heads) have been preserved in the preparation of the specimen, but a white space appears in place of the myelin
sheath. Cells in close proximity with myelin are Schwann cells. Endoneurial connective tissue is outside of the Sch-
wann cells, and the perineurium (arrow) is two cell layers thick. In the region of longitudinal sections through the
nerve fibers is a node of Ranvier (curved arrow), which is an interruption of the myelin sheath in that area of the
axon. Nodes are covered by interdigitations of the plasma membranes of two Schwann cells and by endoneurium.
Adipocytes and blood vessels occupy the epineurium surrounding the fascicle.
Downloaded
At the higher magnification shown in Figure 8-23a, the Schwann cell cytoplasm-endoneurium unit (neurilemma)
is indicated by the arrowhead. The densely stained nuclei associated with this unit are, most likely, those of the
by
Schwann cells. The nuclei (arrows) appear oval or round in longitudinal sections of nerve fibers. The endoneurium
[
consists of delicate reticular fibers that provide a structural support for the nerve fibers and small blood vessels that
Rutgers
supply nutrients.
University
130.219.8.11]
Figure 8­24
at
[04/08/15].
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In this image, one Schwann cell has wrapped itself a number of times around an axon (A). The nucleus of the
permission.
Schwann cell (N) has darkly stained peripheral heterochromatin. Its cytoplasm contains microtubules (arrow) and
mitochondria (M). The Schwann cell-myelin-axon unit is surrounded by the connective tissue of the endoneurium
(CT). Several unmyelinated axons (curved arrow) are visible at the edge of the field and are enclosed by the cyto-
plasm of other Schwann cells. A single Schwann cell can cover a number of smaller, unmyelinated axons, but only
one segment of an axon is myelinated by one Schwann cell. The region between two Schwann cells is the node of
Ranvier, where the myelin sheath is interrupted and the axon is covered by just the endoneurium.
The junctions of Schwann cell cytoplasm and plasmalemma adjacent to the axon form the inner mesaxon (left ar-
rowhead). This is the area where the Schwann cell cytoplasm first encircled the axon. The outer mesaxon (right
arrowhead) is the junction between the two external areas of Schwann cell cytoplasm, which was formed when the
final layer of myelin was added. The outer mesaxon consists of the portion of cytoplasm that was involved in my-
elination and the portion of cytoplasm in the vicinity of the nucleus. Both the inner and outer mesaxons are not
compressed during myelination. (Reproduced with permission from DE Kelly et al, Bailey's Textbook of Micro-
scopic Anatomy, 18/e, Williams and Wilkins, 1984)
376

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