On electron microscopy of a nerve biopsy, which specific ultrastructural feature distinguishes a myelinated from an unmyelinated peripheral nerve fiber, and which cells produce the myelin sheath in the peripheral nervous system?

• A Neurofilaments (intermediate filaments) within the axoplasm; oligodendrocytes produce PNS myelin
• B Dense-core vesicles within the axon terminal; Schwann cells produce myelin for multiple axons simultaneously
• C Concentric compacted lipid lamellae (major dense lines + intraperiod lines) around the axon; Schwann cells produce PNS myelin (one Schwann cell per internode) ✓
• D Schmidt-Lanterman incisures visible as cytoplasmic channels; astrocytes form the peripheral myelin sheath

Explanation

PNS myelin is produced by Schwann cells — each Schwann cell myelinates one internode of one axon by spiralling its plasma membrane around the axon many times. On EM, myelin appears as alternating electron-dense (major dense lines, formed by fused inner cytoplasmic leaflets) and electron-lucent (intraperiod lines, formed by fused outer leaflets) concentric lamellae. Unmyelinated fibers (C fibers) are enveloped by a single Schwann cell cytoplasm without compacted lamellae (Remak bundles). Oligodendrocytes myelinate CNS axons. Schmidt-Lanterman incisures are cytoplasmic channels within compact myelin visible on light microscopy of teased nerve preparations.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.