Haematoxylin and eosin stained histological sections of X-cell pseudotumours. a) Low magnification of a section through an epidermal pseudotumour in P. obscurus. Large numbers of X-cells (x) are in folds of infected host tissues. The dermis (d) and the underlying muscle (m) remain uninfected. b) High power magnification of X-cells in P. obscurus have a characteristic polygonal appearance with a lightly staining large nucleus (black arrows), melanomacrophages are present in high numbers (white arrows). c) Low magnification of a section through a pseudobranchial pseudotumour in juvenile Icelandic cod. X-cell masses (x) form adjacent to pseudobranchial tissues (ps) but are also found surrounding pseudobranchial cartilage containing chondrocytes (ch) deeper into the pseudotumour. d) High power magnification of the boxed section in Figure 3c; numerous X-cells (black arrows) form a distinctive mass that forms the bulk of the pseudotumour, pseudobranchial cartilage containing chondrocytes (ch) is enclosed by X-cells. Scale bars a & c = 400 μm, b & d = 20 μm.