Microglial cells are macrophage-like cells of the central nervous system and the primary form of immunity in the brain. Activated microglia act as the ‘housekeepers’ of the CNS, and have many important roles in maintaining its health and function. Their main role is as phagocytes, and they are responsible for keeping the brain and spinal cord free of cellular debris. They also function as antigen-presenting cells, and secrete pro-inflammatory cytokines in the event of injury or infection.
What is a Microglial Cell?
Microglial cells are specialized, macrophage-like cells of the central nervous system (CNS). They account for up to 16.6% of all cells in the human brain and are the main form of immunity in the CNS. Microglial cells have several functions in maintaining the health of the brain and spinal cord, including phagocytosis of pathogens, apoptotic cells, and damaged and infected neurons. They also function as antigen-presenting cells and have 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, therefore helping to protect the CNS from infection.
Microglial cells exist in the CNS in two main states; these are the resting state and the activated state. In their resting state, microglia function as immune sentinels and continuously probe their surroundings for signs of damage and infection. Once activated, they take on a wide range of other functions including phagocytosis, antigen presentation, and production of pro-inflammatory cytokines.
Functions of Resting Microglia
Resting (or ramified) microglia are characterized by a small cell body and long, branching projections. They use these projections to probe their immediate surroundings for signals of tissue damage. In the event of injury or infection, microglial cells will become activated and rapidly migrate to the affected site.
Functions of Activated Microglia
When microglia become activated, they retract their branches and the cell body becomes larger. Activated microglia migrate to sites of injury and infection, where they rapidly multiply. They can perform several functions that resting microglia can’t, including phagocytosis, antigen-presentation, and cytokine secretion.
Microglial cells are resident macrophages of the CNS, and their primary function is to phagocytize pathogens and apoptotic or damaged neurons. By clearing cellular debris from the CNS, they help to prevent potentially harmful inflammation of the brain and spinal cord. Microglial cells also clear infections from the CNS by phagocytising viruses and bacteria. Therefore, microglia are critically important for maintaining the health and function of the CNS.
Microglia also function as the major antigen-presenting cells of the CNS, and play a central role in stimulating the adaptive immune response. After engulfing and digesting a pathogen, they display the antigens as part of their major histocompatibility complex (MHC) II and present it to naïve CD4+ T cells. The T cells are activated to become helper T cells, which then proliferate and help to stimulate other effector cells of the adaptive immune system.
Activated microglia secrete pro-inflammatory cytokines in response to tissue damage in the CNS. The cytokines activate other immune cells and recruit them to sites of injury or infection, where they further amplify the inflammatory response. By promoting inflammation of infected tissues, microglia help to kill invading pathogens and, therefore, help to clear infections from the CNS. However, prolonged inflammation can have harmful side effects, and uncontrolled activation of the microglia can damage healthy neurons and tissues.
Microglial Cells and Aging
The pro-inflammatory function of microglial cells is important for the removal of infectious agents from the CNS. It also contributes to the healing of damaged tissues. However, the inflammatory response is not always beneficial and has been implicated in the development of age-related, neurodegenerative diseases. Studies have found a strong link between chronic inflammation of the CNS and the onset of conditions such as Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis. Uncontrolled activation of the microglia may damage healthy tissues in the brain, eventually leading to the decline in cognitive function characterized by neurodegenerative disorders.
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 known as macroglia.
Oligodendrocytes are specialized cells that produce a fatty substance called myelin. This forms the myelin sheath; an insulative layer that surrounds the axons of all neurons. The myelin sheath is essential for the proper conduction of nerve impulses in the CNS.
Astrocytes are supportive cells that help to promote healthy neuronal function. Their main role is to maintain the chemical environment around neurons, deliver nutrients to the tissues of the CNS, and regulate blood flow in the brain.
Ependymal cells produce cerebrospinal fluid (CSF), which fills the cavities of 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 with the removal of waste products. It also acts as a shock absorber, and cushions the brain and spinal cord in the event of injury.