The Endocrine Glands - 401 



The Menstrual Cycle. In man and the an- 

 thropoid apes, the period of estrus is not 

 distinct, and the situation is complicated by 

 the occurrence of menstruation. However, the 

 menstrual cycle is likewise dominated partly 

 by the follicular hormone, estrogen, and 

 partly by the luteal hormone, progesterone. 



The average menstrual cycle endures for 

 a total period of 28 days during which only 

 one Graafian follicle, usually, ripens and 

 ovulates. Starting at the time when the men- 

 strual flow has just begun, a new follicle 

 commences to develop. This new follicle 

 reaches maturity usually on the twelfth to 

 sixteenth day, and this is the time when ovu- 

 lation normally occurs (Fig. 22-2). 



The follicle continues to increase its pro- 

 duction of estrogen as it matures, passing the 

 increased amounts of the hormones into the 

 blood stream from the accumulating follic- 

 ular fluid, until the follicle ruptures and 

 liberates the egg. 



In addition to stimulating the growth of 

 the mammary glands, uterus, and vagina at 

 the time of puberty, estrogen exerts a cyclic 

 (monthly) influence upon the growth of the 

 mucosal lining of the uterus. Each month 

 as a result of menstruation, the uterine mu- 

 cosa is virtually destroyed; and in fact, the 

 menstrual discharge consists of mucosal de- 

 bris, together with some blood that escapes 

 from the subjacent capillaries in the uterine 

 wall. Thus an important function of estro- 

 gen is to stimulate the initial stages of re- 



generation and reconstruction in the internal 

 lining immediately after menstruation. How- 

 ever, estrogen alone is inadequate to com- 

 plete the reconstruction of the uterine 

 mucosa; without the subsequent action of 

 progesterone, an implantation of the embryo 

 cannot occur, and pregnancy cannot take 

 place. 



Following ovulation (Fig. 22-2), the corpus 

 luteum develops, usurping the region for- 

 merly occupied by the follicle, and from 

 about the sixteenth to the twenty-third day 

 of the menstrual cycle, the luteal hormone, 

 progesterone, is produced in increasing 

 amounts. 



Progesterone exerts a very important influ- 

 ence upon the structure and activity of the 

 female reproductive organs: (1) it inhibits 

 the ripening of new Graafian follicles, and 

 consequently further ovulations do not occur 

 while the corpus luteum is active; (2) it 

 stimulates a continued growth of the uterus 

 and perfects the reconstruction of the uterine 

 mucosa, which now becomes highly vascular 

 and capable of effecting an implantation of 

 the embryo; (3) it inhibits muscular con- 

 tractions in the uterine wall; and (4) it aug- 

 ments the growth of the mammary glands. 



Obviously all these effects of progesterone 

 are preparations for the event of pregnancy; 

 and in fact, if pregnancy does occur, the cor- 

 pus luteum persists during the first five 

 months of the gestational period. However, 

 if the egg is not fertilized, the corpus luteum 



DAYS 26 28 1 



28 1 2 3 DAYS 



OVULATION 



Fig. 22-2. The menstrual cycle in relation to the ovarian hormones. 



