On the parturition of the experimentally induced superimplanting rat

Abstract

In order to elucidate the mechanism of the initiation of parturition, it seems very suggestive to investigate how the parturition takes place in superimplanting rats.Superimplanting rats were prepared, mostly based on the method of Yoshinaga2), as follows ; into the adipose tissue of mesometrium of a lactating pregnant rat with 6 pups, a small amount of estradiol was locally administered on the 5 th day of the pregnancy and pups were removed on the 9 th day. The mother was laparotomized on the 20 th day, and the fetuses were distinguished into the younger and the elder one(s) by the size of uterine swellings, and the number and location were recorded. The mother was continuously observed for the parturition thereafer.In spite of the above mentioned treatment, there were cases in which only the elder or the younger fetus(es) was conceived. Judging from the day of parturition (shown at the upper column of Fig. 1) and the body weight in these cases, it was assumed that the implantation of the elder was about 5 day earlier than that of the younger.Two types of parturition were observed in these superimplanting rats. The first; the elder(s) was delivered alive and at the same time the younger(s) was born in premature state. The second; the youngers were delivered normally at full term, and the elder(s) delayed in parturition and was dead in uterus or delivered dead together with the youngers. In other words, the parturition took place only once but never twice for the elder(s) and for the younger(s) with a few days interval.Csapo1) has stated that the activity of the uterus of a pregnant animal is supressed more by proge-stin locally secreted from a placenta than by that in systemic circulation, and has concluded that the activity of the pregnant uterus is controlled by the ratio of the uterine volume and the local concen-tration of progestin in uterine tissue. If his theory applied on our experiment, the parturition should have independently taken place at the uterine horn having the youngers only, whether the elder(s) in the other horn was delivered or not. In such cases, however, when the elders were delivered, the parturition always occurred simultaneously to the contralateral horn in which only the youngers were implanted.The number of the elders exceeding three, the first type of parturition mostly took place, although the day of parturition always delayed for about two days, and the second type mostly took place when the number was one. When the number was two, the both types occurred, and it seemed to have a close relation with the number of the youngers whether the first or the second type took place. From these facts, though very complex mechanism was anticipated for the induction of parturition, it is, assumed that the uterine volume, as Csapo suggested, may be one of the important factors which bring about parturition.