The adult human brain contains an estimated 86 to 100 billion neurons, and a roughly equivalent amount of non-neuronal cells, including glial cells and blood vessels. A characteristic mammalian brain, it is composed of a cerebral cortex, cerebellum, thalamus, hypothalamus, hippocampus, amygdala, basal ganglia, olfactory bulb, optic tectum, and the brainstem. The cerebral cortex makes up 82% of the brain mass and contains only about 16 billion neurons. Most of the cortex is actually composed of astrocytes (a type of glial cell) and capillaries. Most of the brain neurons, about 69 billion, are located in the cerebellum. The three major cell types of the brain, neurons, astrocytes, and oligodendrocytes all arise from the common neural stem cell. Notably, this type of adult stem cell can be isolated from the brain and cultured in vitro into free-floating neurospheres.