Germ cell transplantation for the propagation of companion animals, non-domestic and endangered species

Spermatogonial stem cell technologies: applications from human medicine to wildlife conservation†

Spermatogonial stem cell (SSC) technologies that are currently under clinical development to reverse human infertility hold the potential to be adapted and applied for the conservation of endangered and vulnerable wildlife species. The biobanking of testis tissue containing SSCs from wildlife species, aligned with that occurring in pediatric human patients, could facilitate strategies to improve the genetic diversity and fitness of endangered populations. Approaches to utilize these SSCs could include spermatogonial transplantation or testis tissue grafting into a donor animal of the same or a closely related species, or in vitro spermatogenesis paired with assisted reproduction approaches. The primary roadblock to progress in this field is a lack of fundamental knowledge of SSC biology in non-model species. Herein, we review the current understanding of molecular mechanisms controlling SSC function in laboratory rodents and humans, and given our particular interest in the conservation of Australian marsupials, use a subset of these species as a case-study to demonstrate gaps-in-knowledge that are common to wildlife. Additionally, we review progress in the development and application of SSC technologies in fertility clinics and consider the translation potential of these techniques for species conservation pipelines.

Successful xenotransplantation of testicular cells following fractionated chemotherapy of recipient birds

An essential step in the success of germ cell transplantation is the preparation of the recipient's testicular environment to increase the availability of stem cell niches. However, most methods for this purpose in birds face serious limitations such as partial germ cell depletion, high toxicity and mortality, or the need to use expensive technologies. Here, we validated a simple and practical technique of transferring quail testicular cells into chicken testes depleted of endogenous spermatozoa by fractioned chemotherapy (20 mg/kg/week busulfan for 5 weeks). This protocol resulted in a very low mortality of the treated day-old chicks and, despite maintenance of androgenic activity, sperm production was decreased by 84.3% at 25 weeks of age. NANOG immunostaining revealed that very few to no germ cells were present following treatment with 20 and 40 mg/kg, respectively. RT-qPCR data also showed that c-MYC and NANOG expression declined in these treatments, but GRFα1 and BID expressions remained unaltered among groups. After xenotransplantation, quail germ cells were immunodetected in chicken testes using a species-specific antibody (QCPN), and quail ovalbumin DNA was found in seminal samples collected from chicken recipients. Together, these data confirm that fractionated administration of busulfan in hatchlings is a practical, effective, and safe protocol to prepare recipient male birds capable of supporting xenogeneic spermatogenesis.

DAZL Knockout Pigs as Recipients for Spermatogonial Stem Cell Transplantation

Spermatogonial stem cell (SSC) transplantation into the testis of a germ cell (GC)-depleted surrogate allows transmission of donor genotype via donor-derived sperm produced by the recipient. Transplantation of gene-edited SSCs provides an approach to propagate gene-edited large animal models. DAZL is a conserved RNA-binding protein important for GC development, and DAZL knockout (KO) causes defects in GC commitment and differentiation. We characterized DAZL-KO pigs as SSC transplantation recipients. While there were GCs in 1-week-old (wko) KO, complete GC depletion was observed by 10 wko. Donor GCs were transplanted into 18 DAZL-KO recipients at 10-13 wko. At sexual maturity, semen and testes were evaluated for transplantation efficiency and spermatogenesis. Approximately 22% of recipient seminiferous tubules contained GCs, including elongated spermatids and proliferating spermatogonia. The ejaculate of 89% of recipients contained sperm, exclusively from donor origin. However, sperm concentration was lower than the wild-type range. Testicular protein expression and serum hormonal levels were comparable between DAZL-KO and wild-type. Intratesticular testosterone and Leydig cell volume were increased, and Leydig cell number decreased in transplanted DAZL-KO testis compared to wild-type. In summary, DAZL-KO pigs support donor-derived spermatogenesis following SSC transplantation, but low spermatogenic efficiency currently limits their use for the production of offspring.

Assisted Reproductive Technology in Neotropical Deer: A Model Approach to Preserving Genetic Diversity

Simple Summary

Deer species in the Neotropical region have undergone a decline of their populations. Although conservation of their natural habitat is considered the best way to assist the species, the speed of occupation of these areas and the anthropic actions are so fast that the efforts are, at times, insufficient. As free-living populations decrease, there is a descent in the genetic diversity and an increase in crossbreeding between related individuals (inbreeding). Genetic diversity is essential for survival, since it enables natural selection to occur, providing adaptation and maintenance of the species. To protect the genetic diversity, it is possible to use reproductive techniques and conserve different types of cells, which can be used in the future to reestablish any alleles that have been lost by the populations.

Abstract

One of the most significant challenges in deer is the ability to maintain genetic diversity, avoiding inbreeding and sustaining population health and reproduction. Although our general knowledge of reproductive physiology is improving, it appears that the application of assisted reproductive technology (ART) will more efficiently advance wildlife conservation efforts and preserve genetic diversity. The purpose of this review is to present the most important results obtained with the use of ART in Neotropical deer. Thus, the state-of-the-art for estrus synchronization, semen technology, artificial insemination, and in vivo embryo production will be presented. In vitro embryo production (IVP) is also a biotechnology that is taking initial steps in deer. In this aspect, the approach with the proteomics of ovarian follicular fluid is being used as a tool for a better understanding of oocyte maturation. Finally, cell banks and the use of interspecific somatic cell nuclear transfer (iSCNT) as well as the use of stem cells for gametes differentiation are promising techniques.

Involvement of Cytokines and Hormones in the Development of Spermatogenesis In Vitro from Spermatogonial Cells of Cyclophosphamide-Treated Immature Mice

Aggressive chemotherapy treatment may lead to male infertility. Prepubertal boys do not produce sperm at this age, however, they have spermatogonial stem cells in their testes. Here, we examined the effect of intraperitoneal injection of cyclophosphamide (CP) on the capacity of immature mice (IM) to develop spermatogenesis in vivo and in vitro [using methylcellulose culture system (MCS)]. Our results show a significant decrease in testicular weight, total number of testicular cells, and the number of Sertoli, peritubular, premeiotic, and meiotic/post-meiotic cells, but an increase in the percentages of damaged seminiferous tubules in CP-treated IM compared to control. The functionality of Sertoli cells was significantly affected. The addition of testosterone to isolated cells from seminiferous tubules of CP-treated IM significantly increased the percentages of premeiotic (CD9-positive cells) and meiotic/post-meiotic cells (ACROSIN-positive cells) developed in MCS compared to control. The addition of FSH did not affect developed cells in MCS compared to control, but in combination with testosterone, it significantly decreased the percentages of CD9-positive cells and ACROSIN-positive cells. The addition of IL-1 did not affect developed cells in MCS compared to control, but in combination with testosterone, it significantly increased the percentages of VASA-positive cells and BOULE-positive cells compared to IL-1 or testosterone. Addition of TNF significantly increased only CD9-positive cells in MCS compared to control, but in combination with testosterone, it significantly decreased ACROSIN-positive cells compared to testosterone. Our results show a significant impairment of spermatogenesis in the testes of CP-treated IM, and that spermatogonial cells from these mice proliferate and differentiate to meiotic/post-meiotic cells under in vitro culture conditions.

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.

Embryonic diapause in roe deer: A model to unravel embryo-maternal communication during pre-implantation development in wildlife and livestock species

An alarming number of large mammalian species with low reproduction rates is threatened with extinction. As basic knowledge of reproductive physiology is currently lacking in many species, increasing the understanding of reproductive physiology is imperative and includes the development of novel artificial reproduction technologies. Despite the relatively comprehensive knowledge on molecular mechanisms underlying reproduction in livestock species such as cattle, pregnancy failures are likewise far from understood. Contrary to other wildlife species, the European roe deer (Capreolus capreolus) displays a remarkably high pregnancy rate. In parts, cattle and roe deer exhibit comparable features of preimplantation embryo development. Therefore, understanding the high fertility rate in the roe deer holds a great potential for cross-species knowledge gain. As the only known species among the artiodactylae, the roe deer displays a long period of embryonic diapause. The preimplantation blastocyst reaches a diameter of 1 mm only at around 4 months compared to around 13 days post estrus in cattle. The expanded blastocyst survives in a uterine microenvironment that contains a unique set of yet unidentified factors that allow embryonic stem cells to proliferate at low pace without impairing their developmental potential. Upon reactivation, intimate embryo-maternal communication comparable to those reported in cattle is thought to occur. In this review, current knowledge, parallels and differences of reproductive physiology in cattle and roe deer are reviewed. The roe deer is proposed as a unique model species to (1) enhance our knowledge of fertility processes, (2) define factors that support embryo survival for an extended period, (3) advance knowledge on embryonic stem cells, and (4) unravel potential implications for the development of novel strategies for artificial reproductive technologies.

An efficient method for generating a germ cell depleted animal model for studies related to spermatogonial stem cell transplantation

BACKGROUND

Spermatogonial stem cell (SSC) transplantation (SSCT) has become important for conservation of endangered species, transgenesis and for rejuvenating testes which have lost germ cells (Gc) due to gonadotoxic chemotherapy or radiotherapy during the prepubertal phase of life. Creating a germ cell-depleted animal model for transplantation of normal or gene-transfected SSC is a prerequisite for such experimental studies. Traditionally used intraperitoneal injections of busulfan to achieve this are associated with painful hematopoietic toxicity and affects the wellbeing of the animals. Use of testicular busulfan has been reported recently to avoid this but with a very low success rate of SSCT. Therefore, it is necessary to establish a more efficient method to achieve higher SSCT without any suffering or mortality of the animals.

METHODS

A solution of busulfan, ranging from 25 μg/20 μl to 100 μg/20 μl in 50 % DMSO was used for this study. Each testis received two diagonally opposite injections of 10 μl each. Only DMSO was used as control. Germ cell depletion was checked every 15 days. GFP-expressing SSC from transgenic donor mice C57BL/6-Tg (UBC-GFP) 30Scha/J were transplanted into busulfan-treated testis. Two months after SSCT, mice were analyzed for presence of colonies of donor-derived SSC and their ability to generate offspring.

RESULTS

A dose of 75 μg of busulfan resulted in reduction of testis size and depletion of the majority of Gc of testis in all mice within 15 days post injection without causing mortality or a cytotoxic effect in other organs. Two months after SSCT, colonies of donor-derived Gc-expressing GFP were observed in recipient testes. When cohabitated with females, donor-derived offspring were obtained. By our method, 71 % of transplanted males sired transgenic progeny as opposed to 5.5 % by previously described procedures. About 56 % of progeny born were transgenic by our method as opposed to 1.2 % by the previously reported methods.

CONCLUSIONS

We have established an efficient method of generating a germ cell-depleted animal model by using a lower dose of busulfan, injected through two diagonally opposite sites in the testis, which allows efficient colonization of transplanted SSC resulting in a remarkably higher proportion of donor-derived offspring generation.

Stem Cells Applications in Regenerative Medicine and Disease Therapeutics

Regenerative medicine, the most recent and emerging branch of medical science, deals with functional restoration of tissues or organs for the patient suffering from severe injuries or chronic disease. The spectacular progress in the field of stem cell research has laid the foundation for cell based therapies of disease which cannot be cured by conventional medicines. The indefinite self-renewal and potential to differentiate into other types of cells represent stem cells as frontiers of regenerative medicine. The transdifferentiating potential of stem cells varies with source and according to that regenerative applications also change. Advancements in gene editing and tissue engineering technology have endorsed the ex vivo remodelling of stem cells grown into 3D organoids and tissue structures for personalized applications. This review outlines the most recent advancement in transplantation and tissue engineering technologies of ESCs, TSPSCs, MSCs, UCSCs, BMSCs, and iPSCs in regenerative medicine. Additionally, this review also discusses stem cells regenerative application in wildlife conservation.

A Review of New Technologies that may Become Useful for in vitro Production of Boar Sperm

Making sperm cells outside the original testicular environment in a culture dish has been considered for a long time as impossible due to the very complicated process of spermatogenesis and sperm maturation, which altogether, encompasses a 2-month period. However, new approaches in complex three-dimensional co-cell cultures, micro-perfusion and micro-fluidics technologies, new knowledge in the functioning, culturing and differentiation of spermatogonial stem cells (SSC) and their precursor cells have revolutionized this field. Furthermore, the use of better molecular markers as well as stimulatory factors has led to successful in vitro culture of stem cells either derived from germ line stem cells, from induced pluripotent stem cells (iPSC) or from embryonic stem cells (ESC). These stem cells when placed into small seminiferous tubule fragments are able to become SSC. The SSC beyond self-renewal can also be induced into haploid sperm-like cells under in vitro conditions. In mouse, this in vitro produced sperm can be injected into a mature oocyte and allow post-fertilization development into an early embryo in vitro. After transferring such obtained embryos into the uterus of a recipient mouse, they can further develop into healthy offspring. Recently, a similar approach has been performed with combining selected cells from testicular cell suspensions followed by a complete in vitro culture of seminiferous cords producing sperm-like cells. However, most of the techniques developed are laborious, time-consuming and have low efficiency, placing questionable that it will become useful used for setting up an efficient in vitro sperm production system for the boar. The benefits and drawbacks as well as the likeliness of in vitro pig sperm production to become applied in assisted technologies for swine reproduction are critically discussed. In this contribution, also the process of sperm production in the testis and sperm maturation is reviewed.

Beyond the mouse monopoly: studying the male germ line in domestic animal models

Spermatogonial stem cells (SSCs) are the foundation of spermatogenesis and essential to maintain the continuous production of spermatozoa after the onset of puberty in the male. The study of the male germ line is important for understanding the process of spermatogenesis, unravelling mechanisms of stemness maintenance, cell differentiation, and cell-to-cell interactions. The transplantation of SSCs can contribute to the preservation of the genome of valuable individuals in assisted reproduction programs. In addition to the importance of SSCs for male fertility, their study has recently stimulated interest in the generation of genetically modified animals because manipulations of the male germ line at the SSC stage will be maintained in the long term and transmitted to the offspring. Studies performed mainly in the mouse model have laid the groundwork for facilitating advancements in the field of male germ line biology, but more progress is needed in nonrodent species in order to translate the technology to the agricultural and biomedical fields. The lack of reliable markers for isolating germ cells from testicular somatic cells and the lack of knowledge of the requirements for germ cell maintenance have precluded their long-term maintenance in domestic animals. Nevertheless, some progress has been made. In this review, we will focus on the state of the art in the isolation, characterization, culture, and manipulation of SSCs and the use of germ cell transplantation in domestic animals.

Lessons from reproductive technology research

A plethora of assisted reproductive technologies (ARTs) have come into routine use over the past half century. Some of these procedures were used much earlier experimentally. For example, Spallanzani performed artificial insemination in the dog in the late 1700s, and Heape did successful embryo transfer in the rabbit in 1890. Truly revolutionary tools and concepts important for ART occur at approximately half-decade intervals, for example, recombinant DNA procedures, transgenic technology, somatic cell nuclear transplantation, the polymerase chain reaction, and microRNAs. Similarly, obvious technologies sometimes take decades to come into practical use, such as sexing sperm and in vitro fertilization. I have categorized ARTs into five somewhat arbitrary categories in terms of perceived difficulty and feasibility: (a) when the seemingly possible turns out to be (essentially) impossible, e.g., homozygous, uniparental females; (b) when the seemingly impossible becomes possible, e.g., cryopreservation of embryos and transgenesis; (c) when the seemingly difficult turns out to be relatively easy, e.g., cryopreservation of sperm; (d) when the seemingly easy turns out to be difficult in key species, e.g., in vitro fertilization; and (e) when the seemingly difficult remains difficult, e.g., making true embryonic stem cells. The adage that "it is easy when you know how" applies repeatedly. The boundaries between what appears impossible/possible and difficult/easy change constantly owing to new tools and insights, one of the more important lessons learned. ARTs frequently are synergistic with each other. For example, somatic cell nuclear transplantation has made many kinds of experiments feasible that otherwise were impractical. Another example is that sexing sperm is useless for application without artificial insemination or in vitro fertilization. ARTs frequently are perceived as neat tricks and stimulate further thinking. This is useful for both teaching and research.

Morphologic and proliferative characteristics of goat type A spermatogonia in the presence of different sets of growth factors

PURPOSE

The present study by using different growth factors was aimed to develop the best practical culture condition for purification of goat undifferentiated SSCs and their colonization under in vitro and in vivo conditions.

METHODS

The enzymatically isolated SSCs obtained from one month old goat testes were cultured in DMEM plus FCS supplemented with different sets of growth factors (GDNF, LIF, bFGF, and EGF) for 2 weeks. At the end of each week, the morphological characteristics of cells and colonies alongside with purification rate of undifferentiated type A spermatogonia were evaluated by immunocytochemical staining and flow cytometry.

RESULTS

The number and size of colonies in treatment groups were significantly (P < 0.01) higher than corresponding values in control group. In immunocytochemical evaluation, the proportion of KIT and PGP9.5 positive cells were significantly (P < 0.001) higher in control and treatment groups, respectively.

CONCLUSIONS

The culture medium comprising all four growth factors, especially the one supplemented with the higher concentration of GDNF, was superior to the other groups with respect to the population of undifferentiated type A spermatogonia and its propagation in culture system. Additionally, goat SSCs could colonize within the mouse testis following xenotransplantation.

Application of testis germ cell transplantation in breeding systems of food producing species: a review

A major benefit of advanced reproduction technologies (ART) in animal breeding is the ability to produce more progeny per individual parent. This is particularly useful with animals of high genetic merit. Testis germ cell transplantation (TGCT) is emerging as a novel reproductive technology with application in animal breeding systems, including the potential for use as an alternative to artificial insemination (AI), an alternative to transgenesis, part of an approach to reducing generation intervals, or an approach toward development of interspecies hybrids. There is one major difference in TGCT between rodents and some other species associated with immunotolerance in heterologous transplantation. In particular, livestock and aquatic species do not require an immunesuppression procedure to allow donor cell survival in recipient testis. Testicular stem cells from a genetically elite individual transplanted into others can develop and produce a surrogate male-an animal that produces the functional sperm of the original individual. Spermatozoa produced from testis stem cells are the only cells in the body of males that can transmit genetic information to the offspring. The isolation and genetic manipulation of testis stem cells prior to transplantation has been shown to create transgenic animals. However, the current success rate of the transplantation procedure in livestock and aquatic species is low, with a corresponding small proportion of donor spermatozoa in the recipient's semen. The propagation of donor cells in culture and preparation of recipient animals are the two main factors that limit the commercial application of this technique. The current paper reviews and compares recent progress and examines the difficulties of TGCT in both livestock and aquatic species, thereby providing new insights into the application of TGCT in food producing animals.

Somatic cells, stem cells, and induced pluripotent stem cells: how do they now contribute to conservation?

More than a decade has now passed since the birth of the first endangered species produced from an adult somatic cell reprogrammed by somatic cell nuclear transfer. At that time, advances made in domestic and laboratory animal species provided the necessary foundation for attempting cutting-edge technologies on threatened and endangered species. In addition to nuclear transfer, spermatogonial stem cell transplantation and induction of pluripotent stem cells have also been explored. Although many basic scientific questions have been answered and more than 30 wild species have been investigated, very few successes have been reported. The majority of studies document numerous obstacles that still need to be overcome to produce viable gametes or embryos for healthy offspring production. This chapter provides an overview of somatic cell and stem cell technologies in different taxa (mammals, fishes, birds, reptiles and amphibians) and evaluates the potential and impact of these approaches for animal species conservation.

Efficient and safe recipient preparation for transplantation of mouse spermatogonial stem cells: pretreating testes with heat shock

Recipient preparation is of prime importance for the successful transplantation of spermatogonial stem cells (SSCs). Busulfan destroys endogenous germs cells and is commonly used for recipient preparation. However, busulfan produces significant side effects, including systemic toxicity, and it is lethal in certain species. The side effects associated with busulfan compromise the efficiency of SSC transplantation and threaten the safety of recipients. Here, we show that heat shock treatment of testes can be used as an alternative to busulfan treatment. Fourteen days after heat shock treatment, mice received a testicular injection of donor germ cells expressing enhanced green fluorescent protein (EGFP). Busulfan-treated mice were used as controls. Two months after transplantation, the number (12 ± 1 mm) and length (30.46 ± 5.23 mm) of EGFP-expressing testicular colonies in heat shock-treated recipients were not significantly different from those in busulfan-treated recipients. Furthermore, healthy EGFP-expressing offspring were obtained after intracytoplasmic injection of round spermatids recovered from heat shock-treated recipients. This result indicates that donor SSCs undergo complete spermatogenesis in the heat shock-treated testes of recipients. Our findings demonstrate the feasibility of using heat shock for the preparation of recipients before SSC transplantation in mice. Heat shock may prove to be useful for recipient preparation in mammalian species in which busulfan produces significant toxicity.

Stem cells in veterinary medicine

The stem cell field in veterinary medicine continues to evolve rapidly both experimentally and clinically. Stem cells are most commonly used in clinical veterinary medicine in therapeutic applications for the treatment of musculoskeletal injuries in horses and dogs. New technologies of assisted reproduction are being developed to apply the properties of spermatogonial stem cells to preserve endangered animal species. The same methods can be used to generate transgenic animals for production of pharmaceuticals or for use as biomedical models. Small and large animal species serve as valuable models for preclinical evaluation of stem cell applications in human beings and in veterinary patients in areas such as spinal cord injury and myocardial infarction. However, these applications have not been implemented in the clinical treatment of veterinary patients. Reviews on the use of animal models for stem cell research have been published recently. Therefore, in this review, animal model research will be reviewed only in the context of supporting the current clinical application of stem cells in veterinary medicine.

Effects of medium and hypothermic temperatures on preservation of isolated porcine testis cells

The effects of medium and hypothermic temperatures on testis cells were investigated to develop a strategy for their short-term preservation. Testes from 1-week-old piglets were enzymatically dissociated for cell isolation. In Experiment 1, testis cells were stored at either room (RT) or refrigeration (RG) temperature for 6 days in one of 13 different media. Live cell recovery was assayed daily using trypan blue exclusion. In Experiment 2, three media at RG were selected for immunocytochemical and in vitro culture studies. Live cell recovery was also assayed daily for 6 days using both trypan blue exclusion and a fluorochrome assay kit. For all media tested, significantly or numerically more live cells were maintained at RG than RT. On preservation Day 3 at RG (cell isolation day as Day 0), 20% FBS-Leibovitz resulted in the highest live cell recovery (89.5 + or - 1.7%) and DPBS in the lowest (60.3 + or - 1.9%). On Day 6 at RG, 20% FBS- Leibovitz also resulted in the best preservation efficiency with 80.9 + or - 1.8% of Day 0 live cells recovered. There was no difference in live cell recovery detected by the two viability assays. After preservation, the proportion of gonocytes did not change, whereas that of Sertoli and peritubular cells increased and decreased, respectively. After 6 days of hypothermic preservation, testis cells showed similar culture potential to fresh cells. These results show that testis cells can be preserved for 6 days under hypothermic conditions with a live cell recovery of more than 80% and after-storage viability of 88%.

Development of new stem cell-based technologies for carnivore reproduction research

New reproductive technologies based on stem cells offer several potential benefits to carnivore species. For example, development of lines of embryonic stem cells in cats and dogs would allow for the generation of transgenic animal models, which could be used to advance both veterinary and human health. Techniques such as spermatogonial stem cell transplantation (SSCT) and testis xenografting offer new approaches to propagate genetically valuable individual males, even if they should die before producing sperm. These techniques might therefore have application to the conservation of endangered species of carnivores, as well as to biomedical research. Recently, our laboratory has successfully performed SSCT in the dog, with a recipient dog producing sperm of donor genetic origin. Testis xenografting has been used to produce sperm from pre-pubertal testis tissue from both cats and ferrets. These early steps reinforce the need not only for research on stem cell technologies, but also for additional research into complementary technologies of assisted reproduction in carnivores, so that the widest array of research and clinical benefits can be realized.

Development of novel strategies for the isolation of piglet testis cells with a high proportion of gonocytes

Gonocytes have germline stem cell potential and are present in the neonatal testis, comprising 5–10% of freshly isolated testis cells. Maximising the number and proportion of gonocytes among freshly isolated testis cells will greatly facilitate their subsequent purification and in vitro study and manipulation. Seven experiments were conducted to evaluate the effects of multiple factors on the efficiency of testis cell isolation from neonatal pigs. We found that the use of a lysis buffer led to elimination of erythrocytes without adversely affecting testis cell isolation. Approximately ninefold as many live cells could be harvested by enzymatic digestion of testis tissues compared with mechanical methods. Digestion with collagenase–hyaluronidase–DNase followed by trypsin resulted in the highest recovery of live cells. However, the proportion of gonocytes (∼7%) did not differ between the mechanical and enzymatic methods of testis cell isolation. Pretreatment of the tissue with cold enzymes increased the recovery of live testis cells. New strategies of combining a gentle enzymatic digestion with two rounds of vortexing resulted in the isolation of testis cells with very high gonocyte proportion. The efficiency of these novel methods could be further optimised to collect testis cells with a gonocyte proportion of approximately 40%. This novel three-step testis cell isolation strategy can be completed within 1 h and can harvest approximately 17 × 106 live gonocytes per g testis tissue. Therefore, in addition to elucidating the effects of several factors on testis cell isolation, we developed a novel strategy for the isolation of testis cells that yielded approximately 40% gonocytes in the freshly isolated cells (i.e. four- to eight-fold higher than the proportions obtained using current strategies). This strategy has instant applications in the purification of gonocytes.

Recent developments in testis tissue xenografting

Development of the mammalian testis and spermatogenesis involve complex processes of cell migration, proliferation, differentiation, and cell–cell interactions. Although our knowledge of these processes has increased in the last few decades, many aspects still remain unclear. The lack of suitable systems that allow to recapitulate and manipulate both testis development and spermatogenesisex situhas limited our ability to study these processes. In the last few years, two observations suggested novel strategies that will improve our ability to study and manipulate mammalian spermatogenesis: i) testis tissue from immature animals transplanted ectopically into immunodeficient mice is able to respond to mouse gonadotropins and to initiate and complete differentiation to the level where fertilization-competent sperm are obtained, and ii) isolated testis cells are able to organize and rearrange into seminiferous cords that subsequently undergo complete development, including production of viable sperm. The current paper reviews recent advances that have been obtained with both techniques that represent novel opportunities to explore testis development and spermatogenesis in diverse mammalian species.

New horizons for in vitro spermatogenesis? An update on novel three-dimensional culture systems as tools for meiotic and post-meiotic differentiation of testicular germ cells

Culture and differentiation of male germ cells has been performed for various purposes in the past. To date, none of the studies aimed at in vitro spermatogenesis has resulted in a sufficient number of mature gametes. Numerous studies have revealed worthy pieces of information, building up a body of information on conditions that are required to maintain and mature male germ cells in vitro. In this review, we report on previously published and unpublished experiments addressing murine germ cell differentiation in three-dimensional (3D) in vitro culture systems. In a systematic set of experiments, we examined the influence of two different matrices (soft agar and methylcellulose) as well as the need for gonadotrophin support. For the first time, we demonstrate that pre-meiotic male germ cells [revealed by the absence of meiotic marker expression (e.g. Boule)] obtained from immature mice pass through meiosis in vitro. After several weeks of culture, we obtained morphologically normal spermatozoa embedded in the matrix substance. Complete maturation relied on support from somatic testicular cells and the presence of gonadotrophins but appeared independent from the matrix in a 3D culture environment. Further research efforts are required to reveal the applicability of this culture technique for human germ cells and the functionality of the spermatozoa for generating offspring.

Male germ cell transplantation

Transplantation of male germ line stem cells from a donor animal to the testes of an infertile recipient was first described in 1994. Donor germ cells colonize the recipient's testis and produce donor-derived sperm, such that the recipient male can distribute the genetic material of the germ cell donor. Germ cell transplantation represents a functional reconstitution assay for male germ line stem cells and as such has vastly increased our ability to study the biology of stem cells in the testis and define phenotypes of infertility. First developed in rodents, the technique has now been used in a number of animal species, including domestic mammals, chicken and fish. There are three major applications for this technology in animals: first, to study fundamental aspects of male germ line stem cell biology and male fertility; second, to preserve the reproductive potential of genetically valuable individuals by male germ cell transplantation within or between species; third, to produce transgenic sperm by genetic manipulation of isolated germ line stem cells and subsequent transplantation. Transgenesis through the male germ line has tremendous potential in species in which embryonic stem cells are not available and somatic cell nuclear transfer has limited success. Therefore, transplantation of male germ cells is a uniquely valuable approach for the study, preservation and manipulation of male fertility in animals.