A cyst is an abnormal growth consisting of a closed cavity or sac containing solid or liquid material. The material inside a cyst is an accumulation of the products formed by the cells composing the cyst lining, or wall. Draining a cyst is not an effective method of treatment, because the cyst will fill up again. Surgical removal of the entire cyst is the only effective treatment.
Some cysts are present at birth. This type forms during embryonic development when some structure fails to disappear. Most cysts, however, occur after birth. Some, known as retention cysts, develop in glandular organs and are probably due to a blocking of the gland's opening. This type of cyst often occurs in the breast, sweat gland, salivary gland, and prostate gland.
Epidermal cysts occur as firm nodules in the skin. These cysts are often pea-sized or larger, and they are commonly filled with keratin, the homy substance normally present in epidermal cells. Sometimes, such cysts contain sebum, the oily substance produced by the sebaceous glands.
Epithelial implantation cysts are formed when a bit of living epidermis is poked beneath the skin surface. The epidermal tissue develops into a round nodule filled with keratin. Surrounding the cyst is a fibrous capsule made up of the connective tissue fibers that were pushed aside by the enlarging cyst.