Germ cell survival and differentiation after xenotransplantation of testis tissue from three endangered species: Iberian lynx (Lynx pardinus), Cuvier's gazelle (Gazella cuvieri) and Mohor gazelle (G. dama mhorr)

Testis and brown adipose tissue xenografts from yellowish myotis (Myotis levis)

Yellowish myotis present a seasonal reproduction, influenced by rainfall distribution, in which the testis mass, germ cell composition, and brown adipose tissue mass change along the reproductive stages. In the present study, tissue xenografts were performed in immunodeficient mice to investigate spermatogenesis development in a stable endocrine milieu and the possible androgenic role of brown adipose tissue. Forty-one adult male bats were captured in the Santuário do Caraça, Minas Gerais, Brazil. The gonads and brown adipose tissue were collected, weighed, and grafted under the mice's back skin. Mice biometric and hormonal data were evaluated after grafting, and the testis grafts and mice gonads were fixed for histological and immunohistochemical analyses. As a result, testis grafts from adult bats presented a continuous germ cell development in all reproductive phases, showing round spermatids in all testis tissues. Furthermore, testis fragments in the Rest stage presented elongating spermatids as the most advanced germ cell type in the seminiferous epithelium after seven months of grafting. These data indicated that yellowish myotis spermatogenesis could be continued (presenting a constant spermatogonial differentiation) in a stable endocrine milieu, as found in mice. In addition, the best spermatogenic development was achieved when testis fragments were transplanted at their lowest activity (Rest stage). Regarding the brown adipose tissue grafts, the adipose tissue consumption by mice increased seminal vesicle mass and testosterone serum levels. This data proved that the brown adipose tissue is related to testosterone synthesis, which may be critical in stimulating the differentiation of spermatogonia in yellowish myotis.

Male fertility preservation and restoration strategies for patients undergoing gonadotoxic therapies†

Medical treatments for cancers or other conditions can lead to permanent infertility. Infertility is an insidious disease that impacts not only the ability to have a biological child but also the emotional well-being of the infertile individuals, relationships, finances, and overall health. Therefore, all patients should be educated about the effects of their medical treatments on future fertility and about fertility preservation options. The standard fertility preservation option for adolescent and adult men is sperm cryopreservation. Sperms can be frozen and stored for a long period, thawed at a later date, and used to achieve pregnancy with existing assisted reproductive technologies. However, sperm cryopreservation is not applicable for prepubertal patients who do not yet produce sperm. The only fertility preservation option available to prepubertal boys is testicular tissue cryopreservation. Next-generation technologies are being developed to mature those testicular cells or tissues to produce fertilization-competent sperms. When sperm and testicular tissues are not available for fertility preservation, inducing pluripotent stem cells derived from somatic cells, such as blood or skin, may provide an alternative path to produce sperms through a process call in vitro gametogenesis. This review describes standard and experimental options to preserve male fertility as well as the experimental options to produce functional spermatids or sperms from immature cryopreserved testicular tissues or somatic cells.

Sperm bauplan and function and underlying processes of sperm formation and selection

The spermatozoon is a highly differentiated and polarized cell, with two main structures: the head, containing a haploid nucleus and the acrosomal exocytotic granule, and the flagellum, which generates energy and propels the cell; both structures are connected by the neck. The sperm's main aim is to participate in fertilization, thus activating development. Despite this common bauplan and function, there is an enormous diversity in structure and performance of sperm cells. For example, mammalian spermatozoa may exhibit several head patterns and overall sperm lengths ranging from ∼30 to 350 µm. Mechanisms of transport in the female tract, preparation for fertilization, and recognition of and interaction with the oocyte also show considerable variation. There has been much interest in understanding the origin of this diversity, both in evolutionary terms and in relation to mechanisms underlying sperm differentiation in the testis. Here, relationships between sperm bauplan and function are examined at two levels: first, by analyzing the selective forces that drive changes in sperm structure and physiology to understand the adaptive values of this variation and impact on male reproductive success and second, by examining cellular and molecular mechanisms of sperm formation in the testis that may explain how differentiation can give rise to such a wide array of sperm forms and functions.

Oxidative Stress Disrupted Prepubertal Rat Testicular Development after Xenotransplantation

In the past two decades, testicular tissue grafting and xenografting have been well established, with the production of fertilization-competent sperm in some studies. However, few studies have been carried out to observe the development of grafted prepubertal testicular tissue of rats and compare the biological differences between in situ testis and grafted testis. In this study, we established the prepubertal testicular tissue xenografting model using a 22-day-old rat and evaluated certain parameters, including testicular histology, testosterone production, and ultrastructure of the grafted testes. We also assessed gene expression of cell proliferation markers, testicular cell markers, and antioxidative defense system. Our results showed that 47 days after transplantation, intratesticular testosterone concentration was not significantly altered; however, cell proliferation, spermatogenesis, and Sertoli cell markers in the transplanted testes were significantly disrupted compared with the control group, accompanied by aggravated apoptosis and oxidative damage. Moreover, the transplanted testes showed smaller tubular diameter and disrupted spermatogenic epithelium with apparent vacuoles, distorted and degenerated germ cells with obscure nuclear margin, and no spermatids in the center of the tubules. Although testis xenografting has been extensively tested and attained great achievement in other species, the prepubertal rat testicular tissue xenografting to immunodeficient mice exhibited obvious spermatogenesis arrest and oxidative damage. The protocol still needs further optimization, and there are still some unknown factors in prepubertal rat testes transplantation.

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.

The study and manipulation of spermatogonial stem cells using animal models

Spermatogonial stem cells (SSCs) are a rare group of cells in the testis that undergo self-renewal and complex sequences of differentiation to initiate and sustain spermatogenesis, to ensure the continuity of sperm production throughout adulthood. The difficulty of unequivocal identification of SSCs and complexity of replicating their differentiation properties in vitro have prompted the introduction of novel in vivo models such as germ cell transplantation (GCT), testis tissue xenografting (TTX), and testis cell aggregate implantation (TCAI). Owing to these unique animal models, our ability to study and manipulate SSCs has dramatically increased, which complements the availability of other advanced assisted reproductive technologies and various genome editing tools. These animal models can advance our knowledge of SSCs, testis tissue morphogenesis and development, germ-somatic cell interactions, and mechanisms that control spermatogenesis. Equally important, these animal models can have a wide range of experimental and potential clinical applications in fertility preservation of prepubertal cancer patients, and genetic conservation of endangered species. Moreover, these models allow experimentations that are otherwise difficult or impossible to be performed directly in the target species. Examples include proof-of-principle manipulation of germ cells for correction of genetic disorders or investigation of potential toxicants or new drugs on human testis formation or function. The primary focus of this review is to highlight the importance, methodology, current and potential future applications, as well as limitations of using these novel animal models in the study and manipulation of male germline stem cells.

Immunological microenvironment in the testis

BACKGROUND

The testis is specific in that it produces haploid germ cells of which autoantigens newly appear long after the neonatal immune tolerance. Under normal condition, these autoantigens are protected by the blood-testis barrier formed by Sertoli cells. Thus, the testis is an immunologically privileged site where haploid cells are protected from autoimmune attack.

METHODS

The immunological microenvironment in the testis was experimentally investigated using mice and rats.

MAIN FINDINGS

Not only the blood-testis barrier but also various immuno-suppressive factors are involved in the immune-privileged testis. Indeed, germ cells transplanted into the xenogeneic seminiferous tubules could proliferate and differentiate with no aid of artificial immunosuppression. On the other hand, autoimmune orchitis could be experimentally produced by various methods of immunization with syngeneic or xenogeneic germ cell antigens.

CONCLUSION

Our results indicate that the testis is immunologically privileged but also immunologically fragile organ. Therefore, the dual nature is critical for immunoregulation of testicular function.

Altered hormonal milieu and dysregulated protein expression can cause spermatogenic arrest in ectopic xenografted immature rat testis

Testis tissue xenografting complemented with cryopreservation is a feasible technique for fertility preservation in children with malignancy receiving gonadotoxic therapy and for endangered species with high neonatal mortality rate. However, xenografted testis of human and most endangered species are known to undergo spermatogenic arrest. In this study, we xenografted immature rat testis onto immunodeficient male mice to investigate the plausible underlying causes of spermatogenic arrest. Histological analysis of xenografted testes collected 8-wk post-grafting showed incomplete spermatogenesis with pachytene-stage spermatocytes as the most advanced germ cells. Although the levels of serum luteinizing hormone and testosterone were normal in recipient mice, those of follicle stimulating hormone (FSH) were significantly high, and specific receptors of FSH were absent in the xenografts. The xenografts demonstrated dysregulated expression of Sertoli cell-transcriptional regulators (WT1 and SOX9) and secretory proteins (SCF and GDNF). In conclusion, results from our study suggested that an altered hormonal milieu in recipients and dysregulated protein expression in xenografts could be a potential cause of spermatogenic arrest in xenografted immature rat testis. Further stereological analysis of xenografts can demonstrate precise cellular composition of xenografts to decipher interactions between germ and somatic cells to better understand spermatogenic arrest in xenografted testis.

Can we induce spermatogenesis in the domestic cat using an in vitro tissue culture approach?

The reduced number of animals in most wild felid populations implies a loss of genetic diversity. The death of juveniles, prior to the production of mature sperm, represents a loss of potential genetic contribution to future populations. Since 2011 mouse testicular organ culture has introduced an alternative mechanism to produce sperm in vitro from immature tissue. However, extension of this technology to other species has remained limited. We have used the domestic cat (Felis catus) as a model for wild felids to investigate spermatogenesis initiation and regulation, with the mouse serving as a control species. Testicular tissue fragments were cultured in control medium or medium supplemented with knockout serum replacement (KSR), AlbuMax, beta-estradiol or AlbuMax plus beta-estradiol. Contrary to expectations, and unlike results obtained in mouse controls, no germ cell differentiation could be detected. The only germ cells observed after six weeks of culture were spermatogonia regardless of the initial stage of tubule development in the donor tissue. Moreover, the number of spermatogonia decreased with time in culture in all media tested, especially in the medium supplemented with KSR, while AlbuMax had a slight protective effect. The combination of AlbuMax and beta-estradiol led to an increase in the area occupied by seminiferous tubules, and thus to an increase in total number of spermatogonial cells. Considering all the media combinations tested the stimulus for felid germ cell differentiation in this type of system seems to be different from the mouse. Studies using other triggers of differentiation and tissue survival factors should be performed to pursue this technology for the genetic diversity preservation in wild felids.

Saving wild ungulate diversity through enhanced management and sperm cryopreservation

Wild ungulates throughout the world face the impending risk of extinction. Small founding population size, lack of interest in exhibiting wild ungulates and declining space in zoos are not sustaining ex situ populations. Animals managed in ex situ collections continue to experience >20% neonate loss globally. To ensure population sustainability there is a critical need to: (1) manage ungulates in large herds, increasing mate choice and reproductive efficiency; (2) improve husbandry and genetic management; and (3) develop consistent assisted reproductive technologies, including sperm cryopreservation and AI. Recently, new models in the management of ungulates have begun to emerge. Animal managers and researchers are also beginning to exploit advances in genomics to improve genetic management of their collections. Furthermore, the past decade has witnessed significant advances particularly in semen collection and cryopreservation in numerous species. Advances in gonadal tissue cryopreservation now offer additional opportunities to preserve male genomes. The new knowledge generated is enabling the creation of genetic (sperm) banks to rescue and enhance reproductive management of wild ungulates. The present paper reviews the threats to ungulate populations, the status and relevance of animal management and biomaterial banking efforts to ensure long-term survival of these charismatic species.

Slow freezing, but not vitrification supports complete spermatogenesis in cryopreserved, neonatal sheep testicular xenografts

The ability to spur growth of early stage gametic cells recovered from neonates could lead to significant advances in rescuing the genomes of rare genotypes or endangered species that die unexpectedly. The purpose of this study was to determine, for the first time, the ability of two substantially different cryopreservation approaches, slow freezing versus vitrification, to preserve testicular tissue of the neonatal sheep and subsequently allow initiation of spermatogenesis post-xenografting. Testis tissue from four lambs (3-5 wk old) was processed and then untreated or subjected to slow freezing or vitrification. Tissue pieces (fresh, n = 214; slow freezing, then thawing, n = 196; vitrification, then warming, n = 139) were placed subcutaneously under the dorsal skin of SCID mice and then grafts recovered and evaluated 17 wk later. Grafts from fresh and slow frozen tissue contained the most advanced stages of spermatogenesis, including normal tubule architecture with elongating spermatids in ~1% (fresh) and ~10% (slow frozen) of tubules. Fewer than 2% of seminiferous tubules advanced to the primary spermatocyte stage in xenografts derived from vitrified tissue. Results demonstrate that slow freezing of neonatal lamb testes was far superior to vitrification in preserving cellular integrity and function after xenografting, including allowing ~10% of tubules to retain the capacity to resume spermatogenesis and yield mature spermatozoa. Although a first for any ruminant species, findings also illustrate the importance of preemptive studies that examine cryo-sensitivity of testicular tissue before attempting this type of male fertility preservation on a large scale.

Targeted Germline Modifications in Rats Using CRISPR/Cas9 and Spermatogonial Stem Cells

Organisms with targeted genomic modifications are efficiently produced by gene editing in embryos using CRISPR/Cas9 RNA-guided DNA endonuclease. Here, to facilitate germline editing in rats, we used CRISPR/Cas9 to catalyze targeted genomic mutations in rat spermatogonial stem cell cultures. CRISPR/Cas9-modified spermatogonia regenerated spermatogenesis and displayed long-term sperm-forming potential following transplantation into rat testes. Targeted germline mutations in Epsti1 and Erbb3 were vertically transmitted from recipients to exclusively generate "pure," non-mosaic mutant progeny. Epsti1 mutant rats were produced with or without genetic selection of donor spermatogonia. Monoclonal enrichment of Erbb3 null germlines unmasked recessive spermatogenesis defects in culture that were buffered in recipients, yielding mutant progeny isogenic at targeted alleles. Thus, spermatogonial gene editing with CRISPR/Cas9 provided a platform for generating targeted germline mutations in rats and for studying spermatogenesis.

Xenografting of testicular tissue pieces: 12 years of an in vivo spermatogenesis system

Spermatogenesis is a dynamic and complex process that involves endocrine and testicular factors. During xenotransplantation of testicular tissue fragments into immunodecifient mice, a functional communication between host brain and donor testis is established. This interaction allows for the progression of spermatogenesis and recovery of fertilisation-competent spermatozoa from a broad range of mammalian species. In the last few years, significant progress has been achieved in testis tissue xenografting that improves our knowledge about the factors determining the success of grafting. The goal of this review is to provide up to date information about the role of factors such as donor age, donor species, testis tissue preservation or type of recipient mouse on the efficiency of this technique. Applications are described and compared with other techniques with similar purposes. Recent work has demonstrated that testicular tissue xenografting is used as a model to study gonadotoxicity of drugs and to obtain sperm from valuable young males.