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Tumors which start in the brain are called Primary Brain Tumors, and are classified according to the kind of cell from which the tumor seems to originate. The most common Primary Brain Tumor in adults comes from cells in the brain called Astrocytes, that make up the blood–brain barrier and contribute to the nutrition of the central nervous system. These tumors are called gliomas (astrocytoma, anaplastic astrocytoma, or glioblastoma multiforme) and account for 65% of all primary central nervous system tumors. The following table explains other types of brain tumors, the cells from which the tumors most likely come, and the functions of those cells:

Type of Tumor Cell of Origin Function
Oligodendroglioma Oligodendrocyte Produces a substance called myelin, which covers the nerves and helps information to travel quickly between the brain and other parts of the body.  
Ependymoma Ependyma Lines the ventricles and aids in the circulation of cerebrospinal fluid.
Meningioma Meninges Covers and protects the brain and spinal cord.
Lymphoma Lymphocyte Part of the immune system, the body’s primary defense against infection and foreign substances.
Schwannoma Schwann cell Produce the myelin that protects the acoustic nerve, the nerve of hearing.
Medulloblastoma Primitive neuroectodermal Cell or Primitive nerve tumor (PNET) These cells normally do not remain in the body after birth.

Cancer from other parts of the body can spread to the brain and cause secondary tumors through a process called Metastasis. Although it is possible for cancer from anywhere in the body to spread to the brain, it happens most often with cancers of the breast and lung. The cells of a metastatic brain tumor resemble the cells of the organ where the tumor started, not brain cells. For example, if a tumor starts in the breast and spreads to the brain, the cells of the brain tumor will resemble abnormal breast cells, not abnormal brain cells.