Blood cell-producing site in the land slug, Incilaria fruhstorferi

Blood cell-producing site in a yeast-particle-injected slug, Incilaria fruhstorferi, was studied by light and electron microscopy, in vivo and in vitro. Cells lining the vascular system and hemocoel wall were discontinuous, scattered, and morphologically resembled fibroblasts. One hour after injection of yeast-particles as foreign material, the lining cells projected into the lumen and 3 h later, the cells were gradually released from the wall. They were morphologically similar to macrophage-like cells that phagocytose foreign material. In tissue culture of the hemocoel wall from yeast-injected slugs, cells that migrated from the explants consisted mainly of fibroblast- and macrophage-like cells. These two kinds of cells were weakly positive to non-specific esterase reaction, whereas circulating hemolymph cells were strongly positive. After administration of bromodeoxy-uridine (BrdU), labeled cells were most commonly found in the ventral side of the body and around the hemal space of the center of the dorsal wall. These results agree with our hypothesis that fibroblasts in the connective tissue of the hemocoel wall of the land slug can transform into macrophages of hemolymph.