The article was written as part of the Ministry of Science and Higher Education of the Republic of Kazakhstan’s project AP 23486220, titled “Study of the important organ systems reactivity in the main commercial fish species of the Ile-Balkhash basin under anthropogenic load.” (2024-2026). The article represents the study investigating the long-term effects of polyethylene terephthalate (PET) microplastics on zebrafish (Danio rerio). Zebrafish exposed to PET microplastics at concentrations of 1.9 mg/l and 5.64 mg/l exhibited significant kidney and gill damage. This included necrosis, circulatory disorders, and tissue damage. Histological analysis revealed specific pathological changes such as vacuolization and hyaline-droplet degeneration in kidneys, and destruction of epithelium and lamellae in gills. Notably, no compensatory-adaptive reactions in gills were observed, indicating a lack of adaptation to prolonged exposure. These findings suggest that microplastics may migrate through the bloodstream, disrupting normal kidney and gill structures. The study highlights the severe impact of microplastic pollution on fish organs, emphasizing the urgent need for further research to develop strategies for mitigating microplas-tic pollution in aquatic ecosystems. Additionally, the observed lack of adaptive responses underscores the potential for significant ecological consequences, urging immediate attention to address this critical environmental issue.

The gills of teleost fish are multifunctional organs involved in respiration, osmoregulation, and immune defense, and are highly sensitive to environmental stressors. This study evaluated the histological and ultrastructural condition of the gill epithelium of common carp (Cyprinus carpio Linnaeus, 1758) inhabiting the middle reaches of the Ile River (Kazakhstan) under chronic anthropogenic pressure. Although hydrochemical parameters remained within permissible limits for fishery waters, all examined specimens (n = 10) exhibited pronounced structural alterations. Histological analysis revealed edema, epithelial hyperplasia, lamellar fusion, necrosis, and desquamation, indicating substantial impairment of gill integrity. Ultrastructural examination demonstrated advanced developmental stages of rodlet cells, mast cell degranulation, and increased activity of mucous and chloride cells, accompanied by mitochondrial alterations and autophagosome formation. Alongside destructive changes, moderate compensatory reactions were observed, reflecting tissue-level adaptation to prolonged environmental stress. These findings demonstrate that gill histopathology provides a sensitive tool for detecting sublethal effects of chronic anthropogenic impact, even when conventional hydrochemical indicators suggest acceptable water quality. The results support the use of cellular and tissue-level biomarkers in freshwater environmental monitoring programs.

Fish mucosal barriers are sensitive interfaces between the organism and the aquatic environment and may reflect integrated tissue responses in natural populations. This study assessed the reactivity of the gill and intestinal mucosal barriers in common bream, Abramis brama, from the Ile–Balkhash Basin and Lake Alakol, Kazakhstan. A total of 50 individuals were examined using light microscopy, transmission electron microscopy, quantitative cell counting, and semi-quantitative histopathological assessment. Background hydrochemical parameters of the studied waters were within guideline values and were used as environmental context rather than as a control condition for tissue normality. Histopathological alterations were recorded in both organs. The gills showed epithelial desquamation, respiratory epithelial edema, deformation and fusion of secondary lamellae, epithelial hyperplasia, and vascular abnormalities, with a median Gill Lesion Score of 2.00 (IQR 1.86–2.29). The intestine showed epithelial sloughing, intestinal fold disruption, brush-border damage, cellular infiltration, fibrosis/collagenization, and vascular changes, with a median Intestinal Lesion Score of 2.14 (IQR 1.86–2.57). Rodlet cells were more abundant in the gills, whereas mast/eosinophilic granular cells occurred at comparable densities in both organs. These findings indicate shared but organ-specific patterns of mucosal reactivity and support paired gill–intestine analysis as an informative morphological approach for assessing wild fish populations under natural multifactorial conditions.