What’s inside a cyst depends entirely on where it forms and what type it is. Most cysts are fluid-filled sacs, but the fluid ranges from thin and watery to thick and paste-like, and some cysts contain surprisingly solid material like hair, calcium deposits, or even teeth. Here’s what fills the most common types.
Skin Cysts: Keratin and Sebum
The most common cysts people encounter are epidermoid cysts, those firm bumps that develop under the skin on the face, neck, back, or trunk. These are filled with keratin, the same protein that makes up your hair and nails. Mixed with lipid-rich debris, this keratin forms a thick, whitish-yellow paste that looks and smells like cheese. If you’ve ever seen someone squeeze one, that foul-smelling material is compacted keratin that’s been slowly accumulating inside a tiny sac lined with skin cells.
Pilar cysts, which grow almost exclusively on the scalp, contain a denser, more homogeneous form of keratin. The protein hardens differently in these cysts, producing a smoother, more solid mass. Pilar cysts also frequently develop calcium deposits within them, which can make the contents feel gritty or even rock-hard over time.
True sebaceous cysts are less common than most people think (many lumps called “sebaceous cysts” are actually epidermoid cysts). When they do occur, they form in the oil-producing glands of the skin and are filled with sebum, an oily mixture of cholesterol, fatty acids, wax, and a compound called squalene. Sebum is normally secreted onto your skin to keep it moisturized, but when a gland’s duct gets blocked, the oil pools and forms a soft, greasy lump.
Ganglion Cysts: Thick, Jelly-Like Mucin
Ganglion cysts are those rubbery bumps that pop up on wrists, hands, and feet, usually near joints or tendons. Inside them is a clear, thick, jelly-like substance called mucin, which is rich in hyaluronic acid. This is related to synovial fluid, the lubricant your joints naturally produce, but it’s far more viscous. Think of it as a concentrated, gel-like version of joint fluid.
These cysts form when joint stress or a small tear in the joint capsule allows fluid to leak into surrounding tissue. The body walls off that fluid with a pseudocapsule, and nearby cells continue producing mucin, causing the cyst to slowly grow. Unlike skin cysts, ganglion cysts don’t have a true epithelial lining. They’re essentially pockets of trapped gel.
Baker’s Cysts: Excess Joint Fluid
A Baker’s cyst forms behind the knee when the joint produces too much synovial fluid, often because of arthritis or a cartilage tear. That excess lubricating fluid migrates to the back of the knee and fills a small cushioning sac called a bursa, causing it to swell into a visible, fluid-filled bulge. The contents are thinner than what you’d find in a ganglion cyst, closer to a slippery liquid. If a Baker’s cyst ruptures, which happens rarely, the synovial fluid leaks into the calf and is gradually reabsorbed by the body.
Ovarian Cysts: Hormonal Fluid
Functional ovarian cysts, the kind that form during a normal menstrual cycle, contain a clear or slightly yellowish fluid rich in proteins, glucose, cholesterol, and hormones. These cysts develop when a follicle either doesn’t release its egg or doesn’t shrink after releasing one, and the fluid that was nourishing the developing egg remains trapped. Most ovarian cysts rupture on their own and the fluid simply dissipates without causing problems. In rare cases, a ruptured cyst tied to endometriosis can cause internal bleeding, and an infected cyst that bursts can trigger a serious immune response.
Dermoid Cysts: Hair, Teeth, and More
Dermoid cysts, a type of teratoma, contain the most surprising material of any cyst. Because they arise from germ cells (the unspecialized cells that can develop into any tissue type), they can grow hair, fat, muscle, and teeth inside them. Less commonly, teratomas contain remnants of complex structures like thyroid tissue, liver tissue, or even parts of eyes, including recognizable eyeballs. These cysts are present from birth, though they may not be discovered until adulthood. Despite their bizarre contents, most are benign.
What Happens When a Cyst Gets Infected
An uninfected cyst contains sterile material. But when bacteria enter the picture, the contents change dramatically. Your immune system floods the area with white blood cells, primarily neutrophils, which attack the bacteria and die in the process. The buildup of dead neutrophils, bacteria, and tissue debris creates pus, a protein-rich fluid that ranges from whitish-yellow to green depending on which immune proteins are present. A green tint comes from myeloperoxidase, an antibacterial protein produced by certain white blood cells. At this stage, the cyst has essentially become an abscess.
When any cyst ruptures, whether infected or not, the contents can irritate surrounding tissue. Even sterile keratin leaking from a ruptured skin cyst triggers an inflammatory response, because the body treats the material as foreign. This is why a ruptured cyst often becomes red, swollen, and painful even without infection. In skin cysts, the body may send specialized immune cells called giant cells to wall off and digest the leaked keratin, creating a noticeable inflammatory reaction that can last weeks.
How Doctors Tell What’s Inside
Imaging can reveal a lot about a cyst’s contents without opening it. On ultrasound, a simple fluid-filled cyst appears completely black (anechoic), while a cyst with thicker contents, internal walls, or solid components shows brighter areas. On CT scans, fluid-filled cysts appear dark, and denser material shows up lighter. MRI is particularly useful for characterizing complex cysts, since fluid lights up bright white on certain sequences.
Doctors also look at structural clues. Benign cysts tend to have thin, smooth walls, uniform fluid inside, and no blood flow on Doppler ultrasound. Cysts with thick or irregular walls, solid components mixed with fluid, multiple internal dividers, or detectable blood flow raise more concern and typically need further evaluation. A cyst’s shape matters too: smooth, round borders suggest something benign, while irregular or spiky margins do not.
The sac itself, regardless of cyst type, is a key part of the structure. Epidermoid cysts are lined with layers of skin-like cells that continuously shed keratin inward, which is why they keep growing. Pilar cysts have a similar lining but without the granular cell layer found in epidermoid cysts, which accounts for their different, more compact keratin. When a cyst is surgically removed, getting the entire lining out is what prevents it from coming back. If any lining cells remain, they continue producing material and the cyst reforms.