Histopathology of zebrafish infected with IAV. Parasagittal semi-thin sections from PBS-, APR8- or X-31-injected zebrafish at 48 hpi, stained with Toluidine Blue (n=3 per group, 350 sections per fish). Anterior to the left, dorsal top. PBS control (A-D), APR8-infected (E-H) and X-31-infected (I,J) zebrafish. (B) Head kidney (hk), liver (lv) and yolk sac (ys) of a control zebrafish. No degeneration of cells was observed. (C) Gill and pharyngeal arches adjoined to the occipital region of a control zebrafish. All tissues are without pathological anomalies: semicircular canal of the inner ear (scc) with macula (mc), cartilage of fifth pharyngeal arch (pa) and anterior located gill filaments (gf). (D) The same control fish with healthy heart in front of the yolk sac, atrium (at), ventricle (vt), conus ateriosus (ca) and pericardial volume (pc). (F) The liver of APR8-infected zebrafish. The white arrowheads indicate the presence of degenerative liver tissue, indicated by the necrotic disintegration of nuclei. (G) Gill chamber of APR8-infected zebrafish. The white arrowheads indicate necrotic gill filaments that contain many vacuolated cells. (H) Heart region of APR8-infected zebrafish. The white arrowhead indicates the fluid-filled pericardium. The histological analysis confirms the edema seen in IAV-infected zebrafish (Fig. 3B,D,F). (J) The head kidney of X-31-infected zebrafish shows clear signs of necrotic tissue degeneration. The white arrowheads indicate the presence of degenerating cells in the hematopoietic part of the head kidney.
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