Follicles

A woman's reproductive years begin with puberty and end with menopause. Controlled by the hypothalamus in the brain, major changes occur from about the age of eight. The hypothalamus begins to secrete a substance which acts on the pituitary gland, causing it to release various hormones. Follicle-stimulating hormone (FSH) is the first to be released. This brings about the growth of the egg-containing "follicles" and makes them produce estrogen, which encourages the growth of breasts, widens the pelvis, and encourages the development of the external genitals. The rising level of estrogen in the bloodstream causes the hypothalamus to reduce a second substance, luteinizing hormone (LH). This causes one of the follicles to burst and release an ovum for possible fertilization, the escape being known as ovulation. The remains of the follicle stay in the ovary, secreting estrogen and a second hormone, progesterone, which prepares the uterus lining ("endometrium") to receive and nourish a fertilized egg. If the egg is not fertilized, levels of estrogen and progesterone drop and the uterus lining breaks down. It is then shed from the vagina together with mucus and the unfertilized ovum. The resulting bleeding constitutes menstruation.