Spermatogonial stem cells have gained increasing attention during recent years mainly due to methodological breakthroughs which allowed the development of novel strategies in regard to basic research and clinical applications. The principal investigator has described novel in vivo and in vitro tools for the study of spermatogenic stem cells and has applied germ cell transplantation and testicular grafting in rodents and monkeys to determine their clinical applicability. The long term perspective of this research is to understand the important roles of male germ line stem cells for human testis function. This knowledge will allow to describe specific causes of male infertility and to determine the effects of gonadotoxic treatments or aging on male fertility. The first specific aim of the current project is to determine an age-dependent (in) ability of testicular stem cells to recolonize the seminiferous tubules and to restore spermatogenesis after germ cell depletion in mice and monkeys. Preliminary data show that recovery of spermatogenesis after gonadotoxic treatment is disrupted at the level of spermatogonial stem cells in aged, but not in young animals. The second specific aim is to analyse the effect of the recipient's age on the development of newborn testicular grafts and the initiation of testicular stem cell activity. Analysis of spermatogenic activity established in the grafts will be performed to determine the influence of a young or aged physiological environment on the differentiation of the testicular tissue and the activity of spermatogonial stem cells. The proposed studies use the testis and its spermatogonial stem cells as a potentially valuable model system to determine a decline of stem cell function during aging. The results of this exploratory grant will indicate whether the testis is a suitable organ to study stem cell mediated deterioration of organ function. The proposed studies will also reveal the role of spermatogonial stem cells during decline of spermatogenic function in the primate testis which has immediate clinical relevance for patients undergoing oncological therapies.