RESPONSIVENESS IN VERTEBRATES 

 Ad*>nohypr>physie 



Uterus 



(preparation to retain and nourish 



embryo) 



Mammary gland 



(preparation to nourish 



newborn) 



Fig. 4.26. Hormonal regulation of reproduction in manunals. 



as an estrogen is released, and the cells of the mature follicle become luteinized 

 and form a corpus luteum after ovulation occurs. The luteinized cells respond 

 to the luteotropic hormone and secrete progesterone. As soon as estrogen 

 reaches a certain level in the blood, the adenohypophysis is depres.sed by it 

 with respect to the release of FSH. This means that the stimulus to growth of 

 follicles and subsequent formation of corpora lutea is cyclical. 



Estrogen and progesterone cooperate in the synchronization of events lead- 

 ing to fertilization, pregnancy, parturition, and the nouri.shment of the new- 

 born. In placental mammals, other than primates (p. 579), there are 

 p^rifxiic inienals during which the female will copulate with the male. The 

 modification of behavior pattern which makes fertilization possible is regu- 

 lated by the response of the central nervous system to estrogen at the time 

 of ovulation. Under the influence first of estrogen, followed by progesterone. 

 the blood supply of the uterus is greatly increased. Its epithelial lining and 

 muscular wall (see Fig. 5.2, p. 130) become much thicker and the uterine 

 glands differentiate. By the time the fertilized egg has traversed the Fal- 

 lopian tube to reach the uterus, this organ has responded to the sequential 

 influences of estrogen and progesterone. Its wall is ready for implantation of 

 the embrv'o and differentiation of the placenta (p. 163). If fertilization 

 occurs, the hypophysis continues to secrete luteotropin and so the corpus 

 luteum is maintained as an endocrine organ. In most mammals the main- 



123 



