Ovarian follicles are small fluid-filled sacs located in the ovaries, which are part of the female reproductive system. Each follicle contains an immature egg, or oocyte, and plays a crucial role in the development of eggs during the menstrual cycle. As a woman ages, the number of ovarian follicles decreases, impacting fertility. During each menstrual cycle, several follicles begin to mature, but typically only one becomes dominant and releases its egg during ovulation. The remaining follicles undergo a process called atresia, where they degenerate. This cycle is regulated by hormones such as estrogen and luteinizing hormone.