Macroglia
Click on a glial cell to move to the related page. |
Protoplasmic Astrocytes
Figure 8.12 |
Protoplasmic astrocytes are found primarily in gray matter. With silver or glial specific stains, their cell bodies and processes are very irregular. The processes may be large or very fine, sometimes forming sheets that run between axons and dendrites, and may even surround synapses. These fine sheet-like processes give the protoplasmic astrocyte cell body a "fuzzy" or murky appearance under the light microscope. Bundles of fine fibrils may be seen within the cytoplasm. The nucleus of a protoplasmic astrocyte is ellipsoid or bean-shaped with characteristic flecks of chromatin. Specific types of intercellular junctions have been noted between the processes of protoplasmic astrocytes. These probably mediate ion exchange between cells.
Fibrous Astrocytes
Figure 8.13 |
Fibrous astrocytes are found primarily in white matter, have a smoother cell body contour than do protoplasmic astrocytes as seen with glial-specific stains, and have processes that tend to emerge from the cell body radially. These processes are narrower and branch to form end feet on blood vessels, ependyma, and pia. Consequently, the processes of fibrous astrocytes do not form sheets and do not tend to conform to the shape of the surrounding neurons or vascular elements. The major distinguishing feature of fibrous astrocytes, as the name suggests, is an abundance of glial fibrils arranged in parallel arrays in the cytoplasm and extending into the processes.
In Nissl stains, the fibrous astrocytes have a nucleus essentially the same as that of the protoplasmic type with a flecked appearance. Intercellular adherences have also been observed between fibrous astrocytes.
Figure 8.14 |
Both types of astrocytes function to support the neurons in their immediate vicinity. They provide a physical barrier between cells, maintain the ionic and pH equilibrium of the extracellular space around neurons, and continually modify the chemical environment of the neighboring cells. As shown in Figure 8.14, astrocytes form a complete lining around the external surface of the CNS (glial limitans) and around blood vessels (perivascular feet). During development, they form scaffolding along which nerve cells migrate to achieve their mature structure. During injury, the astrocytes proliferate and phagocytize dead cells. This often leads to the formation of glial scar.
In addition to these general functions, astrocytes also act in more specialized ways to facilitate neuron function. They metabolize neurotransmitters by removing them from the synaptic cleft. For example, the amino acid glutamate is taken up by astrocytes and inactivated by conversion to glutamine. Glutamine is then transported to the neuron to be re-synthesized into glutamate (see Chapter 13). More recent evidence indicates that the astrocytes can dramatically change size as part of their physiological regulation of the neuronal environment. These functions will be discussed in later sections.
