Phagocytotic cells in the fish heart

Comparatively little is known about host-defense activities in the fish heart. Investigations showed that intraperitoneally injected carbon particles are actively taken up by the cardiac endothelial cells of the medaka Oryzias latipes, but less so by those of the goldfish Carassius auratus and lemon tetra Hyphessobrycon pulchripinnis. (In vitro experiments confirmed these species differences in endocytic activities by these cells.) Electron microscopy revealed that endothelial cells of the medaka atrium have large cytoplasm with many organelles, and ingested carbon particles were observed within phagosomes of cardiac endothelial cells even at 4 degrees C. Phagocytic cells, which apparently reside in the heart, were found in all the species examined. These cells were located on the endothelial cells and developed cytoplasmic processes extending toward the heart lumen and/or the intercellular spaces of the endothelial cells. The heart with its resident phagocytes is proposed to function as a host defense organ--at least in certain fish species.