Cysts form when fluid, air, or semi-solid material gets trapped in a pocket of tissue that normally wouldn’t be there. The specific cause depends on the type of cyst, but most develop through one of a few common mechanisms: a blocked gland or duct, a disruption in the normal cell turnover cycle, a hormonal process gone slightly off track, or an inherited genetic change. Some cysts are completely harmless and resolve on their own, while others grow slowly over months or years.
Skin Cysts and Blocked Follicles
The most common cysts people notice are the ones that form just under the skin. Epidermoid cysts, the type most people are referring to when they say “skin cyst,” develop when skin cells that normally shed from the surface instead migrate deeper into the second layer of skin. Once trapped there, these cells continue doing what they always do: producing a tough protein called keratin. But with nowhere to go, the keratin accumulates inside a small sac, slowly forming a firm, round lump you can feel under your skin.
This inward migration of skin cells often happens around hair follicles. The follicle’s lining can fold inward or become disrupted, creating a pocket where material collects. Epidermoid cysts are most common between ages 20 and 50, with peak incidence in the 21 to 30 age range. They affect men roughly twice as often as women.
A closely related type, the pilar (or trichilemmal) cyst, originates specifically from the outer root sheath of the hair follicle. These tend to appear on the scalp and have a thicker wall that’s less likely to rupture than an epidermoid cyst. Both types grow slowly and are almost always benign.
How Skin Injuries Create Cysts
Physical trauma is one of the clearest triggers for cyst formation. When skin is cut, punctured, or crushed, surface cells can get pushed below the top layer into the deeper dermis. This misplacement creates the same trapped pocket of keratin-producing cells that forms naturally around follicles. On parts of the body without hair follicles, injury is often the primary reason a cyst develops at all.
Repeated irritation to the same area of skin can produce the same effect over time, even without a single dramatic injury. Friction from tight clothing, chronic rubbing, or surgical scars can all push surface cells inward and set the stage for a cyst to slowly fill and grow.
Gland Blockages
Many cysts form simply because a gland’s drainage pathway gets plugged. The gland keeps producing its normal secretion, but the fluid has no exit. Pressure builds, the gland swells, and a cyst forms around the trapped material.
A clear example is the chalazion, a hard lump that develops on the eyelid. Your eyelids contain dozens of tiny oil glands (meibomian glands) that release a thin film of oil every time you blink, keeping your eyes moist. When one of these glands gets blocked, oil backs up inside it, the surrounding tissue becomes irritated, and a firm cyst filled with retained oil forms on the lid. The same principle applies to cysts in other glandular tissues throughout the body, including the breast, the salivary glands, and sweat glands in the skin.
Ovarian Cysts and Hormonal Cycles
Ovarian cysts are extremely common and usually form as part of the normal menstrual cycle. Each month, the ovaries grow small fluid-filled sacs called follicles. A follicle matures, produces estrogen and progesterone, then ruptures to release an egg during ovulation. When this process doesn’t go exactly as planned, a cyst can result.
There are two main types tied to the cycle. A follicular cyst forms when the follicle fails to rupture or release its egg. Instead of breaking open, it continues growing and fills with fluid. A corpus luteum cyst forms after ovulation, when the opening where the egg exited seals itself off. Fluid builds up inside the now-closed structure, creating a cyst. Both types are called functional cysts because they arise from the ovary’s normal function. Most are small, cause no symptoms, and disappear within one to three menstrual cycles without treatment.
Joint and Tendon Cysts
Ganglion cysts are the rubbery lumps that commonly appear on the wrist, hand, or top of the foot, usually near a joint or tendon. They’re filled with a thick, jelly-like fluid similar to the lubricating fluid found inside joints. No one has identified a definitive cause. They seem to grow out of joint capsules or tendon linings, and they often fluctuate in size, sometimes swelling with activity and shrinking with rest.
Baker’s cysts, which form behind the knee, have a clearer origin. They typically develop as a secondary response to a problem inside the knee joint itself. Osteoarthritis, rheumatoid arthritis, and cartilage tears (especially meniscus tears) all cause the knee to produce excess joint fluid. That fluid can migrate to the back of the knee and pool in a sac, creating a visible bulge. Treating the underlying knee condition often resolves the cyst.
Kidney Cysts and Genetic Causes
Simple kidney cysts are common and become more frequent with age. They’re fluid-filled pockets that form on or inside the kidneys, and most people who have them never know it. They’re typically discovered by accident during an imaging scan for something else, and they rarely need treatment.
Polycystic kidney disease is a different situation entirely. This is a genetic condition where mutations in specific genes cause clusters of cysts to grow throughout both kidneys over time. The dominant form, which is the more common one, develops gradually in adulthood. The recessive form is rarer and appears in infancy or childhood. In both cases, the cysts can eventually crowd out normal kidney tissue and affect function. A small percentage of polycystic kidney cases aren’t inherited at all. They develop in people who have been on dialysis for several years, likely as a response to the long-term filtering process.
Why Fluid Accumulates Inside Cysts
Once a cyst forms, it tends to grow because the cells lining it continue to secrete fluid or protein into a space with no drainage. The lining cells behave like miniature versions of the tissue they came from. Skin cells produce keratin. Glandular cells produce oil or mucus. Joint lining cells produce synovial fluid. The cyst wall acts as a container, and the ongoing secretion slowly expands it from the inside.
Active fluid transport also plays a role. Cells lining a cyst can actively pump water and ions into the cavity, driven by chemical signals within the cell. This pumping mechanism can cause a cyst to enlarge even when there’s no obvious blockage or ongoing injury. It’s one reason cysts sometimes seem to grow in spurts rather than at a steady pace.
Risk Factors That Make Cysts More Likely
Several factors increase the chances of developing cysts in general:
- Age: Simple kidney cysts, joint cysts, and many skin cysts become more common as you get older, partly because tissues lose elasticity and glands become more prone to blockage.
- Skin damage: Past surgeries, acne, or chronic skin irritation can push surface cells into deeper tissue layers where cysts form.
- Hormonal fluctuations: Ovarian cysts are directly tied to reproductive hormone cycles. Conditions that disrupt ovulation, like polycystic ovary syndrome, increase their frequency.
- Joint conditions: Arthritis or cartilage injuries create excess fluid that can pool into cysts around affected joints.
- Genetics: Polycystic kidney disease is inherited. Certain rare genetic syndromes also predispose people to developing many epidermoid cysts throughout the body.
Most cysts aren’t preventable because they stem from normal biological processes that go slightly off course. You can’t stop a hair follicle from occasionally misdirecting cells, or an ovarian follicle from failing to open. What you can do is recognize the difference between a stable, painless lump and one that’s growing rapidly, becoming inflamed, or causing pressure on surrounding structures.