The inside of a cyst depends entirely on what type it is, but most cysts share a basic structure: a thin wall or capsule surrounding a central cavity filled with fluid, semi-solid material, or sometimes surprisingly complex biological contents. The most common skin cysts contain a thick, white or yellowish, cheese-like substance made of a protein called keratin. Other cysts hold clear jelly-like fluid, watery liquid, or in rare cases, hair and even teeth.
The Basic Structure of a Cyst
Every cyst has two main parts: the wall and the contents. The wall is a thin layer of cells that forms a sac, separating the cyst from surrounding tissue. In the most common skin cysts (epidermoid cysts), this wall is made of the same layered skin cells you’d find on the surface of your body. These cells continuously shed inward, building up material inside the cavity the way skin normally flakes off on the outside.
The cavity itself, called the lumen, is where everything accumulates. Think of it like a balloon where the rubber is living tissue and the air inside has been replaced with whatever that particular type of cyst produces. The wall keeps making material, the material has nowhere to go, and the cyst slowly grows.
Skin Cysts: The Thick, Cheesy Material
Epidermoid cysts are the lumps most people picture when they think of a cyst. Cut one open and you’ll find layers of compacted keratin flakes filling the cavity. Keratin is the same protein that makes up your hair and nails, but inside a cyst it looks completely different. It forms a thick, pale, cheese-like paste that often comes out in layers, almost like an onion. Under a microscope, these layers appear as thin, flat sheets stacked on top of each other.
This material has a notoriously strong smell. The odor comes from the breakdown of proteins and fats trapped inside the cyst. Over time, bacteria (particularly Staphylococcus species) colonize the contents and use enzymes to split proteins into amino acids and fats into fatty acids. That process produces volatile sulfur compounds and other organic substances responsible for the pungent, rotten smell many people associate with cyst drainage.
One common misconception: most people call these “sebaceous cysts,” but that’s a misnomer. True sebaceous cysts filled with oily sebum are actually quite rare and mostly occur in a condition called steatocystoma multiplex. The vast majority of skin cysts are epidermoid cysts filled with keratin and cell debris, not oil.
Scalp Cysts: Dense and Compact
Pilar cysts, which grow almost exclusively on the scalp, contain the same basic protein as epidermoid cysts but with a different texture. The keratin inside a pilar cyst is denser and more solidly packed. Rather than the soft, cheesy consistency of an epidermoid cyst, pilar cyst contents tend to form a firm, waxy ball that can sometimes be removed in one piece. This happens because old skin cells and keratin pile up in tightly compressed layers beneath the scalp’s surface, creating a more solid mass over time.
Ganglion Cysts: Clear, Thick Jelly
Ganglion cysts, the rubbery bumps that typically appear on wrists and hands near joints or tendons, contain something entirely different from skin cysts. Their interior holds a clear, viscous, jelly-like fluid rich in hyaluronic acid (the same substance used in joint lubrication and skin care products), along with a protein called albumin and glucosamine. If you were to draw this fluid out with a needle, it would look like a thick, transparent or slightly yellowish gel, similar in consistency to hair gel. It’s nothing like the cheesy material in a skin cyst.
Ovarian Cysts: Watery to Mucus-Like
Internal organ cysts vary widely. Ovarian cysts fall into two broad categories based on what’s inside them. Serous cysts contain thin, watery fluid that looks similar to the clear liquid portion of blood. On imaging, this fluid appears uniform throughout the cyst. Mucinous cysts, by contrast, contain thicker, gel-like material that can vary in density from one pocket of the cyst to another. On a CT scan, different sections of a mucinous cyst often show up at different densities because the mucus-like contents aren’t uniform. Some areas may also contain protein-rich or blood-tinged fluid, giving them a more complex appearance.
Dermoid Cysts: The Strangest Contents
Dermoid cysts are the most unusual cysts in the body. They form from cells that got trapped during embryonic development, and because those cells have the potential to become multiple tissue types, the contents can be genuinely bizarre. Most dermoid cysts contain a greasy, yellow, oily material, but they can also hold fully formed hair, teeth, bone fragments, skin, sweat glands, and even nerve tissue. Opening a dermoid cyst and finding a clump of hair floating in yellow grease alongside a recognizable tooth is not unusual. These cysts most commonly appear on the ovaries, near the tailbone, or around the face and skull in children.
What Changes When a Cyst Gets Infected
An uninfected cyst typically contains pale, off-white or yellowish material with a pasty or gel-like consistency. When infection sets in, the contents change dramatically. Bacteria multiply inside the cyst, breaking down proteins and fats and producing pus in the process. The previously thick, cheesy keratin becomes more liquid, often turning yellow-green, and the smell intensifies significantly as sulfur compounds and fatty acids accumulate. The cyst wall itself becomes inflamed, turning the surrounding skin red, warm, and tender. At this stage, what was once a slow-growing, painless lump starts to behave more like an abscess.
How Cyst Contents Differ From a Lipoma
People often confuse cysts with lipomas, but the two look and feel completely different on the inside. A cyst is a defined sac with a distinct wall, filled with fluid or semi-solid material. A lipoma has no sac or cavity at all. It’s simply a collection of fat cells clumped together under the skin. Where a cyst feels firm when pressed, a lipoma feels soft and doughy and slides easily under your fingers. If you could see inside a lipoma, it would look like a lump of yellowish fat, similar to the fat you’d see on a cut of meat, with no liquid or paste-like material.