An ovarian follicle is a small fluid-filled sac 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 and release of eggs during the menstrual cycle. As the cycle progresses, follicles mature and one typically becomes dominant, leading to ovulation. During ovulation, the dominant follicle releases the mature egg into the fallopian tube, where it may be fertilized by sperm. The remaining follicle transforms into a structure called the corpus luteum, which produces hormones like progesterone to support potential pregnancy.