DNA synthesis and cell proliferation in the simple liver acinus of 10 to 20-day-old rats: evidence for cell fusion

Mechanisms determining phenotypic heterogeneity of hepatocytes

This review summarizes results of biochemical and immunohistochemical studies indicating the existence of functional heterogeneity of hepatocytes depending on their localization in the hepatic acinus; this determines characteristic features of metabolism of carbohydrates, lipids, and xenobiotics. The physiological significance of hepatocyte heterogeneity is discussed. According to the proposed model of intercellular communication, the metabolic specialization of hepatocytes is determined by secretory activity of hepatic resident macrophages (Kupffer cells) localized mainly in the periportal zone of the liver acinus. Macrophages participate in secretion of a wide spectrum of intercellular mediators (cytokines, prostaglandins, growth factors) and also in metabolism of numerous blood metabolites and biologically active substances (hormones, lipoproteins, etc.). In the sinusoid and in the space of Disse (also known as perisinusoidal space) they form a concentration gradient of regulatory factors and metabolites inducing the phenotypic differences between hepatocytes.

Hepatic microvascular development in relation to the morphogenesis of hepatocellular plates in neonatal rats

The development of hepatic microvascular heterogeneity after birth, and its temporal relationship to the development of parenchymal cell plates have received little attention. As a result, the morphogenesis of some of the parameters contributing to this heterogeneity in suckling and weaned rats was studied as a function of time between postpartum days 4 and 30 using in vivo light microscopic, electron microscopic, and immunocytochemical methods. During the early suckling period, the sinusoid network is highly anastomotic, with little evidence of zonation, and the parenchymal cell plates contain multiple cells and are irregularly arranged throughout the lobule. Sinusoidal endothelial fenestration is sparse at 4 days, but phagocytic Kupffer cell (KC) function already exists and exhibits zonal heterogeneity, with more cells located in the periportal zone. With increasing age, endothelial fenestrae increase and organize as sieve plates. Widened centrilobular radial sinusoids form through a loss ("drop-out") of intersinusoidal sinusoids (ISS). Concomitantly, the associated cell plates straighten and become one cell thick. Hepatocyte DNA synthesis and mitosis are higher in the periportal zone, which retains thickened cell plates and anastomotic sinusoids. The centrilobular sinusoids may widen to accommodate the increased volume of blood that results from the loss of ISS as well as the increased numbers of periportal sinusoids containing flow that feed these vessels. KC phagocytic activity increases during the suckling period concomitant with an increase of gut-derived endotoxin in the portal blood, which suggests that the KCs may be releasing mediators that affect sinusoid diameter, blood flow, endothelial fenestration, and perhaps parenchymal growth either directly or through the stimulation of growth factors.

Quantitative fluorescence imaging approach for the study of polyploidization in hepatocytes

We applied automatic quantitative fluorescence imaging of nuclear DNA to rat liver cells obtained from animals at various times after birth up to 3 months of age. We show that, in conditions best preserving the native cellular structures, DNA content measurements, performed on whole single cells in situ after Hoechst staining, were precise and accurate. Cells in the various ploidy and nuclearity classes could thus be identified correctly and their percentages were estimated on a total of 300 cells or more. DNA synthesis was shown to occur asynchronously in all ploidy and nuclearity classes around weaning time. Observation of the labeling patterns, after in vivo BrdU pulse and short-term culture (chase), showed that the cell cycle was shorter in diploid cells compared with cells undergoing polyploidization. These results show that the approach of fluorescence imaging is well suited to investigations on polyploidization mechanisms.

Accumulation of abnormally high ploid nuclei in the liver of LEC rats developing spontaneous hepatitis

Enlarged hepatocytes with huge nuclei were found in LEC rats with hereditary hepatitis. Flow cytometric analysis of the DNA content of nuclei from jaundiced LEC rats revealed the presence of very high polyploids, such as 32n and 64n. At the age of 12 weeks, before the onset of hepatitis, 8n polyploid nuclei were more frequent in LEC rats than in LEA rats, a sibling line of LEC rats. Binucleated hepatocytes were also more frequent in LEC rats than in LEA rats at week 4. Bi-, tri- and tetra-nucleated cells whose nuclei were sometimes different in size were observed when jaundice became manifest. The number of proliferating liver cells, determined by pulse labeling with 5-bromo-2'-deoxyuridine (BrdU), was higher in LEC rats than in LEA rats at 2, 4, 8, 12 and 14 weeks, with a maximum at week 4. A remarkable increase of BrdU uptake was observed at week 16, when jaundice developed. The possible involvement of abnormal cytokinesis and kariokinesis in the manifestation of hepatitis was suggested.

A functional and structural heterogeneity is formed among fetal rat hepatocytes during culture

Fetal rat hepatocytes (22-day-old, full-term) in vivo were homogeneous in ultrastructure and glucose 6-phosphatase (G6Pase) distribution throughout the liver acinus. These cells were isolated and cultured for 17 days in Williams medium E containing 10% fetal calf serum, dexamethasone, insulin, and glucagon. Heterogeneity among hepatocytes appeared progressively during culture. There were variations in numbers of binucleated cells, cytoplasmic ultrastructure (e.g., in the distribution of glycogen and the quantity of ultrastructure (e.g., in the distribution of glycogen and the quantity of mitochondria), and intensity of histochemical reactivity for G6Pase. The cells could be classified generally into two types: type I cells had some characteristics of periportal hepatocytes and type II cells those of centrolobular hepatocytes in in vivo adult liver. The results show that a functional and structural heterogeneity can be formed among fetal hepatocytes in monolayer culture without differences in supply of oxygen, nutrients, and hormones. A hidden heterogeneity might already exist among hepatocytes in full-term fetuses even though it cannot be detected ultrastructurally or cytochemically.

Karyometry of nuclei in liver cells. Heterogeneity of nuclear volume and percentage of binucleate cells associated with veins

In untreated adult male albino rats nuclear volume and the percentage of binucleate cells were determined in the first layer of hepatocytes adjacent to hepatic venous branches of varying diameters (less than 40 microns, 40 microns-80 microns, 80 microns-120 microns, 120 microns-160 microns, greater than 160 microns), and in the third and fourth layer of hepatocytes in the remainder of the perivenous parenchyma. In the first layer of hepatocytes adjacent to the vascular structures means of nuclear volume are significantly lower and percentage of binucleate cells significantly higher than in the cells of the remainder of the perivenous parenchyma. Within each area measured distribution curves of nuclear volume classes were homogeneous but showed heterogeneity in comparison with each other. The morphometric data presented in this study strongly support the opinion of the heterogeneity of liver cells in the perivenous zone, as previously postulated on the basis of histochemical investigations.

Multinucleate ameloblasts in the rat incisor

Cytological examination of the rat incisor enamel organ with the light and electron microscope revealed a small number of ameloblasts which contained two and sometimes three or more nuclei per cell. A multinucleate ameloblast usually contained two vertically apposed nuclei situated near the base of the cell. A narrow cytoplasmic band was interposed between adjacent nuclear envelopes. The apical nucleus was often the more elongated of the two nuclei and it fitted a convexity or a concavity within the more basally positioned nucleus. In serial sections examined with the electron microscope no connections were observed between the nuclei. In animals injected with 3H-thymidine instances of multinucleate ameloblasts were found within the advancing front of labeling where only one of the nuclei contained label. Finally, quantitative analysis by nuclear counting established that multinucleate ameloblasts were 60 times more frequent within the maturation zone as in the secretory zone of amelogenesis. As well, the numbers of multinucleate ameloblasts increased progressively in the course of the maturation stage. It was concluded that multinucleate ameloblasts increase with cell age and likely arise by the process of cell fusion.