OBJECTIVETo investigate the morphological changes and regeneration mechanism of sinusoidal endothelial cell.

METHODSSixty male Wistar rats (bought from SLC company limited of Japan) were divided into three groups. Fifty of them belonged to experiment group, five rats belonged to untreated group, and the rest five ones belonged to normal saline treated group. The experiment group was then divided into ten subgroups. All the rats of the experiment group were killed under anaesthesia using aether at 12, 24, 36 hrs, and 2, 3, 5, 7, 8, 10 and 14 days subsequently after an one-off injection of dimethylnitrosamine (DMN) (50 mg/kg). The liver tissues, bone marrows and peripheral blood of the rats were taken out rapidly. All the tissues received with HE staining, immunohistochemistry staining and double immunofluorescence labelings, and they were observed under a light microscope and electron microscope. The livers, bone marrows and peripheral blood from the rats at 24 hrs to 14 days after an injection of DMN were examined by light microscopic, immunohistochemical, and ultrastructural methods.

RESULTSSmall focal necrosis of the liver tissues was found at 12 hrs after the DMN injection, and gradually becomes more obvious from the 24 hrs. The most obvious necrosis, with lots of ED-1 (monocyte/phagocyte marker of rats) positive cells infiltration, was observed at 36 hrs. On the 2nd day and 3rd day after injection, the necrotic fragments and red cells were phagocyted by ED-1 positive macrophages. On the 5th day, some of the ED-1-positive cells were transformed from round to spindle in shape. On the 7th day, these cells 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. On the 8th day, the histomorphology of liver tissue was similar with that on day 7, except that the range of the lesions had become smaller. On the 10th day, the regeneration of liver tissue increased, filling in the necrosis. On the 14th day, the necrotic tissues were almost replaced by regenerated liver tissues and thin bundles of central-to-central bridging fibrosis. 12 hrs after the DMN injection, bone marrow studies showed an increase in the number of ED-1 positive mononuclear cells, some of which were both BrdU/ED-1 positive. The number of ED-1 positive mononuclear cells reach their highest level at 36 hrs. These cells are morphologically similar to round mononuclear cells in bone marrows and could be found in the peripheral blood from 24 hrs to the 10 days. They reached their highest level in peripheral blood at the same time as in the bone marrow. These cells morphologically resembled ED-1 positive cells in necrotic tissues of the liver.

CONCLUSIONSThese findings suggest that round mononuclear ED-1-positive cells proliferate first in the bone marrow after DMN treatment, reach necrotic areas of livers through circulation, and differentiate to sinusoidal endothelial cells. Namely, hepatic sinusoids in DMN-induced necrotic areas may partly be reorganized possibly by vasculogenesis.