Today, we recognize three broad groups of glial cells: (i) true glial cells or macroglia, such as astrocytes and oligodendrocytes, of ectodermal origin, the stem cell of which is the spongioblast; (ii) microglia, of mesodermal origin; and (iii) ependymal cells, also of ectodermal origin and sharing the same stem cell as true glia. Microglia invade the CNS at the time of vascularization via the pia mater, the walls of blood vessels and the tela choroidea. Glial cells differ from neurons in that they possess no synaptic contacts and retain the ability to divide throughout life, particularly in response to injury. The rough schema represented in Figure 1-3 demonstrates the interrelationships between the macroglia and other CNS components.