Light microscopy of brain tissue. The gray matter of the cerebral cortex has very many neurons but an even larger number of non-neuronal cells called glial cells. One type is the astrocyte seen here as star-shaped cells with many cytoplasmic processes that make contact with blood vessels. These contacts serve to control the passage of molecules from the blood into the brain hence the term blood-brain barrier. Most primary brain tumours arise from glial cells, known collectively as gliomas. Magnification x150 when height printed at 10 cm.