bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effect of microtubule-destroying drugs on the spreading and shape of cultured epithelial cells
Authors: Domnina L. V. , Rovensky J. A. , Vasiliev J. M. , Gelfand I. M. 
Published: April 2021

ABSTRACT
The role of microtubules in the spreading of cells from the liver-derived IAR2 rat cell line was studied. Cells in control medium seeded on a flat isotropic glass surface rapidly spread to form discoid shapes. Spreading in colcemid-containing medium was disorganized and delayed; partial reversal of spreading was observed. Nevertheless, even in the presence of colcemid the cells finally spread to discoid flattened shapes. IAR2 cells in medium without colcemid spread not to discoid but to elongated shapes under three different sets of conditions: (1) when the cells were forced to spread on narrow strips of adhesive glass surface between two non-adhesive lipid films; (2) when the cells spread on the poorly adhesive surface of poly(HEMA)-covered glass; (3) when the cells spread on the usual glass surfaces in medium containing cytochalasin D. Addition of colcemid to the media reversed the polarized spreading under the first two conditions; colcemid did not reverse the formation of the elongated cell shape acquired by the cells spreading in cytochalasin-containing medium. Effects of microtubule-destroying drugs on the spreading of epithelial and fibroblast cells are compared and discussed. It is suggested that microtubules are essential for the stabilization of the spread state of those attached cytoplasmic processes and lamellae that do not have numerous and stable cell-substratum contacts, e.g. the processes formed at the early stages of spreading or the elongated processes of polarized cells. Possibly, microtubules stabilize the non-contracted state of the actin cytoskeleton in these processes.
Microtubules and root protophloem ontogeny in wheat
Authors: Eleftheriou E. P. 
Published: April 2021

ABSTRACT
Protophloem ontogeny in roots of Triticum aestivum has been investigated ultrastructurally. Each protophloem pole consists of three cells, a protophloem sieve element and two companion cells, all originating from a single precursor cell usually having a pentahedral shape. This protophloem mother cell (PMC) undergoes two successive asymmetrical divisions: the first one gives rise to a smaller cell that will differentiate into a companion cell, and a larger one that divides again asymmetrically yielding another companion cell and a protophloem sieve element. The latter divides once more, but now symmetrically, increasing the number of cells.
Both asymmetrical and symmetrical divisions are preceded by preprophase microtubule bands (PMBs), well demarcated by a great number (more than 100 profiles in a single band section) of microtubules (MTs). The plane of a PMB coincides with that of the succeeding cell plate, which fuses with parent walls at sites previously occupied by the PMB. The strict correspondence between PMB and cell plate suggests that at cytokinesis the latter bisects the PMB cortical zone. The possible role of PMB cortical zone in positioning the cell plate and guiding its expanding edges towards predetermined sites is discussed in relation to recent discoveries in other anatomical situations. The plane of PMBs (and hence of divisions) changes from one division to the next, so that the three successive divisions occur in three spatial planes transversely to each other. This change is probably influenced by cell polarity.
Prior to each asymmetrical division peri-nuclear MTs were observed besides the MTs of the PMB. They appear before the PMB organization and persist throughout preprophase, but they change their position and orientation in response to the transition from PMB to the spindle organization.
Interlocked bivalents in reconstructed metaphase i cells of bread wheat
Authors: Heslop-Harrison J. S. , Bennett M.D. 
Published: April 2021

ABSTRACT
Complete reconstructions of all the bivalents were made from electron micrographs of serial sections through six pollen mother cells at metaphase I of meiosis in Triticum aestivum (hexapioid bread wheat). At least two of these metaphases contained interlocked pairs of bivalents. In one, two ring bivalents were interlocked, while in another a rod bivalent ran through the centre of a ring bivalent. Two other groups of bivalents were too closely appressed to allow separation into individual bivalents and may have contained interlocks. Meiosis in other anthers of the same plants examined by light microscopy was considered normal. The frequency of interlocking found was much higher than reported from light-microscope spreads.
Not all interlocks in metaphase I cells need adversely affect meiosis, but knowledge of their regularity and form may facilitate understanding the processes of chromosome pairing.
The aggregation of trypsinized BHK21 cells
Authors: Edwards J. G. , Campbell Janice A. 
Published: April 2021

ABSTRACT
The aggregation in suspension of freshly trypsinized cultured cells (BHK21) has been studied by microscopic and electronic counting techniques. Comparison of cells grown in suspension with cells grown on glass suggests that the aggregation may not be an artifact of tryptic action, but may reflect an adhesive property of the cell surface which survives the dispersal procedure.
The aggregation is very strongly inhibited at low temperature, and resembles in this respect the re-aggregation of cells from trypsinized embryonic tissues. There is no detectable delay in aggregation at 37 °C, and cells pre-incubated at 37 °C do not aggregate at 2 °C. These results are inconsistent with the view that the temperature-dependence of cell aggregation in this system is for resynthesis of surface components.
The aggregates are readily redispersed by proteolytic enzymes. The adhesions in the aggregates thus resemble in this respect the adhesions between cells in many tissues.
Trypsin and chymotrypsin are of similar effectiveness in dispersing the aggregates, but di-isopropylphosphoryl trypsin is completely ineffective, indicating that the enzyme activity of the proteins is involved in dispersal.
A method of obtaining a suspension of intact parenchymal cells from adult rat liver
Authors: Gallai-Hatchard Jennifer J. , Gray G. M. 
Published: April 2021

ABSTRACT
The perfusion of liver with either citrate or tetraphenyl boron to remove Ca2+ or K+ or with a solutionof high osmolarity and alkaline pH yields plenty of cells but they are all damaged.Perfusion of the liver with hyaluronidase and collagenase followed by incubation of liver slices in the same enzyme solution produced a high yield of cells (25 %, w/w, of liver) of which only about 1 % were undamaged. However, perfusion with 0·3 % hyaluronidase, 0·3 % collagenase and 0·1 % trypsin in phosphate-buffered saline (excluding Mg2+ and Ca2+ followed by incuba-tion at 25 °C of the chopped liver gave a small yield (2–4%, w/w) of undamaged cells which were not permeable to eosin for up to an hour when suspended in culture medium containing 2 % bovine serum albumin.
The fine-structural organization of the brush border of intestinal epithelial cells
Authors: Mukherjee T. M. , Staehelin L. A. 
Published: April 2021

ABSTRACT
The fine structure of the brush border of intestinal epithelial cells of the mouse has been studied with both normal sectioning and freeze-etching techniques.
Freeze-etching reveals the plasma membrane of the microvilli as consisting of a continuous layer, that is split during the cleaving process, in which numerous particles, 5–9 nm in diameter, are embedded, while other particle-like structures, with diameters of 7–10 nm, appear attached to the true outer membrane surface. The mucopolysaccharide surface coats of the microvilli show up more clearly in sectioned material than in freeze-etched specimens.
Inside each microvillus 2 different filament systems can be demonstrated: (1) bundles of fairly closely packed and straight core microfilaments, which lead into the tip of the microvillus, and (2) short cross-filaments. Under suitable conditions the core microfilaments display a sub-unit structure with a repeating distance of approximately 6 nm. The diameter of a microfilament can vary along its length from 6 to 11 nm. Two strands of globular particles wound helically around each other seem to make up each microfilament. These and other data support the idea that the core microfilaments are actin-like. No substructure has been found on the cross-filaments, which have an orientation approximately radial to the axis of the microvilli and seem to be attached at one end to the core microfilaments and at the other to the inner surface of the microvillous membrane.
The interwoven terminal web filaments also show no substructure. They form a continuous flexible platform-like structure into which the bundles of core microfilaments extend. Some terminal web filaments appear attached to the plasma membrane between the microvilli. It is suggested that the core microfilaments represent mechanical supporting elements and that the terminal web and cross-filaments are tensile elements of the brush border. In addition all 3 filament systems may also be involved in possible contractile movements of the microvilli.
The anchoring of nuclei by cytoplasmic microtubules inAcetabularia
Authors: Woodcock C. L. F. 
Published: April 2021

ABSTRACTThe giant unicellular alga Acetabularia enters the reproductive phase of growth when the cap has completed its expansion. The primary nucleus then divides and the resulting secondary nuclei take up fixed equidistant positions in the coenocytic cap cytoplasm. These positions are visible as colourless areas in the otherwise green caps.Studies on the cytology and ultrastructure of the caps at this stage showed that each secondary nucleus was surrounded by a layer of cytoplasm which was free from chloroplasts and mitochondria, but which contained many microtubules, some of which were closely associated with the nuclear envelope, while others appeared to touch the plasmalemma adjacent to the cell wall. (Microtubules are not a regular feature of Acetabularia cells; they have not been reported in the stems, young caps or cysts, and vegetative growth is not inhibited by colchicine.)The function of the microtubules was investigated by treatment with colchicine or vinblastine sulphate, both of which depolymerize microtubules in many other systems. Administration of these drugs at 10−3 M and greater concentrations had the following effects: (1) the colourless areas were lost as chloroplasts and mitochondria invaded the cytoplasm around each secondary nucleus; (2) the nuclei began to migrate from their fixed positions; and (3) cyst formation (in which the cytoplasm cleaves into uniformly sized cysts, each containing a single secondary nucleus) either was inhibited, or proceeded abnormally.It is therefore proposed that a major function of these transitory microtubules is to anchor the secondary nuclei at fixed equidistant positions in the cytoplasm. This function is probably mediated by their association both with the nuclear envelope and cell membrane.
The density of ribosomes bearing messenger rna in phage-infected and normal bacteria
Authors: Fenwick M. L. 
Published: April 2021

ABSTRACT
Newly made viral RNA in lysates of Escherichia coli infected with phage R17 was studied as a model messenger RNA. The viral RNA that was associated with groups of 1, 2, 3 and 4 ribosomes banded in caesium chloride at densities of approximately 1·68, 1·65, 1·645 and 1·635 g cm-3 respectively and the single ribosomes of uninfected cells at 1·62. The density of infected ribosomes was restored to 1·62 by treatment with RNase. These values are consistent with the presence of a single molecule of viral RNA in each group.
In uninfected bacteria the mean density of the smaller polysomes exceeded that of actino-mycintreated or RNase-treated ribosomes by about 0·005 g cm−3. Similarly, pulse-labelled RNA associated with single ribosomes or pairs banded at a density only marginally higher than that of the bulk of the single ribosomes. If any RNA was attached to the ribosomes it probably amounted to not more than one fifth of the size of the phage RNA for each ribosome pair.
The analysis of malignancy by cell fusion: II. hybrids between ehrlich cells and normal diploid cells
Authors: Bregula U. , Klein G. , Harris H. 
Published: April 2021

ABSTRACT
When Ehrlich ascites cells were fused with diploid fibroblasts, isolated directly from the animal, the resulting hybrid cells regularly produced progressive tumours. However, an analysis of a range of clonal populations of these hybrid cells, each derived from a separate primary fusion, revealed that the chromosomal constitution of these cells was highly unstable; all cell populations were found to have already undergone substantial chromosome losses by the time enough cells were available to permit chromosomal analysis. Thus, although these hybrid cells were highly tumorigenic, the tumours arising from them were not composed of cells with complete parental chromosome sets, but of cells from which some chromosomes had been eliminated.
The intramembrane structure of septate junctions based on direct freezing
Authors: Kachar Bechara , Christakis Nicholas A. , Reese Thomas S. , Lane Nancy J. 
Published: April 2021

ABSTRACT
Smooth septate junctions from the midgut of the cricket, Acheta, and the horseshoe crab, Limulus, as well as Hydra-type septate junctions from the epidermis of Hydra have been studied by freeze-fracture after direct freezing using the liquid helium-cooled copper block/slam freezing method. The exoplasmic fracture face at both types of septate junction exhibits rows of closely packed but irregularly shaped intramembrane particles. Complementary to these particle rows, on the protoplasmic fracture face, are sharply defined grooves with a periodic variation in depth and width that was conspicuous in Hydra but less well defined in arthropods. The closely packed, irregular particles on the exoplasmic faces could represent plastically deformed portions of transmembrane proteins pulled through the bilayer during freeze-fracture. On the basis of this interpretation, the grooves on the protoplasmic faces represent a confluence of the bilayer disruptions occurring during fracturing. The structures observed here are different from those reported in replicas of glutaraldehyde-fixed and glycerol-cryoprotected tissue, in which the intramembrane junctional components partition with the protoplasmic face and often assume the appearance of continuous cylinders. This comparison illustrates some of the artifacts associated with freezefracturing and shadowing. On the basis of a comparison of freeze-fracture replicas and sections of lanthanum-infiltrated tissues, the relationship between intramembrane junctional components and intercellular septal elements is analysed.
Intracellular ph in Dictyostelium: A 31P nuclear magnetic resonance study of its regulation and possible role in controlling cell differentiation
Authors: Kay R. R. , Gadian D. G. , Williams S. R. 
Published: April 2021

ABSTRACT
Intracellular pH (pHi) has been measured in Dictyostelium discoideum cells by 31P nuclear magnetic resonance. Ax2 cells, newly harvested from growth medium, maintained a pHi of 7·33 ± 0·04 (17) at an extracellular pH ranging from 3·5 to 6·5. Below pH 3·5 the cells tend to lyse, whereas at pH values above 6’5 their pH; rises though they remain viable. pHi regulation in acid medium is not dependent on external Na+ or any other inorganic ion and so most probably involves the electrogenic plasma membrane proton pump.
No significant change in pHi was detected during development through to the slug stage. Mature stalk cells gave a very acidic phosphate signal (pH ⩽5·5) which was probably vacuolar in origin.
Indirect experiments had suggested that pHi might regulate the development of Dictyostelium cells, with low pH, favouring stalk cell and high pHi favouring spore cell differentiation. In particular, two inhibitors of the plasma membrane proton pump, diethylstilbestrol and zearalenone, had been shown to be stalk cell inducers. In the present studies measurements of pHi of cells exposed to these inducers failed to detect the expected drop in pHi. In addition, DIF-1 (a low Mr factor), the natural inducer of stalk cell formation, caused, if anything, a slight alka-linization of the cells. Thus the original theory linking pHi and cell differentiation is not supported by these results and therefore appears to require some modification.
Finally, extract experiments revealed the existence of two unidentified abundant phosphocompounds with resonant frequencies close to inorganic phosphate. The existence of these compounds can complicate the interpretation of spectra gained from living Dictyostelium cells.
The occurrence of lampbrush chromosomes in early diplotene oocytes of Xenopus laevis
Authors: Mitchell E. L. D. , Hill R. S. 
Published: April 2021

ABSTRACT
Transmission electron microscopy has been used to study the chromosomes found in the early diplotene oocytes of Xenopus laevis. Chromosomes from 40 μm early diplotene oocytes were found to possess a normal lampbrush chromosome morphology. The contour length of the loops found on these chromosomes was measured, the mean value determined and compared with that for lampbrush chromosomes found on 300 μm mid-diplotene (Dumont stage II) oocytes. The mean contour length of the loops from 40 μm oocytes (12·33 ± 6·5 μm) was longer than that for the 300 μm oocytes (7·897 ± 5·22 μm). Analysis of variance showed these two values to be significantly different (p&lt;0·01). The mean loop density (number of loops per 25 μm chromosome axis) was also determined for the two size classes of oocytes, being 9·33 ± 2·05 per 25 μm of chromosome axis for the 40 μm size class and 11·2 ± 2·435 per 25 μm of chromosome axis for the 300 μm size class. Analysis of variance showed these values to be significantly different (P&lt; 0·01). The results of the present study demonstrate that the lampbrush chromosomes found in 40 μm early diplotene oocytes possess a greater loop contour length and a lower density of loops than those found in 300 μm, mid-diplotene oocytes. Possible reasons for these observations are discussed.
Kinetic properties of microtubule-activated 13 s and 21 s dynein ATPases: evidence for allosteric behaviour associated with the inner row and outer row dynein arms
Authors: Warner Fred D. , Mcilvain Jane H. 
Published: April 2021

ABSTRACT
The 13 S and 21 S dynein ATPases from Tetrahymena cilia rebind to extracted doublet microtubules as inner row and outer row arms. Rebinding is accompanied by four- to ninefold activation of the ATPase activity. The soluble (microtubule-free) forms of the two dyneins exhibit simple saturation kinetics (h = 1·0) with Vmax ⪡ 1 μmol Pi mg-1 min-1 and Km= 20–40 μM-ATP. Mixing a fixed quantity of free dynein with increasing concentrations of extracted doublets results in systematic increases in all three kinetic parameters for each dynein. At infinite concentrations of doublets and ATP, each enzyme undergoes a significant shift to sigmoid saturation kinetics (h = 2–3), Vmax increases to a turnover rate of about 90mol ATP per mol Es-1 and the Michaelis constant increases to ⪢100 μM-ATP. These data suggest that both enzymes are allosteric and can be interpreted in terms of positive cooperativity relative to a minimum of two or three interacting sites. It is less clear whether this cooperativity is related to subunit interactions within the 21 S or 13 S particles, or to subunit interactions between adjacent particles (arms) on the microtubule lattice.
Structural integrity of the nuclear matrix: Differential effects of thiol agents and metal chelators
Authors: Dijkwel Peter A. , Wenink Paul W. 
Published: April 2021

ABSTRACT
Nuclear matrices, associated with over 80% of the chromosomal DNA, could be isolated from BHK nuclei by extraction with 2M-NaCl. The matrices were found to impose at least two levels of structural order upon nuclear DNA.
From sedimentation studies it was inferred that metal depletion of the salt-extracted nuclei generated matrix structures, which sedimented at significantly lower rates than control matrices. Fluorescence microscopy revealed that the reduced sedimentation rate is a consequence of the increase in the radius of the DNA halo, i.e. the DNA loops emanating from the residual nucleus. Addition of Cu ions to nuclei prior to salt extraction was found to induce contraction of this DNA halo. These results indicate that Cu ions may play an important role in stabilizing one level of DNA folding.
When metal depletion had been brought about by thiol agents, a second effect was observed to occur. Within 15 min, salt-extracted nuclei disintegrated, generating irregularly shaped, slowly sedimenting structures. Disintegration only occurred when the full complement of DNA was still attached to the nuclear matrices.
Analysis by sodium dodecyl sulphate-polyacrylamide gel electrophoresis revealed that treatment with thiols did not detectably alter the polypeptide composition of DNA-depleted residual nuclei.
Results of these experiments suggest that both metal-protein interactions and disulphide bonds are important in maintaining higher-order structure in the nucleus. A model to account for these observations is discussed.
Nuclear division in the marine dinoflagellate Oxyrrhis marina
Authors: Xiao-Ping Gao , Jing-Yan Li 
Published: April 2021

ABSTRACT
The nuclear division of Oxyrrhis marina is a very distinct one among the mitoses of dinoflagellates that have been studied. Using synchronized populations, we have investigated the ultrastructural changes in this nuclear division. In interphase, nuclei can be classified into two groups on the basis of the shapes of the chromosomes. Y- and U-shaped chromosomes have been observed in both types of interphase nuclei. By prophase the nucleus becomes oval, many nuclear plaques appear on the nuclear envelope, and many microtubules radiate from these nuclear plaques within the nucleus. Metaphase can be identified by the characteristic arrangement of the chromosomes; an equatorial metaphase plate is absent. As in many higher organisms, anaphase includes two stages: anaphase A and anaphase B. During anaphase A the nucleus does not apparently elongate and the chromosomes migrate towards the poles by a combination of the shortening of the chromosome-associated microtubules and the elongation of those located between daughter chromosomes. During anaphase B the nucleus elongates to about twice its former length. This elongation may result from growth of the interzonal nuclear envelope. Dividing nucleoli are associated with microtubules, which suggests that microtubules may play an active role in the division of the nucleolus.
The evolution of mitosis and the phylogenetic relationships between Oxyrrhis, typical dinoflagellates and Syndinium are discussed.