Previous studies have shown that the equine endometrium changes morphologically under the influence of progesterone. It also begins to secrete the 19 kDa protein uterocalin, which is believed to act as a carrier or transport protein to provide the conceptus with essential vitamins and minerals during the early stages of development. In the present study endometrial biopsies were used to assess and compare the effects of both endogenous and exogenous progesterone on endometrial morphology, the surface density of endometrial glands and their secretion of uterocalin. An attempt was also made to relate these findings to the duration of progesterone dominance. Therefore, endometrial biopsies were taken at several time points after ovulation and after exogenous progesterone administration. These were processed and sectioned, after which they were either stained with haematoxylin and eosin (H&E) for the morphometric study, or labelled immunohistochemically for the detection of uterocalin. Morphometric analysis indicated that the endometrial changes are comparable after both endogenous and exogenous progesterone dominance and were most noticeable in the epithelium lining the endometrial glands, where the height of the epithelial cells had increased significantly by 3 – 5 days after ovulation and after 3 days of exogenous progesterone treatment. Glandular surface density, however, did not increase after ovulation, nor after treatment with exogenous progesterone. Immunohistochemical staining of the endometrial biopsies showed significant uterocalin secretion after 5 days in both normal dioestrus and after the administration of exogenous progesterone. This suggests that the changes in the morphology of the endometrium are progesterone dependent and that gland surface density is either not progesterone dependent or might need more days of progesterone dominance before significant changes can be seen. It can be concluded that the secretion of uterocalin is initiated by progesterone and that the rate of secretion is sufficient by the time the equine embryo enters the uterine lumen on day 6 after ovulation
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