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In recent years, the progress of the studies on the progestins has been remarkable and many kinds of progestins were put on the market . Their roles in gynecology and obstetrics have become important on the development of progestins , and numerous reports have appeared concerning their effects on mammalian gestation period . But only few reports have been made concerning the mammalian placenta . In our department, studies on the progestins have been done but many questions still remain fundamentally and clinically . The following studies were carried out in order to observe the morphological changes of the rat placenta and pregnancy maintenance ability in ovariectomized rat after admi nistration of several progestins. For many years , the placentas in many kinds of animals were observed anatomically and physiologically by many investigators . In these experiments, rat placentas were chosen . In rats, abortion occurred in almost all cases by ovariectomy . The histochemical and histological changes of succinic dehydrogenase were investigated during abortion . The pregnant rats were divided into 2 groups , according to the day of pregnancy . They were sacrificed and autopsied on the 1st, 2nd and 3rd day, respectively, after ovariectomy . The results of this study are as follows : All fetuses died within 48 to 52 hours after ovariectomy on the 13th , 14th and 15th day of pregnancy. Progestin could prolong the duration of pregnancy , to some extent. Its activity was remarked in the late pregnancy , but in early pregnancy it was not so remarkable. This activity of pregnane derivatives was stronger than that of estran e derivatives. It was difficult to observe the morphological changes of rat placenta . The authors observed the blood vessel system in labyrinth chiefly . Twenty four hours after ovariectomy, the morphological changes of the placenta were very slight only in blood vessels of the labyrinth. After 48 hours , its changes became remarkable. Individual cells of labyrinth and spongy zone were strongly atrophic and there were bleeding areas in the

Histological Study on the Rat Placenta with Special Reference to the Effects of the Administration of Synthetic Progestins