An illustration showing the lining of the uterus. The cells that line the womb are called the endometrium. This tissue is normally shed monthly in response to the hormonal changes of the menstrual period. Ovarian follicles are depicted at top. These hormone-secreting cells are found in the ovaries. Women begin puberty with about 400,000 follicles, each with the potential to release an egg cell (ovum) at ovulation for fertilization. These eggs are developed once every menstrual cycle. Folliculogenesis is continuous, meaning that at any time the ovary contains follicles in many stages of development. The majority of follicles die and never complete development. A few develop fully to produce a secondary oocyte which is released by rupture of the follicle in a process called ovulation.