In vivo imaging of tumor formation in STII medaka: brightfield microscopy. Neoplastic response following early life stage exposure of STII medaka to the reference hepatocarcinogen diethylnitrosamine (DEN). Medaka acutely exposed at early life stages to DEN were followed serially, and at 10 months hepatic tumors were imaged through the abdominal wall. (A and C) In vivo imaging (brightfield) of hepatic tumor formation (green arrowheads) in DEN exposed medaka, showing enlargement of total liver mass and altered vasculature. Histopathological assessment of the tumor showed mixed hepatocellular (B) and cholangiocellular carcinomas (D). Biliary hyperplasia (D) was characterized by a single layer of biliary epithelium lining large cystic spaces in the liver. Opaque white tissue in brightfield images (A&C) is ovary, whereas the gut occupies caudal most region of the abdominal cavity.