Number of nucleoli in various cell types of the mouse

The nucleoli of cells of the adult mouse were examined by staining with toluidine blue after removal of deoxyribonucleic acid from tissue sections by deoxyribonuclease treatment. The nuclei of each cell type examined contained one or more nucleoli. This was observed even in lymphocytes and neuroglia, although these cells have occasionally been described as anucleolated. In mature spermatids and spermatozoa, however, it was not possible to detect a nucleolus. The distribution of the number of nucleoli in many diploid cells exhibited a mode of two or three nucleoli per nucleus, and a range from 1 to 6 nucleoli. In presumedly diploid hepatic nuclei, the maximum number of nucleoli was six; but in presumedly tetraploid hepatic nuclei, it was 11. Thus, nearly twice as many nucleoli are present when the chromosome number is doubled. In view of this observation, it is suggested that six nucleolar organizers are present in the diploid chromosomal complement of the mouse. However, through failure of some nucleolar organizers or more probably through fusion of nucleoli, the number of these organelles in most nuclei is less than six.     

红豆草根瘤侵染细胞核在细胞凋亡中的超微结构变化

用透射电镜观察红豆草根瘤侵染细胞核在细胞凋亡过程中的超微结构,以探讨红豆草根瘤侵染细胞核在发育过程中的超微结构变化及其与细胞凋亡的关系.结果表明,红豆草根瘤侵染细胞核的超微结构随细胞发育程度不同而不同.在幼龄侵染细胞中,细胞核体积较大,近似圆形.在即将成熟和成熟的侵染细胞中,细胞核膜有内陷现象,其核仁常具有核仁泡和核仁联合体.在早期凋亡的侵染细胞中,细胞核体积减小,形状变得不规则,核膜出现大量内陷,在其表面形成许多大的突起和深的沟槽,有时还有内质网、线粒体、小液泡和细菌等位于核膜的内陷处,而且核仁也开始裂解.在后期凋亡的侵染细胞中,除细菌解体外,还出现核仁消失,核膜破裂,核质外流,并在细胞质中形成一些电子密度很高,无一定形状的团块状物质.     

Effects of Copper on Root Growth, Cell Division, and Nucleolus of Zea Mays

The effects of different concentrations (10^–5 – 10^–2 M) of copper sulfate on root growth, cell division and nucleoli in root tip cells of Zea mays L. were investigated. 10^–5 M Cu stimulated root growth, but at higher concentrations (10^–4 – 10^–2 M) inhibited it. Cu had toxic effects on chromosomal morphology: c-mitosis, anaphase bridges, and chromosome stickiness were induced. Some nuclei had irregular shape and particles extruded from nucleoli to nuclei and finally from the nuclei into the cytoplasm.     

Analysis of nucleolus organizer regions (NORs) in mitotic and polytene chromosomes ofPhaseolus coccineus by silver staining and Giemsa C-banding

Chromosomes with active nucleolus organizer regions (NORs) were visualized in root tip metaphases ofPhaseolus coccineus using the silver staining technique. A mean number of 5.5 Ag-NORs per cell was observed in 54 cells from eight plants. In the endopolyploid nuclei of the suspensor the silver technique did not demonstrate the reported specificity for nucleolus organizer activity, because there was usually pale staining of nucleoli and preferential staining of heterochromatic regions in the polytene chromosomes including pericentromeric material, telomeres and NORs. The mean number of NORs per nucleolus as detected by this method was 5.8 (28 nucleoli analysed). Using a modified preparation technique, giant chromosomes stained pale, but nucleoli of suspensor cells displayed darkly silver staining internal domains, each of which originating from a nucleolus organizer.—Giemsa C-banding of endopolyploid suspensor nuclei revealed C-positive nucleolus organizers with darkly staining intranucleolar fibrils. The latter were frequently involved in inter-NOR associations. In 34 nucleoli analysed, the mean number of Giemsa C-positive NORs per nucleolus was 6.0.Dedicated to Professor Dr.Lothar Geitler on the occasion of his 80th birthday.     

Relation between the nuclei and the nucleoli during cell differentiation in roots of Vicia faba volumetric and cytochemical analysis

Summary The behaviour of the nuclei and the nucleoli of roots of Vicia faba during cell differentiation was studied quantitatively. The relations between these cell constituents and the polyploidy was analysed. The study was made on isolated nuclei and nucleoli and on plastic sections. A method for the isolation of nuclei and nucleoli of secondary roots fixed in formol was modified and another developed for material fixed in ethanol/acetic acid mixture. The volumetric investigation showed that the nuclear volume increases while the nucleolar decreases during cell differentiation. The mean number of nucleoli decreases. In Vicia faba there is no relation between the ploidy and the volume of nuclei and nucleoli; the protein synthesis rate has an influence on the size of these organelles. Quantitative investigation has shown the proportionality of dry weight, DNA, total protein, histone, protein-bound lysine and arginine content of the nuclei and their ploidy. The same experiments made on nucleoli showed linear relation between their content and volume. The concentration of analysed substances in constant in nucleoli.     

Effects of Cadmium on Root Growth, Cell Division and Nucleoli in Root Tip Cells of Garlic

The effects of different concentrations (10⁻⁷ to 10⁻² M) of cadmium chloride on root growth, cell division and nucleoli in root tip cells of Allium sativum L. were investigated. At lower concentrations of Cd²⁺ (10⁻⁷ to 10⁻⁶ M), Cd²⁺ did not influence the root growth, even had a stimulation effects during a short treatment. The results showed that the rate of root growth per day at the treatment groups (10⁻⁴ to 10⁻² M Cd²⁺) decreased with increasing duration of the treatment and increasing Cd²⁺ concentration. Cd²⁺ induced c-mitosis, anaphase bridges, chromosome stickiness and on nucleoli, causing some particles of similar silver-stained material scattered in the nuclei and making the silver staining reaction at the periphery of the nucleolus weaker. This revised version was published online in July 2006 with corrections to the Cover Date.     

Isolation of the Chromocenter Fraction from Mouse Liver Nuclei

A new method for isolation of the constitutive heterochromatin (chromocenters) from interphase nuclei of mouse liver has been developed. This method allows separation of chromocenters of different size. Chromocenter fractions are essentially free of nucleoli and other contaminants. In contrast to nuclei and nucleoli, the chromocenter fraction is characterized by simpler protein composition, this fraction having a reduced number of proteins (especially high molecular weight proteins). Chromocenters contain all histone fractions; however, the relative proportion of histone H1 is lower and histone H3 is higher than in the total nuclear chromatin. The amount of non-histone proteins of 51, 63, 73, and 180 kD is higher in the chromocenter fraction than in nuclei and nucleoli. The use of immunocytochemistry and immunoblotting methods revealed the presence of the specific kinetochore component, CENP A protein. This suggests tight association of some molecular kinetochore components with chromocenters in the interphase.     

Macro- and Micronuclei of Tetrahymena pyriformis: A Model System for Studying the Structure and Function of Eukaryotic Nuclei*†

The macro- and micronucleus of Tetrahymena pyriformis are formed from a common diploid synkaryon during conjugation. Shortly after the 2nd postzygotic division, distinct morphologic and physiologic differences develop between the 2 nuclei. Micronuclei remain small, presumably diploid, and electronmicroscopic observations indicate that micronuclear DNA is contained in a dense, fibrous, chromosome-like coil. Macronuclei contain considerably more DNA than micronuclei, and the DNA of the macronucleus is found largely in the chromatin bodies typical of ciliate nuclei. The functional differences between macro- and micronuclei in vegetative cells also are striking. The template activity of DNA in the micronucleus is highly restricted compared to that in the macronucleus. Micronuclei synthesize and contain little RNA, and do not contain either nucleoli or ribonucleoprotein granules. Macronuclei, on the other hand, synthesize and contain large amounts of RNA and have many nucleoli and ribonucleoprotein granules. Macro- and micronuclei also have distinct differences in the timing of DNA synthesis during the cell cycle and in the timing and mechanism of nuclear division. Finally, during conjugation the macronucleus becomes pycnotic and disappears while the micronucleus undergoes meiosis and fertilization, ultimately giving rise to new macro- and new micronuclei. In short, the macro- and micronuclei of Tetrahymena provide an excellent system for studying the molecular mechanisms by which the same (or related) genetic information is maintained in different structural and functional states. Methods have been devised to isolate and purify macro- and micronuclei of Tetrahymena in the hope of correlating differences in the nucleoprotein composition of these nuclei with differences in their structure and function. The DNAs of macro- and micronuclei have been found to differ markedly in their content of a methylated base, N⁶-methyl adenine, and major differences in the histones of the 2 nuclei have been observed. Macronuclei contain histones similar to those found in vertebrate nuclei, while 2 major histone fractions seem to be missing in micronuclei. In addition, histone fraction F2A1 which is found in multiple, acetylated forms in macronuclei, is present only as a single, unacetylated form in micronuclei.     

Multiple nucleoli and enhanced nucleolar activity in the nurse cells of the insect ovary

Origin and function of the nucleolar apparatus in nurse cell nuclei of Calliphora erythrocephala have been investigated by cytological and autoradiographic methods in some inbred lines of laboratory blowflies with well paired polytene chromosomes in the nurse cell nuclei. Besides the nucleolus at chromosome VI large numbers of multiple free nucleoli develop in the highly polyploidized nurse cells during oocyte growth. The nucleoli incorporate H³-uridine in a considerable amount producing a homogeneous and RNase-sensitive label even after short time incubation. Their capacity of RNA synthesis is independent of their spatial relationships to other nuclear components. DNA particles in the nucleoli could be identified by the Feulgen reaction and by fluorescence staining with N,N'-diethylpseudoisocya-ninchloride, which also demonstrates the existence of own templates for autonomous RNA synthesis. There are indications that the nucleolus' own DNA is produced by gene amplification beyond the level of endomitotic polyploidization in the nurse cell nuclei. A quantitative estimation of grain density in the autoradiograms shows a rigorous shift of rRNA synthesis: at least 72% of all newly synthesized macromolecular RNA in nurse cell nuclei as contrasted to 13 % of nucleolar RNA synthesis in bristle forming cells with a similar degree of polyploidy. It seems that the nurse cell nuclei of Calliphora in addition to polyploidization increase their template capacity for synthesizing rRNA in a similar way as has repeatedly been demonstrated for Amphibia. Cytological and physiological peculiarities of the nurse cells have been discussed from the viewpoint of their functional similarity to the oocyte nucleus.     

Nucleolar number variation in Hordeum species,their haploids and interspecific hybrids

Nucleolar number variation has been investigated in root tip cells by cytologically determining the number of nucleoli per cell in autoploids and alloploids of Hordeum species, their haploids and interspecific hybrids. The nucleolus organisers in autoploid types of H. vulgare or H. bulbosum did not show any alterations irrespective of the ploidy level. The nucleolar number variation in these species results from a definite pattern of fusion and the maximum number of nucleoli per nucleus corresponds to the number of secondary constrictions. Nucleolus formation in alloploids and interspecific hybrids is impaired on some of the NORs, suggesting differential amphiplasty or nucleolar dominance. A comparison of nucleolar formation in the alloploid species (brachyantherum, arizonicum, procerum and parodii), their haploids, and the interspecific hybrids revealed different degrees of variation from the expected mean and maximum numbers of nucleoli. While the deviations in hybrids between alloploids (H. arizonicum and H. brachyantherum or H. procerum and H. brachyantherum) are marginal, nucleolar dominance is more pronounced in hybrids involving H. vulgare or H. bulbosum as one of the parents and is invariably associated with the disappearance of the secondary constriction(s) from the NOR(s) contributed by one of the parents, and the number of nucleoli is appropriately reduced.     

Morphology of Nurse Cell Nuclei Induced by Meloidodera floridensis: A Graphics Application

The highly irregular distribution of nuclear material in the host nurse cell induced by Meloidodera floridensis has made it difficult to interpret the number of nuclei from two-dimensional micrographs alone. The primary goal of this investigation was to determine the distribution of nuclear material from a three-dimensional solid surface model of the nurse cell nucleus. This model demonstrated the continuity of nuclear material as a single highly irregular nucleus. Custom computer graphics programs were written to accept digitized tracings of nuclear material. From these digitized tracings, a wireframe or polygonalized mesh model was generated. The model was shaded, colored, rotated, and analyzed. This technique provides controlled transparency of the model to display nucleoli within the nucleus. Photographs of the computer screen, color printouts, and video recordings were used to record final results. These refined computer graphic tools have a range of applications in nematode host-parasite relationships, ontogeny, and morphology.     

MOLECULES AT THE EXTERNAL NUCLEAR SURFACE : Sialic Acid of Nuclear Membranes and Electrophoretic Mobility of Isolated Nuclei and Nucleoli

The molecules occurring as terminal residues on the external surfaces of nuclei prepared from rat liver by either sucrose-CaCl₂ or citric acid methods and nucleoli derived from the sucrose-CaCl₂ nuclei were studied chemically and electrokinetically. In 0.0145 M NaCl, 4.5% sorbitol, and 0.6 mM NaHCO₃ with pH 7.2 ± 0.1 at 25°C, the sucrose-CaCl₂ nuclei had an electrophoretic mobility of -1.92 µm/s/V/cm, the citric acid nuclei, -1.63 µm/s/V/cm, and the nucleoli, -2.53 µm/s/V/cm. The citric acid nuclei and the nucleoli contained no measurable sialic acid. The sucrose-CaCl₂ nuclei contained 0.7 nmol of sialic acid/mg nuclear protein; this was essentially located in the nuclear envelope. Treatment of these nuclei with 50 µg neuraminidase/mg protein resulted in release of 0.63 nmol of sialic acid/mg nuclear protein; treatment with 1 % trypsin caused release of 0.39 nmol of the sialic acid/mg nuclear protein. The pH-mobility curves for the particles indicated the sucrose-CaCl₂ nuclei surface had an acid-dissociable group of pK. ~2.7 while the pK for the nucleoli was considerably lower. Nucleoli treated with 50 µg neuraminidase/mg particle protein had a mobility of -2.53 µm/s/V/cm while sucrose-CaCl₂ nuclei similarly treated had a mobility of -1.41 µm/s/V/cm. Hyaluronidase at 50 µg/mg protein had no effect on nucleoli mobility but decreased the sucrose-CaCl₂ nuclei mobility to -1.79 µm/s/V/cm. Trypsin at 1 % elevated the electrophoretic mobility of the sucrose-CaCl₂ nuclei slightly but decreased the mobility of the nucleoli to -2.09 µm/s/V/cm. DNase at 50 µg/mg protein had no effect on the mobility of the isolated sucrose-CaCl₂ nuclei but decreased the electrophoretic mobility of the nucleoli to -1.21 µm/s/V/cm. RNase at 50 µg/mg protein also had no effect on the electrophoretic mobility of the sucrose-CaCl₂ nuclei but decreased the nucleoli mobility to -2.10 µm/s/V/cm. Concanavalin A at 50 µg/mg protein did not alter the nucleoli electrophoretic mobility but decreased the sucrose-CaCl₂ nuclei electrophoretic mobility to -1.64 µm/s/V/cm. The results are interpreted to mean that the sucrose-CaCl₂ nuclear external surface contains terminal sialic acid residues in trypsin-sensitive glycoproteins, contains small amounts of hyaluronic acid, is completely devoid of nucleic acids, and binds concanavalin A. The nucleolus surface is interpreted to contain a complex made up of protein, RNA, and primarily DNA, to be devoid of sialic acid and hyaluronic acid, and not to bind concanavalin A.     

Heterochromatin,repetitive DNS und Nukleolen in Leberzellen von Microtus agrestis

Zusammenfassung Die Zellstruktur von Leberzellen der Erdmaus, Microtus agrestis, wurde nach Giemsafärbung, Feulgenbehandlung, Behandlung mit Ribonuklease und nach Färbung des konstitutiven Heterochromatins untersucht. Das konstitutive Heterochromatin ist in Leberzellen nicht heteropyknotisch, das fakultative Heterochromatin ist im weiblichen Geschlecht als Sexchromatinkörperchen sichtbar. Bestimmungen des relativen DNS-Gehalts ergaben, daß die Zahl der Sexchromatinkörperchen der Ploidie der Zellkerne proportional ist. Die Nukleolen liegen in Hepatozyten oft randständig; in 59% der diploiden Zellkerne sind 2 Nukleolen enthalten. Nach Anfärbung der repetitiven DNS werden oft auch die Nukleolen gefärbt, nach Ribonukleasebehandlung tritt dieser Effekt nicht auf. Das konstitutive Heterochromatin wird in Form von 2 langen fädigen Strukturen sichtbar.

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Heterochromatin, repetitive DNA and nucleoli in liver cells of Microtus agrestis

Summary The nuclear structure of parenchymal liver cells of embryo and adult Microtus agrestis was studied in smear and section preparations after staining with Giemsa solution and treatment according to Feulgen, after treatment with ribonuclease and after specific staining of constitutive heterochromatin. In liver cell nuclei only the facultative heterochromatin is heteropycnotic, a sex chromatin body is observable in female but not in male animals. Constitutive heterochromatin is not heteropycnotic in liver cells. Measurements of the relative DNA content showed that nuclei with one sex chromatin body are diploid; tetraploid nuclei possess two and octoploid nuclei four sex chromatin bodies. Solely in the diploid cell nuclei of the intrahepatic gall ducts two large chromocenters are found. The nucleoli in hepatocytes often lie at the perimeter of the nucleus. 17% of the diploid nuclei contain one nucleolus, 59% two nucleoli, 23% three and 1% four. After staining of repetitive DNA, the nucleoli often become stained as well; after treatment with ribonuclease this effect does not occur. The constitutive heterochromatin becomes visible in form of two long, threadlike structures. After longer periods of dissociation the sex chromatin body ceases to be visible. Sex chromatin and constitutive heterochromatin are contiguous to the nucleoli.

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Mit dankenswerter Unterstützung durch das Bundesministerium für Bildung und Wissenschaft der Bundesrepublik Deutschland.     

Changes in nurse cell nuclei during synchronous infection with Trichinella spiralis

The rate of enlargement of nuclei was determined on 4-microns-thick sections of synchronously infected mouse thigh muscle. Normal muscle nuclei had a geometric mean volume of 84 microns and a range of 42-170 microns 3. At days 5, 6, 7, 8, and 10 and 6 mo after infection, mean nuclear volume was 177 (100-315) microns 3, 254 (140-462) microns 3, 278 (172-447) microns 3, 681 (407-1,138) microns 3, 512 (326-804) microns 3, and 509 (298-870) microns 3, respectively. Size of nuclei for any given day followed a log normal distribution. On days 7 and 8 after infection, 31% of enlarged nuclei had 2 nucleoli, whereas only 15% had 2 nucleoli on day 10. One percent of enlarged nuclei in 6-mo-old nurse cells had double nucleoli. The number of enlarged nuclei in 6-mo-old nurse cells was determined from serial sections of infected tongue muscle. Each nurse cell contained an average of 40 enlarged nuclei. Sixty-four percent of nurse cells examined (n = 55) had between 30 and 60 enlarged nuclei. However, there was great variation in the range (7-142). These results are discussed in relation to the development of the nurse cell.     

Nuclei isolation protocols for flow cytometry allowing nuclear DNA content estimation in problematic microalgal groups

Microalgae are fundamentally important organisms for global ecosystem functioning with high potential in biotechnology and its applications. The knowledge of their nuclear DNA content has become a prerequisite for many areas of microalgal research. Due to common presence of various pigments, secondary metabolites and complex cell walls, the nuclear DNA content estimation using flow cytometry (FCM) is, however, often laborious or even impossible with the currently used protocols. In this study the performance of six nuclei isolation protocols was compared on various problematic microalgae using FCM. The nuclei isolation methods involved osmotic bursting of cells, razor blade chopping of fresh biomass and two newly introduced protocols, razor blade chopping of desiccated biomass and bead beating. These techniques also involved the use of two different nuclei isolation solutions, Otto I + II solutions, and LB01 buffer. Performance of the particular protocols differed greatly, depending on the used nuclei isolation solution and microalgal group. The most successful method was a newly adopted chopping of desiccated biomass in LB01 buffer. This method seems more appropriate for nuclei isolation in filamentous microalgae; on the other hand, bead beating appears to be more suitable for nuclei isolation in solitarily living algae. Using the optimal protocol for a given species, their nuclear DNA content was estimated, resulting in first DNA content estimates for four investigated taxa (Chlamydomonas noctigama, Gonyostomum semen, Microglena sp. and Stigeoclonium sp.). The estimated DNA content spanned from 0.15 to 32.52 pg.

     

Individual Nuclei Differ in Their Sensitivity to the Cytoplasmic Inducers of DNA Synthesis: Implications for the Origin of Cell Cycle Variability

Nuclei of multinucleate cells generally initiate DNA synthesis simultaneously, suggesting that the timing of DNA synthesis depends upon the appearance of a cytoplasmic signal. In contrast, intact nuclei from quiescent mammalian cells initiate DNA synthesisasynchronouslyin cell-free extracts ofXenopuseggs, despite the common environment. Here we show that the two nuclei of permeabilized binucleate cells enter DNA synthesis coordinately in egg extracts, as they doin vivo,with different pairs of nuclei initiating replication at different times. This indicates that the two nuclei of a binucleate cell are identical in their sensitivity to the inducers of DNA synthesis in egg extracts; this sensitivity varies in general between the nuclei of unrelated cells. The asynchrony of DNA synthesis shown by unrelated nuclei in egg extracts is therefore not an artifact of the cell-free system but a reflection of genuine differences preexisting within the intact cell. Evidence that these differences between nuclei are responsible for a substantial fraction of G₁variability in living cells is presented.     

Dinitrogen fixation (Acetylene reduction) and nitrogen accumulation in peas cultivated under the standard growth conditions

InPisum sativum cultivated under standard growth conditions the extent of N₂ fixation with time estimated by the acetylene reduction assay (PN₂F) and rates of the actual nitrogen accumulation of plant biomass (ANA) were calculated from six independent growth experiments. In the plants inoculated with indigenous soilRhizobium populations and cultivated on 0.63 mmol/L nitrate level the percentage PN₂F:ANA ratios ranged from 25.7 to 61.5%. In peas inoculated with the inoculant strain the PN₂F:ANA ratios were markedly higher, ranging from 59.8 to 65.1%. The plants cultivated on N-free nutrient solutions showed both PN₂F:ANA and C₂H₄N₂ ratios to be somewhat higher compared with the 0.63 mmol/L nitrate cultivated plants.     

Discrete Localization of Different DNA Topoisomerases in HeLa and K562 Cell Nuclei and Subnuclear Fractions

Monoclonal antibodies raised against DNA topoisomerase I and against topoisomerase II α and β isoforms, which have been previously demonstrated to be highly specific and capable of detecting cell cycle-related variations of the topoisomerase II isoforms (Negri et al., 1992, Exp. Cell Res. 200, 452-459), have been utilized for a fine subcellular localization. Immunocytochemistry by confocal and electron microscopy have been used for a topological and quantitative evaluation of the fine distribution of the different topoisomerases in HeLa and K562 cells. Topoisomerase I and topoisomerase II α are present both in the nucleoplasm and in the nucleolus, though at different relative ratios, while topoisomerase II β is exclusively present at the nucleolar level. This is further confirmed by immunoblotting and immunocytochemical quantitative evaluations performed on purified nuclear matrix fractions obtained from K562 cells. In fact, the amount of topoisomerase I and topoisomerase II α present in the whole cell nuclei is partly lost in isolated nuclei but, while topoisomerase I is further significantly reduced in nuclear matrix preparations, the topoisomerase II α content is only slightly decreased. On the other hand, the great majority of topoisomerase II β is retained in the nuclear matrix and can be detected exclusively in association with the nucleolar remnant. These results are consistent with specific functional roles hypothesized for the different topiosomerase types.     

Three-dimensional structure of the ciliate Didinium nasutum nucleoli

The nucleolar organization in ciliate Didinium nasutum somatic interphase nuclei was studied using serial ultrathin sections and compared for various physiological states of the cell, namely, fed ciliates, starved ciliates, and dormant cysts. It has been shown that the interphase nucleoli are large structures with a complex architecture: the fibrillar component forms an intricate network in the macronucleus space, while the granular component is located inside this network. The structures looking as individual nucleoli in single sections are actually parts of branched nucleolar networks. The intricate nucleolar networks do not disintegrate after a 30-h starvation; however, the granular component becomes denser and develops numerous cavities filled with fine fibrils of a nonribonucleoprotein nature. In fed D. nasutum, the fibrillar structures on the periphery of nucleoli contain numerous pores (virtually absent in starved cell nucleoli), which can potentially serve for transporting newly synthesized rRNP. Branched nucleolar networks are undetectable in cysts. Their nucleoli are individual structures consisting mainly of the fibrogranular component.     

Large-Conductance Cation Channels in the Envelope of Nuclei from Rat Cerebral Cortex

Eucaryotic nuclei are surrounded by a double-membrane system enclosing a central cisterna which is continuous with the endoplasmic reticulum and serves as a calcium store for intracellular signaling. The envelope regulates protein and nucleic acid traffic between the nucleus and the cytoplasm via nuclear pores. These protein tunnels cross through both nuclear membranes and are permeable for large molecules. Surprisingly, patch clamp recordings from isolated nuclei of different cell species have revealed a high resistance of the envelope, enabling tight seals and the resolution of single ion channel activity. Here we present for the first time single-channel recordings from nuclei prepared from neuronal tissue. Nuclei isolated from rat cerebral cortex displayed spontaneous long-lasting large conductances in the nucleus-attached mode as well as in excised patches. The open times are in the range of seconds and channel activity increases with depolarization. The single-channel conductance in symmetrical K⁺ is 166 pS. The channels are selective for cations with P _K/P _Na= 2. They are neither permeable to, nor gated by Ca²⁺. Thus, neuronal tissue nuclei contain a large conductance ion channel selective for monovalent cations which may contribute to ionic homeostasis in the complex compartments surrounding these organelles. Received: 12 November 1996/Revised: 18 February 1997