Testicular Maturation in the Rabbit: Secretion of Testosterone, Dihydrotestosterone, 5α-Androstan-3α,17β-diol and 5α-Androstan-3β, 17β-diol by Perfused Rabbit Testes-Epididymides and Spermatogenesis

The rabbit testis-epididymis was perfused in vitro to observe pubertal changes in the secretion of testosterone, dihydrotestosterone, 3α-androstanediol, and 3β-androstanediol. At selected ages between 1 and 52 weeks, steroid secretion was correlated with testicular gonadotropin binding activity, plasma androgen levels, serum gonadotropin levels, daily sperm production and spermatogenesis. Testosterone was secreted throughout the period of testicular maturation and accounted for 50% or more of the mass of total androgens produced. Dihydrotestosterone, 3α-androstanediol, and 3β-androstanediol (5α-reduced steroids) were not detected until 6 weeks of age but were secreted in increasing quantities during the remainder of pubertal development. The combined mass of testosterone and the 3 5α-reduced androgens that was secreted averaged 0.2, 1.5, 7.4 and 7.8 (µg/testis/h) at 4, 6, 10 and 52 weeks, respectively. Increments in plasma androgen titers coincide with the rise in androgen secretion by perfused testes-epididymides. Circulating androgen concentrations were not correlated with serum LH levels but were tightly coupled to circulating FSH titers (r = 0.93). Testicular and seminal vesicle growth and germ cell differentiation were coincident with graded increments in testosterone and 5α-reduced androgen production. Each of the indices of testicular function indicated sexual maturity was achieved by 18 weeks of age. Two important points emerge from the foregoing results. First, testosterone was the chief androgen elaborated throughout pre- and postpubertal testicular development, a finding which contrasts with the predominant formation of 5α-reduced androgens in prepubertal mice and rats. Second, the highly significant correlation between FSH release and plasma androgen levels suggests that FSH may mediate testicular responsiveness to LH during sexual maturation in the rabbit.