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    • Another question to address is the potential influence of th

    2018-10-24

    Another question to address is the potential influence of the degree of maturity of prepubertal donor tissue on the success of transplantation. Donor age-dependent differential gene expression and subsequent protein expression have been reported in tissue grafts. This could alter the initial ability of grafted tissue to survive and differentiate in a host environment (Schmidt et al., 2007). A study by Caires et al. on xenotransplantation of piglet testicular tissue of different maturity to castrated nude mice is also of interest, showing an age-related difference in the capacity of germ and Sertoli rifampicin to achieve complete differentiation after grafting (Caires et al., 2008). This issue was previously raised in the context of two age groups in xenografting experiments of human ITT (Wyns et al., 2008), but requires further investigation. In addition to recipient environment and donor tissue age, poor outcomes of ITT transplants may be due to tissue hypoxia related to the avascular grafting procedure. Tissue encapsulation in hydrogels with incorporation of molecules captured in nanoparticles was therefore evaluated. ITT embedded in alginate hydrogel loaded or not with vascular endothelial growth factor (VEGF) nanoparticles resulted in two-fold increased spermatogonial recovery (Poels et al., 2016), demonstrating the potential of alginate hydrogel to enhance cryopreserved tissue engraftment.
    Towards SSC isolation and transplantation The first success in SSC transplantation was reported by Brinster in 1994. Testicular cell suspensions isolated from transgenic mice expressing LacZ were ubiquitously transplanted to the testes of infertile mice. One month after transplantation, β-galactosidase activity was detected in seminiferous tubules of recipient infertile mice (Brinster and Zimmermann, 1994). Restoration of fertility was evidenced by transgene transmission to offspring. This technique was applied to other animal species, including non-human primates, yielding offspring or embryos (Ogawa et al., 2000; Nagano et al., 2001; Hamra et al., 2002; Honaramooz et al., 2003; Izadyar et al., 2003; Mikkola et al., 2006; Kim et al., 2008; Hermann et al., 2012). Promising results in animals prompted translation to a clinical setting. The rete testis was identified as the optimal injection site for SSC transplantation to large testes (Schlatt et al., 1999). The first attempt in humans was announced when Radford, after cryopreservation of testicular cell suspensions from 12 adult patients affected by non-Hodgkin\'s lymphoma (Radford et al., 1999), transplanted these cell suspensions to seven post-treated patients (Radford, 2003). Unfortunately, no follow-up information was provided possibly because of the difficulty of proving that spermatozoa produced after transplantation were not from residual spermatogonia after chemotherapy. Furthermore, it appears that SSCs maintain their fertilizing potential even after cryopreservation and long-term storage (Wu et al., 2012). Frozen-thawed SSCs isolated from both prepubertal and adult monkeys and transplanted to infertile (post alkylating chemotherapy) adult monkeys resulted in spermatogenesis regeneration and spermatozoa able to fertilize oocytes and develop into embryos (Hermann et al., 2012). Moreover, it was shown that biological activity and fertilization potential of SSCs were not affected by long term culture since spermatogenesis and offspring were obtained when cultured SSCs were transplanted in infertile mice (Kanatsu-Shinohara et al., 2003a; Kubota et al., 2004; Hamra et al., 2005; Ryu et al., 2005). Since the extent of spermatogenesis was shown to depend on the number of transplanted SSCs (Dobrinski et al., 1999) and the SSC engraftment efficiency was estimated to be extremely low in non-human primates (Hermann et al., 2007), propagating SSCs could increase the colonization efficiency with a view to effective clinical application. Several groups reported human SSC culture (Chen et al., 2009; Sadri-Ardekani et al., 2009; Wu et al., 2009; He et al., 2010; Kokkinaki et al., 2011; Liu et al., 2011; Sadri-Ardekani et al., 2011; Mirzapour et al., 2012; Piravar et al., 2013; Conrad et al., 2014; Nickkholgh et al., 2014b; Smith et al., 2014; Baert et al., 2015; Guo et al., 2015), but only two for prepubertal SSCs (Wu et al., 2009; Sadri-Ardekani et al., 2011). So far, no validated method was established for human SSC culture but SSC enrichment by cell sorting or differential plating was generally used before culture.