Ulrich Scheer

The ultrastructure of the nuclear envelope of amphibian oocytes: a reinvestigation. II. The immature oocyte and dynamic aspects.

Different stages of oogenesis of the Alpine newt were compared with respect to the nuclear envelope structure, employing negative staining and sectional work: larval eggs, early and later lampbrush stages, and mature eggs. While in all the stages the general components of the nuclear pore complexes could be observed, differences were found concerning quantitative structural data. In particular the frequency of pores containing a central granule decreased from 61 % in the larval to 36 % in the mature egg. Some new information on the mode of nucleocytoplasmic transit of nucleolus-derived material could be obtained from the lampbrush stage eggs. In this stage, fibrillar strands spinning out from the nucleolar periphery can be seen in contact with the nuclear pore complex, expecially with the inner annular granules. Furthermore, dense spheres of nucleolar material migrate through the central channel of the pore in a rodlike configuration in the very same mode that has been described for salivary glands of Chironomus thummi (69). Alternative relationships of nucleocytoplasmic RNP transport to the constituents of the nuclear pore complex are discussed.

The ultrastructure of the nuclear envelope of amphibian oocytes: a reinvestigation I. The mature oocyte

In order to review the contradictory statements on the ultrastructure of the nuclear envelope, a study was undertaken combining section and negative staining electron microscopy on manually isolated oocyte nuclei and nuclear envelopes from six amphibian species including Anura as well as Urodela. The appearance of the negatively stained isolated nuclear envelopes is described in detail and the dependence on the preparation conditions used is emphasized. Pore complex structures such as pore perimeter, central granule, annular components, internal fibrils, and annulus-attached fibrils could be identified by both techniques, negative staining and sections. Comparative studies show that no marked differences exist in the structural data of the nuclear envelope among the investigated amphibians and the significance of the structural components is discussed. A model of the nuclear pore complex based on the findings of the present investigation is presented.

Nuclear pore flow rate of ribosomal RNA and chain growth rate of its precursor during oogenesis of Xenopus laevis

Abstract The number of ribosomal RNA molecules which are transferred through an average nuclear pore complex per minute into the cytoplasm (nuclear pore flow rate, NPFR) during oocyte growth of Xenopus laevis is estimated. The NPFR calculations are based on determinations of the increase of cytoplasmic rRNA content during defined time intervals and of the total number of pore complexes in the respective oogenesis stages. In the mid-lampbrush stage (500–700 μm oocyte diameter) the NPFR is maximal with 2.62 rRNA molecules/pore/minute. Then it decreases to zero at the end of oogenesis. The nucleocytoplasmic RNA flow rates determined are compared with corresponding values of other cell types. The molecular weight of the rRNA precursor transcribed in the extrachromosomal nucleoli of Xenopus lampbrush stage oocytes is determined by acrylamide gel electrophoresis to be 2.5 × 106 daltons. From the temporal increase of cytoplasmic rRNA (3.8 μg per oocyte in 38 days) and the known number of simultaneously growing precursor molecules in the nucleus the chain growth rate of the 40 S precursor RNA is estimated to be 34 nucleotides per second.

Transcription of ribosomal RNA cistrons: Correlation of morphological and biochemical data

Abstract Electron microscopic spread preparations of oocyte nucleoli (lampbrush stage) of various amphibians are quantitatively evaluated and the length distributions of repeat-, matrix-, and spacer-units along the rRNA cistron containing axes are given. The correlation of the matrix unit data with the gel electrophoretic pattern of labelled nuclear RNA from the same oocytes is examined. The mean value of the matrix unit corresponds fairly well to a 2.6 million D peak of pre-rRNA but the distribution of both matrix units and labelled pre-rRNAs shows an asymmetrical heterogeneity indicating the existence of some larger primary transcription products of rDNA. Novel structural aspects are described in the spacer regions which suggest that transcription does also take place in DNP regions between the matrix units. A special ‘prelude piece’ coding for approx. 0.5 million D of RNA is frequently visualized in the spacer segments at the beginning of a matrix unit. Possible artifacts resulting from the preparation, the relative congruence between the data obtained using both methods, and the functional meaning of the findings are discussed against the background of current concepts of structural organization and transcription products of nucleolar DNA.

Nucleocytoplasmic translocation of RNA in Tetrahymena pyriformis and its inhibition by actinomycin D and cycloheximide

Abstract The nucleocytoplasmic translocation of RNA pulse-labelled with 3H-uridine was studied in logarithmically growing Tetrahymena pyriformis GL cells during a subsequent chase period, without or with actinomycin (AMD) or cycloheximide (CH), by light and electron microscopic autoradiography and analysis of RNA by electrophoresis, and in isolated cell fractions (nuclei, nuclear membranes, nuclear non-membrane bulk material, microsomes, and post-microsomal fractions). Kinetic changes in nucleocytoplasmic transfer of RNA as well as changes of RNA contents and fine structure brought about by AMD and CH are described. Both antibiotics applied at concentrations inhibitory to RNA synthesis effectively inhibited the nucleocytoplasmic translocation of pre-existing nuclear RNA and resulted in relative accumulation of stable RNA moieties, especially of precursors of rRNA, in the macronucleus. The marked stability of the apparent primary transcription product of the nucleoli (about 2.2 million D mol. wt) indicated that neither considerable processing nor degradation took place during 90 min of chase in the presence of both drags. RNA with the molecular weights of mature rRNA (1.32 and 0.7 million D) was not found in substantial amounts in purified macronuclei from normal cells but was detected in the nuclei from AMD-treated cells. AMD completely inhibited nucleocytoplasmic migration of pulse-labelled RNA, the onset of this event being correlated with the time needed for a complete inhibition of 3H-uridine incorporation at a particular drag concentration. With CH, however, even when applied at relatively high concentrations, a residual translocation amounting to 15–20% of that of controls was maintained throughout the chase period.

Annulate lamellae in plant cells: Formation during microsporogenesis and pollen development in Canna generalis Bailey

SummaryThe occurrence of stacked annulate lamellae is documented for a plant cell system, namely for pollen mother cells and developing pollen grains of Canna generalis. Their structural subarchitecture and relationship to endoplasmic reticulum (ER) and nuclear envelope cisternae is described in detail. The results demonstrate structural homology between plant and animal annulate lamellae and are compatible with, though do not prove, the view that annulate lamellar cisternae may originate as a degenerative form of endoplasmic reticulum.

Structural details of dictyosomal pores

Structural details of the dictyosomal pores in several plant cell types are described from tangential and cross sections of Golgi cisternae. Frequency distributions of the sizes of such Golgi pores are given and compared with the corresponding values of nuclear pores in the same cells. Golgi pore inner diameters are less homogeneously distributed and can be as small as 100 A or less. They are not simply cisternal holes, but are often associated with centrally located electron dense granules or rods and with inner pore filaments. This organization, which is very common in dictyosomal pores in plant and animal cells, has some similarities with the structural architecture of nuclear envelope and annulate lamellar pore complexes. The particulate material associated with the dictyosomal pores shows spatial and structural relationship to cytoplasmic ribosomes. Possible modes of Golgi pore formation and some consequences of these observations for interpretation of nuclear pore structures are discussed.

Natural segregation of nucleolar components in the course of a plant cell differentiation.

SummarySegregation of the nucleolar components is described in the differentiated nucleus of the generative cell in the growing Clivia and Lilium pollen tubes. This finding of a natural nucleolar segregation is discussed against the background of current views of the correlations of nucleolar morphology and transcriptional activity.

Entwicklung der Gametogonien in ektopisch transplantierten Gonaden beiTriturus

SummaryHomografts of gonads including fat-bodies show fusion of the fat-body with liver tissue. Thus, contact between gonad and liver is only indirect. The differentiation of the gametogonia follows the normal way of development. In case of ovary homografts auxocytes appear. Not later than 27 days after transplantation vascularization is reestablished.Homo- and autografts of gonads without fat-body show a quite different development of the gametogonia. When the gonads are broadly fused with liver tissue one notices karyolysis of the gametogonia nuclei within the gonad liver contact region already 7 days after transplantation. After 3–4 weeks the graft represents a cyst formed by connective tissue without any germ cells. Erythrocytes indicate vascularization. In the gonad partly fused with liver normal structure and mitosis in the gametogonia appear in that part of the transplant not attached to liver tissue. The present experiments suggest that the degeneration of germ cells is depending on their position to extragonadal tissue.ZusammenfassungNach homoplastischer Transplantation von larvalen Gonaden mit Pettkörper in die vordere Leibeshöhle wächst nur der Fettkörper an der Leber an, so daß die Gonade nur indirekt mit dem Wirtsgewebe verbunden ist. Die Differenzierung der Gametogonien folgt der Normogenese, bei Ovartransplantationen entwickeln sich Auxocyten. Nach spätestens 27 Tagen ist die Blutversorgung wiederhergestellt.Homo- und autoplastische Transplantationen von Gonaden ohne Fettkörper ergeben für die Gametogonien eine völlig andere Entwicklung. Sind die Gonaden mit breiter Fläche angewachsen, läßt sich bereits 7 Tage p.o. im Bereich der Kontaktzone Gonade-Leber die Karyolyse der Gametogonienkerne feststellen. Nach 3–4 Wochen stellt das Transplantat eine bindegewebige Zyste ohne Geschlechtszellen dar. Erythrozyten zeigen die Vaskularisation an. Ist nur ein Teil der Gonade mit der Leber verwachsen, zeigt der frei gebliebene Abschnitt eine normale Struktur mit Mitosen der Gametogonien. Die Degeneration der Geschlechtszellen hängt offenbar von ihrer Lage zum extragonadalen Gewebe ab.Homografts of gonads including fat-bodies show fusion of the fat-body with liver tissue. Thus, contact between gonad and liver is only indirect. The differentiation of the gametogonia follows the normal way of development. In case of ovary homografts auxocytes appear. Not later than 27 days after transplantation vascularization is reestablished.Homo- and autografts of gonads without fat-body show a quite different development of the gametogonia. When the gonads are broadly fused with liver tissue one notices karyolysis of the gametogonia nuclei within the gonad liver contact region already 7 days after transplantation. After 3-4 weeks the graft represents a cyst formed by connective tissue without any germ cells. Erythrocytes indicate vascularization. In the gonad partly fused with liver normal structure and mitosis in the gametogonia appear in that part of the transplant not attached to liver tissue. The present experiments suggest that the degeneration of germ cells is depending on their position to extragonadal tissue.