Histological and ultrastructural examination of immune cells in the gill epithelium of the silver carp (Hypophtalmichthys molitrix).

Fish use their mucosal barriers to interact with their surroundings while maintaining homeostasis. Gill-associated lymphoid tissue (GIALT) provides an important initial line of defense against waterborne infections. We examined the morphology and ultrastructural characteristics of immune cells in the branchial epithelium of silver carp (Hypophthalmichthys molitrix) using histology and transmission electron microscopy. We processed samples from silver carp (n = 10) for semi-thin sectioning and transmission electron microscopy. We stained semi-thin sections with toluidine blue and methylene blue and ultrathin sections with uranyl acetate and lead citrate. We identified several immune cell types in the gill mucosa, including polymorphonuclear leukocytes with multilobed nuclei and three granule types: eosinophilic granular cells with crystalloid cores in their specific granules, and rodlet cells at granular and mature developmental stages. We identified mononuclear phagocytes (macrophages) in the lamina propria and gill lumen. In the basal and intermediate layers of the branchial epithelium, we observed cells with abundant rough endoplasmic reticulum and endo-lysosomal systems. We also observed apoptotic characteristics in the surface layer, including chromatin condensation and the expansion of rough endoplasmic reticulum. Our findings illustrate the complex cellular architecture of the branchial mucosal immune barrier in silver carp and establish a morphological basis for understanding the cellular components that influence gill-associated immunity.