Cysts form when fluid, protein, or other material gets trapped in a pocket under the skin or inside an organ. The specific cause depends on the type of cyst, but most develop from blocked pathways, trapped cells, or normal hormonal cycles. Some are completely harmless and resolve on their own, while others grow slowly over months or years.
How Skin Cysts Form
The most common type of cyst people notice is an epidermoid cyst, a firm, round bump just under the skin. Your skin constantly sheds old cells from its outermost layer. Normally these cells flake off and disappear. But sometimes they move deeper into the skin instead of shedding outward. When that happens, the misplaced cells form a small pocket, or sac, and begin secreting keratin, a thick protein your body normally uses to build skin and hair. That keratin slowly accumulates inside the sac, creating the characteristic lump you can feel.
Several things can redirect skin cells inward and start this process:
- Skin injuries: A scratch, cut, or surgical wound can push surface cells beneath the skin during healing.
- Blocked hair follicles: When the opening at the top of a hair follicle gets plugged, dead cells can’t escape and instead build up into a cyst.
- Acne: Chronic breakouts cause repeated plugging and inflammation around hair follicles, which occasionally leads to cyst formation.
- Sun damage: Years of UV exposure thins the skin and makes follicle openings more prone to plugging, which is why older adults with significant sun exposure develop cysts more frequently.
These cysts are often called “sebaceous cysts,” but that name is misleading. They aren’t filled with oil from your sebaceous glands. They’re filled with keratin and dead cell debris. True sebaceous cysts, which actually contain skin oil, are rare and mostly associated with a genetic condition called steatocystoma multiplex.
How Ovarian Cysts Develop
Most ovarian cysts are a byproduct of the normal menstrual cycle. Each month, the ovaries grow small fluid-filled sacs called follicles. A follicle produces hormones, matures an egg, then ruptures to release that egg during ovulation. When this process doesn’t go exactly as planned, a cyst can form in two ways.
A follicular cyst develops when the follicle never ruptures or releases its egg. Instead of breaking open, it keeps growing and fills with fluid. A corpus luteum cyst happens after the egg has been released: the opening where the egg escaped seals itself shut, trapping fluid inside the now-empty follicle. Both types are called functional cysts, and most shrink and disappear within one to three menstrual cycles without any treatment.
Fertility medications that stimulate ovulation increase the risk of ovarian cysts because they push the ovaries to produce more follicles than usual, giving more opportunities for the process to go sideways. Hormonal imbalances unrelated to medication can have a similar effect.
Ganglion Cysts on Joints and Tendons
Ganglion cysts are the rubbery bumps that commonly appear on the wrist, hand, or top of the foot. They grow out of a joint lining or tendon sheath and are filled with a thick, jelly-like fluid similar to the lubricating fluid that naturally cushions your joints. No one has identified an exact cause. The leading theory is that small tears or irritation in the tissue allow joint fluid to leak out and pool into a sac, but repetitive motion and joint stress seem to play a role. They’re more common in women and in people between 20 and 40.
Cystic Acne Is a Different Process
Cystic acne looks and feels like a cyst, but it forms through a different mechanism than the cysts described above. It starts the same way as a regular pimple: excess oil and dead skin cells clog a pore. The difference is that bacteria also get trapped inside. Your immune system responds with intense inflammation deep in the middle layer of your skin, creating a large, painful, pus-filled lump that sits well below the surface. Unlike an epidermoid cyst, which is typically painless and grows slowly, a cystic acne lesion is red, tender, and develops over days.
Genetics and Cyst Formation
Some people are simply more prone to cysts because of their genes. Steatocystoma multiplex, a condition that causes dozens of small oil-filled cysts across the chest, arms, and neck, is caused by mutations in the KRT17 gene. This gene provides instructions for building keratin 17, a structural protein in skin cells. When the gene is mutated, the keratin network inside cells becomes unstable, disrupting the normal function of oil glands and triggering cyst growth. The condition follows an autosomal dominant inheritance pattern, meaning you only need one copy of the mutated gene from one parent to develop it.
Other genetic syndromes can also increase cyst formation throughout the body, but for most people who develop a single cyst, genetics isn’t the primary driver. It’s usually a combination of skin trauma, follicle blockage, or hormonal fluctuations.
What a Cyst Feels Like
Skin cysts typically present as a smooth, round lump you can move slightly under the skin with your fingers. They grow slowly, sometimes over years, and are usually painless unless they become infected or inflamed. An infected cyst turns red, swollen, and tender, and may drain a thick, yellowish material with a strong odor.
Ovarian cysts often cause no symptoms at all and are discovered during routine imaging. Larger ones can cause a dull ache or pressure on one side of the lower abdomen, bloating, or pain during certain activities. A sudden, sharp pain could signal that a cyst has ruptured or twisted the ovary, which needs prompt medical attention.
Ganglion cysts may or may not hurt. Pain usually depends on location. If the cyst presses against a nerve, you might feel tingling, numbness, or weakness in the affected hand or foot.
How Cysts Are Identified
A doctor can often suspect a skin cyst based on its appearance and feel, but a physical exam alone isn’t enough to confirm what’s inside. Ultrasound is the most common tool for distinguishing a fluid-filled cyst from a solid mass. Breast cysts, for example, are frequently discovered during routine mammograms and then further evaluated with ultrasound. In some cases, a small tissue sample (biopsy) is taken to rule out other growths. For ovarian cysts, a pelvic ultrasound can show the size, location, and whether the cyst is filled with fluid or solid material.
Reducing Your Risk
You can’t prevent every cyst, especially those driven by hormones or genetics. But for skin cysts, the most controllable factor is minimizing the conditions that trap cells under the surface. Managing acne reduces the chronic inflammation and follicle plugging that can lead to cyst formation. Protecting your skin from excessive sun damage helps preserve the normal cell-shedding process as you age. Keeping wounds clean during healing lowers the chance that surface cells get pushed into deeper layers of skin. Avoid squeezing or picking at bumps, which can drive material further into the skin and create a new cyst or cause an existing one to become inflamed.
For cysts that have already formed, the only permanent removal method is surgical excision of the entire sac wall. If just the contents are drained but the sac remains, the cyst will almost always refill over time.