MAMMALIAN REPRODUCTIVE CYCLE 



533 



Fig. 8.12. Experimental dissociation in rats of the ovulation mechanism and that causing 

 pseudopregnancy. A. Control cycles for comparison with B and C . Points on base line 

 represent diestrum, on ascending lines proestrum, on highest level full vaginal estrum. X, 

 ovulation. B. Blockade with Nembutal {'NB) on day of proestrum and following day (see 

 Fig. 8.11E'). Ovulation during third night. C. Same basic procedure as B, but with copulation 

 during first night (M). Ovulation usually failing in this cycle (contrast with B) . Corpora 

 lutea formed after spontaneous ovulation in second cycle regularly become functional with- 

 out further stimulation : the wavy line represents pseudopregnancy. The early copulation has 

 introduced some change in the animal such that this pseudopregnancy "spontaneously" 

 follows ovulation as in the standard mammalian cycle. (From J. W. Everett, Ciba Founda- 

 tion Collofiuia Endocrinol.. 4, 172. 1952.) 



"spontaneously" a]ter the next cyclic estrus 

 (Fig. 8.12). Dissociation of the two mecha- 

 nisms is expressed in another way by cer- 

 tain Mustelidae, e.g., the mink and marten. 

 Ovulation in these forms is invoked by 

 mating, whereas corpus luteum activation 

 awaits appropriate environmental condi- 

 tions, i.e., temperature and length of daily 

 illumination (Pearson and Enders, 1944; 

 Hansson, 1947). In the mink, during the pe- 

 riod of relative luteal inactivity that fol- 

 lows mating early in the season, recurrent 

 estrus continues. If reraating takes place at 

 an interval of 6 days or more, new ovula- 

 tions are induced (Hansson, 1947). Matings 

 late in the season are immediately followed 

 by luteal activity. The pseudopregnant cy- 

 cles of a representative series of mammals 

 are much alike when conditions appropri- 

 ate to the respective species are applied 

 (Fig. 8.1). 



1. Duration of Psciidopregndncn 



The length of time that corpora lutea re- 

 main functional in the pseudopregnant cy- 

 cle is thought to be relatively uniform in 

 the great majority of mammals, usually 

 about 10 to 15 days. Rarely it is shorter, 



e.g., the hamster, 7 days, although usually 

 9 to 10 days (Asdell, 1946). At the other 

 extreme, the corpora lutea remain functional 

 for periods corresponding to the duration of 

 pregnancy, as in the ferret, 5 to 6 weeks. In 

 fact, corpus luteum function lasting over a 

 month is usual in the other two carnivores 

 for which information is at hand: cat, 30 to 

 44 days (Foster and Hisaw, 1935) ; and dog, 

 30 days or more (Evans and Cole, 1931). 



These figures are only approximations, 

 however, as the criteria on which they are 

 based differ. In the rat, in which pseudo- 

 pregnancy is said to last 12 to 14 days, its 

 termination is taken to be the onset of the 

 next estrus, whereas the corpora lutea must 

 have undergone a decline of activity 2 or 3 

 days earlier (Everett, 1948). The decline is 

 probably not abrupt, inasmuch as the vagi- 

 nal smear during the next estrus is very 

 strongly mucified and, as mentioned earlier 

 (p. 519), enough progesterone seems to be 

 secreted by the waning corpora lutea to fa- 

 cilitate ovulation. Morphologic criteria are 

 often employed as indicators of corpus lu- 

 teum regression: characteristically, fatty 

 vacuolation of luteal cells, decrease in size 

 of the individual cells or of the entire corpus 



