Ultrastructure of the secretory endometrium during oestrus in young maiden and foaled mares.

The research examines the structure of the secretory equine endometrium in mares during oestrus and connects its characteristics to the accumulation of uterine fluid, a condition often leading to endometritis.

Introduction and Methodology

• The research addresses the cyclical accumulation of uterine fluid in mares during oestrus, which in excessive quantities, often contributes to endometritis — inflammation of the inner lining of the uterus.
• The main focus of the study lies in understanding the formation of this uterine fluid. For their study, the researchers analyzed the ultrastructure, or the fine structure, of the secretory equine endometrium.
• The secretory equine endometrium is the innermost lining of the uterus of the mares, which undergoes changes during oestrus, or the fertility phase of the mare’s estrous cycle.
• Biopsies were taken from 14 endometritis-free mares, all between the age of 4 to 9 years, during oestrus for this study. The selections included both nulliparous mares (those who have never given birth) and mares that had had 1 or 2 foals.

Findings and Interpretations

• The biopsies displayed noticeable oedema (swelling due to fluid accumulation) in the tunica mucosa, the middle layer of the endometrium.
• The surface epithelium exhibited a mix of both ciliated and nonciliated cells, with the inclusion of intra-epithelial macrophages, predominantly at the uterine body.
• Epithelial cells of the gland duct resembled those of the surface epithelium, apart from the nonciliated cells that were devoid of secretory vesicles.
• The glandular epithelium showed prominent signs of secretory activity, evident by the presence of secretory vesicles containing electron-dense granules in the cytoplasm and a developed supranuclear Golgi apparatus.
• Secretory products and cell debris were routinely observed within the glands’ lumens, however there were no clear signs of apocrine secretion (a specific type of exocrine secretion). The researchers infer that the secretion process is probably merocrine, i.e., involving the release of secretions through exocytosis without loss of cell integrity.
• The researchers concluded that the endometrial oedema and intense secretory activity in the endometrium, influenced by estrogen, contribute to the accumulating uterine fluid during oestrus.
• Significantly, there were no discernible differences in the morphology of the secretory endometrium between nulliparous mares and those that had given birth to 1 or 2 foals.