Cells of the Nervous System.

In order to carry out the functions outlined above a series of highly specialised cells has evolved.

These basically comprise of two groups.

Neuroglia : a collective name given to a series of highly specialised cells which have a supportive function to neurones.

Neurones : these are the functional units of the nervous system. They have the capacity to transmit information to and from various parts of the body and also to communicate with each other.

Neuroglia

The neuroglia are specialised nervous system support cells found only in the brain and the spinal cord (central nervous system).

The central nervous system contains numerous specialised support cells collectively known as neuroglia (this literally means 'nerve glue'). In fact they outnumber the neurones (nerve cells) in a ratio of around 10:1.

Estimates of the actual number vary but there is probably a minimum of 100,000 million neuroglial cells in the brain and spinal cord of a young adult.

Neuroglial cells are classified according to their shape, size and functional relationship with neurones.

They comprise:

• astrocytes
• oligodendrocytes
• microglia
• ependyma

Outside the central nervous system there are some other specialised support cells called Schwann cells.

Astrocytes

Astrocytes are large, multi-processed cells with a star like shape. Their shape gives rise to their name which literally translated means ‘star cell’.

They have several functions:

· in the developing foetus they form a structural framework to guide the migration of developing nerve cells;

· in the developed brain, they form a structural scaffolding for the more specialised neural elements;

· certain astrocytes transport fluid and ions from the extracellular space around neurones to blood vessels.

Astrocytes are characterised by oval or slightly irregular nuclei with an open chromatin pattern, and a spectacular stellate (star shaped) morphology with numerous fine processes radiating in all directions.

These processes contain a specific form of cytoskeletal intermediate filament called Glial Fibrillary Acidic Protein (GFAP).

The stellate morphology is not evident in conventional histology department slides because the processes merge with the processes of other cells, but is seen with special staining methods.

Two types of astrocyte have been identified. Fibrous astrocytes are most evident in the white matter and have long cell processes which are rich in bundles of GFAP. Protoplasmic astrocytes are most evident in the grey matter of the brain and have long thin processes containing few bundles of GFAP.

One important structural adaptation of astrocytes is seen in their interaction with the blood vessels of the brain, which they surround by forming flat plates termed end feet. The interaction induces changes in the structure of the cerebral vascular endothelium, rendering it highly impermeable so that it acts as a barrier between the blood and the brain.

Figure 4. Relationship of neuroglial cells.

Note. P.M. = Pericapillary Macrophage.

As can be seen from the above diagram, the end feet of the astrocytes completely surround any blood capillaries and their associated macrophages (cells of the immune system which devour bacteria and other foreign bodies).

In addition astrocytes form end feet which form a barrier to materials passing into the CNS from the meninges. These end feet fuse with the innermost layer of the meninges called the pia mater and are called sub-pial foot processes.

Figure 5. A slide showing stained specimens of astrocytes (darker) showing their star-like appearance

Astrocytes also have an important role whenever damage occurs to the central nervous system either by injury or infection.

Whenever neurones in the CNS die the dead cells are removed by microglia and macrophages carrying out phagocytosis.

The damaged area is then repaired by the proliferation of astrocytes which fill the gaps forming an astrocytic scar. This process is known as gliosis.

Because the dead neurones are replaced by astrocytes the damaged part of the brain loses the ability to send and receive nerve impulses.

This means that if the function is to be restored other parts of the brain tissue must take over the tasks undertaken by the damaged part.

It is this relearning process which results in a prolonged rehabilitation period for people recovering from head injuries, strokes, CNS infections etc.

Oligodendrocytes

These are cells found within the central nervous system that carry out a similar function to the Schwann cells found in the peripheral nervous system.

The literal translation of their name means ‘few branched cells’.

Each oligodendrocyte has a number of processes that are sent out to the axons of nearby neurones which they wrap themselves around to create a myelin sheath.

Figure 6. A model of an oligodendrocyte myelinating the axons of nearby neurones

This myelin in the CNS is the target for attack by the immune system in the disease called multiple sclerosis. The exact reason why this happens is unknown.

Myelin is vital for the CNS to function correctly and it's destruction in multiple sclerosis results in severe deficits in the ability to transmit nerve impulses in the right way. This can result in paralysis, loss of sensation, loss of co-ordination etc.

The nature of the deficit depends on the area of the CNS affected.

Figure 7. The above picture is a low power micrograph of a section through the pons region of the brain stem of a patient with multiple sclerosis taken after autopsy. The area has been stained with a special dye that is taken up by myelin. The paler regions represent the extent of the loss of myelin caused by the disease.

Microglia

These are basically specialised macrophages unique to the central nervous system. They appear to be similar to the dendritic antigen presenting cells of the immune system.

Figure 8. Microglia shown above as the darker stained cells

When the nervous system becomes infected they become active and increase dramatically in number. They are also joined by monocytes which migrate from the blood to become active macrophages within the nervous tissue.

Their function is to destroy invading organisms and dead nerve tissue cells by phagocytosis.

Ependyma

Ependymal cells are similar to the epithelial cells which line other parts of the body. In the central nervous system they form the lining of the cavities in the brain known as ventricles and the central canal of the spinal cord.

Many of the ependyma cells are also ciliated which helps the flow of the cerebro-spinal fluid which fills the spaces mentioned above. Unlike other epithelial cells however the ependyma cells do not lie on a basement membrane but have tapering processes which merge with the processes of the underlying astrocytes.

Figure 9. The scanning electron microscope picture above shows surface cilia of ependyma cells

Monday, 13 September 1999 09:45:11 +0100