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-1 showed immunoreactivity in monocytes
and macrophages around the ellipsoids. NF-B and TGF- 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.