A Corpus Luteum-Stimulating Substance in the Rat Placenta. ∗

Abstract

In the majority of mammals the length of the pregnancy cycle exceeds that of the normal estrous or pseudopregnant cycles, and this prolongation is due in part, to a sustained function of the corpora lutea. The presence of foetal elements is in some way responsible for this continued activity of the maternal ovary.

The function of the corpora lutea of the rat during the first half of pregnancy is under the control of the hypophysis and the mechanism involved is apparently the same as that in pseudopregnancy. Luteal function is essential throughout pregnancy, however, as shown by the fact that bilateral öophorectomy invariably terminates gestation.^1-4 Removal of the pituitary gland during the first half of pregnancy is likewise followed by death and resorption of the foetuses, whereas pregnancy continues if hypophysectomy is performed after the tenth or eleventh day. 5 , 6 In the rat hypophysectomy results in a persistence of existing corpora lutea, but such persisting corpora are non-functional in the non-pregnant animal. 7 From these considerations it is apparent that some extra-pituitary factor is responsible for the luteal function of late pregnancy.

That the uterus itself, the maternal placenta or the “metrial gland” are not concerned in this mechanism is indicated by the fact that the presence of numerous large deciduomata in the uterus fails to prolong the length of pseudopregnancy. Any influence in this regard of uterine distension or of uterine hormones is thus ruled out. Furthermore, the controlling factor is not resident in the foetuses themselves, for removal of all the foetuses by Caesarian section does not shorten the pregnancy cycle if the placentæ are left intact. 8

Attempts to demonstrate a gonadotropic substance in the placenæ of animals other than those of certain primates and equidæ have thus far been unsuccessful, but the existence of some corpus luteum-stimulating substance of extra-pituitary source during pregnancy has been suspected by nunierous investigators. These failures may be attributed to the type of tests used for the demonstration of a corpus luteum-stimulating principle.