Astrocytes are specialized, support cells found in the human brain. Together with the oligodendrocytes and microglia, they form the neuroglia, which is the supportive tissue of the cerebrum. Astrocytes are star-shaped cells with numerous slender outgrowths that intertwine with the neurons, being the bridge between the nerve cells body and the walls of the capillary blood vessels. Therefore, their main function is to transfer metabolites from capillaries to the neurons and to regulate the water-salt metabolism of nervous tissue as they are the principal element of the blood-brain barrier. They account for about 60 per cent of the total number of neuroglial cells. The network of astrocytes is called 'astroglia'. This important support cell of the brain was discovered in 1911 by the Spanish histologist Nicolas Achucarro, using his own staining method.
There are five types of this supportive cell. There are protoplasmic astrocytes in the gray matter of the cerebral cortex, fibrous astrocytes in white matter of cerebrum, Bergmann cells in cerebellum, pituicytes in the posterior lobe of pituitary gland, and Müller cell in eye retina. The cell bodies and processes of astrocytes are interconnected by gap junctions that form a matrix. This matrix surrounds the neurons, separating them from one another. Each astrocyte in turn has several processes that extend out in every direction among nerve cells and oligodendrocytes to make contact with neurons and the basement membrane of a capillary through their processes' ''end-feet''. Astrocytes allow the interchange of ions and molecules between them and the extracellular fluids. Both astrocyte and neuron have negative membrane potentials, showing that their membranes are permeable to potassium ions.
Astrocytes store and transfer metabolites such as glucose and ketone bodies from the capillaries to the neurons where they are used as fuel by their mitochondria to produce ATPs. They also take up excess potassium from the extracellular potassium sinks through potassium channels. After intense neuronal activity, astrocytes also absorb glutamate and neurotoxins that build up in the extracellular spaces and synaptic clefts. Their intimate contact with neurons, capillaries, and cerebrospinal fluid (CSF) places astroglial cells (astrocytes) in a unique position to control the environment of the nerve cells, that is the extracellular fluid of the brain.
 |
| A photo-micrograph of one astrocyte. |
 |
| A micrograph of a network of astrocytes, which are in contact with capillaries and neurons. |
No comments