The identification of diverse astrocyte roles in both health and disease has fueled interest in the possibility that different astrocyte subtypes may exist that exert different functions. The concept of astrocyte heterogeneity is not new. Over a hundred years ago, classical neuroanatomists described various morphological subtypes in healthy CNS that are common across mammals, including bushy astrocytes in grey matter, fibrous astrocytes in white matter, border-forming glia limitans astrocytes along meninges and many others [2]. Recent and ongoing studies are expanding information about structural, genetic and functional diversity of astrocytes across the healthy CNS [33–41] and are beginning to identify its developmental origins [42, 43]. In addition, there is a steadily growing interest in characterizing the diversity of astrocyte reactivity and understanding how it is regulated. In this regard, information obtained with different methodologies is accumulating rapidly and there is a need for guidelines on assimilating this information. Here, we present a working model and conceptual framework for categorizing diverse forms of astrocyte reactivity, consistent with current information, as discussed below; the model recognizes and encompasses the need for flexibility for future expansion and modification as information accrues (Key Figure, Figure 2).
