Immunohistochemical detection of the estrogen receptor-alpha and progesterone receptor in the canine pregnant uterus and placental labyrinth

The presence of hormone receptors is as important as the amount of hormone to predict hormone action. Therefore, the presence of estrogen receptors of the alpha subtype (ER-alpha) and progesterone receptors (PR) was evaluated in six pregnant uteri including the placenta and in three postpartum uteri of dogs. This preliminary study is part of our immunohistochemical research project on steroid hormone receptor distribution in the canine female genital tract. Specific staining for ER-alpha or PR was found only in cell nuclei. Staining for ER-alpha was rare in the various cell types of pregnant and postpartum uteri. Staining for PR was absent or weak in epithelial cells. Moderate staining for PR was observed in endometrial stromal cells and myometrial smooth muscle cells, two cell types playing an important role in the maintenance of pregnancy. Stromal cells stained more frequently positive for ER-alpha and PR than epithelial cells, indicating that both hormones may act on epithelial cells indirectly via stromal cells. In the placental labyrinth, fetal cells showed no evidence of ER-alpha or PR. In contrast, both receptors were present in maternal mesenchymal cells that were located around the basement membrane of the maternal blood vessels. These cells showed signs of decidualization. No difference in PR distribution was seen between pregnant and postpartum uterine tissue, suggesting that during parturition the decrease in serum progesterone levels and the concomitant increase in the estrogen/progesterone ratio are probably more important than the decline in receptor availability.