GESTATION 



971 



10 

 9 



J' 



w> 7 

 E 6 



BO -* 



o 



3 

 2 



— p o 



Ovariectomy during pregnancy 



_ ] 



— k-(ovariectonny) 



1 - 



114 



125 



Time (days) 



Fig. 16.10. The effect of castration on gestagen levels in the pregnant ewe. Gestagen 

 le\els were obtained by the Hooker-Forbes test. Note that castration failed to interfere 

 with the pregnancy or the level of gestagen in the blood. (From G. M. Neher and M. X. 

 Zarrow, J. Endocrinol., 11, 323, 1954.) 



it might be concluded that here also the 

 placenta has successfully replaced the 

 ovary. In the rabbit, on the other hand, 

 castration at any time during pregnancy 

 vvill cause a decrease in the level of the 

 circulating hormone and terminate the 

 pregnancy. Hence, in this species, the pla- 

 centa has failed to replace completely the 

 ovary. The mouse is another instance in 

 which castration leads to abortion so that 

 one can assume a failure on the part of the 

 placenta to replace the endocrine activity of 

 the ovary. In this case, however, the second 

 peak of circulating gestagen has been as- 

 cribed to the placenta and this presents the 

 possibility of a partial replacement of the 

 ovary by the placenta but a replacement 

 that is not adequate since pregnancy is 

 terminated by ovariectomy. 



As indicated above, a marked discrep- 

 ancy exists between the bioassays and the 

 chemical determinations of gestagens in 

 the blood and other tissues. The chemical 

 determinations of progesterone invariably 

 give results that are far lower than those 

 obtained by bioassay methods. Edgar and 

 Ronaldson (1958) found a maximal con- 

 centration of approximately 2 /xg. proges- 

 terone per ml. ovarian venous blood during 



gestation in the ewe. This concentration 

 was no higher than that seen in the ewe 

 during a normal estrous cycle. The maxi- 

 mal level reached during the estrous cycle 

 was maintained when pregnancy super- 

 vened and remained fairly constant until 

 the last month of pregnancy. Thereafter 

 the concentration fell and no progesterone 

 was detectable at 15 days prepartum (Fig. 

 16.11). Inasmuch as no progesterone was 

 found in the peripheral blood of the ewe, 

 this poses again the following question: 

 What was being measured in the peripheral 

 blood by the bioassay procedure? In addi- 

 tion, a second question is posed by the 

 earlier discussion on the need of the ovary 

 in the maintenance of pregnancy as to the 

 relative contributions of the ovary and the 

 placenta to the concentration of this hor- 

 mone in the body. 



That the biologic methods are measuring 

 more than progesterone is obvious from the 

 many reports emphasizing the high levels 

 obtained by bioassay and the low levels 

 obtained by chemical techniques. In addi- 

 tion to the above data. Short (1957, 1958a, 

 1958b) reported the presence of progester- 

 one in the peripheral blood of the pregnant 

 cow but onlv in the order of 0.0074 to 



