The appearance of endometrial bleeding in VED rats was previously
reported as being representative of spontaneous decidualization
within the pseudopregnant endometrium [8]. Uterine horn
weight was significantly greater in the VED group compared to
controls (Fig. 2A, P < 0.05). Moreover, uterine horns from rats in the
VED group were a darker red compared to the control group
(Fig. 2B). At 5.5 dpc, rats in the VED group exhibited a greater degree
of vasopermeability, as determined by Trypan blue staining. At
7.5 dpc, rats in the VED group exhibited enlarged uterine horns
with few scattering nodes, signifying decidualization. The uterine
horns at 9.5 dpc were darker than those at 7.5 dpc in the VED group,
with some parts appearing black, a feature indicative of endometrial
bleeding. Histological examination of these tissues at 7.5 dpc
revealed a typical manifestation of decidualization with diploid
cells (brick shaped) in the stroma (Fig. 2C). By 9.5 dpc the uterine
horns were decidualized and histological analysis showed clear
evidence of hemorrhage (Fig. 2C). No gross or histological changes
were detected in the control group throughout the experiment. In
addition to individual variation between rats in the VED group, it
was apparent at autopsy that the extent of decidualization varied
among uterine horns, although we commonly observed 4e6 nodes.
The proportion of rats showing histological decidualization in the
VED group (63.3%) was higher than those exhibiting endometrial
bleeding (50%) by vaginal smears, indicating that endometrial
bleeding underestimated the proportion of rats undergoing
decidualization.