Microglial cells are macrophage-like cells in the central nervous system and are the primary form of immunity in the brain. Activated microglia act as “housekeepers” for the CNS and have many important roles in maintaining its health and function. Their main job is that of the phagocytes and they are responsible for keeping the brain and spinal cord free from cell debris. They also act as antigen presenting cells and secrete proinflammatory cytokines in the event of injury or infection.

Microglia are the most important immune cells in the CNS
Microglia are macrophage-like cells in the CNS

What is a microglial cell?

Microglial cells are specialized, macrophage-like cells of the central nervous system (CNS). They make up 16.6% of all cells in the human brain and are the main immunity in the CNS. Microglial cells have multiple functions in maintaining brain and spinal cord health, including phagocytosis of pathogens, apoptotic cells, and damaged and infected neurons. They also act as antigen presenting cells and play a role in promoting inflammation in the CNS.

Functions of microglial cells

Microglial cells are the immune cells of the CNS. They remove damaged neurons and infectious agents from the brain and spinal cord, thus helping to protect the CNS from infection.

Microglial cells exist in the CNS in two main states; These are the Hibernation and the activated state. When at rest, microglia act as immune guards, continuously examining their surroundings for signs of damage and infection. Once activated, they perform a variety of other functions including phagocytosis, antigen presentation, and production of pro-inflammatory cytokines.

Microglial cells are tissue-resident macrophages of the CNS
Microglial cells are the most important immune cells in the CNS

Functions of the dormant microglia

Rest (or branched) Microglia are characterized by a small cell body and long, branched processes. They use these projections to examine their immediate surroundings for signals of tissue damage. In the event of an injury or infection, microglial cells are activated and quickly migrate to the affected area.

Functions of activated microglial cells

When microglia are activated, they retract their branches and the cell body grows larger. Activated microglia migrate to sites of injury and infection, where they multiply quickly. They can perform several functions that dormant microglia cannot perform, including phagocytosis, antigen presentation, and cytokine secretion.

Microglia withdraw their branches when activated
Activated microglia have indented branches

Phagocytosis

Microglial cells are resident macrophages of the CNS and their primary function is to phagocytize pathogens and apoptotic or damaged neurons. By clearing debris from the CNS, they help prevent potentially harmful inflammation of the brain and spinal cord. Microglial cells also clear infections from the CNS by phagocytizing viruses and bacteria. Therefore, microglia are vital to the maintenance of the health and function of the CNS.

Antigen presentation

Microglia also act as the main antigen-presenting cells of the CNS and play a central role in stimulating the adaptive immune response. After engulfing and digesting a pathogen, they present the antigens as part of their major histocompatibility complex (MHC) II and present them to naive CD4 + T cells. The T cells are activated to become T helper cells, which then multiply and help stimulate other effector cells of the adaptive immune system.

inflammation

Activated microglia secrete proinflammatory cytokines in response to tissue damage in the CNS. The cytokines activate other immune cells and recruit them to injury or infection sites, where they further intensify the inflammatory response. By promoting inflammation in infected tissues, microglia help kill invading pathogens, thereby removing infections from the CNS. However, prolonged inflammation can have harmful side effects, and uncontrolled activation of the microglia can damage healthy neurons and tissues.

Microglia cells promote inflammation in the CNS
Activated microglial cells secrete proinflammatory cytokines

Microglial cells and aging

The pro-inflammatory function of microglial cells is important for the removal of infectious agents from the CNS. It also helps heal damaged tissue. However, the inflammatory response is not always beneficial and has been linked to the development of age-related neurodegenerative diseases. Studies have found a strong link between chronic inflammation of the CNS and the occurrence of diseases such as Alzheimer’s, Parkinson’s, and multiple sclerosis. Uncontrolled activation of the microglia can damage healthy tissues in the brain and ultimately lead to a decline in cognitive function, which is characterized by neurodegenerative disorders.

Microglia have been linked to Alzheimer's, Parkinson's, and multiple sclerosis
Inflammation caused by microglia has been linked to neurodegenerative diseases

What are macroglia?

Microglial cells are one of four types of glial cells in the CNS. The other three (oligodendrocytes, astrocytes and ependymal cells) are collectively referred to as macroglia.

Oligodendrocytes

Oligodendrocytes are specialized cells that produce a fatty substance called myelin. This forms the Myelin sheath; an insulating layer that surrounds the axons of all neurons. The myelin sheath is essential for the proper conduction of nerve impulses in the CNS.

Astrocytes

Astrocytes are supportive cells that promote healthy neural function. Their main job is to maintain the chemical environment of the neurons, to supply the tissues of the CNS with nutrients and to regulate the blood flow in the brain.

Ependymal cells

Ependymal cells produce cerebrospinal fluid (CSF), which fills the cavities in the brain and spinal cord. CSF has several important roles in maintaining healthy brain function. It delivers vitamins and nutrients to the CNS and helps remove waste products. It also acts as a shock absorber and cushions the brain and spinal cord in the event of injury.

The CNS contains four types of glial cells
There are four types of glial cells in the CNS

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