Comparatively little is known about host-defense activities in the fis
h heart. Investigations showed that intraperitoneally injected carbon
particles are actively taken up by the cardiac endothelial cells of th
e medaka Oryzias latipes, but less so by those of the goldfish Carassi
us auratus and lemon tetra Hyphessobrycon pulchripinnis. (In vitro exp
eriments confirmed these species differences in endocytic activities b
y these cells.) Electron microscopy revealed that endothelial cells of
the medaka atrium have large cytoplasm with many organelles, and inge
sted carbon particles were observed within phagosomes of cardiac endot
helial cells even at 4 degrees C. Phagocytic cells, which apparently r
eside in the heart, were found in all the species examined. These cell
s were located on the endothelial cells and developed cytoplasmic proc
esses extending toward the heart lumen and/or the intercellular spaces
of the endothelial cells. The heart with its resident phagocytes is p
roposed to function as a host defense organ-at least in certain fish s
pecies.