Histological and Ultrastructural Studies of the Unique Hemopoietic-Endocrine Organ of the Grass Carp, Ctenopharyngodon idella (Valenciennes, 1844)

Functional morphology of cardiac stomach of Nile catfish (Clarias gariepinus): Histological, scanning, and ultrastructural studies

The present study was done on 20 adult specimens of Nile catfish (Clarias gariepinus) to demonstrate the morphological characteristics of the cardiac region of the stomach. The cardiac mucosa was characterized by a large number of well-defined long folds. The surface epithelial cells were simple columnar type covered with distinct microvilli and connected by desmosomes. Few PAS- and AB- positive goblet cells were found between the surface epithelium. In addition, many lymphocytes, macrophages, and blood capillaries were seen in the epithelial layer. The lamina propria was exclusively occupied by simple branched gastric (cardiac) glands that fill most of the thickness of the mucosa and open into gastric pits. The gastric glands were composed of numerous secretory tubules that were lined with one type of cells with a cytoplasm containing numerous electron-dense granules, well-developed rER, mitochondria, and a large number of free ribosomes. Moreover, macrophages were distributed in the lamina propria and submucosa. Telocytes were observed in the cardiac region for the first time around the glands, blood vessels, between the muscular layer, and in the serosa. A large number of mast cells could be identified in the submucosa around the blood vessels. The presence of many immune cells in the wall of the cardiac stomach suggests involvement in immune response in addition to its digestive function.

An overview of the structural and functional aspects of immune cells in teleosts

The immune system of fish consists of two main components, innate and adaptive immunities. Innate immunity is non-specific and acts as the primary line of protection against pathogen invasion, while adaptive immunity is more specific to a certain pathogen/following adaptation. The adaptive immune system consists of the humoral and cellular components. Cytotoxic T-lymphocyte cells are the major component of the cellular immunity that frequently kills viral-, bacterial- or parasitic-infected cells. According to the anatomical location, the mucosal-associated lymphoid tissue (MALT) in teleost fish subdivides into gut-associated lymphoid tissue (GALT), gill-associated lymphoid tissue (GIALT), and skin-associated lymphoid tissue (SALT). The MALTs contain various leukocytes; including, but not limited to, lymphocytes (T and B cells), plasma cells, macrophages, and granulocytes. Macrophages are multifunctional cells that are mainly involved in the immune response, including; phagocytosis and degradation of foreign antigens, tissue remodeling, and production of cytokines, chemokines and growth factors. An interesting feature of teleost macrophages is their ability to form melanomacrophage centers (MMC) in the hemopoietic tissues. Dendritic cells, rodlet cells, mast cells, eosinophilic granular cells (ECGs), telocytes, osteoclasts, club cells, as well as, barrier cells have been recorded in many fish species and have many immunological roles. This paper aims to summarize the current knowledge of the immune cells present in fish tissues serving as anatomical and physiological barriers against external hazards. Increased knowledge of fish immune systems will facilitate the development of novel vaccination strategies in fish.