Male fertility restoration: in vivo and in vitro stem cell-based strategies using cryopreserved testis tissue: a scoping review

IMPORTANCE

Advances in the treatment of childhood cancer have significantly improved survival rates, with more than 80% of survivors reaching adulthood. However, gonadotoxic cancer treatments endanger future fertility, and prepubertal males have no option to preserve fertility by sperm cryopreservation. In addition, boys with cryptorchidism are at risk of compromised fertility in adulthood.

OBJECTIVE

To investigate current evidence for male fertility restoration strategies, explore barriers to clinical implementation, and outline potential steps to overcome these barriers, a scoping review was conducted. This knowledge synthesis is particularly relevant for prepubertal male cancer survivors and boys with cryptorchidism.

EVIDENCE REVIEW

The review was conducted after the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews criteria and previously published guidelines and examined studies using human testis tissue of prepubertal boys or healthy male adults. A literature search in PubMed was conducted, and 72 relevant studies were identified, including in vivo and in vitro approaches.

FINDINGS

In vivo strategies, such as testis tissue engraftment and spermatogonial stem cell transplantation, hold promise for promoting cell survival and differentiation. Yet, complete spermatogenesis has not been achieved. In vitro approaches focus on the generation of male germ cells from direct germ cell maturation in various culture systems, alongside human induced pluripotent stem cells and embryonic stem cells. These approaches mark significant advancements in understanding and promoting spermatogenesis, but achieving fully functional spermatozoa in vitro remains a challenge. Barriers to clinical implementation include the risk of reintroducing malignant cells and introduction of epigenetic changes.

CONCLUSION

Male fertility restoration is an area in rapid development. On the basis of the reviewed studies, the most promising and advanced strategy for restoring male fertility using cryopreserved testis tissue is direct testis tissue transplantation.

RELEVANCE

This review identifies persistent barriers to the clinical implementation of male fertility restoration. However, direct transplantation of frozen-thawed testis tissue remains a promising strategy that is on the verge of clinical application.

Frequency, morbidity and equity - the case for increased research on male fertility

Currently, most men with infertility cannot be given an aetiology, which reflects a lack of knowledge around gamete production and how it is affected by genetics and the environment. A failure to recognize the burden of male infertility and its potential as a biomarker for systemic illness exists. The absence of such knowledge results in patients generally being treated as a uniform group, for whom the strategy is to bypass the causality using medically assisted reproduction (MAR) techniques. In doing so, opportunities to prevent co-morbidity are missed and the burden of MAR is shifted to the woman. To advance understanding of men's reproductive health, longitudinal and multi-national centres for data and sample collection are essential. Such programmes must enable an integrated view of the consequences of genetics, epigenetics and environmental factors on fertility and offspring health. Definition and possible amelioration of the consequences of MAR for conceived children are needed. Inherent in this statement is the necessity to promote fertility restoration and/or use the least invasive MAR strategy available. To achieve this aim, protocols must be rigorously tested and the move towards personalized medicine encouraged. Equally, education of the public, governments and clinicians on the frequency and consequences of infertility is needed. Health options, including male contraceptives, must be expanded, and the opportunities encompassed in such investment understood. The pressing questions related to male reproductive health, spanning the spectrum of andrology are identified in the Expert Recommendation.

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.

Advances of three-dimensional (3D) culture systems for in vitro spermatogenesis

The loss of germ cells and spermatogenic failure in non-obstructive azoospermia are believed to be the main causes of male infertility. Laboratory studies have used in vitro testicular models and different 3-dimensional (3D) culture systems for preservation, proliferation and differentiation of spermatogonial stem cells (SSCs) in recent decades. The establishment of testis-like structures would facilitate the study of drug and toxicity screening, pathological mechanisms and in vitro differentiation of SSCs which resulted in possible treatment of male infertility. The different culture systems using cellular aggregation with self-assembling capability, the use of different natural and synthetic biomaterials and various methods for scaffold fabrication provided a suitable 3D niche for testicular cells development. Recently, 3D culture models have noticeably used in research for their architectural and functional similarities to native microenvironment. In this review article, we briefly investigated the recent 3D culture systems that provided a suitable platform for male fertility preservation through organ culture of testis fragments, proliferation and differentiation of SSCs.

In Vitro and In Vivo Models for Drug Transport Across the Blood-Testis Barrier

The blood-testis barrier (BTB) is a selectively permeable membrane barrier formed by adjacent Sertoli cells (SCs) in the seminiferous tubules of the testes that develops intercellular junctional complexes to protect developing germ cells from external pressures. However, due to this inherent defense mechanism, the seminiferous tubule lumen can act as a pharmacological sanctuary site for latent viruses (e.g., Ebola, Zika) and cancers (e.g., leukemia). Therefore, it is critical to identify and evaluate BTB carrier-mediated drug delivery pathways to successfully treat these viruses and cancers. Many drugs are unable to effectively cross cell membranes without assistance from carrier proteins like transporters because they are large, polar, and often carry a charge at physiologic pH. SCs express transporters that selectively permit endogenous compounds, such as carnitine or nucleosides, across the BTB to support normal physiologic activity, although reproductive toxicants can also use these pathways, thereby circumventing the BTB. Certain xenobiotics, including select cancer therapeutics, antivirals, contraceptives, and environmental toxicants, are known to accumulate within the male genital tract and cause testicular toxicity; however, the transport pathways by which these compounds circumvent the BTB are largely unknown. Consequently, there is a need to identify the clinically relevant BTB transport pathways in in vitro and in vivo BTB models that recapitulate human pharmacokinetics and pharmacodynamics for these xenobiotics. This review summarizes the various in vitro and in vivo models of the BTB reported in the literature and highlights the strengths and weaknesses of certain models for drug disposition studies. SIGNIFICANCE STATEMENT: Drug disposition to the testes is influenced by the physical, physiological, and immunological components of the blood-testis barrier (BTB). But many compounds are known to cross the BTB by transporters, resulting in pharmacological and/or toxicological effects in the testes. Therefore, models that assess drug transport across the human BTB must adequately account for these confounding factors. This review identifies and discusses the benefits and limitations of various in vitro and in vivo BTB models for preclinical drug disposition studies.

Mice 3D testicular organoid system as a novel tool to study Zika virus pathogenesis

Zika virus (ZIKV) poses a serious threat to global public health due to its close relationship with neurological and male reproductive damage. However, deficiency of human testicular samples hinders the in-depth research on ZIKV-induced male reproductive system injury. Organoids are relatively simple in vitro models, which could mimic the pathological changes of corresponding organs. In this study, we constructed a 3D testicular organoid model using primary testicular cells from adult BALB/c mice. Similar to the testis, this organoid system has a blood-testis barrier (BTB)-like structure and could synthesize testosterone. ZIKV tropism of testicular cells and ZIKV-induced pathological changes in testicular organoid was also similar to that in mammalian testis. Therefore, our results provide a simple and reproducible in vitro testicular model for the investigations of ZIKV-induced testicular injury.

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.

Regulation of Cell Types Within Testicular Organoids

Organoids are 3-dimensional (3D) structures grown in vitro that emulate the cytoarchitecture and functions of true organs. Therefore, testicular organoids arise as an important model for research on male reproductive biology. These organoids can be generated from different sources of testicular cells, but most studies to date have used immature primary cells for this purpose. The complexity of the mammalian testicular cytoarchitecture and regulation poses a challenge for working with testicular organoids, because, ideally, these 3D models should mimic the organization observed in vivo. In this review, we explore the characteristics of the most important cell types present in the testicular organoid models reported to date and discuss how different factors influence the regulation of these cells inside the organoids and their outcomes. Factors such as the developmental or maturational stage of the Sertoli cells, for example, influence organoid generation and structure, which affect the use of these 3D models for research. Spermatogonial stem cells have been a focus recently, especially in regard to male fertility preservation. The regulation of the spermatogonial stem cell niche inside testicular organoids is discussed in the present review, as this research area may be positively affected by recent progress in organoid generation and tissue engineering. Therefore, the testicular organoid approach is a very promising model for male reproductive biology research, but more studies and improvements are necessary to achieve its full potential.

Microfluidics in male reproduction: is ex vivo culture of primate testis tissue a future strategy for ART or toxicology research?

The significant rise in male infertility disorders over the years has led to extensive research efforts to recapitulate the process of male gametogenesis in vitro and to identify essential mechanisms involved in spermatogenesis, notably for clinical applications. A promising technology to bridge this research gap is organ-on-chip (OoC) technology, which has gradually transformed the research landscape in ART and offers new opportunities to develop advanced in vitro culture systems. With exquisite control on a cell or tissue microenvironment, customized organ-specific structures can be fabricated in in vitro OoC platforms, which can also simulate the effect of in vivo vascularization. Dynamic cultures using microfluidic devices enable us to create stimulatory effect and non-stimulatory culture conditions. Noteworthy is that recent studies demonstrated the potential of continuous perfusion in OoC systems using ex vivo mouse testis tissues. Here we review the existing literature and potential applications of such OoC systems for male reproduction in combination with novel bio-engineering and analytical tools. We first introduce OoC technology and highlight the opportunities offered in reproductive biology in general. In the subsequent section, we discuss the complex structural and functional organization of the testis and the role of the vasculature-associated testicular niche and fluid dynamics in modulating testis function. Next, we review significant technological breakthroughs in achieving in vitro spermatogenesis in various species and discuss the evidence from microfluidics-based testes culture studies in mouse. Lastly, we discuss a roadmap for the potential applications of the proposed testis-on-chip culture system in the field of primate male infertility, ART and reproductive toxicology.

A novel xeno-organoid approach: exploring the crosstalk between human iPSC-derived PGC-like and rat testicular cells

Specification of germ cell-like cells from induced pluripotent stem cells has become a clinically relevant tool for research. Research on initial embryonic processes is often limited by the access to foetal tissue, and in humans, the molecular events resulting in primordial germ cell (PGC) specification and sex determination remain to be elucidated. A deeper understanding of the underlying processes is crucial to describe pathomechanisms leading to impaired reproductive function. Several protocols have been established for the specification of human pluripotent stem cell towards early PGC-like cells (PGCLC), currently representing the best model to mimic early human germline developmental processes in vitro. Further sex determination towards the male lineage depends on somatic gonadal cells providing the necessary molecular cues. By establishing a culture system characterized by the re-organization of somatic cells from postnatal rat testes into cord-like structures and optimizing efficient PGCLC specification protocols, we facilitated the co-culture of human germ cell-like cells within a surrogate testicular microenvironment. Specified conditions allowed the survival of rat somatic testicular and human PGCLCs for 14 days. Human cells maintained the characteristic expression of octamer-binding transcription factor 4, SRY-box transcription factor 17, and transcription factor AP-2 gamma and were recovered from the xeno-organoids by cell sorting. This novel xeno-organoid approach will allow the in vitro exploration of early sex determination of human PGCLCs.

Testicular organoid formation is a property of immature somatic cells, which self-assemble and exhibit long-term hormone-responsive endocrine function

Testicular organoid models are tools to study testicular physiology, development, and spermatogenesis in vitro. However, few side-by-side comparisons of organoid generation method have been evaluated. Here, we directly tested whether the culture microenvironment is the prime determinant promoting testicular organoid self-assembly. Using Matrigel as a representative extracellular matrix (ECM), we compared multiple culture environments, 2D and 3D, ECM-free and ECM, for organoid self-assembly with immature murine testicular cells. De novo tissues were observed to self-assemble in all four culture environments tested within 72 h, however, these tissues only met requirements to be named organoids in 2D ECM and 3D ECM-free (3DF) culture methods. Based on these results, 3DF was selected for further study, and used to examine animal age as an independent variable. Organoid assembly was significantly delayed when using pubertal murine cells and entirely absent from adult murine and adult human cells. Organoid-conditioned medium and medium supplemented with 1% Matrigel did not improve organoid assembly in pubertal murine cells, but immature murine cells rescued the assembly of adult murine cells when cultured together as age-chimeric cell mixtures. In murine organoids cultured for 14 d, tubule-like structures exhibiting a highly biomimetic architecture were characterized, including some rare germ and spermatogonial stem cells. These structural organoids secreted high levels of testosterone and inhibin B over 12 weeks with preserved responsivity to gonadotropins. Collectively these studies, in which cellular self-assembly and organoid formation was achieved independent of the culture microenvironment, suggest that self-assembly is an innate property of immature testicular cells independent from, but capable of being promoted by, the culture environment. This study provides a template for studying testicular organoid self-assembly and endocrine function, and a platform for improving the engineering of functional testicular tissues.

Challenging human somatic testicular cell reassembly by protein kinase inhibition -setting up a functional in vitro test system

Signalling pathways and cellular interactions defining initial processes of testis morphogenesis, i.e. cord formation, are poorly understood. In vitro cell-based systems modelling cord formation can be utilised as platforms to interrogate processes of tubulogenesis. We aimed at testing our established cord formation in vitro model using adult human testicular cells as a quantitative assay that can facilitate future studies on cord morphogenesis. We challenged the responsiveness of our system with a broad-spectrum protein kinase inhibitor, K252a. Cultured testicular cells were treated with various K252a concentrations under constant exposure and compound withdrawal. To quantify cell reaggregation changes, we performed computer-assisted phase-contrast image analysis of aggregate size and number. Cell reaggregation was analysed in detail by categorisation of aggregates into size groups and accounting for changes in aggregate number per size category. We found a dose-related disturbance of testicular cell reaggregation. K252a decreased aggregate size (IC50 of 203.3 nM) and reduced the large aggregate numbers. Video recordings revealed that treatment with K252a at a concentration above IC50 interfered with aggregate coalescence into cords. Short-term exposure and compound wash-out induced irreversible decrease in large aggregates. We propose our in vitro model as a functional platform to quantitatively investigate seminiferous tubulogenesis under pharmacological impact.

In-vitro spermatogenesis through testis modelling: Toward the generation of testicular organoids

Background

The testicular organoid concept has recently been introduced in tissue engineering to refer to testicular cell organizations modeling testicular architecture and function. The testicular organoid approach gives control over which and how cells reaggregate, which is not possible in organotypic cultures, thereby extending the applicability of in‐vitro spermatogenesis (IVS) systems. However, it remains unclear which culture method and medium allow reassociation of testicular cells into a functional testicular surrogate in‐vitro.

Objective

The aim of this paper is to review the different strategies that have been used in an attempt to create testicular organoids and generate spermatozoa. We want to provide an up‐to‐date list on culture methodologies and media compositions that have been used and determine their role in regulating tubulogenesis and differentiation of testicular cells.

Search method

A literature search was conducted in PubMed, Web of Science, and Scopus to select studies reporting the reorganization of testicular cell suspensions in‐vitro, using the keywords: three‐dimensional culture, in‐vitro spermatogenesis, testicular organoid, testicular scaffold, and tubulogenesis. Papers published before the August 1, 2019, were selected.

Outcome

Only a limited number of studies have concentrated on recreating the testicular architecture in‐vitro. While some advances have been made in the testicular organoid research in terms of cellular reorganization, none of the described culture systems is adequate for the reproduction of both the testicular architecture and IVS.

Conclusion

Further improvements in culture methodology and medium composition have to be made before being able to provide both testicular tubulogenesis and spermatogenesis in‐vitro.

Induction of Sertoli-like cells from human fibroblasts by NR5A1 and GATA4

Sertoli cells are essential nurse cells in the testis that regulate the process of spermatogenesis and establish the immune-privileged environment of the blood-testis-barrier (BTB). Here, we report the in vitro reprogramming of fibroblasts to human induced Sertoli-like cells (hiSCs). Initially, five transcriptional factors and a gene reporter carrying the AMH promoter were utilized to obtain the hiSCs. We further reduce the number of reprogramming factors to two, NR5A1 and GATA4, and show that these hiSCs have transcriptome profiles and cellular properties that are similar to those of primary human Sertoli cells. Moreover, hiSCs can sustain the viability of spermatogonia cells harvested from mouse seminiferous tubules. hiSCs suppress the proliferation of human T lymphocytes and protect xenotransplanted human cells in mice with normal immune systems. hiSCs also allow us to determine a gene associated with Sertoli cell only syndrome (SCO), CX43, is indeed important in regulating the maturation of Sertoli cells.

Rebuilding the human testis in vitro

Increasing rates of male infertility have led to a greater need for relevant model systems to gain further insight into male fertility and its failings. Spermatogenesis and hormone production occur within distinct regions of the testis. Defined by specialized architecture and a diverse population of cell types, it is no surprise that disruption of this highly organized microenvironment can lead to infertility. To date, no robust in vitro system has facilitated full spermatogenesis resulting in the production of fertilization‐competent human spermatozoa. Here, we review a selection of current in vitro systems available for modelling the human testis microenvironment with focus on the progression of spermatogenesis and recapitulation of the testis microenvironment.

Propagation of Spermatogonial Stem Cell-Like Cells From Infant Boys

Background

Gonadotoxic treatment of malignant diseases as well as some non-malignant conditions such as cryptorchidism in young boys may result in infertility and failure to father children later in life. As a fertility preserving strategy, several centers collect testicular biopsies to cryopreserve spermatogonial stem cells (SSCs) world-wide. One of the most promising therapeutic strategies is to transplant SSCs back into the seminiferous tubules to initiate endogenous spermatogenesis. However, to obtain sufficient numbers of SSC to warrant transplantation, in vitro propagation of cells is needed together with proper validation of their stem cell identity.

Materials and Methods

A minute amount of testicular biopsies (between 5 mg and 10 mg) were processed by mechanical and enzymatic digestion. SSCs were enriched by differential plating method in StemPro-34 medium supplemented with several growth factors. SSC-like cell clusters (SSCLCs) were passaged five times. SSCLCs were identified by immunohistochemical and immunofluorescence staining, using protein expression patterns in testis biopsies as reference. Quantitative polymerase chain reaction analysis of SSC markers LIN-28 homolog A (LIN28A), G antigen 1 (GAGE1), promyelocytic leukemia zinc finger protein (PLZF), integrin alpha 6 (ITGA6), ubiquitin carboxy-terminal hydrolase L1 (UCHL1) and integrin beta 1 (ITGB1) were also used to validate the SSC-like cell identity.

Results

Proliferation of SSCLCs was achieved. The presence of SSCs in SSCLCs was confirmed by positive immunostaining of LIN28, UCHL1 and quantitative polymerase chain reaction for LIN28A, UCHL1, PLZF, ITGA6, and ITGB1, respectively.

Conclusion

This study has demonstrated that SSCs from infant boys possess the capacity for in vitro proliferation and advance a fertility preservation strategy for pre-pubertal boys who may otherwise lose their fertility.