Ovarian and testicular tissue xenografting: its potential for germline preservation of companion animals, non-domestic and endangered species

Regeneration of testis tissue after ectopic implantation of porcine testis cell aggregates in mice: improved consistency of outcomes and in situ monitoring

Ectopic implantation of donor testis cell aggregates in recipient mice results in de novo formation or regeneration of testis tissue and, as such, provides a unique invivo model for the study of testis development. However, currently the results are inconsistent and the efficiency of the model remains low. This study was designed to: (1) examine several factors that can potentially improve the consistency and efficiency of this model and (2) explore the use of ultrasound biomicroscopy (UBM) for the non-invasive invivo evaluation of implants. Testis cell aggregates, containing ~40% gonocytes, from 1-week-old donor piglets were implanted under the back skin of immunodeficient mice through skin incisions using gel matrices or through subcutaneous injection without using gel matrices. The addition of gel matrices led to inconsistent tissue development; gelatin had the greatest development, followed by collagen, whereas agarose resulted in poor development. The results also depended on the implanted cell numbers since implants with 100×106 cells were larger than those with 50×106 cells. The injection approach for cell implantation was less invasive and resulted in more consistent and efficient testis tissue development. UBM provided promising results as a means of non-invasive monitoring of implants.

Developmental ability of oocytes retrieved from Meishan neonatal ovarian tissue grafted into nude mice

Ovarian xenografting makes it possible to obtain oocytes with fertilization ability from immature pigs of Western breeds. In this study, we applied these methods to the Meishan, an indigenous Chinese pig breed, and investigated the developmental competence of oocytes grown in their neonatal tissue after grafting into nude mice. First, mice harboring neonatal ovarian tissue were infused with follicle stimulating hormone (FSH) (62.5 U/ml) for 13 days starting at 10, 30, and 60 days after vaginal opening (D10‐, D30‐, and D60‐FSH groups, respectively). Development of antral follicles and their oocytes was most enhanced in the D60‐FSH group. For the next step, we examined the in vitro maturation ability of the oocytes recovered from host mice after infusion with FSH at a dose of 62.5 U/ml or 125 U/ml (FSH‐62.5 or ‐125 group) for 13 days starting at 60 days after vaginal opening. Many more oocytes with maturation ability were obtained from the FSH‐125 group. The FSH‐125 mature oocytes were fertilized in vitro, as shown by formation of male and female pronuclei, but did not reach the blastocyst stage. These results indicate that Meishan neonatal ovaries are able to produce oocytes with fertilization ability after being grafted into nude mice.

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)

The use of assisted reproductive techniques for endangered species is a major goal for conservation. One of these techniques, testis tissue xenografting, allows for the development of spermatozoa from animals that die before reaching sexual maturity. To assess the potential use of this technique with endangered species, testis tissue from six Iberian lynxes (one fetus, two perinatal cubs, two 6-month-old and one 2-year-old lynx), two Cuvier's gazelle fetuses and one 8-month-old Mohor gazelle were transplanted ectopically into nude mice. Tissue from the lynx fetus, perinatal cubs and 2-year-old donors degenerated, whereas spermatogonia were present in 15% of seminiferous tubules more than 70 weeks after grafting in transplanted testis tissue from 6-month-old donors. Seminal vesicle weights (indicative of testosterone production) increased over time in mice transplanted with tissue from 6-month-old lynxes. Progression of spermatogenesis was observed in xenografts from gazelles and was donor age dependent. Tissue from Cuvier's gazelle fetuses contained spermatocytes 40 weeks after grafting. Finally, round spermatids were found 28 weeks after transplantation in grafts from the 8-month-old Mohor gazelle. This is the first time that xenotransplantation of testicular tissue has been performed with an endangered felid and the first successful xenotransplantation in an endangered species. Our results open important options for the preservation of biological diversity.

Normal reproductive development of offspring derived by intracytoplasmic injection of porcine sperm grown in host mice

For establishment of gonadal xenografting, it is essential to clarify whether offspring derived from gametes grown in host mice harboring xenografts have normal reproductive development. This study examined the secretory profiles of gonadal hormones in relation to sexual maturation or ovarian cyclicity in pigs generated by intracytoplasmic sperm injection using xenogeneic sperm (Xeno-ICSI pigs, four males and one female). We also assessed the developmental activity of gametes obtained from these pigs using in vitro culture systems, or by mating with conventionally produced (conventional) pigs. During the growth of male Xeno-ICSI pigs, serum inhibin and testosterone concentrations were generally within ranges for those hormones in conventional pigs. Histologically, there were no differences in the growth and differentiation of seminiferous tubules between Xeno-ICSI and conventional pigs. Parameters of semen quality, including volume, pH, sperm concentration, and the percentage of motile sperm were not different from those in conventional pigs. Among the Xeno-ICSI pigs, individual differences were noted in the ability of sperm to penetrate oocytes and to produce blastocysts. However, oocytes after in vitro fertilization using these sperm developed into blastocysts containing more than 31 cells. One conventional sow delivered 12 piglets after being mated with a male Xeno-ICSI pig. During growth of the female Xeno-ICSI pig, serum progesterone concentrations had a sudden increase at 41 wk of age, suggesting CL formation. After puberty, this animal showed cyclic changes in the serum concentrations of progesterone and inhibin, and delivered 10 piglets after AI using fresh sperm obtained from a conventional boar. In conclusion, these findings demonstrated that both male and female Xeno-ICSI pigs had normal reproductive abilities.

Gonadal status of male recipient mice influences germ cell development in immature buffalo testis tissue xenograft

Growth and development of immature testis xenograft from various domestic mammals has been shown in mouse recipients; however, buffalo testis xenografts have not been reported to date. In this study, small fragments of testis tissue from 8-week-old buffalo calves were implanted subcutaneously onto the back of immunodeficient male mouse recipients, which were either castrated or left intact (non-castrated). The xenografts were retrieved and analyzed 12 and 24 weeks later. The grafted tissue survived and grew in both types of recipient with a significant increase in weight and seminiferous tubule diameter. Recovery of grafts from intact recipients 24 weeks post-grafting was significantly lower than that from the castrated recipients. Seminal vesicle indices and serum testosterone levels were lower in castrated recipients at both collection time points in comparison to the intact recipients and non-grafted intact mouse controls. Pachytene spermatocytes were the most advanced germ cells observed in grafts recovered from castrated recipients 24 weeks post-grafting. Complete spermatogenesis, as indicated by the presence of elongated spermatids, was present only in grafts from intact recipients collected 24 weeks post-grafting. However, significant number of germ cells with DNA damage was also detected in these grafts as indicated by TUNEL assay. The complete germ cell differentiation in xenografts from intact recipients may be attributed to efficient Sertoli cell maturation. These results suggest that germ cell differentiation in buffalo testis xenograft can be completed by altering the recipient gonadal status.

Effects of recipient mouse strain, sex and gonadal status on the outcome of testis tissue xenografting

The aim of the present study was to examine factors that may affect the outcome of testis tissue xenografting. Recipient factors were examined by grafting small fragments of testis tissue from newborn piglets under the back skin of immunodeficient mice of different strains (severe combined immunodeficiency (SCID) v. nude), sex (male v. female) and gonadal status (intact v. gonadectomised) using a factorial design (eight groups; n = 7 mice per group). Recipient mice were killed after 8 months to compare the gross and histological attributes of the recovered grafts. Overall, approximately 94% of grafts were recovered. Gonadectomy of male or female recipients did not affect any of the measured outcomes of testis tissue xenografting, therefore data were pooled. Overall, in terms of sex, male mice and, in terms of strain, SCID mice tended to show higher gross and histological development of grafts. The group of female nude mice had the lowest graft recovery rate (75%) compared with the other groups (95-100%; P < 0.05). The grafts from male SCID mice were, on average the largest and had the highest percentage of spermatozoa-containing seminiferous tubules among all the groups (P < 0.05). These results suggest that male SCID mice provide a suitable recipient model for testis tissue xenografting and that the mice do not need to be castrated for optimal results.

Xenografting as a tool to preserve endangered species: outcomes and challenges in model systems

The use of testis tissue xenografting as a valuable tool to rescue endangered and genetically valuable individuals that die young or otherwise fail to produce sperm has been the subject of much interest. Although the technique has been successfully applied to a wide variety of species, little is known about what determines the outcome. Furthermore, to improve the applicability of xenografting, new methods to preserve and transport testis tissue from valuable animals are emerging. However, one major issue remains: the application of xenografting implies the development of subsequent ART techniques to produce offspring from the recovered material. This paper focuses on these three aspects of testis tissue xenografting as a tool for rescuing endangered and valuable genetic pools.

The number of grafted fragments affects the outcome of testis tissue xenografting from piglets into recipient mice

To optimize the procedure for testis tissue xenografting, we grafted 2, 4, 8, or 16 small fragments of immature porcine testis tissue under the back skin of immunodeficient castrated mice (n = 10 mice/group). At 8 months post grafting, the graft recovery rate did not differ between groups; however, not only the total but also the average graft weights were higher (by ∼12-fold and ∼2.5-fold, resp.) in mice receiving 16 fragments than those receiving 2 fragments (P < .05). The recipient mice with 16 fragments had the largest vesicular glands (indicators of testosterone release by the grafts) compared with those with 2 fragments (P = .007). The grafts in the group of 16 fragments also had more (P < .05) percentage of tubules with round spermatids than those of the group of mice receiving 2 fragments. Therefore, recipient mice can be grafted with at least 16 testis tissue fragments for optimal results.

Donor-host involvement in immature rat testis xenografting into nude mouse hosts

Immature testicular tissue of a wide variety of mammalian species continues growth and maturation when ectopically grafted under the dorsal skin of adult nude mouse recipients. Tissues from most donor species fully mature, exhibiting complete spermatogenesis within months. The connection to the recipient's vascular system is mandatory for graft development, and failure of vascularization leads to necrosis in the grafted tissue. In the present study, we analyze to what extent 1) the xenografted immature donor tissue and 2) the recipient's cells and tissues contribute to the functional recovery of a "testicular xenograft." We address whether recipient cells migrate into the testicular parenchyma and whether the circulatory connection between the donor testicular tissue and the recipient is established by ingrowing host or outgrowing donor blood vessels. Although this issue has been repeatedly discussed in previous xenografting studies, so far it has not been possible to unequivocally distinguish between donor and recipient tissues and thus to identify the mechanisms by which the circulatory connection is established. To facilitate the distinction of donor and recipient tissues, herein we used immature green fluorescent protein-positive rat testes as donor tissues and adult nude mice as graft recipients. At the time of graft recovery, donor tissues could be easily identified by the GFP expression in these tissues, allowing us to distinguish donor- and recipient-derived blood vessels. We conclude that the circulatory connection between graft and host is established by a combination of outgrowing small capillaries from the donor tissue and formation of larger vessels by the host, which connect the graft to subcutaneous blood vessels.

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.

Testicular stem cells for fertility preservation: preclinical studies on male germ cell transplantation and testicular grafting

Spermatogonial stem cells open novel strategies for preservation of testicular tissue and fertility preservation in boys and men exposed to gonadotoxic therapies. This review provides an update on the physiology of spermatogonial stem cells in rodent and primate testes. Species-specific differences must be considered when new technologies on testicular stem cells are considered. Germ cell transplantation is presented as one novel and promising strategy. Whereas this technique has become an important research tool in rodents, a clinical application must still be regarded as experimental and many aspects of the procedure need to be optimized prior to a safe and efficient clinical application in men. Testicular grafting opens another exciting strategy for fertility preservation. Autologous and xenologous transfer of immature tissue revealed a high regenerative potential of immature testicular tissue. Grafting was applied in rodents and primates and resulted in the generation of sperm. Further research is needed before an application in humans can be considered safe and efficient. Despite the current limitations in regard to the generation of sperm from cryopreserved male germline cells and tissues, protocols for cryopreservation of testicular tissue are available and reveal a promising outcome. Since future improvements of germ cell transplantation and grafting approaches can be assumed, bioptic retrieval and cryopreservation of testicular tissue fragments should be performed in oncological patients at high risk of fertility loss since this is their only option to maintain their fertility potential.

Animal models for fertility preservation in the male

Fertility preservation in the male is routinely focused on sperm. In clinical and veterinary settings, cryopreservation of sperm is a widely used tool. However, the goals for male fertility preservation differ between experimental models, maintenance of livestock, conservation of rare species, and fertility protection in men. Therefore very different approaches exist, which are adapted to the specialized needs for each discipline. Novel tools for male fertility preservation are explored targeting immature germ cells in embryonic or immature testes. Many options might be developed to combine germline preservation and generation of sperm ex vivo leading to interesting new perspectives. This review highlights current and future options for male fertility preservation with a special focus on animal models and a consideration of the various disciplines in need of novel tools.

Modelling germ cell development in vitro

Germ cells have a critical role in mediating the generation of genetic diversity and transmitting this information across generations. Furthermore, gametogenesis is unique as a developmental process in that it generates highly-specialized haploid gametes from diploid precursor stem cells through meiosis. Despite the importance of this process, progress in elucidating the molecular mechanisms underpinning mammalian germ cell development has been retarded by the lack of an efficient and reproducible system of in vitro culture for the expansion and trans-meiotic differentiation of germline cells. The dearth of such a culture system has rendered the study of germ cell biology refractory to the application of new high-throughput technologies such as RNA interference, leaving in vivo gene-targeting approaches as the only option to determine the function of genes believed to be involved in gametogenesis. Recent reports detailing the derivation of gametes in vitro from stem cells may provide the first steps in developing new tools to solve this problem. This review considers the developments made in modelling germ cell development using stem cells, and some of the challenges that need to be overcome to make this a useful tool for studying gametogenesis and to realize any future clinical application.

Xenografting of sheep testis tissue and isolated cells as a model for preservation of genetic material from endangered ungulates

Recovery of germ cells could be an option for preservation of the genetic pool of endangered animals. In immature males, xenografting of testis tissue provides the opportunity to recover sperm from these animals. In adult animals, xenografting has been less successful, but de novo morphogenesis of functional testis tissue from dissociated testis cells could be an alternative. To assess the potential use of these techniques in endangered bovid species, the domestic sheep was used as a model. Testes from 2-week-old lambs were grafted as tissue fragments or cell suspensions into nude mice. Grafts were recovered at 4, 8, 12 and 16 weeks post grafting. For isolated cells, two additional time points at 35 and 40 weeks after grafting were added. In addition, to analyse the possible effect of social stress among mice within a group on the development of the grafts, testis tissue grafts were recovered 13 weeks post grafting from mice housed individually and in groups. Complete spermatogenesis occurred in sheep testis xenografts at 12 weeks, similar to the situation in situ. Isolated sheep testis cells were able to reorganize and form functional testicular tissue de novo. Housing mice individually or in groups did not have any effect on the development of xenografts. Xenografting of testis tissue might be useful to obtain sperm from immature endangered ungulates that die prematurely. Testis tissue de novo morphogenesis from isolated cells could open interesting options to recover germ cells from mature males with impaired spermatogenesis.