A cyst /sɪst/ is a closed sac, having a distinct envelope and division compared with the nearby tissue.Cancer-related cysts are formed as a defense mechanism for the body following the development of mutations that lead to an uncontrolled cellular division.That said, the cancerous cells still may mutate further and gain the ability to form their own blood vessels, from which they receive nourishment before being contained.Pseudocysts are similar to cysts in that they have a sac filled with fluid, but lack an epithelial lining.[17] This is just one example of how the Greek root cyst-, which simply means a fluid-filled sac, also is found in medical terms that relate to the urinary bladder and the gallbladder, neither of which involve cysts.