Reactivity of Gill and Intestinal Mucosal Barriers in Common Bream (Abramis brama): A Comparative Histological and Ultrastructural Study of Natural Populations

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.