An ovarian follicle is a small, fluid-filled sac that contains an immature egg (oocyte) surrounded by specialized cells. These follicles are found within the ovaries, which are a pair of female reproductive organs located on either side of the uterus. The ovaries are responsible for producing and releasing eggs, as well as the hormones estrogen and progesterone.
During each menstrual cycle, several follicles begin to grow and develop under the influence of hormones such as follicle-stimulating hormone (FSH). As these follicles mature, one usually becomes dominant and continues to grow, while the others degenerate. The dominant follicle secretes estrogen, which thickens the uterine lining and prepares it for potential pregnancy.
Around the middle of the menstrual cycle, luteinizing hormone (LH) surges, triggering the release of the mature egg from the dominant follicle. This process is known as ovulation, and the egg is released into the fallopian tube where it may be fertilized by sperm.
If the egg is not fertilized, the menstrual cycle continues, and the follicle that released the egg transforms into the corpus luteum. The corpus luteum produces progesterone, which helps to maintain the uterine lining in case a fertilized egg implants. If pregnancy does not occur, the corpus luteum breaks down, hormone levels drop, and a new menstrual cycle begins.
Overall, ovarian follicles play a critical role in female reproductive biology, providing the site for egg development and regulating the menstrual cycle.