Abstract
Photoperiod‐induced cycles of gonadal regression and recrudescence in the Syrian hamster were used to determine if epididymal growth in adults involves mitotic activity of principal cells. In Experiment 1, the following groups of adult hamsters were examined: induced recrudescing (5L:19D [5 hr light and 19 hr dark] for 13 wk followed by 14L:10D for at least 3 wk), spontaneous recrudescing (5L:19D for 25 wk), and active gonadal state (14:10D). In Experiment 2, adult hamsters were divided into the following groups: induced recrudescing, active, and regressed (5L:19D for 16 wk). Hamsters received subcutaneous injections of 0.5 μCi ^3^H‐thymidine/g body weight three times/wk for 3 wk. The epididymis was fixed in a glutaraldehyde followed by osmium, embedded in Epon 812, and sectioned at 1 μm. Slides were dipped in Kodak NTB‐3 emulsion, exposed for 2 or 3 months, developed, and evaluated for isotopic labeling of principal and basal cell nuclei by scoring 500 to 1,000 nuclei. In Experiment 1, the percentages of labeled principal cell nuclei for the induced recrudescing, spontaneous recrudescing, and active groups were 26 ± 2%, 23 ± 5%, and 9 ± 1%, respectively. Considering the intermittent availability of ^3^H‐thymidine during 21 days, this represents daily recruitment of 6.3%, 5.6%, and 2.2%, respectively. In Experiment 2, the percentages of labeled principal cell nuclei for induced recrudescing, active, and regressed groups were 12 ± 4%, 3 ± 1%, and 4 ± 1%, respectively. There was no effect of photoperiod on labeling pattern of basal cells (1.5 ± 0.6%, 1.2 ± 0.1%, 0.4 ± 0.1% for the three photoperiod groups, respectively). Regardless of photoperiod, there was no difference in the labeling pattern of principal or basal cells between the head and the tail of the epididymis. In conclusion, photoperiod influenced the recruitment of epididymal principal cells throughout the epididymis, and new principal cells were added continually to the existing population of cells even in gonadally active adult hamsters.