Maturation of the pituitary-gonadal system in the male rat.

Serum FSH and testosterone concentrations reached maximum levels between 35 and 45 days of age, which coincided with the appearance of mature spermatozoa in the majority of seminiferous tubules. Spermatozoa were not observed in sections of the urethra until the age of 46 days. Serum LH concentrations were low (5-6 ng/ml) before Day 25, became highly variable (12-57 ng/ml) between Days 25 and 53 and remained consistently above 35 ng/ml thereafter. Serum prolactin levels rose significantly between 30 and 43 days of age. Maximum prolactin levels coincided with the start of accelerated growth in the prostate and seminal vesicle glands. Testicular weights relative to body weight reached a plateau by 35 days of age, while relative pituitary and adrenal weights decreased throughout the study period. It is suggested that spermatogenesis is not complete until FSH and testosterone reach maximum levels, while prolactin may be involved in the stimulation of accessory sex organ growth. The pronounced variation in serum LH concentrations during the maturation period may reflect a progressive change in the sensitivity of the hypothalamic-pituitary axis to the negative feedback of gonadal steroids.