An ovarian follicle is a round cellular structure found in the ovaries that contains an egg, or oocyte, which matures inside the follicle and is eventually released during ovulation. It also secretes hormones that influence stages of the ovarian cycle. Human women are born with over a million ovarian follicles, each with the potential to release an egg for fertilization. Abnormalities in the life-cycle of this structure can lead to health problems, such as ovarian cysts.
There are three main segments of an ovarian follicle that each play a specific role in the maturation of the egg. The egg itself grows inside the follicle and is eventually released during ovulation. Surrounding the egg is a single layer of cells called granulosa cells, which respond to hormones in the body, producing certain steroids and promoting the growth of the egg. The theca, which compose an inner and outer layer surrounding the granulosa, also produces steroid hormones. Working together, the theca and the granulosa also synthesize estrogen, preventing the development of additional follicles and influencing the ovarian cycle.
Although women are born with a huge supply of ovarian follicles, many of them are reabsorbed by the body during childhood. About half a million immature eggs are left at puberty, when the body begins a cycle of growth and release that typically occurs on a monthly basis. This cycle repeats until a woman reaches menopause and no longer menstruates.
The ovarian cycle begins on the first day of menstruation, when an increased amount of follicular stimulating hormone (FSH) is secreted from the pituitary gland. Primary follicles begin to mature as the theca produce steroid hormones. Only a single group will develop each cycle because the granulosa cells secrete a substance that reduces other follicles’ sensitivity to FSH. In addition, increasing levels of estrogen cause a reduction in FSH secretion.
By the end of this phase, typically only one follicle is still developing. The primary egg within it divides into a secondary oocyte and a polar body. The mature follicle secretes collagenase, a substance that breaks down the tissue holding the cells together. Cells in the outer theca contract, causing the follicle to burst open. The oocyte is released into the fallopian tube and is ready to be fertilized.
The thecal and granulosa cells that once made up the ovarian follicle transform into luteal cells and become part of the corpus luteum, a structure that produces progesterone and estrogen after ovulation. These hormones prepare the uterus for pregnancy by thickening the inner lining, which provides nourishment for an embryo. If pregnancy does not occur, after about 12 days, the cells of the corpus luteum die, and the structure becomes inactive. Without the influence of the hormones secreted by the corpus luteum, menstruation begins, discharging the extra lining in the uterus, and the ovarian cycle repeats.
If an ovarian follicle does not release an egg during ovulation, it continues to grow within the ovary, becoming a cyst. Follicles that do release the egg can also become cystic if the sac does not dissolve and fluid becomes trapped under the cellular wall. Ovarian cysts are typically benign and quite common in women after puberty, usually causing little to no discomfort and often disappearing without treatment.
In some cases, ovarian cysts can cause considerable pain and discomfort if they burst, and may even require surgical removal. Cysts are most likely to burst during ovulation, causing a sharp pain or dull ache in the ovary. For benign cysts, many healthcare professionals recommend taking over-the-counter pain relievers, applying heating pads, or taking warm baths.
If cysts get too large, they can push the ovary out of its normal position, which increases the chances that it could twist. This causes sudden, extreme pain, and requires surgery to correct. The twisting can also cut off blood to the ovary, in which case it would need to be removed if it cannot be untwisted before the tissue dies.
Polycystic Ovary Syndrome
While many women have cysts at some point, some women's bodies produce more androgen, a male sex hormone, than normal. As a result, eggs are repeatedly not released from the follicles, and cysts build up in the ovaries, leading to polycystic ovary syndrome (PCOS). Women with this condition typically have problems with fertility, and they also often suffer from increased hair growth and an higher risk of diabetes, heart attack, and endometrial cancer. Regular exercise and a healthy diet may help keep PCOS under control, and birth control pills may be prescribed to help control hormone levels and decrease the chance of developing new cysts.
In rare cases, ovarian cysts can be cancerous and require medical treatment. They are often detected during a pelvic exam and identified using procedures such as ultrasound examinations. Although cysts are usually benign, many medical professionals recommend visiting a healthcare provider if unexplained pelvic or abdominal pain occurs in order to rule out cancer. In the case of a cancerous cyst, surgery may be necessary to remove the structure. In some cases, a hysterectomy, a procedure that removes the ovaries and uterus, may be required.