A ganglion cyst is filled with a thick, gel-like fluid that’s rich in hyaluronic acid, the same molecule your body uses to lubricate joints. The fluid is often described as having the consistency and appearance of “apple jelly,” ranging from clear to slightly yellowish. It’s not pus, blood, or anything solid, and a ganglion cyst is not cancerous.
The Fluid Inside
The main ingredient inside a ganglion cyst is hyaluronic acid mixed with other sugar-protein compounds called mucopolysaccharides. Together, these create a slippery, gelatinous substance that’s much thicker than the fluid normally found in your joints. If you’ve ever seen joint lubricant drawn from a knee during a medical procedure, the fluid inside a ganglion is similar but noticeably more viscous, almost like a thick jelly rather than a liquid.
This fluid is similar to, but not identical to, normal joint fluid. Your joints naturally contain a thinner version of this same lubricating substance. Inside a ganglion cyst, the concentration of hyaluronic acid is higher, which is why the contents are so thick and sticky. When doctors drain a ganglion cyst with a needle, the fluid can be difficult to extract because of this gel-like consistency.
The Cyst Wall
Surrounding the fluid is a wall made of dense connective tissue, primarily layers of collagen fibers stacked loosely on top of one another. This is an important detail: a ganglion cyst has no true lining. Unlike other cysts in the body that are enclosed by a layer of specialized cells (epithelial cells), a ganglion lacks this feature entirely. Technically, that means it isn’t a “true cyst” in the pathological sense, even though everyone calls it one.
The inner surface of the wall does contain scattered cells, but they’re not the same type found lining a joint. Under a microscope, these cells resemble fibroblasts or early muscle-like cells rather than the synovial cells that line joint capsules. This distinction is what separates a ganglion cyst from a synovial cyst. A synovial cyst is lined with actual joint tissue and filled with standard joint fluid. A ganglion cyst is its own structure, walled off by connective tissue and filled with a concentrated version of joint lubricant.
How the Fluid Gets There
A ganglion cyst grows out of the tissues around a joint or tendon, almost like a balloon inflating on a stalk. The American Academy of Orthopaedic Surgeons describes it as emerging from ligaments, tendon sheaths, or joint linings. The exact trigger for why a ganglion forms isn’t fully understood, but the leading theory involves repetitive small injuries to soft tissue around a joint.
Here’s what researchers believe happens: repeated stress on a ligament or tendon sheath causes a type of tissue breakdown called myxoid degeneration. The connective tissue in the area begins to deteriorate at a microscopic level, and fibroblasts (the cells responsible for building and repairing connective tissue) start producing excess hyaluronic acid. This accumulates in a pocket, gradually expanding as more gel-like mucin collects. Over time, the pocket forms a defined wall and becomes the firm, round bump you can feel under the skin.
The connection to the joint or tendon sheath explains why ganglion cysts tend to change size. When you use the joint more, fluid may be pushed into the cyst through its stalk, making it swell. During periods of rest, it can shrink. This is also why they most commonly appear on the wrist, where repetitive motion is constant.
What’s Not Inside
Because the word “cyst” can sound alarming, it helps to know what a ganglion cyst doesn’t contain. There are no cancer cells. There’s no infection. The fluid isn’t blood, and the cyst isn’t connected to a tumor. It’s a self-contained pocket of concentrated joint lubricant held together by fibrous tissue. It’s entirely benign.
A ganglion cyst also differs from an abscess, which is filled with pus from a bacterial infection and surrounded by inflamed tissue. And it’s distinct from a lipoma, which is a lump of fat cells. The contents of a ganglion are uniquely its own: that thick, clear, jelly-like fluid that your body produced in the wrong place, in too high a concentration, likely in response to mechanical stress on the joint.