Follicular release in women.

From birth, a woman contains a lifetime supply of follicles (envelopes of cells that contain immature eggs; Derry, 2006) which gradually lessen as menstrual cycles occur, however this process accelerates powerfully around the age of 38, in a way that doesn’t happen in chimps, monkeys or rodents (Cant and Johnstone, 2008). If this acceleration never evolved, menopause would occur around the age of 70 due to the earlier and slower rate of follicle loss, which is predicted by trends in other primates of the same age. 

What happens to accelerate this process of follicular release in women? Once puberty is reached, the ovary contains about an average of 200,000 follicles (Block E, 1952). During a reproductive lifespan, a gradual decrease in the original follicle pool is caused by continuing growth of primordial and primary follicles into secondary and larger follicles. The primordial follicle pool could also decrease due to apoptosis (a tightly regulated cell suicide process that destroys cells that are no longer needed or of threat) of resting follicles.

Figure 1. Development of an Ovarian Follicle. M.H. Ross (2015).

About 10 years before menopause, increasing percentages of follicles are lost from the resting pool (Richardson et al. 1987, Faddy & Gosden 1996, Faddy et al. 1992, Gougeon et al. 1994) of cells due to association with increased serum follicle-stimulating hormone (FSH) and decreases in circulating inhibin hormones. The decreasing follicle reserve serves as a ticking clock to time the commencement of menopause.

References:

Block E 1952 Quantitative morphological investigations of the follicular system in women. Variations at different ages. Acta Anat (Basel) 14:108–123.

Derry, P.S. (2006). A Lifespan biological theory of menopause. Sex Roles. 54, 393-399.

Faddy MJ, Gosden RG, Gougeon A, Richardson SJ, Nelson JF 1992 Accelerated disappearance of ovarian follicles in mid-life: implications for forecasting menopause. Hum Reprod 7:1342–1346.

2015

Faddy MJ, Gosden RG 1996 A model conforming the decline in follicle numbers to the age of menopause in women. Hum Reprod 11:1484–1486.

Gougeon A, Echochard R, Thalabard JC 1994 Age-related changes of the population of human ovarian follicles: increase in the disappearance rate of non-growing and early-growing follicles in aging women. Biol Reprod 50:653–663.

Richardson SJ, Senikas V, Nelson JF 1987 Follicular depletion during the menopausal transition: evidence for accelerated loss and ultimate exhaustion. J Clin Endocrinol Metab 65:1231–1237.

 Ross, M.H. (2015). Schematic drawing of the stages in the development of ovarian follicles, beginning with the primordial follice and ending with the pre-ovulatory of Grannfian. History A Text and Atlas 3rd Ed. p. 684 Fig. 22.3.