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Abstract
In rodents, steroids are able to pass between fetuses. Although not previously examined
in mice, we have provided evidence that blood flow in the uterine loop artery in rats
is bidirectional and that steroids are transported between fetuses by diffusion via
the uterine lumen, not via the uterine blood vessels of the mother. The direction
of blood flow in the uterine loop artery and vein feeding each uterine horn in house
mice was examined on day 17 of pregnancy. Dye was injected into the heart to determine
the direction of blood flow in the uterine artery while injection of dye into individual
placentae was used to determine the direction of blood flow in the uterine vein. Blood
entered the loop artery from both the dorsal and caudal ends and was thus bidirectional.
Venus blood flow from placentae was in a rostral direction from placentae in the rostral
portion of a uterine horn and in a caudal direction from placentae in the caudal portion
of a uterine horn. Comparison of anogenital distance and body weight at birth using
a variety of classification schemes, based on different assumptions about the mechanism
and direction of transport of steroids between fetuses, showed that the only scheme
which accounted for variation in anogenital distance at birth in female mice was that
which was based on the number of directly adjacent male siblings in utero. Taken together
with recent findings in rats reported elsewhere, we conclude that blood flow in the
uterine blood vessels is bidirectional, and steroids are transported between fetuses
by diffusing through the amniotic fluid and across the fetal membranes of adjacent
fetuses.(ABSTRACT TRUNCATED AT 250 WORDS)