Microglia beyond boundaries: Paradigm shifts in origin, distribution, state diversity, and therapeutic regeneration

Microglia have undergone a fundamental redefinition, transitioning from traditional CNS-restricted scavengers to a highly dynamic and systemically distributed immune lineage. This review synthesizes recent paradigm shifts that challenge long-standing concepts in neuroimmunology. We first discuss the revision of the classical vascular extravasation model by a newly identified integrin-dependent pial surface migration route for embryonic microglial progenitors. Second, we introduce the expanded concept of the "microglial lineage," which includes transcriptionally and ontogenetically homologous cells residing not only in the CNS but also in peripheral tissues such as the skin, heart, and peripheral nervous system, suggesting broader physiological functions. Third, we highlight the transition from the obsolete M1/M2 polarization model to a microenvironment-driven, dynamic multimodal framework that captures the complex and context-dependent nature of microglial states in health and disease. Finally, we review breakthroughs in regenerative therapy, from pharmacologically induced repopulation to exogenous replacement using iPSC-derived or gene-corrected microglia, offering new hope for genetic microgliopathies such as adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP). Collectively, these advances position microglia as central mediators of neuro-immune crosstalk and open novel avenues for treating neurodegenerative, neuroinflammatory, and developmental brain disorders.