There have been tremendous advances in both the diagnosis and treatment of male factor infertility; however, the mechanisms responsible to recreate spermatogenesis outside of the testicular environment continue to elude andrologists. Having the ability to 'grow' human sperm would be a tremendous advance in reproductive biology with multiple possible clinical applications, such as a treatment option for men with testicular failure and azoospermia of multiple etiologies. To understand the complexities of human spermatogenesis in a research environment, model systems have been designed with the intent to replicate the testicular microenvironment. Currently, there are both in vivo and in vitro model systems. In vivo model systems involve the transplantation of either spermatogonial stem cells or testicular xenographs. In vitro model systems involve the use of pluripotent stem cells and complex coculturing and/or three-dimensional culturing techniques. This review discusses the basic methodologies, possible clinical applications, benefits and limitations of each model system. Although these model systems have greatly improved our understanding of human spermatogenesis, we unfortunately have not been successful in demonstrating complete human spermatogenesis outside of the testicle. Asian Journal of Andrology (2011) 13, 677-682; doi:10.1038/aja.2011.88; published online 18 July 2011 Keywords: fertility preservation; spermatogenesis models; spermatogonial stem cells
INTRODUCTIONRecreating human spermatogenesis outside of its original environment remains the 'Holy Grail' for generations of andrologists. To grow sperm is not only a matter of scientific curiosity, but also a quest for the treatment of male infertility. A well-characterized model for spermatogenesis in humans will have a huge impact on our understanding on the physiological and genetic pathways of male reproduction. Its clinical applications also extend to the study of human reproductive toxicology and preservation of fertility in treated cancer patients.While significant progress has been made in the laboratory, multiple obstacles remain. Current knowledge on the control of gonadogenesis, spermatogenesis and steroidogenesis are based on the examination of histology, immunohistochemistry, hormonal assays, and phenotypes of single gene or small groups of gene alterations in animal models. Human studies in male reproduction are hindered by the lack of human testicular specimens, which are not as readily available as our animal counterparts, for ethical and practical reasons. In this review, we will examine the existing literature on the experimental models and their limitations in growing sperm to provide a foundation for future investigation and clinical application ( Table 1).