Fig. 1

Evolution of demyelination in VB₂⁻ chickens. Light micrographs. A Panel 1 (P1) and P2 show part of a TF from a VB₂⁻PH11d chicken. The nodes of Ranvier are indicated by arrows (N₁ to N₅). Paranodal demyelination is noted at 2 sites (N₃ and N₄) and normal node of Ranvier at 3 sites (N₁, N₂ and N₅). Schwann cell nuclei (white arrowheads) are located in the middle of each internode. P3 to P5 are longitudinal sections through designated segments. P3 is through the first paranodal demyelinating area. No discernible myelin sheath is seen in the demyelinated segment. Myelin degeneration (large arrowheads in P3) is seen in the internodal region adjoining the paranodal demyelination (small arrowheads in P2). P4 is through the central part of the internode with paranodal demyelination. The myelin sheath is well preserved in this segment and a Schwann cell nucleus (SN) is present. P5 is through the second paranodal demyelinating area. No discernible myelin sheath is seen in the demyelinated segment. Myelin degeneration (large arrowheads in P5) is seen in longitudinal section through the segment (small arrowheads in P2) adjoining the paranodal demyelination. The axon is attenuated in demyelinating regions. B (adapted from Cai et al., 2007 [14] with permission from the author and publisher): P1 to P3 show a TF from a VB₂⁻PH16d chicken displaying paranodal demyelination (N₁ and N₄) and partial internodal demyelination (N₂, and N₃, N₅ and N₆). Paranodal swellings (small arrowheads) and surrounding pale-osmicated material representing the fibroblastic onion bulb like proliferation [14]. P4 to P6 are longitudinal sections through designated segments in the three upper panels. In the demyelinated segments, there is no discernible myelin sheath and axon (a) is attenuated. Redundant myelin foldings with varying degrees of myelin splitting and degeneration (large arrowheads) are found in longitudinal sections through paranodal swellings while the myelin sheaths in the neighbouring non-demyelinated internodal regions are intact. The fibre is surrounded by a variably thick fibroblastic proliferation consisting of fibroblast processes and collagen fibres. Numerous fibroblast nuclei (arrows), some with multiple distinct nucleoli (double arrows), are noted at both demyelinated and non-demyelinated segments. C P1 and P2 show part of a TF from a VB₂⁻PH21d chicken displaying internodal demyelination. P3 and P4 are cross sections through designated sites of the TF, showing denuded axon in the demyelinated segments (L1, 5–19), redundant myelin foldings with myelin splitting and degeneration at both ends of the myelin-maintained segment (L2 and 4). The axon size in the region with intact myelin (L3) is larger than that of demyelinated regions. Fibroblast processes and collagen ensheath the TF at all levels to some degree