Morphological and histochemical features of the luminal epithelium of caudal region of the equine oviduct

The caudal region of the mammalian oviduct includes the isthmus and utero-tubal junction segments. In equine oviduct a few studies have been performed on the isthmus by means of scanning electron microscopy (SEM) and lectin histochemistry, whereas studies on the interstitial segment of isthmus (ISI) (corresponding to the cow and pig utero-tubal junction) are lacking. These two segments of mare oviducts were processed for SEM and electron transmission microscopy as well as for lectin histochemistry. The ISI showed a narrower lumen and a taller epithelium than isthmus. The epithelium of the both regions contained electron-lucent ciliated cells and two sub-types of non-ciliated (secretory) cell. Most secretory cells in the ISI showed granules characterized by a dense core surrounded by less electron-dense material, whereas a few cells showed an electron-dense cytoplasm containing small electron-dense granules. In the isthmus non-ciliated cells were moderately electron-dense and the two sub-types differed in the number and the electron-density of the granules. Lectin histochemistry showed Galβ1,3GalNAc and sialic acid-Galβ1,3GalNAc in the ISI and Neu5Acα2,6Gal/GalNAc in both segments. These results suggest that during their passage through the ISI and the isthmus spermatozoa move in two different intraluminal milieu which contribute to ensuring the effective condition of the sperm when ovulation occurs