A cyst is a closed sac with a distinct wall, and what fills it depends entirely on where it forms and what type it is. Most cysts contain some combination of fluid, protein, dead skin cells, or oily substances, but some hold surprisingly complex materials like hair, bone, or even teeth. Understanding what’s inside a cyst also helps explain why they form in the first place.
The Outer Wall
Every cyst has a lining that separates its contents from the surrounding tissue. In the most common skin cysts (epidermoid cysts), this wall is made of the same layered skin cells that form your outer skin, called stratified squamous epithelium. The wall actively produces material that fills the interior, which is why cysts can slowly grow over time. This lining is also the reason cysts tend to come back if they’re drained but not fully removed: as long as the sac remains, it keeps producing its contents.
What’s Inside Skin Cysts
The lumps most people call “cysts” on their skin are usually epidermoid cysts, and they’re filled with keratin, the same tough protein that makes up your hair and nails. Under a microscope, this keratin appears as thin, layered flakes packed tightly together. In person, it looks and feels like a thick, off-white, cheese-like paste with a distinctly unpleasant smell.
Despite being commonly called “sebaceous cysts,” most skin cysts don’t actually involve oil glands and don’t contain sebum. That name is a holdover from older medical terminology. True sebaceous cysts are far less common and form specifically within oil-producing glands. Their contents are genuinely oily, a mix of lipids including cholesterol, fatty acids, squalene, and wax. The distinction matters because the two types look similar from the outside but have different compositions and origins.
Joint and Tendon Cysts
Ganglion cysts, the firm bumps that commonly appear on wrists and hands, contain a thick, clear, jelly-like fluid. This gel is rich in hyaluronic acid and other sugar-based molecules called mucopolysaccharides. It’s similar in composition to the lubricating fluid inside your joints but much thicker and more concentrated, which gives ganglion cysts their characteristic firmness when you press on them.
Ovarian Cysts
Functional ovarian cysts, the kind that form as part of a normal menstrual cycle, are typically filled with clear fluid. These follicular cysts develop when the egg-releasing sac on the ovary doesn’t open as expected and instead continues to grow. After ovulation, the sac can become a corpus luteum cyst, which actively produces progesterone to prepare the uterus for pregnancy. If these cysts get bumped or twisted, they can bleed internally, filling rapidly with blood and causing sudden, sharp pain.
Dermoid Cysts: The Strangest Contents
Dermoid cysts are a category unto themselves. They form when skin cells, tissues, and glands that would normally be on the body’s surface get trapped in a sac during fetal development. Because these trapped cells can develop into multiple tissue types, dermoid cysts can contain a remarkably strange assortment of materials. Most commonly, they hold a greasy yellow substance, but they may also contain fully formed hair, teeth, bone fragments, nerve tissue, sweat glands, and skin. These cysts are present from birth, though they may not become noticeable until later in life.
How Cysts Form in Glands and Ducts
Many cysts throughout the body form through a simple mechanism: a duct or gland opening gets blocked, and the material that would normally drain out builds up behind the blockage. Salivary duct cysts, for example, form when mucus gets trapped inside a saliva gland duct due to an obstruction. The blockage might be a tiny stone, a mucus plug, or scar tissue from inflammation or surgery. As secretions continue to accumulate with nowhere to go, the duct stretches and a fluid-filled sac develops. The same basic process explains many cysts in the breast, liver, kidneys, and pancreas.
How Cysts Differ From Abscesses
Cysts are sometimes confused with abscesses, but their contents are fundamentally different. A cyst is a closed sac containing debris, fluid, or air. Its contents are typically sterile. An abscess, by contrast, is a pocket of pus caused by a bacterial infection. Pus is a mixture of living and dead white blood cells, bacteria, and destroyed tissue. An abscess is hot, red, and painful because the immune system is actively fighting an infection inside it. A cyst can become infected and turn into an abscess, but on its own, a cyst is not an infection.
What Doctors Learn From Cyst Fluid
When a cyst forms on an internal organ like the pancreas, its contents can reveal whether it’s harmless or potentially dangerous. Doctors use a needle to draw out a small sample of fluid and test it for specific proteins and enzymes. In pancreatic cysts, for instance, high levels of certain digestive enzymes suggest an inflammatory cyst, while elevated levels of specific tumor markers point toward a cyst with the potential to become cancerous. The fluid’s chemical profile, combined with imaging, helps determine whether a cyst needs to be monitored or removed. For most external cysts, though, this level of analysis isn’t necessary since a physical exam or simple ultrasound is usually enough.