Melano-macrophage centre-like structure in the heart of the medaka, Oryzias latipes.

Endocardial lining cells in the heart of certain teleosts are phagocytic against foreign materials. In the medaka Oryzias latipes, in addition to the endocardial lining cells, cells in the subendocardial spaces are also phagocytic. Most subendocardial cells contain yellowish-brown pigments and are often aggregated to form nodules resembling splenic and kidney melano-macrophage centres. Subendocardial cells are positive to PAS and Schmorl's reaction. Macrophages containing large heterogeneous inclusions together with lymphocytes, were observed with the TEM. The definitive characteristics of reticular cells, including the cell sheaths, were not observed. Because of these two characteristics they were regarded as more primitive aggregates of macrophages than the ordinary melano-macrophage centres. The possible analogy between these structures and MMCs is discussed.