Fig. 1.

Macroscopical and histological location and appearance of the interbranchial lymphoid tissue (ILT), here in sexually mature salmon. (A) In a transversal section of a gill arch, the interbranchial septum (is) is proximally attached to the bone of the gill arch (b) and continues approximately 1/3 along the length of the primary gill lamella terminating in the ILT, which may be observed as a greyish structure (arrow) by the naked eye. The lumen of the branchial chamber (lu) is indicated. (B) Samples were collected from the dorsal (I), mid- (II) and ventral (III) portion of each gill arch; the location of the lymphoid aggregate in this projection is indicated (arrow), as is the location of the bone (b). (C) Histological image showing the terminal end of the interbranchial septum (is) and the ILT (arrow). HE. Scale bars: A,B = 1 cm, C = 500 μm.