53^ 



PHYSIOLOGY OF GONADS 



less than 1.0 mg. progesterone would be in- 

 hibited by the periodic rise in estrogen secre- 

 tion. 



Evidence that some rats, but not all, ac- 

 tually experience low-grade corpus luteum 

 activity during the short cycle was furnished 

 by Everett (1945). In the comparison of 

 ovaries from females of two strains of rats, 

 it was noted in the supposedly normal Van- 

 derbilt strain that on the two days im- 

 mediately following ovulation the corpora 

 lutea of the next youngest generation con- 

 tained a great quantity of cholesterol, giv- 

 ing a strong Schultz reaction. By contrast, 

 comparable corpora lutea of the DA strain 

 were usually free of visible lipid in Sudan 

 preparations or the Schultz test. Adminis- 

 tration of small amounts of lactogen (luteo- 

 trophin I during the cycle preceding the 

 current one, amounts inadequate to cause 

 l)seudopregnancy, resulted in the rich depo- 

 sition of cholesterol in these otherwise lipid- 

 free corpora lutea. The conclusion was 

 reached that the corpora lutea of the Van- 

 derbilt rat must be slightly active during the 

 short cycle and those of the DA rat less so, if 

 at all. This would easily explain the relative 

 indifference of the Vanderbilt rat to contin- 

 uous light and the ease with which persistent 

 cstrus could be induced in the DA rat by 

 such treatment (Everett, 1942a, b). In fact, 

 the low dosages of lactogen mentioned sub- 

 stituted for progesterone treatment in main- 

 taining regular cycles in persistent-estrous 

 rats of the DA strain (Everett, 1944b). Sig- 

 nificantly, the treatment was effective in 

 only those animals in which a set of corpora 

 lutea had been induced by other means at 

 the beginning of the experiment. 



To be correlated with the above indica- 

 tions of low-grade function during tlie short 

 cycle, is the finding that corpora lutea of 

 the Vanderl)ilt rat retain full responsiveness 

 to luteotrophin throughout most of the di- 

 estrous interval (Nikitovitch-Winer and 

 Everett, 1958a). Responsiveness diminishes 

 near the onset of i:)roestrum. Once the rat 

 has entered proestrum these older corpora 

 lutea are not capable of sustained function. 

 The loss is not a function of time per se, but 

 of stage of the cycle. 



.\. I'.^KTI)OPRE(;\.\XrY 



The terms coi-pu,^ hitcuni of ()^•ulation and 

 corpus luteum of pscudojji'cgnancy are com- 



monly u.sed to differentiate the luteal bodies 

 occurring during the normal cycles from 

 those found during some unusually long 

 period of luteal activity. However, the 

 terms deny the fundamental similarity of 

 the luteal phase in the cycles of such ani- 

 mals as the guinea pig and the luteal phase 

 induced by sterile mating or its equivalent 

 in animals like the rat. In the unmated bitch 

 the spontaneous luteal phase of the cycle is 

 commonly called pseudopregnancy, yet it 

 is equally common to say that the guinea 

 pig does not experience pseudopregnancy. 

 The truth is that the luteal phase of the 

 canine cycle is simply longer than the luteal 

 phase in the guinea pig and may be marked 

 by a period of lactation near its close. In 

 the present discussion, the expression pseu- 

 dopregnancy will be equivalent to saying 

 the luteal phase of the infertile cycle. Under 

 experimental conditions it will refer to any 

 period of sustained luteal function similar 

 to that of the normal progestational state. 

 Wherever appropriate, the distinction will 

 be made between a pseudopregnancy that is 

 spontaneous and one that is induced. 



In most of the familiar animals that ovu- 

 late spontanously corpus luteum function 

 also begins spontaneously and continues for 

 at least several days after ovulation. With 

 respect to the rabbit, cat, and ferret, it is 

 often said that pseudopregnancy is invoked 

 by sterile copulation, whereas strictly speak- 

 ing it is only ovulation and corpus luteum 

 formation which are invoked. The pseudo- 

 pregnancy then follows automatically. This 

 interpretation seems appropriate, inasmuch 

 as in all three species the formation of 

 corpora lutea by l)rief treatment with hy- 

 pophyseal or chorionic gonadotrophin is fol- 

 lowed by long periods of progesterone secre- 

 tion which can hardly be the direct effect 

 of the injected substances (Hill and Parkes, 

 1930a, b; Foster and Hisaw, 1935; van 

 Dyke and Li, 1938). Quite different is the 

 pseudopregnancy of the rat, mouse, and 

 hauistei', in \\liich progestational activity is 

 invoked by stinudation of the cervix uteri, 

 l^verett (1952a) described the ex])eriiuental 

 dissociation in rats of the o^•^llati()n and 

 luteotr()])hic mechanisms. respectively. 

 When o\-uhition is blocked (by pentobarbi- 

 tal) in the cycle dui'ing which controlled 

 uiatinu occurs, pseudopi-egnancy begins 



