What comes out of a cyst depends entirely on what type of cyst it is. The most common type, an epidermoid cyst (often called a sebaceous cyst), releases a thick, cheesy, often foul-smelling material made of keratin, the same protein found in your hair and fingernails. But other cysts can contain anything from clear jelly to fully formed teeth and hair. Here’s what to expect from each type.
Epidermoid Cysts: Cheesy Keratin
Epidermoid cysts are the lumps most people picture when they think of a cyst. They sit just under the skin and are lined with cells that continuously produce keratin, which accumulates inside the sac over time. The contents are often described as cheesy, fatty, or oily in texture, and they come out as a thick paste ranging from white to yellowish in color.
The material is made of lamellated keratin flakes, essentially compressed layers of dead skin cells and skin secretions packed tightly together. When this material is exposed to air, either through drainage or rupture, it frequently smells terrible. The odor comes from the breakdown of trapped proteins and oils that have been sealed inside the cyst for months or even years. The longer the material has been accumulating, the stronger the smell tends to be.
Pilar Cysts: Denser and Firmer
Pilar cysts (also called trichilemmal cysts) grow almost exclusively on the scalp and originate from the outer root sheath of hair follicles. Like epidermoid cysts, they’re filled with keratin, but the consistency is noticeably different. Pilar cyst contents are denser and more compact, sometimes almost solid. Under a microscope, the keratin appears as a very dense pink material rather than the flaky layers seen in epidermoid cysts. These cysts can also calcify over time, making the contents feel gritty or chalky when removed.
Ganglion Cysts: Clear Jelly
Ganglion cysts form near joints and tendons, most commonly on the wrist. Instead of keratin, they contain a thick, clear, jelly-like fluid made of hyaluronic acid, glucosamine, and albumin. The fluid is highly viscous, closer to the consistency of hair gel than water. It’s essentially a concentrated version of the lubricating fluid your joints naturally produce. When a doctor aspirates a ganglion cyst with a needle, the fluid is often so thick it comes out slowly and may need a larger-gauge needle to extract.
Baker’s Cysts: Joint Fluid
A Baker’s cyst forms behind the knee when excess synovial fluid, the natural lubricant inside your knee joint, pushes into a pouch at the back of the leg. The fluid inside is the same joint fluid your knee normally contains, just in the wrong place. It’s typically thin and straw-colored. Baker’s cysts often develop because of an underlying knee problem like a cartilage tear or arthritis that causes the joint to overproduce fluid. If the cyst ruptures on its own, the fluid leaks into the calf, causing swelling and pain that can mimic a blood clot.
Dermoid Cysts: Hair, Teeth, and More
Dermoid cysts are the most unusual. Found most often on the ovaries, these cysts develop from germ cells that have the ability to grow into almost any type of human tissue. The contents can include fully formed skin, hair, teeth, sweat glands, nerve tissue, and even brain tissue. This happens because germ cells contain three developmental layers (ectoderm, mesoderm, and endoderm) that would normally build organs and body systems during fetal development. Inside a dermoid cyst, these cells mature on their own in a disorganized way, producing a bizarre mix of recognizable tissues floating in an oily fluid.
Despite their startling contents, ovarian dermoid cysts are almost always benign. They’re typically discovered incidentally during imaging for something else or when they grow large enough to cause pelvic pain.
Eye Area: Oil Versus Pus
Small cysts near the eyelid come in two common varieties, and their contents are quite different. A chalazion forms when an oil-producing gland in the eyelid clogs up. The trapped material is thickened, hardened oil that can sometimes be coaxed out with warm compresses as the gland opens and drains. A stye, by contrast, is a bacterial infection at the base of an eyelash. It produces actual pus, visible as a small yellowish-white spot at the center of a red, painful bump.
Normal Contents Versus Infection
One of the most important distinctions is between a cyst’s normal contents and signs that it has become infected. Uninfected epidermoid cysts release that characteristic thick, white-to-yellow cheesy material. When infection sets in, the contents change. You’ll see thinner, more liquid drainage that may be greenish or blood-tinged. The area around the cyst becomes red, swollen, warm, and painful to touch. The drainage from an infected cyst is pus, a mix of dead white blood cells, bacteria, and destroyed tissue, rather than the keratin paste of a healthy cyst.
Color is a useful guide. White or pale yellow, thick material from a skin cyst is typically normal keratin. Green, gray, or brown liquid drainage with increasing pain signals infection. A cyst that was previously painless and suddenly becomes tender and warm has likely become inflamed or infected, even if it hasn’t started draining yet.
Why Cysts Smell
The notorious smell of cyst contents comes from the breakdown of keratin, lipids, and other organic material trapped in an enclosed space with no oxygen. Bacteria that naturally live on your skin can colonize the inside of a cyst, further breaking down proteins and fats into sulfur-containing compounds. These are the same types of molecules responsible for the smell of rotten eggs. The odor is often strongest when a cyst ruptures suddenly or is squeezed, releasing material that has been decomposing in a sealed environment for a long time. Cysts that have been present for years tend to smell worse than newer ones simply because the contents have had more time to break down.