Razi University

  Introduction and Objective: The skin is an important organ in aquatic animals. Owing to its specialized cells and structures, in addition to protecting against environmental factors, it contributes to osmotic balance, sensory perception, and the secretion of certain substances. Skin health and structural changes reflect the overall health of fish and are important for understanding disease processes and environmental effects. Therefore, understanding fish skin and its structural components is important. In this study, the skin of the Mesopotamian barb was investigated using histological, histochemical, immunohistochemical, and surface ultrastructural approaches.Materials and Methods: Six adult female Mesopotamian barb specimens were collected from the Sirwan River at the Qeshlaq Dam, Sanandaj (latitude: 35.4167° N; longitude: 46.9833° E). Approximately 1 × 1 cm skin specimens were collected from the dorsal and ventral regions of the mid-body and placed in fixative. Following fixation and tissue processing, sections were stained using routine and special techniques, including Masson’s trichrome, periodic acid–Schiff (PAS), and Alcian blue, and examined by light microscopy. Tissue sections were also subjected to immunohistochemical staining to investigate S100, Vimentin, and CK AE1 expression. Images were captured using a digital camera. Histometric measurements were performed using ImageJ, and the data were statistically analyzed. Additional dorsal and ventral skin samples were collected and, following fixation and dehydration, sent to the scanning electron microscopy laboratory for processing and imaging.Results: The skin of the Mesopotamian barb consisted of three structural layers: the epidermis, dermis, and hypodermis. The epidermis consisted of living, non-keratinized stratified epithelial tissue composed of columnar to cuboidal, polygonal, and flattened squamous cells. Goblet and club secretory cells were also observed. The dermis consisted of loose and dense connective tissue and contained blood vessels, pigmented cells, and scales. The hypodermis connected the dermis to the muscular bundles of the trunk. Histometric results showed no significant difference in epidermal thickness between dorsal and ventral surfaces (p > 0.05). Goblet cells were more numerous dorsally, whereas club cells were more numerous ventrally; however, neither difference was significant (p > 0.05). Melanophores were significantly more numerous dorsally (p < 0.05). Goblet cells were significantly more numerous than club cells in both dorsal and ventral epidermis (p < 0.05).Immunohistochemical reactivity of CK AE1, Vimentin, and S100 was evaluated in dorsal and ventral skin. CK AE1 showed the most prominent reaction, with moderate to strong cytoplasmic immunoreactivity predominantly in epidermal epithelial cells. Vimentin immunoreactivity was weak and mainly observed in scattered cell populations in the dermis and among epithelial cells near the basement membrane. S100 immunoreactivity was negative.Scanning electron microscopy revealed the arrangement of scales on the skin surface and their posterior regions covered by the epidermis, as well as microridges of surface cells and apical pores between them. Scale surface features included circular ridges and transverse grooves, with an arrangement similar to cycloid scales of other species of the same family.Overall, the general histological structure of the skin was similar between dorsal and ventral surfaces. The histological, histochemical, and ultrastructural characteristics were also similar to those reported in other freshwater fish. Differences in the intensity and distribution of immunostaining indicate that IHC results in fish should be interpreted considering the marker type, target cell population, and species-specific characteristics.