Aspects of ovarian follicle development throughout life.

The pool of primordial follicles present in the female ovary reaches its maximum number around 20 weeks of gestational age and then decreases in a logarithmic fashion throughout life until complete depletion occurs around the age of the menopause. Reproductive life is initiated when less than 10% (0.5 million) of primordial follicles are left. The entire growth trajectory of the follicle takes at least 3 months. Follicle growth up to the antral stage occurs during fetal life and infancy. While the role of gonadotropins in early follicular development remains controversial, the last 2 weeks of development are FSH dependent. The intercycle rise in FSH and decreasing levels thereafter are crucial for recruitment of a cohort of healthy, early antral follicles and subsequent single dominant selection. Following puberty, anovulation may persist for years and this may presage the development of adult anovulatory infertility. The menopause is preceded by a period of reduced fertility. The development of reliable and sensitive markers for ovarian ageing will be the challenge of the near future.