A cyst forms when cells, fluid, or other material becomes trapped in a pocket of tissue with no way to drain. The specific trigger depends on where in the body the cyst develops, but the underlying pattern is remarkably consistent: something that should flow outward or be reabsorbed gets sealed off, and a sac builds around it. Cysts can appear in skin, ovaries, joints, kidneys, and nearly every other organ.
How Skin Cysts Form
The most common cysts people notice are the firm, round bumps that develop just beneath the skin. These are usually epidermoid cysts, and they start with a simple misdirection of skin cells. Your outer layer of skin constantly sheds dead cells outward. When those cells move inward instead, deeper into the skin, they create a small pocket. The displaced cells form the walls of the cyst and begin secreting keratin, a thick protein your body normally uses to build hair and nails. That keratin fills the pocket, producing the dense, cheese-like substance inside.
Several things can push skin cells in the wrong direction. A scratch, surgical wound, acne, or chronic sun damage can disrupt the normal path cells take to leave your body. The injury traps cells and keratin below the surface, where they collect into a growing sac. Even a clogged hair follicle can start the process. The blockage happens at the top of the follicle, sealing off the opening where dead cells would normally exit. On areas of the body without hair follicles, trauma alone can shove skin cells into the deeper layer of skin, creating a pocket where keratin accumulates over weeks or months.
These cysts typically grow slowly and are painless unless they become inflamed or infected. The thick keratin inside is what gives them their characteristic firmness when you press on them.
How Ovarian Cysts Form
Ovarian cysts follow a different mechanism, driven by the monthly cycle of egg release. Each month, an egg grows inside a small fluid-filled sac called a follicle. Normally, the follicle ruptures to release the egg, then shrinks. A follicular cyst forms when that rupture never happens. The follicle simply keeps growing, filling with fluid.
A second type, called a corpus luteum cyst, develops after the egg has been released successfully. Once the egg leaves, the empty follicle is supposed to shrink and produce hormones needed for conception. Sometimes the opening where the egg exited seals shut. Fluid builds up inside the collapsed follicle, inflating it into a cyst. Most ovarian cysts of both types resolve on their own within a few menstrual cycles without treatment.
How Joint and Tendon Cysts Form
Ganglion cysts, the rubbery lumps that often appear on wrists or hands, form from joint or tendon tissue rather than skin cells. The leading explanation is that a small tear develops in the tissue covering a joint or tendon. That tear allows the tissue to bulge outward and create a sac, which fills with a thick, jelly-like fluid normally found inside joints.
Repetitive stress plays a role. An injury like tendonitis from overusing your wrist can weaken the tissue enough to allow a cyst to develop later. Some ganglion cysts appear without any obvious injury, but the underlying process is the same: tissue that should contain joint fluid develops a weak spot, and fluid pushes through it into a new pocket.
The Common Pattern Across All Cysts
Despite the variety of locations and triggers, cyst formation follows a core sequence. First, a normal pathway gets disrupted. Skin cells that should shed outward get pushed inward. A follicle that should rupture stays sealed. Joint tissue that should stay intact develops a tear. Second, material that would normally move freely, whether keratin, reproductive fluid, or joint lubricant, becomes enclosed. Third, a lining or wall forms around that trapped material, creating a self-contained sac. The sac often continues to accumulate its contents slowly, which is why many cysts grow over time.
A cyst can form in virtually any part of the body, including bones, organs, and soft tissues. What unites them is that they are sacs filled with air, fluid, or other material rather than solid masses of cells.
Genetic Conditions That Cause Cysts
Some people develop cysts not from a single injury or blockage but because of inherited genetic mutations. Von Hippel-Lindau syndrome is one example. People with this condition carry a mutation in a gene that normally acts as a brake on cell growth. When that brake fails, cells grow and divide in an uncontrolled way, forming cysts and tumors across the kidneys, pancreas, and genital tract. Only one copy of the mutated gene needs to be inherited from a parent to put someone at risk, though cysts develop in specific organs only after the second copy of the gene is also damaged during a person’s lifetime.
Polycystic kidney disease is another hereditary condition where fluid-filled sacs gradually replace healthy kidney tissue. In these genetic conditions, cyst formation isn’t a one-time event but an ongoing process that requires monitoring throughout life.
How Cysts Differ From Tumors and Abscesses
A cyst is a fluid-filled or material-filled sac. A tumor is a solid mass of tissue. An abscess is a pocket of pus caused by infection. These three can feel similar under the skin, but their internal structure is different. On ultrasound or CT scan, a cyst that appears uniform and fluid-filled is almost always benign. If a cyst contains solid components, further imaging or a biopsy may be needed to rule out something more serious.
Most cysts are not cancerous and never become cancerous. The rare exceptions tend to involve cysts with irregular walls or solid areas inside, which is why imaging sometimes leads to follow-up appointments to track whether a cyst changes over time.
What Happens When a Cyst Ruptures
Skin cysts that rupture typically cause local redness, swelling, and soreness as their contents leak into surrounding tissue. The bigger concern is internal cysts, particularly ovarian cysts. When an ovarian cyst ruptures, it often causes sudden, sharp pain in the lower belly or back, sometimes with vaginal spotting or bloating. Most women feel pain at the moment of rupture and then discomfort for a few days afterward, though some don’t notice it at all.
Certain ruptures carry serious risks. If an infected cyst breaks open, it can trigger sepsis, a dangerous immune overreaction. A large cyst can also cause the ovary to twist on itself, cutting off its blood supply, a condition called ovarian torsion that requires surgery. Warning signs that a rupture needs emergency attention include severe nausea and vomiting, fever, heavy vaginal bleeding, or faintness.
For skin cysts that become inflamed or repeatedly fill back up, minor surgical removal of the entire cyst wall is the most reliable way to prevent recurrence. Simply draining a cyst often leads to regrowth because the lining that produces the cyst’s contents remains intact.