Most cysts are fluid-filled sacs lined with a thin layer of skin-like cells, but what’s actually inside varies widely depending on the type. Some contain watery fluid nearly identical to water. Others are packed with thick, paste-like protein, oily fat, or even fully formed hair and teeth. The outer wall and the inner contents are two distinct parts of a cyst, and both differ based on where in the body the cyst forms and what triggered it.
The Outer Wall: What Holds a Cyst Together
A true cyst has an outer wall lined with epithelial cells, the same type of cells that form the surface of your skin and line your organs. This lining is what separates a true cyst from a pseudocyst, which is simply a pocket of fluid held in place by scar tissue and inflammation without any organized cellular lining. The distinction matters because the epithelial wall is what allows a true cyst to keep producing and accumulating material inside it.
In epidermoid cysts (the most common type found under the skin), the wall is made of stratified squamous epithelium, essentially the same layered structure as the outer layer of your skin. These cells continuously shed inward, filling the cyst cavity the same way your skin constantly sheds dead cells outward. Kidney cysts, by contrast, are lined with a single layer of cells that actively pump fluid into the enclosed space, causing them to gradually enlarge over time.
Epidermoid Cysts: Keratin, Not Fat
The most common skin cysts are often called “sebaceous cysts,” but that name is misleading. Most of these lumps are actually epidermoid cysts, and they don’t contain sebum (skin oil) at all. Instead, the interior is packed with keratin, the same tough protein that makes up your hair and fingernails.
Inside an epidermoid cyst, keratin accumulates in thin, layered sheets that compress into a semi-solid paste. It typically looks white or yellowish and has a texture somewhere between cottage cheese and toothpaste. The smell is often the most memorable feature. As keratin breaks down, bacteria in the cyst produce hydrogen sulfide, ammonia, and short-chain fatty acids, creating a strong, unpleasant odor that intensifies if the cyst ruptures or becomes infected.
When an epidermoid cyst gets infected, the contents change noticeably. Pus, a mix of dead white blood cells, bacteria, and cellular debris, turns the material more liquid and yellow or green. The cyst becomes tender, swollen, and the odor worsens.
True Sebaceous Cysts: Oil and Wax
Genuine sebaceous cysts are far less common than epidermoid cysts. They form directly from sebaceous glands, the tiny oil-producing glands attached to hair follicles. The contents are actual sebum: an oily mixture of lipids including cholesterol, squalene, and wax. This material tends to be more liquid and greasy than the thick keratin paste found in epidermoid cysts, though it can still have a yellowish color and unpleasant smell.
Ganglion Cysts: Thick, Clear Jelly
Ganglion cysts, the firm bumps that commonly appear on wrists and hands, contain something entirely different from skin cysts. Their interior is filled with a thick, clear, jelly-like substance called mucin. This gel forms when the connective tissue near a joint or tendon degenerates. Collagen fibers break down, and surrounding cells produce mucin in response to chronic irritation or repetitive stress. The result is a viscous, sticky material with a consistency similar to hair gel, enclosed in a fibrous sac rather than a true epithelial lining.
Dermoid Cysts: Hair, Teeth, and More
Dermoid cysts are among the most unusual types because they can contain fully formed tissues. These cysts develop from germ cells, the early embryonic cells that normally differentiate into every tissue type in the body. When germ cells get trapped in the wrong location, typically in an ovary, they can still mature into recognizable structures.
Opening a dermoid cyst might reveal hair, skin, sweat glands, teeth, nerve tissue, and even small amounts of brain tissue, all floating in an oily fluid produced by the sweat glands inside the cyst. The three embryonic cell layers (ectoderm, mesoderm, and endoderm) can each contribute different tissues, which is why the contents are so varied. Despite sounding alarming, ovarian dermoid cysts are almost always benign. They grow slowly as the internal glands secrete sebum, gradually expanding the sac.
Kidney Cysts: Mostly Water
Simple kidney cysts contain the least dramatic material of any cyst type. The fluid inside is a clear or straw-colored liquid with a consistency and density close to water. It’s essentially a filtrate, similar in composition to very dilute urine, with low protein content and low viscosity. These cysts form when a segment of a kidney tubule becomes blocked and the lining cells continue pumping fluid into the sealed-off space. Most simple kidney cysts cause no symptoms and are discovered incidentally on imaging done for other reasons.
Baker’s Cysts: Excess Joint Fluid
A Baker’s cyst forms at the back of the knee and contains synovial fluid, the slippery lubricant your joints naturally produce. The fluid itself isn’t abnormal. The problem is volume. When arthritis, a torn meniscus, or another knee injury triggers inflammation, the joint produces more lubricating fluid than it can reabsorb. Because of the anatomy of the knee, the excess fluid can only escape in one direction: backward, through a natural weak point in the joint capsule. The fluid pools into a sac behind the knee, forming the cyst. The contents feel like a thick, slightly viscous liquid, similar to egg white.
Why Cyst Contents Vary So Much
The material inside a cyst is determined by the cells that line it. Skin cells produce keratin. Oil gland cells produce sebum. Joint-lining cells produce synovial fluid. Kidney cells produce a watery filtrate. Germ cells can produce nearly anything. This is why “cyst” is such a broad term. The only thing all cysts truly share is a basic structure: a closed sac, separated from surrounding tissue, with something accumulating inside. What that something is depends entirely on where the cyst forms and which cells built the wall.
Pseudocysts, like those that develop in the pancreas after inflammation, skip the cellular lining altogether. Their walls are made of fibrin, scar tissue, and inflammatory debris rather than organized epithelial cells. The contents are typically enzyme-rich fluid from the damaged organ, enclosed by the body’s attempt to wall off the irritation. Without an epithelial lining actively producing material, pseudocysts are more likely to resolve on their own than true cysts are.