A meningioma is a brain tumor that grows in the meninges, a thin layer of tissue covering the brain and spinal cord. Some meningiomas contain cysts or calcified mineral deposits, and others contain hundreds of tiny blood vessels. Because meningiomas tend to grow inward, they commonly cause pressure on the brain or spinal cord. Infrequently meningiomas will grow outward, causing the skull to thicken. Meningiomas grow very slowly and may not produce symptoms for many years.