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A single injection of thioacetamide (TAA) induces a significant increase in hepatocytes' DNA synthesis and mitosis within 48 hours without accompanying pathology. The present study explores this mitogen system by characterizing several changes in the livers of young adult SD rats following an injection of TAA under controlled conditions. Radioautography of ('3)H-thymidine labeled livers showed an initial depression in the number of hepatocytes in S phase after TAA. This was followed by a significant increase in DNA synthesis beginning by 24 hours. The hepatocytes entering DNA synthesis and subsequently undergoing mitosis were localized initially to zone 1, followed by zone 2. The labeled hepatocytes remained localized for at least 8 weeks. The hepatocyte mitotic index was slightly depressed for 30 hours after TAA and then increased for 36 hours. The increase in the mitotic index followed that in the DNA synthetic index by 8 to 12 hours. The natural synchrony of hepatocytes' cell cycles was verified. The increased proliferative response after TAA was also synchronous, although a slight lengthening of the S + G(,2) interval may have occurred. The percentage of binucleated hepatocytes decreased between 24 and 30 hours after TAA; possibly indicating that binucleated cells enter a round of cell division more readily than other hepatocytes. Plasma alpha-fetoprotein (AFP) concentrations increased significantly during the first and third days after TAA. The first wave was a G(,1) event indicating that hepatocytes in young adult rats remain capable of synthesizing AFP and that they need not have entered a round of DNA synthesis and mitosis prior to secreting AFP. Immunofluorescent labeling specific for AFP verified that virtually all of the differentiated hepatocytes were responsible for the AFP synthesis. This mitogen system to explore liver proliferation and growth control is proposed as an alternative to partial hepatectomy. The regimen of a single low dose of TAA creates a model system of cellular proliferation with which to study the earliest changes leading to DNA synthesis and mitosis and the enhanced expression of AFP genes without the requirement of extended time periods as with chronic administration of hepatocarcinogens.

Three (intact) Angus males and three females which were half-sibs and born within 21 days of each other were selected for this study. Each animal was bled and biopsied periodically from suckling calves to mature slaughter weights, to determine the qualitative composition of lipoproteins and to differentiate the lactate lipogenic activity of subcutaneous tissue during growth and development. At slaughtered, a sample of intramuscular adipose tissue was taken to determine the lactate lipogenic activity at this location. Two days later, one side of each carcass was separated into wholesale cuts. Each wholesale cut was dissected into separable bone and soft tissue and sampled for protein, lipid and moisture determinations. The elution profiles of lipoproteins were similar for all animals. Major peaks observed were (1) very low density (VLDL), (2) low density (LDL) and (3) high density lipoproteins (HDL). Triglycerides, cholesterol and protein were not significant (P < .05) between males and females for the VLDL. At one year of age, females had large (P < .05) amounts of protein for the HDL. In both groups of cattle, largest (P > .05) amounts of protein were greater in the HDL at 9 months of age. Profiles of HDL apoproteins at all ages showed that in both groups of cattle, a distinct band with a weight of about 28,000 was present representing apo-AI. Apo-protein components of pooled LDL fractions showed a protein which was unable to enter the acrylamide gel (7.5 – 20%) used. The component may represent apo-B with a molecular weight of about 250,000. The lactate lipogenic activity of subcutaneous adipose tissue was larger in the males and only significant (P < .05) at 9 months of age. The lipogenic activity was higher (P > .05) in the subcutaneous tissue when compared to the intramuscular tissue at slaughter. In both cases, males showed to use more (P > .05) lactate for fatty acid synthesis in intramuscular and subcutaneous tissue than the females. Magnitude of quality and yield for carcass traits were better for the males than females. Bone, meat and lean weights were significantly (P < .05) greater for the males, however, on a percentage basis per side weight, differences were eliminated. In addition, no significant (P > .05) effect was present between male and female wholesale and side carcass composition.