Abstract
Aquatic ecosystems are contaminated by many endocrine disrupting chemicals and pollutants such as selenium. Selenium is highly toxic to aquatic organisms like fish when present in increased concentrations. In fish, selenium accumulates in organs like the kidney and liver, leading to morphological alterations in the various tissues, and subsequently affecting the overall functioning of the respective organs. Fish are used as sentinel organisms in aquatic health research with many sacrificed during experimentation. This practise is potentially disadvantageous when analysing brood stock or endangered species. The current study aims to implement image-guided non-lethal sampling techniques to demonstrate that adequate test specimens can be collected without having to sacrifice the test organism. Fine needle aspiration cytology (FNAC) and fine needle aspiration biopsy (FNAB) are examples of such techniques and have the added benefit of sampling the same organ multiple times in a single individual.
Thirty-two adult female fish of the Oreochromis mossambicus species were used in the current study. Four different study groups were set up in order to consider all possible variables that could impact the results. The variables analysed in this study were (A) fish exposed to sodium selenate and biopsied, (B) fish exposed to sodium selenate and not biopsied, (C) fish not exposed to a chemical and biopsied and lastly, (D) fish not exposed to a chemical and not biopsied. A sub-lethal concentration (0.7715 mg/L) of sodium selenate was used for the 67 days of the exposure experiment. At the end of the exposure period 10 fish were randomly selected from all of the experimental groups and killed. The hepatopancreata were excised and examined both macro- and microscopically to determine if the sodium selenate exposure and FNAB method caused any damage to the organs. The collected cell and tissue samples from the hepatopancreas were processed according to standard micro-techniques. Histopathology, if any was observed under the light microscope, was enhanced with various staining techniques (H & E, Periodic Acid- Schiff, Gordon and Sweets and Perl’s Prussian blue). The histology was analysed qualitatively and semi-quantitatively. Stereological and statistical analysis of the data were also done. It was expected that the hepatopancreas would regenerate after each aspiration and biopsy procedure and that the fish would most likely succumb to chronic sodium selenate exposure as opposed to the sampling procedures.
Various histological alterations were observed in the hepatopancreatic tissue of the exposed fish. The hepatopancreas index for the exposed fish was greater than 26 and less than 35. Based
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on the liver index classification these values indicate tissue with pronounced histological alterations. The prevalent alterations observed in the hepatopancreas of the exposed fish were nuclear alterations, regenerative hyperplasia, an increased amount of melanomacrophage centers (MMCs), glycogen deposits and cytoplasmic vacuolation. The control fish had a liver index greater than 10 and less than 25 which shows tissue with moderate histological alterations. The results of the current study indicated that the sodium selenate exposure affected the hepatopancreas of the female O. mossambicus fish, and in turn impaired organ functioning. Aquatic ecosystems are continuously contaminated by pollutants and prolonged exposure could lead to deleterious effects of the organisms and, subsequently, the entire aquatic ecosystem.
Key words: Oreochromis mossambicus, hepatopancreas, histomorphology, sodium selenate, non-lethal sampling techniques