Age dependent changes in the sperm population and fertility in the male rat.

The entire reproductive tract was devoid of spermatozoa during the first 42 days of life. The first appearance of spermatozoa was detected in the caput epididymidis of 45-day old rats and in the cauda epididymidis of 52-day or older rats. The number of spermatozoa in the reproductive tract increased with age. The sperm population reached its maximum in the caput epididymidis by day-72 and in the caud epididymidis by day-100. These high levels were maintained beyond 450 days of age. Fifty-two day old male rats failed to impregnate the female. Although male rats of 62-500 days were able to sire litters, the peak reproductive period was found to be between days 100-270, during which the number of young per litter ranged between 11-16. Towards advanced age the litter size dropped to 8. The lower fertility in the males of advanced age might be due to a lower proportion of fertile spermatozoa, a change in the physiology of epididymis or an altered sperm transport in the reproductive tract.