| CELLS OF THE NERVOUS SYSTEM
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Fewer than 10% of the cells of the nervous system are large neurons. The three major cell types in the nervous system (which each constitute about 30%) are:
- astrocytes, which also make up part of the blood/brain barrier;
- oligodendrocytes, which are principally composed of fat, and serve to insulate the axons;
- microglia, which are essentially resident macrophages (scavengers).
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| REGIONAL FLUIDS WITHIN THE BRAIN |
| It is essential, on clinical grounds, to distinguish CSF rhinorrhea from local nasal secretions caused by, say, influenza infection. The ENT surgeon must know whether the fluid present is CSF, as any leak must be surgically repaired lest it remain a chronic potential source of meningitis as a result of the migration of nasal flora into the subarachnoid space. One characteristic and useful marker protein in the CSF is asialotransferrin, which is transferrin lacking sialic acid. In the systemic circulation, this absence of sialic acid gives a molecular signal for the protein to be recycled, and it is thus immediately removed from the systemic circulation by all reticuloendothelial cells. The brain has no true reticuloendothelial cells along the path of CSF flow, and hence asialotransferrin is present in quite high concentrations. The aqueous humor of the anterior chamber of the eye also produces the characteristic asialotransferrin, and the same asialotransferrin can also be found in the perilymph of the semicircular canals. |
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| These different cell types are associated with predominant protein molecules that are important in various brain pathologies (Table 39.1). Other minor constituents of the
nervous system include the ependymal cells, which are ciliated cells secreting brain-specific proteins such as prostaglandin synthase. The brain endothelial cells also merit special comment because, unlike other tissue capillaries, these contiguous cells have tight junctions that bind them together; this feature is believed also to contribute to the blood/brain barrier, although it is the basement membrane which is the major source of molecular sieving of the different-sized proteins.
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