Cysts form when fluid, air, or semi-solid material accumulates inside a closed sac of tissue. The specific trigger depends on the type of cyst, but the general pattern is consistent: something causes cells to create a pocket, and that pocket fills with material that has no way to drain. Some cysts develop from blocked pores, others from hormonal imbalances, joint stress, or inherited genetic mutations. Most are benign, but understanding how they form helps explain why some disappear on their own while others grow or cause problems.
How Skin Cysts Develop
The most common cysts people notice are the firm, round bumps that appear under the skin. These are almost always epidermoid cysts, which account for about 74% of all benign skin cysts. Despite being widely called “sebaceous cysts,” they don’t actually involve oil glands. They start in hair follicles.
The process begins when the opening of a hair follicle becomes plugged. Skin cells normally shed and cycle out of the follicle, but when the opening is blocked, those cells have nowhere to go. Instead of reaching the surface, they accumulate beneath the skin and continue producing keratin, the tough protein that makes up your outer skin layer. Over weeks or months, this buildup creates a sac lined with skin cells, filled with soft, yellowish keratin. The cyst stays connected to the skin surface through a tiny plugged opening, which sometimes appears as a small dark dot.
These cysts are usually painless unless they rupture. When the sac wall breaks, keratin spills into the surrounding tissue, and your immune system reacts to it as a foreign substance. That’s what causes the sudden redness, swelling, and tenderness that sends many people to the doctor.
Ovarian Cysts and Hormonal Triggers
Functional ovarian cysts are among the most common cysts in the body, and most women develop them without ever knowing. Each month, an ovary grows a small fluid-filled follicle that houses a maturing egg. Normally, the follicle ruptures to release the egg during ovulation, then shrinks. A cyst forms when this process stalls: either the follicle doesn’t rupture and keeps growing (a follicular cyst), or it releases the egg but then seals shut and fills with fluid (a corpus luteum cyst). Both types typically resolve on their own within one to three menstrual cycles.
Polycystic ovary syndrome (PCOS) involves a different mechanism. Overproduction of androgens by the ovaries, and sometimes the adrenal glands, disrupts normal follicle development. Instead of one follicle maturing and releasing an egg each cycle, multiple follicles begin growing but stall at a small size, typically 2 to 9 millimeters in diameter. These arrested follicles accumulate on the ovaries, creating the characteristic “string of pearls” appearance on ultrasound. The underlying problem involves changes in how certain regulatory molecules in the follicle’s supporting cells behave, leading to higher rates of follicle failure and a lack of ovulation.
If an ovarian cyst ruptures unexpectedly, it can cause sudden, sharp pain along with nausea and vomiting. Most ruptured cysts cause only mild to moderate symptoms and resolve without treatment. Rarely, a rupture leads to significant internal bleeding that requires emergency care.
Ganglion Cysts and Joint Stress
Ganglion cysts, the rubbery lumps that appear near wrist and finger joints, form through a less straightforward process. The exact origin of the thick, jelly-like fluid inside them is still debated, but joint stress plays a central role in all leading theories.
One explanation is that repetitive motion or acute injury creates a small tear in the joint capsule, allowing synovial fluid (the lubricant inside your joints) to leak into surrounding tissue. The body reacts to this displaced fluid by forming a wall around it, and wrist motion acts like a pump, pushing more fluid into the growing sac. A second theory suggests the process works in reverse: joint stress causes the connective tissue near the joint to break down, forming a cyst that only later develops a connection to the joint itself. A third possibility is that stressed tissue cells actively secrete the gel-like substance, building the cyst from scratch rather than filling it with leaked joint fluid.
Regardless of the mechanism, ganglion cysts fluctuate in size with activity. They often grow larger with repetitive use and may temporarily shrink with rest.
Genetic Cyst Formation in the Kidneys
Autosomal dominant polycystic kidney disease (ADPKD) is the clearest example of genetically driven cyst formation. It affects roughly 1 in 400 to 1,000 people and involves mutations in one of two genes. Mutations in the first gene account for about 78% of cases and cause more aggressive disease, with kidney failure occurring on average around age 58. Mutations in the second gene cause a slower progression, with kidney failure averaging around age 80.
These genes produce proteins that help kidney tubule cells maintain their shape and structure. Cyst formation follows what researchers call a “threshold model.” Every cell has two copies of the gene, and the kidneys function normally as long as the combined protein activity stays above a critical level. When activity drops below that threshold in an individual cell, the cell begins to multiply. Rather than forming a solid mass like a tumor, the proliferating cells create a small outpouching from the kidney tubule, lined by a single layer of mutant cells. This pouch eventually pinches off and becomes a self-contained cyst.
Once detached, the cyst grows through two parallel processes. The cells lining the cyst wall keep dividing, expanding the sac. At the same time, those cells actively pump fluid into the cyst’s interior. The mutant cells also shift their energy metabolism in ways that resemble cancer cells, relying on a less efficient but faster form of energy production that supports rapid growth. Over decades, hundreds or thousands of these cysts enlarge, gradually replacing functional kidney tissue.
Trauma-Related Cysts
Physical injury can also trigger cyst formation. Post-traumatic cysts of soft tissue typically form at the boundary between the fat layer under your skin and the tough connective tissue beneath it. Crushing or shearing forces, like those from a direct blow or a fall, damage tissue in this zone. As the body repairs the damage, fluid collects in the disrupted space, and a fibrous wall forms around it. These cysts may appear weeks or even months after the original injury, which is why people sometimes don’t connect the bump to the event that caused it.
How Cysts Differ From Tumors
Cysts and tumors can feel similar under the skin, but they have fundamentally different structures. A cyst is a sac filled with fluid, air, or semi-solid material. A tumor is a solid mass of tissue. On ultrasound or CT scan, cysts that appear uniform and fluid-filled are almost always benign. Cysts with solid components need closer evaluation, often through repeat imaging to check whether they change over time. When imaging alone can’t determine whether a growth is harmless, a biopsy (removing a small tissue sample for lab analysis) is the most reliable way to rule out cancer.
The distinction matters because the management is different. Simple fluid-filled cysts are often left alone and monitored. Cysts that grow, cause symptoms, or show worrisome features on imaging may be drained or surgically removed. Solid tumors follow an entirely separate evaluation and treatment path.
Why Some Cysts Resolve and Others Don’t
Whether a cyst disappears on its own depends on what created it. Functional ovarian cysts resolve because the hormonal cycle that produced them moves on, and the fluid is gradually reabsorbed. Ganglion cysts sometimes shrink when the joint stress that feeds them decreases. Epidermoid cysts, on the other hand, rarely go away without removal because the sac lining continues producing keratin indefinitely. The sac itself is the problem: even if a cyst is drained, the lining remains and will refill unless it’s completely excised.
Genetically driven cysts like those in polycystic kidney disease never resolve spontaneously. The underlying mutation continues driving cell growth and fluid secretion in every affected cell, and new cysts keep forming throughout a person’s life. This is why genetic cyst diseases are progressive conditions rather than one-time events.