Zahl und Ploidiegrad der Zellkerne der Leber unter dem Einflu� k�rperfremder Stoffe

Hormesis and dose-response-mediated mechanisms in carcinogenesis: evidence for a threshold in carcinogenicity of non-genotoxic carcinogens

Recently the idea of hormesis, a biphasic dose-response relationship in which a chemical exerts opposite effects dependent on the dose, has attracted interest in the field of carcinogenesis. With non-genotoxic agents there is considerable experimental evidence in support of hormesis and the present review highlights current knowledge of dose-response effects. In particular, several in vivo studies have provided support for the idea that non-genotoxic carcinogens may inhibit hepatocarcinogenesis at low doses. Here, we survey the examples and discuss possible mechanisms of hormesis using phenobarbital, 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT), alpha-benzene hexachloride (alpha-BHC) and other non-genotoxins. Furthermore, the effects of low and high doses of non-genotoxic and genotoxic compounds on carcinogenesis are compared, with especial attention to differences in mechanisms of action in animals and possible application of the dose-response concept to cancer risk assessment in humans. Epigenetic processes differentially can be affected by agents that impinge on oxidative stress, DNA repair, cell proliferation, apoptosis, intracellular communication and cell signaling. Non-genotoxic carcinogens may target nuclear receptors, cause aberrant DNA methylation at the genomic level and induce post-translational modifications at the protein level, thereby impacting on the stability or activity of key regulatory proteins, including oncoproteins and tumor suppressor proteins. Genotoxic agents, in contrast, cause genetic change by directly attacking DNA and inducing mutations, in addition to temporarily modulating the gene activity. Carcinogens can elicit a variety of changes via multiple genetic and epigenetic lesions, contributing to cellular carcinogenesis.

Toxicological studies on a benzofurane derivative. I. A comparative study with phenobarbital on rat liver

The benzofurane derivative benzbromarone (BBR) previously has led to liver tumor formation after long-term treatment of rats, but no indications of genotoxicity were detected. The present studies were designed to elucidate the mechanism(s) possibly involved in liver tumor formation by BBR. Female Wistar rats were used. Phenobarbital (PB) served as a positive control. (1) Short-term treatment (7 days) with daily doses of 2 to 100 mg/kg BBR led to adaptive responses in the liver, i.e., growth (increases in DNA, RNA, and protein) and induction of monooxygenases. These changes were also observed after feeding BBR for 8, 33, 77, and 102 weeks at doses of 2, 10, and 50 mg/kg/day but tended to weaken with time. Similar effects were obtained with PB fed at 2, 10, or 50 mg/kg/day. However, unlike PB, BBR did not enhance the expression of cytochrome P450-PB as demonstrated by immunostaining of histological liver sections. (2) BBR feeding for 102 weeks, but not for 77 weeks, produced some neoplastic liver nodules and at 50 mg/kg produced one hepatocellular carcinoma (HCC). Thus, BBR was tumorigenic in the present study, but was clearly weaker than PB which had induced liver nodules and HCCs at 77 weeks and even more markedly at 102 weeks. (3) To check for tumor-initiating activity 100 mg/kg BBR was given 14 hr after a two-thirds hepatectomy followed by promotion with PB (50 mg/kg) for 15 weeks. No phenotypically altered liver foci were detected. (4) To test for tumor-promoting activity rats received a single dose of N-nitrosomorpholine (250 mg/kg), and subsequently BBR or PB at doses of 2, 10, and 50 mg/kg/day. While PB markedly enhanced the development of neoplastic nodules and HCCs, BBR had only a weak enhancing effect on the induction of HCC, which was not dose related. gamma-glutamyl transpeptidase-positive foci dramatically increased in PB-treated animals, in contrast they showed no response after 2 and 10 mg/kg BBR and even decreased after 50 mg/kg BBR. (5) With PB changes in liver growth, monooxygenase activity, foci expansion, and tumor promotion all correlating with tumorigenesis in a quantitative manner, apparent no-observed-effect-levels are somewhat below 2 mg/kg (or 10 mg/kg for liver enlargement). (6) These studies suggest that BBR belongs to a group of nongenotoxic, growth-stimulating drugs with tumorigenic potential in rat liver. Its effects on the liver are different from those of PB, but seemed to resemble those of peroxisome proliferators, a hypothesis studied in the subsequent papers.

Development of in vitro toxicity tests with cultures of freshly isolated rat hepatocytes

Freshly isolated and cultured hepatocytes were analyzed by two-parameter flow cytometry. The combined analysis of DNA and cellular protein content allowed the contribution of ploidy classes and of subpopulations within a ploidy class to be defined. Analysis of hepatocytes during exposure to dimethylsulfoxide (DMSO), phenobarbital (PB), low oxygen tension (4% O2) or fetal calf serum (FCS), provided insight into the dynamic response of individual ploidy classes as a function of culture time. By analogy with the age-dependent ploidy shifts in vivo, hepatocyte-cultures shift towards adult animals during exposure to DMSO and towards young animals when cultured at low pO2 (4% O2). FCS and phenobarbital disturb this constitutive ploidy balance. FCS increased the 2 N cell population, where stem cells probably respond to the proliferative stimuli provided by growth factors in the serum. Phenobarbital affects the liver-specific 4 N hepatocytes, which agrees with effects seen in liver after exposure in vivo. It is suggested that drug-induced pathological alterations in ploidy in hepatocyte cultures could serve as indicators of compounds, such as liver tumor promoters, which interfere with cell differentiation in liver. The heterotypic cell-cell interaction of freshly isolated hepatocytes with isolated, in vitro cultured, rat liver epithelial cells in co-cultures proved to be a valuable concept in toxicity testing: aldrin epoxidase, an enzyme system involved in xenobiotic metabolism, was stabilized for more than two weeks. After exposure to the three chemicals, 2-acetylaminofluoren, procarbazine and cyproterone-acetate, a preferential toxicity for each compound and cell population was established. Thus heterotypic cell cultures can considerably increase the amount of information available from in vitro studies. The final concept, combining monitoring of cellular DNA (ploidy) and protein content in hepatocyte cultures during and after exposure to a given test compound at tissue oxygen tension with the heterotypic cell-cell interaction, would create a more in vivo-like culture system. This would enhance the predictability of hepatocyte cultures and contribute to a more widespread use of the test system and as a result help to reduce the number of whole-animal tests.

Circadian rhythm of liver parameters (cellular structures, mitotic activity, glycogen and lipids in liver and serum) during three consecutive cycles in phenobarbital-treated rats

The circadian rhythm of gastric content, serum alkaline phosphatase (alk.P.), serum lipids, body weight (wt), relative (rel.) liver wt, cellular structures (by light- and electron-microscopy), mitotic activity of hepatocytes, glycogen content, protein and lipids in liver was studied in 180 male Sprague-Dawley rats orally treated at 0830-1030 with 50 mg/kg phenobarbital (PB) for 7 days. Thereafter, five PB-treated males and five controls each were studied at 4-hr intervals at 0600, 1000, 1400, 1800, 2200 and 0200 on 3 consecutive days. The lighting schedule in the colony was 12:12 = light/dark (light from 0600 to 1800). Following the rhythm of gastric emptying, the rel. liver wt showed a clear circadian rhythm with a peak at 0800. The rel. liver wt was raised in PB-treated rats at all times of the day. The circadian rhythm of cellular structures was closely related to the hepatic glycogen content which exhibited a clear rhythm with the peak also at 0800, but lowered values were found in PB-treated rats. The mitotic activity of hepatocytes was significantly increased in PB-treated rats but displayed the same circadian rhythm as controls with peaks at noon and troughs at midnight. The well-known hypertrophy of the smooth endoplasmic reticulum in PB-treated rats was not found at 0600, but was fully developed at 1400 and 2200. PB-treatment increased significantly the liver content of cholesterol, triglycerides and phospholipids. Liver cholesterol showed a clear circadian rhythm with peaks at 1800. No rhythm of liver protein, triglycerides and phospholipids was observed. In serum, levels of cholesterol were significantly elevated, those of triglycerides and alk.P. significantly lowered, while those of phospholipids were not affected by the treatment. The three serum lipids, alk.P. and beta-lipoprotein exhibited a clear circadian rhythm, while serum glucose and non-esterified fatty acids did not.

Circadian rhythm of the liver of male rats dosed with phenobarbital--I. Organ weight, cellular structures, glycogen contents and mitotic activity

The circadian rhythm of the liver, namely organ weight, cellular structures (by light-microscopy), glycogen content (by periodic acid-Schiff (PAS) reaction) and mitotic activity, was studied in 166 male Sprague-Dawley rats orally treated daily at 0800-0900 with 70 (study 1) or 50 (study 2) mg/kg phenobarbital (PB) for 7 days. Thereafter, eight (study 1) or five (study 2) rats each were studied at 4-hr intervals at 1000, 1400, 1800, 2200, 0200, 0600 and 1000 through till the following day. The lighting schedule in the colony was 12:12, light:dark (light from 0600 to 1800). The liver weight was raised in PB-treated rats at all times of the day compared to controls and showed a distinct circadian rhythm with a peak at 1000 and a minimum at 2200 in PB-treated rats and the controls. The circadian rhythm of cellular structures was closely related to the hepatic glycogen content which was in good agreement with the controls, but at 1400 and 1800 the glycogen particles were more distinctly diminished in the enlarged centrilobular hepatocytes of PB-treated rats. The mitotic activity of hepatocytes was markedly increased in rats treated with PB but showed the same circadian rhythm as controls with a peak at 1000.

Growth, regression and cell death in rat liver as related to tissue levels of the hepatomitogen cyproterone acetate

Previous studies have shown that "xenobiotic" compounds such as the environmental pollutant alpha-hexachlorocyclohexane (alpha-HCH) and the synthetic sex steroid cyproterone acetate (CPA) induce growth of rat liver by hypertrophy and hyperplasia. After withdrawal of the growth stimuli, liver hypertrophy was usually found to be readily reversible. Conflicting observations were made concerning the fate of liver hyperplasia: hepatic hyperplasia persisted when induced by alpha-HCH but was found to be partially reversible when induced by CPA. The present study confirms the reversibility of hepatic hyperplasia induced by CPA in rats: about 30% of liver DNA present at maximal liver enlargement disappeared within 6 days after cessation of CPA treatment. Simultaneously, a dramatic increase in the rate of cell elimination by apoptosis was found. Glutamate-pyruvate transaminase and alkaline phosphatase in serum did not show major increases, suggesting that cell death was not due to lytic membrane damage. Furthermore, if treatment with CPA was continued or resumed, the enhanced DNA content persisted and the number of apoptotic bodies was greatly reduced. These observations suggest that the occurrence of cell death is due to withdrawal of the growth stimulus CPA. It may reflect a regulatory phenomenon serving to maintain homeostasis of cell number. Further studies showed that CPA is rapidly eliminated from rat liver and serum: t 1/2 in the liver is about 11 h. In contrast, alpha-HCH was previously found to be eliminated more slowly: t 1/2 approximately 144 h. The present study revealed that alpha-HCH, CPA and nafenopin lower the number of apoptotic bodies. This suggests that inducers of liver growth can inhibit hepatocellular death by apoptosis.(ABSTRACT TRUNCATED AT 250 WORDS)

[Comparison of the distribution and toxicity of alpha-, beta-, and gamma-hexachlorocyclohexane (HCH) following 30 days of administration to rats]

Administration of 1000 ppm alpha-HCH, 600 ppm beta-HCH and greater than or equal to 125 ppm gamma-HCH to rats caused a growth retardation. alpha-HCH and beta-HCH led to livermass enlargement. beta-HCH caused a decrease of absolute brain-mass. In case of alpha-HCH and gamma-HCH fat tissue, kidneys and adrenals had a high burden in residues. Fat tissue and adrenals had high residues of beta-HCH. The level of HCH-residues during the test period was gamma much less than beta less than alpha. The results are discussed taking into account the contradictory knowledge from literature.

Karyometry of pseudostratified, metaplastic and dysplastic nasal epithelium by morphometry and stereology. 2. Automated image analysis (IBAS) of the basal layer of nickel workers

A mathematical model for the stereological analysis of parallel-oriented, spheroidal nuclei has been applied to the basal layers of respiratory, metaplastic and dysplastic nasal epithelia. Nuclear profiles seen on tissue sections were measured with an automatic image analyser (IBAS). Nuclear profile area distributions were used to assess possible polyploidies. The result are compared to the histopathological grading, consisting of pseudostratified (respiratory), cuboidal, mixed cuboidal/squamous, squamous, and dysplastic epithelia. The estimated nuclear axis lengths, volumes, surface areas and volume/surface ratios are, in the great majority of cases, significantly smaller in pseudostratified or cuboidal epithelium than in squamous metaplasia and dysplasia. Correspondingly, the numerical density of the nuclei is lower in the latter cases, in which is also noted a smaller nuclear eccentricity. No significant difference is found between the nuclei of squamous and dysplastic epithelia. There is a clear correlation between the mean profile areas and the stereologically estimated volumes of the nuclei. When classifying the mean profile areas into four classes, a progressive shift from the smaller towards the larger size classes is observed when passing from pseudostratified to dysplastic epithelia, through the different metaplastic stages. The nuclear profile area plots often show several peaks, even in some pseudostratified and cuboidal epithelia, probably reflecting polyploidy. There is a marked tendency towards larger profile areas in squamous metaplasia and dysplasia. One histopathologically typical dysplasia showed only small-sized nuclei and another had a notable contribution of those in its profile area plot, in contrast with all the other dysplasias and squamous metaplasias, which displayed practically no small nuclei. The possible implications of the existence of "small-sized-nuclei-dysplasias" are discussed. The literature dealing with nuclear volumes or DNA quantities in putative preneoplastic situations in reviewed. Our method adds some valuable objective criteria to those used in classical histopathological grading. It should be possible to apply this method to other epithelial tissues.

Excessive reversible phenobarbital induced nuclear DNA-polyploidization in the growing mouse liver

Phenobarbital was injected intraperitoneally into male white NMRI mice aged 0.5, 1. 1.5, 3, 6 and 12 months at a dose of 120 mg/kg body weight for 10 consecutive days. The 0.5 month-old mice did not tolerate the phenobarbital dose and died. The experimental animals and one of the controls were sacrificed 1, 3, 5, 10, 15 and 20 days after phenobarbital administration was started. Liver weights were recorded and liver cells were isolated. The number of nuclei per cell was determined and the DNA-content of each single nucleus was measured by Feulgen fluorescence cytophotometry. Liver weights showed an increase of 25--30% during phenobarbital treatment and returned slowly to lower values after cessation of drug application. The hepatic enlargement was accompanied by an excessive and likewise reversible nuclear and whole cell DNA-polyploidization, i.e. polyploidization beyond the physiological age-dependent ploidy level; for example, mean values of 7.7 c per nucleus (versus 4.2 c in the controls) and 14.3 c for whole liver cells (versus 7.5 c in the controls) were found in 3 months-old animals after 5 days of treatment. As with the induction of microsomal enzymes, the augmentation of smooth endoplasmic reticulum, and the increase of RNA- and protein-synthesis, excessive DNA-polyploidization of liver cell nuclei appears to be an expression of hepatocellular hypertrophy due to the functional or metabolic stress imposed upon the liver by large quantities of phenobarbital. After cessation of drug administration the abnormally high ploidy cells are eliminated - probably by necrobiosis - and the liver cells return to their normal age-dependent DNA-ploidy level. The liver cells of the one-month-old animals revealed the physiological polyploidization to be slightly inhibited. This is probably due to some toxic effect of phenobarbital. Phenobarbital did not alter the number of nuclei per liver cell.

Adaptive responses of rat liver to the gestagen and anti-androgen cyproterone acetate and other inducers. III. Cytological changes

Treatment of rats with cyproterone acetate (CPA) induces liver growth. Intact hepatocytes and cell nuclei were isolated from enlarged livers and their volumes or diameters were determined by electronic and microscopic methods. No changes in mean hepatocyte volume or ploidy were observed. However, there was a marked fall in the frequency of binuclear hepatocytes (from 43% to 7%) and a concomitant increase of nuclear ploidy. This effect probably resulted from CPA-induced replication of binuclear hepatocytes. The total number of hepatocytes replicating in response to CPA was estimated on the basis of these data and was found to be up to 75% of all parenchymal cells. Similar cytological changes were observed in the liver after treatment with pregnenolone-16 alpha-carbonitrile (PCN) and, to a lesser extent, with alpha-hexachlorocyclohexane (alpha-HCH). In contrast, physiological liver growth in adolescent rats was characterized by only small changes in binuclearity and nuclear ploidy, and by increases of cellular ploidy. Thus, ploidy analyses may be a useful tool to characterize the type of growth stimulation. Following discontinuation of treatment the cytological changes induced by CPA or alpha-HCH were not reversible in a matter of 3 weeks.

Adaptive responses of rat liver to the gestagen and anti-androgen cyproterone acetate and other inducers. II. Induction of growth

Treatment of female Wistar rats with cyproterone acetate (CPA) leads to considerable enlargement of the liver. The organ content of water, dry mass, protein, RNA, and DNA increased in parallel with the enlargement; only lipid accumulation showed a slight excess. The changes were maximal after treatment for 3 days and increased in a dose-dependent manner, the threshold dose being 5--10 mg CPA/kg DNA synthesis was strongly enhanced after a lag phase of 12--14 h; a maximal rate of synthesis was attained after 18--24 h. The number of parenchymal cells involved in DNA synthesis and mitosis were increased up to 20-fold. Sinusoidal cells participated only slightly in the growth process, and their number decreased relative to the number of parenchymal cells. These results indicate that CPA induces (presumably adaptive) liver growth essentially by parenchymal hyperplasia; of the model inducers used for comparison only pregenolone-16 alpha-carbonitrile (PCN) produced a similarly strong hyperplastic response while liver enlargement elicited by a alpha-hexachlorocyclohexane (alpha-HCH), phenobarbital (PB) and 3-methylcholanthrene (3-MC) was partly or exclusively due to hypertrophy. Liver growth was also observed in male rats treated with CPA, but was less pronounced in this sex. After discontinuation of treatment, liver enlargement and the increase of DNA regressed partially within 1--3 weeks; this regression seemed to be due to sequestration of old cells which were not involved in replication after CPA treatment. The relationship between induction of liver growth by CPA and hepatoma formation in rats is discussed.

Circadian rhythm of hepatocellular structures in rats dosed phenobarbital

The circadian rhythm of liver weight and hepatic structures was studied in rats given a maximum tolerated dose of 70 or 80 mg/kg/day Phenobarbital during 7 days and killed at four different hours of the following day. The drug-induced increase in liver weight in % of body weight of rats was about 1% in the morning and in the afternoon in comparison with controls. At 6.00--7.00 during the glycogen maximum, the fine structure of hepatocytes was normal except enlargement and glycogen depletion of cells in the near neighbourhood of central veins. From 9.00--10.00 the mitotic activity was markidly increased and parallel to the progressive disappearance of glycogen the smooth endoplasmic reticulum (SER) became prominent. Hypertrophy of SER was fully developed at 15.00--16.00 and 17.00--18.00.

Liver growth, biosynthesis of cytidine nucleotides and level of cytochrome P-450 in rat liver after administration of alpha-hexachlorocyclohexane

The biosynthesis of cytidine nucleotides and the level of microsomal cytochrome P-450 in intact and regenerating rat liver after repeated administration of alpha-hexachlorocyclohexane (alpha-HCH) were compared. In alpha-HCH treated animals the utilization of [2-14C] orotic acid for the synthesis of cytidine nucleotides is suppressed. In 24-h regenerating liver the incorporation of labelled orotic acid into cytidine nucleotides is markedly activated; the degree of activation is lower in regenerating livers of alpha-HCH treated animals. The changes in the level of cytochrome P-450 vary inversely with the changes in the utilization of [2-14C] orotic acid for the synthesis of cytidine nucleotides. The activity of cytidine triphosphate synthetase of liver cytosol increases shortly after the administration of alpha-HCH; uridine-cytidine kinase is enhanced in the later stages of the drug action. Within 15-45 min after the administration of alpha-HCH the uptake of [U-14 C] cytidine into the liver and its incorporation into RNA cytosine are increased. After the administration of the drug the uptake of [2-14 C] uridine and its incorporation into RNA uracil is also enhanced whereas its utilization for the synthesis of cytidine nucleotides of the acid-soluble extract as well as for the RNA cytosine are suppressed.

Effect of hypophysectomy on the stimulation of liver growth by alpha-hexachlorocyclohexane, phenobarbital, and partial hepatectomy in the rat

alpha-Hexachlorocyclohexane (alpha-HCH) or phenobarbital (PB) elicit growth and cell multiplication in rat liver. In hypophysectomized rats, alpha-HCH and PB induce an increase in liver mass, but no increase in liver DNA. Hypophysectomy without additional treatment results in a decrease of liver size and RNA, while the DNA content remains unchanged, thereby leading to a relative DNA surplus. 1/3-hepatectomy in hypophysectomized animals leads to a small increase of hepatic DNA only; after 2/3-hepatectomy 75-80% of the original liver DNA are restored. In rats with intact hypophysis losses of liver DNA are known to be restored completely. The findings suggest that the relative DNA surplus in hypophysectomized rats prevents the stimulation of DNA synthesis by weak growth stimuli such as alpha-HCH, PB, and 1/3-hepatectomy. If the relative DNA surplus is eliminated by partial hepatectomy, the inducers do produce DNA multiplication. It is concluded that the induction of liver growth and cell multiplication by alpha-HCH and PB does not require the presence of the hypophysis or one of its hormones.

[Feeding rhythms and the diurnal rhythm of cell proliferation in pharmacologically induced liver growth (author's transl)]

Stimulation of hepatic DNA synthesis can be achieved in the intact rat by alpha-hexachlorocyclohexane (alpha-HCH = alpha-benzene hexachloride). The extent of stimulation is high in the morning and low in the evening. These rhythmic variations in the rate of DNA synthesis are synchronized indirectly by the light-dark rhythm, but directly by the animal's feeding habits: Rats eat preferentially during the night. If the diurnal rhythm of food intake is abolished, the rhythmic fluctuations in the rate of DNA synthesis are no longer detectable; if rats are adapted to daily feeding periods of only 5 h, these fluctuations are pronounced and almost synchronized. Further experiments show that the time of feeding determines the time of DNA replication. It is concluded that food intake provides a "2nd stimulus" or permissive factor, which is required for the induction of DNA synthesis in a certain critical stage of the prereplicative phase. Labelling experiments with orotic acid suggest that foot intake initially induces an increase of RNA synthesis. The results indicate that controlled feeding schedules provide the possibility to synchronize, in the living animal, a proliferating population of hepatocytes. A hypothesis is derived which offers an explanation for the generation of the diurnal rhythm of cell proliferation in the liver.

Stimulation of cell proliferation in rat liver by alpha-hexachlorocyclohexane or partial hepatectomy and end points during G1 of the inhibitory action of beta-diethyl-aminoethylphenyldiallyl acetate-HC1 (CFT 1201), beta-diethylaminoethyldiphenylpropyl acetate. HC1 (SKF 525-A) and actinomycin D

The present work was designed to study the nature, sequence and temporal position of some inhibitor-sensitive events of the replicative cycle in rat liver. Hepatocyte proliferation was induced by alpha-hexachlorocyclohexane and by partial hepatectomy; the onset of DNA synthesis and of mitotic activity were determined and used as reference points in the cell cycle. Inhibition of cell proliferation was achieved by CFT 1201, SKF 525-A, and actinomycin D. It was found that the inhibitory action of the three agents ends at the same stage of the replicative cycle, 0--2 h before the G1/S transition, in both alpha-hexachlorocyclohexane-stimulated and regenerating rat liver. It is concluded that the molecular events sensitive to CFT 1201, SKF 525-A or actinomycin D are either identical or temporally closely associated; they do not figure in the metabolic activation of alpha-hexachlorocyclohexane.

Changes in size, DNA content, and nuclear ploidy of rat liver produced by the environmental microflora

Effects of the environment microflora on rat liver were studied in the present investigation. Rats born under sterile conditions were bred either in a germfree or in a "SPF" environment. At 12 weeks of age the liver of the SPF rats was larger by 20% and contained more DNA than germfree rat liver. Volume analysis, by means of a particle counter, of isolated nuclei suggested that the increase of DNA was due to an increase of hepatic nuclear ploidy. The findings show that infections may influence liver size and ploidy. They indicate that the state of infection should be controlled in animal studies which concern the regulation of liver growth and ploidy or which use liver weight and DNA as reference standards.

Investigations on d-galactosamine hepatitis after pretreatment with alpha-hexachlorocyclohexane

D-Galactosamine hepatitis cannot be induced in rapidly replicating liver tissue at various times after induction of proliferation. Proliferation was induced by administration of alpha-hexachlorocyclohexane. The morphological features of galactosamine hepatitis do not appear or are very mild. The onset of DNA synthesis is delayed to about 12 hrs as also shown in partially hepatectomized rats.