Local Action of Progesterone Leading to Polyspermic Fertilization in Pigs

Summary. Microdroplets of progesterone were injected unilaterally beneath the serosa of the tubal isthmus and uterotubal junction in thirty pigs, a total of 1 mg in 0·1 ml of oil being deposited 8 to 12 hr before ovulation. A similar volume of the vehicle was injected as a control into the contralateral tubal structures. All animals were inseminated intracervically approximately 2 hr before these surgical procedures and slaughtered between 6 hr 20 min and 14 hr 30 min after completion of ovulation. Polyspermic fertilization was found in 32·3% of the eggs recovered from the progesterone-treated tubes compared with 9·7% of those from the control tubes (P<0·01). The number of tubes yielding polyspermic eggs was also significantly higher on the progesterone-treated side (P<0·01). Injection of progesterone 8 hr before ovulation resulted in 38·2% polyspermic eggs from the treated tubes, dispermy being the principal anomaly of fertilization. In both dispermic and trispermic eggs, the female and a single male pronucleus were uniting, the accessory male element(s) remaining in the opposite hemisphere of the egg. It is concluded that relaxation of the oedematous tissues of the isthmus and uterotubal junction occurs after local injection of progesterone. In these circumstances, polyspermy is thought to originate principally from increased numbers of spermatozoa reaching the site of fertilization, although a direct effect of progesterone on the membranes of the gametes cannot be excluded. The disposition of the accessory male pronuclei in the polyspermic eggs raises the question whether polyploids usually arise from polyspermy or whether delayed cleavage and cytoplasmic fragmentation are more frequent sequelae to this condition in pigs.