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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.

Migration of spermatozoa in the female genital tract will be strongly influenced by the viscosity of the fluids encountered, yet little systematic analysis has been given to such a consideration. This essay reviews the series of milieux confronting a fertilising sperm during its progression to the oviduct ampulla. Two groups are discussed, first those in which ejaculation is into the vagina, second those in which semen enters the uterus during a protracted mating. Viscous glycoprotein secretions that accumulate in the oviduct isthmus of both groups before ovulation are highlighted, as is the environment generated in the ampulla by the post-ovulatory suspension of oocyte(s), cumulus cells and spermatozoa; follicular and peritoneal fluids may also be present. The viscosity of all female tract fluids responds to cyclical variations in temperature, and these exist within the oviduct near the time of ovulation. Gradations in viscosity influence the pattern and strength of sperm flagellar activity and the rate of forward movement. Measurements of sperm motility are currently made in a physiological medium of constant viscosity and temperature, thereby overlooking changes in the female genital tract. A more sophisticated approach might reveal an adequate fertilising potential in a proportion of putatively poor semen samples.

This essay considers the means whereby sperm/egg ratios close to unity are generated during the initial stages of fertilisation in placental mammals. Pre-ovulatory graafian follicles and their contents are seen to be key structures orchestrating the events of sperm progression and coordinating the subsequent meeting of male and female gametes. Three levels of control over the numbers of spermatozoa activated and released from the functional reservoir in the caudal region of the fallopian tube isthmus are proposed. A primary control would be obtained by means of a countercurrent transfer of ovarian follicular progesterone from the ovarian vein into the tubal branch of the ovarian artery. The concentration of progesterone so transferred would be proportional to the number of preovulatory follicles, and thus to the number of eggs to be shed, and would act progressively to reduce sperm binding to the endosalpinx of the caudal isthmus. Differential timing of the release from epithelial binding may be a crucial means of achieving the initial low sperm/egg ratios. a secondary regulation of the release of graded numbers of viable spermatozoa towards the ampullary-isthmic junction of the fallopian tubes would be by means of molecular messages derived from the mucified oocyte-cumulus complex shortly before and after the time of ovulation. Third would be reorientation of sperm trajectories by molecular gradients within the cumulus cell mass to direct competent spermatozoa to those oocytes as yet unpenetrated. Together these differing levels of control would impose low sperm/egg ratios during the initial stages of fertilisation, such strict quantitative regulation of male gametes lasting at least until the block to polyspermy is fully established and the vitellus is no longer at risk from further sperm penetration.

In order to study how soon a population of spermatozoa capable of fertilisation could progress into the cervical canal, the vaginal contents were flushed with a detergent solution at different intervals after mating at the onset of oestrus. Eggs recovered 1 or 2 days later from the oviducts were stained and examined by phase-contrast microscopy. Flushing at 3 minutes after mating did not prevent fertilisation in 44% of instances, whereas similar flushes performed at 6 or 9 minutes completely prevented fertilisation in a total of 26 animals. In the groups flushed at 12 and 15 minutes, the proportion of eggs fertilised was 21% and 24%, respectively, and this increased to 37% after flushing at 18 minutes. An explanation is offered for the paradoxical finding at 3 minutes, and it is concluded that at least 30-60 minutes would be required for adequate colonisation of the cervix with a fertilising population of spermatozoa under the conditions employed in the experiment. However, multiple matings or mating closer to the time of ovulation might act to reduce this interval.

Because the exceptionally high incidence of polyspermic fertilisation has been emphasised as a major defect in systems of in vitro fertilisation in pigs, the aetiology of the condition has been analysed in a series of experiments in vivo in the search for a common underlying cause and possible means of mitigation. Whereas the defense mechanism against polyspermy in pig oocytes is classically viewed as zona reaction, more recent evidence suggests a secondary block at the vitelline surface. Both blocks may be compromised in situations leading to polyspermy, although deleterious influences seem to be expressed principally in an inadequate zona block, as judged by the presence of perivitelline spermatozoa. Postovulatory aging of mammalian oocytes prior to sperm penetration leads to polyspermy, as can be demonstrated in pig eggs. The primary lesion may concern the cortical reaction, owing to a delayed and incomplete exocytosis of the vesicular contents. Eggs ovulated after gonadotrophin treatment during the luteal phase of the cycle show a high incidence of polyspermic penetration (60.6%), as do those shed at estrus in animals treated with progesterone systemically (40%) or by local microinjections in the oviduct wall (32.3%). Whereas progesterone may be modifying interactions of the gametes and responses of the egg organelles in all four above experimental situations, enhanced numbers of spermatozoa ascending a more patent isthmus appear to be the principal cause of polyspermy.(ABSTRACT TRUNCATED AT 250 WORDS)

Whilst very large numbers of spermatozoa accumulate in the cervical mucus and crypts in the hours following coitus, evidence is offered from species with comparable gonadotrophin surge-to-ovulation intervals to suggest that the cervix is not the site of the functional sperm reservoir in women. On the contrary, human spermatozoa involved in the events of fertilization may spend the greater part of the pre-ovulatory interval in the distal portion of the Fallopian tube, that is in the intra-mural segment. This would enable spermatozoa to be arrested within reach of the site of fertilization in the ampullary region of the tube, and to be activated and released close to the time of ovulation by means of a local transfer of high concentrations of ovarian follicular hormones. The clinical importance of confirming this proposed role for the intra-mural segment of the tube is stressed, as is the possibility of specific micro-environments within the lumen of the duct.

The technique of post-coital transection of the oviducts from the uterus at various intervals after mating has been used to establish how soon a population of spermatozoa competent to fertilize the egg(s) first appears in the oviduct of the ewe. Fertilised eggs were not found as a sequel to transection at 4 or 6 hrs after mating, whereas the incidence of fertilization in a small series of ewes was 30 p. 100 following transection at 8 hrs and 100 p. 100 at 10 hrs. The mean number of spermatozoa associated with the zona pellucida at these later times increased from 2 to 13.4. The results are interpreted as indicating only a gradual progression of viable spermatozoa from the cervix to the oviduct, although the possible contribution of a rapid phase of sperm transport to the events of fertilisation is discussed. Finally, the isthmus rather than the cervix is reasoned to act as the functional sperm reservoir at the time of ovulation, with some form of peri-ovulatory programming of sperm release from this region of the tract.

The process of incorporation and metamorphosis of the sperm head within the vitellus has been examined by phase-contrast microscopy in a large series of pig eggs exhibiting either normal monospermic fertilization or extensive polyspermy. This latter condition was induced in vivo after increasing the numbers of capacitated spermatozoa in the Fallopian tubes by pre-ovulatory surgical insemination. Attention was drawn in monospermic eggs to the initial fusion between the spermatozoon and vitelline surface which produced a characteristic constriction in the equatorial region of the head. Immediately following cytoplasmic contact with the sperm nucleus, an increase in size was detectable in this structure, remnants of the inner acrosomal membrane having apparently been displaced during incorporation. In fixed preparations of activated eggs, there was some evidence that the morphologically transformed sperm nucleus underwent a brief period of shrinkage before commencing pronuclear formation. The most striking feature of the polyspermic condition was the number of spermatozoa that had entered the vitellus (2 to greater than 80), and the formation of aggregates of sperm head chromatin in eggs penetrated by more than 20-30 spermatozoa; the heads of at least 8 or 9 spermatozoa could participate in the formation of such an aggregate. Various unusual configurations were noted during breakdown of the midpiece in polyspermic eggs, and degeneration was also a general feature of the cytoplasm in situations of excessive polyspermy. Aspects of the block to polyspermy are discussed, and it is inferred that highly polyspermic pig eggs can still exhibit a zona reaction.

In an attempt to demonstrate limitations in the capacitating potential of the Fallopian tube, ejaculated boar spermatozoa were introduced directly into the isthmus at varying intervals before ovulation. The incidence and degree of polyspermy subsequently observed were taken as indicators of the population of capacitated spermatozoa confronting the newly ovulated eggs: the more extensive the condition of polyspermy, the greater the number of capacitated spermatozoa presumed to have been available at the site of fertilization. Results are based on 673 eggs from 53 animals. When suspensions containing 2.21-3.87 x 10(8) sperm per ml were introduced 36-40 hours and 26-30 hours before ovulation, 85% and 61% respectively of the eggs were polyspermic, such eggs exhibiting mainly dispermy and trispermy. By contrast, when comparable sperm suspensions from the same boar were instilled 17-18 hours before ovulation, 70% of the eggs were polyspermic but the degree of polyspermy had increased dramatically: most eggs contained 40 or more sperm heads in the vitellus, invariably forming swollen chromatin aggregates rather than male pronuclei. Surgical insemination at times closer to ovulation significantly reduced the incidence and degree of polyspermy, reaching a low of 2% with insemination 1-2 hours before ovulation. These results therefore support the concept of a limited capacitation potential of the Fallopian tube. In a separate series of observations, mating animals shortly before surgical insemination with sperm suspensions from the same boar markedly reduced the incidence of polyspermy. This latter observation may be of clinical significance in procedures of laparoscopic or transcervical insemination into the tubes to alleviate human infertility. The manner whereby myosalpingeal physiology could be modified in response to coital stimulation is discussed.