Fig. 3

Evolution of demyelination and remyelination in VB₂^−/+ chickens. Light micrographs except for electron micrograph at P6 in A. A P1 and P2 are part of a TF from a VB₂^−/+PH14d chicken with partial internodal demyelination. P3 to P5 are longitudinal sections through designated sites of the TF. Redundant myelin foldings are seen in externally normal paranodal regions (P3 left) and paranodal regions adjoining partial internodal demyelination (P4 and P5). A centrally located Schwann cell nucleus (SN) is noticed in the demyelinated regions (P4 and P5) and neighbouring myelin-maintained segment (P3 right). Supernumerary fibroblasts (Fi) are seen attached to the TF preparation at paranodal and internodal regions. Electron microscopy (P6) further demonstrates the fibroblast with enriched rough reticulum (white rectangle in P3 left). B P1 and P2 are part of a TF from a VB₂^−/+PH16d chicken, showing remyelination. P3 to P5 are longitudinal sections of designated segments. The nodes of Ranvier (N₁–N₁₃ arrows) are hardly seen from the surface appearance of the TF, but are clearly identified in longitudinal sections. The 1st (N₁–N₂), 4th (N₄–N₅), 8th (N₈–N₉) and 11th (N₁₁–N₁₂) internodes are original internodes with thick myelin sheaths. The 2nd (N₂–N₃), 3rd (N₃–N₄), 5th (N₅–N₆), 6th (N₆–N₇), 7th (N₇–N₈), 9th (N₉–N₁₀), 10th (N₁₀–N₁₁) and 12th (N₁₂–N₁₃) internodes are remyelinating internodes with thin myelin sheaths. Substantial variation of the internodal length is seen in both original and remyelinating internodes. The length of some remyelinating internodes, such as the 2nd (N₂–N₃), 3rd (N₃–N₄), 7th (N₇–N₈) and 10th (N₁₀–N₁₁) internodes, are similar or even longer than some original internodes, such as the 4th (N₄–N₅) and 8th (N₈–N₉) internodes. Focal myelin swellings (arrowheads) are present at the paranodal regions of original internodes. The nodal gap is not extra ordinarily large, implicating that the length of the remyelinating internode is already fixed at this stage. A Schwann cell nucleus (SN) and lipid deposition (asterisk) are present in the middle of an original internode. Myelin debris is seen in the paranodal Schwann cell cytoplasm of the original internode (double arrows). The TF is surrounded by a variably thick fibroblastic proliferation consisting of fibroblast processes and collagen fibres. Numerous fibroblast nuclei (white arrows) are noted in the longitudinal section at both original and remyelinating internodes. C P1 is part of a TF from a VB₂^−/+PH21d chicken, showing remyelination. The nodes of Ranvier are indicated by arrows (N₁–N₆). There is no considerable variation of the internodal length between the original (N₂–N₃ and N₄–N₅) and remyelinating internodes (N₁–N₂, N₃–N₄ and N₅–N₆). One of the remyelinating internode (N₁–N₂, 194 µm in length) is even longer than that of original internodes (N₂–N₃, 176 µm; N₄–N₅, 159 µm). P2 and P3 are longitudinal sections through the full length of a remyelinating and a neighbouring original internode in the first panel. Schwann cell nucleus (SN) is present in the middle of both the remyelinating and original internodes. Degenerating myelin of varying stages (double arrows) is noticed in Schwann cell cytoplasm of both original and remyelinating internodes. White arrows indicate fibroblast nuclei