Concepts of what might constitute reactive astrocyte subtypes are not well developed and We posit that there is a need to establish criteria for discriminating amongst them. Considerable information is available from decades of structural and cell biological studies that should not be neglected. The first clues for distinguishing among different mammalian reactive astrocytes subtypes derived from examining their structure, proliferative state, the types of cells they interacted with, and the tissue architecture to which they contributed [1, 30, 44]. Based on these criteria, we propose that at least two fundamentally different subtypes can be readily discriminated at this time: (i) astrocytes that are newly proliferated and organized into a new and permanent tissue architecture that forms borders around areas of overt tissue damage or inflammation (Key Figure, Figure 2C), and (ii) astrocytes that do not proliferate and retain the basic cell structure, tissue architecture, and functional interactions they exhibited in healthy tissue (Key Figure, Figure 2D). Basic features of these two broad subtypes can be summarized as follows: Proliferative, border-forming reactive astrocytes surround damaged tissue that contains leukocyte infiltration,
