Page 359 - Atlas of Histology with Functional Correlations

FIGURE 9.18 ■ Ultrastructure of an oligodendrocyte in the CNS with

myelinated axons. Transmission electron micrograph. Courtesy of Dr. Mark
DeSantis, Professor Emeritus, WWAMI Medical Program, University of Idaho,
Mascow, ID. ×25,000.

FIGURE 9.19 | Ultrastructure of CNS Myelinated

Axons with Node of Ranvier

A transmission electron micrograph shows a myelinated axon (7) sectioned in a

longitudinal plane and a cross section of a myelinated axon (2) in close
associated with the cytoplasm and the organelles of the myelinating cell, the
oligodendrocyte. Because the myelin sheath is not continuous along the entire

axon length, there is a small nodal gap called the node of Ranvier (4). This
region is located where myelin sheaths (5, 6) are absent, and the axon is
surrounded by the processes or loops containing the cellular cytoplasm (3, 8) of
the oligodendrocyte that covers and contacts the axon. At the node of Ranvier
(4), the oligodendrocyte cell cytoplasm (3, 8) was not completely displaced

during the wrapping of the cell around the axon and formation of the myelin
sheath (5, 6). Located in the myelinated axons (2) are numerous neurofilaments
(2, 7) and dark-staining mitochondria (1). Located near the myelinated axon (7)

are cross sections of unmyelinated axons (9) and the cytoplasm (10) of an
adjacent cell.

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