MAMMALIAN OVARY 



461 



chapter, in pseudopregnancy tlie hormonal 

 aspects of pregnancy are duplicated, but no 

 fetal tissues are present. In the pseudopreg- 

 nant bitch, for example, the hormonal as- 

 pects of pregnancy are so nearly duplicated 

 that lactation begins at the time a normal 

 gestation would have terminated. 



The duration of pseudopregnancy in dif- 

 ferent species offers evidence of adaptive or 

 evolutionary mechanisms to control the du- 

 ration of corpus luteum function, mecha- 

 nisms that must be endogenous to the uterus. 

 Rats and mice have a pseudopregnancy of 

 12 days duration after a sterile mating, cer- 

 vical stimulation, or injection of estrogen at 

 estrus. There is no comparable condition in 

 guinea pigs, monkeys, or man. However, if 

 rabbits, rats, or guinea pigs are hysterecto- 

 mized, any subsequent corpora lutea will 

 function for a time equivalent to the dura- 

 tion of gestation in each species (Chu, Lee 

 and You, 1946; Bradbury, Brown and Gray, 

 1950), although Velardo, Olsen, Hisaw and 

 Dawson (1953) stated that, in the rat, hys- 

 terectomy has no effect on the length of 

 pseudopregnancy. Hysterectomy in the cow 

 and sow will prolong the life of the corpus 

 luteum (Melampy, personal communica- 

 tion). Experimental distention of the uterus 

 l)y beads has resulted in alteration of the 

 length of the estrous cycle in ewes (Nal- 

 l)andov, Moore and Norton, 1955). The only 

 explanation which seems to account for 

 these results is that there is a luteolytic 

 agent in the uterus (probably in the endo- 

 metrium) of some polyestrous species which 

 shortens the life of the corpora lutea in non- 

 pregnant animals. In pregnancy, or when 

 massive deciduomas are present, if Velardo, 

 Olsen, Hisaw and Dawson are correct, the 

 conversion of endometrium to decidual tis- 

 sue may cause it to lose its luteolytic ability. 

 In future studies on the duration of the 

 functional span of cor]5ora lutea, the possi- 

 bility of luteotrophic and luteolytic mecha- 

 nisms should be considered. On the other 

 hand, a fresh start may be advisable. Few- 

 problems in reproductive and clinical endo- 

 crinology (Marx, 1935) seem to have been 

 as resistant to clarification. 



In unmated females of species not having 

 a spontaneous "pseudopregnancy," the cor- 

 lius luteum involutes shortly after its forma- 

 tion. The rat, in which 4 to 8 corpora lutea 



are formed in each ovary at intervals of 4 

 to 5 days, has recognizable involuting cor- 

 pora lutea from the two preceding cycles, 

 but no remnants of older ones. The early 

 stages of involution of the corpus luteum 

 have been described (Brewer, 1942; Boling, 

 1942; Dawson, 1946; Duke, 1949; Moss, 

 Wrenn and Sykes, 1954; Corner, Jr., 1956; 

 Rowlands, 1956; Dickie, Atkinson and Fe- 

 kete, 1957). The timetables of cellular 

 changes given by Brewer and by Corner, Jr. 

 are of interest for the comparison they per- 

 mit with physiologic estimates of the dura- 

 tion of secretory activity by the human cor- 

 pus luteum. On day 7 the corpus luteum 

 seems to have reached its peak of activity, 

 as judged by the vacuolation of its cells in 

 Bouin's or Zenker's fluid-fixed and hematox- 

 ylin and eosin-stained preparations. Corpora 

 lutea of days 9 to 12 show evidence of i)ro- 

 gressive secretory exhaustion. 



The later stages of cori:)ora lutea degenera- 

 tion have not received the same careful at- 

 tention. In women the corpus luteum under- 

 goes a slow hyaline degeneration and the 

 corpora albicantia persist as old scars for 

 months or years. They may be present in 

 ovaries 15 to 20 years after the menopause. 

 Whether the final stage of degeneration is a 

 process of lysis, phagocytosis, or transfor- 

 mation into connective tissue has not been 

 studied. 



IV. Follicular Atresia 



It was long ago estimated that the infan- 

 tile liuman ovary contains about 400,000 

 oocytes (Fig. 7.4). In the 30 years of repro- 

 ductive life about 400 ova may mature and 

 ovulate. On this basis about 1 oocyte in 1000 

 achieves ovulation; the other 999 are lost 

 through a degenerative process known as 

 atresia. The problem is not different in any 

 other species. Whether it is monotocous or 

 polytocous, there is always an enormous 

 wastage of oocytes in each cycle of follicu- 

 logenesis. Atresia may have its onset at any 

 stage of follicular growth or maturation and 

 oocytes may degenerate before they have 

 acquired a distinct membrana granulosa 

 (Mandl and Zuckerman. 1950; de Wit, 1953; 

 Payne, Hellbaum and Owens, 1956; Wil- 

 liams, 1956). In advanced stages of follicu- 

 lar development the granulosa cells may 

 show pycnotic changes before any degenera- 



