MAMMALIAN REPRODUCTIVE CYCLE 



51)5 



VI. Follicle Maturation and Ovulation 



A variety of evidence indicates disconti- 

 nuity between growth of large follicles, on 

 the one hand, and their preovulatory matu- 

 ration, on the other. Such is clearly the case 

 among "reflex ovulators." Evidence that the 

 same is true for spontaneous ovulators will 

 be outlined below. Follicle maturation, ovu- 

 lation, and structural transformation of the 

 follicles to corpora lutea seem to represent 

 successive stages in a distinct physiologic 

 process, superimposed on the follicle growth 

 cycle and brought about by a relatively 

 abrupt increase in circulating gonadotrophin 

 (theoretically LH). Since there is evidence 

 (p. 519) that progesterone secretion may 

 become detectable as this process begins, 

 there might be justification for regarding it 

 as merely the first portion of the luteal 

 phase. However, the fact that luteinization 

 ( i.e., the organization per se of luteal tissue) 

 does not necessarily lead to functional cor- 

 |)ora lutea warrants treatment of the ovula- 

 tion-luteinization phase as a distinct phe- 

 nomenon. 



Although it is customary to state that the 

 hypopliysis invokes ovulation by release of 

 LH, there is considerable question about 

 the auxiliary roles played by other gonado- 

 trophic hormones (Hisaw, 1947). Inasmuch 

 as the time of release has been known in 

 only the reflex ovulators, one might look to 

 them for information. However, the avail- 

 able data (Hill, 1934) pertain only to the 

 ovulating i)otency of the total gonado- 

 trophin content of the hypophysis at various 

 times after coitus. Substitution experiments 

 are unsatisfactory because the presence of 

 competent follicles implies the presence of 

 l)oth FSH and a small amount of LH. The 

 substitution of even the purest hormone 

 preparations immediately after hypophy- 

 sectomy leads to equivocal results inasmuch 

 as it must be assumed that some FSH and 

 LH of intrinsic origin remain in circulation. 

 Talbert, Meyer and McShan (1951) deter- 

 mined that in rats, when hypophysectomy 

 is performed at the onset of proestrum, the 

 follicles remain capable of responding to in- 

 jected LH for about 6 hours. Morphologic 

 signs of follicle deterioration do not appear 

 until nuich later. Adding to the uncertainty 



is the fact that relatively pure preparations 

 of either FSH or LH will ovulate an estrous 

 rabbit (Greep, van Dyke and Chow, 1942). 

 On the other hand, until the recent use of 

 species-specific gonadotrophins (van Wage- 

 nen and Simpson, 1957), the primate ovary 

 was notoriously difficult to ovulate thera- 

 peutically. Until effluent blood from the hy- 

 pophysis can be assayed, there is little 

 likelihood that the gonadotrophin complex 

 that is normally responsible for ovulation 

 can be known. Thus, whereas the expression, 

 LH-release, will be employed occasionally 

 to refer to the release of gonadotrophin that 

 in^•okes ovulation, the term is used purely 

 for convenience and brevity, and should be 

 ai^propriately qualified by the reader. 



A. TIME OF OVUL.\TION 



The time of ovulation with respect to 

 other events of the cycle is relatively easy 

 to determine in reflex ovulators, but in spon- 

 taneous ovulators has proven to be more 

 elusive. In the former, laparotomy at vari- 

 ous intervals after the stimulus enables ex- 

 act measure to be made of the time required 

 to accomplish ovulation. For most of the 

 spontaneous ovulators, save the few in 

 which the ripening follicles can be palpated 

 as in monkeys and cattle, it has been neces- 

 sary to attempt to correlate ovulation with 

 some easily detectable external sign. Inas- 

 much as the ovulation stimulus to the hy- 

 j)oiihysis in these animals is probably in- 

 voked by ovarian hormones and these are 

 equally responsible for phenomena such as 

 vaginal cornification and behavioral estrus, 

 a considerable degree of correlation might 

 be expected between ovulation and a given 

 change in the vaginal smear or onset of es- 

 trous behavior. The predictability of the 

 relationship, however, must depend in great 

 measure on the degree of correlation among 

 thresholds of response in the various tissues 

 concerned. In the primates that have no 

 sharply limited period of sex desire the 

 i:)roblem is even more troublesome. When 

 reference is made to the date of the last 

 menstruation, prediction is erratic because 

 of the variable occurrence of postmenstrual 

 quiescence (Rossman and Bartelmez, 1943; 

 Young and Yerkes, 1943). Consequently, 

 attempts must be made to find indicato:- 



