Oligodendrocytes and microglia cells were discovered the latest due to resistance to metal impregnation staining. Early in 1858, Rudolph Virchow had coined ''myelin'' to designate the sheath around stained axons, and in 1878 Louis Ranvier described interruptions in the myelin sheath (nodes of Ranvier). Camillo Golgi developed in 1873 a silver impregnation staining neurons and partially astrocytes. In 1913, Ram on y Cajal improved Golgi staining by gold and described astrocytes. He named ''third element'' the cells resistant to staining. In 1918, CajalΓs student P Δ±o del R Δ±o-Hortega improved Golgi staining by silver carbonate and discovered the ''third element'' as two new cell types. One type had short processes that he named ''microglia'' from the Greek ''micro'' [small] and ''glia' ' [glue] in 1919; and another cell type had few processes: ''oligodendroglia'' for ''oligo'' [few], ''dendro'' [tree], and ''glia'' [glue] in 1921. However, R Δ±o-Hortega had to wait for Wilder Penfield to travel to Madrid in1924 and corroborate his observations, forcing Cajal to accept microglia and oligodendroglia. The function of microglia remained obscure until the end of the 20th century, despites the early observations by Nissl in 1899 and Merzbacher in 1909 of a rapid transformation from ramified to activated state. R Δ±o-Hortega had recognized the role of microglia in pathology as well as their mesodermic origin. In the late 1980Γs microglia function made a breakthrough with Georg Kreutzberg who demonstrated that activated microglia expressed immunological markers. Microglia is currently viewed as the first effector of the brain. Meanwhile, the function of oligodendrocytes producing the myelin sheath remained hypothetical until after World War II. The definite proof was provided in 1962 by Richard Bunge with electron microscopy showing a continuity between oligodendrocyte and myelin. The chemical analysis of myelin started in the 1950Γs followed by sequencing of proteins in the 1970Γs. A decade later Helmut Kettenmann showed oligodendrocytes were more than insulator as endowed with current channels. Oligodendrocytes are now considered the survival of neurons. The future will unveil the full extent of their functions.
THE CURIOUS ODYSSEY OF THE UNDERAPPRECIATED