Structure of the vaginal plugs generated by normal rats and by rats with partially removed seminal vesicles

Normal male rats generate vaginal plugs that appear to be firmly apposed to the vagino-cervical junction and permit a large number of spermatozoa to reach the uterus. A few spermatozoa form entangled masses inside these plugs, as revealed by light microscopy. Males in which the seminal vesicles have been partially removed produce plugs that are smaller and softer than those generated by normal males, and the plugs display a cup-like structure at the proximal end. The cup-like structure is completely filled with spermatozoa that exhibit a characteristic arrangement in relation to the plug material. In this situation, the number of spermatozoa that reach the uterus is very much reduced. Experiments were also carried out to explore the restoration of sperm transport by addition of a vaginal plug. Such experiments involved successive matings of individual females with a seminal vesicle-deprived male and with a vasectomized male (which generated the plug) and also the intravaginal injection of seminal vesicle secretions after mating with a seminal vesicle-deprived male. In none of the experimental situations was transport of spermatozoa to the uterus restored, and the plug consisted of a large quantity of trapped spermatozoa inside a mass of coagulated proteins. The results suggest that the structure of the plug depends on the amount of seminal vesicle secretion present in the ejaculate and that the vaginal plug must be formed immediately after deposition of the sperm if spermatozoa are to reach the uterus.