Glial Cells

Glial Cells

CNS tissue contains several types of non-neuronal, supporting cells, neuroglia.

Astrocytes (or astroglia) are star-shaped cells. Their processes are often in contact with a blood vessel (perivascular foot processes). Astrocytes provide mechanical and metabolic support to the neurones of the CNS. They participate in the maintenance of the composition of the extracellular fluid. Although not themselves directly involved in the process of communication between neurones, they may be involved in the removal of transmitters from synapses and the metabolism of transmitters. Astrocytes are also the scar-forming cells of the CNS.

Oligodendrocytes (or oligoglia) have fewer and shorter processes. Oligodendrocytes form myelin sheath (see below) around axons in the CNS and are the functional homologue of peripheral Schwann cells. Oligodendrocytes may, in contrast to Schwann cells in the periphery, form parts of the myelin sheath around several axons.

Microglia are small cells with complex shapes. Microglia are, in contrast to neurones and the other types of glial cells, of mesodermal origin. They are derived from the cell line which also gives rise to monocytes, i.e. macrophage precursors which circulate in the blood stream. In the case of tissue damage, microglia can proliferate and differentiate into phagocytotic cells.

The ventricles of the brain and the central canal of the spinal cord are lined with ependymal cells. The cells are often cilated and form a simple cuboidal or low columnar epithelium. The lack of tight junctions between ependymal cells allows a free exchange between cerebrospinal fluid and nervous tissue. Ependymal cells can specialise into tanycytes, which are rarely ciliated and have long basal processes. Tanycytes form the ventricular lining over the few CNS regions in which the blood-brain barrier is incomplete. They do form tight junctions and control the exchange of substances between these regions and surrounding nervous tissue or cerebrospinal fluid.

Many glial cells do express neurotransmitter receptors. Neuronal activity may regulate glial function by a spillover of transmitter from synaptic sites, which are typically surrounded by fine processes of glial cells. Occasionally, neurones also make synapse-like contacts with glia cells. Glial cells may also communicate with each other via GAP junctions.

 

Suitable Slides

sections of the forebrain - toluidine blue, Giemsa, luxol fast blue/cresyl violet

Forebrain, Cortex, mouse - Giemsa and Forebrain, Hippocampus, mouse - Giemsa
Most glial cells are much smaller than neurones. Their nuclei are generally much smaller than neuronal nuclei, and they rarely contain an easily visible nucleolus. Other aspects of their morphology are variable. The glial cytoplasm is, if visible at all, very weakly stained. Different types of glial cells cannot be easily distinguished by their appearance in this type of preparation. Most of the small nuclei located in the white matter of the CNS, where they may form short rows, are likely to represent oligodendrocytes.
Browse through the sections at low or medium magnification and try to get a feeling for the structural diversity visible in the section available to you - parts of the section that look different from others are very likely to have different functions.
Find a spot that appears interesting (or least boring) to you and sketch its structure at low magnification. Choose a spot for high magnification, and draw some of the visible neurones and glial cells. Note the difference in the size and number of glial cells and neurones.