On electron microscopy of a nerve biopsy, the myelin sheath appears as alternating electron-dense and electron-lucent lines. The major dense line (MDL) is formed by apposition of which surface of Schwann cell membranes?
- A Extracellular surfaces (outer leaflets)
- B Intramembranous particles only
- C Cytoplasmic surfaces (inner leaflets) — due to extrusion of cytoplasm as the membrane wraps ✓
- D Basement membrane layers
Explanation
The myelin sheath is formed by tight wrapping of the Schwann cell membrane. As the membrane spirals, the cytoplasm is extruded and the cytoplasmic faces (inner/intracellular leaflets) of adjacent membrane turns come into close apposition, forming the major dense line (MDL). The intraperiod line (IPL — lighter, electron-lucent) represents apposition of the extracellular surfaces (outer leaflets). Myelin basic protein (MBP) is the principal protein responsible for holding the cytoplasmic faces together in the MDL; P0 protein maintains the intraperiod line compaction in peripheral myelin.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.