Struktur und Verhalten der Nukleolen von H�hnerherzmyoblasten in Gewebekultur w�hrend des Interphasewachstums und der Mitose

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 These rounded skeins are tentatively assumed to represent the withdrawn organizers.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 establishment of polarity and bilateral symmetry in the oocytes of Gryllus domesticus L.]

1. The origin of polarity and bilateral symmetry of oocytes inGryllus domesticus L. has been studied. 2. The histology of the ovary and the process of oogenesis are described. 3. The germinal vesicle shifts into the posterior half of the oocyte, when these cells are 60 to 100 μm in length, thus indicating for the first time oocytic anteroposterior polarity. 4. Bilateral symmetry first appears, when the follicle reaches a length of 450 to 550 μm. This change from radial to bilateral symmetry is the result of the germinal vesicle leaving its position in the long axis of the cell and moving perpendicularly to this axis towards the follicular epithelium. 5. In follicles 60 to 100 μm in length the nucleolus comes to lie, under the influence of gravity, in the geopetal quarter of the germinal vesicle, and remains there. 6. No correlation can be found between geopetal position of the nucleolus and bilateral symmetry of the oocyte. 7. Oogenesis and the establishment of axes in the oocytes are not affected by centrifuging adults and last instars. 8. When the oocyte nucleus touches the follicular epithelium, changes occur in the structure and function of some epithelial cells. Those close to the germinal vesicle, with flattened nuclei, show reduced yolk secretion. Those at the opposite side show enhanced secretory activity. Presumably as a result of this differential secretory activity the shape of the oocyte becomes curved. 9. Only intact vitellogenetic oocytes show a banana-like form. 10. The ultrastructural examination of the previtellogenetic follicle indicates no predisposition for the direction of displacement of the oocyte nucleus, and therefore no predisposition of the plane of bilateral symmetry. 11. The hypothesis is discussed that the germinal vesicle itself initiates and directs the appearance of bilateral symmetry in the oocyte.