On electron microscopy of a peripheral nerve biopsy from a patient with Charcot-Marie-Tooth disease type 1, onion bulb formations are seen. What do these onion bulbs represent histologically?

• A Perineurial cells surrounding individual axons as a protective response
• B Proliferating Schwann cells and axons forming concentric whorls due to repeated demyelination and remyelination ✓
• C Axonal regeneration sprouts forming multiple myelinated fibers around a central demyelinated axon
• D Amyloid deposition within the endoneurium compressing axons

Explanation

Onion bulb formations in CMT-1 (and other hypertrophic neuropathies) represent concentric layers of Schwann cell processes with collagen deposition surrounding a central axon. They result from repeated cycles of segmental demyelination and attempted remyelination by Schwann cells that fail to maintain compact myelin. Each 'layer' of the onion is a Schwann cell cytoplasmic process. This is a pathognomonic feature of hypertrophic demyelinating neuropathy (CMT type 1, Dejerine-Sottas). Axonal CMT-2 does NOT show onion bulbs.

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

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