Modulating testicular mass in xenografting: a model to explore testis development and endocrine function.

The hypothalamic-pituitary-gonadal (HPG) axis is involved in both the regulation of growth of the developing testis and in controlling spermatogenic and steroidogenic activity in the adult testis. Here, we develop a novel testicular xenografting model to examine to which degree testicular growth and function are controlled by intra- and extratesticular factors. Two or eight halves of neonatal Djungarian hamster testes were implanted into intact, hemicastrated, or castrated nude mouse recipients, and the development of the grafts under reduced or increased competition of testicular tissue was monitored and analyzed. We hypothesized that the outgrowth of the testicular grafts is influenced by the total amount of testicular tissue present in a host and that less testicular tissue in a host would result in more extended outgrowth of the grafts. Our results reveal that the hypothesis is wrong, because implanted hamster testis tissue irrespectively of the grafting condition grows to a similar size revealing an intrinsic mechanism for testicular growth. In contrast, similar size of seminal vesicle as bio-indicator of androgen levels in all hosts revealed that the steroidogenic activity is independent from the mass of testicular tissue and that steroid levels are extrinsically regulated by the recipient's HPG axis. We propose that the model of testicular xenografting provides highly valuable options to explore testicular growth and endocrine regulation of the HPG axis.