Vascularization in tissue remodeling after rat hepatic necrosis induced by dimethylnitrosamine

We observed postnecrotic tissue remodeling to examine vascularization in adult rat livers. Livers, bone marrow, and peripheral blood from rats at 24 h to 14 days after an injection of dimethylnitrosamine (DMN) were examined by light microscopic, immunohistochemical, and ultrastructural methods. Numerous ED-1 (a marker for rat monocytes/macrophages)-positive round mononuclear cells infiltrated in the necrotic areas at 36 h after DMN treatment. On day 5, when necrotic tissues were removed, some of the cells were transformed from round to spindle in shape. On day 7, these cells were contacted with residual reticulin fibers and became positive for SE-1, a marker of hepatic sinusoidal endothelial cells and Tie-1, an endothelial cell-specific surface receptor, associated with frequent occurrence of ED-1/SE-1 and ED-1/Tie-1 double-positive spindle cells. Ultrastructurally, the spindle cells simultaneously showed phagocytosis and endothelial cell-like morphology. With time necrotic areas diminished, and on day 14, the necrotic tissues were almost replaced by regenerated liver tissues and thin bundles of central-to-central bridging fibrosis. Bone marrow from 12 h to day 2 showed an increase of BrdU-positive mononuclear cells. Some of them were positive for ED-1. The BrdU-labeled and ED-1-positive cells appeared as early as 12 h after DMN injection and reached a peak in number at 36 h. They were similar in structure to ED-1-positive cells in necrotic liver tissues. These findings suggest that round mononuclear ED-1-positive cells proliferate first in bone marrow after DMN treatment, reach necrotic areas of the liver through the circulation, and differentiate to sinusoidal endothelial cells. Namely, hepatic sinusoids in DMN-induced necrotic areas may partly be reorganized possibly by vasculogenesis.