A cyst is a closed sac, having a distinct membrane and division compared to the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (not unlike the manner in which water molecules group together, forming a bubble); however, the distinguishing aspect of a cyst is the cells forming the “shell” of such a sac, being distinctly abnormal (in both appearance and behaviour) when compared to all surrounding cells for that given location. It may contain air, fluids, or semi-solid material.
see Tumor cyst.
Cystic lesions in the atrium (trigone) of the lateral ventricle may become symptomatic due to obstruction of physiological CSF circulation and/or from mass effect on adjacent structures.