Ovarian cyst

“Cyst” comes from kustis, which means “pouch”, so ovarian cysts are fluid-filled sacs on or in the ovaries. They are very common in females of reproductive age, but can affect females of any age.

The ovaries are a pair of white-ish organs about the size of walnuts. They’re held in place, slightly above and on either side of the uterus and fallopian tubes by ligaments.

Specifically, there’s the broad ligament, the ovarian ligament, and the suspensory ligament. And the suspensory ligament is particularly important because the ovarian artery, ovarian vein, and ovarian nerve plexus pass through it to reach the ovary.

If you slice the ovary open and look at it (don’t try this at home) there’s an inner layer called the medulla, which contains most of the blood vessels and nerves and an outer layer called the cortex, which has ovarian follicles scattered throughout it.

Each follicle is initially made up of an immature sex cell, or primary oocyte, which is the female sex cell, and layers of theca and granulosa cells surrounding the oocyte.

Now, there’s actually loads going on with the ovaries throughout the menstrual cycle, which is controlled by the hypothalamus and the pituitary up in the brain.

The hypothalamus secretes gonadotropin-releasing hormone, or GnRH, which makes the nearby anterior pituitary gland release follicle-stimulating hormone, or FSH, and luteinizing hormone, or LH.

In the first two weeks of an average 28-day cycle, the ovaries go through the follicular phase, meaning that out of the many follicles scattered throughout the ovaries, a couple of them enter a race to become the dominant follicle, that will be released at ovulation, while the rest regress and die off.

All the developing follicles secrete loads of estrogen, which negatively inhibits pituitary FSH, and they also make a few androgens like testosterone.

At ovulation, the oocyte is released into the fallopian tube, and the luteal phase begins - which lasts for the last 2 weeks of an average 28-day cycle.

During the luteal phase, the remnant of the ovarian follicle, called the corpus luteum, makes progesterone, which negatively inhibits pituitary LH.

If fertilization occurs then the corpus luteum continues making progesterone until the placenta forms.

If fertilization doesn’t happen, then the corpus luteum stops making hormones after approximately 10 days, becomes fibrotic, and is called the corpus albicans.

So as far as ovarian cysts go, there are 2 broad categories. First, there are functional cysts, which result when the normal, cyclic development of the ovarian follicles is disrupted.

One type of functional cyst that can form before ovulation is a follicular cyst, which is a dominant follicle that fails to rupture and keeps growing.

This can happen, if say, the normal surge of LH that causes ovulation just doesn’t happen during a given menstrual cycle.

A condition where you might have multiple follicular cysts is polycystic ovary syndrome, which is caused by a dysfunction in the hypothalamic-pituitary-ovarian axis that causes chronic anovulation, which may lead to amenorrhea, or absent menstruation, and excess androgen production.

Alternatively, if the dominant follicle ruptures but then closes up again after ovulation, this is called a corpus luteal cyst. In this case, the corpus luteum doesn’t dissolve but instead continues to grow.

As it grows, the arteries nourishing it can rupture and hemorrhage into the cyst, which is why corpus luteal cysts are also called hemorrhagic cysts.

Both follicular and corpus luteal cysts are usually around 2 to 3 centimeters, but they can get as big as 10 cm in diameter, they contain a clear serous liquid, and have a smooth internal lining - so they are referred to as “simple cysts”.

The last kind of functional cysts are theca lutein cysts. These are caused by overstimulation by human chorionic gonadotropin or hCG, a hormone that’s produced by the placenta, so they’re only seen in pregnancy.

hCG stimulates growth of the follicular theca cells, so these cysts are usually bilateral, since resting follicles can be found on both ovaries.

Theca lutein cysts are more likely to develop when there’s more hCG than usual, like when there are multiple fetuses, or with gestational trophoblastic disease, where tumours made up of placental cells causes higher than normal hCG levels.

Ok, now, the second category of cysts are neoplastic cysts - which is not to say all of them are cancer, but rather that they’re caused by the abnormal reproduction of cells on or in the ovaries, which can result in a benign or malignant mass.

Broadly speaking, neoplastic cysts are usually complex, meaning that they’re larger than 10 centimeters, have irregular borders, and have internal septations creating a multilocular appearance.

The fluid inside these cysts tends to be heterogeneous, meaning there’s something other than fluid inside it.

Benign neoplastic cysts include endometriomas, which occur in endometriosis, and it’s where endometrial tissue from the uterus grows on the ovary.

Because they’re functionally the same as the endometrium inside the uterus, endometriomas respond to hormones just like the uterus would.

Because of this, endometriomas tend to bleed within the cyst cavity during menstruation, and over time, they fill up with old, dark blood and shed tissue - hence why they’re sometimes also called “chocolate cysts”.

Endometriomas also release proinflammatory factors which cause inflammation, which leads to cyst growth. When they get too large, they can rupture and spill their contents inside the peritoneal cavity.