558 



PHYSIOLOGY OF GONADS 



is reproduction wliicli obviously cannot l)0 

 fulfilled unless an ovum is made available 

 for fertilization. Therefore, in this sense it 

 seems quite clear that anovulatory cycles 

 should be considered incomplete and ab- 

 normal. 



The investigation of changes taking place 

 in the uterine endometrium at various pe- 

 riods of the menstrual cycle in women was 

 confronted with many difficulties, the chief 

 one being that of obtaining normal tissue 

 representative of specific times of the cycle. 

 The entire uterus and both ovaries are es- 

 sential for proper evaluations and it was 

 rarely possible to meet these requirements. 

 The material for such studies came from 

 autopsies and surgery and tissues usually 

 had suffered postmortem changes or the 

 surgical condition was one involving serious 

 pelvic disease. There have been, however, 

 a goodly number of instances in which 

 these difficulties were adequately overcome 

 (Stieve, 1926, 1942, 1943, 1944; Allen, Pratt, 

 Newell and Bland, 1930) and the clinic will 

 continue to make important contributions 

 (Rock and Hertig, 1942; Hertig and Rock, 

 1944), but quite early the need became ob- 

 vious for a suitable primate that could be 

 used as an experimental animal for research 

 on the different aspects of the physiology 

 of reproduction. 



Since the initial observations by Corner 

 (1923) on the menstrual cycles of captive 

 rhesus monkeys iMacaca mulatta) , more 

 has been learned about the physiology of 

 reproduction of this animal than any other 

 primate. Monkeys of this species thrive un- 

 der laboratory conditions, which has made 

 it possible to devise accurately controlled 

 experiments on normal healthy animals and 

 obtain reliable information on the men- 

 strual cycle, gestation, fetal development, 

 and the interaction of hormones concerned 

 with regulating reproductive processes. 



Other features that make the rhesus mon- 

 key such an attractive animal for these pur- 

 poses are the many morphologic and physi- 

 ologic attributes that are strikingly like 

 those of the human being. Tiic modal length 

 of their menstrual cycles is 28 days but 

 there is wide variation (Corner, 1923; Hart- 

 man, 1932; Zuckcrman, 1937a). From an 

 analysis of 1000 cycles recorded for some 

 80 females of different ages, Zuckerman 



(1937a) found an average cycle length of 

 33.5 ± 0.6 days, and the mode 28 days with 

 an over-all range of 9 to 200 days. Ovulation 

 occtu's api:)roximately midway between two 

 menstrual periods, most between tlie 11th 

 and 14th days (Hartman, 1932, 1944; van 

 Wagenen, 1945, 1947), and although these 

 animals breed at all seasons of the year 

 many cycles are anovulatory, especially 

 during the hot summer months (Eckstein 

 and Zuckerman, 1956). A method developed 

 by Hartman for detecting the exact time of 

 ovulation by palpation of the ovaries in the 

 unanesthetized animal greatly facilitated 

 the timing of events of the menstrual cycle. 

 This procedure also made it possible to de- 

 termine the age of corpora lutea with great 

 accuracy (Corner, 1942, 1945) and corre- 

 late their develojiment and involution with 

 corresponding changes in the endometrium 

 (Bartelmez, 1951 ) and, in ju-egnancy, with 

 the exact age of developing embryos ( Wis- 

 locki and Streeter, 1938; Heuser and 

 Streeter, 1941). 



The primary purpose of the present dis- 

 cussion is to review the results of experi- 

 mental investigations of physiologic proc- 

 esses occurring in the female reproductive 

 tract of lower primates during the menstrual 

 cycle, and particularly those processes that 

 are under hormonal control. The brief in- 

 troductory presentation of basic observa- 

 tions could be greatly extended and we take 

 up the discussion of endocrine problems 

 knowing that we must return often to the 

 work of these authors and that of others to 

 be cited, as conclusions based on experi- 

 mental data take on meaning only in terms 

 of normal function. 



II. Ovarian Hormones and Growth 

 of the Genital Tract 



The changes that are repeated in differ- 

 ent parts of the reproductive tract with each 

 menstrual cycle are produced by ovarian 

 hormones, estrogens, and progesterone. The 

 dominant hormone of the follicular phase is 

 estradiol-17/y, which is secreted by the 

 Graafian follicle, and in the tissues is read- 

 ily transformed in i)art to estrone, an estro- 

 genic metabolite. Progesterone, secreted by 

 the corinis luteum, is jirimarily a hormone 

 of the luteal phase of the cycle. However, 

 small amounts of progesterone may appear 



