Microglia

Microglia account for 0.5-16.6% of the total cell population in the human brain and 5-12% in the mouse brain. Under physiological conditions, the number and function of microglia are strictly controlled by the local microenvironment and the interaction with surrounding cells. In response to an insult, microglia have the ability to transform into different functional states, thereby modifying their morphology, proliferation, phagocytic activity, antigen presentation and the release of inflammatory factors.

Microglia generally polarize in two directions from a resting state. The classical activation is known as M1, which is the mediator of pro-inflammatory responses, producing and releasing ROS, nitrogen reactive species (NRS) and cytokines. The alternative activation, known as M2, is responsible for resolution and repair, involving in the production and release of trophic factors. The polarization of microglia has been clarified by measuring the markers both in vitro and in vivo.

Table 1. Polarization of activated microglia in CNS.

Microglia are the main immune cells of the CNS and are highly similar to macrophages. They act as the major inflammatory cell type in the brain and activate to respond to pathogens and injury. Although seeking out and eliminating pathogens is an important role, microglia have also been extensively studied for their harmful roles in neurodegenerative diseases and brain injuries, such as Alzheimer's disease, Parkinson's disease, ischemic injury, and traumatic brain injuries.