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The lining of a joint has three cell types. One secretes, one absorbs and a third creates a envelope. A cystic mass which emerges from the distal interphalangeal joint is known as a mucous cyst. These lesions emerge from the joint. One theory as to their origin has it that an envelope cell most likely undergoes an metaplasia and begins to function as a fluid producing cell, but due to its location on the surface of the joint the fluid thereby generated produces an extra-articular mass. A breakdown of apoptosis with persistence of a cell leading to errors in genetic expression might be responsible for such a scenario, though what factors might influence this seeming desire of the cell to persist when it should desist making room for new cells is unknown. The treatment is to remove the offending cells and thereby eliminate the cyst.