br Towards successful transplantation of

Towards successful transplantation of human ITT Where there is no risk of neoplastic contamination of the tissue that could lead to retransmission of cancer thrombin inhibitors back to the cured patient, autotransplantation of testicular tissue pieces may be considered for fertility restoration. It has the advantage of preserving the integrity of the tissue, thereby maintaining molecular interactions within the SSC niche and with the surrounding interstitial testicular compartment. Multiple studies have investigated grafting of ITT from different animal species under the skin of immunodeficient mice (Honaramooz et al., 2002; Schlatt et al., 2002; Nakai et al., 2010), which resulted in tissue maturation and complete spermatogenesis. However, grafting efficiency was shown to be poor e.g. in colts where only a small percentage of grafts produced spermatozoa (Rathi et al., 2006). Although resumption of spermatogenesis was also achieved in slow-frozen ITT xenografts from rabbits (Shinohara et al., 2002), pigs (Honaramooz et al., 2002; Caires et al., 2008; Zeng et al., 2009; Abrishami et al., 2010), lambs (Pukazhenthi et al., 2015) and primates (Jahnukainen et al., 2012), the cryopreservation protocols used may have negatively impacted the efficiency of the procedure (Frederickx et al., 2004), emphasizing the need to further improve these protocols. Use of the vitrification technique also leads to complete spermatogenesis after allografting of Japanese quail ITT (Liu et al., 2013) and xenografting of piglet ITT (Kaneko et al., 2013). Finally, healthy offspring were obtained by micro-insemination of sperm extracted from frozen-thawed allografted mouse ITT, fresh xenografting rabbit ITT (Shinohara et al., 2002) and vitrified xenografted piglet ITT (Kaneko et al., 2014), the latter showing normal reproductive abilities. While xenotransplantation is a useful tool to investigate the potential of ITT grafts, it is not an option to produce sperm for clinical use because of animal contaminants (Patience et al., 1998) and infectious agents (Kimsa et al., 2014). Complete spermatogenesis has not yet been achieved after xenotransplantation of human ITT, neither from human foetuses (Yu et al., 2006) nor prepubertal boys (Wyns et al., 2008; Goossens et al., 2008). Goossens et al. observed mainly Sertoli cell-only tubules and only a few surviving spermatogonia 4 and 9months after grafting (Goossens et al., 2008). Besides low spermatogonial recovery rates, xenotransplantation of human ITT also showed limited differentiation up to the pachytene spermatocyte stage, regardless of whether it has been cryopreserved or not, or the cryopreservation technique used (Wyns et al., 2007; Wyns et al., 2008; Poels et al., 2013). Moreover, supplementation with antioxidants (Poels et al., 2014), follicule-stimulating hormone (Van Saen et al., 2011) or testosterone (Poels et al., 2014) did not improve xenograft outcomes. An inappropriate recipient environment and phylogenetic distance between species probably play a role. For this reason, while cost and ethical concerns may be limiting factors, better study models allowing autotransplantation experiments, such as non-human primates, should be favored. Jahnukainen et al. used cryopreserved testicular tissue from prepubertal monkeys and an irradiated and castrated pubertal rhesus monkey to assess autologous orthotopic grafts. Although they obtained complete spermatogenesis five months after autografting (Jahnukainen et al., 2012), they found Sertoli cell-only tubules in their concomitant xenografting experiments, demonstrating that host conditions are critical for germ cell maturation. Their results are a big step forward in terms of translating experimental cryopreserved ITT transplantation to a clinical setting. Indeed, these authors showed that it is possible to achieve spermatogenesis in an individual presenting with hypergonadotropic hypogonadism after gonadotoxic treatment.