Comparative assessment of the timing of sexual maturation in male Wistar Han and Sprague-Dawley rats

Given the increasing use of Wistar Han (WH) rats in regulatory toxicology studies, these studies were performed to characterize the onset of sexual maturation in maturing WH rats as compared to Sprague-Dawley (SD) rats. Beginning on postnatal day (PND) 38 through PND 91 groups (n = 8) of untreated WH rats were evaluated for maturation of the male reproductive system. Testicular spermatid head counts increased beginning on PND 42 until PND 70. Sperm were detected in the caput, corpus, and cauda epididymis on PND 45, 49, and 49, respectively, and counts increased through PND 91. Sperm motility was at adult levels by PND 63. The morphology of the testis/epididymis of all animals at day 70 or older was consistent with qualitative sexual maturity. Based on these endpoints, WH rats were determined to be sexually mature at PND 70, and many of these endpoints evaluated in SD rats exhibited nearly identical trends.

► In maturing male Wistar rats testicular spermatid head counts increased beginning on PND 42 until PND 70.
► Sperm were detected in the caput, corpus, and cauda epididymis on PND 45, 49, and 49, respectively, and counts increased through PND 91.
► Sperm motility was at adult levels by PND 63.
► The morphology of the testis/epididymis of all animals at day 70 or older was consistent with qualitative sexual maturity.
► Based on all of the endpoints evaluated, Wistar rats were determined to be sexually mature at PND 70.
► Many of the same endpoints were evaluated in Sprague-Dawley rats in a separate study, and they exhibited nearly identical trends to the Wistar rats.