Ans. D: Kupffer cellsGlial cells are very numerous, there are 10-50 times as many glial cells as neurons. The Schwann cells that invest axons in peripheral nerves are classified as glia. In the CNS, there are three main types of neuroglia:Microglia consists of scavenger cells that resemble tissue macrophages. They probably come from the bone marrow and enter the nervous system from the circulating blood vessels.Oligodendrogliocytes are involved in myelin formation.Astrocytes, which are found throughout the brain, are of two subtypes. Fibrous astrocytes, which contain many intermediate filaments, are found primarily in white matter. Protoplasmic astrocytes are found in gray matter and have granular cytoplasm. Both types send processes to blood vessels, where they induce capillaries to form the tight junctions that form the blood-brain barrier. They also send processes that envelop synapses and the surface of nerve cells. They have a membrane potential that varies with the external K+ concentration but do not generate propagated potentials. They produce substances that are tropic to neurons, and they help maintain the appropriate concentration of ions and neurotransmitters by taking up K+ and the neurotransmitters glutamate and -aminobutyrate.