The manner in which the microglia cells react in cases of injury to the brain was first brought to my attention by Hortega in 1926. He had perfected a stain for the demonstration of microglia and had shown by its use the life history of these mesodermal cells, which have been called the "third element" in the nervous system.1
In Hortega's drawing (fig. 1), one sees the microglia cells appearing in the brain about the time of birth in the form of rounded cells. These cells soon acquire small buds or processes, and, as they migrate to their final positions in the nervous system, they gradually acquire the long, thin, spiky processes that enable one to distinguish them from all other cells in the nervous system when properly stained by Hortega's method.
In the case of an injury to the brain, the microglia cells undergo a change that is
STEVENSON LD. HEAD INJURIES: EFFECTS AND THEIR APPRAISALII. THE RÔLE OF THE MICROGLIA. Arch NeurPsych. 1932;27(4):784-790. doi:10.1001/archneurpsyc.1932.02230160025003