Structural and Functional Aspects of the Spleen in Molly Fish Poecilia sphenops (Valenciennes, 1846): Synergistic Interactions of Stem Cells, Neurons, and Immune Cells

SIMPLE SUMMARY: Fish are free-living organisms who are dependent on their innate immune system for survival. This system is fundamental for the defense mechanisms against invading pathogens. The head kidney, spleen, and thymus are the main immune organs in fish, albeit with slightly different roles between species. This study aims to highlight the cellular components of the spleen in the molly fish (Poecilia sphenops) by providing some background on their potential role in the immune system of fish. The splenic parenchyma of molly fish is comprised mainly of red pulp intermingled with clusters of white pulp cells. The main cells identified in this study included: erythrocytes, neurons, dendritic cells, stem cells, epithelial reticular cells, lymphocytes, ellipsoids, and granular leukocytes (eosinophils, basophils, neutrophils), and macrophages. The current study demonstrated a variety of immune cell types with specific functions in the spleen of molly fish. The strong positive immunoreactivity of the cellular constituents of the spleen for APG-5, IL-1β, NF-κB, and TGF-β suggests its critical role in immunity. ABSTRACT: In fish, the spleen is the prime secondary lymphoid organ. It has a role in the induction of adaptive immune responses, in addition to its significance in the elimination of immune complexes. This study was conducted on 18 randomly obtained adult molly fish (Poecilia sphenops) of both sexes using histological, immunohistochemical, and ultrastructural studies to highlight the cellular components of the spleen and their potential role in the immune system. The spleen of molly fish was characterized by the presence of well-distinct melanomacrophage centers, and other basic structures present in higher vertebrates including red and white pulps, blood vessels, and ellipsoids. Some mitotic cells could also be identified in the red pulp. Mast cells with characteristic metachromatic granules could be seen among the splenic cells. Rodlet cells were randomly distributed in the spleen and were also observed around the ellipsoids. The white pulp of the spleen expressed APG5. The expressions were well distinct in the melanomacrophages, leukocytes, and macrophages. Myostatin was expressed in leukocytes and epithelial reticular cells. IL-1β showed immunoreactivity in monocytes and macrophages around the ellipsoids. NF-κB and TGF-β were expressed in macrophages and epithelial reticular cells. Nrf2 expression was detected in stem cells and rodlet cells. Sox-9 had a higher expression in epithelial reticular cells and stem cells. The high frequency of immune cells in the spleen confirmed its role in the regulation of both innate and adaptive immunity, cell proliferation, and apoptosis.