What is an ovarian follicle?

Q: What is an ovarian follicle and what does it contain?

A: An ovarian follicle is a tiny cavity in the ovary that serves as an egg sac.

Women are born with all the eggs they will ever have. Baby girls come into the world with about two million immature eggs, each residing in its own ovarian follicle. Some of those eggs are destroyed in the body and by the time a girl reaches puberty, roughly half a million eggs remain.

That?s still vastly more than she?ll need. Over the course of her reproductive years, she?ll have about 400 menstrual cycles. Each cycle involves ovulation, the release of one mature egg ready for fertilization.

A complex hormonal process orchestrates ovulation and governs the menstrual cycle. The cycle begins in the brain, where the hypothalamus gland sends out a hormone that instructs the pituitary gland to produce luteinizing hormone and follicle-stimulating hormone. These hormones help launch the follicular phase of the cycle, during which several follicles and their eggs begin to mature. Several egg-bearing follicles start the journey, but only one will fully ripen and make it across the ovarian finish line.

At ovulation, the egg ruptures the follicle as it emerges from the ovary. The follicle then transforms into a corpus luteum, and begins to secrete progesterone to help prepare the uterus for implantation of the fertilized egg. If conception does not occur, the corpus luteum deteriorates in two weeks — the time of menstruation. But if there is a pregnancy, the corpus luteum continues secreting progesterone as well as another hormone for the first two trimesters. After that point, the fetus begins producing its own hormones.