structure of the spleen of the Nile tilapia (oreochromis niloticus

A histological investigation of the possible structure and functions of the spleen of the Niletilapia Oreochromis niloticus was cond ucted by light (LM}, and transmission electron microscopy (TEM). The spleen is divisible into red and white pulp and a stroma consisting of a reticular network, a collagenous capsule, and trabeculae containing smooth muscle bundles. The parenchyma of the organ is predominantl y made up of the red pulp, a system of splenic cord s and sinuses. White pulp areas appear to be devoid of lymphoid follicles and consist mainly of periarteriolar lymphatic sheaths (PALS). Fil­ tering of particulate matter from the blood occurs in the red pul p by phagocytes of the pulp cord s and ellipsoid s (periarterial macrophage sheaths). The ellipsoid s are pale­ staining cu.Ifs of macrophages and reticular cells in a framework qf reticular.fibres sur­ rounding the arterial capillaries. The melano-macrophage centres (MMCs) are found throughout the parenchyma of spleen and show a close association with the vascular system. i.e. splenic ellipsoid s,. sinusoid s of red pulp and blood sinuses. They exhibit d istinct degree of development from small groups of actively phagocytic macrophages to large. totally or partiall y encapsulated centres, where effete phagocytic cells are filled by lipqfuscin and melanin. The ultrastructural data presented here suggest various specific physiological roles for the Nile tilapia Oreochromis niloticus spleen, incltLd ­ ing hematopoiesl.s , phagocytosis. tissue breakdown and erythrocyte catabolism. Only sparse lymphopoiesis and plasmapoiesis were recognized