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The aim of this immunohistochemical study is to describe the normal distribution of progesterone receptors in the various cell types of the canine uterine horns, body and cervix. The results can be used for research on uterine and endocrinological pathology, since the impact of progesterone on different uterine cell types is partly determined by the receptor availability. Nuclear staining for progesterone receptors was observed in epithelial cells of the surface epithelium, glandular ducts and basal glands of the endometrium, in endometrial stroma cells and in myometrial smooth muscle cells. This staining was positively correlated with the estradiol-17 beta:progesterone ratio, and reflects the positive effect of estradiol-17 beta and the negative influence of progesterone on the receptors. Staining scores were high during proestrus and decreased through estrus to early metestrus. In late metestrus, staining scores of the stromal and smooth muscle cells increased again. In anestrus, high scores of the surface-epithelial cells contrasted with minimal scores of the basal glands. This finding suggests a different hormonal regulation of the progesterone receptor expression in both epithelial cell groups. The higher staining intensities for progesterone receptors in stromal cells compared with epithelial cells might be explained by the fact that stromal cells mediate some effects of steroid hormones on the epithelial cells in the genital tract. Therefore, the role of stromal cells in regulation of the cyclic endometrial changes and in pathologic changes of uterine tissue should not be underestimated.

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