. Nevertheless, specific genes and signaling transduction pathways in mediating fate decisions of human SSCs remain elusive. Here, we have demonstrated for the first time that OIP5 (Opa interacting protein 5) controlled the self-renewal and apoptosis of human SSCs. RNA sequencing identified that NCK2 was a target for OIP5 in human SSCs, and interestingly, OIP5 could interact with NCK2 as shown by Co-IP (co-immunoprecipitation), IP-MS (mass spectrometry), and GST pulldown assays. NCK2 silencing decreased human SSC proliferation and DNA synthesis but enhanced their apoptosis. Notably, NCK2 knockdown reversed the influence of OIP5 overexpression on human SSCs. Moreover, OIP5 inhibition decreased the numbers of human SSCs at S and G2/M phases, while the levels of numerous cell cycle proteins, including cyclins A2, B1, D1, E1 and H, especially cyclin D1, were remarkably reduced. Significantly, whole-exome sequencing of 777 patients with nonobstructive azoospermia (NOA) revealed 54 single-nucleotide polymorphism mutations of the OIP5 gene (6.95%), while the level of OIP5 protein was obviously lower in testes of NOA patients compared to fertile men. Collectively, these results implicate that OIP5 interacts with NCK2 to modulate human SSC self-renewal and apoptosis via cell cyclins and cell cycle progression and that its mutation and/or lower expression is correlated with azoospermia. As such, this study offers novel insights into molecular mechanisms underlying the fate determinations of human SSCs and the pathogenesis of NOA, and it provides new targets for treating male infertility.

Human organoid systems in modeling reproductive tissue development, function, and disease

Research focused on human reproductive biology has primarily relied upon clinical samples affording mainly descriptive studies with limited implementation of functional or mechanistic understanding. More importantly, restricted access to human embryonic material has necessitated the use of animals, primarily rats and mice, and short-term primary cell cultures derived from human patient material. While reproductive developmental processes are generally conserved across mammals, specific features unique to human reproduction have resulted in the development of human-based in vitro systems designed to retain or recapitulate key molecular and cellular processes important in humans. Of note, major advances in 3D epithelial stem cell-based systems modeling human reproductive organ development have been made. These cultures, broadly referred to as organoids, enable research aimed at understanding cellular hierarchies and processes controlling cellular differentiation and function. Moreover, organoids allow the pre-clinical testing of pharmacological substances, both from safety and efficacy standpoints, and hold large potential in driving aspects of personalized medicine that were previously not possible with traditional models. In this mini-review, we focus on summarizing the current state of regenerative organoid culture systems of the female and male reproductive tracts that model organ development, maintenance, and function. Specifically, we will introduce stem cell-based organoid models of the ovary/fallopian tube, endometrium, cervix, prostate gland, and testes. We will also describe organoid systems of the pre-implanting blastocyst and trophoblast, as the blastocyst and its extraembryonic trophectoderm are central to fetal, maternal, and overall pregnancy health. We describe the foundational studies leading to their development and outline the utility as well as specific limitations that are unique and common to many of these in vitro platforms.

Biomaterials for Testicular Bioengineering: How far have we come and where do we have to go?

Traditional therapeutic interventions aim to restore male fertile potential or preserve sperm viability in severe cases, such as semen cryopreservation, testicular tissue, germ cell transplantation and testicular graft. However, these techniques demonstrate several methodological, clinical, and biological limitations, that impact in their results. In this scenario, reproductive medicine has sought biotechnological alternatives applied for infertility treatment, or to improve gamete preservation and thus increase reproductive rates in vitro and in vivo. One of the main approaches employed is the biomimetic testicular tissue reconstruction, which uses tissue-engineering principles and methodologies. This strategy pursues to mimic the testicular microenvironment, simulating physiological conditions. Such approach allows male gametes maintenance in culture or produce viable grafts that can be transplanted and restore reproductive functions. In this context, the application of several biomaterials have been proposed to be used in artificial biological systems. From synthetic polymers to decellularized matrixes, each biomaterial has advantages and disadvantages regarding its application in cell culture and tissue reconstruction. Therefore, the present review aims to list the progress that has been made and the continued challenges facing testicular regenerative medicine and the preservation of male reproductive capacity, based on the development of tissue bioengineering approaches for testicular tissue microenvironment reconstruction.

Organoids of the male reproductive system: Challenges, opportunities, and their potential use in fertility research

Organoids are units of function of a given organ able to reproduce, in culture, a biological structure similar in architecture and function to its counterpart in vivo. Today, it is possible to develop an organoid from a fragment of tissue, a stem cell located in an adult organ, an embryonic stem cell, or an induced pluripotent stem cell. In the past decade, many organoids have been developed which mimic stomach, pancreas, liver and brain tissues, optic cups, among many others. Additionally, different male reproductive system organs have already been developed as organoids, including the prostate and testis. These 3D cultures may be of great importance for urological cancer research and have the potential to be used in fertility research for the study of spermatozoa production and maturation, germ cells-somatic cells interactions, and mechanisms of disease. They also provide an accurate preclinical pipeline for drug testing and discovery, as well as for the study of drug resistance. In this work, we revise the current knowledge on organoid technology and its use in healthcare and research, describe the male reproductive system organoids and other biomaterials already developed, and discuss their current application. Finally, we highlight the research gaps, challenges, and opportunities in the field and propose strategies to improve the use of organoids for the study of male infertility situations. This article is categorized under: Reproductive System Diseases > Stem Cells and Development Reproductive System Diseases > Biomedical Engineering.

In vitro complete differentiation of human spermatogonial stem cells to morphologic spermatozoa using a hybrid hydrogel of agarose and laminin

Spermatogenesis refers to the differentiation of the spermatogonial stem cells (SSCs) located in the base seminiferous tubules into haploid spermatozoa. Prerequisites for in vitro spermatogenesis include an extracellular matrix (ECM), paracrine factors, and testicular somatic cells which play a supporting role for SSCs. Thus, the present study evaluated the potential of co-culturing Sertoli cells and SSCs embedded in a hybrid hydrogel of agarose and laminin, the main components of the ECM. Following the three-week conventional culture of human testicular cells, the cells were cultured in agarose hydrogel or agarose/laminin one (hybrid) for 74 days. Then, immunocytochemistry, real-time PCR, electron microscopy, and morphological staining methods were applied to analyze the presence of SSCs, as well as the other cells of the different stages of spermatogenesis. Based on the results, the colonies with positive spermatogenesis markers were observed in both culture systems. The existence of the cells of all three phases of spermatogenesis (spermatogonia, meiosis, and spermiogenesis) was confirmed in the two groups, while morphological spermatozoa were detected only in the hybrid hydrogel group. Finally, a biologically improved 3D matrix can support all the physiological activities of SSCs such as survival, proliferation, and differentiation.

Sertoli cell-only syndrome: advances, challenges, and perspectives in genetics and mechanisms

Male infertility can be caused by quantitative and/or qualitative abnormalities in spermatogenesis, which affects men's physical and mental health. Sertoli cell-only syndrome (SCOS) is the most severe histological phenotype of male infertility characterized by the depletion of germ cells with only Sertoli cells remaining in the seminiferous tubules. Most SCOS cases cannot be explained by the already known genetic causes including karyotype abnormalities and microdeletions of the Y chromosome. With the development of sequencing technology, studies on screening new genetic causes for SCOS are growing in recent years. Directly sequencing of target genes in sporadic cases and whole-exome sequencing applied in familial cases have identified several genes associated with SCOS. Analyses of the testicular transcriptome, proteome, and epigenetics in SCOS patients provide explanations regarding the molecular mechanisms of SCOS. In this review, we discuss the possible relationship between defective germline development and SCOS based on mouse models with SCO phenotype. We also summarize the advances and challenges in the exploration of genetic causes and mechanisms of SCOS. Knowing the genetic factors of SCOS offers a better understanding of SCO and human spermatogenesis, and it also has practical significance for improving diagnosis, making appropriate medical decisions, and genetic counseling. For therapeutic implications, SCOS research, along with the achievements in stem cell technologies and gene therapy, build the foundation to develop novel therapies for SCOS patients to produce functional spermatozoa, giving them hope to father children.

Human in vitro spermatogenesis as a regenerative therapy - where do we stand?

Spermatogenesis involves precise temporal and spatial gene expression and cell signalling to reach a coordinated balance between self-renewal and differentiation of spermatogonial stem cells through various germ cell states including mitosis, and meiosis I and II, which result in the generation of haploid cells with a unique genetic identity. Subsequently, these round spermatids undergo a series of morphological changes to shed excess cytoplast, develop a midpiece and tail, and undergo DNA repackaging to eventually form millions of spermatozoa. The goal of recreating this process in vitro has been pursued since the 1920s as a tool to treat male factor infertility in patients with azoospermia. Continued advances in reproductive bioengineering led to successful generation of mature, functional sperm in mice and, in the past 3 years, in humans. Multiple approaches to study human in vitro spermatogenesis have been proposed, but technical and ethical obstacles have limited the ability to complete spermiogenesis, and further work is needed to establish a robust culture system for clinical application.

The Role of Promyelocytic Leukemia Zinc Finger (PLZF) and Glial-Derived Neurotrophic Factor Family Receptor Alpha 1 (GFRα1) in the Cryopreservation of Spermatogonia Stem Cells

The cryopreservation of spermatogonia stem cells (SSCs) has been widely used as an alternative treatment for infertility. However, cryopreservation itself induces cryoinjury due to oxidative and osmotic stress, leading to reduction in the survival rate and functionality of SSCs. Glial-derived neurotrophic factor family receptor alpha 1 (GFRα1) and promyelocytic leukemia zinc finger (PLZF) are expressed during the self-renewal and differentiation of SSCs, making them key tools for identifying the functionality of SSCs. To the best of our knowledge, the involvement of GFRα1 and PLZF in determining the functionality of SSCs after cryopreservation with therapeutic intervention is limited. Therefore, the purpose of this review is to determine the role of GFRα1 and PLZF as biomarkers for evaluating the functionality of SSCs in cryopreservation with therapeutic intervention. Therapeutic intervention, such as the use of antioxidants, and enhancement in cryopreservation protocols, such as cell encapsulation, cryoprotectant agents (CPA), and equilibrium of time and temperature increase the expression of GFRα1 and PLZF, resulting in maintaining the functionality of SSCs. In conclusion, GFRα1 and PLZF have the potential as biomarkers in cryopreservation with therapeutic intervention of SSCs to ensure the functionality of the stem cells.

MAP4K4/JNK Signaling Pathway Stimulates Proliferation and Suppresses Apoptosis of Human Spermatogonial Stem Cells and Lower Level of MAP4K4 Is Associated with Male Infertility

Spermatogonial stem cells (SSCs) serve as a foundation for spermatogenesis and they are essential for male fertility. The fate of SSC is determined by genetic and epigenetic regulatory networks. Many molecules that regulate SSC fate determinations have been identified in mice. However, the molecules and signaling pathways underlying human SSCs remain largely unclear. In this study, we have demonstrated that MAP4K4 was predominantly expressed in human UCHL1-positive spermatogonia by double immunocytochemical staining. MAP4K4 knockdown inhibited proliferation of human SSCs and induced their apoptosis. Moreover, MAP4K4 silencing led to inhibition of JNK phosphorylation and MAP4K4 phosphorylation at Ser801. RNA sequencing indicated that MAP4K4 affected the transcription of SPARC, ADAM19, GPX7, GNG2, and COLA1. Interestingly, the phenotype of inhibiting JNK phosphorylation by SP600125 was similar to MAP4K4 knockdown. Notably, MAP4K4 protein was lower in the testes of patients with non-obstructive azoospermia than those with normal spermatogenesis as shown by Western blots and immunohistochemistry. Considered together, our data implicate that MAP4K4/JNK signaling pathway mediates proliferation and apoptosis of human SSCs, which provides a novel insight into molecular mechanisms governing human spermatogenesis and might offer new targets for gene therapy of male infertility.

De novo testicular tissue generation from non-testicular cell lines, biologic and synthetic scaffolds: Current findings and future translational applications

In recent decades, reproductive science has revolutionized the options for biological parenthood for the 20-50% of infertility cases affected by male factors. However, current solutions exclude those who are infertile due to absent testicular tissue. This includes anorchic 46, XY individuals due to trauma or congenital factors and transgender men with a 46, XX genotype. There is a clinical need for methods to restore testicular function independent of pre-existing testicular tissue. This mini-review analyzes studies that have applied non-testicular cell lines to generate germline and non-germline testicular parenchymal components. While only 46, XY cell lines have been evaluated in this context to date, the potential for future application of cell lines from 46, XX individuals is also included. Additionally, the role of varied culture methods, media supplementation, and biologic and synthetic scaffolds to further support testicular parenchyma generation are critiqued. De novo testicular tissue generation in this manner will require a focus on both cellular and environmental aspects of tissue engineering. Put together, these studies highlight the future potential for expanded clinical, reproductive, and endocrine management options for individuals who are currently excluded from aspects of biologic reproduction most consistent with their gender identity and reproductive preferences.

Differentiation of neonate mouse spermatogonia on two-dimensional and three-dimensional culture systems supplemented with d-Serine and Dizocilpine (MK-801)

N-methyl-d-aspartate (NMDA) modulates the spermatogenesis process through stimulating the steroid hormone biosynthesis. The aim of this study was to evaluate the effects of NMDA receptors agonists (d-Serine) and antagonists (MK801) on spermatogonia differentiation on decellularization testicular matrix (DTM) hydrogel scaffold. Four treatment groups were planned: 2D + D-Serine, 3D + D-Serine, 2D + MK801, and 3D + MK801. Results showed that cell viability was significantly decreased after 48 h in the 3D + D-Serine group and after 24 and 48 h in the 3D + MK801 group compared to the controls. The spermatogonia proliferation after two, four, and eight weeks was significantly increased in the 3D + D-Serine culture, while it was significantly reduced in the 2D + MK801 and 3D + MK801 groups after four and eight weeks. Real-time PCR results demonstrated that pre-meiotic gene (Plzf) expression was significantly increased only in the 3D + D-Serine culture compared to the control groups after four weeks of culture. The meiotic gene (Sycp3) expression was significantly increased in the 2D + D-Serine and 3D + D-Serine compared to the 2D controls after four and eight weeks. The post-meiotic gene (Tnp1) level in the 3D + D-Serine was significantly higher than the other groups. Flow-cytometry results indicated that the protein expression of Plzf (after four and eight weeks), Sycp3 (after eight weeks), and Tnp1 (after eight weeks) in the d-Serine-treated groups was significantly increased compared with the 2D control groups. There were not any significant changes in the gene expression of spermatogenic-related markers in MK801 culture media. However, a significant decrease in the protein levels of Plzf after eight weeks and Sycp3 after four and eight weeks was observed. In conclusion, the addition of NMDARs agonists (d-Serine) could be used to regulate the differentiation of spermatogonia in the 3D culture system.

Generation and Characterization of Bovine Testicular Organoids Derived from Primary Somatic Cell Populations

Organoids are 3D-culture systems composed of tissue-specific primary cells that self-organize and self-renew, creating structures similar to those of their tissue of origin. Testicular organoids (TOs) may recreate conditions of the testicular niche in domestic and wild cattle; however, no previous TO studies have been reported in the bovine species. Thus, in the present study, we sought to generate and characterize bovine TOs derived from primary testicular cell populations including Leydig, Sertoli and peritubular myoid cells. Testicular cells were isolated from bovine testes and cultured in ultra-low attachment (ULA) plates and Matrigel. TOs were cultured in media supplemented from day 3 with 100 ng/mL of BMP4 and 10 ng/mL of FGF2 and from day 7 with 15 ng/mL of GDNF. Testicular cells were able to generate TOs after 3 days of culture. The cells positive for STAR (Leydig) and COL1A (peritubular myoid) decreased (p < 0.05), whereas cells positive for WT1 (Sertoli) increased (p < 0.05) in TOs during a 28-day culture period. The levels of testosterone in media increased (p < 0.05) at day 28 of culture. Thus, testicular cells isolated from bovine testes were able to generate TOs under in vitro conditions. These bovine TOs have steroidogenic activity characterized by the production of testosterone.

Biomaterial strategies for the application of reproductive tissue engineering

Human reproductive organs are of vital importance to the life of an individual and the reproduction of human populations. So far, traditional methods have a limited effect in recovering the function and fertility of reproductive organs and tissues. Thus, aim to replace and facilitate the regrowth of damaged or diseased tissue, various biomaterials are developed to offer hope to overcome these difficulties and help gain further research progress in reproductive tissue engineering. In this review, we focus on the biomaterials and their four main applications in reproductive tissue engineering: in vitro generation and culture of reproductive cells; development of reproductive organoids and models; in vivo transplantation of reproductive cells or tissues; and regeneration of reproductive tissue. In reproductive tissue engineering, designing biomaterials for different applications with different mechanical properties, structure, function, and microenvironment is challenging and important, and deserves more attention.

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Various biomaterials have been developed and used in reproductive tissue engineering.

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3D culture systems can lead to better cell-cell interactions for in vitro production of reproductive cells.

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Reproductive organoids and models are formed by biomaterials to simulate the environment of natural reproductive organs.

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Biomaterials should promote vascular regeneration and resist inflammation for in-situ reproductive tissue regeneration.

In vitro production of mouse morphological sperm in artificial testis bioengineered by 3D printing of extracellular matrix

Since autologous stem cell transplantation is prone to cancer recurrence, in vitro sperm production is regarded a safer approach to fertility preservation. In this study, the spermatogenesis process on testicular tissue extracellular matrix (T-ECM)-derived printing structure was evaluated. Ram testicular tissue was decellularized using a hypertonic solution containing triton and the extracted ECM was used as a bio-ink to print an artificial testis. Following cell adhesion and viability examination, pre-meiotic and post-meiotic cells in the study groups (as testicular suspension and co-culture with Sertoli cells) were confirmed by real-time PCR, flow-cytometry and immunocytochemistry methods. Morphology of differentiated cells was evaluated using transmission electron microscopy (TEM), toluidine blue, Giemsa, and hematoxylin and eosin (H&E) staining. The functionality of Leydig and Sertoli cells was determined by their ability for hormone secretion. The decellularization of testicular tissue fragments was successful and had efficiently removed the cellular debris and preserved the ECM compounds. High cell viability, colonization, and increased expression of pre-meiotic markers in cultured testicular cells on T-ECM-enriched scaffolds confirmed their proliferation. Furthermore, the inoculation of neonatal mouse testicular cells onto T-ECM-enriched scaffolds resulted in the generation of sperm. Morphology evaluation showed that the structure of these cells was quite similar to mature sperm with a specialized tail structure. The hormonal analysis also confirmed production and secretion of testosterone and inhibin B by Leydig and Sertoli cells. T-ECM printed artificial testis is a future milestone that promises for enhancing germ cell maintenance and differentiation, toxicology studies, and fertility restoration to pave the way for new human infertility treatments in the future.

In vitro spermatogenesis: Why meiotic checkpoints matter

Successful in vitro spermatogenesis would generate functional haploid spermatids, and thus, form the basis for novel approaches to treat patients with impaired spermatogenesis or develop alternative strategies for male fertility preservation. Several culture strategies, including cell cultures using various stem cells and ex vivo cultures of testicular tissue, have been investigated to recapitulate spermatogenesis in vitro. Although some studies have described complete meiosis and subsequent generation of functional spermatids, key meiotic events, such as chromosome synapsis and homologous recombination required for successful meiosis and faithful in vitro-derived gametes, are often not reported. To guarantee the generation of in vitro-formed spermatids without persistent DNA double-strand breaks (DSBs) and chromosomal aberrations, criteria to evaluate whether all meiotic events are completely executed in vitro need to be established. In vivo, these meiotic events are strictly monitored by meiotic checkpoints that eliminate aberrant spermatocytes. To establish criteria to evaluate in vitro meiosis, we review the meiotic events and checkpoints that have been investigated by previous in vitro spermatogenesis studies. We found that, although major meiotic events such as initiation of DSBs and recombination, complete chromosome synapsis, and XY-body formation can be achieved in vitro, crossover formation, chiasmata frequency, and checkpoint mechanisms have been mostly ignored. In addition, complete spermiogenesis, during which round spermatids differentiate into elongated spermatids, has not been achieved in vitro by various cell culture strategies. Finally, we discuss the implications of meiotic checkpoints for in vitro spermatogenesis protocols and future clinical use.

Perfusion in Organ-on-Chip Models and Its Applicability to the Replication of Spermatogenesis In Vitro

Organ/organoid-on-a-chip (OoC) technologies aim to replicate aspects of the in vivo environment in vitro, at the scale of microns. Mimicking the spatial in vivo structure is important and can provide a deeper understanding of the cell–cell interactions and the mechanisms that lead to normal/abnormal function of a given organ. It is also important for disease models and drug/toxin testing. Incorporating active fluid flow in chip models enables many more possibilities. Active flow can provide physical cues, improve intercellular communication, and allow for the dynamic control of the environment, by enabling the efficient introduction of biological factors, drugs, or toxins. All of this is in addition to the fundamental role of flow in supplying nutrition and removing waste metabolites. This review presents an overview of the different types of fluid flow and how they are incorporated in various OoC models. The review then describes various methods and techniques of incorporating perfusion networks into OoC models, including self-assembly, bioprinting techniques, and utilizing sacrificial gels. The second part of the review focuses on the replication of spermatogenesis in vitro; the complex process whereby spermatogonial stem cells differentiate into mature sperm. A general overview is given of the various approaches that have been used. The few studies that incorporated microfluidics or vasculature are also described. Finally, a future perspective is given on elements from perfusion-based models that are currently used in models of other organs and can be applied to the field of in vitro spermatogenesis. For example, adopting tubular blood vessel models to mimic the morphology of the seminiferous tubules and incorporating vasculature in testis-on-a-chip models. Improving these models would improve our understanding of the process of spermatogenesis. It may also potentially provide novel therapeutic strategies for pre-pubertal cancer patients who need aggressive chemotherapy that can render them sterile, as well asfor a subset of non-obstructive azoospermic patients with maturation arrest, whose testes do not produce sperm but still contain some of the progenitor cells.

Using clinically derived human tissue to 3-dimensionally bioprint personalized testicular tubules for in vitro culturing: first report

OBJECTIVE

To study the feasibility and spermatogenic potential of 3-dimensional (3D) bioprinting personalized human testicular cells derived from a patient with nonobstructive azoospermia (NOA).

DESIGN

A human testicular biopsy from a single donor with NOA was dissociated into single cells, expanded in vitro, and 3D bioprinted into tubular structures akin to the seminiferous tubule using AGC-10 bioink and an RX1 bioprinter with a CENTRA coaxial microfluidic printhead from Aspect Biosystems. Three-dimensional organoid cultures were used as a nonbioprinted in vitro control.

SETTING

Academic medical center.

PATIENT(S)

A 31-year-old man with NOA with testis biopsy demonstrating Sertoli cell-only syndrome.

INTERVENTION(S)

Three-dimensional bioprinting and in vitro culturing of patient-derived testis cells.

MAIN OUTCOME MEASURE(S)

Cellular viability after printing was determined, along with the expression of phenotypic and spermatogenic functional genetic markers after 12 days of in vitro culture.

RESULT(S)

Testicular cultures were expandable in vitro and generated sufficiently large numbers for 3D bioprinting at 35 million cells per mL of bioink. Viability 24 hours after printing was determined to be 93.4% ± 2.4%. Immunofluorescence staining for the phenotype markers SRY-Box transcription factor 9, insulin-like 3, actin alpha 2 smooth muscle, and synaptonemal complex protein 3 after 12 days was positive, confirming the presence of Sertoli, Leydig, peritubular myoid, and meiotic germ cells. Reverse transcription qualitative polymerase chain reaction analysis showed that after 12 days in spermatogenic media, the bioprints substantially up-regulated spermatogenic gene expression on par with nonbioprinted controls and showed a particularly significant improvement in genes involved in spermatogonial stem cell maintenance: inhibitor of deoxyribonucleic acid binding 4 by 365-fold; fibroblast growth factor 3 by 94,152-fold; stem cell growth factor receptor KIT by twofold; stimulated by retinoic acid 8 by 125-fold; deleted in azoospermia-like by 114-fold; synaptonemal complex protein 3 by sevenfold; zona pellucida binding protein by twofold; transition protein 1 by 2,908-fold; and protamine 2 by 11-fold.

CONCLUSION(S)

This study demonstrates for the first time the feasibility of 3D bioprinting adult human testicular cells. We show that the bioprinting process is compatible with high testicular cell viability and without loss of the main somatic phenotypes within the testis tissue. We demonstrate an increase in germ cell markers in the 3D bioprinted tubules after 12 days of in vitro culture. This platform may carry future potential for disease modeling and regenerative opportunities in a personalized medicine framework.

Potential clinical value of in vitro spermatogenesis

Infertility has become the third most common disease threatening human health, immediately after tumors and cardiovascular diseases. Male infertility is primarily caused by spermatogenesis disorders which may be classified as either genetic or non-genetic. For part of non-genetic disorders, in vitro spermatogenesis can be induced by adjusting the microenvironment of the testis culture. Establishing the in vitro spermatogenic induction system helps to clarify the critical molecular mechanisms in spermatogonia self-renewal, spermatocyte meiosis, and sperm formation during spermatogenesis. In this review, we summarize recent advances in the field of in vitro sperm cells induction. Therefore, we hope to provide ideas and solutions for the clinical treatment of male infertility.

Spermatogonial Stem Cell-Based Therapies: Taking Preclinical Research to the Next Level

Fertility preservation via biobanking of testicular tissue retrieved from testicular biopsies is now generally recommended for boys who need to undergo gonadotoxic treatment prior to the onset of puberty, as a source of spermatogonial stem cells (SSCs). SSCs have the potential of forming spermatids and may be used for therapeutic fertility approaches later in life. Although in the past 30 years many milestones have been reached to work towards SSC-based fertility restoration therapies, including transplantation of SSCs, grafting of testicular tissue and various in vitro and ex vivo spermatogenesis approaches, unfortunately, all these fertility therapies are still in a preclinical phase and not yet available for patients who have become infertile because of their treatment during childhood. Therefore, it is now time to take the preclinical research towards SSC-based therapy to the next level to resolve major issues that impede clinical implementation. This review gives an outline of the state of the art of the effectiveness and safety of fertility preservation and SSC-based therapies and addresses the hurdles that need to be taken for optimal progression towards actual clinical implementation of safe and effective SSC-based fertility treatments in the near future.

Progress in germline stem cell transplantation in mammals and the potential usage

Germline stem cells (GSCs) are germ cells with the capacities of self-renewal and differentiation into functional gametes, and are able to migrate to their niche and reconstitute the fertility of recipients after transplantation. Therefore, GSCs transplantation is a promising technique for fertility recovery in the clinic, protection of rare animals and livestock breeding. Though this novel technique faces tremendous challenges, numerous achievements have been made after several decades' endeavor. This review summarizes the current knowledge of GSCs transplantation and its utilization in mammals, and discusses the application prospect in reproductive medicine and animal science.

Proliferation of human spermatogonial stem cells on optimized PCL/Gelatin nanofibrous scaffolds

Improvement of culture system and increasing the proliferation of spermatogonia stem cells under in vitro condition are the essential treatment options for infertility before autologous transplantation. Therefore, the present study aimed to evaluate the proliferation of human spermatogonia stem cells on the electrospun polycaprolactone/gelatin nanocomposite. Therefore, for this purpose, nanofiber porous scaffolds were prepared using the electrospinning method and their structures were then confirmed by SEM. After performing swelling, biodegradability and cell adhesion tests, human spermatogonia stem cells were cultured on scaffolds. In addition, both cell viability and proliferation were assessed using immunocytochemistry, flow cytometry and real-time PCR techniques in culturing during a 3-week period. SEM images indicated the presence of fibres with suitable diameters and arrangement as well as a sufficient porosity in nanocomposite scaffolds, showing good biocompatibility and biodegradability. The results show a significant increase in the number of spermatogonia stem cells in the cultured group on scaffold compared with the control group (p ≤ 0.05). As well, the results show that the expressions of integrin ɑ6 and β1 and Plzf genes estimated using real-time PCR in nanofiber scaffolds were significantly higher than those of the control group (p ≤ 0.05). However, the expression of c-Kit gene in the 3D group showed a significant decrease compared with the 2D group. Flow cytometry analysis also showed that the number of Plzf-positive cells was significantly higher in nanofiber porous scaffolds compared with the control group (p ≤ 0.05). Additionally, immunocytochemistry findings confirmed the presence of human spermatogonia stem cell colonies. In general, it seems that the designed nanocomposite scaffold could provide a suitable capacity for self-renewal of human spermatogonia stem cells, which can have a good application potential in research and reconstructive medicine related to the field of male infertility.

Direct reprogramming of human Sertoli cells into male germline stem cells with the self-renewal and differentiation potentials via overexpressing DAZL/DAZ2/BOULE genes

We propose a new concept that human somatic cells can be converted to become male germline stem cells by the defined factors. Here, we demonstrated that the overexpression of DAZL, DAZ2, and BOULE could directly reprogram human Sertoli cells into cells with the characteristics of human spermatogonial stem cells (SSCs), as shown by their similar transcriptomes and proteomics with human SSCs. Significantly, human SSCs derived from human Sertoli cells colonized and proliferated in vivo, and they could differentiate into spermatocytes and haploid spermatids in vitro. Human Sertoli cell-derived SSCs excluded Y chromosome microdeletions and assumed normal chromosomes. Collectively, human somatic cells could be converted directly to human SSCs with the self-renewal and differentiation potentials and high safety. This study is of unusual significance, because it provides an effective approach for reprogramming human somatic cells into male germ cells and offers invaluable male gametes for treating male infertility.

Integration and gene co-expression network analysis of scRNA-seq transcriptomes reveal heterogeneity and key functional genes in human spermatogenesis

Spermatogenesis is a complex process of cellular division and differentiation that begins with spermatogonia stem cells and leads to functional spermatozoa production. However, many of the molecular mechanisms underlying this process remain unclear. Single-cell RNA sequencing (scRNA-seq) is used to sequence the entire transcriptome at the single-cell level to assess cell-to-cell variability. In this study, more than 33,000 testicular cells from different scRNA-seq datasets with normal spermatogenesis were integrated to identify single-cell heterogeneity on a more comprehensive scale. Clustering, cell type assignments, differential expressed genes and pseudotime analysis characterized 5 spermatogonia, 4 spermatocyte, and 4 spermatid cell types during the spermatogenesis process. The UTF1 and ID4 genes were introduced as the most specific markers that can differentiate two undifferentiated spermatogonia stem cell sub-cellules. The C7orf61 and TNP can differentiate two round spermatid sub-cellules. The topological analysis of the weighted gene co-expression network along with the integrated scRNA-seq data revealed some bridge genes between spermatogenesis's main stages such as DNAJC5B, C1orf194, HSP90AB1, BST2, EEF1A1, CRISP2, PTMS, NFKBIA, CDKN3, and HLA-DRA. The importance of these key genes is confirmed by their role in male infertility in previous studies. It can be stated that, this integrated scRNA-seq of spermatogenic cells offers novel insights into cell-to-cell heterogeneity and suggests a list of key players with a pivotal role in male infertility from the fertile spermatogenesis datasets. These key functional genes can be introduced as candidates for filtering and prioritizing genotype-to-phenotype association in male infertility.

Self-organising human gonads generated by a Matrigel-based gradient system

BACKGROUND

Advances in three-dimensional culture technologies have led to progression in systems used to model the gonadal microenvironment in vitro. Despite demonstrating basic functionality, tissue organisation is often limited. We have previously detailed a three-dimensional culture model termed the three-layer gradient system to generate rat testicular organoids in vitro. Here we extend the model to human first-trimester embryonic gonadal tissue.

RESULTS

Testicular cell suspensions reorganised into testis-like organoids with distinct seminiferous-like cords situated within an interstitial environment after 7 days. In contrast, tissue reorganisation failed to occur when mesonephros, which promotes testicular development in vivo, was included in the tissue digest. Organoids generated from dissociated female gonad cell suspensions formed loosely organised cords after 7 days. In addition to displaying testis-specific architecture, testis-like organoids demonstrated evidence of somatic cell differentiation. Within the 3-LGS, we observed the onset of AMH expression in the cytoplasm of SOX9-positive Sertoli cells within reorganised testicular cords. Leydig cell differentiation and onset of steroidogenic capacity was also revealed in the 3-LGS through the expression of key steroidogenic enzymes StAR and CYP17A1 within the interstitial compartment. While the 3-LGS generates a somatic cell environment capable of supporting germ cell survival in ovarian organoids germ cell loss was observed in testicular organoids.

CONCLUSION

The 3-LGS can be used to generate organised whole gonadal organoids within 7 days. The 3-LGS brings a new opportunity to explore gonadal organogenesis and contributes to the development of more complex in vitro models in the field of developmental and regenerative medicine.

In Vitro Meiosis of Male Germline Stem Cells

In vitro spermatogenesis has been achieved by culturing mouse embryonic stem cells (ESCs) together with a cell suspension of male juvenile gonad. However, for human fertility treatment or preservation, patient-specific ESCs or juvenile gonad is not available. We therefore aim to achieve in vitro spermatogenesis using male germline stem cells (GSCs) without the use of juvenile gonad. GSCs, when cultured on immortalized Sertoli cells, were able to enter meiosis, reach the meiotic metaphase stages, and sporadically form spermatid-like cells. However, the in vitro-formed pachytene-like spermatocytes did not display full chromosome synapsis and did not form meiotic crossovers. Despite this, the meiotic checkpoints that usually eliminate such cells to prevent genomic instabilities from being transmitted to the offspring were not activated, allowing the cells to proceed to the meiotic metaphase stages. In vitro-generated spermatid-like cells should thus be thoroughly investigated before being considered for clinical use.

The mechanisms and functions of microRNAs in mediating the fate determinations of human spermatogonial stem cells and Sertoli cells

Human spermatogonial stem cells (SSCs) and Sertoli cells might have the applications in reproduction and regenerative medicine. Abnormal spermatogenesis results in male infertility, which seriously affects human reproduction and health. Spermatogenesis depends on the epigenetic and genetic regulation of male germ cells and somatic cells. A number of microRNAs (miRNAs) have been identified in human testicular tissues, and they are closely related to male fertility. Significantly, we and peers have recently demonstrated that numerous miRNAs are essential for regulating the self-renewal and apoptosis of human SSCs and Sertoli cells through controlling their mRNA and lncRNA targets. In this review, we critically discuss these findings regarding the important functions and mechanisms of miRNAs in mediating the fate determinations of human SSCs and Sertoli cells. Meanwhile, we illustrate the regulatory networks for miRNAs by forming the upstream and downstream regulators of mRNAs and lncRNAs in human SSCs, and we address that miRNAs regulate the decisions of Sertoli cells by targeting genes and via N6-methyladenosine (m6A). We also point out the future directions for further studies on this field. This review could offer an update on novel molecular targets for treating male infertility and new insights into epigenetic regulation of human spermatogenesis.

Is It Possible to Treat Infertility with Stem Cells?

Infertility is a major health problem, and despite improved treatments over the years, there are still some conditions that cannot be treated successfully using a conventional approach. Therefore, new options are being considered and one of them is cell therapy using stem cells. Stem cell treatments for infertility can be divided into two major groups, the first one being direct transplantation of stem cells or their paracrine factors into reproductive organs and the second one being in vitro differentiation into germ cells or gametes. In animal models, all of these approaches were able to improve the reproductive potential of tested animals, although in humans there is still too little evidence to suggest successful use. The reasons for lack of evidence are unavailability of proper material, the complexity of explored biological processes, and ethical considerations. Despite all of the above-mentioned hurdles, researchers were able to show that in women, it seems to be possible to improve some conditions, but in men, no similar clinically important improvement was achieved. To conclude, the data presented in this review suggest that the treatment of infertility with stem cells seems plausible, because some types of treatments have already been tested in humans, achieving live births, while others show great potential only in animal studies, for now.

SCF Improves In Vitro Differentiation of SSCs Through Transcriptionally Up-regulating PRTM1, STRA8, c-KIT, PIWIL2, and OCT4 Genes

Several lines of evidence strongly suggest that retinoic acid (RA) and stem cell factor (SCF)/c-Kit signal transduction pathways are involved in the differentiation of spermatogonial stem cells (SSCs). This study was aimed to investigate the effect of RA and SCF on in vitro differentiation of SSCs via evaluation of the mRNA expression of meiosis-specific genes in cultured testicular tissues. Testicular tissue samples were obtained from bilaterally vasectomized rats and also healthy adult rats and then were cultured for 25, 30, and 35 days on different conditions. The cultured testicular pieces were sectioned and stained with PAS to histological analysis. The total RNA was extracted from cultured testicular samples, and the expression of ACR, PRTM1, SYCP3, STRA8, c-KIT, PIWIL2, and OCT4 genes at mRNA level was quantified using real-time polymerase chain reaction (qPCR) procedure. After 1-month surgery, bilateral testicular weight showed a significant decrease in vasectomized adult rats compared with healthy adult rats (P < 0.05). Reduction in the diameter of the seminiferous tubules and depletion of advanced germinal elements in vasectomized rats compared with healthy adult rats were also observed. Our findings also demonstrated that the mRNA expression level of PRTM1, STRA8, c-KIT, PIWIL2, and OCT4 genes in cultured testicular tissues significantly up-regulated in experimental group II compared with the control group (P < 0.001). Our findings lead us to conclude that SCF improves in vitro differentiation of SSCs in the OA rats, at least partially, by transcriptionally upregulating PRTM1, STRA8, c-KIT, PIWIL2, and OCT4 genes.

Advanced bioengineering of male germ stem cells to preserve fertility

In modern life, several factors such as genetics, exposure to toxins, and aging have resulted in significant levels of male infertility, estimated to be approximately 18% worldwide. In response, substantial progress has been made to improve in vitro fertilization treatments (e.g. microsurgical testicular sperm extraction (m-TESE), intra-cytoplasmic sperm injection (ICSI), and round spermatid injection (ROSI)). Mimicking the structure of testicular natural extracellular matrices (ECM) outside of the body is one clear route toward complete in vitro spermatogenesis and male fertility preservation. Here, a new wave of technological innovations is underway applying regenerative medicine strategies to cell-tissue culture on natural or synthetic scaffolds supplemented with bioactive factors. The emergence of advanced bioengineered systems suggests new hope for male fertility preservation through development of functional male germ cells. To date, few studies aimed at in vitro spermatogenesis have resulted in relevant numbers of mature gametes. However, a substantial body of knowledge on conditions that are required to maintain and mature male germ cells in vitro is now in place. This review focuses on advanced bioengineering methods such as microfluidic systems, bio-fabricated scaffolds, and 3D organ culture applied to the germline for fertility preservation through in vitro spermatogenesis.

Artificial testis: a testicular tissue extracellular matrix as a potential bio-ink for 3D printing

A summary of the study design showing the extraction of extracellular matrix of testicular tissue and the printing of hydrogel scaffolds and the interaction of testicular cells on three-dimensional scaffolds.

Meiotic Chromosome Synapsis and XY-Body Formation In Vitro

To achieve spermatogenesis in vitro, one of the most challenging processes to mimic is meiosis. Meiotic problems, like incomplete synapsis of the homologous chromosomes, or impaired homologous recombination, can cause failure of crossover formation and subsequent chromosome nondisjunction, eventually leading to aneuploid sperm. These meiotic events are therefore strictly monitored by meiotic checkpoints that initiate apoptosis of aberrant spermatocytes and lead to spermatogenic arrest. However, we recently found that, in vitro derived meiotic cells proceeded to the first meiotic division (MI) stage, despite displaying incomplete chromosome synapsis, no discernible XY-body and lack of crossover formation. We therefore optimized our in vitro culture system of meiosis from male germline stem cells (mGSCs) in order to achieve full chromosome synapsis, XY-body formation and meiotic crossovers. In comparison to previous culture system, the in vitro-generated spermatocytes were transferred after meiotic initiation to a second culture dish. This dish already contained a freshly plated monolayer of proliferatively inactivated immortalized Sertoli cells supporting undifferentiated mGSCs. In this way we aimed to simulate the multiple layers of germ cell types that support spermatogenesis in vivo in the testis. We found that in this optimized culture system, although independent of the undifferentiated mGSCs, meiotic chromosome synapsis was complete and XY body appeared normal. However, meiotic recombination still occurred insufficiently and only few meiotic crossovers were formed, leading to MI-spermatocytes displaying univalent chromosomes (paired sister chromatids). Therefore, considering that meiotic checkpoints are not necessarily fully functional in vitro, meiotic crossover formation should be closely monitored when mimicking gametogenesis in vitro to prevent generation of aneuploid gametes.

Fertility preservation for prepubertal boys: lessons learned from the past and update on remaining challenges towards clinical translation

BACKGROUND

Childhood cancer incidence and survivorship are both on the rise. However, many lifesaving treatments threaten the prepubertal testis. Cryopreservation of immature testicular tissue (ITT), containing spermatogonial stem cells (SSCs), as a fertility preservation (FP) option for this population is increasingly proposed worldwide. Recent achievements notably the birth of non-human primate (NHP) progeny using sperm developed in frozen-thawed ITT autografts has given proof of principle of the reproductive potential of banked ITT. Outlining the current state of the art on FP for prepubertal boys is crucial as some of the boys who have cryopreserved ITT since the early 2000s are now in their reproductive age and are already seeking answers with regards to their fertility.

OBJECTIVE AND RATIONALE

In the light of past decade achievements and observations, this review aims to provide insight into relevant questions for clinicians involved in FP programmes. Have the indications for FP for prepubertal boys changed over time? What is key for patient counselling and ITT sampling based on the latest achievements in animals and research performed with human ITT? How far are we from clinical application of methods to restore reproductive capacity with cryostored ITT?

SEARCH METHODS

An extensive search for articles published in English or French since January 2010 to June 2020 using keywords relevant to the topic of FP for prepubertal boys was made in the MEDLINE database through PubMed. Original articles on fertility preservation with emphasis on those involving prepubertal testicular tissue, as well as comprehensive and systematic reviews were included. Papers with redundancy of information or with an absence of a relevant link for future clinical application were excluded. Papers on alternative sources of stem cells besides SSCs were excluded.

OUTCOMES

Preliminary follow-up data indicate that around 27% of boys who have undergone testicular sampling as an FP measure have proved azoospermic and must therefore solely rely on their cryostored ITT to ensure biologic parenthood. Auto-transplantation of ITT appears to be the first technique that could enter pilot clinical trials but should be restricted to tissue free of malignant cells. While in vitro spermatogenesis circumvents the risk linked to cancer cell contamination and has led to offspring in mice, complete spermatogenesis has not been achieved with human ITT. However, generation of haploid germ cells paves the way to further studies aimed at completing the final maturation of germ cells and increasing the efficiency of the processes.

WIDER IMPLICATIONS

Despite all the research done to date, FP for prepubertal boys remains a relatively young field and is often challenging to healthcare providers, patients and parents. As cryopreservation of ITT is now likely to expand further, it is important not only to acknowledge some of the research questions raised on the topic, e.g. the epigenetic and genetic integrity of gametes derived from strategies to restore fertility with banked ITT but also to provide healthcare professionals worldwide with updated knowledge to launch proper multicollaborative care pathways in the field and address clinical issues that will come-up when aiming for the child's best interest.

Spermatogonial stem cells are a promising and pluripotent cell source for regenerative medicine

Regenerative medicine has been shown to hold enormous potential to treat traumatic and degenerative diseases, and substantial advancements have been made in the recent decades. In particular, different cell types were evaluated in basic research and preclinical studies on cell-based therapy applications. Despite the extraordinary achievements made in experimental studies and clinical practice, a considerable number of obstacles, such as the cellular source, ethical and safety issues, hinder further clinical applications. Spermatogonial stem cells (SSCs) are gradually becoming the research focus of cell-based regenerative medicine owing to their unique merits over other types of stem cells, particularly the lack of ethical concerns and lower immunogenicity. In addition, SSCs have been successfully induced to differentiate into other cell types under different appropriate conditions in compelling studies. Based on these properties, we systemically reviewed the development of SSCs as an attractive cell source for cell-based regenerative medicine.

Derivation and propagation of spermatogonial stem cells from human pluripotent cells

OBJECTIVES

This study is designed to generate and propagate human spermatogonial stem cells (SSCs) derived from human pluripotent stem cells (hPSCs).

METHODS

hPSCs were differentiated into SSC-like cells (SSCLCs) by a three-step strategy. The biological characteristics of SSCLCs were detected by immunostaining with antibodies against SSC markers. The ability of self-renewal was measured by propagating for a long time and still maintaining SSCs morphological property. The differentiation potential of SSCLCs was determined by the generation of spermatocytes and haploid cells, which were identified by immunostaining and flow cytometry. The transcriptome analysis of SSCLCs was performed by RNA sequencing. The biological function of SSCLCs was assessed by xeno-transplantation into busulfan-treated mouse testes.

RESULTS

SSCLCs were efficiently generated by a 3-step strategy. The SSCLCs displayed a grape-like morphology and expressed SSC markers. Moreover, SSCLCs could be propagated for approximately 4 months and still maintained their morphological properties. Furthermore, SSCLCs could differentiate into spermatocytes and haploid cells. In addition, SSCLCs displayed a similar gene expression pattern as human GPR125⁺ spermatogonia derived from human testicular tissues. And more, SSCLCs could survive and home at the base membrane of seminiferous tubules.

CONCLUSION

SSCLCs were successfully derived from hPSCs and propagated for a long time. The SSCLCs resembled their counterpart human GPR125⁺ spermatogonia, as evidenced by the grape-like morphology, transcriptome, homing, and functional characteristics. Therefore, hPSC-derived SSCLCs may provide a reliable cell source for studying human SSCs biological properties, disease modeling, and drug toxicity screening.

Epigenetic Regulation of Spermatogonial Stem Cell Homeostasis: From DNA Methylation to Histone Modification

Spermatogonial stem cells(SSCs)are the ultimate germline stem cells with the potential of self-renewal and differentiation, and a dynamic balance of SSCs play an essential role in spermatogenesis. During the gene expression process, genomic DNA and nuclear protein, working together, contribute to SSC homeostasis. Recently, emerging studies have shown that epigenome-related molecules such as chromatin modifiers play an important role in SSC homeostasis through regulating target gene expression. Here, we focus on two types of epigenetic events, including DNA methylation and histone modification, and summarize their function in SSC homeostasis. Understanding the molecular mechanism during SSC homeostasis will promote the recognition of epigenetic biomarkers in male infertility, and bring light into therapies of infertile patients.Graphical Abstract.

Expression and localization of retinoid receptors in the testis of normal and infertile men

Retinoic acid (RA), the active metabolite of vitamin A, is one of the most important factors regulating spermatogenesis. RA activates downstream pathways through its receptors (retinoic acid receptor alpha [RARA], retinoic acid receptor beta, and retinoic acid receptor gamma [RARG]) and retinoid X receptors (retinoid X receptor alpha [RXRA], retinoid X receptor beta [RXRB], and retinoid X receptor gamma [RXRG]). These receptors may serve as therapeutic targets for infertile men. However, the localization and expression of retinoid receptors in normal and infertile men were unknown. In this study, we found RARA and RARG were mostly localized in spermatocytes and round spermatids, RXRB was mainly expressed in Sertoli cells, and RXRG was expressed in most cell types in the fertile human testis. The localization of RARA, RARG, RXRB, and RXRG in men with hypospermatogenesis (HYPO) was similar to that of men with normal fertility. In addition, the messenger RNA expression levels of RARA, RARG, RXRA, RXRB, and RXRG were significantly decreased in men with Sertoli cell-only syndrome (SCOS) and maturational arrest (MA), but not in men with HYPO. These results suggest that reduced levels of RARA, RARG, RXRB, RXRA, and RXRG are more closely associated with SCOS and MA spermatogenetic failure. These results could contribute to the development of new molecular indicators of spermatogenic dysfunction and might provide novel therapeutic targets for treating male infertility.