Melano-macrophage centers in the aglomerular kidney of the sea horse (teleosts) : morphologic studies on its formation and possible function

In order to elucidate the process of the formation and possible function of the renal “melano-macrophage centers (MMC)” of fish, light and electron microscopic observations were carried out on the aglomerular kidney of the sea horse, Hippocampus kuda BLEEKER, injected with antigenic horse-spleen ferritin (AF) and nonantigenic carbon particles (CP). Kidneys of the sea horse have well-developed hemopoietic foci in the interstitial tissues surrounding the bundles of renal tubules. Each hemopoietic focus has a small artery in the central area and is framed with the densely arranged sinusoids lined with monolayered macrophages which have no melanin pigments. In the hemopoietic foci free macrophages (Mϕ) were hardly found, but large clusters or MMC filled with densely packed masses of cell debris with some melanin pigments were encountered. Some of them had degenerated Mϕ with fibrotic change surrounded by fibroblasts. AF and CP injected intraperitoneally entered quickly into blood vessels and were taken up actively by the sinusoidal Mϕ. The Mϕ full of AF or CP left the sinusoidal wall and moved gradually into hemopoietic foci to form the MMC or to fuse with the preexisting MMC. The melanophores in the interstitial connective tissue joined to form the MMC. No specific histologic change suggestive of immune response to the AF was present in and around the MMC. The results indicate that the MMC of fish should be only the aggregates of Mϕ to digest the ingested materials effectively. The process of formation of MMC and their possible function is discussed.