What’s inside a cyst depends entirely on the type of cyst, but most contain some combination of protein, fluid, cellular debris, or tissue. The most common skin cysts are filled with a soft, cheese-like material made of keratin, the same protein that makes up your hair and nails. Other cysts contain clear fluid, thick mucus, blood, or in rarer cases, fully formed tissues like hair and teeth.
Inside a Common Skin Cyst
The cysts most people encounter are epidermal inclusion cysts, the firm bumps that form just beneath the skin’s surface (often mistakenly called “sebaceous cysts”). These are not filled with oil or fat. Instead, the interior is packed with keratin and dead skin cell debris. This material builds up in layers, like pages in a book, and has a distinctive pasty, cheese-like consistency. It’s typically white or yellowish and often has a strong, unpleasant smell.
These cysts form when skin cells get trapped beneath the surface, usually after a scratch, surgical wound, or skin condition like acne. Normally, dead skin cells migrate to the surface and shed. When that path gets disrupted, the cells collect in a pocket within the deeper layer of skin, where keratin accumulates over time. The cyst wall itself is made of epithelial cells, the same type of cells that form the outer layer of your skin, and they keep producing keratin into the enclosed space.
Fluid-Filled Cysts
Not all cysts contain that thick, cheesy material. Many are filled with fluid, and the type of fluid varies based on where the cyst forms.
Ganglion cysts, the rubbery lumps that appear near wrist or hand joints, contain a thick, jelly-like substance rich in hyaluronic acid (the same molecule used in skin products and joint injections). This gelatinous fluid also contains smaller amounts of proteins like globulins and albumin. It’s clear or slightly translucent and much thicker than water.
Baker’s cysts, which form behind the knee, are filled with synovial fluid, the natural lubricant your joints produce. These develop when synovial fluid accumulates in a small pouch behind the knee, often because of arthritis or a cartilage tear. The tissue acts like a one-way valve, allowing fluid to flow into the cyst but preventing it from draining back out.
What’s Inside Ovarian Cysts
Ovarian cysts are among the most varied in terms of their contents. Functional cysts, the kind that form during a normal menstrual cycle, typically contain clear fluid or a mixture of fluid and blood. A corpus luteum cyst, for example, forms when the structure that releases an egg each month fails to shrink back down and instead fills with fluid or blood.
Serous ovarian cysts are usually filled with a clear, watery fluid. Mucinous cysts, by contrast, contain a thick, sticky mucus produced by the cells lining the cyst wall. This mucus is rich in glycoproteins with a high carbohydrate content, giving it a viscous, gel-like quality that’s quite different from the thin fluid in serous cysts.
Dermoid Cysts Contain Actual Tissue
Dermoid cysts are the most unusual type. Also called mature cystic teratomas, these ovarian cysts develop from germ cells, the precursors that normally differentiate into various body tissues during fetal development. Because germ cells have the potential to become almost anything, dermoid cysts can contain fully mature, recognizable tissue: skin, hair, teeth, sweat glands, nerve tissue, and even brain tissue.
This happens because germ cells have three foundational layers. One layer can become skin, hair, and teeth. Another becomes muscle and connective tissue. The third forms gut tissue and internal organs. A dermoid cyst may contain material from any or all of these layers, which is why surgeons sometimes open one to find a clump of hair, a formed tooth, and a patch of skin all enclosed in the same sac. Despite their strange contents, the vast majority of dermoid cysts are benign.
How Cysts Expand
A cyst isn’t just a static pocket. It actively grows through two mechanisms working together. First, the cells lining the cyst wall transport ions into the enclosed space. This creates an osmotic pull that draws water inward, stretching the cyst and increasing its internal pressure. Second, as the wall stretches, the lining cells divide and multiply, making the sac larger and capable of holding even more fluid or debris. The result is a self-reinforcing cycle: fluid accumulates, the wall stretches, new cells grow to accommodate the expansion, and the cyst keeps enlarging.
Cyst Contents vs. Abscess Contents
People sometimes confuse cysts with abscesses, but what’s inside each is fundamentally different. A cyst contains sterile material: keratin, fluid, mucus, or tissue, depending on the type. An abscess contains pus, which is a mixture of living and dead white blood cells, bacteria, and damaged tissue. Pus forms because your immune system sends white blood cells to fight a bacterial infection, and those cells accumulate at the infection site.
The distinction matters because a cyst can sit under your skin for years without causing problems, while an abscess is an active infection that tends to be painful, warm, and red. If a cyst becomes infected, its contents can shift from sterile keratin or fluid to something resembling pus, but a true cyst starts out without any bacterial involvement.
Can Cyst Contents Be Cancerous?
For common skin cysts, the risk of cancer is extremely low. Most epidermal cysts are entirely benign, with only rare case reports of malignant transformation. When it does happen, squamous cell carcinoma is the most commonly reported type, though other cancers like basal cell carcinoma and melanoma have been documented in isolated cases. Ovarian cysts carry a slightly higher level of clinical attention, which is why doctors sometimes use imaging or fluid analysis to distinguish between benign and concerning types, but the majority of ovarian cysts are also noncancerous.