THE ENDOCRINE SYSTEM 



633 



for the action of prolactin secreted by the pituitary, which stimulates 

 the flow of milk. 



257. The Hormones of Pregnancy 



The placenta, which develops in part from the extraembryonic 

 membranes of the fetus and in part from the lining of the uterus (p. 

 639), is primarily an organ for the support and nourishment of the 

 fetus. It is also an endocrine organ, which produces hormones similar 

 to those of the ovary, the adrenal cortex and the pituitary. These 

 placental hormones, together with those of the maternal endocrine 

 glands, control the many adaptations necessary for the continuation and 

 successful termination of pregnancy. 



The placenta secretes a protein hormone, chorionic gonaclotropin, 

 which is produced by the cells of the chorionic villi. Its effects are 

 similar to, yet distinct from, those of the pituitary gonadotropins. It is 

 known that the placenta secretes this, and does not merely accumulate 

 a hormone made elsewhere, for bits of placenta grown in tissue culture 

 produce the hormone. One of the earliest signs of pregnancy is the 

 appearance of this hormone in the blood and urine. The peak of 

 chorionic gonadotropin production is reached in the second month of 

 pregnancy, after which the amount in blood and urine decreases to low 

 levels (Fig. 30.14). Several pregnancy tests involve the effect of this 

 gonadotropin, obtained from a sample of urine from the woman to be 

 tested, on sperm release in the frog or African toad or on the production 

 of corpora lutea in rats or rabbits. These tests are quite accurate and 

 make possible a diagnosis of pregnancy within a few weeks of concep- 

 tion. Chorionic gonadotropin stimulates the corpus luteum to remain 

 functional and not regress as it would in the absence of pregnancy. 



The placenta also secretes estrogens and progesterone which rein- 

 force the ovarian hormones in the maintenance of pregnancy. There is 

 good evidence that the placenta actually produces these hormones and 

 does not accumulate them from the blood. There is a considerable body 



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iree/CS OF PREeitANCY 



35 



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Figure 30.14. Hormone levels in blood and urine during pregnancy. 



