Cyst << sihst >> is a sac in the body that contains fluid and has no outside opening. Cysts of the skin occur more often than others. These usually form when the opening of an oil gland or hair follicle (baglike structure that surrounds a hair) becomes blocked. Some cysts form around a foreign substance that gets into the body. Others form around blood, after an injury. A cyst that forms in the salivary gland under the tongue is called a ranula. Cysts of internal organs are usually caused by abnormal development of the organ. Cysts may be removed by surgery. See also Polycystic kidney disease; Wen.
