Differentiation of Urovaginal Septum and its Critical Period of Abridgment by Testosterone in Female Rat Fetuses

The chronological differentiation of the urovaginal septum of the female rat fetus and its critical period for the virilizing activity of testosterone were examined by measuring the urovagival septum length directly under microscope equipped with a micrometer. Until the 18th day of gestation, definite differentiation of the urovaginal septum was not observed, but thereafter the septum developed rapidly and on the 21st day it was built up almost completely to separate the urethra and vagina. Following a consecutive maternal subcutaneous administration of various doses of testosterone in olive oil for 4 days between the 17th and 20th day of gestation, a significant abridgment in urovaginal septum length was noted at a dose level of 0.01 mg/rat. In the chronological examination, a significant inhibition of urovaginal septum development was observed 3 days after a single dose of 5 mg/rat of testosterone on the 17th day of gestation. When the same dose of testosterone was given on the 16th day of gestation, no effect was observed in the urovaginal septum length of female fetus on the 21st day of gestation. In the 17th-day-treatment group, however, a highly significant abridgment in the urovaginal septum was noticed compared with the control. Further abridgment was observed in the 18th day treatment group, and the 19th-day-treatment group showed the largest abridgment, but the length change in the 20th-day-treatment group was comparable to that in the 17th-day-treatment group. On these findings, it was concluded that the critical day of urovaginal septum abridgment by the virilizing activity of exogenous testosterone appeared to be the 19th day of gestation in female rat fetuses.