<P> Pituicytes from the posterior pituitary are glia cells with characteristics in common to astrocytes . Tanycytes in the median eminence of the hypothalamus are a type of ependymal cell that descend from radial glia and line the base of the third ventricle . </P> <P> In general, neuroglial cells are smaller than neurons; there are about 86 billion neurons and 85 billion "non-neuronal" (glia) cells in the human male brain . Glial cells make up about half the total volume of the brain and spinal cord. ^ The ratio varies from one part of the brain to another . The glia / neuron ratio in the cerebral cortex is 3.72 (60.84 billion glia (72%); 16.34 billion neurons), while that of the cerebellum is only 0.23 (16.04 billion glia; 69.03 billion neurons). The ratio in the cerebral cortex gray matter is 1.48, with 3.76 for the gray and white matter combined . The ratio of the basal ganglia, diencephalon and brainstem combined is 11.35 . </P> <P> Most cerebral cortex glia are oligodendrocytes (75.6%); astrocytes account for 17.3% and microglia for 6.5% . </P> <P> Most glia are derived from ectodermal tissue of the developing embryo, in particular the neural tube and crest . The exception is microglia, which are derived from hemopoietic stem cells . In the adult, microglia are largely a self - renewing population and are distinct from macrophages and monocytes, which infiltrate the injured and diseased CNS . </P>

Are the most abundant cells in the human brain