A Graafian follicle, or ovarian follicle, is found inside an ovary and is a tiny cyst inside which an egg, or ovum, develops. Women are born with many immature follicles, each containing an undeveloped ovum. Once puberty is reached and menstruation begins, one ovum matures each month. The follicle grows larger and ruptures at around day 14 of the cycle, releasing the egg in a process known as ovulation.
These follicles start out as immature structures called primordial follicles. These are present in females from birth, and each contains a single egg cell, or oocyte, surrounded by a layer of follicular cells. At the beginning of every menstrual cycle, a number of the primordial follicles grow larger, each one creating a fluid-filled space around its ovum. Within a week, one of the follicles becomes the dominant one, growing rapidly while the others shrink away. In the middle of the cycle, the dominant Graafian follicle bursts to release its ovum, which is then carried along the Fallopian tube to the womb.
After the follicle has ruptured, it fills with blood, sometimes causing slight abdominal discomfort. This blood is quickly replaced by cells that are rich in fat, called luteal cells, and the follicle becomes the corpus luteum. The luteal cells produce estrogen and progesterone, and this is known as the luteal phase of the menstrual cycle. If pregnancy follows, the corpus luteum remains, but if there is no pregnancy, it starts to break down a few days before the start of the next menstrual bleed, ending up as scar tissue.
A Graafian follicle may be seen on an ultrasound scan, appearing as what is called a functional cyst. It is a part of normal ovarian function and is benign. Treatment is not generally required because functional cysts tend to disappear on their own.
Sometimes, an ovarian follicle fails to shrink after the ovum is released, or does not release it at all. This causes the follicle to remain full of fluid and to continue to grow into what is known as a follicular ovarian cyst. These cysts generally disappear after a number of weeks without any treatment being needed.
Occasionally, the corpus luteum does not break down but becomes a blood-filled cyst, called a luteal cyst or corpus luteum cyst. Such cysts are commonly seen in women undergoing fertility treatment. Normally, these cysts go away by themselves, but sometimes they burst, giving rise to sharp abdominal pain and bleeding. The symptoms may be confused with appendicitis or ectopic pregnancy, but the cause can be determined by an ultrasound scan. Usually, the condition resolves on its own, but in rare cases with excessive bleeding, surgery might be required.