Cysts form when fluid, air, or other material gets trapped inside a pocket of tissue. They can develop almost anywhere in the body, from just beneath the skin to deep inside organs like the kidneys or liver. The causes vary widely depending on where the cyst appears, but most fall into a few categories: blocked ducts, hormonal cycles, joint or tendon injuries, genetic conditions, and infections.
How Cysts Form at a Basic Level
A cyst is essentially a sac lined by a thin layer of cells, filled with fluid or semi-solid material. The two forces that drive cyst growth are cell multiplication along the cyst wall and fluid secretion into the space inside it. Once that enclosed pocket forms, fluid pressure causes it to expand over time. Some cysts stay tiny and never cause symptoms. Others grow large enough to press on surrounding tissue or become painful.
Skin Cysts: Blocked Follicles and Trapped Cells
The most common cysts people notice are the firm, round lumps that appear under the skin. These are usually epidermal inclusion cysts, often mistakenly called sebaceous cysts. They form when a hair follicle gets blocked at its opening, trapping skin cells and a tough protein called keratin beneath the surface. Instead of shedding normally, those cells accumulate in a pocket and slowly expand.
Skin injuries can trigger this process too. A scratch, surgical wound, or chronic condition like acne or repeated sun damage can disrupt the normal path skin cells take as they migrate outward. The injury pushes surface cells down into a deeper layer of skin, creating a pocket where keratin collects. This is why cysts sometimes appear at the site of an old scar or along areas prone to friction and irritation. On parts of the body without hair follicles, trauma is the primary trigger.
Ovarian Cysts: A Normal Part of the Cycle
Most ovarian cysts are a direct result of the menstrual cycle. Each month, the ovaries grow small fluid-filled sacs called follicles that produce estrogen and progesterone. Normally, a follicle breaks open to release an egg during ovulation. When that process doesn’t go as planned, a functional cyst develops.
There are two main types. A follicular cyst forms when the follicle fails to rupture and release its egg, so it keeps growing instead. A corpus luteum cyst forms after the egg is released but the follicle’s opening seals shut, trapping fluid inside. Both types are usually harmless, rarely cause pain, and tend to resolve on their own within two to three menstrual cycles. Hormonal conditions like polycystic ovary syndrome can also lead to clusters of small cysts on the ovaries, though the underlying mechanism there involves persistent hormonal imbalances that prevent follicles from maturing properly.
Ganglion Cysts: Joint Stress and Tendon Tears
Ganglion cysts are the rubbery, round lumps that pop up near joints or tendons, most commonly on the wrist or hand. They form when the tissue covering a joint or tendon tears or weakens, allowing the tissue to bulge outward and create a sac filled with thick, jelly-like fluid.
No one knows exactly why some people develop them and others don’t, but a few patterns are clear. A previous joint injury, including overuse injuries like tendonitis, increases the risk. Arthritis in the hands also makes ganglion cysts more likely. Repetitive motion may play a role, which is why they’re common in people who use their wrists heavily. Many ganglion cysts fluctuate in size and can disappear without treatment, though they frequently come back.
Baker’s Cysts: Knee Problems Spilling Over
A Baker’s cyst is a fluid-filled swelling behind the knee. It’s almost always a secondary problem, caused by an existing issue inside the knee joint itself. Inflammation from osteoarthritis or rheumatoid arthritis is one of the most common triggers. Cartilage tears, particularly meniscus injuries, are another. The underlying condition causes excess fluid to build up in the joint, and that fluid gets pushed into a pocket at the back of the knee. Treating the Baker’s cyst alone rarely works unless the root cause in the joint is also addressed.
Genetic Causes: Kidney and Liver Cysts
Cysts inside the kidneys or liver often have a genetic origin. Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited form. In this condition, a genetic mutation causes kidney cells to multiply abnormally, forming small outpouchings along the tubes inside the kidney. These outpouchings eventually pinch off and become independent cysts that grow as cells lining them continue to divide and secrete fluid.
ADPKD accounts for roughly 70% of polycystic liver disease cases as well, since people with the condition typically develop cysts in both the kidneys and the liver. The remaining 30% of polycystic liver cases occur in isolation, without kidney involvement, caused by mutations in different genes that disrupt normal bile duct development. Isolated polycystic liver disease tends to appear later in life, with symptoms usually showing up in adulthood. Several genes have been linked to these conditions, all affecting how cells communicate and grow within the organ.
Simple kidney or liver cysts can also develop with age in people without any genetic condition. These are extremely common in older adults and are almost always harmless.
Infections and Parasites
Some cysts form as a direct result of infection. One of the most notable examples is cystic echinococcosis, caused by accidentally swallowing eggs from the Echinococcus granulosus tapeworm. The eggs hatch inside the body and develop into cyst-like larvae, most often in the liver or lungs, where they can grow slowly for years before causing symptoms.
People typically become infected through direct contact with dogs that carry the tapeworm, or by ingesting soil, water, or vegetables contaminated with infected dog feces. The disease is most common in rural areas where dogs have access to home-slaughtered livestock. Bacterial infections can also produce cysts or cyst-like structures, though these are more accurately called abscesses when they fill with pus rather than clear fluid.
Cysts vs. Tumors vs. Abscesses
Not every lump is a cyst. A tumor is any abnormal mass of tissue, and unlike a cyst, it can be solid rather than fluid-filled. An abscess is a pocket of pus caused by infection, typically warm, red, and painful. A cyst, by contrast, is a sac filled with air, fluid, or semi-solid material, and it can form in bones, organs, or soft tissue.
Imaging helps sort them out. Cysts that appear uniform on an ultrasound or CT scan are almost always benign and simply monitored over time. If a cyst contains solid components, further evaluation is needed, often through repeat imaging to see if it changes. The definitive way to determine whether any lump is benign or cancerous is a biopsy, where a small tissue sample is examined under a microscope. Most cysts never require this step, but any lump that grows rapidly, feels hard, or doesn’t match the typical pattern deserves a closer look.