A light microscopic and ultrastructural study on the presence and location of oxytocin in the equine endometrium

It has been reported that oxytocin is produced not only in the hypothalamus and posterior pituitary but also in outside the classical hypothalamo–neurohypophyseal axis such as the ovary, testis, placenta and in some nonreproductive sites. In the mare, oxytocin-mRNA has been identified in the endometrium, and oxytocin and its neurophysin have been identified in the uterus. In the present study, oxytocin was localised in the endometrium of the mare at the light microscopic and ultrastructural level by immunostaining and immunogold labelling of endometrial biopsy specimens collected during estrus. Strong positive immunostaining for oxytocin was found in the secretory vesicles of the secretory (nonciliated) epithelial cells of the uterine lumen and of the superficial glands. Using immunogold labelling, oxytocin was detected in the secretory vesicles of secretory epithelial cells. The vesicles containing immunoreactive oxytocin were present on the luminal surface suggesting that oxytocin is secreted into the uterine lumen by apical exocytosis. There was no positive immunostaining in ciliated epithelial cells of the uterine lumen and endometrial glands, in the stromal cells, or in the basal endometrial glands. To our knowledge, this is the first report of the location of oxytocin in specific secretory cells in the endometrium of any domestic species. This locally synthesised uterine oxytocin may have an important role in the autocrine/paracrine control of uterine contractility and luteolysis in the mare.