Follicle dynamics during ovarian ageing.

The human ovary is endowed at birth with a fixed number of primordial follicles which steadily declines throughout life as a result of atresia and recruitment towards ovulation. The pattern of this decline is not exponential, but more bi-exponential corresponding to a 'broken-stick' regression of logged total numbers of follicles against age. Such a model implies an abrupt change in the exponential rate of follicle loss at age 38 years, and is thus rather implausible biologically. A more refined model with an exponential rate of follicle loss that changes gradually throughout life also describes the data on declining follicle numbers but in addition leads to a distribution of age at menopause, corresponding to follicle numbers falling below a critical threshold, that shows quite remarkable agreement with independent data on menopausal ages of American women. When the follicles are classified into resting and growing stages, it is found that any changes in the consequent process of follicle development as the ovary ages relate mainly to the small resting follicles and not the larger growing ones.