Microglia: Driving critical periods and sexual differentiation of the brain.

The proverbial role of microglia during brain development is shifting from passive members of the brain's immune system to active participants that are able to dictate enduring outcomes. Despite these advances, little attention has been paid to one of the most critical components of early brain development-sexual differentiation. Mounting evidence suggests that the normal developmental functions microglia perform-cell number regulation and synaptic connectivity-may be involved in the sex-specific patterning of the brain during these early sensitive periods, and may have lasting sex-dependent and sex-independent effects on behavior. In this review, we outline the known functions of microglia during developmental sensitive periods, and highlight the role they play in the establishment of sex differences in brain and behavior. We also propose a framework for how researchers can incorporate microglia in their study of sex differences and vice versa. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 580-592, 2018.