ABSTRACT
INTRODUCTION In the mid-nineteenth century, Virchow provided an early description of neuroglial. Following this, others observed non-neural cells in the brain that were thought to be due to the infi ltration of the developing nervous system by mesodermally derived cells. In 1899, Nissl identifi ed cells in the brain with rod-shaped nuclei and named them Staebchenzellen (1) . Ramon y Cajal (2) identifi ed this group of cells as the third element of the central nervous system as they were morphologically distinct from neurons and astrocytes. These cells were further distinguished as microglia and oligodendrocytes by del Rio Hortega. Microglia were identifi ed as a distinct cell type (3) originating from mononuclear cells of the circulating blood (4) . The current view is that microglia are derived from circulating monocytes or precursor cells in the monocytesmacrophage lineages, which originate in the bone marrow ( 5-8 ). The early work of del Rio Hortega suggested that microglia have the ability to transform from a resting ramifi ed phenotype into an amoeboid phagocytic phenotype. In this early work on microglia, morphological classifi cations fell into three types: ramifi ed, intermediate forms, and amoeboid ( 4 , 9 ). More recent studies have used a similar classifi cation but these terms have lead to the attachment of an additional functional connotation with resting, activated, and amoeboid phagocytic microglia ( 10 , 11 ). The accumulation of more recent data suggests that it is likely that the earlier classifi cation relying on morphological features will end up being the more correct form of a classifi cation system given the heterogeneity of functional activities of the cells ( Fig. 1 )
IDENTIFICATION OF MICROGLIA In his initial work, del Rio Hortega (12) used a weak silver carbonate method to selectively stain microglia that required very specifi c fi xation procedures. While this method can be used for electron microscopy examination, it is variable with regards to the number of microglia stained and across species. Enzyme histochemical methods have been successfully employed for staining microglia. Thiamine pyrophosphatase activity is associated with the plasma membrane of microglial cells and with blood vessels in the CNS (13) . Nonspecifi c esterase is an enzyme used to identify microglia in cultures (14) ; however, it has limited use in tissue sections. For example, it does not label reactive microglia in the hypoglossal nucleus after peripheral nerve transection (15) , yet it can be found in brain macrophages within a stab-wound. This distinct staining pattern in vivo raised concerns that microglia isolated and cultured represent cells that have transformed into brain macrophages as a consequence of the culture process (see following section). Activated phagocytic microglia (macrophages) both in vivo and in culture can be detected based upon increased enzyme activity. Such enzymes include acid phosphtase, 5 ′ -nucleotidase, and oxidoreductase (16) . Activated microglia can express other markers such as nitric oxide synthase, lysosomal proteinases, plasminogen activator, lysozyme, urine nucleoside phosphorylase, and elastase ( 17-19 ); however, these enzymes are not specifi c for microglia and can be expressed by other glial cell types.
