Changes in nuclear non-histone protein composition during normal differentiation and carcinogenesis of intestinal epithelial cells

Nuclei from colonic epithelial cells were isolated and fractionated by centrifugation in discontinuous sucrose density gradients. Nuclei differing in buoyant density differ in size, non-histone protein to DNA ratio, and capacity for DNA synthesis in vivo. They do not differ in histone content or in proportions of the major histone classes. The distribution of cell nuclei after density gradient centrifugation corresponds functionally to their histological localization in the colonic mucosa, as judged by the nuclear capacity for DNA synthesis in both normal and tumor tissues. The nuclei of colonic epithelial cells contain a heterogeneous set of non-histone proteins which change in total amount and in relative proportions during normal differentiation. The complement of nuclear proteins differs in normal intestinal epithelial cells and in colon tumors induced by the carcinogen, 1,2-dimethylhydrazine. There is a striking increase in the nuclear content of two major protein classes (of mol. wt ca 44 000 and 62 000) during carcinogenesis. The accumulation of these proteins in the nuclei of carcinogen-treated cells follows early, selective increases in their rates of synthesis.     

Rat luminal cell nuclear area changes correlated with uterine growth responses induced by a low dose infusion or injection of estradiol-17 beta

Rat uterine luminal epithelial cells (LEC) responded differently when exposed to an injection of 1.0 microgram estradiol-17 beta (E2) compared to a continuous infusion of E2 at the rate of 1.0 microgram/24 hours. After injection or beginning infusion, LEC mean nuclear area significantly decreased by 4 h, then increased thereafter. After injection, nuclear area distributions were determined at each time point. The percentage of large nuclei (greater than 40 mu 2) decreased by 4h postinjection and remained a relatively small proportion of the population, while the percentage of nuclei of 20-30 mu 2 areas increased throughout the experiment. During infusion, the percentage of large nuclei decreased by 4h after pump implantation, then increased. Only infusion induced sustained, increased uterine protein content, DNA synthesis and ornithine decarboxylase activity. This study suggests that E2 treatment modality induces differences in nuclear size in target cells as well as in biochemical parameters.     

Cytophotometric determination of unscheduled DNA synthesis

We describe a cytophotometric assay for unscheduled DNA synthesis (UDS) in asynchronously growing cells. Monolayer cultures of human HEp-2 and mouse MCa-11 cells were incubated with the carcinogen methyl-methane sulfonate (MMS), as well as with hydroxyurea and (3H)thymidine. Slides were prepared, and the DNA contents and areas of nuclei were measured by absorption cytophotometry. The labeling of the nuclei, determined on the basis of their DNA content to be in G0/G1, was selectively measured after the preparation of autoradiographs. The labeling of the G0/G1 cells increased with increasing doses of MMS. We also found that the increased nuclear labeling after MMS treatment was not due to induction of replicative DNA synthesis or selective destruction of G0/G1 cells. The results of this assay compared favorably with a standard biochemical method for measuring unscheduled DNA synthesis by benzoylated naphthoylated DEAE cellulose chromatography.     

Nuclear proteins of quiescent Xenopus laevis cells inhibit DNA replication in intact and permeabilized nuclei

Quiescent cells from adult vertebrate liver and contact-inhibited or serum-deprived tissue cultures are active metabolically but do not carry out nuclear DNA replication and cell division. Replication of intact nuclei isolated from either quiescent Xenopus liver or cultured Xenopus A6 cells in quiescence was barely detectable in interphase extracts of Xenopus laevis eggs, although Xenopus sperm chromatin was replicated with approximately 100% efficiency in the same extracts. Permeabilization of nuclei from quiescent Xenopus liver or cultured Xenopus epithelial A6 cells did not facilitate efficient replication in egg extracts. Moreover, replication of Xenopus sperm chromatin in egg extracts was strongly inhibited by a soluble extract of isolated Xenopus liver nuclei; in contrast, complementary-strand synthesis on single-stranded DNA templates in egg extracts was not affected. Inhibition was specific to endogenous molecules localized preferentially in quiescent as opposed to proliferating cell nuclei, and was not due to suppression of cdk2 kinase activity. Extracts of Xenopus liver nuclei also inhibited growth of sperm nuclei formed in egg extracts. However, the rate and extent of decondensation of sperm chromatin in egg extracts were not affected. The formation of prereplication centers detected by anti-RP-A antibody was not affected by extracts of liver nuclei, but formation of active replication foci was blocked by the same extracts. Inhibition of DNA replication was alleviated when liver nuclear extracts were added to metaphase egg extracts before or immediately after Ca++ ion-induced transition to interphase. A plausible interpretation of our data is that endogenous inhibitors of DNA replication play an important role in establishing and maintaining a quiescent state in Xenopus cells, both in vivo and in cultured cells, perhaps by negatively regulating positive modulators of the replication machinery.     

Correlation between nuclear morphology and rate of deoxyribonucleic acid synthesis in a normal cell line.

Chinese hamster fibroblasts were investigated for the existence of correlations between proliferative activity and nuclear morphology. As a proliferative parameter, the rate of DNA synthesis of individual cells was determined by quantitative 14C-autoradiography. In a second step the images of the Feulgen-stained nuclei were digitized for extraction of features of morphology and texture. These features were correlated with the corresponding DNA synthesis rate values. The following relationships were found: Round nuclei have higher rates of DNA synthesis than flat ones. The more chromatin is packed at the nuclear rim, possibly representing heterochromatin, the lower the rate of DNA synthesis. The DNA synthesis rate also correlates with the graininess of chromatin. Larger areas of condensed chromatin are associated with lower rate values. A fine and irregular network of chromatin, as is typical of immature cell types, is associated with a high rate of DNA synthesis. Although these results are presently confined to the cell line investigated, parallels seem to exist to other cell types, such as erythropoietic cells, which await further investigation.     

The sperm nuclear matrix is required for paternal DNA replication

The mammalian sperm nucleus provides an excellent model for studying the relationship between the formation of nuclear structure and the initiation of DNA replication. We previously demonstrated that mammalian sperm nuclei contain a nuclear matrix that organizes the DNA into loop domains in a manner similar to that of somatic cells. In this study, we tested the minimal components of the sperm nucleus that are necessary for the formation of the male pronucleus and for the initiation of DNA synthesis. We extracted mouse sperm nuclei with high salt and dithiothreitol to remove the protamines in order to form nuclear halos. These were then treated with either restriction endonucleases to release the DNA not directly associated with the nuclear matrix or with DNAse I to digest all the DNA. The treated sperm nuclei were injected into oocytes, and the paternal pronuclear formation and DNA synthesis was monitored. We found that restriction digested sperm nuclear halos were capable of forming paternal pronuclei and initiating DNA synthesis. However, when isolated mouse sperm DNA or sperm DNA reconstituted with the nuclear matrices were injected into oocytes, no paternal pronuclear formation or DNA synthesis was observed. These data suggest that the in situ nuclear matrix attachment organization of sperm DNA is required for mouse paternal pronuclear DNA synthesis.     

Induction of host nuclear DNA synthesis in coccidia-infected chicken intestinal cells

When Eimeria maxima (gamonts) infects villus epithelial cells of the chicken duodenum there is extensive cellular enlargement with no alteration in nuclear size. Feulgen DNA microspectrophotometric measurements indicated that the infected host-cell nucleus contains the same amount of DNA as an uninfected cell nucleus. Evidence is presented to indicate that second generation schizonts of E. necatrix develop in crypt epithelial cells that are displaced/migrate into the lamina propria. The developing parasite causes cellular and nuclear hypertrophy in these cells as does E. tenella in cecal cells of the chicken. In these two cases nuclear enlargement is accompanied by induced rounds of DNA synthesis in the host-cell. Analyses indicated that the DNA content of enlarged nuclei does not fall into classes that correspond to a geometric series 2:4:6:8:16: etc. times the DNA content of a 2C equivalent, and that nuclear size and DNA content in infected cells are not significantly correlated. Autoradiographic studies on E. necatrix infected chicks administered ³H-thymidine show that DNA synthesis takes place in the nuclei of cells containing all developing stages but not mature schizonts, and that this synthesis is not a continuous process. The data suggest that intestinal cells that are capable of undergoing cell division and therefore additional rounds of DNA synthesis, can be induced by coccidial infection in the absence of concomitant cell division.     

Initiation of DNA synthesis and uptake of T antigen by chick erythrocyte nuclei in hterokaryons with SV40-transformed human cells.

Nonsynchronized and hydroxyurea (HU)-synchronized SV40-transformed human cells (W98VaD) were fused with chick embryo erythrocytes (CE). The uptake of T antigen by CE nuclei was compared with initiation of chick nuclear DNA synthesis. Uptake of T antigen by CE nuclei occurred at about the same time after fusion with asynchronous as with HU-synchronized cells. CE nuclei rapidly became T antigen-positive between 16 h and 28 h after fusion and usually almost all CE nuclei were T antigen-positive by 48 h after fusion. In contrast, initiation of chick nuclear DNA synthesis occurred as a function of time after reversal of the HU block, when the host cell nuclei were also synthesizing DNA. Chick nuclear DNA synthesis occurred in many heterokaryons before the CE nuclei became T antigen-positive by immunofluorescence.     

Regulatory role of endogenous regucalcin in the enhancement of nuclear deoxyribonuleic acid synthesis with proliferation of cloned rat hepatoma cells (H4-II-E)

The role of endogenous regucalcin in the regulation of deoxyribonuleic acid (DNA) synthesis in the nuclei of the cloned rat hepatoma cells (H4-II-E) with proliferative cells was investigated. Cells were cultured for 6-96 h in a alpha-minimum essential medium (alpha-MEM) containing fetal bovine serum (FBS; 1 or 10%). Cell number was significantly increased between 24 and 96 h after culture with 10% FBS; cell proliferation was markedly stimulated by culture with 10% FBS as compared with that of 1% FBS. In vitro DNA synthesis activity in the nuclei of cells was significantly elevated 6 h after culture with 10% FBS and its elevation was remarkable at 12 and 24 h after the culture. Nuclear DNA synthesis activity was significantly reduced in the presence of various protein kinase inhibitors (PD98059, staurosprine, or trifluoperazine) in the reaction mixture containing the nuclei of cells cultured for 12 and 24 h with FBS (1 and 10%). The addition of regucalcin (10(-7) and 10(-6)M) in the reaction mixture caused a significant inhibition of nuclear DNA synthesis activity. The presence of anti-regucalcin monoclonal antibody (25-100 ng/ml) in the reaction mixture containing the nuclei of cells cultured for 24 h with 10% FBS resulted in a significant increase in nuclear DNA synthesis activity. This increase was completely blocked by the addition of regucalcin (10(-6) M). The effect of anti-regucalcin antibody (100 ng/ml) in increasing nuclear DNA synthesis activity was significantly inhibited in the presence of various protein kinase inhibitors. DNA synthesis activity was significantly enhanced in the presence of anti-regucalcin antibody (100 ng/ml) in the reaction mixture containing the nuclei of cells cultured for 24 h with 10% FBS in the presence of Bay K 8644 (2.5 x 10(-6) M). Culture with Bay K 8644 did not cause a significant increase in DNA synthesis activity in the absence of anti-regucalcin antibody. The present study demonstrates that endogenous regucalcin plays a suppressive role in the enhancement of nuclear DNA synthesis with proliferative cells.     

DNA profiles in dysplasia and carcinoma of the human esophagus

The nuclear DNA content was microspectrophotometrically measured in 16 resected esophagi having dysplasia, carcinoma in situ and/or early invasive squamous carcinoma. First, the epithelial thickness in (1) normal squamous esophageal epithelium and (2) dysplasia-carcinoma in situ areas was divided into three equal compartments (i.e., basal-parabasal, intermediate and superficial) in five cases. In the normal epithelium, while some of the nuclei in basal-parabasal normal squamous cells had elevated DNA values (corresponding to the natural DNA replication in these cells), intermediate and superficial (nonreplicating) normal squamous cells showed a more definite clustering about the 2c value. In the nonnormal epithelium, the percentage of cells with DNA levels exceeding the normal tetraploid value was highest for the intermediate zone. Therefore, in all 16 cases, normal intermediate cells were measured as internal controls, against which the DNA levels of cells in the intermediate compartment in the areas of dysplasia and/or carcinoma in situ were compared. In areas of dysplasia, two different DNA patterns were observed: one clustering around the normal diploid region and the other with aneuploid values. While the former corresponded to some of the lesions considered by conventional histologic examination to be slight and moderate dysplasias, the aneuploid pattern corresponded to the remaining slight and moderate dysplasias as well as to the severe dysplasias. The possibility that "diploid dysplasias" are reactive (i.e., nonneoplastic) lesions due to chronic inflammation or are "dormant" nonprogressive dysplasias, while aneuploid dysplasias are more aggressive lesions, seems to be substantiated by the fact that all areas with carcinoma in situ or with microinvasive squamous carcinoma had aneuploid nuclei.     

Effect of poly(ADP-ribose) formation on DNA synthesis and DNA fragmentation in nuclei of rat liver and rat ascites hepatoma AH-130 cells

To elucidate the role of poly(ADP-Rib) in the nucleus, DNA synthesis and DNA fragmentation were studied in isolated nuclei of rat liver and rat ascites hepatoma AH-130 cells. Liver and hepatoma cell nuclei formed the same amount of poly(ADP-Rib) per mg of nuclear DNA from NAD. Preincubation of liver nuclei with NAD repressed DNA polymerase activity to 30% of that of the control, but preincubation of hepatoma cell nuclei with NAD did not affect DNA polymerase activity. It was also found that incubation of liver nuclei with NAD prevented the fragmentation of nuclear DNA which occurred without NAD. Incubation of hepatoma cell nuclei with or without NAD did not result in fragmentation of DNA. The role of endonuclease in primer formation for DNA synthesis is discussed.     

Cellular changes in cultured lenses.

Observations were made on the frog lens epithelium after culture of the entire lens or of capsular explants. General deviations from normal lens structure as well as specific changes in two media were studied. DNA synthesis and mitosis were induced in the central epithelial cells. Disruption of the orderly, single, epithelial layer that is characteristic of normal lenses was accompanied by the appearance of multilayered plaques of epithelial cells and invasion of vacuolated regions of the lens fibers by epithelial cells. Cells that are fibroblast-like in appearance were observed in regions of the capsule depleted of cells and at the free edges of epithelial sheets in cell culture. Epithelial cells were surrounded by capsule-like material even situated in the lens interior. Nuclie derived from central epithelial cells of lenses cultured in L-15 medium and medium 199 had served as donors in previous nuclear transfer experiments in this laboratory. In our current observation of L-15-cultured lenses, cells were sparsely distributed on the capsule and nuclei were abnormally shaped; in 199-cultured lenses, cells were more densely distributed and nuclei resembled those of normal lenses. Medium 199 without serum could better maintain normal lens structure than L-15 medium without serum. In addition, the percentage of epithelial explants demonstrating cellular outgrowth was greater in medium 199. The differences in cellular behavior were shown not to be the result of different sugars, pH, or the presence of CO2. The nuclear transfer results may reflect the structural changes in the epithelium after lens culture in the two media.     

Involvement of deoxyribonucleic acid polymerase beta in nuclear deoxyribonucleic acid synthesis.

The effects on DNA synthesis in vitro in mouse L929-cell nuclei of differential extraction of DNA polymerases alpha and beta were studied. Removal of all measurable DNA polymerase alpha and 20% of DNA polymerase beta leads to a 40% fall in the replicative DNA synthesis. Removal of 70% of DNA polymerase beta inhibits replicative synthesis by 80%. In all cases the nuclear DNA synthesis is sensitive to N-ethylmaleimide and aCTP (arabinosylcytosine triphosphate), though less so than DNA polymerase alpha. Addition of deoxyribonuclease I to the nuclear incubation leads to synthesis of high-molecular-weight DNA in a repair reaction. This occurs equally in nuclei from non-growing or S-phase cells. The former nuclei lack DNA polymerase alpha and the reaction reflects the sensitivity of DNA polymerase beta to inhibiton by N-ethylmaleimide and aCTP.     

Cellular changes in cultured lenses

Summary Observations were made on the frog lens epithelium after culture of the entire lens or of capsular explants. General deviations from normal lens structure as well as specific changes in two media were studied. DNA synthesis and mitosis were induced in the central epithelial cells. Disruption of the orderly, single, epithelial layer that is characteristic of normal lenses was accompanied by the appearance of multilayered plaques of epithelial cells and invasion of vacuolated regions of the lens fibers by epithelial cells. Cells that are fibroblast-like in appearance were observed in regions of the capsule depleted of cells and at the free edges of epithelial sheets in cell culture. Epithelial cells were surrounded by capsule-like material even when situated in the lens interior. Nuclei derived from central epithelial cells of lenses cultured in L-15 medium and medium 199 had served as donors in previous nuclear transfer experiments in this laboratory. In our current observation of L-15-cultured lenses, cells were sparsely distributed on the capsule and nuclei were abnormally shaped; in 199-cultured lenses, cells were more densely distributed and nuclei resembled those of normal lenses. Medium 199 without serum could better maintain normal lens structure than L-15 medium without serum. In addition, the percentage of epithelial explants demonstrating cellular outgrowth was greater in medium 199. The differences in cellular behavior were shown not to be the result of different sugars, pH, or the presence of CO₂. The nuclear transfer results may reflect the structural changes in the epithelium after lens culture in the two media. This work was supported by grants 2RO1 EY 00555-06 and 5SO1 RR 05510-10 from the National Institutes of Health.     

Extra DNA synthesis in the dedifferentiating cells ofVicia faba roots

Summary Cell dedifferentiation was induced inVicia faba root tissues by removing the whole root meristem (decapitation) and the behaviour of the nuclear DNA in the dedifferentiating cells was studied by means of cytophotometric and autoradiographic analyses. Cytophotometric determination after Feulgen-staining showed that: 1. the vast majority of nuclei in differentiated cells were in the DNA postsynthetic phase, but their Feulgen absorption was lower than that of DNA postsynthetic nuclei (G₂, 4 C) in the meristem; 2. such a Feulgen absorption was detected in certain nuclei after root decapitation; 3. all the mitoses in the dedifferentiating tissues were diploid, fully matching the Feulgen absorption of mitoses in the meristem.After³H-thymidine (³H-T) feeding of the decapitated roots and autoradiography, the following results were obtained: 1. two populations of labeled nuclei, characterized by two different levels of scattered labeling occurred in dedifferentiating tissues, slightly labeled nuclei being much more numerous than heavily labeled nuclei; 2. the percentage of labeled nuclei was much greater than that of DNA presynthetic nuclei in the root tissues; 3. almost all the mitoses were labeled after a 16-hour³H-T feeding; 4. the percentage of slightly labeled nuclei paralleled that of dedifferentiating cells; 5. the duration of the DNA synthesis phase and that of the gap between completion of DNA synthesis and mitosis differed in heavily and slightly labeled nuclei; 6. all nuclei which entered DNA synthesis also entered mitosis.These results are interpreted to mean that: 1. after decapitation, two different DNA syntheses occur in the dedifferentiating root tissues ofV. faba: DNA reduplication in cells which dedifferentiate starting from a DNA presynthetic nuclear condition (heavily labeled nuclei) and extra DNA synthesis in cells which dedifferentiate starting from a DNA postsynthetic nuclear condition (slightly labeled nuclei); 2. extra DNA synthesis is required in these dedifferentiating cells for entry into mitosis.     

Site-specific initiation of DNA replication in Xenopus egg extract requires nuclear structure.

Previous studies have shown that Xenopus egg extract can initiate DNA replication in purified DNA molecules once the DNA is organized into a pseudonucleus. DNA replication under these conditions is independent of DNA sequence and begins at many sites distributed randomly throughout the molecules. In contrast, DNA replication in the chromosomes of cultured animal cells initiates at specific, heritable sites. Here we show that Xenopus egg extract can initiate DNA replication at specific sites in mammalian chromosomes, but only when the DNA is presented in the form of an intact nucleus. Initiation of DNA synthesis in nuclei isolated from G1-phase Chinese hamster ovary cells was distinguished from continuation of DNA synthesis at preformed replication forks in S-phase nuclei by a delay that preceded DNA synthesis, a dependence on soluble Xenopus egg factors, sensitivity to a protein kinase inhibitor, and complete labeling of nascent DNA chains. Initiation sites for DNA replication were mapped downstream of the amplified dihydrofolate reductase gene region by hybridizing newly replicated DNA to unique probes and by hybridizing Okazaki fragments to the two individual strands of unique probes. When G1-phase nuclei were prepared by methods that preserved the integrity of the nuclear membrane, Xenopus egg extract initiated replication specifically at or near the origin of bidirectional replication utilized by hamster cells (dihydrofolate reductase ori-beta). However, when nuclei were prepared by methods that altered nuclear morphology and damaged the nuclear membrane, preference for initiation at ori-beta was significantly reduced or eliminated. Furthermore, site-specific initiation was not observed with bare DNA substrates, and Xenopus eggs or egg extracts replicated prokaryotic DNA or hamster DNA that did not contain a replication origin as efficiently as hamster DNA containing ori-beta. We conclude that initiation sites for DNA replication in mammalian cells are established prior to S phase by some component of nuclear structure and that these sites can be activated by soluble factors in Xenopus eggs.     

Synthesis of herpes simplex virus, vaccinia virus, and adenovirus DNA in isolated HeLa cell nuclei. I. Effect of viral-specific antisera and phosphonoacetic acid.

Purified nuclei, isolated from appropriately infected HeLa cells, are shown to synthesize large amounts of either herpes simplex virus (HSV) or vaccinia virus DNA in vitro. The rate of synthesis of DNA by nuclei from infected cells is up to 30 times higher than the synthesis of host DNA in vitro by nuclei isolated from uninfected HeLa cells. Thus HSV nuclei obtained from HSV-infected cells make DNA in vitro at a rate comparable to that seen in the intact, infected cell. Molecular hybridization studies showed that 80% of the DNA sequences synthesized in vitro by nuclei from herpesvirus-infected cells are herpesvirus specific. Vaccinia virus nuclei from vaccinia virus-infected cells, also produce comparable percentages of vaccinia virus-specific DNA sequences. Adenovirus nuclei from adenovirus 2-infected HeLa cells, which also synthesize viral DNA in vitro, have been included in this study. Synthesis of DNA by HSV or vaccinia virus nuclei is markedly inhibited by the corresponding viral-specific antisera. These antisera inhibit in a similar fashion the purified herpesvirus-induced or vaccinia virus-induced DNA polymerase isolated from infected cells. Phosphonoacetic acid, reported to be a specific inhibitor of herpesvirus formation and the herpesvirus-induced DNA polymerase, is equally effective as an inhibitor of HSV DNA synthesis in isolated nuclei in vitro. However, we also find phosphonoacetic acid to be an effective inhibitor of vaccinia virus nuclear DNA synthesis and the purified vaccinia virus-induced DNA polymerase. In addition, this compound shows significant inhibition of DNA synthesis in isolated nuclei obtained from adenovirus-infected or uninfected cells and is a potent inhibitor of HeLa cell DNA polymerase alpha.     

Heterogeneity in nuclear transport does not affect the timing of DNA synthesis in quiescent mammalian nuclei induced to replicate in Xenopus egg extracts

Intact G0 nuclei from quiescent mammalian cells initiate DNA synthesis asynchronously in Xenopus egg extracts, despite exposure to the same concentration of replication factors. This indicates that individual nuclei differ in their ability to respond to the inducers of DNA replication. Since the induction of DNA synthesis requires the accumulation of replication factors by active nuclear transport, any variation in the rate of transport among nuclei could contribute to the variability of DNA replication. Using the naturally fluorescent protein allophycocyanin (APC) coupled with the nuclear localization sequence (NLS) of SV40 T antigen, as a marker of nuclear uptake, we show here that individual G0 nuclei differ in their rate of transport over a range of more than 20-fold. Surprisingly, this variation has no direct influence on the timing or extent of DNA synthesis. Similar results were obtained by monitoring the uptake of nucleoplasmin, a nuclear protein present at high levels in egg extracts. These experiments show that the initiation of DNA synthesis is not driven merely by the accumulation of replication factors to some threshold concentration. Instead, some other explanation is needed to account for the timing of initiation.     

Localization of HPV-16 DNA sequence in CaSki cells by electron microscopic hybridocytochemistry.

We investigated the HPV-16 DNA sequence in the CaSki cervical carcinoma cell line by electron microscopic hybridocytochemistry using biotinylated HPV-16/18 probes. At the light microscopic level, reaction product of hybridized HPV-16 DNA sequence was not seen in the cytoplasm but appeared as spots or rods randomly distributed in the nuclei. By electron microscopy, reaction product was seen aggregated in several regions in the nuclei. Most of the stained areas did not reveal particular architecture but showed part of the chromatin structure. In other nuclei, reaction product was observed to be associated with strings of loop-like structure, and some stained loops were seen to be connected directly to the nuclear filamentous chromatin structure. The skeletonized images of hybridized HPV-16 DNA in the nuclei were illustrated by computerized image analysis. In conclusion, we have demonstrated the HPV-16 DNA sequence in the nuclei of CaSki cells by electron microscopy. The identification of stained areas localized only in the chromatin suggests an integrated form of HPV-16 DNA sequence in the cells. This method could be used to identify an integrated or episomal form of viral DNA in the virus-containing cells.