Effect of oocyte number and rate of atresia on the age of menopause.

Menopause occurs when oocyte number falls below the threshold required for ovarian function. The age of menopause is decreased by some drugs, some occupational exposures, and cigarette smoke (both active and passive). Using data collected by Block (Acta Anat (Basel) 1954; 14:208), we have constructed four, two parameter models to explore the effect of oocyte number and rate of atresia on age at menopause. The models used are: linear [O(A) = OB + R * A]; natural log [O(A) = R * In (A)]; exponential [O(A) = OB * exp(R * A)], and power [O(A) = OB * Age(R)]. OB = oocyte number at birth, ATR = rate of atresia, and O(A) = number of oocytes at age A. The parameters for the four models are: (formula; see text) Each model has different behavior with respect to alterations in oocyte number and rate of atresia on age at menopause. Animal studies suggest that ovarian failure is only weakly dependent on oocyte number, consistent with the power and exponential models. These suggest that the decrease in age at menopause following xenobiotic exposures results from an increase in the rate of atresia.