Current progress, challenges, and future prospects of testis organoids†

Generation of mouse testicular organoids with highly compartmentalized tubular lumen structure and their cryopreservation

Testicular organoids have great potential for maintaining male fertility and even restoring male infertility. However, existing studies on generating organoids with testis-specific structure and function are scarce and come with many limitations. Research on cryopreservation of testicular organoids is even more limited, and inappropriate cryopreservation methods may result in the loss of properties in resuscitated or regenerated organoids, rendering them unsuitable for clinical or research needs. In this paper, we investigated the effects of mouse age and cell number on the self-aggregation of testicular cells into spheres in low-adsorption plates. Various media compositions, culture systems, and cell numbers were used to culture cell spheres for 14 days to form testicular organoids, and the self-organization of the organoids was assessed by histological and immunofluorescence staining. We determined the appropriate cryopreservation conditions for testicular cells, cell spheres, and tissues. Subsequently, organoids derived from cryopreserved testicular tissues, testicular cells, and testicular cell spheres were compared and evaluated by histological and immunofluorescence staining. The results indicate that testicular cell spheres consisting of 30 × 104 testicular cells from 2-week-old mice were able to form organoids highly similar to the luminal structure and cell distribution of natural mouse testicular tissues. This transformation occurred over 14 days of incubation in α-MEM medium containing 10 % knockout serum replacer (KSR) using an agarose hydrogel culture system. Additionally, the Sertoli cells were tightly connected to form a blood-testis barrier. The relative rates of tubular area, germ cells, Sertoli cells, and peritubular myoid cells were 36.985 % ± 0.695, 13.347 % ± 3.102, 47.570 % ± 0.379, and 27.406 % ± 1.832, respectively. The optimal cryopreservation protocol for primary testicular cells involved slow freezing with a cryoprotectant consisting of α-MEM with 10 % dimethyl sulfoxide (DMSO). Slow freezing with cryoprotectants containing 5 % DMSO and 5 % ethylene glycol (EG) was optimal for all different volumes of testicular cell spheres. Compared to testicular organoids generated from frozen testicular tissue and cell spheres, freezing testicular cells proved most effective in maintaining organoid differentiation characteristics and cell-cell interactions. The findings of this study contribute to a "universal" testicular organoid in vitro culture protocol with promising applications for fertility preservation and restoration in prepubertal cancer patients and adult infertile patients.

Whole exome sequencing analysis of 167 men with primary infertility

BACKGROUND

Spermatogenic failure is one of the leading causes of male infertility and its genetic etiology has not yet been fully understood.

METHODS

The study screened a cohort of patients (n = 167) with primary male infertility in contrast to 210 normally fertile men using whole exome sequencing (WES). The expression analysis of the candidate genes based on public single cell sequencing data was performed using the R language Seurat package.

RESULTS

No pathogenic copy number variations (CNVs) related to male infertility were identified using the the GATK-gCNV tool. Accordingly, variants of 17 known causative (five X-linked and twelve autosomal) genes, including ACTRT1, ADAD2, AR, BCORL1, CFAP47, CFAP54, DNAH17, DNAH6, DNAH7, DNAH8, DNAH9, FSIP2, MSH4, SLC9C1, TDRD9, TTC21A, and WNK3, were identified in 23 patients. Variants of 12 candidate (seven X-linked and five autosomal) genes were identified, among which CHTF18, DDB1, DNAH12, FANCB, GALNT3, OPHN1, SCML2, UPF3A, and ZMYM3 had altered fertility and semen characteristics in previously described knockout mouse models, whereas MAGEC1,RBMXL3, and ZNF185 were recurrently detected in patients with male factor infertility. The human testis single cell-sequencing database reveals that CHTF18, DDB1 and MAGEC1 are preferentially expressed in spermatogonial stem cells. DNAH12 and GALNT3 are found primarily in spermatocytes and early spermatids. UPF3A is present at a high level throughout spermatogenesis except in elongating spermatids. The testicular expression profiles of these candidate genes underlie their potential roles in spermatogenesis and the pathogenesis of male infertility.

CONCLUSION

WES is an effective tool in the genetic diagnosis of primary male infertility. Our findings provide useful information on precise treatment, genetic counseling, and birth defect prevention for male factor infertility.

In vivo and in vitro sperm production: an overview of the challenges and advances in male fertility restoration

Male infertility can be caused by genetic anomalies, endocrine disorders, inflammation, and exposure to toxic chemicals or gonadotoxic treatments. Therefore, several recent studies have concentrated on the preservation and restoration of fertility to enhance the quality of life for affected individuals. It is currently recommended to biobank the tissue extracted from testicular biopsies to provide a later source of spermatogonial stem cells (SSCs). Another successful approach has been the in vitro production of haploid male germ cells. The capacity of SSCs to transform into sperm, as in testicular tissue transplantation, SSC therapy, and in vitro or ex vivo spermatogenesis, makes them ideal candidates for in vivo fertility restoration. The transplantation of SSCs or testicular tissue to regenerate spermatogenesis and create embryos has been achieved in nonhuman mammal species. Although the outcomes of human trials have yet to be released, this method may soon be approved for clinical use in humans. Furthermore, regenerative medicine techniques that develop tissue or cells on organic or synthetic scaffolds enriched with bioactive molecules have also gained traction. All of these methods are now in different stages of experimentation and clinical trials. However, thanks to rigorous studies on the safety and effectiveness of SSC-based reproductive treatments, some of these techniques may be clinically available in upcoming decades.

An in vitro testicular organoid model for the study of testis morphogenesis, somatic cell maturation, endocrine function, and toxicological assessment of endocrine disruptors

Male reproductive capacity has fallen considerably in recent decades; in addition, the incidence of testicular cancer has increased in many developed countries. The cause of this phenomenon is unknown, but environmental toxicants are considered a major contributing factor. To study potential reproductive toxicants, robust in vitro testis models are needed. We have recently established a porcine testis organoid system with a high resemblance to the architectures of innate testis tissue. Here, we further investigated the testis morphogenesis, cell maturation, and endocrine function of the testis organoids. We also challenged this system with abiraterone, a steroidogenic inhibitor, to validate its suitability as an in vitro platform for endocrine toxicology tests. Our results showed that the testis cells in the organoids reorganize into testis cordal structures, and the cordal relative areas increase in the organoids over time of culture. Moreover, the diameters and cell numbers per cross-section of the cordal structures increased over time. Interestingly, Sertoli cells in the organoids gradually underwent maturational changes by showing increased expression of androgen receptors, decreased expression of the anti-müllerian hormone, and formation of the blood-testis barrier. Next, we confirmed that the organoids respond to hormonal stimulation and release multiple sex hormones, including testosterone, estradiol, and progesterone. Finally, we showed that the production of testosterone and estradiol in this system can be inhibited in response to the steroidogenic inhibitor. Taken together, our organoid system provides a promising in vitro platform for male reproductive toxicology studies on testis morphogenesis, somatic cell maturation, and endocrine production.

The limitations of testicular organoids: are they truly as promising as we believe?

Organoid systems have revolutionised various facets of biological research by offering a three-dimensional (3D), physiologically relevant in vitro model to study complex organ systems. Over recent years, testicular organoids have been publicised as promising platforms for reproductive studies, disease modelling, drug screening, and fertility preservation. However, the full potential of these systems has yet to be realised due to inherent limitations. This paper offers a comprehensive analysis of the current challenges associated with testicular organoid models. Firstly, we address the inability of current organoid systems to fully replicate the intricate spatial organisation and cellular diversity of the in vivo testis. Secondly, we scrutinise the fidelity of germ cell maturation within the organoids, highlighting incomplete spermatogenesis and epigenetic inconsistencies. Thirdly, we consider the technical challenges faced during organoid culture, including nutrient diffusion limits, lack of vasculature, and the need for specialised growth factors. Finally, we discuss the ethical considerations surrounding the use of organoids for human reproduction research. Addressing these limitations in combination with integrating complementary approaches, will be essential if we are to advance our understanding of testicular biology and develop novel strategies for addressing reproductive health issues in males.

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.

Biomaterials / bioinks and extrusion bioprinting

Bioinks are formulations of biomaterials and living cells, sometimes with growth factors or other biomolecules, while extrusion bioprinting is an emerging technique to apply or deposit these bioinks or biomaterial solutions to create three-dimensional (3D) constructs with architectures and mechanical/biological properties that mimic those of native human tissue or organs. Printed constructs have found wide applications in tissue engineering for repairing or treating tissue/organ injuries, as well as in vitro tissue modelling for testing or validating newly developed therapeutics and vaccines prior to their use in humans. Successful printing of constructs and their subsequent applications rely on the properties of the formulated bioinks, including the rheological, mechanical, and biological properties, as well as the printing process. This article critically reviews the latest developments in bioinks and biomaterial solutions for extrusion bioprinting, focusing on bioink synthesis and characterization, as well as the influence of bioink properties on the printing process. Key issues and challenges are also discussed along with recommendations for future research.

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.

Se(XY) matters: the importance of incorporating sex in microphysiological models

The development of microphysiological models is currently at the forefront of preclinical research. Although these 3D tissue models are being developed to mimic physiological organ function and diseases, which are often sexually dimorphic, sex is usually neglected as a biological variable. For decades, national research agencies have required government-funded clinical trials to include both male and female participants as a means of eliminating male bias. However, this is not the case in preclinical trials, which have been shown to favor male rodents in animal studies and male cell types in in vitro studies. In this Opinion, we highlight the importance of considering sex as a biological variable and outline five approaches for incorporating sex-specific features into current microphysiological models.

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.

Behavior and Functional Roles of CD34+ Mesenchymal Cells in Mammalian Testes

Mammalian testes consist of seminiferous tubules within which Sertoli cells line up at the periphery and nurse germ cells, and of interstitia that harbor various cells such as peritubular myoid cells (PMCs), Leydig cells (LCs), vascular endothelial cells, immune cells such as macrophages, and mesenchymal (stromal) cells. Morphological studies have recently reported the presence of telocytes with telopodes in the interstitium of adult mouse, rat, and human testes. CD34+PDGFRα+ telocytes with long and moniliform telopodes form reticular networks with various cell types such as LCs, PMCs, and vessels, indicating their potential functions in cell-cell communications and tissue homeostasis. Functional studies have recently been performed on testicular interstitial cells and CD34+ cells, using 3D re-aggregate cultures of dissociated testicular cells, and cell cultures. Direct observation of CD34+ cells and adult LCs (ALCs) revealed that CD34+ cells extend thin cytoplasmic processes (telopodes), move toward the LC-CD34+ cell-re-aggregates, and finally enter into the re-aggregates, indicating the chemotactic behavior of CD34+ telocytes toward ALCs. In mammalian testes, important roles of mesenchymal interstitial cells as stem/progenitors in the differentiation and regeneration of LCs have been reported. Here, reports on testicular telocytes so far obtained are reviewed, and future perspectives on the studies of testicular telocytes are noted.

Generation of a Highly Biomimetic Organoid, Including Vasculature, Resembling the Native Immature Testis Tissue

The creation of a testis organoid (artificial testis tissue) with sufficient resemblance to the complex form and function of the innate testis remains challenging, especially using non-rodent donor cells. Here, we report the generation of an organoid culture system with striking biomimicry of the native immature testis tissue, including vasculature. Using piglet testis cells as starting material, we optimized conditions for the formation of cell spheroids, followed by long-term culture in an air–liquid interface system. Both fresh and frozen-thawed cells were fully capable of self-reassembly into stable testis organoids consisting of tubular and interstitial compartments, with all major cell types and structural details expected in normal testis tissue. Surprisingly, our organoids also developed vascular structures; a phenomenon that has not been reported in any other culture system. In addition, germ cells do not decline over time, and Leydig cells release testosterone, hence providing a robust, tunable system for diverse basic and applied applications.