Astrocytes

All the organs in the body are made up of two types of cells; the parenchymal cells that perform the actual function of that organ, and the supporting cells that provide support and nutrition to the parenchymal cells.

The same is true for the brain and spinal cord. The
nervous system consists of neurons that perform the actual function of the
system. The supporting cells that provide nutrition, support, and protection to
the neurons are called the glial cells.

Astrocytes are a subtype of glial cells present in the brain and spinal cord.  They are the most abundant glial cells present in brain. Astrocytes have two subtypes and are performing a number of functions. In the article, we will review different aspects related to astrocytes.

Structure

As the name suggests, astrocytes are star-shaped cells. They have several processes radiating from the central body. There are two types of astrocytes. Each type has its own structure depending on the function performed by it.

Generally, astrocytes have sparse organelles and have several processes radiating from the central cell body. These processes may be branched or unbranched. Each process of the astrocytes shows extensive terminal branching that allows a single astrocyte to be associated with a number of neuronal synapses.

Some processes have vascular feet at their end and take part in forming the blood-brain barrier (BBB).

Types

Astrocytes are divided into two broad categories:

Fibrous astrocytes:

The fibrous astrocytes have long processes radiating from the central cell body. These processes are unbranched. They have only a few organelles. The long processes have vascular feet that encircle the capillaries present in the vicinity of the astrocytes.

These astrocytes are typically seen in the white matter of the brain and spinal cord. They are less common as compared to the protoplasmic astrocytes.

Protoplasmic astrocytes:

The protoplasmic astrocytes have short processes. The processes of protoplasmic astrocytes are branched. They have abundant organelles and cytoplasm, as evident from the name.

This type of astrocytes is more abundantly present in
the CNS. They are seen in the grey matter of the brain and spinal cord.

Embryologic Development

Astrocytes are derived for the cells of neuroepithelium present in developing neural tube. The neuroepithelial cells upon differentiation produce glioblasts, also called the spongioblasts.

These glioblasts differentiate into oligodendroblasts and astroblasts. The oligodendroblasts give rise to oligodendrocytes. The astroblasts further differentiate into protoplasmic astrocytes or fibrous astrocytes.

Thus, the precursor of astrocytes is the same
neuroepithelium that gives rise to other glial cells as well as neurons present
in the CNS.

Functions

Astrocytes are the most abundant and the most diverse glial cells present in the CNS. They perform a number of functions that are essential for the normal functioning of neurons. Below is given a brief detail of the functions of astrocytes.

Regulation of ionic concentration:

Astrocytes play the most important role in the regulation of extracellular ionic concentration around the neurons. The concentration of various ions in the extracellular fluid controls the nerve impulse generation and transmission in the neurons.

If the ionic concentration gets disturbed, the neurons may fail to generate or propagate nerve impulses.

Astrocytes regulate the extracellular ionic concentration by buffering the extracellular potassium (K+) ions.

Physical support:

The cytoplasmic processes of the astrocytes provide
physical support to the neurons. They play an important role in deciding the
structure of brain. They provide the physical support for the movement of
differentiating neurons in the developing brain.

Blood-brain barrier:

The Blood-brain barrier prevents the entry of large sized particles, ions or proteins from blood into the extracellular material of the brain. It is very essential for keeping the brain in a separate compartment.

Astrocytes play an important role in forming the blood-brain barrier. The protoplasmic processes of astrocytes have vascular feet. They encircle the endothelial cells of the capillaries present in the brain.

They prevent any leakage or movement of any unwanted substance from the blood in the capillaries to the extracellular matrix of brain. Thus, astrocytes make an important component of the blood-brain barrier.

Fuel reserve:

Astrocytes are the glial cells that act as fuel
reserve to the neurons. These cells are capable of storing glycogen and providing
it to the neurons when necessary.

The astrocytes not only store glucose in the form of
glycogen but can also produce glucose from non-carbohydrate sources. Astrocytes
are the only glial cells that are capable of gluconeogenesis. Thus, they can
provide newly formed glucose to the neurons.
They also provide other nutrients to the neurons such as lactate etc.

Regulation of blood flow:

Astrocytes can also control the blood flow in the
brain. They act as vaso-modulators. These cells regulate the vasodilation in
the CNS thus controlling blood flow.

Excretion of wastes:

Astrocytes collect metabolites and other waste
products and move them to the capillaries. Thus, they have a role in the
removal of harmful metabolites and waste products from the CNS.

Synapse structure:

The protoplasmic processes of astrocytes cover the various synapses present in the CNS. They have a role in the structure and formation of synapses. They also affect the functioning of synapses and their plasticity.

Glial limiting membrane:

The foot processes of astrocytes make glial limiting membrane. It is the outer most layer of neuronal tissue in the brain and spinal cord. The glial limiting membrane lines the meninges on the outer surface of the CNS.

It prevents the migration of neurons and glial cells and neurons into the meninges. It also regulates the movement of small molecules between the meninges and the parenchyma of the brain and spinal cord.

Repair:

Astrocytes are also thought to play a role in the repair of the neuronal tissues in CNS. When injury occurs to the nervous tissue in the brain or spinal cord, astrocytes migrate to that place and form glial scar.

The research studies have shown that this glial scar plays an important role in the process of regeneration. It also allows the axons to grow and pass through the injured tissue present in the spinal cord.

Cellular network:

Astrocytes can freely communicate with one another
through gap junctions. In this way, they form a very large highly coordinated
cellular network throughout the CNS. It helps in the regulation of various
activities in the brain and spinal cord.

Pathologies associated with Astrocytes

Different pathologies associated with the astrocytes
are given below.

Astrocytoma

Most of the brain tumors are astrocytomas. These are the cancerous cells derived from astrocytes. They occur mostly in the brain but are sometimes seen in the spinal cord.

Depending on the malignancy and severity, the
astrocytomas are of two types:

Pilocytic astrocytoma: They are the slow growing tumors of astrocytes that are mostly benign. They most commonly occur in the cerebellum and produce symptoms related to balance and gait disturbances.

Anaplastic astrocytoma: They are the malignant tumors of astrocytes that show rapid growth. They can rapidly invade the surrounding healthy tissue. They rapidly grow into larger size tumors that cannot be removed by surgery.

Alzheimer’s disease

Astrocytes are considered to play an important role in
the pathogenesis of Alzheimer’s disease. They produce a large amount of
beta-amyloid proteins. The deposition of these beta-amyloids is the most
significant factor that contributes to the pathogenesis of Alzheimer’s disease.

Alexander’s disease:

It is a rare disease characterized by destruction of
myelin sheath around the neurons and deposition of abnormal protein deposits.
It is associated with the mutation in the glial fibrillary acid protein (GFAP)
gene specifically associated with the astrocytes.

The abnormalities of astrocytes are also associated
with some developmental disorders of the nervous system.

Conclusion/Summary

Astrocytes are the star-shaped supporting cells present in the brain and spinal cord. They are the most abundant and diverse glial cells present in the CNS.

They are the star-shaped cells having a central body
with radiating protoplasmic processes. They may be protoplasmic or fibrous in
shape.

Protoplasmic astrocytes have abundant organelles and small branching protoplasmic processes. They are abundantly present in the grey matter.

Fibrous astrocytes have large unbranched protoplasmic processes. They have limited organelles and are abundantly present in the white matter of CNS.

Like other cells of the CNS, they are also derived
from the neuro-epithelium of the neural tube.

Astrocytes perform a number of functions that are essential for the normal functioning of neurons. These include:

• Regulation of ionic concentration
• Providing nutrients to the neurons
• Removing the metabolites and waste
  products
• Contributing in the blood-brain
  barrier
• Repair of the CNS
• Regulation of blood flow
• Physical support to the developing
  brain
• Synapse structure and functioning
• Formation of glial limiting membrane

The pathologies of astrocytes include astrocytomas,
the most abundant tumors of the brain.

They also include other diseases such as Alzheimer’s disease and Alexander’s disease, etc as well as some developmental disorders of the CNS. 

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