The mean weight of rats in the VED group was consistently
lower compared to rats in the control group until 2 months of age
(data not shown). Rats in the VED group generally looked healthy
but had some degree of fur loss. The mean proportion of rats in the
VED group exhibiting a normal estrous cycle (78.43 ± 1.70%) was
similar to that in the control group (78.89 ± 1.92%).
At around 4 months of age, rats were mated with vasectomized
males to induce pseudopregnancy. Endometrial bleeding was then
monitored daily from 8.5 dpc by vaginal smears (Table 2). Over
three consecutive pseudopregnancy cycles, the proportion of rats
exhibiting endometrial bleeding gradually increased (5.9%, 32.3%,
and 50%). In contrast, endometrial bleedingwas not detected in any
of the rats in the control group. Collectively, 18 of the 34 rats in the
VED group showed endometrial bleeding by the end of three successive
cycles. A rat showing endometrial bleeding in one of the
earlier cycles was more likely, but not guaranteed, to exhibit
bleeding in a subsequent cycle. During the second cycle, two rats
showing endometrial bleeding were sacrificed for histological examination,
and subsequently proven to be undergoing a typical
decidual reaction (Fig. 2C and D). Following three pseudopregnancies,
a pregnancy cycle was initiated in all rats within the two
groups in order to evaluate fertility status. Pregnancy rate in the
VED groupwas 14.3% compared to 100% in the control group. These
results concur with a previous report stating that VED rats experienced
spontaneous decidualization during pseudopregnancy [8].
In the present study, we were able to identify uterine decidualization
by endometrial bleeding, but not by uterine histomorphology,
thus proving that endometrial bleeding represents a simple
and economical method that enables continuous tracking of uterine
condition.