The Epibranchial Organ of Heterotis niloticus: Integrated Histological and Immunohistochemical Insights into Its Gustatory, Neural, Immune-Associated, and Stromal Structures

The epibranchial organ is a specialized anatomical structure in certain teleost fishes, but its histological organization and the distribution of cellular markers remain incompletely characterized in Heterotis niloticus. This study investigated the histological architecture of the epibranchial organ and the localization of CD68, interleukin-1β (IL-1β), inducible nitric oxide synthase (iNOS), and S100 immunoreactivity. Histological examination showed a folded mucosal surface lined by stratified epithelium containing taste buds and mucous cells. The epithelium was supported by connective tissue and underlain by a prominent layer of striated skeletal muscle, with nerve bundles present among the deeper tissues. CD68-immunoreactive cells were observed in the epithelium and connective tissue, particularly around blood vessels; many had irregular outlines and multiple cytoplasmic processes, consistent with macrophage-like morphology. IL-1β immunoreactivity was detected in epithelial cells, including cells surrounding taste buds, and in cells within connective and muscular tissues. iNOS immunoreactivity occurred in epithelial cells and some cells in the submucosal connective tissue. S100 immunoreactivity highlighted nerve fibers and also labeled elongated or stellate cells closely associated with neural bundles. These associated cells had features compatible with Schwann-like cells. Overall, the findings describe a close spatial association among gustatory structures, mucous cells, neural elements, and immune-associated cells in the epibranchial organ of H. niloticus. This combined histological and immunohistochemical characterization provides a basis for further investigation of the sensory and mucosal components of this specialized organ.