Ultrastructure and behavior of the nucleoli and fibrillar centers during cell differentiation of liver erythroblasts of 12-day-old mouse embryos

The transformation of nucleolus and its structural components in the main groups of erythroid cells (from pronormoblasts to reticulocytes and dividing ones) has been studied. It is shown that during inactivation of the nucleolus, the granular component is reduced, and the degree of chromatin condensation increases. Enlargement and "naking" of fibrillar centres are also observed. At the stage of basophilic and polychromatophilic erythroblasts, the nucleolus has a mushroom-like shape with well developed fibrillar centres, which lie at the border of the nucleolus. Nucleolar RNP components consist predominantly of a fibrillar component and forms "caps" of these mushroom-like structures. Therefore, at this stage "free" fibrillar centres are found on ultrathin sections, if the section plane runs only through the fibrillar centre, or through ring-shaped nucleoli, i.e. the fibrillar centre surrounded by sheet of nucleolar RNP fibrilles, when the mushroom-like nucleolus is cut tangentially. Using serial section technique, small round nucleoli with an extremely weakly developed RNP material or free fibrillar centres, resembling those in telophase nuclei, are shown on the terminal stage of nucleolus transformation. It is noted that the main groups of erythroid cells differ from each other not only in the chromatin condensation degree, but also in the development of nucleolus material and in the size of fibrillar centres. However, such differences exist in either cell group. Consequently, we can distinguish between cell populations being on different stages of maturation. On this basis, we described on intermediate population of cells, which possess signs of pronormoblasts and basophilic erythroblasts. In all the cases, strands of electron opaque material bound with the condensed chromatin are present in fibrillar centres.(ABSTRACT TRUNCATED AT 250 WORDS)     

黄瓜根尖分生组织细胞核仁超微结构类型及缺硼时的变化

硼是植物必需的一种微量元素,缺乏时植物根的生长被抑制。黄瓜根尖分生组织细胞核中核仁有四种类型。不缺硼的核仁中,丝状成分、粒状成分、丝状中心及核仁液泡能够明显地看到。缺硼的核仁中,丝状成分萎缩,变得粗短。看不见丝状中心、核仁液泡等。环形核仁中的中央大液泡变小。     

Correlation of nucleolar activity and nucleolar vacuolation in plant cells

In root meristematic cells nucleolar structure varies with the cell cycle. Apart from normal meristematic nucleoli one finds nucleoli with a big central vacuole surrounded by a loose cortex with individual fibrillar centres [22] clearly visible within it. There are also intermediate structures between both nucleolar types. In Pisum sativum nuclear tissue, the structure of the vacuolated nucleoli is similar and appears in periods of high metabolic activity during megasporogenesis. In both tissues, vacuolated nucleoli incorporate tritiated uridine more actively than 'normal' nucleoli. In this work the structure of spontaneous nucleolar vacuoles is compared with that induced by drugs such as cordycepin, and FUdR. The vacuolated nucleolus with its increased surface corresponds to a transient structure which not only shows higher metabolic activity but also supplies a storing and/or transporting mechanism for nucleolar products.     

Electron microscopic study of endopolyploid nuclei in rat trophoblast giant cells. IV. Changes in nucleolar ultrastructure during cell differentiation

The ultrastructure of the nucleolus of highly differentiated trophoblast giant cells has been studied on the 17th day of the foetus development. Changes in its morphology have been followed in relation to the degree of nuclear chromatin condensation and to the cell differentiation level. The nucleoli have a reticular structure in the nuclei with dispersed and condensed chromatin. In both the cases the nucleoli involve the four components: fibro-granular, fibrillar (of moderate and normal density) and lacunar regions; fibrillar centres are distinguished within the regions. In the nucleoli with condensed chromatin, unlike those with dispersed chromatin, the perinuclear chromatin is clearly seen, and the penetration of nucleolus-organizer threads along lacunae and deep into the nucleolus can be easily followed. The fibrillar centres are more obvious. With the run of a progressive differentiation of the trophoblast cells, the number of granules is reduced; first, the fibro-granular component covers a significant part of the nucleolus, then granules become visible only in the cortical zone of the nucleolus; in the nuclei with strongly condensed chromatin no granules are seen in the nucleolus.     

Increased functional load on mouse kidney proximal tubule epithelial cells causes changes in nucleolar 3-D architecture

Ultrastructural 3-D analysis of nucleolar architecture and Ag-NOR protein distribution in mouse kidney-cortex proximal-tubule epithelium has been performed. A principal scheme of structural changes of the nucleolus and organization of its components during the intensification of pre-rRNA synthesis (dynamic model of a nucleolus) based on computer spatial modelling has been advanced. According to the nucleolar composition, three groups of cells, which differ from each other by rRNA synthesis, are defined in normal kidney. Most nephron proximal-section cells (about 52%) are characterized by lower activity of RNA synthesis. Such kind of cells are defined as group I (nucleolar diameter 0.7–1.5 µm) and always contain resting, ring-shaped or close to ring-shaped dense nucleoli, which have 2 or 3 fibrillar centers. Nucleoli of group II cells (about 37%, nucleolar diameter 1.5–2.5 µm) have a higher level of activity, contain 4–7 fibrillar centers, and their structural organization is close to reticulated forms due to the first indications of vacuolar network (identified as prereticulated nucleoli). The most active cells of group III (about 11%, nucleolar diameter 2.5–3.5 µm) include cells with typical reticulated nucleoli with a well expressed vacuolar network and numerous fibrillar centers (18–22). Increased functional load of the epithelium caused by unilateral nephrectomy and diuretic (4-chlor-H [2-furylmethyl] 5-sulphamyl-antranic acid) injection changed the proportion of the different cell groups: group I decreased (about 25%), whereas groups II and III increased (about 8% and 17%, respectively). The increase of nucleolar activity first causes a deformation of the individual fibrillar centers as well as complication and growth of their surface. Further, a progressive fragmentation of the fibrillar centers and the growth of their total volume is observed. The complication and growth of the total volume of Ag-positive zones is another indication of the nucleolar activation. The vacuolar system develops by a gradual fusion of small isolated cavities into a united vacuolar network. Nucleoli with 2–7 fibrillar centers are considered to be intermediate forms reflecting successive stages of its activation or inactivation: from the resting ring-shaped nucleolus via transient stages of increasing functional activity to the active reticulated nucleoli and vice versa. The observed differences in the nucleolar ultrastructure are regarded as evidence of the functional heterogeneity of cell populations within one functional segment of nephron.     

ULTRASTRUCTURAL ASPECTS OF EARLY STAGES IN COTTON FIBER ELONGATION

Fine structural alterations associated with early stages of cotton fiber elongation in Gossypium hirsutum L. var. dunn 56 C occur rapidly following anthesis and appear to be correlated with the formation of the central vacuole, plasma membrane, and primary cell wall as well as with increased protein synthesis necessary for cell elongation. Association of dilated cisternae of the endoplasmic reticulum with the tonoplast suggests that the endoplasmic reticulum is involved in the formation of the central vacuole. Dictyosome involvement in both plasma membrane and primary cell wall formation was suggested from observations of similarities between dictyosome associated vesicles, containing fibrils appearing similar in morphology to fibrils found in the primary cell wall, and plasma membrane associated vesicles. The single nucleolus found in cotton fibers enlarges following anthesis, shows segregation of granular and fibrillar components by 1 day postanthesis, develops a large “vacuole,” thus appearing ring-shaped, and occupies much of the nuclear volume by 2 days postanthesis. Prominent nucleoli were not observed in nuclei after 10 days postanthesis.     

3-dimensional organization of the nucleolus and the nucleolus-organizing regions in differentiated cells. I. Ring-shaped nucleoli in the endotheliocytes, smooth-muscle cells and fibroblasts in the mouse

The method of ultra-thin serial sections was used to study the three-dimensional structure and to perform the quantitative analysis of ring-shaped nucleoli of kidney and liver endotheliocytes, smooth muscle cells of kidney arterioles and fibroblasts of mice. Spatial models of ring-shaped nucleoli with one fibrillar centre are given. For the quantitative analysis the following parameters were measured: the number and volumes of nucleoli, fibrillar centers, RNP-containing structures, the vacuolar system and the RNP-index (the latter is a ratio of RNP-part and fibrillar center volumes). Nucleoli of the same type of cells, occasionally in the same nucleus, were found to differ sharply in their fibrillar center shape. Differences in the mean volume values of nucleoli, fibrillar centers and the RNP-part between some cell populations are sufficiently well pronounced. Within the same population ring-shaped nucleoli have, as a rule, specific volume values of nucleoli, RNP and fibrillar centers. The comparison of quantitative data obtained on different cell types showed that the mean RNP-index values were the most stable parameter. The structural relation between fibrillar centers, intra- and perinucleolar chromatin and lacunar region is shown. The structural organization of intranucleolar chromatin and rRNA in the nucleolar body and in fibrillar centers is discussed.     

丁酸钠对人胃腺癌MGC-803细胞核仁纤维中心和银染蛋白的影响

用透射电镜观察到MGC-803细胞的核仁是网织型的,在网眼内分布有电子密度低的纤维中心。MGC-803细胞经丁酸钠作用后,其核仁的类型发生了改变,多呈环型的,核仁的中央有一个大的纤维中心;纤维中心和银染颗粒的大小和数目明显减低;用图像分析仪测得核仁银染蛋白所占面积与核总面积的比值也明显降低。结果提示:丁酸钠可能通过抑制rRNA合成和rDNA转录活性调控MGC-803细胞的增殖。     

Analysis of ring-shaped nucleoli in serially sectioned human lymphocytes

Summary Serial section analysis has demonstrated that ring-shaped nucleoli of mature human lymphocytes are spherical structures consisting of a peripheral ribonucleoprotein shell that surrounds one large fibrillar center. The shell exhibits usually one or, less frequently, two openings. The fibrillar center is in contact with the nucleoplasm and perinucleolar condensed chromatin, which frequently appears as a pedicle-like structure. Several chromocenters are associated with the ring-shaped nucleolus.     

The reaction of Lupinus angustifolius L. root meristematic cell nucleoli to lead

The effect of 2–48 h treatment of Lupinus angustifolius L. roots with lead nitrate at the concentration of 10⁻⁴ M on the nucleoli in meristematic cells was investigated. In the lead presence the number of ring-shaped as well as segregated nucleoli increased especially after 12–48 h of treatment, while spindle-shaped nucleoli appeared after 24 h and 48 h. Lead presence also increased the frequency of cells with silver-stained particles in the nucleus and the number of these particles especially from the 12th hour of treatment. It was accompanied by significant decline of nucleolar area. Analysis of these cells in transmission electron microscope confirmed the presence of ring-shaped and segregated nucleoli. Moreover, electron microscopy revealed compact structure nucleoli without granular component. Additionally, one to three oval-shaped fibrillar structures attached to nucleolus or lying free in the nucleoplasm were visible. The possible mechanism of lead toxicity to the nucleolus is briefly discussed.     

Relative position of nucleolar chromatin and nucleolar components in ciliate <Emphasis Type="Italic">Didinium nasutum</Emphasis> somatic nuclei

According to our computer modeling data obtained earlier, nucleoli in interphase ciliates Didinium nasutum are complex netlike structures, in which the trabeculumor lamella-shaped fibrillar component is located on the periphery, and the granular component in the central part of the nucleolus. Chromatin bodies connected with nucleoli act as the nucleolar organizers in D. nasutum. In the present work, the arrangement of all chromatin bodies, which could correspond to nucleolar organizers by morphological criteria, is studied by means of a 3D-reconstruction. It is shown that all of these chromatin bodies are localized outside the nucleoli, on the fibrillar component’s periphery. Even those chromatin bodies which appeared to be completely surrounded by the fibrillar nucleolar component on single ultrathin sections are actually settled down in nucleolus cavities open to the nucleoplasm. This proves that the RNA processing in D. nasutum nucleoli is directed toward the center of nucleoli, where the granular component is located. The analysis of the nucleolar chromatin distribution made it possible to conclude that different parts of the complex interfase netlike nucleoli of D. nasutum have approximately the same activity.     

Ultrastructural Distribution of DNA within Plant Meristematic Cell Nucleoli during Activation and the Subsequent Inactivation by a Cold Stress

We have investigated the precise location of DNA within the meristematic cell nucleolus ofZea maysroot cells andPisum sativumcotyledonary buds, in the course of their activation and induced inactivation following a subsequent treatment at low temperature. For this purpose, we combined the acetylation method, providing an excellent distinction between the various nucleolar components, with thein situterminal deoxynucleotidyl transferase-immunogold technique, a highly sensitive method for detecting DNA at the ultrastructural level. In addition to the presence of DNA in the condensed chromatin associated with the nucleolus, we demonstrated that a significant label was detected in the nucleolus of quiescent cells in both plant models. Evident labels were also found in the dense fibrillar component of actived nucleoli. Whereas in inactivated nucleoli no significant label was observed within the dense fibrillar component, an intense label was seen over the large heterogeneous fibrillar centres only during inactivation. The granular component was never significantly labelled. These results appear to indicate that the DNA present in the dense fibrillar component of activated nucleoli withdraws from this structure during its inactivation and becomes incorporated in the large fibrillar centres. These observations suggest that in plant cells inactivation of rRNA genes is clearly accompanied by changes in the conformation of ribosomal chromatin.     

Ultrastructure of the ring-shaped nucleoli in the hepatocytes of chick embryos]

The fine structure of ring-shaped nucleoli in hepatocytes of chick embryos at different terms of development was studied. Structural differences were shown between annular nucleoli and nucleoli with typical nucleolonemal organization. Ring-shaped nucleoli, as a rule, are devoid of nucleolonemal structure and their fibrillar component is reduced. The material which fills the central cavity always appears similar to the nucleoplasm in its structure. On the basis of serial sections, we propose that central cavity is isolated from the nucleoplasm. From the hypotonical treatment and EDTA staining application it is concluded that the central cavity of ring-shaped nucleoli contains DNP associated with the intranucleolar chromatin. The number of these nucleoli increased after the injection of a liver homogenate cytoplasmic fraction extracted from adult hen. The physiological significance of such nucleoli is discussed.     

Activation of transcription function in lymphocytes after irradiation with He-Ne-laser

The influence of He-Ne-laser irradiation (lambda = 632.8 nm) in dose 56 J/m2 on the ultrastructure of the nucleolus from human peripheral lymphocytes was studied electronmicroscopically. After 1 h irradiation a well-expressed reaction of the nucleolus was observed in 70% of the lymphocytes under examination. Changes consist in the appearance of a wrong-shaped fibrillar center or in its fragmentation, the increase of RNP-containing fibrillar and granular components, and also in expansion of vacuoli. In a number of irradiated lymphocytes nucleoli with several fibrillar centres and with a strand-like organization of RNP part were observed. The size of these nucleoli increases. Following the accepted functional interpretations the observed changes can be connected with the intensification of RNA metabolism including the synthesis, processing of pre-rRNA and preribosome transport from the nucleolus. Similar rearrangements of the nucleoli were revealed in parallel experiments with phytohemagglutinin-treated lymphocytes. They were observed 1 h after the stimulation of lymphocytes. Taking into account the absence of mitogenic action of He-Ne-laser irradiation on lymphocytes, the ultrastructural changes of nucleoli under the action of irradiation are considered as functional activation of rRNA synthesis in the Go-period.     

Cytochemical distinction of various nucleolar components in insect cells.

The fine structure of the insect Sf9 cell nucleolus has been investigated by means of different cytochemical and immunocytochemical techniques at the electron microscope level. Apart from a few perinucleolar condensed chromatin clumps, the insect cell nucleolus comprises two compartments. The first of these consists of a roundish compact zone formed of fibrillar material. The other is composed of fibrillar and granular structures organized into a network separated by interstitial spaces. But, unlike mammalian cell nucleoli, any fibrillar center has been observed in the Sf9 cell nucleolus, even after actinomycin D treatment. We also show that the compact fibrillar zone of Sf9 cell nucleoli contains silver-stainable material and DNA. In actinomycin D-treated cells, a preferential contact of this compact fibrillar zone with condensed chromatin has been visualized. Finally, silver-stainable material has been found to persist throughout the whole mitosis. These results suggest that the compact fibrillar zone at the insect Sf9 cell nucleolus should, at least partly, correspond to the fibrillar center of mammalian cell nucleoli.     

Fine structure of nucleoli in micronucleated cells

The correlation between the number of nucleolus organizing regions (NOR) on metaphase chromosomes and the number of nucleoli was studied in normal and micronucleate cells. Many micronuclei, but not all, were able to form complete nucleoli with fibrillar and granular RNP components and fibrillar centers. Micronuclei which failed to form complete nucleoli often contained multiple electron-dense bodies of fibrillar material. These structures, which were much smaller than nucleoli, reacted with nucleolus-specific antibodies and the Ag-As method in the same way as complete nucleoli, but lacked fibrillar centers and granular RNP components. The data suggest that these nucleolus-like ‘blobs’ contain nucleolar material which, following mitosis, has been enclosed in micronuclei which do not contain nucleolus organizing chromosomes. No evidence was found for the activation of latent NORs not expressed in mononucleate cells.