Capacitation of boar spermatozoa: Synergism between uterine and tubal environments

The influence of the site of semen deposition on the rate of sperm capacitation was studied in 36 oestrous gilts in which ovulation had been induced at a precise time. Aliquots of semen were instilled into the proximal uterine horn on one side of the tract and into the isthmus of the contralateral Fallopian tube in 24 animals. Eggs were recovered in vivo, and examined by phase‐contrast microscopy for the number of spermatozoa associated with the zona pellucida and the stage of fertilization. A total of 420 eggs was examined. The mean number of spermatozoa per egg increased from 3.3 at four hours after uterine insemination to 27.9 at six hours, compared with 0.6 to 6.3 at corresponding intervals after tubal insemination (p<0.05). The mean proportion of fertilized eggs increased from 54.3% at four hours to 95.9% at six hours after uterine insemination, compared with 0.0 to 51.0% at these intervals after tubal insemination (p<0.001). The temporal advantage accruing from uterine insemination was also seen in the developmental stages of the penetrated eggs. In a further six animals in which seminal plasma was introduced into the isthmus at the time of uterine insemination, fertilization was retarded by approximately two hours, although when a similar volume of physiological medium was introduced into the isthmus as a control, no such delay was observed. The advantage of uterine over tubal insemination was attributed to a synergistic effect of the uterus and Fallopian tubes on those spermatozoa that penetrated the eggs. The nature of the membrane‐related changes in the spermatozoon initiated during passage through the uterus and utero‐tubal junction that facilitate rapid capacitation remains unknown, but is thought to involve modification or removal of seminal plasma constituents from the sperm surface. It was inferred that seminal plasma does not enter the Fallopian tubes of oestrous pigs in significant amounts after mating.