Apical surface morphology of the mare isthmic oviduct during anoestrus and oestrus: a scanning electron microscopy study

The oviduct isthmus is considered to be a sperm reservoir prior to ovulation in the reproductive tract of mares. This segment is able to i) prevent polyspermic fertilization, ii) maintain the fertility of sperm, and iii) regulate capacitation and motility hyperactivation in order to ensure an effective sperm condition when ovulation occurs. The isthmus epithelium undergoes morphological and functional changes in response to oestrogen and progesterone fluctuation during the oestrus cycle. Ovarian steroids regulate the synthesis and secretion of specific molecules such as glycoproteins that are involved in the interactions between spermatozoa and oviductal epithelial cells. The objective of the present study was to investigate the apical surface morphology of the isthmus epithelium of mares during different physiological conditions. Isthmus fragments were separated from oviducts of anoestrus (n=2, collected in November) and oestrus (n=3, one of them mated) (with a follicle > 35 mm) mares butchered in a local slaughterhouse. The tissues were fixed in 2.5 % glutaraldehyde in 0.1 M PBS, post-fixed in 1% OsO4 and processed for scanning electron microscopy investigations. The luminal epithelium of the isthmic oviduct from anoestrus and unmated oestrus mares revealed two types of cells: ciliated and non-ciliated. The non-ciliated cells showed the apical surface covered with microvilli in anoestrus mares whereas exhibited a luminal surface with irregular apical protrusions in unmated oestrus mares. The isthmus luminal surface from mated mare did not show cilia and/or microvilli but was characteristically covered by a granular and irregular material where spermatozoa bound. These findings suggest that the apical surface of mare isthmus epithelium undergoes clear morphological changes related to various phases of the reproductive cycle.