A cyst is a fluid-filled sac, lined by fibrous connective tissue or surface epithelium. It may form in an enlarged normal cavity (for example, a sebaceous cyst), it may arise in an embryonic remnant (for example, a branchial cyst), or it may occur as part of a disease process. Cysts may present as swellings or may cause pain, as in the case of some ovarian cysts. Multiple cysts in the kidney and liver occur in inherited polycystic diseases; here kidney failure may develop.