Gamete Structure and Fertilization in the Barents Sea Sponge Leucosolenia complicata

The ultrastructure of male and female gametes of asconoid sponge Leucosolenia complicata(Calcispongiae, Calcaronea), a hermaphrodite species that reproduces in autumn, is described. The mature sponge's oocytes were up to 70 m in diameter, had no coatings, and contained a nucleus about 31 m in diameter with large nucleoli (up to 6.6 m). There were vacuoles with fibrillar contents typical of calcareous sponges in ooplasm. During vitellogenesis, a cluster of a great number of nurse cells developed above each oocyte from transformed choanocytes. Mature spermia of L. complicatalooked like orbicular cells about 2.5 m in diameter, with no acrosome or tail. The spermium nucleus (diameter about 2.2 m) was formed by incompletely condensed chromatin and was surrounded with a thin layer of cytoplasm of nonuniform thickness. In the thick layer of cytoplasm beyond the ribosomes, there were two or three mitochondria, dictyosomes, and electron-dense protein bodies lying freely under the nucleus. Fertilization occurred with the aid of a carrier cell. During spawning (mass release of spermia), any nurse cell complex can seize a spermium and transform into a carrier cell in situ. The transformation of a seized spermium into a spermiocyst was connected with the rapid isolation of the spermium nucleus from the protein body. Fertilization began with the penetration of the protein body into the oocyte cytoplasm. Only after this did the spermium's nucleus penetrate into the oocyte.     

γ-Tubulin in tobacco pollen tubes: association with generative cell and vegetative microtubules

-Tubulin was localized in tobacco pollen tubes using an antibody raised against a peptide conserved in all known -tubulins. Antibody staining occurs in a primarily punctate pattern along the length of the microtubule bundles in generative cells and along cortical microtubules in the vegetative cytoplasm. During generative cell division, -tubulin is localized in the forming mitotic apparatus. By metaphase, it is present along kinetochore fibers except at their plus ends located at the kinetochores. By telophase, staining is observed in the phragmoplast, where it again avoids the plus ends of microtubules at the cell plate. -Tubulin is also present at the periphery of the sperm nuclei. A patch of intense staining on the distal side of each nucleus marks the site of assembly of a new population of sperm microtubules. No specific fluorescence is present in control pollen tubes treated with preimmune IgG. These localization patterns bear similarities to those seen in somatic cells and in addition may help explain changes in microtubule arrays between generative cells and sperm.     

Transformation of the Golgi apparatus in the cell cycle of a green alga,Micrasterias americana

Summary Transformation of the Golgi apparatus in the stages of the cell cycle ofMicrasterias americana was investigated with an electron microscope. In the resting cell, dictyosomes consisted of eleven cisternae, the distal-most cisterna of which was partially network-shaped. During the developmental stages of the new half cell followed by nuclear division, dictyosomes produced dark vesicles and large vesicles at the peripheries of the distal cisternae, thus diminishing the diameter of the distal cisternae and their numbers. Finally, dictyosomes with six or seven cisternae of small diameter were found when the new half cell reached to full size as a mother half cell. At this stage, the small dictyosomes produced flat vesicles. Thereafter, dictyosomes recovered the size and number of their constituent cisternae, being supplied a membrane and substrate from the primary vesicles. Lysosomes divided the dictyosomes in two, and these divided dictyosomes seemed to regenerate in one case and to disorganize in another. The occurrence of large vesicles was also confirmed using the negative staining method in growing cells.     

Nuclear ploidy is contingent on the microtubular cycle responsible for plant cytokinesis

Summary. Division of the plant cell relies on the preprophase band of microtubules (PPB)-phragmoplast system. Cells of onion (Allium cepa L.) root meristems were rendered binucleate by preventing the consolidation of cell plate formation in telophase with 5mM caffeine. These binucleates developed either a single PPB around one of their two nuclei or two PPBs, one per nucleus, in the prophase of the ensuing mitosis. Prophase cells developing one single PPB were shorter in length (42.3±4.1µm) than those developing 2 PPBs (49.8±4.1µm), and interphase duration was inversely related to cell length. Cells whose length was less than or equal to 42µm, i.e., which had not even reached the mean size of the small binucleates in prophase, were followed throughout mitosis. In metaphase, they always assembled two mitotic spindles (one per nucleus). However, the cells that had assembled a single PPB also developed a single phragmoplast in telophase, leading to polyploidization. As these meristematic cells were not wide enough to accommodate the midzones of both mitotic spindles in any single plane transversal to the cell elongation axis, the spindles tilted until their midzones formed a continuum where the single common phragmoplast assembled. Its position was thereby uncoupled from that of the preceding PPB. Subsequently, the chromosomes from two different half-spindles were included, by a common nuclear envelope, in a single tetraploid nucleus. Finally, the cytokinetic plate segregated the two tetraploid nuclei formed at each side of the phragmoplast into two independent sister cells.Correspondence and reprints: Centro de Investigaciones Biológicas, CSIC, Ramiro de Maeztu 9, 28040 Madrid, Spain.     

Synaptic junction development in the spinal cord of an amphibian embryo: An electron microscope study

Summary An electron microscopical study has been made of the cervical spinal cord of Xenopus laevis embryos, from the time that the neural tube closes until the larvae were hatched and could swim. Sections of the whole cord were searched for intercellular junctions during this period. Two nonsynaptic types were found, the first were widely distributed puncta adherentia, the second were rare and similar to gap junctions. Membrane specializations with synaptic vesicles were first found when the neural folds had fused; membrane-vesicle clusters which looked like the presynaptic half of a synaptic junction were present, together with synaptic junctions lacking any postsynaptic membrane thickening or cytoplasm density. About four hours later, mature synaptic junctions with full thickening of the postsynaptic membrane, dense cytoplasm and striated or dense material in the synaptic cleft were present. Presynaptic mitochondria, dense-cored and flattened vesicles, fibre to fibre and fibre to cell body synapses were present from the first, as were synapses onto very fine dendrites which might be filopodia from dendritic growth cones. Synaptogenesis may start with the accumulation of vesicles in dense cytoplasm near a thickened cell membrane; the postsynaptic element becomes associated with this membrane-vesicle cluster and matures by increasing cleft and cytoplasmic density, and by membrane thickening.     

Nuclear behavior in multinucleate Schizophyllum commune germlings

Summary The ultrastructure of a fir morphological mutant containing multinucleate cells is described in Schizophyllum commune. The germlings of basidiospores which arose from mating fir with wild-type mycelium were studied in culture by phase contrast microscopy to elucidate behavior of multinucleate cells. Nuclear division appeared synchronous from two nuclei yielding four progeny through six nuclei producing twelve products, beyond which loss of synchrony was indicated. Compensatory nuclear migration into an anucleate cell was presumed during synchronous division of nuclear aggregates in the adjacent cell of an individual germling. The migrant nucleus eventually returned to the cell of origin. However, the return route was not via the central pore of the septum but rather occurred at the juncture of the cross-wall with the germling-periphery. Ultrastructure of a partial septum in fir which could accommodate nuclear passage of this sort is described.     

Ribonucleic acid transfer from nucleus to cytoplasm during interphase and mitosis in mouse somatic cells cultured in vitro

Summary Kidney cells from primary cultures of 15-day old mouse embryos were incubated for 2, 5 or 10 min with H³-uridine, then either fixed immediately or incubated again for various periods in a chase medium containing an excess of unlabeled uridine and cytidine. The number of grains over the non-nucleolar part of the nucleus (chromatin), the nucleolus and the cytoplasm were counted on the autoradiograms.The grain count showed that both chromatin and nucleolus incorporate very rapidly H³-uridine from the medium, whereas a time lag elapses before any H³-radioactivity above background is detected in the cytoplasm. Incorporation of H³-uridine into the RNA of the nucleus and the nucleolus is not immediately blocked after chase, suggesting that the labeled precursor pool is not completely washed out from the living cell, or diluted by the excess of unlabeled uridine present in the medium. The grain count over the nucleus and the nucleolus rises for a certain time after chase and then gradually declines; H³-radioactivity appears in the cytoplasm 10 min after chase and keeps rising through a 110-min interval. The experiment, then — even though it suggests that the bulk of cellular RNA is synthesized in the chromatin and the nucleolus and then continuously released into the cytoplasm — does not rule out the possibility that some RNA fraction, characterized by a low turnover rate, is synthesized independently in the cytoplasm.Synthesis of RNA is a continuous process throughout the cell cycle, except during metaphase and anaphase. It ceases at prometaphase after the disappearance of the nucleolus and disintegration of the nuclear membrane, and resumes in early telophase. Part of the chromosomal RNA does not remain associated with the chromosomes through division, but is suddenly released into the cytoplasm when the cell enters metaphase.     

Ultrastructural aspects of cell wall synthesis inSpirogyra

Summary Filaments ofSpirogyra were fixed in 2% osmium tetroxide dehydrated in alcohol and embedded in Araldite. The fine structure of cells with regard to wall synthesis was studied. The cell wall was shown to have four layers. The inner one contains microfibrils and is considered to be the cell wall proper. The outer three layers are components of the slime layer. The innermost of these, the second layer of the wall, was shown to be between 1m to 3m and the third 0.3m to 1m. The fourth layer appears as no more than a dark black line measuring 10 nm across. In the cytoplasm two types of vesicles were seen. The largest of these has contents similar in appearance to the slime layer of the wall. This same material was also seen in the large vesicles attached to the Golgi bodies. It is suggested that the smaller vesicles are derived from the larger vesicles and later fuse with the cell membrane. The Golgi bodies were found to be fairly large measuring up to 5m across. Small electron opaque blobs and flecks on the outside of the plasmalemma and in between the microfibrils of the cell wall proper are considered to be mucilage droplets travelling to the slime layer. It cannot be excluded that some of the material of the large vesicles is released directly into the cytoplasm and is transferred without vesicles through the plasma membrane. The negative contrast appearance of the microfibrils seen in the cell wall is thought to be due to the spaces between them being filled with this electron opaque mucilage.Intercisternal rodlets measuring 2.5 nm across were seen in the Golgi bodies.Transverse microtubules were found to occur near the plasmalemma having the same orientation as some of the microfibrils.Lomasome-like structures sometimes with many 5 nm fibrils in their vicinity were seen.     

Growth,acetylene reduction activity and localization of nitrogenase in relation to vesicle formation in Frankia strains Cc1.17 and Cp1.2

A comparative study was conducted on the effect of NH₄Cl on growth, vesicle formation and formation of nitrogenase of Frankia strains Cc1.17 and Cp1.2, derived from root nodules of Colletia cruciata and Comptonia peregrina, respectively. On a medium without combined nitrogen (P-N), both strains formed spherical cells, called vesicles, like many other Frankia strains. Data are presented on the number of vesicles per mg protein, after cultivation in media with sodium propionate as C-source without combined nitrogen (P-N) or with 0.2 g NH₄Cl/l (P+N). Strain Cp1.2 as may other Frankia strains, showed on P+N medium a very strong reduction of vesicle formation of 99% relative to the number of vesicles formed on P-N medium, after 11 days growth. However, in strain Cc11.17 this reduction was only 70%. The occurence of relatively large numbers of vesicles in P+N media has not yet been reported for other Frankia strains. No acetylene reduction activity was found in NH ₄ ⁺ -grown cells. The regulation of induction of nitrogenase in Frankia by NH₄Cl was tested by immuno-gelectrophoresis using antisera against nitrogenase of Rhizobium leguminosarum PRE. The component I of the enzyme showed crossreactivity while the component II had only a weak crossreaction. The experiments indicated that no nitrogenase was detectable in the NH ₄ ⁺ -grown cells. For the localization of nitrogenase, relative amounts of the enzyme were compared in whole cells and vesicle-enriched fractions. Western blots showed a significant enrichment of nitrogenase in the vesicle fractions, which indicated that most of the nitrogenase was localized in the vesicle.     

Gamma-tubulin colocalizes with microtubule arrays and tubulin paracrystals in dividing vegetative cells of higher plants

Summary The distribution of -tubulin throughout cell division is studied in several taxa of higher plants. -Tubulin is present along the whole length of microtubules (Mts) in every cell stage-specific Mt array such as the preprophase band, the preprophase-prophase perinuclear Mts, the kinetochore Mt bundles, the phragmoplast, and the telophase-interphase transition Mt arrays. -Tubulin follows with precision the Mt pattern, being absent from any other, Mt-free, cell site. In cells treated with anti-Mt drugs, -tubulin is present only on degrading or on reappearing Mt arrays, while it is totally absent from cells devoid of Mts. -Tubulin is also present in tubulin paracrystals, which are formed in colchicine-treated cells. These observations support the view that in higher plants -tubulin may not be a microtubule-organizing-center-specific protein, but it may play a certain structural and/or functional role being related to - and -tubulin.Abbreviations Mt microtubule - MTOC microtubule-organizing center - PPB preprophase band     

Further cytogenetic analyses of the Croatian triploid shallot “Ljutika” (Allium cepa var.viviparum, Alliaceae) and its comparison with the Indian triploid “Pran”

Feulgen and silver-stained karyotypes and meiosis of two triploid viviparous onion forms (Allium cepa var.viviparum), the Croatian Ljutika and the Indian Pran, were comparatively analyzed. The results of chromosome measurements show that Ljutika and Pran are karyologically not identical, although significant similarities were found in the morphology of their chromosomes. Five geographically distant clones of Ljutika showed good agreement in the number and gross morphology of the chromosomes and in the number and position of NORs and interphase nucleoli. Heterotrivalents were predominant in meiosis of Ljutika but a relatively high frequency of higher multivalents together with univalents and bivalents were also observed. The relationship between Ljutika and Pran and their possible origin are discussed.     

A cytokinin-sensitive mutant of the moss,Physcomitrella patens,defective in chloroplast division

Summary An X-ray induced mutant (PC22) of the moss,Physcomitrella patens was analysed with respect to its morphology, physiology and suitability for microculture techniques. The mutant protonemata are defective in bud formation and in chloroplast division. As a consequence of the latter, giant chloroplasts are formed which disturb the development of the phragmoplast, the formation of regular cross walls, and cell division. Abnormal cross walls are rich in callose. The actin cytoskeleton was found to be less regularly developed in the mutant than in the wild type. Three-dimensional analysis of the microtubular arrangement with confocal laser scan microscopy demonstrates a close association between spindle- or phragmoplast- and interphase-microtubules. The deficiencies in chloroplast division and in bud formation can partly be compensated for by exogeneously applied cytokinin. The suitability of this particular developmental mutant for further studies was shown by regeneration of protoplasts in microculture and microinjection of the fluorochrome Lucifer yellow into the chloroplast.Abbreviations CLSM confocal laser scan microscope - DAPI diamidinophenyl indole - DiOC 3,3-dihexyloxacarbocyanine iodide - EGTA ethylene glycol-bis-(-amino-ethylether-N,N,N,N-tetraacetic acid - i⁶Ade N⁶-(²-isopentenyladenine) - PIPES piperazine-N, N-bis-2-ethanesulfonic acid - ptDNA chloroplast DNA Devoted to the memory of Prof. Dr. O. Kiermayer, our colleague and friend.     

Ultrastructural analysis ofSpinacia oleracea sperm cells isolated from mature pollen grains

Summary In isolated condition, the sperm cells ofSpinacia oleracea are no longer arranged in pairs as in the pollen grain. The vegetative membrane, which surrounds a sperm cell pair in a mature pollen grain, is lost during the isolation procedure. The sperm cells become spherical in shape.The isolated sperm cell is surrounded by an intact plasma membrane. The heterochromatic or euchromatic sperm cell nucleus is located in the cell center. Mitochondria are round to oval and have distinct cristae. Often they are clustered in groups of 5 to 10 mitochondria. Dictyosomes are present in the cytoplasm and consist of 4 to 5 cisterns. Endoplasmatic reticulum is mostly situated at the sperm cell periphery, as single cisterns very near the plasma membrane.From diameters of sectioned sperm cells in electron micrographs, it is possible to calculate the average diameter of the whole sperm cell. This average diameter is 3.66 m with a variation of 3.0 m to 4.2 m, resulting in an average volume of 25.6 m³. The nuclear volume is 12.8 m³ (50.0% of the whole cell) and the mitochondrial volume is 0.7 m³ (2.5% of the whole cell). The frequency distribution of the isolated sperm cells diameters shows only one peak with a normal distribution, indicating that there is no dimorphism in volume.     

The spermatozoon of Oikopleura dioica Fol (Larvacea,Tunicata)

Summary The spermatozoon of Oikopleura dioica is about 30 m long, with a spherical head, about 1 m wide, a 3 m long and 1 m wide midpiece, and a 25 m long tail with a tapered end piece. The head contains a nucleus with the chromatin volume limited to about 0.1 m³. A small acrosome is found in an anterior inpocketing, and a flagellar basal body in a posterior inpocketing of the nucleus. The midpiece contains a single mitochondrion with the flagellar axoneme embedded in a groove along its medial surface. The flagellar axoneme has the typical 9 + 2 substructure, and the basal body the typical 9+0 substructure. A second centriole and special anchoring fibres are absent.     

Ultrastructural localization of cellular compartments involved in secretion of the low molecular weight,alkaline xylanase by Trichoderma reesei

The intracellular location of the low-molecular weight, alkaline xylanase (XYN II) of Trichoderma reesei RUT C-30 was investigated during growth on xylan, using immunoelectron microscopy. A monoclonal antibody, produced against XYN II, was used for this purpose. The enzyme was found at the endoplasmic reticulum and in electron dense 0.2 to 0.8 m vesicles, as well as in the vacuole, at the plasma membrane and in the fungal cell-wall. No staining occured in the cytoplasm, the mitochondria and the nucleus. No Golgi-like structures could be seen. Addition of the carboxylic ionophore monensin blocked xylanase as well as total protein secretion. The results are discussed with respect to XYN II being secreted by T. reesei via a pathway involving the endoplasmic reticulum and secretory vesicles and/or the vacuole.     

Inhibitory effect of menaquinone-7 (vitamin K2) on osteoclast-like cell formation and osteoclastic bone resorption in rat bone tissues in vitro

Heme oxygenase1, the major inducible isoform of heme oxygenase (HO), can be induced by heme and numerous other physical and chemical factors, many of which cause cellular stress. This has led to the realization that HO1 is a major highly conserved stress or heat shock protein. Recent work has implicated activation of mitogenactivated protein kinases and other kinases in the mechanism of induction of HO1, and suggested that signal transduction pathways through tyrosine kinases are involved in induction of HO1 gene expression by stress inducers. We hypothesized that phenylarsine oxide (PAO), an inhibitor of protein tyrosine phosphatases (PTPs), might up-regulate the HO1 gene. Here, we show that a remarkably brief (1–15 min) exposure of normal hepatocytes to low concentrations (0.5–3 M) of PAO produces a marked increase in mRNA and protein of HO1. This increase is comparable to the level obtained by addition of heme (20 M), and occurs without producing changes in cellular glutathione levels or stabilization of HO1 message. Preincubation of cells with inhibitors of protein synthesis decreased the ability of PAO to increase levels of HO1 mRNA, suggesting that the inductive effect requires de novo protein synthesis. Addition of thiol donors abrogated the PAOmediated induction of HO1 in a dose dependent fashion. Addition of genistein, a tyrosine kinase inhibitor, blunted the induction produced by both PAO and heme. After brief incubations with PAO or heme, cell extracts showed comparable increases in levels of protein tyrosine phosphorylation in general, and specifically in ZAP70 kinase. Our results are consistent with the proposition that induction of HO1 by PAO involves inhibition of specific PTP(s), and that the mechanisms of induction of HO1 by PAO and by heme may share some common pathways.     

Three-dimensional organization of the plasmalemmal vesicular system in directly frozen capillaries of the rete mirabile in the swim bladder of the eel

Summary Several recent studies comparing chemically fixed and cryofixed endothelium have indicated that glutaraldehyde fixation may result in increases in the population of vesicles in the cytoplasm. Other reports based on ultrathin serial-section reconstruction of chemically fixed endothelium have revealed that the vesicular system is comprised of interconnected membranous compartments, which are ultimately continuous with either cell surface but do not extend across the endothelial cell. In this study, we have investigated the three-dimensional organization of the vesicular system in directly frozen, freeze-substituted capillaries of the rete mirabile from the swim bladder of the eel, specifically using the same block of embedded capillaries in which frozen capillaries had previously been found to contain less vesicles than chemically fixed capillaries. The results show that essentially all vesicles remain inter-connected with each other and are part of two separate sets of invaginations from the luminal and abluminal cell surface like in chemically fixed tissue. Any increase in vesicle number resulting from glutaraldehyde fixation does not affect the overall three-dimensional organization of the vesicular system in these endothelial cells.     

Plasma membrane-cell wall connections: Roles in mitosis and cytokinesis revealed by plasmolysis ofTradescantia virginiana leaf epidermal cells

Summary Tradescantia virginiana leaf epidermal cells were plasmolysed by sequential treatment with 0.8 M and 0.3 M sucrose. Plasmolysis revealed adhesion of the plasma membrane to the cell wall at sites coinciding with cytoskeletal arrays involved in the polarisation of cells undergoing asymmetric divisions — cortical actin patch — and in the establishment and maintenance of the division site —preprophase band of microtubules and filamentous (F) actin. The majority of cells retained adhesions at the actin patch throughout mitosis. However, only approximately 13% of cells formed or retained attachments at the site of the preprophase band. After the breakdown of the nuclear envelope, plasmolysis had a dramatic effect on spindle orientation, cell plate formation, and the plane of cytokinesis. Spindles were rotated at abnormal angles including tilted into the plane of the epidermis. Cell plates formed but were quickly replaced by vacuole-like intercellular compartments containing no Tinopal-stainable cell wall material. This compartment usually opened to the apoplast at one side, and cytokinesis was completed by the furrow extending across the protoplast. This atypical cytokinesis was facilitated by a phragmoplast containing microtubules and F-actin. Progression of the furrow was unaffected by 25 g of cytochalasin B per ml but inhibited by 10 M oryzalin. Phragmoplasts were contorted and misguided and cytokinesis prolonged, indicating severe disruption to the guidance mechanisms controlling phragmoplast expansion. These results are discussed in terms of cytoskeleton-plasma membrane-cell wall connections that could be important to the localisation of plasma membrane molecules defining the cortical division site and hence providing positional information to the cytokinetic apparatus, and/or for providing an anchor for cytoplasmic F-actin necessary to generate tension on the phragmoplast and facilitate its directed, planar expansion.Abbreviations ADZ actin-depleted zone - DIC differential interference contrast - GMC guard mother cell - MT microtubule - PPB preprophase band - SMC subsidiary mother cell Dedicated to Professor Brian E. S. Gunning on the occasion of his 65th birthday     

Pollen development in orchids 1. Cytoskeleton and the control of division plane in irregular patterns of cytokinesis

Summary The cytokinetic apparatus in microsporogenesis lacks a preprophase band of microtubules and the selection of cytokinetic planes is dependent upon disposition of nuclei which define cytoplasmic domains via post-meiotic radial systems of microtubules. Meiotic cytokinesis was investigated in hybrid moth orchids (Phalaenopsis) exhibiting irregular patterns of cytokinesis. In these polliniate orchids, spindle orientation is imprecise, and the tetrad nuclei (therefore the microspores) may be in rhomboidal, tetrahedral or linear arrangement. The hybrid Sabine Queen (section Phalaenopsis) regularly undergoes simultaneous cytokinesis, as is common in orchids. The hybrid Vista Rainbow (section Amboinenses) produces either a complete dyad wall, a partial wall, or no wall after first nuclear division. In all cases, a first division phragmoplast is initiated in the interzonal region and expands centrifugally into the peripheral cytoplasm. Fluorescence microscopy shows that the phragmoplast consists of fusiform bundles of microtubules and Factin bisected by a non-fluorescent zone. If a cell plate fails to form, a band of organelles polarized in the equatorial region effectively divides the cell into two domains. The organelles disperse when a dyad wall is complete, but tend to remain polarized around an incomplete wall. In four-nucleate coenocytes, the usual interzonal microtubules between sister nuclei (primary) form slightly in advance of secondary arrays between non-sister nuclei. Phragmoplasts are initiated in sites defined by the post-meiotic microtubule arrays.Abbreviations CLSM confocal laser scanning microscope/microscopy - DMSO dimethylsulfoxide - FITC fluorescein isothiocyanate - PPB preprophase band of microtubules - TEM transmission electron microscope/microscopy     

Induction,polarity and spatial control of cytokinesis in some abnormal subsidiary cell mother cells ofZea mays

Summary Cytokinesis in the subsidiary cell mother cells (SMCs) ofZea mays leaves grown in the presence of 5 mM of caffeine solution is usually partially inhibited. A continuous wall strip, resembling a portion of the subsidiary cell (SC) wall, is laid down in the preprophase microtubule band (PMB) cortical zone. Sometimes, the incomplete SC (SC) wall grows centripetally in the absence of a phragmoplast and the gap becomes smaller or closes. The SC nucleus escapes through the SC wall gap into the larger SMC compartment and may fuse with the other nucleus.The aberrant SMCs (a-SMCs) pass through another division cycle, reattempting to produce a SC. A typical PMB is found in the SC space, in the site of the previous PMB. Moreover, in some preprophase SMCs, the cytoplasm adjacent to the SC wall is traversed by a small number of microtubules. The preprophase nuclei are partly or totally separated from the PMB by the perforated SC wall and may lie far from the latter.Usually, one mitotic spindle is assembled. The cycling paired polarized nuclei appear to synchronize and their chromosomes line up together on a single metaphase plate. Although the mitotic spindle axis is diversely oriented, one of its poles tends to be stabilized in the proximity of the SC wall gap. These divisions separate abnormal cells. Most or all the cell plate edges fuse with wall regions far from the PMB cortical zone. However, when some of them approach the SC wall strips, they are attracted and intersect their rims. In rare occasions the cell plate, invading the SC space is guided by the PMB cortical zone to create a SC-like curved wall portion, in absence of a daughter nucleus.Observations show that the cell plate arrangement in redividing aberrant SMCs is not subjected to a strict spatial control. The disorder of polarization sequence generated by the SC wall ring and especially the perturbation of the spatial (and functional?) relationship between PMB-PMB cortical zone and the nucleus—mitotic spindle is a causal factor of the variable cell plate arrangements.