THE UTERINE CYCLE 259 



The uterine cycle 



We can divide these changes into two major processes. The first is a 

 growth of the glandular portion of the uterine wall during the period just 

 preceding ovulation. The wall undergoes some thickening, and the walls of 

 the vagina shed a layer of epithelial cells. In man this is the preovulatory 

 period. In lower mammals the female becomes receptive to the male toward 

 the end of this period and the condition is termed estrus. Ovulation occurs at 

 the end of the preovulatory period in man and at estrus in lower mammals. 

 Following ovulation is a second period during which the uterine wall thickens 

 as a result of secretion and some growth of the glands. This is known as a 

 postovulatory period and also as the progestational stage of the uterus. If the 

 egg is not fertilized the uterine wall breaks down and menstruation results. 

 The cycle of preovulatory growth, postovulatory growth and secretion, and 

 menstruation constitutes the menstrual cycle in the human. In lower mam- 

 mals a similar cycle of growth and regression of the uterine wall is termed 

 the estrous cycle. We may use the general term uterine cycle to include both 

 the menstrual and estrous cycles. 



Control of the cycles 



The over-all control of the ovarian and the uterine cycles is quite clear, 

 since, if the pituitary body is removed, both cycles stop. Under these condi- 

 tions no ovulation takes place; no growth of the uterine wall is observed. 

 The pituitary does not act directly on the uterine cycle, for if the pituitary is 

 left intact but the ovaries are removed, the uterine cycle will stop. The 

 sequence of events, then, is, first, the pituitary controls the changes in the 

 ovary and, second, the changes in the ovary control those in the uterus. 



How is this control brought about? Extracts of the anterior lobe of the 

 pituitary gland yield two hormones, which can be separated by chemical 

 means and which differ in their effects on the ovary. One of these hormones 

 (FSH, follicle-stimulating hormone) acts on the growth of the follicle (Fig. 

 169, 1). If the pituitary of an animal is removed, the follicles in the ovary all 

 remain small. If then FSH is injected into the animal, a tremendous growth 

 of the follicles results. However, no corpora lutea are formed. There is 

 simply a growth of the follicles. 



A second extract, with different chemical properties, controls the growth 



