THE ENDOCRINE SYSTEM 631 



tilization takes place it regresses after about two weeks to a small patch 

 of whitish scar tissue, the corpus albicans. 



Histochemical evidence indicates that the thecal cells are the source 

 of estrogen and that these plus the granulosa cells of the corpus luteum 

 are the source of progesterone. The primary estrogen is probably es- 

 tradiol; other estrogens such as estrone and estriol may be metabolites 

 of estradiol. Estradiol stimulates the changes which occur at sexual ma- 

 turity: the growth of the accessory sex organs, uterus and vagina, the 

 development of the breasts, changes in skeletal structure such as the 

 broadening of the pelvis, the change in voice quality, the growth of 

 pubic hair and the onset of the menstrual cycle. Progesterone together 

 with estradiol is required for the growth of the uterine lining in each 

 menstrual cycle to the stage at which implantation of the fertilized egg 

 is possible. It is also necessary for the maintenance of the developing 

 embryo in the uterus. Progesterone along with estradiol causes develop- 

 ment of the breasts during pregnancy. 



Progesterone is related chemically to the adrenal cortical hormones 

 and is believed to be an intermediate in their synthesis, as well as an 

 intermediate in the synthesis of estradiol and testosterone. 



256. Estrous and Menstrual Cycles 



The females of most mammalian species show cyclic periods of the 

 sex urge and will permit copulation only at certain times, known as 

 periods of estrus or "heat," when conditions are optimal for the union of 

 egg and sperm. Most wild animals have one estrous period a year, the 

 dog and cat have two, and rats and mice have estrous periods every five 

 days. Estrus is characterized by heightened sex urge, ovulation, and 

 changes in the lining of the uterus and vagina. The uterine lining 

 thickens, and its glands and blood vessels develop to provide optimal 

 conditions for implantation. 



The menstrual cycle of the primates is characterized not by periods 

 of mating urge, but by periods of bleeding caused by the degeneration 

 and sloughing of the uterine lining. Ovulation occurs about midway 

 between two successive menstruations, or periods of bleeding. Primates, 

 unlike other mammals, permit copulation at any time in the menstrual 



cycle. 



The menstrual cycle is controlled by the interaction of ovarian and 

 pituitary hormones, and includes events in the ovary, uterus and vagina. 

 One menstrual cycle, from the beginning of one period of bleeding to 

 the next, lasts 28 to 30 days in the human female (Fig. 30.13). 



The lining of the uterus is almost completely sloughed off at each 

 menstruation and thus is thinnest just after the menstrual flow. At that 

 time, under the influence of FSH from the pituitary, one or more of 

 the follicles in the ovary begin to giow rapidly. The follicular cells 

 produce estradiol, which stimulates the growth of the uterme Immg 

 (the endometrium), and some growth of the uterine glands and blood 

 vessels. The rupture of the follicle in ovulation does not occur auto- 

 matically when a certain size is reached, but is induced by the proper 



