Tomacula in Charcot-Marie-Tooth disease type 1B (electron microscopy)

Ultrastructure of a representative tomaculum. A redundant fold of myelin is wrapped nearly twice around the axon. The membranes of this fold are compacted about the original myelin sheath (19 lamellae) to form a hypermyelinated structure that is 97 lamellae thick along most of its circumference. The radial periodicity of these myelin lamellae is normal (14 to 15 nm). Where the extracellular surfaces of the original myelin sheath and the redundant fold fuse, a thin zone of reduced staining is formed (large arrows), which is about 3 myelin lamellae thick. The altered staining is mostly accounted for by the seam and the 2 adjacent intraperiod lines that are more lucent than elsewhere. The seam follows each turn of the redundant loop to form a giant spiral that ends in the abaxonal Schwann cell cytoplasm. The paired membranes of these intermediate lines become indistinct within the cytoplasm, obscuring their connection with the surface membrane of the cell. Compaction of the internal surfaces of the redundant fold forms a second seam that is less distinct than the first one. It is formed by the line of fusion of the cytoplasmic surfaces of the Schwann cell membrane, thus, producing an additional major dense line for each turn of the myelin fold. At the terminus of this seam a small pocket of cytoplasm marks the site where the membrane of the fold loops back on itself (asterisk). This remnant of cytoplasm is derived from the adaxonal Schwann cell cytoplasm. (magnification (A) x13,000; (B) x39,000) (Contributed by Dr. Florian Thomas.)