Electron microscopic spread pre parations of oocyte nucleo li (I a mpbrush stage) of va ri o us a... more Electron microscopic spread pre parations of oocyte nucleo li (I a mpbrush stage) of va ri o us am phibia ns are quantitativel y evaluated and the length distributions of repeat-, mat ri x-, a nd space r-units a long the rRNA cistron containing axes are given. The co rrela ti o n of the matrix unit da ta with the gel electrophoretic pattern of labelIed nucl ear R NA fr om the sa me oocytes is exam ined. The mean value of the mat rix unit cOITesponds fa irly weil to a 2.6 million 0 peak of pre-rRNA but the distribution o f bot h ma trix units a nd labeli ed pre-rRNAs shows an asymmetrical heterogeneity indicating the ex istence of some large r primary tra nsc ription produc ls of rDNA. Novel structura l aspects a re described in the spacer regions which suggcst lh a t lranscripli o n does also ta ke place in DNP regions between the matrix unit s. A spec ial ' prelude piece' cod in g for approx. 0.5 million D o f RNA is frequently visua li zed in the spacer segments at the beginning of a matrix unit. Possible artifacts res ulting fr om the prepa ralion , lhe re lat ive congruence belwee n the data obtained usin g bOlh meth od s, a nd lhe funclional mea nin g o f the findings are discussed aga inst the backgro und o f c urrent conce pts of str uctura l o rga ni za tion a nd lra nscript io n products o f nucleolar DNA .
Three different calls of the clawed toadXenopus laevis are described and their sound spectrograms... more Three different calls of the clawed toadXenopus laevis are described and their sound spectrograms are presented. The male and female have one characteristic call each, which is heard during clasping after stimulating the mating behaviour with chorionic gonadotropin. A second call of the male is heard without hormone treatment, often after feeding or change of water.
Proceedings of the National Academy of Sciences, 1974
The structural organization of transcriptionally active DNA that contains cistrons for precursor ... more The structural organization of transcriptionally active DNA that contains cistrons for precursor molecules of ribosomal RNA is described in positively stained spread preparations from nuclei and nucleoli isolated from the green alga, Acetabularia mediterranea Lmx. These nuclei contain large aggregates of nucleolar subunits in which fibril-covered regions, the putative active cistrons for precursors of ribosomal RNA, alternate with fibril-free intercepts, the “spacers.” The length distribution of the different intercepts of this DNA is given, and the pattern is compared with those shown in animal cell systems. The data are discussed in relation to problems of transcription and of amplification of ribosomal RNA genes.
The tumor suppressor p53 has been implicated in the regulation of ribosome biogenesis based on it... more The tumor suppressor p53 has been implicated in the regulation of ribosome biogenesis based on its inhibitory effect on RNA polymerase I (pol I)-dependent transcription. Consistent with this, p53 has been described in nucleoli, albeit under specific experimental conditions. Since data on the intranucleolar localization of p53 are controversial, we have analyzed in detail its subnucleolar distribution. Our results show that p53 does not localize to one of the well-known structural components of the nucleolus involved in ribosome biogenesis, but rather occupies distinct intranucleolar regions that constitute nucleolar cavities. When cells were treated with the proteasome inhibitor MG132, the size and frequency of p53-containing nucleolar cavities increased, and the protein partially colocalized with inactivated proteasomes. Importantly, p53 did not colocalize with pol I at the transcription sites in fibrillar centers (FCs) as has previously been reported. The observed intranucleolar d...
The fate of defined nucleolar constituents during o-galactosamine-induced inhibition of transcrip... more The fate of defined nucleolar constituents during o-galactosamine-induced inhibition of transcription and the accompanying extensive structural changes such as nucleolar segregation, fragmentation and disappearance of the granular components was studied by light and electron microscopic immunolocalization , using antibodies to different nucleolar components. In contrast to other inhibitors such as actinomycin D, we show that preribosomal components as monitored by a ribosomal protein leave the nucleolus , while a large proportion of RNA polymerase I remains associated with the nucleolar chromatin, i.e. probably the pre-rRNA genes , during inactivation of transcription. These small structures containing the RNA polymerase I are characterized by low electron density and resemble the 'fibrillar centers ' of normal nucleoli. The results are discussed in relation to current concepts of the functional topology of the nucleolus.
The high mobility group proteins A2 (HMGA2) have been implicated in the control of cell prolifera... more The high mobility group proteins A2 (HMGA2) have been implicated in the control of cell proliferation and differentiation, in particular during embryogenesis. Here, we used Xenopus laevis to analyze HMGA2 gene expression patterns during oogenesis and early embryogenesis. We found two functional XlHMGA2 isoforms, which we named XlHMGA2alpha and XlHMGA2beta. As revealed by RT-PCR, real-time PCR and whole-mount in situ hybridization both mRNAs are maternally produced and stored in eggs. Whole-mount in situ hybridizations revealed a conspicuous redistribution of the XlHMGA2 transcripts during early embryogenesis. Initially, during oogenesis and in eggs, the transcripts are uniformly distributed in the cytoplasm. With activation of the eggs the transcripts accumulate near the animal pole and remain in the juxtanuclear regions of animal pole blastomeres until midblastula transition. According to real-time PCR data, XlHMGA2alpha appears to be preferentially expressed during oogenesis and after midblastula transition, whereas XlHMGA2beta expression predominates after neurulation, suggesting an individual transcriptional regulation.
Glucocorticoid hormones regulate the transcription of nuclear genes by way of their cognate recep... more Glucocorticoid hormones regulate the transcription of nuclear genes by way of their cognate receptors. In addition, these hormones also modulate mitochondrial gene transcription by mechanisms which are as yet poorly understood. Using immunofluorescence labeling and confocal laser scanning microscopy we show that the glucocorticoid receptor of HeLa and Hep-2 cells is specifically enriched at the sites of the mitochondria which were visualized by labeling with the vital dye CMX and antibodies against cytochrome oxidase subunit I. Immunogold electron microscopy demonstrated that the receptor was located within the inner space of the mitochondria. Immunoblotting experiments also revealed the presence of glucocorticoid receptor in mitochondria isolated from HeLa and Hep-2 cells. Finally, living HeLa cells expressing green fluorescent-glucocorticoid receptor fusion protein revealed a distinct mitochondrial GFP fluorescence. Our results support the concept of a receptor-mediated direct action of steroid hormones on mitochondrial gene transcription.
The transcriptionally active rRNA genes have the remarkable ability to organize and integrate the... more The transcriptionally active rRNA genes have the remarkable ability to organize and integrate the biochemical pathway of ribosome production into a structural framework, the nucleolus. The past year has seen numerous advances in our understanding of the relationships between nucleolar substructures, the site of ribosomal RNA frRNA) gene transcription and the pathway of ribosome maturation. Progress has also been made both in the molecular identification of nucleolar constituents and in our understanding of the interactions between these components and their assembly into higher order structures.
One of the significant steps in the past decade of chromatin research has been the finding of the... more One of the significant steps in the past decade of chromatin research has been the finding of the nucleosome as the common principle form of DNA-histone organization (Hewish and Burgoyne 1973; Kornberg 1974; Olins and Olins 1974; for review see Chambon 1978). In most eukaryotic nuclei the bulk of the chromatin is associated with histones in a regular pattern and is organized, in the non-transcribed chromatin regions, in nucleosomal particles, which can be visualized by electron microscopy of spread chromatin in extended nucleosomal filaments, i. e. 10-12 nm large beads-on-a-string (Fig. I a). This principle of packing in globular repeating units results in an overall foreshortening of the DNA by a factor of 5-6. However, some other forms of DNA arrangement in chromatin have also been reported: (i) the dinoflagellate chromosome (for references see Rae and Steele 1978); (ii) DNA newly replicated in the absence of protein synthesis (Riley and Weintraub 1979); (iii) actively transcribed chromatin in which the nucleohistone is organized in such a way that the contained DNA is in a predominantly extended configuration (Fig. 1 b;
Antibodies to calf thymus histone H2B were purified by chromatography on DEAE-cellulose and injec... more Antibodies to calf thymus histone H2B were purified by chromatography on DEAE-cellulose and injected into oocyte nuclei of Pleurodeles waltlii. As shown by indirect immunofluorescence these antibodies cross-reacted strongly with corresponding histones associated with lampbrush chromosomes. Shortly after injection the lateral loops of the chromosomes retracted into the chromomeres and by 3 h postinjection the ‘lampbrush’ appearance was completely lost and the chromosomes appeared in light-microscopic preparations as rod-like structures consisting of longitudinally coalesced chromomeres. In control oocytes injected with non-immune immunoglobulins or antibodies against a ubiquitous transcript-associated protein no morphological alterations of the lampbrush chromosomes could be observed. Electron microscopic spreads of chromosomes prepared at various times after injection of anti-H2B revealed a progressive loss of transcriptional complexes from the loop axes. Finally, higher-order chrom...
Proceedings, annual meeting, Electron Microscopy Society of America, 1978
A significant contribution to the understanding of chromatin organization was the discovery of th... more A significant contribution to the understanding of chromatin organization was the discovery of the nucleosome as a globular repeating unit of the package of DNA (Hewish and Burgoyne, 1973; Woodcock, 1973; Kornberg, 1974; Olins and Olins, 1974; for review see Oudet et al., 1978a). In accord with the original definition and in agreement with most workers in this field of research we identify a nucleosome as a spherical or slightly oblate granular particle 10-13 nm in diameter, containing about 200 base pairs of DNA and two of each of the four histones H2a, H2b, H3 and H4. It is this structure in which the bulk of the nuclear chromatin is organized in most eukaryotic cells, with the exception of the dinoflagellates (Rae and Steele, 1977; dinoflagellate DNA, however, can be packed into nucleosomal structures in vitro by addition of the appropriate amounts of histones;the same reference).
PtK2 cells in which pore complex-mediated transport is blocked by microinjection early in mitosis... more PtK2 cells in which pore complex-mediated transport is blocked by microinjection early in mitosis of a monoclonal antibody (specific for an Mr 68,000 pore complex glycoprotein) or of wheat germ agglutinin (WGA) complete cytokinesis. However, their nuclei remain stably arrested in a telophase-like organization characterized by highly condensed chromatin and the absence of nucleoli, indicating a requirement for pore-mediated transport for the reassembly of interphase nuclei. We have now examined this requirement more closely by monitoring the behavior of individual nuclear macromolecules in microinjected cells using immunofluorescence microscopy and have investigated the effect of microinjecting the antibody or WGA on cellular ultrastructure. The absence of nuclear transport did not affect the sequestration into daughter nuclei of components such as DNA, DNA topoisomerase I and the nucleolar protein fibrillarin that are carried through mitosis on chromosomes. On the other hand, lamins...
Purified mitochondrial DNA (mitDNA) from ovaries ofXenopus lae vis was injected into the nuclei (... more Purified mitochondrial DNA (mitDNA) from ovaries ofXenopus lae vis was injected into the nuclei (germinal vesicles) of large viteUogenic oocytes of the same organism and examined by electron microscopy ofthe spread nuclear contents. Normally located nuclei of untreated oocytes as weil as peripherally translocated nuclei of centrifuged oocytes were used. In addition, oocyte nuclei isolated and incubated under liquid paraffin oil were injected with DNA. The integrity oftranscriptional structures of endogenous chromosomal (Iampbrush chromosomes) and extrachromosomal (nucleoli) genes of the injected nuclei was demonstrated. Microinjected mitDN A was identified as circles of chromatin exhibiting polynucleosome-like organization and a me an contour length of 2.6 J.Lm, corresponding to a compaction ratio of the mitDN A of about 2 : I. This DNA packing ratio is similar to that observed after preparation of various kinds of native chromatin in low salt buffers. The chromatin circles formed from injected mitDNA only very rarely exhibited lateral fibrils suggestive of transcriptional activity. These results suggest that purified mitDNA can be transformed to normally structured chromatin when exposed to oocyte nuclear contents but is rarely , if at all , transcribed in this form and in this environment. MitDNA injected into nuclei 365
We describe novel nucleolar structures, observed by thin section electron microscopy in oocyte nu... more We describe novel nucleolar structures, observed by thin section electron microscopy in oocyte nuclei of the grashopper Locusta migratoria, which we interpret, based on morphological and compositional criteria, as rDNA transcription units. Morphologically they resemble the condensed and foreshortened "Christmas trees" seen in Miller spreads of nucleolar chromatin prepared from the same biological material. They contain DNA and rRNA as shown by immunocytochemistry and in situ hybridization and are concentrated in several intranucleolar cavities. The presumptive rDNA transcription units extend throughout the interior of these nucleolar pockets or are selectively enriched at their outermost zones in close contact with the surrounding fibrillarinpositive dense component. We suggest that the nucleolar pockets of Locusta oocytes are equivalent to the fibrillar centers of somatic nucleoli and discuss possible implications for the current understanding of the functional organization of nucleoli.
Electron microscopic spread pre parations of oocyte nucleo li (I a mpbrush stage) of va ri o us a... more Electron microscopic spread pre parations of oocyte nucleo li (I a mpbrush stage) of va ri o us am phibia ns are quantitativel y evaluated and the length distributions of repeat-, mat ri x-, a nd space r-units a long the rRNA cistron containing axes are given. The co rrela ti o n of the matrix unit da ta with the gel electrophoretic pattern of labelIed nucl ear R NA fr om the sa me oocytes is exam ined. The mean value of the mat rix unit cOITesponds fa irly weil to a 2.6 million 0 peak of pre-rRNA but the distribution o f bot h ma trix units a nd labeli ed pre-rRNAs shows an asymmetrical heterogeneity indicating the ex istence of some large r primary tra nsc ription produc ls of rDNA. Novel structura l aspects a re described in the spacer regions which suggcst lh a t lranscripli o n does also ta ke place in DNP regions between the matrix unit s. A spec ial ' prelude piece' cod in g for approx. 0.5 million D o f RNA is frequently visua li zed in the spacer segments at the beginning of a matrix unit. Possible artifacts res ulting fr om the prepa ralion , lhe re lat ive congruence belwee n the data obtained usin g bOlh meth od s, a nd lhe funclional mea nin g o f the findings are discussed aga inst the backgro und o f c urrent conce pts of str uctura l o rga ni za tion a nd lra nscript io n products o f nucleolar DNA .
Three different calls of the clawed toadXenopus laevis are described and their sound spectrograms... more Three different calls of the clawed toadXenopus laevis are described and their sound spectrograms are presented. The male and female have one characteristic call each, which is heard during clasping after stimulating the mating behaviour with chorionic gonadotropin. A second call of the male is heard without hormone treatment, often after feeding or change of water.
Proceedings of the National Academy of Sciences, 1974
The structural organization of transcriptionally active DNA that contains cistrons for precursor ... more The structural organization of transcriptionally active DNA that contains cistrons for precursor molecules of ribosomal RNA is described in positively stained spread preparations from nuclei and nucleoli isolated from the green alga, Acetabularia mediterranea Lmx. These nuclei contain large aggregates of nucleolar subunits in which fibril-covered regions, the putative active cistrons for precursors of ribosomal RNA, alternate with fibril-free intercepts, the “spacers.” The length distribution of the different intercepts of this DNA is given, and the pattern is compared with those shown in animal cell systems. The data are discussed in relation to problems of transcription and of amplification of ribosomal RNA genes.
The tumor suppressor p53 has been implicated in the regulation of ribosome biogenesis based on it... more The tumor suppressor p53 has been implicated in the regulation of ribosome biogenesis based on its inhibitory effect on RNA polymerase I (pol I)-dependent transcription. Consistent with this, p53 has been described in nucleoli, albeit under specific experimental conditions. Since data on the intranucleolar localization of p53 are controversial, we have analyzed in detail its subnucleolar distribution. Our results show that p53 does not localize to one of the well-known structural components of the nucleolus involved in ribosome biogenesis, but rather occupies distinct intranucleolar regions that constitute nucleolar cavities. When cells were treated with the proteasome inhibitor MG132, the size and frequency of p53-containing nucleolar cavities increased, and the protein partially colocalized with inactivated proteasomes. Importantly, p53 did not colocalize with pol I at the transcription sites in fibrillar centers (FCs) as has previously been reported. The observed intranucleolar d...
The fate of defined nucleolar constituents during o-galactosamine-induced inhibition of transcrip... more The fate of defined nucleolar constituents during o-galactosamine-induced inhibition of transcription and the accompanying extensive structural changes such as nucleolar segregation, fragmentation and disappearance of the granular components was studied by light and electron microscopic immunolocalization , using antibodies to different nucleolar components. In contrast to other inhibitors such as actinomycin D, we show that preribosomal components as monitored by a ribosomal protein leave the nucleolus , while a large proportion of RNA polymerase I remains associated with the nucleolar chromatin, i.e. probably the pre-rRNA genes , during inactivation of transcription. These small structures containing the RNA polymerase I are characterized by low electron density and resemble the 'fibrillar centers ' of normal nucleoli. The results are discussed in relation to current concepts of the functional topology of the nucleolus.
The high mobility group proteins A2 (HMGA2) have been implicated in the control of cell prolifera... more The high mobility group proteins A2 (HMGA2) have been implicated in the control of cell proliferation and differentiation, in particular during embryogenesis. Here, we used Xenopus laevis to analyze HMGA2 gene expression patterns during oogenesis and early embryogenesis. We found two functional XlHMGA2 isoforms, which we named XlHMGA2alpha and XlHMGA2beta. As revealed by RT-PCR, real-time PCR and whole-mount in situ hybridization both mRNAs are maternally produced and stored in eggs. Whole-mount in situ hybridizations revealed a conspicuous redistribution of the XlHMGA2 transcripts during early embryogenesis. Initially, during oogenesis and in eggs, the transcripts are uniformly distributed in the cytoplasm. With activation of the eggs the transcripts accumulate near the animal pole and remain in the juxtanuclear regions of animal pole blastomeres until midblastula transition. According to real-time PCR data, XlHMGA2alpha appears to be preferentially expressed during oogenesis and after midblastula transition, whereas XlHMGA2beta expression predominates after neurulation, suggesting an individual transcriptional regulation.
Glucocorticoid hormones regulate the transcription of nuclear genes by way of their cognate recep... more Glucocorticoid hormones regulate the transcription of nuclear genes by way of their cognate receptors. In addition, these hormones also modulate mitochondrial gene transcription by mechanisms which are as yet poorly understood. Using immunofluorescence labeling and confocal laser scanning microscopy we show that the glucocorticoid receptor of HeLa and Hep-2 cells is specifically enriched at the sites of the mitochondria which were visualized by labeling with the vital dye CMX and antibodies against cytochrome oxidase subunit I. Immunogold electron microscopy demonstrated that the receptor was located within the inner space of the mitochondria. Immunoblotting experiments also revealed the presence of glucocorticoid receptor in mitochondria isolated from HeLa and Hep-2 cells. Finally, living HeLa cells expressing green fluorescent-glucocorticoid receptor fusion protein revealed a distinct mitochondrial GFP fluorescence. Our results support the concept of a receptor-mediated direct action of steroid hormones on mitochondrial gene transcription.
The transcriptionally active rRNA genes have the remarkable ability to organize and integrate the... more The transcriptionally active rRNA genes have the remarkable ability to organize and integrate the biochemical pathway of ribosome production into a structural framework, the nucleolus. The past year has seen numerous advances in our understanding of the relationships between nucleolar substructures, the site of ribosomal RNA frRNA) gene transcription and the pathway of ribosome maturation. Progress has also been made both in the molecular identification of nucleolar constituents and in our understanding of the interactions between these components and their assembly into higher order structures.
One of the significant steps in the past decade of chromatin research has been the finding of the... more One of the significant steps in the past decade of chromatin research has been the finding of the nucleosome as the common principle form of DNA-histone organization (Hewish and Burgoyne 1973; Kornberg 1974; Olins and Olins 1974; for review see Chambon 1978). In most eukaryotic nuclei the bulk of the chromatin is associated with histones in a regular pattern and is organized, in the non-transcribed chromatin regions, in nucleosomal particles, which can be visualized by electron microscopy of spread chromatin in extended nucleosomal filaments, i. e. 10-12 nm large beads-on-a-string (Fig. I a). This principle of packing in globular repeating units results in an overall foreshortening of the DNA by a factor of 5-6. However, some other forms of DNA arrangement in chromatin have also been reported: (i) the dinoflagellate chromosome (for references see Rae and Steele 1978); (ii) DNA newly replicated in the absence of protein synthesis (Riley and Weintraub 1979); (iii) actively transcribed chromatin in which the nucleohistone is organized in such a way that the contained DNA is in a predominantly extended configuration (Fig. 1 b;
Antibodies to calf thymus histone H2B were purified by chromatography on DEAE-cellulose and injec... more Antibodies to calf thymus histone H2B were purified by chromatography on DEAE-cellulose and injected into oocyte nuclei of Pleurodeles waltlii. As shown by indirect immunofluorescence these antibodies cross-reacted strongly with corresponding histones associated with lampbrush chromosomes. Shortly after injection the lateral loops of the chromosomes retracted into the chromomeres and by 3 h postinjection the ‘lampbrush’ appearance was completely lost and the chromosomes appeared in light-microscopic preparations as rod-like structures consisting of longitudinally coalesced chromomeres. In control oocytes injected with non-immune immunoglobulins or antibodies against a ubiquitous transcript-associated protein no morphological alterations of the lampbrush chromosomes could be observed. Electron microscopic spreads of chromosomes prepared at various times after injection of anti-H2B revealed a progressive loss of transcriptional complexes from the loop axes. Finally, higher-order chrom...
Proceedings, annual meeting, Electron Microscopy Society of America, 1978
A significant contribution to the understanding of chromatin organization was the discovery of th... more A significant contribution to the understanding of chromatin organization was the discovery of the nucleosome as a globular repeating unit of the package of DNA (Hewish and Burgoyne, 1973; Woodcock, 1973; Kornberg, 1974; Olins and Olins, 1974; for review see Oudet et al., 1978a). In accord with the original definition and in agreement with most workers in this field of research we identify a nucleosome as a spherical or slightly oblate granular particle 10-13 nm in diameter, containing about 200 base pairs of DNA and two of each of the four histones H2a, H2b, H3 and H4. It is this structure in which the bulk of the nuclear chromatin is organized in most eukaryotic cells, with the exception of the dinoflagellates (Rae and Steele, 1977; dinoflagellate DNA, however, can be packed into nucleosomal structures in vitro by addition of the appropriate amounts of histones;the same reference).
PtK2 cells in which pore complex-mediated transport is blocked by microinjection early in mitosis... more PtK2 cells in which pore complex-mediated transport is blocked by microinjection early in mitosis of a monoclonal antibody (specific for an Mr 68,000 pore complex glycoprotein) or of wheat germ agglutinin (WGA) complete cytokinesis. However, their nuclei remain stably arrested in a telophase-like organization characterized by highly condensed chromatin and the absence of nucleoli, indicating a requirement for pore-mediated transport for the reassembly of interphase nuclei. We have now examined this requirement more closely by monitoring the behavior of individual nuclear macromolecules in microinjected cells using immunofluorescence microscopy and have investigated the effect of microinjecting the antibody or WGA on cellular ultrastructure. The absence of nuclear transport did not affect the sequestration into daughter nuclei of components such as DNA, DNA topoisomerase I and the nucleolar protein fibrillarin that are carried through mitosis on chromosomes. On the other hand, lamins...
Purified mitochondrial DNA (mitDNA) from ovaries ofXenopus lae vis was injected into the nuclei (... more Purified mitochondrial DNA (mitDNA) from ovaries ofXenopus lae vis was injected into the nuclei (germinal vesicles) of large viteUogenic oocytes of the same organism and examined by electron microscopy ofthe spread nuclear contents. Normally located nuclei of untreated oocytes as weil as peripherally translocated nuclei of centrifuged oocytes were used. In addition, oocyte nuclei isolated and incubated under liquid paraffin oil were injected with DNA. The integrity oftranscriptional structures of endogenous chromosomal (Iampbrush chromosomes) and extrachromosomal (nucleoli) genes of the injected nuclei was demonstrated. Microinjected mitDN A was identified as circles of chromatin exhibiting polynucleosome-like organization and a me an contour length of 2.6 J.Lm, corresponding to a compaction ratio of the mitDN A of about 2 : I. This DNA packing ratio is similar to that observed after preparation of various kinds of native chromatin in low salt buffers. The chromatin circles formed from injected mitDNA only very rarely exhibited lateral fibrils suggestive of transcriptional activity. These results suggest that purified mitDNA can be transformed to normally structured chromatin when exposed to oocyte nuclear contents but is rarely , if at all , transcribed in this form and in this environment. MitDNA injected into nuclei 365
We describe novel nucleolar structures, observed by thin section electron microscopy in oocyte nu... more We describe novel nucleolar structures, observed by thin section electron microscopy in oocyte nuclei of the grashopper Locusta migratoria, which we interpret, based on morphological and compositional criteria, as rDNA transcription units. Morphologically they resemble the condensed and foreshortened "Christmas trees" seen in Miller spreads of nucleolar chromatin prepared from the same biological material. They contain DNA and rRNA as shown by immunocytochemistry and in situ hybridization and are concentrated in several intranucleolar cavities. The presumptive rDNA transcription units extend throughout the interior of these nucleolar pockets or are selectively enriched at their outermost zones in close contact with the surrounding fibrillarinpositive dense component. We suggest that the nucleolar pockets of Locusta oocytes are equivalent to the fibrillar centers of somatic nucleoli and discuss possible implications for the current understanding of the functional organization of nucleoli.
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Papers by U. Scheer