REGULATION OF TESTOSTERONE SYNTHESIS IN THE FETAL MOUSE TESTIS

Plasma testosterone (T) levels were determined by radioimmunoassay in male and female mouse fetuses from day 13 (1 day after sexual differentiation) to day 18 (1 day before parturition). T levels in plasma of male fetuses were significantly higher than those of female. In male fetuses, plasma T levels rose from day 13 to a peak level at 16-17 days, before declining on day 18. A large amount of progesterone (P) was present in pregnant mouse blood over the observed period. P of maternal origin may be available as a potential precursor for T biosynthesis by the fetal testis. The dissociation between maternal P and fetal T levels from day 15-day 16 of gestational age may suggest a more complex regulation of fetal T synthesis.