Microglia heterogeneity along a spatio-temporal axis: More questions than answers.

Microglia are resident macrophages of the central nervous system; they arise during early embryonic development and persist throughout adulthood. These unique cells provide developmental support, contribute to adult brain homeostasis and impart immune protection during infection. Dysregulated microglia are implicated in the pathophysiology of several neurological disorders, including Alzheimer disease, and as such, a better understanding of their regulation and function is required for rational therapeutic design. Recent studies have highlighted the various heterogeneous aspects of microglia, such as their wide differentiation spectrum from early embryogenesis to adulthood, their location in different brain regions and their responses to ageing, infection and inflammation. In this review, we discuss microglial heterogeneity in time and space and highlight the remaining questions arising from such heterogeneity.