The modulation of sperm function by the oviductal epithelium.

During their sojourn in the female tract, sperm come into contact with the epithelial cells that line the tract and their secretions. In the past, much attention has been given to the nature and composition of uterine and oviductal fluid [1-3] with the aim of understanding what components of these fluids may affect the events associated with fertilization and preimplantation embryo development. A number of secretory glycoproteins have been identified in reproductive tract fluids of several species [3]. However, with the exception of two estrus-associated oviductal glycoproteins that may play a role in the capacitation [4, 5] and/or motility [6] of bovine sperm, the function of these glycoproteins in relation to sperm physiology remains unclear. One remarkable aspect of sperm behavior, widely observed across species, is a tendency to directly contact and adhere to the epithelial cells that line the tract. Evidence is accumulating that such contacts are physiologically significant. Thus, examination of the role of direct contact between sperm and the epithelial cells that line the tract may provide additional insight into the mechanisms that control sperm biology in vivo. Although sperm come into contact with cervical and uterine epithelial cells during passage through the tract, oviductal epithelial-sperm cell interactions are of particular importance. In many species, the isthmic region of the oviduct is an important site of sperm storage prior to fertilization [7]. While resident in the lower isthmus, many sperm adhere to the apical plasma membrane of the ciliated and secretory epithelial cells that line this region. For all species studied so far, adhesion is specific to the rostral region of the sperm head. Sperm adhesion to isthmic epithelial cells is temporary, and adherent sperm are capable of detaching from the epithelium and reaching the oviductal ampulla to participate in fertilization [8]. Current evidence indicates that temporary adherence to the oviductal epithelium has a beneficial effect on sperm viability both in vivo [9] and in vitro [10-15]. The ability of sperm to adhere to and detach from the epithelium appears to be related to their capacitation status. Capacitated hamster and bull sperm bind to oviductal epithelial cells with much lower frequency than uncapacitated sperm [8, 16, 17]. This may be due to changes in the head plasma membrane that occur during capacitation or due to