22 EMBRYOLOGY 



new follicle grows in size and another ovulation occurs. If pregnancy does 

 not intervene this cycle is repeated over and over. 



There are two distinct processes in this cycle: (l) a growth of the follicle, 

 and (2) the formation of the corpus luteum. These two processes are con- 

 trolled by two hormones liberated from the pituitary gland. One hormone, a 

 follicle-stimulating hormone (FSH), stimulates the growth of follicles. The 

 effect of this hormone is shown by the fact that when the pituitary gland of 

 an animal is removed the follicles in the ovary remain very small. The injection 

 of the FSH hormone into such hypophysectomized animals stimulates follicu- 

 lar growth, and numerous large follicles result. 



A second hormone from the pituitary is responsible for the formation of 

 the corpus luteum. When this luteinizing hormone (LH) is injected, it pro- 

 duces an increase in the number of lutein cells, which form the corpus luteum, 

 but it has no effect on the follicle growth itself. These two pituitary hormones 

 are proteins and one of them, LH, has been highly purified. The luteinizing 

 hormone is also known as an interstitial-cell-stimulating hormone (ICSH). 



What controls the secretion of the two pituitary hormones, FSH and 

 LH (ICSH) ? Since the corpus luteum in some animals inhibits ovulation, a 

 reasonable explanation at present is the following: Beginning with the secre- 

 tion of FSH by the pituitary, the next step is the growth of the follicle and 

 ovulation. LH then stimulates the growth of the corpus luteum, and further 

 ovulation is inhibited until the corpus luteum decreases in size and its 

 inhibiting effect is lost. The growth of the next follicle and ovulation can 

 then proceed. 



We may ask what causes the growth of the corpus luteum after ovulation. 

 A hormone from the follicle, estrone, when injected into some animals causes 

 an enlargement of the pituitary and also the formation of corpora lutea in 

 the ovary. Since the formation of a corpus luteum is the result of the action 

 of LH, it is reasonable to suppose that estrone stimulates the pituitary to 

 release LH and inhibits the formation of FSH. The whole sequence of events 

 may be summarized in the diagram on the facing page. 



When the supply of estrone decreases owing to its elimination in urine, 

 the stimulation of the secretion of LH ceases and the corpus luteum degener- 

 ates so that follicle growth is no longer inhibited, and the cycle begins anew. 

 It is interesting that during pregnancy, when the corpus luteum persists 

 because of the presence of an embryo, there is no follicle growth. Estrone 

 produced by the placenta is said to inhibit both FSH and LH secretion. But 



