microglial

Microglial cells are the resident immune effector cells of the vertebrate central nervous system. They derive from yolk sac progenitors and migrate into the brain early in development, where they persist throughout life with limited turnover. In health, microglia monitor the CNS environment and contribute to tissue homeostasis and development.

In the normal brain, microglia exhibit a ramified morphology with branching processes that continually survey tissue. Upon activation, they retract processes and adopt an amoeboid shape. They express markers such as IBA1, TMEM119, and P2RY12, though marker expression varies with state and region.

Functions include debris clearance, synaptic pruning during development, release of signaling molecules, and modulation of neuroinflammation. They phagocytose apoptotic cells, clear protein aggregates, and secrete cytokines and chemokines that influence neurons, astrocytes, and the vasculature. Microglia also participate in synaptic remodeling and neurogenesis in certain contexts.

Microglia respond to injury, infection, and pathology through complex signaling networks. The traditional M1/M2 polarization is an oversimplification; microglia show a spectrum of activation states with diverse gene expression. Chronic microglial activation is linked to neuroinflammation and may contribute to neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and demyelinating disorders.

Research aims to modulate microglia for therapy, including imaging markers, CSF biomarkers, and strategies to limit deleterious activation or promote protective phenotypes. Distinct CNS macrophage populations, including meningeal and perivascular macrophages, are related but separable from parenchymal microglia.