Nucleolus activation in pea cotyledonary buds during 24 hours after decapitation of the main stem: Cytochemical studies

Summary Several cytochemical techniques and routine electron microscopy were used to detect changes in the pea cotyledonary bud nucleolus, from the G_0–1 resting state until the first mitoses (24th hour) following removal of the main stem. In the G_0–1 nucleus the fibrillar RNP nucleolus, shaped like a hollow sphere, possessed clumps of DNA connected to the compact peripheral nuclear chromatin by DNA fibres. The large central space, surrounded by the fibrillar component, showed the characteristics of a heterogeneous fibrillar center. By the 6th hour, this heterogeneous fibrillar center broke up, into small scattered elements in which DNA was detected. Between the 9th and 12th hours, RNA maturation produced a granular zone. Inter- and peri-chromatin granules of RNP were then present. In the bud, which was either in the resting state or in the process of reactivation, Ag-NOR proteins were located throughout the nucleolar mass, but were heavily concentrated in its peripheral region; they constituted a binding network between the various nucleolar components. The role and importance of the G_0–1 nuclei karyosome, present until the 9th hour, is discussed.     

Fate of the nucleolar vacuole during resumption of cell cycle in pea cotyledonary buds

Summary Meristematic cells of pea cotyledonary buds blocked in G_0–1 state contain a small nucleolus with a large central clear area surrounded by a fibrillar rim. The nucleolar structure varies according to the cell cycle from the G_0–1-blocked state until the first mitoses occurring between 24 and 27h after removal of the main stem. In order to better identify and understand the role of the central area in the nucleolar function, its content was investigated by cytochemical and terminal deoxynucleotidyl transferase-immunogold methods. The central area showed the characteristics of a vacuole commonly constituted of the condensed chromatin, ribonucleoprotein granules, and lack of argyrophilic proteins. 3 h after decapitation, a thickening of the fibrillar rim occurred, accompanied by an increase of granules in the vacuole. After 6h, the unique vacuole broke up into two to four small vacuoles in which the granules are more abundant. After 12 h the nucleolus acquired compact structure with few minute vacuoles dispersed over the fibrillar component. During the whole cell cycle, the condensed chromatin is always observed in the vacuole. Our findings suggest that the appearance of the vacuoles is subsequent to the output of preribosomes from nucleolus. These vacuoles might play a role in condensation and decondensation of the chromatin.     

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 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)     

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 G₁. During very early G₁, a dense, fibrous material characteristic of interphase nucleoli was noted in the nucleoplasm of the cells. As the cells progressed through G₁, a granular component appeared which was intimately associated with the fibrous material. By the middle of G₁, complete, mature nucleoli were present. The nucleolar volume enlarged by a factor of two from the beginning of G₁ to the middle of S primarily due to the accumulation of the granular component. During the G₂ 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).     

Subnucleolar distribution and organization of Vicia faba L. rDNA in situ

The distribution and organization of nucleolar DNA in Vicia faba L. was analyzed by specific cytochemical staining using NAMA-Ur. The results showed that nucleolar DNA was distributed in the FCs and at the FC/DFC junctions. Statistical analysis showed that the rRNA genes occupied about one-third of the total dense fibrillar component region. The rDNA was condensed in some regions and uncondensed in others. Nucleolus-associated chromatin extended from outside the nucleolus to the periphery of the FCs via nucleolar channels, suggesting a possible origin for nucleolar DNA.     

A HEAT-SENSITIVE CELLULAR FUNCTION LOCATED IN THE NUCLEOLUS

Striking nucleolar lesions occur in cultured cells after exposure to supranormal temperatures. These lesions appear at 42°C and consist of a loss of the granular ribonucleoprotein (RNP) component and intranucleolar chromatin, and a disappearance of the nucleolar reticulum. The material remaining in the morphologically homogeneous nucleolus is a large amount of closely packed fibrillar RNP. The lesions remain identical as temperature increases to 45°C. These alterations are reversible when the cells are returned to 37°C and are associated with the reappearance of an exaggerated amount of intranucleolar chromatin and granular RNP. High-resolution radioautography indicates that after thermic shock nucleolar RNA synthesis is inhibited whereas extranucleolar sites are preserved: it also suggests that the granular RNP is reconverted to fibrillar RNP probably by simple unraveling. The results prove the existence of heat-sensitive cellular functions in the nucleolus which deal with the DNA-dependent RNA synthesis. The precise site of action is assumed to involve hydrogen bonds, resulting in configurational changes in nucleolar RNP and affecting the stability of the DNA molecule. The subsequent events in nucleolar RNA synthesis are discussed in light of the morphologic and biochemical effects of actinomycin D on the nucleolus.     

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。