Control gills morphology
Gills of Metynnis roosevelti had 4 pairs of cartilagenous branchial arches. Gill filaments or primary lamellae, were lined by a stratified squamous epithelium. At the epithelial surface, the polygonal cells showed concentric microridges. The apical portion of chloride cells was rarely observed among the epithelial cells of the filament (Fig. 1).
Respiratory or secondary lamellae were leaf like structures in an oblique display to the primary lamellae. They were lined by a smooth squamous epithelium, without microridges at the cell surfaces, and with a prominent nucleus (Figures 1 and 2). There was a clear transition from the branchial filament to the respiratory lamella (Fig. 1). Respiratory lamellae had a thin epithelium, sustained by pillar cells that surround blood spaces (Fig. 2).
Effects of the organophosphorous compound on the gills of Metynnis roosevelti
The effect of methyl parathion on the branchial epithelium was quite drastic: structural changes of the gill lamellae organization, epithelial detachment, necrosis, hyperplasia, loss of the microridges, altered cellular morphology. These features were observed in both concentrations.
Time of exposure - 1h. Respiratory lamellae and interlamellar epithelium disorganization was observed. In addition, epithelium wrinkling of the respiratory lamellae was noticed (Fig. 3).
Time of exposure - 4h. Epithelial detachment in the respiratory lamellae was observed (Fig. 4).
Time of exposure - 8h. Hyperplasia was quite frequent, and cellular proliferation ocurred mainly on the surface of the respiratory lamellae (Fig. 5), that tended to fuse.
Even at the concentration of 1 ppm, methyl parathion caused changes in the branchial epithelium of M. roosevelti.
Time of exposure - 1h. All branchial tissues were altered. The interlamellar epithelium became irregular, the surface corregated, and the cells thin, with flat nucleus. The shape of pillar cells was altered, and consequently the size and shape of blood spaces. The shape of erytrocytes became irregular, crenated with disform nucleus (Fig. 6).
Time of exposure - 4h. Degeneration of lamellar and interlamellar cells was observed. The blood spaces collapsed and the erytrocytes were weakly stained by eosine.
Time of exposure - 8h. The respiratory lamellae were quite narrowed with a significant flattening of the squamous cells nucleus. The cytoplasm of erytrocytes was weakly stained by eosine.
Time of exposure - 24h. Epithelium wrinkling of respiratory lamellae, cellular degeneration and collapse of the blood spaces were common. Disorganization and decrease in the amount of microridges at the epithelial cells surface of the primary lamellae, as well as generalized wrinkling was observed (Figures 7 and 8).
Time of exposure - 48h. Intense tissue degeneration and disorganization occurred, with consequent loss of blood spaces. Wrinkled, irregular lamellae, with discontinuous aspect, contrasted with their aspect in the control branchial epithelium (Fig. 9).
Time of exposure - 72h. At this time, epithelium detachment of respiratory lamellae was observed. However, the most evident alteration was observed at the surface of the filament epithelium, where microridges were substituted by cell membrane folds (Figures 10 and 11).
Time of exposure - 96 h. The effects of the OP were similar to those observed after 72h. The detachment of the lamellar epithelium was intensified. At the surface of the filament cells showed changes in the shape of the microridges that either became punctiform or disappeared in some areas, giving place to a fold that covered the whole extension of the cells (Figures 12 and 13).
Some behavioural symptoms of Metynnis roosevelti after contamination with OP
The concentration of 7 ppm of methyl parathion was lethal for M. roosevelti. All fish died after 8 hours of exposure. After 15 minutes in water with OP, fish lost balance and intensified respiratory frequency. After 8 hours, the opercular and jaws movements connected to respiration stopped. Fishes kept the mouth constantly opened, and swam without rest, subsequently becoming lethargic and dying.
The concentration of 1ppm of methyl parathion was sublethal to M. roosevelti, and all fishes survived for more than 96 hours of exposure. However, some behavioural changes were observed in the first 8 hours. All fishes remained close to the water surface. They did not form groups and the decrease of activity was evident. The fishes were most of the time in rest, while their respiratory movements were accelerated. There was recovery and noticeable return to the normal in the subsequent period until the end of the experiment.