Abstract. To investigate the spatial relationship between the nucleus and the cortical division site, epidermal cells were selected in which the separation between these two areas is large. Avoiding enzyme treatment and air drying, Datura stramonium cells were labeled with antitubulin antibodies and the threedimensional aspect of the cytoskeletons was reconstructed using computer-aided optical sectioning.In vacuolated cells preparing for division, the nucleus migrates into the center of the cell, suspended by transvacuolar strands. These strands are now shown to contain continuous bundles of microtubules which bridge the nucleus to the cortex. These nucleus-radiating microtubules adopt different configurations in cells of different shape. In elongated cells with more or less parallel side walls, oblique strands radiating from the nucleus to the long side walls are presumably unstable, for they are progressively realigned into a transverse disc (the phragmosome) as broad, cortical, preprophase bands (PPBs) become tighter. The phragmosome and the PPB are both known predictors of the division plane and our observations indicate that they align simultaneously in elongated epidermal cells. These observations suggest another hypothesis: that the PPB may contain microtubules polymerized from the nuclear surface. In elongated cells, the majority of the radiating microtubules, therefore, come to anchor the nucleus in the transverse plane, consistent with the observed tendency of such cells to divide perpendicular to the long axis.In nonrectangular isodiametric epidermal cells, which approximate regular hexagons in section, the radial microtubular strands emanating from the nucleus tend to remain associated with the middle of each subtending cell wall. The strands are not reorganized into a single dominant transverse bar, but remain as a starlike array until mitosis. PPBs in these cells are not as tight; they may only be a sparse accumulation of microtubules, even forming along nondiametrical radii. This arrangement is consistent with the irregular division patterns observed in epidermal mosaics of isodiametric D. stramonium cells.The various conformations of the radial strands can be modeled by springs held in two-dimensional hexagonal frames, and by soap bubbles in three-dimensional hexagonal frames, suggesting that the division plane may, by analogy, be selected by minimal path criteria. Such behavior offers a cytoplasmic explanation of long-standing empirically derived "rules" which state that the new cell wall tends to meet the maternal wall at right angles. The radial premitotic strands and their analogues avoid taking the longer path to the vertex of an angle where a cross wall is already present between neighboring cells. The resultant tendency to form three-rayed vertices rather than intercrossing septa helps explain the hexagonal packing of plant cells.
There are two conflicting ideas about the site of reassembly of cortical microtubules following cytokinesis. Some observations indicate that microtubules (MTs) radiate from the surface of the postcytokinetic nuclear envelope, before becoming organized at the cortex. On the other hand, results of regrowth experiments, following MT depolymerization by drugs, suggest that the array may assemble directly upon the cortex. In this study, we have taken advantage of the significant separation between nucleus and cortex, in large, vacuolated epidermal cells, to determine which of these two potential sites supports the earliest stages of regrowth in the undrugged, native state. MTs in stem epidermis of Datura stramonium L. were stained by indirect immunofluorescence. This was performed on hand-cut sections of tissue in which the cells were not separated by enzymes or distorted by air- drying. Epidermal cells with these sheets were optically sectioned by confocal laser scanning microscopy and three-dimensional images reconstructed, rotated and viewed stereoscopically using computer methods. During metaphase, no MTs can be detected at the cortex but MTs begin to re-colonize the cell surface during early cytokinesis. Thick cables of MTs radiate from the nucleus parallel to the cell plate as well as in other directions, along transvacuolar strands, out to the cortex. Microtubules grow out over the cortex where the thick bundles make contact, as well as from the edges of the fully developed phragmoplast. These early cortical MTs do not form regular transverse arrays: they either appear to be random or to grow in branching V- shaped patterns. The cortical array is therefore not organized immediately but at a later stage. It is concluded that MT bundles, radiating from the nucleus, are involved in the earliest stages of cortical array formation.
From immunofluorescence microscopy it has been cortical microtubules form whole-cell arrays. This has clearly seen in cylindrical hairs where the existence testifies to the continuity of the array around the cell. It is not, however, clear how microtubules pack polyhedral cells with multiple, angled facets. In problem, elongated and isodiametric cells in the stramonium L. were subjected to anti-tubulin avoiding distortion by cellulase treatment and air- sections were then deblurred by computer, the digitized, reconstructed and then rotated in order to arrangement of microtubules along the anticlinal walls This established several things. Microtubules tend to any one cell face; they form transverse, oblique or except that some walls bear a crisscross arrangement. cells, microtubules clearly form helices. In the cells, transversely wound microtubules are confirmed continuous from one face to another and probably, constitute helices. Microtubules on oblique end walls continue onto the side walls and do not form a microtubules can be ordered upon two adjacent facets, with respect to the stem's axis need not necessarily both facets, i.e. overall alignment can change at the isodiametric epidermal cells, microtubules can one cell facet to another. However, where microtubules anticlinal walls spill over onto a periclinal wall at a crisscross arrangement is set up. This is attributed geometrical problem of fitting parallel lines around polyhedra. Despite crossing over one another, the walls are nevertheless continuous with MTs on the side conclusion, in elongated cells the arrays still various pitch: in isodiametric cells (where the walls non-orthogonal angles to one another) the integrity of appears to be preserved by microtubules crossing over what is termed a ‘sacrificial’ face. The overriding microtubules to form an integral array regardless of
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