Cerebral cysts are abnormal structures that can occur in the brain and gradually enlarge by producing or entrapping fluid. Think of it as a balloon being filled with water.
There are several types of cerebral cysts that are known to occur in the brain. All types of cerebral cysts generally arise because of an error in the formation of brain structures during prenatal development. These cysts are nearly always benign, but not necessarily innocent if they get big enough to press against important parts of the brain.
Arachnoid cysts (sometimes called leptomeningeal cysts)
These occur within the subarachnoid space around the outside of the brain where the membranes, which form the subarachnoid space, cause extra fluid to collect in one specific location.
Neuroepitheleal cysts (sometimes called ependymal cysts)
Lined by the same type tissue that normally lines the brain ventricles, these cysts usually occur within the same area but are separate from the brain ventricles.
These fluid filled membranous sacs that occur within the normal brain ventricles are formed from the ependymal tissue that normally lines the ventricle or from arachnoid tissue that became entrained into the ventricle through the choroid fissure (the middle, vascular coat of the eye).
Choroid plexus cysts
Material within the normal brain ventricle called the choroid plexus is very vascular and normally produces the cerebral fluid. Occasionally a portion of the choroid plexus entraps the fluid it is producing which then forms a cyst that is attached to the choroid plexus.
These fluid containing structures typically occur in the roof of the third ventricle and contain a fluid that is different from the normal cerebral fluid.
Dermoid cysts and epidermoid cysts
Early in embryological development one layer of tissue, called the ectoderm, transforms in various ways to produce both the neurological structures and the skin structures. An error of that process can cause "skin-like" tissues, such as dermis or epidermis, to become entrapped within brain tissue, producing benign cysts.