Dynamic glia in the living brain: insights into glial physiology and function from in vivo imaging

Glial cells play an essential role in the development, function, and plasticity of the central nervous system (CNS). Once regarded as passive support cells, glia are now recognized as highly dynamic and responsive participants in neural circuitry, a shift in understanding largely attributed to recent advances in in vivo imaging. This review highlights key discoveries from the past three decades that have transformed our understanding of glial cell functions, while also addressing the key obstacles ahead. We first discuss the technical challenges in imaging glial cells, including glial reactivity, accessing deeper brain regions, phototoxicity, anesthesia effects, and the need for glia-specific analysis tools. We then review major imaging-based discoveries across the four major glial subtypes in the mammalian CNS. For astrocytes, genetically encoded calcium indicators have visualized intracellular Ca²⁺ activity linked to neuromodulation and behavior. Microglia display continuous surveillance and intimate interactions with neurons and vasculature, contributing to synapse formation and network regulation. Oligodendrocyte precursor cells (OPCs), once considered mere resident progenitors for myelinating oligodendrocytes, exhibit rich structural and Ca²⁺ dynamics modulated by neurotransmitters. Finally, longitudinal imaging of oligodendrocytes has revealed ongoing myelin remodeling throughout life, supporting the theory that myelin is a plastic structure influenced by circuit activity. Together, these in vivo imaging approaches have illuminated the highly dynamic nature of glial cells, revealing multifunctional roles beyond structural support. Continued technological innovation will be critical to fully uncovering the multifaceted contributions of glia to brain function and plasticity.