8 Acta Neuropathol (2010) 119:7–35 normal astrocyte functions or gain of abnormal effects to contribute to, or play primary roles in, disease processes, and there are now numerous examples of astrocyte contributions to clinical and pathological mechanisms [15, 52, 212, 224– 226, 237]. This article summarizes recent advances in (1) astrocyte functions in healthy tissue, (2) the gradations, mechanisms, functions, and effects of reactive astrogliosis, and (3) the appearance and potential roles of reactive astro- cytes in a variety of CNS disorders and pathologies. Astrocyte biology in healthy CNS Basic astrocyte morphology Since the late nineteenth century, astrocytes have been divided into two main subtypes, protoplasmic or ﬁbrous, on 123 b Fig. 1 Astrocyte morphology and interactions with blood vessels in healthy and diseased tissue. a Protoplasmic astrocyte giving rise to a dense network of ﬁnely branching processes throughout its local gray matter neuropil, as well as to a large stem branch that extends foot processes along a blood vessel (bv). b Two color ﬂuorescence showing astrocytes in healthy mouse gray matter stained immuno- histochemically for GFAP (red) as well as the transgene-derived reporter molecule GFP (green). Note that in these transgenic mice [95], GFP reporter is present in all of the ﬁne processes of the protoplasmic astrocytes throughout the neuropil, whereas the GFAP is present only in the large stem astrocyte processes and endfeet (which appear yellow where green and red staining overlap). Note that endfeet from many astrocytes contact and envelop bv. c Two color ﬂuorescence showing dense accumulations of GFAP-positive (green) endfeet and processes of reactive astrocytes lining up along perivas- cular cuffs or clusters (pvc) of inﬂammatory cells stained with DAPI (blue) in a mouse with experimental autoimmune encephalomyelitis (EAE). Transgenic disruption of this reactive astrocyte barrier leads to widespread invasion of inﬂammatory cells away from perivascular clusters into CNS parenchyma during EAE [251]. d Two color brightﬁeld staining showing human autopsy specimen with reactive astrocytes lining their processes along perivascular cuffs of inﬂam- matory cells as if forming perivascular scar-like barriers similar to those observed in experimental animal models [251]. Scale bars a 3 lm, b 7.5 lm, c 15 lm, d 5 lm the basis of differences in their cellular morphologies and anatomical locations [191]. These two main categories retain their validity and usefulness today. Protoplasmic astrocytes are found throughout all gray matter and, as ﬁrst demonstrated using classical silver impregnation tech- niques, exhibit a morphology of several stem branches that give rise to many ﬁnely branching processes in a uniform globoid distribution (Fig. 1a). Fibrous astrocytes are found throughout all white matter and exhibit a morphology of many long ﬁber-like processes [191]. Classical and modern neuroanatomical studies also indicate that both astrocyte subtypes make extensive contacts with blood vessels (Fig. 1a, b). Electron microscopic analyses of the mid- twentieth century revealed that the processes of proto- plasmic astrocytes envelop synapses and that the processes of ﬁbrous astrocytes contact nodes of Ranvier, and that both types of astrocytes form gap junctions between distal processes of neighboring astrocytes [183]. These basic attributes of astrocyte morphology and some of their functional interactions are summarized schematically in (Fig. 2a). Molecular markers and proteomic characterization Immunohistochemical techniques that enable the detection of speciﬁc molecular markers at the single-cell level are essential tools for identifying and characterizing cells in healthy and pathological tissue. Expression of glial ﬁbrillary acid protein (GFAP) has become a prototypical marker for immunohistochemical identiﬁcation of astrocytes.