Hepatocarcinogenesis in the mouse. Combined morphologic-stereologic studies.

The histogenesis of trabecular hepatocellular carcinomas was studied in male B6C3 F1 mice that were given injections of 5 micrograms diethylnitrosamine (DENA)/g body wt when they were 15 days old. Fully developed trabecular carcinomas with characteristically thickened hepatic plates were not seen until 44 weeks after DENA injection. However, focal microscopic collections of RNA-rich hepatocytes, referred to as basophilic hepatic foci, were first noted at 10 weeks after DENA injection. Hepatocytes in the foci were characterized by a twofold increase in the nuclear to cytoplasmic ratios, which imparted an easily recognized crowded appearance to the lesions, and by a marked tendency to invade hepatic vein branches. Blocks of liver from 16 mice, killed at 10, 20, and 28 weeks, were serially sectioned; and all foci and nodules with a diameter greater than 80 mu were identified. At 20 weeks, only 6 of 51 foci (12%) with diameters between 160 and 224 mu showed venous invasion; whereas 15 of 20 (75%) in the range 320-450 mu showed this feature. Thus, the predisposition to invade hepatic vein branches correlated with an increase in the size of foci and preceded their development of thickened hepatic plates. Since other studies have documented the presence of alpha-fetoprotein, an oncofetal marker, in some invasive foci, and since the late-appearing trabecular carcinomas metastasize to the lungs, we have suggested that the tiny infiltrating lesions be classified as microcarcinomas that are predisposed to develop into trabecular hepatocellular carcinomas. Proliferated bile ductules were found in some of the larger foci and nodules. This feature also became more prevalent with an increase in the size of the lesions. The ductules were derived from bile ducts, which, in association with portal vein branches, entered the lesions at localized areas at their peripheries. Thus, the presence of ductules within foci appeared to result from an encroachment on bile ducts of the enlarging foci. Since the foci were generally spherical and did not disseminate within the liver, this single-dose carcinogen model should be particularly useful for further studies of tumor growth kinetics.