Within the transilluminated oviducts of naturally mated mice, some hyperactivated sperm were seen swimming freely while all nonhyperactivated sperm appeared tightly bound to the epithelium. Moreover, Demott et al. showed that fetuin interfered with hamster sperm attachment to the oviductal epithelium by binding to the acrosomal region of fresh epididymal sperm. However, fetuin did not bind to hyperactivated sperm. These results imply that there is a preferential binding of uncapacitated spermatozoa to oviductal epithelium and that capacitation or switch to hyperactivated motility allows spermatozoa in a condition appropriate for fertilization to be released from the reservoir. However, the capac-itation status of spermatozoa bound to OEC and the changes in functional status of sperm bound to OEC have never been directly visualized and determined.
The main difficulty encountered in identifying the ca-pacitation status of spermatozoa bound to OEC is that no obvious morphological changes accompany the changes in capacitation status of spermatozoa. Yet during the past few years, the availability of the fluorescent antibiotic chlortetracycline (CTC) has proven useful in this regard. Ward and Storey reported that three fluorescence staining patterns could be identified on mouse sperm heads, with the population of cells in each category changing as the suspensions underwent capacitation and the acrosome reaction. The advantage of CTC analysis over other more commonly used techniques is that it not only identifies acrosome-re-acted cells (AR pattern), but it also allows the acrosome-intact sperm population to be subdivided into two groups, namely the uncapacitated (F pattern) and capacitated (B pattern) spermatozoa. Since its introduction, CTC analysis has been successfully employed to identify the capacitation status of spermatozoa in several species; such studies have been done in the mouse, stallion, human, bull, boar, ram, and buck.