The growth of liver parenchymal nuclei and its endocrine regulation

Amino acid environment determines expression of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in embryonic rat hepatocytes

A completely defined medium (EHM-1), which reflects the amino acid composition of fetal rat serum and contains albumin as the sole proteinaceous compound, allows the accumulation of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in the presence of dexamethasone, dibutyryl cyclic AMP, and triiodothyronine to approximately twice the level attained in a standard culture medium (RPMI 1640) supplemented with 10% fetal bovine serum (and hormones). Using the EHM-1 medium we could show that the capacity of hepatocytes to synthesize phosphoenolpyruvate carboxykinase in the presence of hormones is manifest as soon as the cells differentiate from the embryonic foregut (embryonic Day 11). Furthermore we could show that embryonic hepatocytes can become binuclear or polyploid when cultured in the presence of thyroid hormone.

Liver DNA synthesis after phenytoin injection in adult NMRI mice

Stimulation of hepatocyte DNA synthesis was observed in adult mice, as measured by the number of parenchymal cells in S phase, at two different times after a single i.p. injection of diphenylhydantoin (DPH): 18 and 25 hr. No such stimulation was obtained for Kupffer or endothelial cells.

Growth and aging in the rat: changes in total protein, cellularity, and polyploidy in various organs

The objectives of this study were to determine the influence of growth and aging on ploidy, cell number, and protein content of various organs. Tissue homogenates were prepared at 3, 8, 25, 50, and 100 weeks of age. Samples were analyzed for DNA per nucleus (by flow cytofluorometry), nuclei number, and protein content. Livers of 8- and 100-week-old animals were also perfused with collagenase and the released cells separated into parenchymal and nonparenchymal populations by unit gravity sedimentation. Nuclei of these cells were also analyzed for DNA. In all four zones of the kidney and in thyroid, 4n nuclei diminished in percentage between 3 and 50 weeks and increased at 100 weeks. In the growth phase these probably are cycling cells and after 50 weeks represent an increasing population of nuclei arrested after synthesis of DNA. Constant levels of ploidy were found in brain, heart, rectus abdominis, and adrenal throughout the 3-100 weeks. A dramatic increase in 4n nuclei occurred between 3 and 8 weeks in liver with little change occurring thereafter. Ploidy is a property of only parenchymal cells in liver and this probably is also true in other organs. The 4n nuclei that remain in constant proportion to the total population are established early in life and are not related to aging. They are probably tetraploid and replicate into 4n daughter cells during growth. Cerebrum shows no changes in nuclei number but exhibits a 70% increase in protein between 3 and 100 weeks. Although kidney, liver and adrenal show large increases in number of nuclei (approximately equal to fourfold) with growth, these are not as great as increases in body weight (approximately equal to 11-fold). With regard to organ protein, only liver shows increases approximating those in body weight. Increases in organ nuclei appear to occur in concert for adrenal, kidney, and liver whereas increases in organ protein bear no relationship to each other. Protein content remains at stable levels in organs of 100-week-old animals and little (adrenal, liver) or no (brain, kidney) diminution occurs in nuclei numbers.

Nuclear ploidy of neonatal rat livers: effects of two hepatic carcinogens (mirex and dimethylnitrosamine)

The effect of two hepatic carcinogens, dimethylnitrosamine (DMN) (genotoxic) and mirex (epigenetic), on polyploidization in 12-d-old neonatal rats was investigated by Coulter counteranalysis and [3H] thymodine uptake in isolated hepatic nuclear classes. DMN disturbed the normal ploidy development in the neonatal liver and the proportion of nuclei in the ploidy classes by inducing the premature formation of a significant population of tetraploids with a concomitant reduction in diploids. A great proportion of the replicative activity was present in tetraploid nuclei as measured by the incorporation of [3H] thymidine. The labeling index and number of mitoses were also increased. In contrast to DMN, mirex had no influence on polyploidization. The neonatal rats used in these studies thus offer an opportunity to investigate in vivo the mode of action of genotoxic versus epigenetic compounds with reference to their effect on DNA.

The relation of rat liver wet weight to dry weight

To assess a reliable relation between the dry and wet weight of rat livers, the water content of liver samples was determined by freeze drying. The ratio between wet and dry weight of the livers turned out as 3.33 +/- 0.3 for male and 3.28 +/- 0.24 for female rats. Thus for calculations a value of 3.3 can be used irrespective of the sex of rats.

Ploidy class-dependent variations during 24 h of glucose-6-phosphate and succinate dehydrogenase activity and single-stranded RNA content in isolated rat hepatocytes

The time-dependent variations over 24 h of glucose-6-phosphate dehydrogenase (G6PDH) activity, succinate dehydrogenase (SDH) activity and single-stranded RNA (ssRNA) content have been investigated by cytophotometric analysis of cytochemically stained isolated hepatocytes of different ploidy classes from adult male rats. A marked variation of 48% over the day in G6PDH activity of the mononuclear diploid cells was revealed, but no significant variation in the binuclear tetraploid cells. The cells of the inbetween ploidy classes showed an amplitude of variation of 38% (binuclear diploid cells) and 24% (mononuclear tetraploid cells), respectively. All cells showed a maximum activity of the enzyme at the middle of the day and a minimum during the night. The relative enzyme activity per mononuclear diploid cell was significantly higher than the relative activity in the other cells, especially at its maximum. The variation of the SDH activity in hepatocytes isolated from the same rats was similar in all cells, irrespective of their ploidy class. The activity was highest at the end of the activity phase of the animals. The SDH activity per cell was directly proportional to the quantity of genome copies. The ssRNA content of the hepatocytes showed a time-dependent variation with a maximum during the resting phase of the animals and a minimum during their activity phase. The variation was larger in the mononuclear diploid cells than in the cells of other ploidy classes and the ssRNA content was also significantly higher in these cells than in the hepatocytes of other ploidy classes when calculated on the basis of genome copies. It is concluded that the large amplitude of variation over the day and the high relative amount of G6PDH activity and ssRNA content in mononuclear diploid cells is related to the function of these cells as stem cells of the liver parenchyma.

Flow cytometric analysis of mouse hepatocyte ploidy. II. The development of polyploidy pattern in four mice strains with different life spans

The development of liver ploidy in mice aged up to 24 months was investigated by flow cytometry in four mouse strains. A mathematical procedure was applied for correction of flow cytometry histograms. In two of the mouse strains, C3H and DBA, both cellular and nuclear ploidy proceed in the same way. The octoploid cell with two tetraploid nuclei is the most numerous cell type in adulthood. On the other hand, strain NZB and the out-bred strain NMRI show at the corresponding age a higher proportion of diploid cells with strikingly low proportions of 4c cells. In addition, high values of 16c cells and nuclei are present in NMRI. In all strains the proportion of binucleate hepatocytes is in the same range (60%). However, the strains differ in ploidy classes of binucleate cells. Development of liver polyploidization does not depend on life span of the specific strain.

Occurrence of polyploidy and multinuclearity in the differentiating liver of chick embryo

Increase in nuclear size in liver has been used as an index of polyploidy. It has long been considered that the occurrence of polyploidy and multinuclearity are characteristics of mammalian liver. The present study shows the occurrence of these phenomena in the liver of birds, so these features are not confined to mammals. 3 classes of nuclear size groups have been identified. The simultaneous occurrence of polyploidy and binuclearity indicates some sort of interrelationship between them.

Lysosomal activity in the aging rat liver: II. Morphometry of acid phosphatase positive dense bodies

Hepatocytes from 3-, 12- and 36-month-old rats were studied in situ with cytochemical and morphometrical techniques to obtain information about the intralobular and age-dependent variation in lysosomal volume. An increase with age was found for the lysosomal volume density. This increase is evident throughout the lobule, but occurs at different rates in the centrolobular and peripheral areas. It is most pronounced in the centrolobular area, resulting in a lysosomal volume density of 7.1% in old rats, compared to 3.6% in the peripheral area of the liver lobule. Both the size and the number of individual lysosomes contribute to the overall effect. Most dense bodies, including lipofuscin-containing aggregates in senescent animals, exhibit acid phosphatase activity. Our results suggest that the use of enzyme activity as a marker may yield higher values for lysosomal volume density than morphologic characteristics alone. The alleged homogeneity of the liver as a source of hepatocytes for studies on isolated cells must be questioned when lysosomal functions are concerned.

DNA and protein content as cellular biochemical parameters. A discussion with two examples: acid phosphatase and cathepsin D in rat liver and hepatoma and acid phosphatase in human breast normal tissue and adenocarcinoma

The advantage of using DNA content as a biochemical parameter because the results it gives are directly related to cellularity is discussed. As examples, comparisons of acid phosphatase and cathepsin D activities in rat liver and hepatoma and of acid phosphatase in human normal breast tissue and adenocarcinoma are considered. Contradictory results are obtained, depending whether they are related to DNA content, fresh tissue weight, or protein content.

Histochemistry and cytochemistry of glucose-6-phosphate dehydrogenase

Histochemistry and cytochemistry of glucose-6-phosphate dehydrogenase has found many applications in biomedical research. However, up to several years ago, the methods used often appeared to be unreliable because many artefacts occurred during processing and staining of tissue sections or cells. The development of histochemical methods preventing loss or redistribution of the enzyme by using either polyvinyl alcohol as a stabilizer or a semipermeable membrane interposed between tissue section and incubation medium, has lead to progress in the topochemical localization of glucose-6-phosphate dehydrogenase. Optimization of incubation conditions has further increased the precision of histochemical methods. Precise cytochemical methods have been developed either by the use of a polyacrylamide carrier in which individual cells have been incorporated before staining or by including polyvinyl alcohol in the incubation medium. In the present text, these methods for the histochemical and cytochemical localization of glucose-6-phosphate dehydrogenase for light microscopical and electron microscopical purposes are extensively discussed along with immunocytochemical techniques. Moreover, the validity of the staining methods is considered both for the localization of glucose-6-phosphate dehydrogenase activity in cells and tissues and for cytophotometric analysis. Finally, many applications of the methods are reviewed in the fields of functional heterogeneity of tissues, early diagnosis of carcinoma, effects of xenobiotics on cellular metabolism, diagnosis of inherited glucose-6-phosphate dehydrogenase deficiency, analysis of steroid-production in reproductive organs, and quality control of oocytes of mammals. It is concluded that the use of histochemistry and cytochemistry of glucose-6-phosphate dehydrogenase is of highly significant value in the study of diseased tissues. In many cases, the first pathological change is an increase in glucose-6-phosphate dehydrogenase activity and detection of these early changes in a few cells by histochemical means only, enables prediction of other subsequent abnormal metabolic events. Analysis of glucose-6-phosphate dehydrogenase deficiency in erythrocytes has been improved as well by the development of cytochemical tools. Heterozygous deficiency can now be detected in a reliable way. Cell biological studies of development or maturation of various tissues or cells have profited from the use of histochemistry and cytochemistry of glucose-6-phosphate dehydrogenase activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Some observations on the structure of Suncus liver with special reference to the vitamin A-storing cell

The structure of the liver of the thick-tailed shrew (Suncus murinus) was studied with special reference to the vitamin A-storing cell. The macroscopic arrangement of hepatic lobes was the same as those of rodents. Connective tissues of Glisson's sheath were not prominent. There were gradual differences in the size and the cytoplasmic contents of hepatocytes among perihepatic, intermediate, and periportal regions. These correspond to the acinar zones 3, 2, and 1 of Rappaport, respectively. The ultrastructural features of the hepatocyte were numerous microvilli, a large mass of glycogen particles (alpha-particles), and well-developed Golgi complexes and lysosomes in the peribiliary region. Cellular elements of the sinusoid were the same as those of rodents. Vitamin A-storing cells were demonstrated by autoradiography of tritiated retinyl acetate administered orally. They were distributed in the perihepatic and intermediate regions. The number of vitamin A-storing cells was much smaller than that in the mouse liver and did not increase after excess vitamin A was given to the animal. Cytological features of the vitamin A-storing cells in Suncus liver were similar to those for other mammalian livers. The scarceness of vitamin A-storing cells and the low amount of vitamin A esters suggest that Suncus does not store much vitamin A in the liver.

The induction of chromosomal damage in rat hepatocytes and lymphocytes. I. Time-dependent changes of the clastogenic effects of diethylnitrosamine, dimethylnitrosamine and ethyl methanesulfonate

Male Wistar rats received a single injection of diethylnitrosamine (DEN), dimethylnitrosamine (DMN) or ethyl methanesulfonate (EMS). After a number of time intervals (up to 56 days) liver cells were assayed for the presence of possible preclastogenic damage by performing partial hepatectomy and subsequent analysis of chromosomal damage (micronucleus formation) in isolated hepatocytes. Peripheral blood lymphocytes from the same animals were collected, stimulated to proliferate and assayed for the frequency of sister-chromatid exchanges (SCEs). Whereas all agents significantly increased frequencies of SCEs in lymphocytes up to at least 28 days (EMS) or 56 days (DMN, DEN) after injection, only the latter 2 compounds gave rise to significantly increased incidences of micronucleated hepatocytes. DMN-induced preclastogenic damage in hepatocytes was lost between 28 and 56 days after injection. After DEN, this type of damage was persistent over the entire experimental period (56 days). When rats treated with DEN did not undergo partial hepatectomy, the frequencies of micronuclei at different time intervals after treatment were at control level. This result, together with those from hepatectomized DEN-treated rats, suggests that it is the persistent character of the preclastogenic damage that is responsible for the occurrence of micronucleated hepatocytes at later time intervals after treatment with DEN, rather than the stability of micronuclei which might eventually have been formed soon after injection.

Association of thyroid hormone receptors with chromatin

A large body of circumstantial evidence indicates that receptors located in nuclei of T3 responsive tissues represent a site of initiation of thyroid hormone action at the cellular level. Partial characterization of T3 receptors indicates that these proteins are monomeric structures in nuclei and are chromatin-associated non-histone proteins. Treatment of rat liver nuclei with either pancreatic DNase I or micrococcal nuclease releases T3 receptors from nuclei in two forms: a predominant (95 400 Mr; 5.5-6.0S) and a minor (265 000-365 000 Mr; 12.5S) nucleoprotein complex. Similar structures are excised from rat kidney, brain, and heart nuclei and from GH1 pituitary cell nuclei by micrococcal nuclease digestion. These endonuclease-excised receptor-containing complexes are significantly larger than the salt-extracted receptor (50 000 Mr; 3.5S). The presence of DNA and other non-receptor proteins in these structures indicates that T3 receptors probably function within multimeric complexes in vivo. Although T3 receptors appear to be associated with DNA between nucleosomes, i.e. linker DNA, it is not entirely clear whether all or only a fraction of T3 receptors interact with nucleosomal components. The 12.5S receptor-containing nucleoprotein complex may represent T3 receptors in association with linker DNA and nucleosomal components. T3 receptors do not appear to be uniformly distributed to all chromatin fractions, but are associated with structures having characteristics of transcriptionally active chromatin. They are found in a region of chromatin which is enriched in RNA polymerase activity, rapidly labeled RNA and non-histone proteins, and depleted of histone Hl. This region is also highly sensitive to both micrococcal nuclease and pancreatic DNase I digestion. The association of receptors with transcriptionally active chromatin, however, must be considered provisional until additional details of the precise receptor-chromatin interaction have been established. The recent demonstration of a 20-fold increase in a specific hepatic mRNA four hours following administration of T3 to hypothyroid rats indicates that thyroid hormone potentially has very rapid effects on hepatic gene expression. However, significant changes in nuclear protein phosphorylation, nuclear protein composition, and chromatin structure have not been detected within this four-hour period. Thus, effects of T3 on hepatic gene expression are brought about by local and presumably subtle changes in nuclear function.

Liver polyploidy: influence of age and of dietary restriction

Polyploidy increases with age in mammalian liver. Since dietary restriction increases lifespan, the question posed in this study was whether liver polyploidy would be influenced by dietary protein restriction. Restricted mice were fed a 4% protein diet, while control mice received a 26% protein diet. Polyploidy was determined from measurements of nuclear diameter on liver of 3 week, 1 1/2, 3, 10, 20 and 24-1/2 month old Swiss albino mice. The percentage of polyploid cells increased steadily with age. The rate of increase, expressed as the polyploidization index, was greater in the fully fed control animals as compared to the animals on protein restriction. Protein restriction thus retards the age associated process of polyploidization of liver nuclei. Nuclear diameter increased slightly with age, but was not influenced by protein restriction.

The effect of sodium butyrate on acetylation in vitro of chromosomal proteins in three classes of liver nuclei from different ages of rats

The optimum conditions of in vitro incorporation of sodium [3H]acetate into sliced rat liver were studied. The incubations with sliced liver from three different ages of rats were performed in the presence of sodium n-butyrate. It was found that butyrate decreases the incorporation of sodium [3H]acetate into the homogenate, isolated nuclei, non-histone chromosomal proteins and histones for all age groups. The acetylations of non-histone chromosomal proteins and histones increase with age upto 2-months and decrease in 4-month-old rats both in the absence and presence of butyrate. Liver nuclei were fractionated by the simple method of zonal centrifugation into three classes, namely diploid stromal, diploid parenchymal and tetraploid parenchymal nuclei. The acetylations of non-histone chromosomal proteins and histones in three classes of nuclei of three ages of rats were studied in the presence and absence of butyrate. Butyrate can decrease the overall acetylations of non-histone chromosomal proteins and histones but increase the amount of polyacetylated histone H4 in all classes of nuclei of the three ages.

Hepatocyte euploidization is a typical mammalian physiological specialization

1. Quantitative cytochemical studies of hepatocytes from a variety of species representing the various sub-classes of mammals have shown that euploidization of hepatocytes is a typical feature of mammalian livers. 2. Euploidization occurs during early development and frequently involves a large proportion of the hepatocyte population. 3. Comparison with livers from birds and diploid fish indicates that euploidization is a special mammalian feature.

The relationship between cell volume, ploidy. and functional activity in differentiating hepatocytes

Liver cells were isolated by collagenase perfusion from rats of 1 day, and 1, 3, 5, and 12 weeks of age, fractionated by velocity sedimentation at 1 g (STAPUT), and the major cell types were identified in terms of specific functions. Alphafetoprotein and albumin were used as markers of differentiating hepatocytes and these functional activities were evaluated in a quantitative manner using a radioimmunoassay. The capacity of this cell type to store 35S-BSP, an indicator of bile formation, was also evaluated. Sinusoidal cells and hematopoietic cells were identified on the basis of their ability to take up 99mTC-colloid sulfur and to incorporate 59Fe, respectively. The fractionation procedure allowed a good separation of sinusoidal cells from hepatocytes at all postnatal ages and also of erythroid cells still present during the first week after birth. With increasing age, alphafetoprotein-producing hepatocytes exhibited changes in sedimentation velocities that parallelled those of albumin-producers. In turn, the latter hepatocyte subpopulation underwent gradual shifts in modal peak velocities similar to those of bile-forming hepatocytes. The fractionated hepatocytes obtained at different ages were further analyzed in terms of cell volume and nuclear ploidy using a Coulter counter system. This quantitative analysis obtained at the cellular level demonstrated that during the age-related differentiation of hepatocytes, which occurs during the postnatal period and results in the gradual appearance of cells of higher ploidy levels, the extent of albumin production and bile formation can be correlated with the hepatocyte volume.

Age-related changes in isolated rat hepatocytes. Comparison of size, morphology, binucleation, and protein content

Hepatocytes isolated from 6-, 12-, 18-, and 30-month-old female Fischer F344 rats were examined by scanning electron microscopy. No significant change in cell size with age was observed. However, the surface morphology of the cells isolated from the older animals exhibited a significant increase in surface folds. This feature did not exceed 10% of the cell population until 12 months of age and continued to increase to 31% of the cells in 30-month-old rats. From 6 to 12 months of age, there was a significant increase in protein content of the hepatocytes. No further increase in protein content occurred during senescence. An increase in percentage of binuclear cells occurred after 24 months of age. Because ploidy and binucleation increase with increasing age, it appears that the nuclear/cytoplasmic ratio changes as a function of age.

Morphological changes in the mammalian vas deferens during aging. I. The occurrence of binuclear cells in the mouse

The number of nuclei per principal cell was determined in the vas deferens of sexually immature, young sexually mature and old male mice. The incidence of binucleation increased with age. In 21-day-old mice approximately 99% of the principal cells were mononucleated and 9%-20% of the principal cells were binucleated. In the oldest animals, the ampulla and testis end of the vas usually had more binuclear cells than the middle segment. The biological significance and the mechanism of this phenomenon are not known; however, binucleation may prove to be a useful indicator of aging in the vas of this species. In addition, the mouse vas may provide a useful model for studies of binucleation and mitosis.

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.

Polyploidy in mammalian urothelial cells

The mitotic indices and the extent of polyploidy in urothelial cells of baboons, dogs and swine were studied. All three species had very low mitotic activity in vivo but short-term culturing of these cells in vitro stimulated mitosis thus enabling chromosome counts. Tetraploid cells were found in the urothelium of all three species, and higher ploidies also in dog and swine. There were substantial differences in the proportions of diploidy and higher ploidies among the three species and among individuals within each species. Dog urothelial cells were predominantly tetraploid (70%) while more swine cells were diploid (68%). Baboon urothelial cells had only two ploidy classes and 92% were diploid.

Histochemical determination of histone and non-histone protein content in rat liver nuclei

The purpose of this study is to compare the protein content of parenchymal and non-parenchymal nuclei, as isolated from rat liver. The nuclei have been separated by means of a 1 g-sedimentation technique. The protein content of the separated nuclei has been determined cytophotometrically using the Naphthol Yellow S staining procedure after TCA-extraction (corresponding with the total protein content) and directly (corresponding with the non-histone proteins). The ratio of the total protein content of non-parenchymal, parenchymal diploid and parenchymal tetraploid nuclei respectively was found to be 0.65:100:1.90. The ratio of non-histone protein a total protein was the same for all types of nuclei investigated, namely about 55%.

A quantitative cytochemical study of acid phosphatases in rat liver parenchymal cells of different ploidy values

The Vickers M86 integrating microdensitometer has been used to quantify the cytochemical reaction for acid naphthol AS-BI phosphatase activity in isolated, rat liver parenchymal cells. Data are presented which validate the method. The use of this method together with that of the Feulgen reaction to estimate nuclear ploidy value in the same cell, has shown that there is an increase in acid phosphatase activity of up to 100% when the euploidy value of the liver cell doubles. It has been further shown that 70--80% of this enzyme activity is ouabain-sensitive, regardless of the euploidy value. The data may be interpreted to indicate that the extra gene copies of the polyploid cells are operative.

Normal human endometrium in cell culture. II. A microspectrophotometric study of polyploid nuclei in short-term primary epithelial cultures

The growth of short-term primary cultures of endometrial epithelium has been studied using Feulgen microspectrophotometry. A gradual increase in the number of polyploid nuclei up to 64C has been observed and is associated with a decline in the growth capacity of the cultures. The specific mechanism(s) of this polyploidization is not known.

The development of polyploidy in two classes of rat liver nuclei

Two classes of nuclei from livers of Sprague-Dawley rats were isolated, one pelleting in 2.3 M sucrose (H nuclei) and the second class sedimenting through 1.6 and 1.8 M sucrose and banding at the 1.8/2.3 M sucrose interface (L nuclei) of a three-step discontinuous gradient. In younger animals, the L nuclear fraction was the major fraction, but the percentage of nuclei found in the L fraction decreased as the animals grew. Nuclear ploidy was determined by flow microfluorometry using propidium iodide as a DNA stain. Both the H and L nuclear fractions contained diploid, tetraploid and octaploid nuclei; but the degree of polyploidy was greater in the H fraction. Concomitant with the change in distribution of nuclei between the H and L fractions with increasing age was a progressive increase in the degree of polyploidy in the H fraction. Polyploidy did not increase linearly with age in the H nuclear fraction but increased in cycles marked by large changes in the numbers of nuclei found in H and L nuclear fractions. By 12 weeks of age, 4n-H nuclei were the largest single population of nuclei in rat liver. These observations suggested that the shift of liver nuclei from the L fraction to the H fraction was associated with the development of polyploidy and with the differentiation of hepatocytes.

Growth fraction and cycle duration of hepatocytes in the three-week-old rat

The proliferation of hepatocytes in the liver of 3-week-old rats has been investigated by autoradiographic methods. This investigation is a continuation of earlier work on the same topic (Schultze & Maurer, 1972; 1973). 21 days after birth, 102 rats received a single injection of 3H-TdR. The percentage of labelled mitoses was then determined 1 hr later and at various times throughout the interval up to 12 days after application of 3H-TdR. In agreement with earlier work, a first peak of labelled mitoses was found 7 hr after 3H-TdR injection. The area under the peak indicates an S phase duration of 8 hr. In addition a second very broad peak of labelled mitoses was found between 2 and 12 days after pulse labelling. The analysis of the results leads to the conclusion that the hepatocytes of the 3-week-old rat have a growth fraction close to 1 and a doubling time of 6-7 days. This is at variance with earlier results of Post, Huang & Hoffman (1963) and Grisham (1969) who had derived a value of 21.5 hr for the duration of the cell cycle and a value of only 0.1-0.2 for the growth fraction of the hepatocytes.

The effect of carcinogens on the nuclear size of HeLa cells

The size of HeLa S-3 cell nuclei was measured after 1--3 days in culture. After exposure to carcinogens, nuclear size increased and the frequency distribution pattern of nuclear size changed. The optimal conditions for these changes were the use of concentrations of chemicals which had a low order of cytotoxicity and the measurement of nuclear size 48 h after pulse treatment of the cells for 30--60 min. These effects were not seen with non-carcinogens. The results obtained suggest that changes in nuclear size may be an index of chemical carcinogenicity.

Age-related changes in chromatin of liver cell nuclei of different ploidity

A comparison of the Feulgen hydrolysis curves and the chromatin compactness of the liver cell nuclei of young and old rats was made. It was found that the rate of DNA depurination and chromatin compactness are higher in the liver cell nuclei of old rats, both in di-and tetraploidal cells. The effect of fixation upon the course of the hydrolysis curves is discussed.

Reassociation kinetics of polyploid hepatocyte DNA

The state of ploidy of 4c hepatocytes is examined by kinetic analysis of DNA reassociation. The nucleotide sequences present in 4c hepatocyte DNA are found to be complementary to, and present in the same relative abundance as, those of the normal diploid genome. Therefore, these experiments indicate that the increased DNA content of 4c hepatocytes is due to the presence of four haploid genomes and not to selective replication of a subfraction of the diploid genome. The concept of ploidy, typically defined in terms of karyotypic and/or DNA content multiples is thus corroborated and extended by these experiments.