Preovulatory binding of viable spermatozoa in the oviduct isthmus is widely accepted as a preliminary to fertilization, but details of physiological events associated with epithelial binding and release from binding are themselves little understood. Important questions include the potential number, distribution and stability of such sites in the caudal isthmus, whether multiple molecular forms of binding exist within a single-mated individual, and whether some sites are more favourable than others for the maintenance of preovulatory sperm viability. Also to be resolved is whether spermatozoa interact with the first available binding sites in the isthmus, whether spermatozoa from second or subsequent matings bind closer to the site of fertilization, and whether the first spermatozoa entering the oviduct are those that will be released first with impending ovulation. Ideally, future research needs to focus on a fertilizing spermatozoon monitored in vivo and not on spermatozoa destined to remain on or in the zona pellucida or in the lower reaches of the oviduct.

In endeavouring to understand the nature of sperm–oviduct interactions in mammals, attention was focused on experimental models in which fertilization can occur without a preliminary phase of sperm head binding to the isthmus epithelium. The ovarian endocrine milieu imposed on the oviduct tissues plays an important role in the binding phenomenon, although less so after the time of ovulation. Nonetheless, a sperm suspension introduced into the peritoneal cavity or surgical insemination directly into the oviduct ampulla before ovulation can result in fertilization, as can a surgical model in which the isthmus has been resected and the remaining portions of the duct reanastomosed. Mating or artificial insemination after ovulation in pigs permits rapid sperm transport to the site of fertilization, and the frequency of polyspermic penetration increases with the post-ovulatory age of eggs.

Strategies underlying sperm binding were considered, especially in terms of preovulatory sperm storage and suppression of full membranous maturation. These, in turn, raised the problem of how sperm binding in vitro to oviduct cells from prepuberal animals or to cells harvested during the luteal phase of the estrous cycle, or to cells from the ampulla or even the tracheal epithelium, can act to regulate sperm storage and maturation with precision. In an evolutionary perspective, preovulatory binding of diverse populations of cells to the endosalpinx may have developed as a form of fine tuning to assist in sperm selection, to synchronize completion of capacitation with the events of ovulation, and to promote monospermic fertilization by a controlled release of competent gametes.

That a mammalian embryo might influence the maternal endocrinological and/or immunological systems whilst still in the Fallopian tube has long seemed possible (Fig. 1). Under normal circumstances, the embryo contains a full diploid complement of genetic information and thus the point at issue is just how soon instructions in the genome are expressed for trophic purposes. It could be argued from several points of view that it would be biologically prudent for an embryo to influence the mother whilst still at an early stage of development. Consideration of the extremely intimate apposition of the zona pellucida and the tubal epithelium - perhaps best appreciated in scanning electron micrographs (Fig. 2) - also suggests scope for a mutual exchange of signals. Even so, one of the earliest recorded maternal responses to a developing embryo is the change in permeability of the capillary bed of the uterus that precedes implantation in rodents: this can be demonstrated by the Pontamine blue reaction, a series of bands of dye in the uterine wall corresponding to the location of individual embryos, and is detectable in rats on the fourth day after mating (Psychoyos, 1960, 1973).