Melanie Klose

• In mammals, the duration of the cycle of the seminiferous epithelium (DCSE) is characteristic for each species, but varies greatly between species, e.g. in rodents between 6.7 days and 17.0 days. Within a species, in contrast, the DCSE is remarkably stable with variations of 1 % - 3 %. This accuracy is very conspicuous since other biological processes are much more variable. It is furthermore difficult to change the DCSE experimentally, e.g. by hormones or chemicals. One reason for the precise timing could be the existence of a testicular peripheral clockwork, as identified previously in other tissues. To test this hypothesis, adult male Djungarian hamsters (n = 20 per group) were exposed to normal day length (24 h), one hour shortened (23 h), or one hour prolonged (25 h) day lengths. Exposure lasted for 43 days and the entrainment to the photoperiods was verified individually by passive infrared detectors (PID). The DCSE was estimated by immunocytochemical localization of 5-bromo-2- deoxyuridine (BrdU)-labeled cells and microscopical identification of the 12 stages of the seminiferous epithelium. The results for the normal day length (24 h: 7.98 ± 0.05 days) are congruent with previous findings in this species. No effects of shortened or prolonged day lengths on the DCSE could be identified (23 h: 7.94 ± 0.04 days; 25 h: 7.91 ± 0.03 days; p = 0.50), despite the low variability of 1.4 % and 2.0 %, respectively. A further experiment was performed to investigate the diurnal variations of expression of two core clock genes haBmal1 and haPer1 in testes and kidneys of Djungarian hamsters using quantitative RT-PCR. Hamsters (n = 4 per time point) were sacrificed every 3 h during 24 h. While in kidneys typical anti-cyclical expressions of haPer1 and haBmal1 were observed, a completely different expression pattern was found in the testes. Here, levels of haBmal1 were almost constant, while the expression of haPer1 showed a distinct diurnal variation. These findings strongly support the hypothesis that a peripheral clock is responsible for the low variability of the DCSE.