Microglia: main functions and associated diseases

The human immune system is made up of many different structures and processes. Organs such as the bone marrow, thymus, spleen or lymph nodes are involved in this function, essential for the production of immune cells.

In this article we will describe the functions and diseases related to microglia, one of these cells.

• Related article: "Glial cells: much more than the glue of neurons"

What is microglia?

Microglia is a type of glial cell found in the central nervous system. The term is used to speak of a set of cells that fulfill similar functions, mainly related to immune defense and phagocytization of potentially harmful elements for neurons.

The term “microglia” was coined in 1920 by Pío del Río Hortega, a disciple of neuroscience pioneer Santiago Ramón y Cajal. The immune functions of these cells have been known since the time of their discovery, although knowledge about their characteristics has advanced in recent decades.

It is a very versatile type of glia: the structure of the microglia varies according to the functions performed by each cell

, where you are, and the chemical signals you receive from neighboring neurons. We speak of "phenotype" to refer to the specific form that each microglia takes.

They originate from progenitor cells of the same lineage as those that make up the blood, probably located in the bone marrow or in the yolk sac attached to the embryo. Some of these cells migrate to the brain during intrauterine development; once they have reached this structure they differentiate as microglia.

Glial cells

Glial or glial cells are located in the nervous system, that is, in the brain, in the spinal cord and in the cranial nerves and spinals. They support neurons in different ways: they provide physical support, nourish them and eliminate pathogens, tissues damaged and waste products, promote the transmission of neuronal impulses through the formation of myelin ...

Among the cell types that are classified as glia, the most prominent are astrocytesessential for the structure and function of the blood-brain barrier, the oligodendrocytes, which create the myelin sheaths of the central nervous system, and Schwann cells, which do so in the peripheral.

Functions of these cells

Microglia are known primarily for their immune and hygienic roles; However, it also fulfills other varied functions, such as maintaining the balance of the extracellular environment of the nervous system or repairing damaged tissues.

1. Phagocytosis (removal of waste)

These cells engulf ("eat") different types of compounds in the central nervous system: injured and dead cells, debris, viruses, bacteria, neurofibrillary tangles, neuritic plaques... After phagocytization, both the microglia and its target remain inactive, thus reducing the risk of altering the functioning of the nervous system.

2. Maintenance of homeostasis

Microglia send signals through cytokines to other cell types, such as neurons, astrocytes, and T lymphocytes, which are also involved in the immune system. Among the consequences of this function, the regulation of homeostasis of the extracellular environment stands out, as well as the promotion of inflammation.

3. Inflammation and damage repair

When a tissue of the central nervous system is damaged or infected, the microglia facilitate it to become inflamed; in this way, the process of repairing the injured cells begins, during which these cells are very important.

In addition, if damage occurs to the spinal cord, the microglia removes the affected neural branches, allowing new nerve connections to be created.

4. Presentation of antigens

As a tissue becomes inflamed, T lymphocytes cross the blood-brain barrier and enter the central nervous system. Once here they unite with microglial cells that have engulfed antigens (particles from which antibodies are produced); This enhances threat removal and injury recovery.

5. Cell destruction (cytotoxicity)

Microglia have the ability to destroy bacteria, viruses, infected neurons, and other cell types by releasing hydrogen peroxide and nitric oxide. Sometimes this response is excessively aggressive and damages significant amounts of healthy tissue, causing even greater brain damage.

Diseases related to microglia

Microglial dysfunctions are associated with very diverse alterations. These cells appear to be involved in a relevant way in neurodegenerative diseases such as Alzheimer's, in which neuritic plaques and neurofibrillary tangles accumulate in the brain: microglial cytotoxicity attacks healthy neurons adjacent to damaged tissue.

Microglia cells play a similar role in the development of dementia due to infection by HIV, the AIDS virus. In fact, this disease also directly affects the microglia, infecting it and promoting neurotoxicity. Microglia are also involved in other infectious diseases, such as herpetic encephalitis and meningitis bacterial.

Research reveals that the glia is important in the development of neuropathic pain, which manifests itself in alterations such as allodynia or phantom limb syndrome. This is because they are activated in response to nerve damage and promote the chronic release of chemical compounds associated with the sensation of pain.