Introduction

The reactive gliosis, sometimes called Astrogliosis or simply Gliosis is refered to a physiological process appearing as a result of damage to the CNS (eg. traumatic brain Injury “TBI”, ischemia, neurodegenerative diseases). Reactive gliosis is as response by glial cells, mainly astrocytes, reacting to an injury and can be reviewed as a healing process. After a CNS injury, the astrocytes around the lesion respond to injury and undergo a typical change of hyperthrophy. These reactive astrocytes are gradually intergrated and form a physical barrier, commonly refered to as glial scar.

Glial Cells

The brain tissue contain two categories of cells, nervous cells and glial cells or, neuroglia. Glial cells are about 10-50 time more numerous than nerve cells and are involved in processes like:

Supply nutrients and oxygen

• Regulation of homeostasis
• Neurotransmission
• Structural support
• Insulation (Myelinating the axons)
• Destroy pathogens and remove dead neurons and metabolites

Following glial cells are the main cells involved in reactive gliosis:

Astrocytes

This is the most abundant glial cell type in the CNS. There two different types of astrocytes according to their location and morphology: protoplasmic astrocytes with characteristic branched processes, which are mainly found in the grey matter, and fibrous astrocytes found mainly in the white matter with fiber-like processes. Astrocytes act as the main regulator of the CNS to sustain homeostatic environment, which is a nessesity for regular neurological cell activity. This includes the ionic concentration regulation and secretion and degeneration of neurotransmitters. Astrocytes are connected to eachother through gap junctions forming a network called astrocyte syncytium which enables them to directly communicate which eachother. They also interact as a bidirectional bridge between the neurons and bloodstream. One of the functions of this communication is energy supply of the neurons, astrocytes is therefore highly related with the neuronal metabolism. Glucose and it´s metabolites are transmitted from the blood to the neurons and neighbouring through glucose transporters. Another function related to its close situation with blood vessels it the aid of the blood brain barrier. Astrocytes take part in numerous actions of the brain and the view and describtion of it´s functions have changed over time along with knowledge and research to a cell type with much more and advanced functions than earlier believed.

Microglia

Microglia is a local type of macrophage in the CNS with its main function directed to the immune defense. It constitutes approximately 5-20% of the total glial cell count and is most abundant in the grey matter. During normal conditions the microglia is in a "resting stage" where it have a ramified morphology with small processes, a unique look of the macrophage family. The microglia are extremely sensitive to any disturbance of the internal environment that can indicate on a potential danger to the CNS. In case of such alteration of homeostasis microglia will be activated. The cell will change in shape, gene expression and its function to counter the potential danger, this is what is called the “microglial activation”

Oligodendrocytes

Possessing few processes they are specializes in myelination of neurons’ axons. Their processes extend toward axons and wrap around them. Then they produce myelin to create the myelin sheet that accelerate neuronal transmission. One oligodendrocyte can myelinate several axons and are more abundant in the white matter.

NG2-Expressing Cells

Discovered a few years ago and today a fairly new accepted member of the glial cells gives the explanation to that their function in the brain is not yet fully understood. Their name derives from the ability to express at their surface chondroitin sulfate proteoglycan (NG2). They have several functions including being a precursor of other glial cells.