ULTRASTRUCTURE OF THE SECRETORY ENDOMETRIUM DURING ESTRUS IN YOUNG MAIDEN AND FOALED MARES

Cyclical accumulation of uterine fluid occurs during oestrus and is of ten seen in excessive volumes in mares considered susceptible to endom etritis. Since the mechanisms behind the formation of free uterine flu id remain to be clarified, the fine structure of the secretory equine endometrium was studied in biopsies collected during videoendoscopy fr om 14 endometritis free, 4-9-year-old mares during oestrus. A distinct oedema of the tunica mucosa was evident. The surface epithelium had b oth ciliated and nonciliated cells and, particularly at the uterine bo dy, often presented intra-epithelial macrophages. The epithelial cells of the gland duct were similar to the surface epithelium, except that the nonciliated cells lacked secretory vesicles. The mid-deep glandul ar segment was, however, loaded with apical secretory vesicles in the nonciliated cells. This glandular epithelium presented clear signs of secretory activity with conspicuous secretory vesicles holding electro n-dense granula in the adluminal cytoplasm and a well developed supran uclear Golgi apparatus. Secretory products as well as cell debris were commonly found in the lumen of the glands. No clear signs of apocrine secretion were found and it seemed therefore, that the mechanism of s ecretion is merocrine, i.e. by exocytosis. The endometrial oedema and intense secretory activity, both under oestrogenic influence, contribu te to the building up of the uterine fluid during oestrus. No differen ces in morphology of the secretory endometrium could be noticed betwee n nulliparous mares and mares that had had 1 or 2 foals.