Histological characteristics of the liver of Azov roach (Rutilus rutilus Linnaeus, 1758) as a quality bioindicator of water bodies

Purpose. To assess the variability of the histological parameters of the medial zone of roach (Rutilus rutilus) liver. To establish the level of effect of the transformed conditions of existence on fish physiological status. To determine the prospect of using histological monitoring data in ichthyological practice to assess the morphofunctional state of fish. Methodology. Primary materials were obtained in different parts of the Southern Bug River, which differ in their degrees of pollution. Lab processing of ichthyological samples was performed in accordance with generally recognized methods and manuals. Histological studies were carried out using proprietary equipment and original techniques specifically designed for diagnosing fish tissues. The obtained results were processed by the methods of variation statistics using MS Excel with an emphasis on the errors of the average values. Findings. Microanatomical studies of roach hepatic parenchyma allowed establishing that the most significant changes occured in the transformed conditions of existence and were contrastingly recorded in the medial zone of the organ, which could conditionally be considered a bioindicative zone. With the potential effect of toxicants dissolved in water, a shift in cell function was observed in the liver. Visually, this was manifested as a change in the structuring of cytoplasmic inclusions and in the intensity of nucleus staining, which simultaneously occured with a direct shift in cytological characteristics. An 1.20-fold increase in cell area adequately leads to an 1.18-fold increase in the cytoplasm area, as well as the Hertwig constant or nuclear-cytoplasmic ratio (0,25-0,28), which was statistically significant. The obtained data indicate a change in the nature of intracellular metabolism. An increase in the number of intracellular cavities is a direct evidence of the attenuation of the glycogen-accumulative function of hepatocytes. An increase in the nucleolus/nucleus diameter ratio (0.28) indicates a change in cellular function, which occurs through the activation of lipid synthesis. No typical necrosis foci were found in the parenchyma of the liver of fish from the places of concentration of commercial effluents, which are an evidence of a moderate accumulation of fat-soluble pollutants in the liver. The absence of pronounced pathologies in most individuals indicates significant reserve-compensatory potencies of the organ. The presence of individual hepatocytes with two nucleoli in the nuclei in the liver parenchyma allows judging the borderline, i.e. the prepathological state of the organ. In some cases, an increase in the proportion of dystrophic hepatocytes with large vacuoles filled with an optically transparent liquid was established, which allowed identifying an irreversible pathology - liver balloon dystrophy, resulting from a severe impairment of cellular metabolism. Histological analysis of roach liver parenchyma from conditionally clean and polluted waters allowed, in parallel, establishing relatively low levels of intraspecific and individual variability of cell populations. The revealed fact testifies to the plasticity of the liver as an organ with significant reserve potencies, which are contrastingly manifested under conditions of technogenic pressure in waters of various origins and intended purposes. This perspective provides evidence for the evolutionary definition of the liver as the body's universal detoxification system. Originality.