Peculiarities of liver regeneration in Chinese hamster <Emphasis Type="Italic">Cricetulus griseus</Emphasis>

With the aid of cytofluorimetry and interference microscopy, the ploidy level and the hepatocyte ploidy class distribution were studied and the dry mass of hepatocytes was measured in hepatocytes in liver of Chinese hamsters Cricetulus griseus and of Balb/c mice before and one month after partial hepatectomy. The mean ploidy level in hepatocytes of the Chinese hamster normal liver amounted to 2.35 ± 0.03 c. The modal class was mononuclear hepatocytes with diploid nuclei (82.4 ± 1.3%). The mean dry mass of hepatocytes amounted to 605.2 ± 4.8 pg. In the process of liver regeneration in the Chinese hamsters, the ratio of ploidy classes and the hepatocyte dry mass did not change. After a similar liver resection in the mice, a significant polyploidization of liver parenchyma occurred. The mean ploidy level in hepatocytes rose by 32%. Instead of 4cx2-hepatocytes, the modal class became mononuclear octaploid cells the relative portion of which increased, on average, by five times. The portion of binuclear hepatocytes with octaploid nuclei in mouse liver rose by more than five times. Thus, in the Chinese hamsters Cricetulus griseus, unlike mice, regeneration of liver occurred exclusively at the expense of proliferation of hepatocytes.     

Cellular mechanisms of cirrhotic rat liver regeneration. II. Proliferation, polyploidization and hypertrophy after partial hepatectomy

Using cytofluorimetry and absorptional cytophotometry, hepatocyte DNA and total protein contents were measured in intact and cirrhotic rats in 1, 3 and 6 months after partial hepatectomy (PH). It has been found that within one month of intact rat liver regeneration the level of hepatocyte ploidy rised by 25% to remain elevated for the next 6 months. This was due mainly to reducing the number of cells with diploid nuclei (2c 2-fold, 2c x 2 - 6.6-fold) and to rising the number of octaploid hepatocytes. In cirrhotic animals the ploidy level in hepatocytes increased in 3 months after PH, and decreased by 15% in 6 months. The number of hepatocytes with diploid nuclei (2c and 2c x 2) increased within 3-6 months in both control and cirrhotic rats. The protein content per diploid hepatocyte rised by 30% within 3-6 months of liver regeneration after PH. Special calculations have shown that within 3 months after PH the increase in the liver mass of control and cirrhotic rats was due completely to hepatocyte DNA synthesis, i. e. proliferation and polyploidization. Within the next 3 months of liver regeneration after PH, the contribution of polyploidization to liver mass increase was negative because of depolyploidization of liver parenchyma cell population. At this time hypertrophy was the main process determining the liver mass increase.     

The kinetics of the cell population of human liver parenchyma at different periods of life]

Processes of polyploidization in the liver parenchyma were investigated in the course of postnatal organism growth, stabilization of growth and ageing, using cytophotometry on the slides of isolated hepatocytes from normal livers of 140 donors aged from 1 day to 92 years. In addition, livers of human embryos (4, 5, 6 and 7 month old) were investigated. It is concluded that polyploid cells in the human liver appear in individuals aged from 1 to 5 years. However, during the postnatal development their relative number increases insignificantly. At the end of the intensive postnatal growth period the share of polyploid human liver cells is less than 3%. Binuclear cells with diploid nuclei are seen as early as in the embryonic liver. After birth their number increases slowly to reach 7.1% in the 16-20 year age group. The postnatal growth of human liver is due mainly to mitotic divisions of mononuclear diploid hepatocytes whose relative number is more than 90% during the postnatal growth. During the period of maturity (from 21 to 50 years), when the liver practically stops to grow, the levels of hepatocyte ploidy are changed insignificantly: part of 2c-hepatocytes decreases slowly (up to 84.8% by the end of period) and (2c x 2)-hepatocyte number increases slowly too. The number of polyploid cells increases by several times, but is equal only to 6.6% of all the hepatocytes counted. Under ageing, on the background of human liver atrophy, acceleration of hepatocyte polyploidization takes place. In the age group of 86-92 years parts of 2c- and (2c x 2)-hepatocytes reach 60.3 and 14.3%, resp., and the total share of polyploid cells is as much as near 25%, calculated from the cell population of liver parenchyma. The maximum ploidy levels in hepatocytes of normal human liver during ageing is becoming 16c and 8c x 2 for mononuclear and binuclear cells, resp. Transition rates among hepatocytes of different ploidy classes (2c--2c, 2c--2c x 2, 2c x 2--4c, 2c--4c) were calculated in addition to the coefficient of changing of the hepatocyte proliferative activity with the increase in its ploidy and cell death rate in different periods of human life. A rather high hepatocyte proliferative activity in the early postnatal period of human life was seen to lower during the following years of life. In maturity it is the lowermost to make less than 5% of that in newborns. During ageing the hepatocyte DNA-synthesizing activity being almost 1.6-1.7 times as much as in maturity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Progressive polyploidy in mouse liver following repeated hepatectomy]

Mouse liver regeneration after partial hepatectomy results in sharp changes of ploidy classes towards the increase of high ploidy cells and the decrease low ploidy ones. These changes retain during three months. Each following partial hepatectomy (till 3 times) intensifies the hepatocyte polyploidy with appearance of cells with 32--64 ploidy nuclei. The cell polyploidization stimulated by repeated regenerations is similar to that observed in normal postnatal liver growth.     

Cellular mechanisms of rat liver regeneration after experimental myocardial infarction

Morphological changes and regeneration activity of rat liver after experimental myocardial infarction (MI) caused by a permanent left coronary artery occlusion were investigated. It was shown that, 6 months after MI, considerable changes were observed in the rat liver circulatory system: the vessel amount per unit area increased by 118%, the thickness of their walls increased by 19%, and the average area of vessel lumens increased by 159%. The contribution of connective tissue 6 months after MI increased by more than one- and-a-half times in comparison with control. Inflammatory and necrotic changes in rat liver remained for 6 months after MI. The liver injury caused by MI leads to activation of regeneration processes in its parenchyma. Six months after MI, the number of 4c-hepatocytes decreased by 12% in comparison with control and the number of 4c×2- and 8c-hepatocytes increased by 45 and 71%, respectively. Six months after MI, the hepatocyte ploidy increased by 11%. In this period, the dry mass of rat hepatocytes increased by 19%. Thus, liver regeneration after MI is stipulated by hepatocyte hypertrophy rather than their polyploidization.     

Hepatocyte ploidy in normal young rat

The aim of the present study was to examine the relation between hepatocyte size and ploidy in Sprague-Dawley rat liver. Therefore, subpopulations of hepatocytes of various sizes were separated from the isolated crude hepatocyte population either mechanically or by using centrifugal elutriation. Hepatocyte size was determined on scanning electron microscopy photographs. Ploidy of hepatocytes was assessed by flow cytometry. The crude hepatocyte population was very heterogeneous in sizes, with diameters ranging from 8 to 39 microm. Hepatocyte ultrastructure was well preserved as demonstrated by transmission electron microscopy. The distribution of hepatocytes within the ploidy classes was the following: 19.6+/-3.6% diploid, 56.2+/-3.2% tetraploid and 3.4+/-0.6% octoploid mononucleated cells. Thus approximately 79% of hepatocytes appeared mononucleated. The binucleated hepatocytes (21%) had two diploid nuclei (18.7+/-2.9%) or two tetraploid nuclei (2.1+/-0.6%). A similar distribution of hepatocytes into ploidy classes was obtained in subpopulations of hepatocytes of various sizes. Our findings suggest that distribution into ploidy classes is not strictly correlated with hepatocyte size. In accordance with previous observations, our results on hepatocyte ploidy from periportal or perivenous origin using digitonin perfusion, is in favour of the existence of ploidy zonation within the rat hepatic lobule.     

CXC Chemokines Function as a Rheostat for Hepatocyte Proliferation and Liver Regeneration

Background

Our previous in vitro studies have demonstrated dose-dependent effects of CXCR2 ligands on hepatocyte cell death and proliferation. In the current study, we sought to determine if CXCR2 ligand concentration is responsible for the divergent effects of these mediators on liver regeneration after ischemia/reperfusion injury and partial hepatectomy.

Methods

Murine models of partial ischemia/reperfusion injury and hepatectomy were used to study the effect of CXCR2 ligands on liver regeneration.

Results

We found that hepatic expression of the CXCR2 ligands, macrophage inflammatory protein-2 (MIP-2) and keratinocyte-derived chemokine (KC), was significantly increased after both I/R injury and partial hepatectomy. However, expression of these ligands after I/R injury was 30-100-fold greater than after hepatectomy. Interestingly, the same pattern of expression was found in ischemic versus non-ischemic liver lobes following I/R injury with expression significantly greater in the ischemic liver lobes. In both systems, lower ligand expression was associated with increased hepatocyte proliferation and liver regeneration in a CXCR2-dependent fashion. To confirm that these effects were related to ligand concentration, we administered exogenous MIP-2 and KC to mice undergoing partial hepatectomy. Mice received a “high” dose that replicated serum levels found after I/R injury and a “low” dose that was similar to that found after hepatectomy. Mice receiving the “high” dose had reduced levels of hepatocyte proliferation and regeneration whereas the “low” dose promoted hepatocyte proliferation and regeneration.

Conclusions

Together, these data demonstrate that concentrations of CXC chemokines regulate the hepatic proliferative response and subsequent liver regeneration.     

Heparin-binding epidermal growth factor-like growth factor links hepatocyte priming with cell cycle progression during liver regeneration

The mechanisms that regulate the transition between the initial priming phase and DNA replication in liver regeneration are poorly understood. To study this transition, we compared events occurring after standard two-thirds partial hepatectomy, which elicits full regeneration, with response to a reduced hepatectomy, one-third partial hepatectomy (1/3PH), which leads to little DNA replication. Although the initial response to partial hepatectomy at the priming phase appeared to be similar between the two procedures, cell cycle progression was significantly blunted in 1/3PH mice. Among the main defects observed in 1/3PH mice were an almost complete deficiency in retinoblastoma phosphorylation and the lack of increase in kinase activity associated with cyclin E. We report that, in two-thirds partial hepatectomy mice, the expression of heparin-binding epidermal growth factor-like growth factor (HB-EGF) preceded the start of DNA replication and was not detectable in 1/3PH animals. Injection of HB-EGF into 1/3PH mice resulted in a >15-fold increase in DNA replication. Moreover, we show that hepatocyte DNA replication was delayed in HB-EGF knock-out mice. In summary, we show that HB-EGF is a key factor for hepatocyte progression through G(1)/S transition during liver regeneration.     

Liver regenerative potential of the lake frog Rana ridibunda after partial hepatectomy

A histomorphological study of the regenerating liver of Rana ridibunda, within 2 months after partial hepatectomy, shows that regenerative processes on the wound surface are slowly proceeding. Processes of reticular fiber reconstruction occurred in the composition of the basal membrane of liver sinusoids. A cytophotometric study shows that glandular cells in R. ridibunda liver are commonly tetraploid. The post-traumatic regeneration of the liver after partial hepatectomy involves activation of DNA synthesis in hepatocytes, leading to increase in their ploidy. Within the 1st month of regeneration, the mitotic index of hepatocytes substantially increased. Regeneration of glandular parenchyma of the liver is accompanied by a quantitative increase in binucleate hepatocytes, which is most highly expressed within 5-20 days after partial hepatectomy.     

Analysis of the DNA content of urethane-induced liver tumors in mice

Administration of urethan (three times per 1 mg calculated per 1 g of the animal mass) after partial hepatectomy resulted in the development of liver tumours classified as adenomas in 62.8 per cent of mice 12 months after treatment. In the cells isolated from 85 adenomas and from the surrounding liver, DNA-fuchsin content was determined cytophotometrically. Three types of DNA distribution were distinguished, with the mode in the region of near-diploid (26%), near-tetraploid (71.8%), or near-octaploid (2.2%) DNA value. Most commonly DNA distributions were polymodal, but unimodal ones also occurred. The number of binucleated cells in tumours was significantly decreased. In the liver of one of the same animal, morphologically similar tumours of all the three types of DNA distribution could be found. The increase in the hepatocyte ploidy level in the initial parenchyma by preliminary repeated treatments with CCl4, had no effect either on the occurrence of tumours, or on their ploidy level. No correlation was found between the DNA content, the size and histological structure of tumours.     

中国地鼠线粒体Cyt b基因测序及其分子进化

目的测定中国地鼠线粒体DNA细胞色素b基因部分序列,分析其分子系统进化关系。方法提取中国地鼠肝脏的总基因组DNA。设计合成特异引物进行PCR扩增,经检测进行测序。用Blast与GenBank中啮齿类其他常用实验动物的物种细胞色素b基因进行同源序列比较,分析其碱基组成及变异情况,并用邻接法、最大简约法、最小进化法构建了分子系统树,在分子水平上探讨中国地鼠和常用啮齿类实验动物的进化关系。结果获得了中国地鼠线粒体Cytb基因的部分序列,共936bp。结论中国地鼠和金黄地鼠的亲缘关系最近,与小鼠、大鼠存在的差异相对大,与豚鼠的亲缘关系最远,与传统的分类地位基本吻合。     

Hepatocyte classes during liver atrophy due to starvation in the golden hamster

Summary The total dry masses of normal hepatocytes isolated from adult golden hamsters form a multimodal frequency distribution curve (10–11 cell classes with a period of 132 pg). During starvation the hepatocytes maintain the arrangement in classes, the number of which, however, decreases. The cell percentage of the lightest classes progressively increases, but no change occurs in the class period. A similar behaviour is shown by the aqueo-insoluble dry masses of the hepatocytes. The nuclear dry masses increase until the 4th day of starvation. Later on, they decrease. The nucleo-cytoplasmic ratio progressively increases. The total number of hepatocytes per liver and the binuclear cell percentage do not show any significant changes during starvation. The mitotic index is lower in starved animals. The kinetics of decrement in dry mass of the hepatocytes during starvation and the possibility that the hepatocyte class series are the resultant of a dynamic balance of single hepatocyte mass are discussed.This work was supported by Consiglio Nazionale delle Ricerche, Roma, Italia (Grant No. 70/01811/04).The author is deeply grateful to Prof. Enrico Puccinelli for his encouragement and advice during this work. The skilful technical assistance of Mrs. Lucia Giaccardo and Mr. Emilio Madrigali is gratefully acknowledged.     

Gene expression in regenerating liver in relation to cell proliferation and stress

When hepatocyte proliferation is stimulated in the liver by partial hepatectomy, messenger RNAs coding for fibrinogen, actin, c-myc and topoisomerase I are rapidly accumulated. We distinguish an early phase of accumulation (0-3 h after partial hepatectomy) which is also observed after a sham operation for the four genes, and during inflammation produced by Freund's adjuvant in the case of fibrinogen and c-myc genes. The hepatic response to inflammation appears therefore to mimic events characteristic of the G0/G1 transition, such as the accumulation of the c-myc mRNA. The late phase of mRNA accumulation (beyond 3 h after partial hepatectomy) is typical of liver regeneration. The level of c-myc mRNA is transiently increased (20-fold over normal) 20 h after partial hepatectomy, that is, at the time of DNA synthesis. Topoisomerase-I mRNA level increases between 3 and 24 h after partial hepatectomy (5-10-fold over normal). These results suggest that accumulation of c-myc and topoisomerase-I mRNAs is associated with DNA replication in regenerating liver.     

Analysis of the polyploidization kinetics of the parenchymal cells in the rat liver

Methodological approaches to kinetics of cell polyploidization in the rat liver parenchyma are discussed. Different ways of hepatocyte polyploidization in the course of postnatal liver growth have been assessed. The intensities of hepatocyte transitions from one ploidy class to another were determined. On the basis of literary experimental data the following is summarized: With the increase in the animal age, there is a decrease in hepatocyte transition from one ploidy class to and ther; in young animals the intensity of formation of tetraploid hepatocytes through the stage of binuclear cells (2c----2c X 2----4c) is 0.39-0.55 within two weeks, the intensity of direct transitions (2c----4c) being 0.00-0.19 within the same time. The intensity of entering to DNA synthesis is reduced with the increase in hepatocyte ploidy levels; in this case the coefficient of the reducing of mitotic activity is calculated as 0.10-0.22, and 0.01-0.05 for 4c- and 8c-hepatocytes, resp. The factors stimulating proliferation in the liver increase the intensity of the direct cell transition (2c----4c) by several times which can exceed the intensity of transition through the binuclear cell stage.     

A method for investigating hepatocyte polyploidization kinetics during postnatal development in mammals.

A method for investigating weakly-proliferating cell populations of liver parenchyma on the basis of a quantitative analysis of hepatocyte polyploidization during postnatal development is described. The method uses a mathematical model which characterizes the hepatocyte polyploidization process, and incorporates data concerning the time course for relative frequencies of hepatocytes in different ploidy classes. As a result of these measurements and calculations for rat liver, transition rates of hepatocytes (the relative number of cells during a given time unit) from one ploidy class to another, and a coefficient for the reduction of hepatocyte mitotic activity with an increase in its ploidy class were obtained. Calculated curves show a good correspondence with the real process of hepatocyte frequency changes as they relate to changes in the age of the animals. To check this method, experiments investigating time changes of autoradiographic label content in the different ploidy classes of hepatocytes were carried out. By mathematically modeling the label diluting process resulting from cell proliferation and polyploidization, transition rates of hepatocytes were calculated, and they reflect values calculated from the model according to changes in occurrence frequencies.     

A method for determining ploidy distributions in liver tissue by stereological analysis of nuclear size calibrated by flow cytometric DNA analysis

A method is presented for determining ploidy distributions in mouse liver from image analysis with stereological estimations of nuclear size in tissue sections. Nuclear profile distributions obtained from profile measurements were subjected to a mathematical unfolding procedure in order to obtain the nuclear size distributions. Based on the assumption that nuclear size increases monotonically with nuclear DNA content, flow cytometric DNA analysis of suspensions of liver cell nuclei was used to calibrate the method, thus yielding the mean nuclear size of each ploidy class, i.e., diploid, tetraploid, and octaploid nuclei. After the size interval for each of the ploidy classes was determined, the method allowed determination of ploidy distributions in mouse liver by stereological image analysis alone. The method was established from combined stereological and flow cytometric measurements on liver tissue representing two different stages of liver regeneration after two-thirds partial hepatectomy, and it was tested against an independent set of data representing a marked increase in the portion of S-phase cells.     

Analysis of the postnatal growth of the mouse liver by estimating the hepatocyte count, their weight and ploidy

Changes in the total number of hepatocytes, their distribution by the ploidy classes, as well as changes in the protein content of the cells were studied in 0.5-6 month old mice. The data obtained made it possible to estimate quantitatively the contribution of different growth components: increase in cell number, hypertrophy and polyploidization of cells, to the total increase of the liver mass. From 2 weeks to 1 month, the liver mass is increased via polyploidization (by 70%) and hypertrophy (by 30%). From 1 to 2 months, the liver mass increases due to hyperplasia (by 65%) and polyploidization (35%). After 2 months, the liver growth is practically terminated. The calculated equivalent mass of the liver, i. e. derivative of all three growth components, coincides fairly well with the factual changes in the liver mass.     

Development of binuclearity and DNA-polyploidization in the growing mouse liver

Summary Suspensions of intact liver cells were prepared from 36 male NMRI mice of different age after perfusion of the liver with ice-cold calcium- and magnesium-free phosphate buffer (CMF). The suspensions of the isolated hepatocytes were smeared on slides, fixed, hydrolized and stained by fluorescent acriflavine-Schiff-Feulgen reaction. The number of nuclei per cell was determined in a phase-contrast microscope. Quantitative fluorescent cytophotometric measurements of nuclear Feulgen-DNA were performed on individual nuclei. At the age of 0.5 month, 55% of the hepatocytes were found to be mononuclear, 45% binuclear. In the animal groups aged 1 month, 1.5 months, 3 months, 6 months and 12 months, the percentage of binuclear hepatocytes remained constant at about 80%. Very few liver cells with 3 or 4 nuclei were detected. Feulgen-DNA-measurements revealed a predominance of 2c and 4c nuclei at ages 1 month and 1.5 months with logarithmic increase of 8c nuclei and a decrease of the 2c nuclei. From 1.5 months on 16c nuclei were found, which never exceeded 8%. When total DNA-ploidy of the hepatocytes was calculated similar kinetics at a higher ploidy level were observed. 2c hepatocytes existed in small percentages at very young ages up to 1.5 months, but were also occasionally found in older animals. With increasing age the number of 16c hepatocytes increased logarithmically with a concomitant decrease of the 4c hepatocytes. The percentage of 8c liver cells remained more or less constant. Few hepatocytes with a 32c total DNA content were found in mice aged 3 months and older. In one-year-old mice the mean DNA-ploidy was calculated to be 5.8c per liver nucleus and 10.0c per whole hepatocyte.Supported by Deutsche Forschungsgemeinschaft, Grant No Bo 395/5     

Proto-oncogene expression in regenerating liver is simulated in cultures of primary adult rat hepatocytes

Proto-oncogene fos mRNA levels are rapidly and transiently elevated 12-fold in regenerating liver 10-60 min following partial hepatectomy. This response, and the induction of fos protein synthesis, has been simulated qualitatively and quantitatively in long term primary cultures of quiescent adult rat hepatocytes where proliferative transitions can be initiated directly in serum-free medium by known hepatocyte mitogens like epidermal growth factor. Expression of a second proto-oncogene, c-rasH, in proliferatively activated hepatocyte cultures between 6 and 24 h also simulates the delayed hepatic response that occurs in vivo following partial hepatectomy. These results suggest that sequential proto-oncogene expression during liver regeneration is caused directly by hepatocellular interactions with specific mitogens. In addition, a role for monovalent cations in the regulation of hepatocyte gene expression is implicated from findings that Na+ deprivation inhibits induction of fos expression in cultured hepatocytes by epidermal growth factor under chemically defined conditions.