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Summary. In litter-bearing mammals, the course of development of male and female fetuses is affected by the presence of other fetuses of the same or opposite sex located nearby within the uterus. The transport of testosterone between rat fetuses was examined by implanting a Silastic capsule containing [3H]testosterone into the amniotic sac of a fetus at either the ovarian or cervical end of a uterine horn on days 19 and 20 of pregnancy. The amount of testosterone that was recovered from the amniotic fluid of other fetuses 12 h later was determined. The amniotic fluid surrounding the adjacent fetus on the cervical side of the implanted fetus contained three times as much [3H]testosterone as did the adjacent fetus on the ovarian side, regardless of where in the uterus the implant was made. The movement of dye injected into the uterine lumen was towards the cervix. Intraluminal fluid movement may thus mediate the greater transport of [3H]testosterone towards the cervix than towards the ovary.
Our findings support the hypothesis that transport of testosterone between fetuses occurs across the fetal membranes via diffusion, such that any fetus (male or female) located between male fetuses receives the greatest supplement of testosterone.
Keywords: intrauterine position; intrauterine transport; sexual differentiation; rat
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The effects of location within the left or right uterine horn, position within each uterine horn, and fetal sex on fetal bodymass, blood flow to individual uterine segments associated with fetuses, and blood flow to the maternal portion of the placenta were investigated in rats. Sprague–Dawley rats were anaesthetized on day 5, 10, 15, 20, 21 or 22 of pregnancy, and radioactive microspheres with diameters of 15 mm were injected via a left ventricular cannula to measure blood flow to tissues. Tissues were weighed wet, and the rate of blood flow, corrected for wet mass (ml min−1 g−1 tissue), was calculated. Microspheres were not detected in fetuses, suggesting that they did not pass from maternal into fetal blood. Uterine blood flow was greater at the cervical and ovarian ends than in the middle of the uterus; on day 15 the rate of blood flow at the cervical and ovarian ends of each uterine horn was over twice that in the middle. The blood flow to the right uterine horn was greater than to the left horn. Blood flow to placentae increased dramatically between day 15 and day 20. There were marked differences in architecture between the uterine artery feeding the ovarian end of the right and left uterine horn, and blood flow to placentae located at the ovarian end of the right uterine horn was greater than to placentae in the same location in the left uterine horn. The blood flow to placentae and fetal bodymass were greater for female than for male fetuses on day 20, but on day 22 the reverse was observed. There was no effect of the number of fetuses in the same uterine horn on blood flow to the uterus or placentae, or on fetal or placental mass.
