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Peripheral Nerve Histology
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Peripheral Nerve Histology

Nerve vs Nerve fibers (axons)
  • A peripheral nerve encompasses bundles of nerve fibers (axons) and has various connective tissue coverings (superficial epineurium, perineurium, and endoneurium).
Nerve Coverings
Outside to Inside:
Superficial epineurium
  • Superficial epineurium is the outer covering of the nerve.
  • It is a supporting coat: a cylindrical, dense connective tissue sheath.
Perineurium
  • Perineurium encases separate nerve fascicles.
  • It is a mechanically strong sheath that is dense and forms a protective barrier around the nerve fascicle: a blood-nerve barrier.
    • It comprises a flattened form of epithelial cells that are joined by special junctions, which helps it withstand tremendous pressure.
Endoneurium
  • Endoneurium is a loose connective tissue; it comprises collagenous fibers.
Additional, less often addressed terminology:
Deep epineurium
  • Deep epineurium accounts for the connective tissue sandwiched between the nerve fascicles.
  • We find vasculature in this region.
Perineurial septa
  • Perineurial septa pass through the nerve fascicles and carry vasculature to the nerve fibers.
Nerve fibers (Axons)
  • The nerve fiber is the nerve cell axon.
  • Myelin sheath surrounds myelinated axons.
  • Schwann cells: each myelinates at most one axon internode.
Myelin
Unmyelinated cell
In an unmyelinated cell, we see:
  • The Schwann cell nucleus lies intermixed with axons within the Schwann cell cytoplasm.
  • The mesaxon is the zone of apposition for the Schwann cell membrane and the axon.
  • Multiple axons can pass through a Schwann cell, when the nerve fiber is unmyelinated.
Myelinated cell
In a myelinated cell, we see:
  • The Schwann cell nucleus lies off to the side of the axon, which is enveloped in concentric circles of myelin: a myelinated sheath.
Schwann cells vs oligodendrocytes
  • Unlike oligodendrocytes, which can myelinate up to 50 individual axons, a Schwann cell only myelinates one axon, as shown here (specifically only one peripheral nervous system internode).
Dorsal root ganglion
We highlight the following:
  • Ganglion cell
  • Cell nucleus
  • Surrounding satellite cells (which support the ganglionic neurons).
    • Satellite cells are derived from neural crest cells in the same way that Schwann cells are.
Peripheral nerve in longitudinal view
We highlight the following:
  • Schwann cells
  • Myelin sheath
    • The myelin sheath is vacuolated because its high lipid content creates this artifact upon paraffin embedding.
  • Deep stained nerve fibers (axons).
Clinical Correlation
Related Tutorials
Histological Images
Histological images modified with permission from:
  • http://medsci.indiana.edu/a215/virtualscope/docs/chap6_3.htm
  • http://medsci.indiana.edu/c602web/602/c602web/toc.htm

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