FIGURE 12.12 • Electron micrograph of a mature myelinated axon. The myelin sheath (M) shown here consists of 19 paired layers of Schwann cell membrane. The pairing of membranes in each layer is caused by the extrusion of the Schwann cell cytoplasm. The axon displays an abundance of neurofilaments, most of which have been cross-sectioned, giving the axon a stippled appearance. Also evident in the axon are microtubules (MT ) and several mitochondria (Mit). The outer collar of Schwann cell cytoplasm (OCS) is relatively abundant compared with the inner collar of Schwann cell cytoplasm (ICS). The collagen fibrils (C) constitute the fibrillar component of the endoneurium. BL, basal (external) lamina. 70,000. Inset. Higher magnification of the myelin. The arrow points to cytoplasm within the myelin that would contribute to the appearance of the Schmidt-Lanterman cleft as seen in the light microscope. It appears as an isolated region here because of the thinness of the section. The intercellular space between axon and Schwann cell is indicated by the arrowhead. A coated vesicle (CV) in an early stage of formation appears in the outer collar of the Schwann cell cytoplasm. 130,000. (Courtesy of Dr. George D. Pappas.) inner collar of Schwann cell cytoplasm
FIGURE 12.14 • Three-dimensional diagrams conceptualizing the relationship of myelin and cytoplasm of a Schwann cell.
This diagram shows a hypothetically uncoiled Schwann cell. Note how the inner collar of the Schwann cell cytoplasm is continuous with the outer collar of Schwann cell cytoplasm via Schmidt-Lanterman clefts.
the fibrous astrocytes, however, hence the name. Antibodies to GFAP are used as specific stains to identify astrocytes in sections and tissue cultures (see Fig. 12.18b). Tumors arising from fbrous astrocytes, fibrous astrocytomas, account for about 80% of adult primary brain tumors. They can be identified microscopically and by their GFAP specificity.
Astrocytes play important roles in the movement of metabolites and wastes to and from neurons. They help maintain the tight junctions of the capillaries that form the blood–brain barrier (see page 385). In addition, astrocytes provide a covering for the “bare areas” of myelinated axons— for example, at the nodes of Ranvier and at synapses. They may confine neurotransmitters to the synaptic cleft and remove excess neurotransmitters by pinocytosis. Protoplasmic astrocytes on the brain and spinal cord surfaces extend their processes (subpial feet) to the basal lamina of the pia mater to form the glia limitans, a relatively impermeable barrier surrounding the CNS (Fig. 12.19).
Astrocytes modulate neuronal activities by buffering the K concentration in the extracellular space of the brain.
It is now generally accepted that astrocytes regulate K concentrations in the brain’s extracellular compartment, thus maintaining the microenvironment and modulating activities of the neurons. The astrocyte plasma membrane contains an abundance of K pumps and K channels that mediate the transfer K ions from areas of high to low concentration. Accumulation of large amounts of intracellular K in astrocytes decreases local extracellular K gradients. The astrocyte membrane becomes depolarized, and the charge is dissipated over a large area by the extensive network of astrocyte processes. The maintenance of the K concentration in the brain’s extracellular space by astrocytes is called potassium spatial buffering.
Oligodendrocytes produce and maintain the myelin sheath in the CNS.