Inhibin B is a more sensitive marker of spermatogenetic damage than FSH in the irradiated non-human primate model

This study evaluated the effect of bilateral testicular irradiation (2 Gy) on reproductive hormones, testicular volume (TV) and sperm parameters in six adult cynomolgus monkeys. Hormone levels (FSH, inhibin B and testosterone (T)) were determined to find the most valuable endocrine marker of irradiation-induced damage. All parameters were analysed at weekly intervals for 14 weeks. Histological evaluation of both testes was performed at week 14 after irradiation when one monkey was castrated and at week 27 when the remaining five monkeys were bilaterally biopsied. A decrease in body weight, TV (30% of the pre-treatment size) and sperm count was observed after irradiation. Severe oligozoospermia was achieved throughout the study but azoospermia was recorded only occasionally. Histological evaluation revealed a heterogeneous picture with patchy arrangement of seminiferous tubules containing advanced germ cell types. An increase (P<0.05) in FSH levels and, to a lesser degree also in T levels, occurred several weeks after irradiation. Inhibin B levels showed a sharp decline (P<0.001) as soon as 1 week after irradiation. FSH and inhibin B did not return to baseline levels during the observation period. A negative correlation was found between FSH and inhibin B values (r=-0.35, P<0.001). Inhibin B correlated positively with testis volume (r=0.73, P<0.001) and sperm counts (r=0.55, P<0.01). In conclusion, this study shows that inhibin B represents an early and more sensitive marker of testicular damage than FSH. Furthermore, the rapid fall of inhibin B after irradiation suggests that this hormone is a direct parameter of premeiotic germ cell proliferation.