New horizons for in vitro spermatogenesis? An update on novel three-dimensional culture systems as tools for meiotic and postmeiotic differentiation of testicular germ cells.

Culture and differentiation of male germ cells has been performed for various purposes in the past. To date, none of the studies aimed at in-vitro spermatogenesis have resulted in a sufficient number of mature gametes. Numerous studies have revealed worthy pieces of information, building up a body of information on conditions that are required to maintain and mature male germ cells in-vitro. In this review, we report on previously published and unpublished experiments addressing murine germ cell differentiation in three-dimensional in-vitro culture systems. In a systematic set of experiments, we examined the influence of two different matrices (soft agar and methylcellulose) as well as the need for gonadotropin support. For the first time, we demonstrate that pre-meiotic male germ cells (revealed by the absence of meiotic marker expression (e.g. Boule)) obtained from immature mice pass through meiosis in vitro. After several weeks of culture, we obtained morphologically normal spermatozoa embedded in the matrix substance. Complete maturation relied on support from somatic testicular cells and the presence of gonadotropins but appeared independent from the matrix in a three-dimensional culture environment. Further research efforts are required to reveal the applicability of this culture technique for human germ cells and the functionality of the spermatozoa for generating offspring.