A CORRELATED LIGHT AND ELECTRON MICROSCOPE STUDY OF THE NUCLEOLAR MATERIAL DURING MITOSIS IN VICIA FABA

Abstract

Root meristematic cells of Viola faba were examined, with both light and electron microscopes, in order to study the behaviour of the nucleolar material during the mitotic process. Under light microscopy, the preprophase nucleolus is seen to consist of a densely stained material in which are embedded several unstained vacuole-like structures of varying size. The electron microscope reveals that the dense nucleolar material is formed of two struc turally distinct components, each segregated into irregularly shaped zones blending with one another. One of these components is represented by 150 A granules which, in places, are arranged into thread-like structures approximately 0.1 # in diameter; the other component apparently consists of fibrils 60 to 100 A in diameter. The large and medium sized intranucleolar vacuoles contain loosely scattered granules and fibrils similar to those just described. The granular and fibrillar components of the denser portion of the nucleolus persist as such during prophase and disperse throughout the nuclear cavity at the time of nucleolar disintegration. After nuclear membrane breakdown, these granules and fibrils, as well as those of the nucleoplasm, mix freely with similar elements already present within the forming spindle. No evidence has been obtained that, during or after nucleolar disintegration, the structural components of the nucleolus become associated as such with the chromosomes to form an external or internal matrix. Our observations suggest the existence, of a matrix substance within late prophase, metaphase, and anaphase chromosomes, the fine structure of which bears strong resemblance to that of their constituent coiled chromonemata. Data are presented, moreover, that indicate that part of this matrix substance, presumably formed at some time during prophase, is released from the chromosomes during their anaphasic movement. A number of observations indicate that the main bulk of the next nucleolus is derived from a prenucleolar fibrillogranular material, arranged into thread-like structures some 0.1 # in diameter, which collect in the interchromosomal spaces during early and midtelophase. Finally, our data would seem to favour the view that most of this prenucleolar material results from a resumption of the synthetic activity of the early and midtelophase chromosomes rather than from a mere shedding of a preexisting matrix substance.

The Distribution of the Water-Soluble Inorganic Orthophosphate Ions Within the Cell: Accumulation in the Nucleus

“…Regarding this subject, it is interesting to remark that dense electron-opaque inclusions-the "foamy particles"-have been reported in the nucleoli of some cells . Particularly striking are the cases of onion and broad bean root tip cells, in which these "particles" were reported by Lafontaine (36) ; in later work, he could not find them (37) . An examination of his papers reveals that these "particles" were found in those cases in which the plants were grown in tissue medium culture, which contains relatively large amounts of divalent cations (e .g ., calcium) .…”

The ultrastructure of the nuclear division of Basidiobolus ranarum Eidam

“…The large elliptical nucleolus (IO-I5 -um long and 5-7/um in diameter) (Figures 2, 3, 8, and 9) is similar in sub structure to nucleoli of other plant cell types (Lafontaine and Chouinard, 1963;Chouinard and Leblond, 196? ;Lord and Lafontaine, 1969).…”

“…;Lord and Lafontaine, 1969). At the fine structure level, the nucleolus consists of three distinct structural components : a granular zone made up of dense particles 15-20 nm in diame ter; a fibrillar zone intermingled with the granular zone and consisting predominantly of densely packed fibrillar material 6-8 nm in diameter ( Figures 2 and 6); and a third zone, the proteinaceous matrix, containing lightly stained amorphous granules ( Figure 6) (Hay, 1968i, The proteinaceous matrix also appears identical with the nucleolar vacuole described by Robinow (1963) with the light microscope and "by Lafontaine and Chouinard (1963) with the electron micro scope. Less dense areas are found in the fibrillar zone and correspond to the DNA-containing lacunae described in nucleoli of Allium cepa (Chouinard, 1966al.…”

“…Prophase nucleoli in D. ranarum behave much like those in grasshopper neuroblasts (Stevens, 1965) and in Vicia faba (Lafontaine and Chouinard, 1963). The pars granulosa and the pars fibrosa disperse into 15-20 nm nucleolar granules and 6-8 nm nucleolar fibrils respectively.…”

Mitosis in a Green AlgaTriploceras GracileBailey with Particular Reference to the Behaviour of Nuclear Envelope

“…Nucleolar organising chromosomes were reported in Spirogyra (GonwARD 1954), Micrasterias (W ARIS 1950) and in some desmids (KING 1959;BRANDHAM 1965). A number of workers have presented data suggesting that the nucleolar material produced over the surface of the anaphase or telophase chromosomes simply collected at the nucleolar sites to form a nucleolus (TANDLER 1959;DAs and ALFERT 1959;DAs 1962;and LAFONTAINE and CHOUINARD 1963). From the present findings it seems likely that in Triploceras gracile, most of the chromosomes have the ability to produce the prenucleolar material over their surface in early telophase, which was later collected during the growth of the nucleolus.…”