Insights into the stomach of tiger shark, Galeocerdo cuvier (Péron & Lesueur, 1822): Histochemical, ultrastructural, and phylogenetic analysis.

The tiger shark (Galeocerdo cuvier), a large apex predator fish, inhabits marine waters. It is also known for its dietary diversity and ecological importance as it plays a crucial role in maintaining the balance of the marine ecosystem. Despite the importance of this fish, no previous studies have reported the microscopic anatomy of its digestive system. The morphohistological features of the juvenile tiger shark stomach were thoroughly analyzed via histological, immunohistochemical, and ultrastructural procedures, as well as phylogenetic analysis. Histological results exhibited that the gastric mucosa was thrown into short, thick primary folds lined by columnar epithelium. Three types of cells lining the gastric glands: mucous neck cells, parietal cells, and chief cells. Histochemical staining revealed differential mucin distribution, as demonstrated by periodic acid-Schiff (PAS) and Alcian blue techniques (AB). Bromophenol blue (BPB) staining indicated high protein content in the keratin layer and mucosal epithelium. Immunohistochemical analysis displayed strong positive immunoreactivity in the surface epithelial cells and keratin layer, while the muscular layer showed no immunoreactivity. In semithin sections, telocytes (TCs) were identified across all gastric layers, recognized by their unique morphology and common distribution within the tunica muscularis. Scanning electron microscopy (SEM) demonstrated multiple well-developed longitudinal folds and concavities, and numerous openings were detected within the gastric mucosa. The study demonstrated the effectiveness of integrating multi-gene datasets and advanced phylogenetic methods in clarifying the complex evolutionary relationships of sharks. In conclusion, this study provides novel insights into juvenile tiger shark stomach morphology, highlighting the keratinized mucosal surface and the widespread presence of TCs, which underscore the potential protective and regulatory functions within the gastric wall.