Cysts form when fluid, air, or semi-solid material accumulates inside a membrane-lined sac somewhere in the body. The specific trigger depends on the type of cyst, but the underlying pattern is remarkably consistent: something creates a pocket, and that pocket fills. This can happen in skin, ovaries, kidneys, joints, and dozens of other locations, each with its own cause but a shared basic architecture.
The Basic Structure of Every Cyst
At the cellular level, cyst formation requires two things: a lining of cells that creates a boundary, and material that fills the space inside. In most tissues, the surrounding environment establishes a kind of polarity, an inside and an outside, that guides cells into forming a hollow cavity. The cells organize themselves around this cavity, secrete or trap material within it, and a cyst takes shape. Whether it’s filled with keratin, synovial fluid, or hormonal byproducts, the structural principle is the same.
Skin Cysts: Blocked Follicles and Trapped Keratin
Epidermoid cysts, the firm lumps you can feel just under the skin, are among the most common types. They originate from the upper portion of a hair follicle. The cyst wall is made of the same layered skin cells that line the follicle, and the interior fills with laminated keratin, the same protein that makes up your outer skin and nails. Essentially, the follicle turns inward and keeps producing skin cells with nowhere to go.
Several things can set this process in motion. Minor damage to a hair follicle, whether from friction, a scratch, or even accumulated sun damage over decades, can push surface skin cells deeper into the tissue where they don’t belong. Once implanted there, those cells continue doing what skin cells do: they multiply and produce keratin. The result is a slowly growing, painless lump. In people with acne, the process is similar. Excess keratin plugs the follicle opening, trapping oil and skin debris below the surface. Occasionally, this progresses from a simple blocked pore into a true cyst.
Most skin cysts cause no symptoms unless they rupture. When a cyst wall breaks, the keratin inside leaks into the surrounding tissue, and the immune system treats it as foreign material. That triggers a granulomatous inflammatory response, producing the redness, swelling, and tenderness people associate with an “infected” cyst. In many cases, it’s not actually infected at all. It’s an immune reaction to the cyst’s own contents.
Ovarian Cysts: When Hormonal Timing Goes Wrong
Functional ovarian cysts form as part of the normal menstrual cycle, then fail to resolve. Each month, a follicle in the ovary swells with fluid as it prepares to release an egg. Normally, a surge of luteinizing hormone (LH) from the pituitary gland triggers ovulation, the follicle ruptures, and the egg is released. If that hormonal surge doesn’t arrive on time or isn’t strong enough, the follicle keeps growing instead of opening. It becomes a fluid-filled cyst.
The hormonal chain that leads to this failure can break at several points. Disruptions in the signaling between the hypothalamus and pituitary gland can reduce LH output. Stress plays a measurable role here: cortisol interferes with the estrogen feedback loop that normally triggers the LH surge, effectively blocking ovulation. Cortisol also shifts hormone production in the ovary itself, increasing androgen output while suppressing the enzyme that converts androgens to estrogen. The net effect is a follicle that grows but never gets the signal to release its egg.
Simple ovarian cysts are extremely common. In a large screening study of over 15,000 women older than 55, simple cysts appeared in 14% of women at their first ultrasound. Among those without a cyst initially, 8% developed a new one within a year. And among women who had a cyst, about a third saw it disappear on its own within 12 months, while just over half retained it. These cysts are overwhelmingly benign and often go unnoticed.
Ganglion Cysts: Fluid Leaking From Joints
Ganglion cysts, the rubbery bumps that commonly appear on wrists and hands, form when thick, jelly-like fluid herniates out of a joint capsule or tendon sheath. The leading explanation is that a small defect or weakness in the connective tissue surrounding a joint allows synovial fluid to push through and pool in a sac just beneath the skin. Repetitive stress on a joint may contribute to that weakness, though many ganglion cysts appear without any obvious cause.
The fluid inside a ganglion is similar to normal joint lubricant but thicker and more viscous. Because the sac connects to the joint through a narrow stalk, fluid can flow in but has difficulty flowing back out. This one-way valve effect explains why ganglion cysts tend to grow over time, especially with increased joint use, and may temporarily shrink with rest.
Kidney Cysts: Genetic Errors in Cell Growth
Polycystic kidney disease (PKD) illustrates how cysts can arise from inherited genetic mutations. In the most common form, autosomal dominant PKD, mutations in either of two genes (PKD1 or PKD2) disrupt proteins that normally help kidney tubule cells communicate and organize correctly. These proteins form a signaling complex that regulates cell growth, cell death, and how cells adhere to one another.
When this signaling complex malfunctions, kidney tubule cells lose their normal restraint. They proliferate when they shouldn’t, resist the programmed cell death that normally keeps tissue in balance, and begin forming fluid-filled pockets along the tubules. Over years, these pockets expand into cysts that can eventually crowd out functional kidney tissue. Unlike a skin cyst that forms and then sits quietly, kidney cysts in PKD tend to multiply and enlarge progressively because the underlying genetic defect affects every kidney cell from birth.
How Inflammation and Injury Create Cysts
The body’s wound-healing response can also produce cyst-like structures. When tissue is injured or a foreign substance gets lodged in the body, the immune system walls it off. Specialized cells called myofibroblasts and fibrocytes lay down collagen fibers around the site, building a fibrous capsule. This encapsulation is the body’s containment strategy: if it can’t eliminate the irritant, it seals it away.
In tissues made up of cells that don’t regenerate easily, like nerve tissue or heart muscle, the inflammatory response almost always ends in fibrosis and capsule formation rather than true healing. The result is a tough-walled pocket that may contain fluid, dead cells, or debris. Chronic infections can trigger the same process, with the immune system forming a walled-off abscess that, once the acute infection resolves, leaves behind an encapsulated cyst.
Why Cysts Come Back After Treatment
Simple drainage of a cyst provides quick relief but frequently leads to recurrence. The reason is straightforward: draining removes the contents but leaves the cyst wall intact. That wall is a living layer of cells that will continue producing whatever filled the cyst in the first place. For skin cysts, this means keratin. For ganglion cysts, synovial fluid. As long as the lining remains, the cyst refills.
Complete surgical excision, removing the entire cyst sac along with its contents, has a significantly lower recurrence rate. The procedure is typically done under local anesthesia. Success depends on getting the wall out in one piece. If fragments of the lining remain embedded in the surrounding tissue, they can seed new cyst growth.
Reducing Your Risk of Skin Cysts
You can’t prevent every type of cyst, particularly those driven by genetics or hormonal shifts, but skin cysts respond to a few practical measures. Keeping skin clean with gentle, non-comedogenic cleansers reduces the pore blockages that can initiate cyst formation. Regular exfoliation with products containing salicylic acid or glycolic acid helps clear dead skin cells before they accumulate in follicles. Avoiding repeated friction or pressure on the same area of skin also helps, since mechanical irritation is a known trigger for pushing surface cells into deeper tissue. Loose-fitting clothing and avoiding tight hairstyles that pull on follicles can make a difference in cyst-prone areas.
Signs a Cyst Needs Attention
Most cysts are benign and painless. The ones worth paying attention to are those that grow steadily, cause pain, or show signs of infection like redness, warmth, swelling, or fever. A ruptured skin cyst usually causes mild discomfort or none at all. A ruptured ovarian cyst is a different matter, potentially causing sharp abdominal pain and internal bleeding that warrants prompt evaluation. Any new lump that doesn’t shrink after a few weeks, or one that keeps growing, is worth having assessed. Imaging and, in some cases, a needle biopsy to sample the fluid can clarify whether a cyst is simple and harmless or something that needs treatment.