Purpose. To investigate the state of the liver of wild carp (Cyprinus carpio Linnaeus, 1758) and bream (Abramis brama Linnaeus, 1758) from sites of the Zaporizhzhia reservoir with different human pressures based on histological and cytometric parameters Methodology.The study was carried out in two zones of the Zaporizhzhia reservoir with different degrees and types of human impact - the lower part, located in the agrarian zone (conditionally "environmentally clean" site), and in the Samara Bay with high content of heavy metals in water. The objects of the study were age-4 bream and wild carp. The fish livers for histological studies were obtained from freshly caught fish (in the spring-summer period) by anatomical dissection. Fragments of organs of 0.3 – 0.5 cm were taken for preservation. For the preparation of histological preparations, the liver tissue was stained with hematoxylin and eosin. The pictures of histological preparations were made with the aid of a digital camera connected to the microscope. Histologic sections were investigated with 40X magnification using a microscope with the Sciencelab T500 5.17 M digital camera. Findings. Human impact on the hydroelectric system causes signs of hepatocyte hypertrophy and a number of histopathologies in cyprinids in the Samara Bay. The cytometric analysis of the liver showed that the area of hepatocytes in the bay was larger in comparison with the the individuals of similar age from the lower part of the reservoir by 20 and 10% for the wild carp and bream, respectively. In this case, a 14% increase in the small diameter of cells was observed the in hepatocytes of carp, and a 10% increase in large diameter and a 19% increase in nucleus area was observed in bream. The results of the histological invesatigation of the morphological structure of the liver of cyprinids inhabiting the Samara Bay have revealed a number of pathological changes. Fatty liver dystrophy was observed in 37% of bream and 12% of wild carp. The bream liver (2%) had necrotic areas of parenchyma. In the 50% of bream and 25% of wild carp, the initial necrosis stages occurred, which were manifested as changes in the nucleus and cytoplasm. Frequently, karyopicnosis and cariolysis and destruction of cellular integrity were observed, which may indicate on a chronic damage and a progressive pathological process. There were no pathologies in the fish inhabiting the lower part of the reservoir. Originality. For the first time a cytometric and histological study of the liver of age-4 bream and wild carp from the Zaporizhian reservoir was performed. Practical Value. The obtained results can be used for the detection of histological biomarkers, the most sensitive to the effect of toxic substances. Fish liver is the most appropriate organ for such researches, since changes in its cytometric and histological parameters are a prerequisite for a number of changes in the metabolism as a separate organ and other interconnected systems of fish organism
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