Origin of Brain Macrophages and the Nature of the So-Called Microglia

Lymphoreticular cells in human brain tumours and in normal brain

“…Kitamura et al (1977) and Blakemore (1975) also claim to be able to differentiate resident microglial cells from macrophages, by silver staining and the presence of microtubules respectively. There is, on the other hand, equally convincing data from other workers that microglial cells and brain macrophages are either one and the same cell or are derived from the same cell precursor in the brain; the stem cell being initially derived from, and possibly subsequently replenished by, cells of the haematogenous compartment (Lantos, 1975;Das, 1976;Fujita & Kitamura, 1976).…”

Migration and Proliferation of Motile Immature Glial Cells in the Developing Cerebral Cortex of Infantile Rat.

“…ectodermal) amoeboid glial cells, since it is well known that differentiation of endogenous amoeboid microglia and hematogenous macrophages is extremely difficult and it is practically impossible to differentiate them without specific techniques of separate markings [20,30]; they could only be differentiated using cumulative labeling of tritiated thymidine to mark specifically only hematogenous cells [14] or by using chimeric animals with transplantation of bone marrow from a mouse of the same strain with tansfected DNA markers [16]. These techniques of separate markings are not feasible for studying newborn animals since such marking technique requires handling of the fetus in utero if we want to study the fate of marked cells in early postnatal periods.…”

“…Previous investigators of gliogenesis in the rat brain have pointed out [14,16,20,28,32] that immature astrocytes first become identifiable in the first postnatal week [16,32], followed by the appearance of oligodendroblasts and oligodendrocytes at the second postnatal week [16,33] and that typical resting microglia appear toward the end of the second postnatal week in the rat cerebral cortex [13,15,20,24,29]. Imamoto and Leblond [20], using tritiated thymidine autoradiography and electron microscopy together with semithin section morphology, pointed out that, in the developing rat cerebral hemisphere, typical microglia became detectable, in substantial number, first at P12.…”

“…After P11, they become differentiated into quiescent microglia and their migration and proliferation rapidly subside, resulting in homogeneous distribution in the adult cortical pattern. The mature microglia are potentially proliferative [14,16,20], as evidenced by the presence of Tb4, and can readily return to an active proliferative state when the brain is damaged [16].…”

Proliferation and differentiation of glial cells in the developing and mature rodent brains.

“…Recently however, the latter have been proved to be mononuclear leukocytes which have entered the brain parenchyma in response to injuries (3,17). Therefore, the term "microglia" will be used only for the resting microglia in this paper.…”