Gray matter heterotopias are neurological disorders caused by clumps of gray matter (nodules of neurons) located in the wrong part of the brain. A grey matter heterotopia is characterized as a type of focal cortical dysplasia. The neurons in heterotopia appear to be normal, except for their mislocation; nuclear studies have shown glucose metabolism equal to that of normally positioned gray matter. The condition causes a variety of symptoms, but usually includes some degree of epilepsy or recurring seizures, and often affects the brain's ability to function on higher levels. Symptoms range from nonexistent to profound; the condition is occasionally discovered as an incidentaloma when brain imaging performed for an unrelated problem and has no apparent ill effect on the patient. At the other extreme, heterotopia can result in severe seizure disorder, loss of motor skills, and mental retardation. Fatalities are practically unknown, other than the death of unborn male fetuses with a specific genetic defect. Background The development of the brain in the human fetus is extraordinarily complex and is still not fully understood. Neural matter originates in the outer, ectodermic layer of the gastrula; thus, it originates from the cell layer primarily responsible for skin, hair, nails, etc., rather than from the layers that develop into other internal organs. The nervous system originates as a tiny, simple open tube called the neural tube; the front of this tube develops into the brain (and retinas of the eye), while the spinal cord develops from the very back end. Neurons begin to form early, but most of them become structural rather than active nerve cells. The brain generally forms from the inside-out, especially in the case of the neocortex. The difficulties arising from this are readily apparent, as each successive layer of cells must travel through the previous layer to reach its destination. Therefore, nervous tissue develops ladders made of radial glial cells that neurons climb, through the previous layers, to reach their proper destination. Some destinations, such as the cerebral cortex, even have "placeholder" neurons that travel up the ladder to form a structure; when the final neurons germinate, they find a correct placeholder and then the placeholder cell dies. The complexity of neural development makes it fraught with opportunities for error. Grey matter heterotopia is such an example. It is believed that gray matter heterotopia are caused by arrested migration of neurons to the cerebral cortex; that is,