Cell surface morphology identifies microglia as a distinct class of mononuclear phagocyte

Abstract

To investigate differences among brain-derived microglia and other classes of immune cells, we compared the morphologies and growth properties of mononuclear phagocytes isolated from tissues of the newborn rat. Scanning EM shows that microglia from postnatal rat brain are covered with spines (typically > 20 per cell body) in a distinctive manner which contrasts the smooth surfaces of bone marrow cells and the ruffled surfaces of tissue macrophages from spleen, liver, and peritoneum. The spine-bearing surface of microglia is a specific cell marker, for it does not change with age or after exposure to cytokines or other immunostimulants. Approximately 99% of mononuclear phagocytes cultured from normal adult rat brain are spinous microglia. Five days after injury to rat brain, cells at sites of Wallerian degeneration are essentially all spinous ones while nearly 30% of cells found within areas of infarction or penetrating trauma are invading macrophages. In a similar way, nearly all cells isolated from normal, postmortem adult human brain are spine-bearing microglia (> 99% homogeneity). Brains from patients with amyotrophic lateral sclerosis contain only spinous microglia whereas cells from HIV-1 infected brains include significant numbers of invading ruffled macrophages. Cultured microglia, unlike cultured bone marrow precursors, monocytes, or tissue macrophages, spontaneously develop long, thin processes that extend hundreds of microns in length. Microglia retract these processes after exposure to fetal bovine serum, laminin, or such immunostimulants as recombinant murine interferon gamma (rmIFN gamma) and lipopolysaccharide. Of all types of mononuclear phagocytes tested, only microglia differentiate into quiescent ramified cells when in contact with astrocytes. Thus, microglia represent a unique class of cell maintained, in part, by astroglia as dormant, ramified mononuclear phagocytes in mature CNS. Application of cell surface criteria described here will allow study of distinct populations of mononuclear phagocytes associated with neurologic disorders.

The cellular response in neuroinflammation: The role of leukocytes, microglia and astrocytes in neuronal death and survival

“…1). These flow cytometric based observations are consistent with previous reports by Guilian and colleagues that microglia and macrophages isolated from CNS tissue can be distinguished by their differential size and density of cell processes [39]. These FACs and EM detectable differences in morphology of CNS-resident microglia and CNS-infiltrating macrophages are evident in ex vivo analysis.…”

The origin and differentiation of microglial cells during development

“…Second, some markers that label macroglial cells have been reported also to label microglia (Dickson and Mattiace, 1989;Wolswijk, 1995). And third, microglial cells have features di erent from other cells of the mononuclear phagocytic system: they display particular morphological features in vitro (Giulian et al, 1995), proliferate spontaneously in culture (Giulian et al, 1995), and have a distinct ion channel pattern di erent from that of monocytes and macrophages in non-nervous tissues (Kettenmann et al, 1990;Brockhaus et al, 1993).…”

“…At present, it is not known whether all cells of the macrophage/monocyte lineage can produce microglia cells, or whether this ability is limited to a special subset of such cells. Giulian et al (1995) found that mononuclear phagocytes isolated from the brain of newborn rats gave rise to cells with particular morphological features which did not appear in cultured mononuclear phagocytes from non-nervous sources; these authors concluded that microglial precursors are a unique class of cell di erent from precursors of other types of mononuclear phagocytes. In this connection, it was found that a subpopulation of bone marrow-derived cells showed the same ion channel pattern as microglial cells, suggesting that the bone marrow contains precursors that are committed to produce microglia and that they are di erent from the precursors which produce macrophages for other body regions (Banati et al, 1991).…”

“…The transformation of ameboid microglia into rami®ed microglia is now largely accepted (Perry and Gordon, 1988;Ling and Wong, 1993;Davis et al, 1994;Barron, 1995). In vitro studies have shown that cultured microglial cells become rami®ed under certain conditions, such as culture on astrocyte monolayers (Sievers et al, , 1996Tanaka and Maeda, 1996), when certain factors are added to the culture medium (Suzumura et al, 1991;Giulian et al, 1995;Fujita et al, 1996;Eder et al, 1997), and in the presence of certain extracellular matrix molecules Giulian et al, 1995). The addition of serum to the culture medium inhibits the appearance of the rami®ed phenotype Giulian et al, 1995;Fujita et al, 1996;Ilschner and Brandt, 1996), although one recent study reported that microglial cells do not ramify when serum is absent (Wilms et al, 1997).…”

“…In vitro studies have shown that cultured microglial cells become rami®ed under certain conditions, such as culture on astrocyte monolayers (Sievers et al, , 1996Tanaka and Maeda, 1996), when certain factors are added to the culture medium (Suzumura et al, 1991;Giulian et al, 1995;Fujita et al, 1996;Eder et al, 1997), and in the presence of certain extracellular matrix molecules Giulian et al, 1995). The addition of serum to the culture medium inhibits the appearance of the rami®ed phenotype Giulian et al, 1995;Fujita et al, 1996;Ilschner and Brandt, 1996), although one recent study reported that microglial cells do not ramify when serum is absent (Wilms et al, 1997). The distribution and properties of microtubules change during the transformation of microglial cells in culture: rami®ed microglia show more stable microtubules, with a higher rate of acetylation and detyrosination than in ameboid microglia, and microtubules in rami®ed cells are apparently not anchored to microtubule organizing centers (Ilschner and Brandt, 1996).…”