Testosterone in the Fetal Rat Testis

Testes of fetal rats possess the enzymes necessary for converting radioactive acetate to radioactive testosterone as early as 15.5 days of intrauterine life. The fetal testis is, therefore, capable of formation de novo of testosterone through the period of sexual differentiation of the internal genitalia. Testosterone was measured in testicular tissue of fetal rats. Values ranged from as low as 0.09 ng/mg at 15.5 days of gestation to 2.76 ng/mg at 18.5 days of gestation. The sharp rise at 18.5 days of fetal age coincides with morphological evidence that relatively large amounts of testosterone are necessary for Wolffian duct stabilization at this time. Fetal testes were also incubated with [ 14 C] acetate or [ 3 H] pregnenolone. Whereas conversion of both substrates to radioactive testosterone occurred at 15.5 and 16.5 days of gestation, no such conversion was evident at 14.5 days of fetal age with either substrate. Nonradioactive testosterone was, however, measurable in incubation from all three time periods. Neither radioactive nor nonradioactive testosterone was detectable in incubations of fetal ovaries with radioactive acetate or pregnenolone at 15 to 20 days of gestation.