Nucleolar structure during the meiotic prophase in Allium cepa anthers

Three structures associated with the nucleolus in male rat germinal cells: round body, coiled body, and "nubecula" and general presence of round body at male meiosis

In addition to chromosomes and nucleoli, three structures, i.e., round body, coiled body, and nubecula, are encountered in the nucleus during the meiotic prophase in male rats. These structures have been examined by electron microscopy in random and serial sections. The round body is a finely fibrillar, proteinaceous structure closely associated with the granular component of a nucleolus in rat spermatocytes and young spermatids. A similar structure has been observed in man, the monkey Macaca mulatta, the gastropod Achatina fulica, and the insect Locusta migratoria. Together with evidence from the literature, these results support the view that the round body is of general occurrence in the male meiocytes of eukaryotes and may, therefore, play a role in meiosis. The coiled body is a group of electron-dense elements called "coils", which average 35 nm in width, except after mid-pachytene when their size almost doubles. The coils are composed of 2-nm-wide filaments and 8 to 10-nm-wide granules, both of which are ribonucleoprotein. The coiled bodies are interpreted to be groups of "spliceosomes", that is, structures containing heterogeneous RNA and small nuclear RNA. A remarkable feature of the coiled body is its temporary disappearance at early pachytene and its reappearance at late pachytene, possibly due to drastic changes in the turnover rate of its component RNAs. The nubecula is a newly identified nuclear inclusion, composed of weakly staining threads loosely organized into a 560 nm-wide spheroid. It has been observed only in early pachytene nuclei.

Structure, development, and cytochemical properties of the nucleolus-associated "round body" in rat spermatocytes and early spermatids

The "round body," a spherical structure typically associated with a nucleolus in male germ cells of the rat, has been examined in the electron microscope using routine and cytochemical methods to determine its structure, composition, and mode of development. Cytochemical analysis indicates that the round body includes neither nucleic acid nor lipid, but is composed of nonhistone protein which appears in the form of 1.6-nm-wide fibrils. Development begins in late leptotene, when a single round body appears in each spermatocyte as an irregular spheroid located along the inner surface of the nuclear envelope. During subsequent stages of the meiotic prophase, the round body leaves the nuclear envelope, becomes a regular sphere, and gradually enlarges from a diameter of 0.4 micron in leptotene to 1.6 micron in diplotene. Concurrently, lacunae appear within its substance and enlarge. At each maturation division, the amount of round-body material is decreased by about half, presumably because the constituent proteins are dissociated at metaphase, distributed between the two daughter cells at telophase, and reconstituted into half-sized round bodies. As spermiogenesis proceeds, the round body shrinks gradually and disappears at step 8. Soon after its appearance at leptotene, the round body becomes associated with and is surrounded by the pars granulosa of one of the nucleoli. Moreover, 3H-uridine incorporation into nucleolar RNA is high as long as the size of the round body increases, but is low or absent when it decreases. It is possible, therefore, that the round body exerts some control on nucleolar activity in meiotic cells.

Position of the nucleolus within the nuclei of pachytene spermatocytes of Dromiciops australis and Marmosa elegans (Didelphoidea-Marsupialia)

The location of the nucleoli within the nuclei of pachytene spermatocytes, and their relation with the position of the nucleolar organizer region (NOR) was studied. It appears that a terminal NOR determines a peripheral location of the nucleolus, due to the position of the NOR over the synaptonemal complex and to the attachment of the nucleolar chromosome telomeres at the nuclear membrane.

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.

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

Segregation 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.