Cytochimie ultrastructurale du nucléole

Nucleolus and chromatin

The nucleolus as site of ribosome biogenesis holds a pivotal role in cell metabolism. It is composed of ribosomal DNA (rDNA), which is present as tandem arrays located in nucleolus organizer regions (NORs). In interphase cells, rDNA can be found inside and adjacent to nucleoli and the location is indicative for transcriptional activity of ribosomal genes-inactive rDNA (outside) versus active one (inside). Moreover, the nucleolus itself acts as a spatial organizer of non-nucleolar chromatin. Microscopy-based approaches offer the possibility to explore the spatially distinct localization of the different DNA populations in relation to the nucleolar structure. Recent technical developments in microscopy and preparatory methods may further our understanding of the functional architecture of nucleoli. This review will attempt to summarize the current understanding of mammalian nucleolar chromatin organization as seen from a microscopist's perspective.

ELECTRON MICROSCOPIC EXAMINATION OF THE SITES OF NUCLEAR RNA SYNTHESIS DURING AMPHIBIAN EMBRYOGENESIS

The site of H³-uridine incorporation and the fate of labeled RNA during early embryo-genesis of the newt Triturus pyrrhogaster were studied with electron microscopic autoradiography. Isolated ectodermal and mesodermal tissues from the embryos were treated in H³-uridine for 3 hours and cultured in cold solution for various periods before fixation with OsO₄ and embedding in Epon. At the blastula stage, the only structural component of the nucleus seen in electron micrographs is a mass of chromatin fibrils. At the early gastrula stage, the primary nucleoli originate as small dense fibrous bodies within the chromatin material. These dense fibrous nucleoli enlarge during successive developmental stages by the acquisition of granular components 150 A in diameter, which form a layer around them. Simultaneously larger granules (300 to 500 A) appear in the chromatin, and they fill the interchromatin spaces by the tail bud stage. Autoradiographic examination has demonstrated that nuclear RNA synthesis takes place in both the nucleolus and the chromatin, with the former consistently showing more label per unit area than the latter. When changes in the distribution pattern of radioactivity were studied 3 to 24 hours after immersion in isotope at each developmental stage, the following results were obtained. Labeled RNA is first localized in the fibrous region of the nucleolus and in the peripheral region of chromatin material. After longer culture in non-radioactive medium, labeled materials also appear in the granular region of the nucleolus and in the interchromatin areas. Further incubation gives labeling in cytoplasm.

Methods in ultrastructural cytochemistry of the cell nucleus

The electron microscopical study of the cell nucleus as observed in thin sections requires the use of cytochemical methods because of the intricate pattern of the nuclear components. The in situ techniques based on electron staining and enzymatic digestion are reviewed, excluding autoradiography, cytoenzymology and immunocytochemistry. A tentative classification has been adopted according to the chemical nature of the revealed component. Thus, the staining procedures for the nucleoproteins in general, for both nucleic acids, for the proteins, and finally for the deoxyribonucleoproteins and DNA are considered separately. 1--Stains for the nucleoproteins include simple reagents such as the uranyl and lead salts which are largely used in electron microscopy but are of limited specificity. 2--A variety of methods, some of them specific, is available for the simultaneous visualization of DNA and RNA which is based on common properties: basophilia, ability to bind diaminoacridines, presence of hydroxyl groups. However, due to the recent development of specific and preferential methods for each nucleic acid, we feel that among the older methods, only rapid and simple procedures for the detection of both nucleic acids remain of interest. 3--Proteins being ubiquitous, the useful techniques must reveal subsets within the total nuclear proteins. Apart from some endogeneous enzymes, basic proteins -- practically histones -- so far represent the only group for the detection of which reliable methods exist. 4--Several techniques developed recently are available for the specific detection of DNA. In favourable cases, methods derived from the Feulgen reaction allow its visualization at a molecular level. In addition, standard procedures for the preparation of mammalian cells and tissues are described. Each staining method is at least briefly discussed, but emphasis has been placed on a small number of techniques described in detail. They comprise the EDTA regressive stain for the ribonucleoproteins, several reactions of the basic proteins and the Feulgen-like osmium ammine reaction for DNA.

Nucleolar reorganization in liver cells of the aging rat

The nucleoli of rat liver cells duplicate in great detail the life-long series of reorganizational changes encountered in kidney and intestinal epithelial cells. The ultrastructural components of the large, loosely organized polymorphous nucleoli, which are dominant in the rapidly multiplying stem cells of embryos, are readily accessible for chemical activities. Smaller, more compact amphinucleoli are dominant in more mature cells, which were characterized by Smetana ('70) as "idling" cells, showing slowly continuing ribosome formation and RNP synthesis. In older cells bipartite nucleoli become dominant and are reorganized in increasing numbers from the younger amphinucleoli. These, however, are not replaced in equal numbers from the shrinking pool of polymorphs of young cells which have greatly reduced mitotic potential. Paralleling the shifts in dominant nucleolar types, the high level of protein synthesis declines in older cells not only in the quantity of proteins synthesized but also in kinds of enzymes produced. These fail to meet the structural and functional requirements of aging cells leading ultimately to the onset of age-related degenerative changes. Again it is noted that separation of the karyosomal DNA from the plasmosomal RNA-protein complex of the nucleolus may lead to possible breakdown of the DNA-dependent RNA-protein transcription system ultimately bringing protein synthesis to a very low level in the senescent animal.

Enzyme histochemical investigation of glycol methacrylate embedded chick embryonic tissue

The advantages of the water-soluble glycol methacrylate (GMA) embedding procedure make it highly applicable for use with fragile early embryonic material. Not only can one obtain tissue sections containing excellent histological detail, but numerous enzymes are retained for subsequent histochemical localization. For the purpose of establishing a methodology whereby concomitant histology and histochemistry could be obtainable, various fixatives and fixation times have been evaluated on GMA embedded chick embryonic mesonephros and gonad. It was found that fixing the tissues for 1 h in a solution of 95% ethanol, 5% acetic acid and 10% neutral-buffered formalin resulted in the retention of not only excellent histology but also alkaline and acid phosphatase. Thus, with this procedure, more specific investigations of early embryonic tissue can be performed.

Quantitative ultrastructural characterisation of rabbit erythroblasts and of 3H uridine incorporation into their nuclear components

Electron microscopic-autoradiographic and morphometric investigations were carried out in erythroblasts from rabbit bone marrow. 1. to obtain quantitative data for the erythroblast classes. 2. to determine quantitative-morphometrically the incorporation rate of 3H uridine into the cell components of the different erythroblast classes. 3. to evaluate the influence of UV irradiation on the incorporation of 3H uridine. According to ultrastructural criteria erythroblasts are divided into 3 classes: basophilic erythroblasts (BE), polychromatic erythroblasts (PE) and orthochromatic erythroblasts (OE). The areas of the 3 classes of erythroblasts are in a ratio of 1:0.5:0.43. In all classes the portion of the nucleus is between 56 and 58.8%. The portion of mitochondria decreases from 13.3% (BE) to 9.3% (PE), and to 6.3% (OE) respectively. Simultaneously the portion of heterochromatin in the nuclear area rises from 24.2% (BE) to 49.8% (PE), and to 63.2% (OE) whereas the percentage of euchromatin decreased correspondingly. Nucleoli were found only in 78% of the BE and in 3.4% of the PE. For the quantitative determination of the silver grains and silver filaments, respectively, the morphometric procedures have been adapted to the particular conditions. So it was necessary to estimate the areas covered by silver grains of filaments. The silver grains or filaments localised in the nuclear area are in the range from 84.6 to 93.4%. The percentage of silver grains of the total cellular area is 14.4% in BE, 6.8% in PE and 4% in OE. In a similar way the relative values of the nucleolus (total) and the euchromatin and heterochromatin are reduced. UV irradiation of bone marrow in vitro for 10 to 20 min does not cause any significant changes in the incorporation of 3H uridine. This result is in contrast to other published experiments on the influence of UV on the RNA synthesis. It is assumed that the irradiation dose was too low to induce a measurable effect on the RNA synthesis or the point of investigation was too early.

Nucleolar reorganization in cells of the kidney of the rat and its relation to aging

A process of nucleolar reorganization apparently identical to that encountered in intestinal epithelial cells (Adamstone and Taylor, '72) develops in kidney cells of aging rats. The polymorphic nucleoli of young tubule cells soon change to amphinucloeli and, while terminal nucleolar reorganization is delayed in cells of collecting tubules, in the nephrons nucleoli soon begin to undergo terminal reorganization becoming bipartite structures with separate plasmosomes and karyosomes. This suggests disruption of the DNA-dependent RNA protein transcription system and failure to maintain the flow of messenger RNA into the cytoplasm. Old cells are not discarded immediately from the kidney tubules and they retain much rough endoplasmic reticulum, numerous ribosomes and polysomes and large plasmosomes. Thus a high RNA concentration is known to develop in old kidney tissue while protein synthesis is also known to be low (Kanungo et al., '70; Buetow and Ghandi, '73). Nucleolar counts show gradual increase in bipartite nucleoli at the expense of amphinucleoli and in the senescent kidney bipartite nucleoli predominate. It is suggested that nucleolar reorganization, with final separation of plasmosomes and karyosomes, includes the process of nucleolar segregation and is triggered by some innate nucleolar mechanism in response to encoded genetic information stored in the nucleolus during nucleogenesis. At this time both DNA and RNA are incorporated into the developing nucleolus. It is also to be noted that two shifts in nucleolar dominance occur with advancing age. These may be fundamental to the process of aging and to the onset of senescence. Furthermore, the changes in dominant nucleolar types are the direct result of the process of nucleolar reorganization.

The nucleolus at prophase of meiosis in three plants: an ultrastructural study

The nucleolus and nucleolar organizing region have been studied with the electron microscope in respect of (a) the fine structure of the nucleolar organizer during zygotene in pollen mother cells ofPhaedranassa viridiflora, as compared with that seen in interphase nuclei of ovary wall and tapetum of this plant; (b) in relation to premature dissolution of the nucleolus at zygotene in pollen mother cells of an F1 hybrid ofLilium aureliensis×Lilium henryiand the hybrid lily ‘Formobel’; (c) in respect of pseudonucleoli found in association with the nucleolus in pollen mother cells of all three plants. In the pollen mother cells of thePhaedranassa, as distinct from a simple arrangement of intranucleolar chromatin observed in the nucleoli in interphase nuclei of tapetum and ovary wall, the organizer is shown to consist of rounded electron-opaque filamentous aggregations about 40-50 nm in diameter, joined by a fine filament seemingly as in a string of beads, and, toward the periphery of the nucleolus, arranged equidistant from each other in columns originating from the organizing chromosomes. Within the core of the nucleolus the aggregations, though still joined, were widely dispersed and contained within lacunae. Axial cores were closely aligned with the organizer, but the completion of the synaptonemal complex in the organizing region was apparently delayed. The premature dissolution of the nucleoli in the pollen mother cells of the two lilies led to a displacement of their characteristic zones: the peripheral particulate zones were dispersed and the weakly staining organizing region became external to therelicnucleolar cores. The relegated organizing regions, fringed lightly at the sides by chromatin, invariably contained 1-2 rounded areas comprised of fine filaments with an electron opacity equivalent to that of the coarser fibrils in the chromatin of the organizing chromosomes to which they were seen to be joined. These rounded skeins are tentatively assumed to represent the withdrawn organizers. No synaptonemal complexes or relics of its components were observed in the organizing regions. Round bodies (pseudonucleoli) with a maximum observed incidence of 2 per nucleus inPhaedranassaand 1 per nucleus in the lilies were frequently seen. They were either totally enclosed within the nucleolus, as was usually the rule in the former, or partly engulfed by it as in the latter. They were composed of filaments about 25 nm wide and their diameter ranged between 0.6-2.5 μm. Their close association with the nucleolus and confinement to meiosis, suggests that they are formed on the nucleolar chromosomes at loci close to the nucleolar organizer, which may be metabolically active only at prophase of meiosis. Various aspects concerning the nucleolar organizing region and organizer are discussed in the light of the present observations.

The use of glycol methacrylate embedding for studies of chromatin distribution and fine structure in salmon spermatid nuclei

Glycol methacrylate has been used as an embedding medium for studies of spermiogenesis in the salmon. DNA and basic proteins are shown to stain with the same specificity in thick (0.5-1.0 mu) sections of GMA-embedded salmon testes as in sections of comparably fixed, paraffin-embedded testes. Stain can be localized far more precisely in GMA sections than in paraffin sections due to the thinness of the sections and to the excellent structural preservation of nuclei. In addition, ultra-thin sections of GMA-embedded salmon testes can be observed with the electron microscope, and this permits exact correlation between nuclear fine structure and chemical composition in consecutive sections of the same nuclei.

Ultrastructure of the nucleolus during the Chinese hamster cell cycle

Changes in the structure of the nucleolus during the cell cycle of the Chinese hamster cell in vitro were studied. Quantitative electron microscopic techniques were used to establish the size and volume changes in nucleolar structures. In mitosis, nucleolar remnants, "persistent nucleoli," consisting predominantly of ribosome-like granular material, and a granular coating on the chromosomes were observed. Persistent nucleoli were also observed in some daughter nuclei as they were leaving telophase and entering G(1). During very early G(1), a dense, fibrous material characteristic of interphase nucleoli was noted in the nucleoplasm of the cells. As the cells progressed through G(1), a granular component appeared which was intimately associated with the fibrous material. By the middle of G(1), complete, mature nucleoli were present. The nucleolar volume enlarged by a factor of two from the beginning of G(1) to the middle of S primarily due to the accumulation of the granular component. During the G(2) period, there was a dissolution or breakdown of the nucleolus prior to the entry of the cells into mitosis. Correlations between the quantitative aspects of this study and biochemical and cytochemical data available in the literature suggest the following: nucleolar reformation following division results from the activation of the nucleolar organizer regions which transcribe for RNA first appearing in association with protein as a fibrous component (45S RNA) and then later as a granular component (28S and 32S RNA).

A cytochemical and radioautographic study of human tissue culture cell nucleoli

Fine structural aspects of human tissue culture cell nucleoli were studied by cytochemical and radioautographic methods. Ribonuclease and pepsin digestions were carried out on glutaraldehyde-fixed cells that, in some instances, were labeled with thymidine-(3)H prior to digestion. Double digestion by ribonuclease and pepsin revealed a fine fibrillar reticulum that appears to be the supportive structure of nucleolonemal threads. The nature of the reticulum remains to be determined. The question of whether it may represent a dispersed form of chromatin was raised. Structural findings suggested such an hypothesis but the results of radioautographic studies do not support it. The reticulum showed a striking absence of radioactive labeling following a 3 hr incorporation of thymidine-(3)H. Only few silver grains were observed occasionally in the fibrillar nucleolonema that may or may not be significant. The radioautographic results are believed to be inconclusive for the various reasons discussed. The possibility that the reticulum is composed of proteins has to be considered. It appears that basic proteins can resist pepsin digestion in aldehyde-fixed cells. Individual chromatin fibrils were found to be associated with the nucleolar reticulum. It is possible that these alone represent the dispersed genetically active chromatin of nucleoli.

The organization of the nucleolus in meristematic plant cells. A cytochemical study

The architecture of the nucleolus in Allium porum and Triticum vulgare meristematic cells has been investigated by means of digestions with various enzymes. After staining with azure B at pH4, plant nucleoli exhibit lighter regions which, under electron microscopy, correspond to the fibrillar zones characterizing these organelles. Evidence is presented indicating that these latter zones contain coarse convoluted filaments quite similar to the loops first demonstrated by La Cour (24) and which are assumed to originate from the nucleolar-organizing chromosomes. These coarse, 0.2micro wide filaments are remarkably resistant to the action of deoxyribonuclease, ribonuclease, pepsin, trypsin, or of various combinations of these enzymes and, moreover, they show insignificant incorporation of labeled thymidine even after long exposure to this DNA precursor. The clearing action of pepsin on different regions of the nucleolus lends support to the hypothesis that an amorphous material or matrix pervades the mass of this organelle. This effect is particularly striking within the particulate nucleolar zones themselves. Both ribonuclease and trypsin disorganize the RNP (ribonucleoprotein) nucleolar particles. The effect of the latter enzyme on the RNP particles is taken to indicate that they contain proteins particularly susceptible to trypsin which are essential for maintenance of their morphological integrity. Trypsin also interferes with azure B-staining of the nucleolar mass as a whole and, according to radioautographic data, extracts RNA throughout this organelle. Accordingly, the hypothesis is considered that RNA is complexed with proteins not only within the particulate nucleolar portions, as is already well known, but also in the fibrillar zones.

Fine structural alterations of interphase nuclei of lymphocytes stimulated to grwoth activity in vitro

This report describes fine structural changes of interphase nuclei of human peripheral blood lymphocytes stimulated to growth by short-term culture with phytohemagglutinin. Chromatin is found highly labile, its changes accompanying the sequential increases of RNA and DNA synthesis which are known to occur in lymphocyte cultures. In "resting" lymphocytes, abundant condensed chromatin appears as a network of large and small aggregates. Early in the response to phytohemagglutinin, small aggregates disappear during increase of diffuse chromatin regions. Small aggregates soon reappear, probably resulting from disaggregation of large masses of condensed chromatin. Loosened and highly dispersed forms then appear prior to the formation of prophase chromosomes. The loosened state is found by radioautography to be most active in DNA synthesis. Small nucleoli of resting lymphocytes have concentric agranular, fibrillar, and granular zones with small amounts of intranucleolar chromatin. Enlarging interphase nucleoli change chiefly (1) by increase in amount of intranucleolar chromatin and alteration of its state of aggregation and (2) by increase in granular components in close association with fibrillar components.