Fig. 1

D. rerio gill apparatus in basal conditions: (A) SEM micrographs showing concentric microridges of pavement cells (PVCs). F = filament and L = lamellae. Bar 50 µm; (B) light micrographs in toluidine blue showing general morphological organization of branchial epithelium; F = filament and L = lamellae. Bar 50 µm.
Fig. 2

TEM micrographs of the gill apparatus in D. rerio under basal conditions: (A) cellular organization in the primary epithelium. CC = chloride cell, PVC = pavement cell; BC = basal cell; (B) high magnification of a mucous cell (MC); (C) ultrastructural organization of the secondary epithelium. PVC = pavement cell; BC = basal cell; PC = pillar cell. All bars 2 µm.
Fig. 3

SEM micrographs of the primary and secondary epithelium in D. rerio after 96 h of exposure to 7.7 µg/L of HgCl2: (A) degeneration in PVCs microridges in the primary epithelium (arrow); folding in the distal portion of lamellae (asterisk) and lamellar fusion (star). Bar 50 µm; (B) higher magnification of lamellar fusion (asterisk). Bar 20 µm.
Fig. 4

Sagittal section in toluidine blue of the gill apparatus in D. rerio after 96 h of exposure to 7.7 µg/L of HgCl2: (A) detachment of epithelium from connective tissue that create wide lacunae in gills lamellae (asterisk). Bar 20 µm; (B) hypertrophy in endothelial cells and blood congestion (black arrow). Bar 50 µm.
Fig. 5

TEM micrographs of the gill apparatus in D. rerio after 96 h of exposure to 7.7 µg/L of HgCl2: (A) early degeneration in the inner layers of filaments; (B) detachment of epithelium (asterisk) and hypertrophy of chloride cell (CC); (C) high magnification of detachment in secondary epithelium (asterisk) and degeneration of PVCs (star); (D) loss of PVCs connections with the basal lamina (asterisk). All bars 2 µm.
Fig. 6

SEM micrographs of the gill apparatus in D. rerio after 96 h of exposure to 38.5 µg/L of HgCl2: (A) degeneration of microridges in primary epithelium (arrow). Bar 20 µm; (B) wrinkled surface of primary and secondary epithelium with swelling in the distal portion of lamellae (star). Bar 20 µm; (C) increase in mucous secretion (asterisk). Bar 100 µm.
Fig. 7

Cross section in toluidine blue of the gill apparatus in D. rerio after 96 h of exposure to 38.5 µg/L of HgCl2: hyperplasia of secondary epithelium with the appearance of CCs and MCs. Bar 20 µm.

Fig. 8

TEM micrographs of the gill apparatus in D. rerio after 96 h of exposure to 38.5 µg/L of HgCl2: (A) degeneration in primary epithelium with degeneration of mucous cells (asterisk), epithelial gaps (arrow), and macrophage infiltrations (star); (B) folding of secondary lamellae, hypertrophic mucous cell (asterisk), and tissue hyperplasia; (C) appearance of CCs in secondary lamellae and formation of deep invagination in CC (star). All bars 2 µm.

Fig. 9

TEM micrographs of the gill apparatus in D. rerio after 96 h of exposure to 38.5 µg/L of HgCl2: (A) high magnification of long processes in PVCs (asterisks) and edema formation in the distal portion of lamellae (star); (B) degenerated and apoptotic PVCs (asterisks) with the disappearance of PCs. All bars 2 µm.

Fig. 10

Confocal micrographs of the D. rerio gill apparatus. Sections labeled with a mouse monoclonal antibody against metallothionein (MT) (green–Fluorescein Isothiocyanate (FITC) labeled); nuclei labeled with propidium iodide (red); (A) no MTs expression in the gills of the control group; (B) after 96 h of exposure to 7.7 µg/L of HgCl2, MTs immunoreactivity strongly appear in both the primary and secondary epithelium; (C) after 96 h of exposure to 38.5 µg/L of HgCl2, the intensity of staining lightly decrease compared to the basal condition in both the filament and lamellar epithelium. All bars 75 µm.

Fig. 12

Confocal micrographs of D. rerio gill apparatus. Sections labeled with a mouse monoclonal antibody against Na+/K+-ATPase (green–FITC labeled); nuclei labeled with propidium iodide (red); (A) detection of Na+/K+-ATPase in the CCs of the interlamellar region in the basal condition; (B) after 96 h of exposure to 7.7 µg/L of HgCl2, Na+/K+-ATPase immunoreactivity strongly decrease compared to the basal condition but the fluorescence labeling appears at the level of the secondary epithelium; (C) After 96 h of exposure to 38.5 µg/L of HgCl2, the expression for Na+/K+-ATPase increase in both the filament and lamellar epithelium. All bars 75 µm.

Acknowledgments
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