Factors Influencing Ovulation and Atresia of Ovarian Follicles 1\1 



in healthy granulosa tissue. It is only seen in the granulosa of follicles that 

 are starting to degenerate. It is tempting to believe that this substance might 

 be mobilized as a precursor to progesterone production by thecaluteal or 

 corporaluteal tissue. Let us now trace the sequence of events that appear to 

 be validated by the evidence reviewed above. Throughout the first ten days 

 or so of the proliferative phase of the menstrual cycle in the primate, many 

 small follicles are developing and contributing in a minor degree to the slowly 

 increasing level of circulating estrogen before they are lost in the process of 

 atresia. By the eleventh or twelfth day of the cycle, perhaps a dozen or so 

 follicles achieve a major degree of development, but only one of these generally 

 proceeds with a tremendous spurt of growth towards maturation on day 

 14. A rapid increase in circulating level of estrogens is noted at this i 

 time, and coincident with this or shortly after it, there is found a first peak of 

 gonadotropins producing LH effect. In this upsurge of follicular growth, only 

 the major follicle survives and achieves maturation while all others of second 

 rank are lost within twenty-four hours of ovulation by successive stages in 

 the atretic process. One of the most striking features of this is the proliferation , 

 of theca interna which appears to coincide with the elevation in LH excretion 

 and it subsides as this LH peak flattens out by forty-eight hours after ovula- 

 tion presumably due to the inhibiting action of progesterone from the 

 developing corpus luteum. The steps that lead the mature follicle to ovulate, 

 a separation of cumulus and granulosa cells by intracellular edema, extrusion 

 of the first polar body and development of the second maturation spindle, 

 the migration of the follicle towards the cortex and actual extrusion of the 

 egg through the stoma, all have been explained as functions of the peak of 

 LH at mid-cycle. Possibly, these are steps that attend the fully developed 

 follicle which may be relatively autonomous and independent of this 

 gonadotropin by this time. Perhaps a more important eff'ect of this peak of 

 LH is that of instituting the process of atresia in the second rank follicles. ' 



SUMMARY 



The interplay between pituitary gonadotropins and the ovary of the primate j 

 at time of ovulation not only may insure that one follicle achieves maturation, 1 

 but also may precipitate dissolution of the next largest follicles in both 

 ovaries at that time. It is suggested that this is the mechanism through which 

 ovulation is generally limited to one or two follicles each month. It has been 

 emphasized that this atresia of contending follicles occurs prior to ovulation, 

 and thus cannot be associated with the function of the post-ovulatory 

 corpus luteum. The institution of atresia might well be due to the action of 

 LH on these "second rank" follicles that are immature and unable to with- ' 

 stand such stimulation. It is at the time of the mid-cycle peak of LH eff'ect 

 that these show dissolution of granulosa and hypertrophy of theca interna. 

 A striking example of this is sometimes seen in the neonatal ovary where 



