Abstract

Background: Teleost fish possess gills that are very complex and multifunctional structures. The histopathological and histochemical integrity of teleost gills is an excellent indicator of overall fish physiological health status and the health of the aquatic environment. Objective: The study aimed to characterize the microstructure of gill tissue and the distribution of mucopolysaccharides in healthy Cyprinus carpio gills using routine and specialized staining methods as a baseline. Methodology: A total of fifteen adult male common carp (approximately 280 g each and approximately four months average age) were collected from commercial aquaculture farms located along the Euphrates River in Anbar Province, Iraq. Gill arches were removed from each specimen, immersed in 10% neutral buffered formalin and subsequently processed by routine procedures. The histologic evaluation of gill sections stained with hematoxylin and eosin (H&E), periodic acid–Schiff (PAS), and Alcian blue (pH 2.5) under the light microscope demonstrated normal gill architecture (primary and secondary lamellae) and normal cell morphology (pavement, chloride, pillar, mucus, chondrocyte, and erythrocytes) to be found in all fifteen specimens examined (100% concordance across all specimens). Results: Mucous cells in the gill sections demonstrated strong magenta PAS staining in all specimens, which indicates a large amount of neutral mucopolysaccharides with consistent staining intensity of 3+ (strong). Alcian blue staining of the gill epithelial chloride cells and gill cartilage demonstrated the presence of the acidic mucopolysaccharides, which showed moderate-to-strong staining intensity of 2+ to 3+. The absence (0% incidence) of pathological changes (lamellar fusion, epithelial lifting, or necrosis) in any of the (n=15) specimens further validates the findings contained in this study as a healthy reference standard for subsequent studies concerning Cyprinus carpio gills regarding toxicity testing, environmental monitoring, and comparative evaluations of aquatic species health. Conclusion: the current study’s findings were suggestive of a predominantly neutral mucin phenotype in healthy common carp gills from aquaculture sources during non-challenged conditions.

Keywords: Cyprinus carpio, Gill histology, Histochemistry, PAS staining, Alcian blue, Mucopolysaccharides, Baseline characterization