Phylogenetic development of melano–macrophage centres in fish

The haemopoietic tissues of 72 species of fish representative of Agnatha, Chondrichthyes and Osteichthyes were examined by light microscopy for melanin–containing macrophages. All were found to possess these pigmented cells except for the lamprey Lampetra fluviatilis Linnaeus. An evolutionary pattern was evident in both their distribution and degree of organization. There was a progressive increase in the abundance of pigment cells and their proclivity for the main lymphoid organs. Melano–macrophage centres appear to evolve structurally from the random distribution of individual pigmented macrophages observed in Agnatha and Chondrichthyes to the organized centres characteristic of all Osteichthyes except the salmonids. It is hypothesized that the change in organ location from the liver in Agnatha, Chondrichthyes and the primitive bony fishes to the spleen and kidney in the advanced bony fishes follows closely upon the evolution of the lymphoid system and represents a major advance in the evolution of the lympho–reticular relationships. The increasing sophistication in cytological organization of the melano–macrophage centres is concomitant with the increasing levels of complexity of the cytoarchitecture of the lymphoid system. These analogies provide additional evidence that these centres are of a lymphatic nature and may well represent the primitive analogues of germinal centres of birds and mammals.