bluuebunny/crossref_metadata_2025_split huggingface dataset:
Disassembly of the mammalian metaphase chromosome into its subunits: Studies with ultraviolet light and repair synthesis inhibitors
Authors: Mullinger Ann M. , Johnson Robert T. 
Published: April 2021

ABSTRACT
Metaphase chromosomes of a simian virus-transformed Indian muntjac cell line have been examined by scanning electron microscopy of material in which the fully packed metaphase structure is progressively relaxed. Such chromosomes are seen in standard, spread preparations of ultraviolet light-irradiated, metaphase-arrested cells, which have been incubated in the presence of inhibitors of DNA synthesis; they are processed for electron microscopy by trypsinization, further fixation and osmium impregnation.
Decondensation is initially associated with a gradual elongation and loosening of the chromosome axis and, as loosening proceeds, the appearance of unexpected higher order structures - clusters of 20 –40 nm diameter fibres. The arrangement of the clusters shows much variation between spreads. In the most fully extended chromosomes clusters are arranged in two longitudinal series with pairing between sister chromatids; the diameter of the majority of clusters in such chromosomes is in the range 0 ·4 –0 ·6 μm. In the final stages of decondensation, clusters separate and individual chromosomes are no longer recognizable. Similar fibre clusters are found in interphase nuclei prepared by the same method.
We suggest that the clusters of chromatin fibres may assemble as intermediates in the construction of an axial structure, which is further compacted in the fully condensed metaphase chromosome.
Weak bases partially activate Xenopus eggs and permit changes in membrane conductance whilst inhibiting cortical granule exocytosis
Authors: Charbonneau Michel , Webb Dennis J. 
Published: April 2021

ABSTRACT
At extracellular pH values close to their pKa values the weak bases, ammonia and procaine, elicited a series of events in non-activated Xenopus eggs, some of which resembled those normally occurring at fertilization. These included: (1) a transient increase in membrane conductance; (2) modification of the microvilli; (3) thickening of the cortical cytoplasm and displacement of the cortical granules; (4) pigment accumulation; (5) contractions and shape changes. However, these eggs did not undergo the cortical reaction nor emit the second polar body.
Cortical granule exocytosis of inseminated or artificially stimulated eggs was inhibited if the eggs had been previously treated for 15 min with the weak base and subsequently rinsed. Multiple sperm-entry sites were exhibited by the inseminated eggs, suggesting polyspermy. However, such eggs did not cleave and although sperm had fused with the egg membrane, they were not incorporated. Nevertheless, a transient increase in membrane conductance was evoked, which was longer in duration and had a slightly different form from that normally accompanying fertilization. In these eggs cortical granules were intact but displaced away from the plasma membrane. Prolonged contact with the weak base rendered eggs totally unresponsive to sperm or artificial stimuli but eggs recovered when rinsed sufficiently.
These effects of weak bases on unfertilized Xenopus eggs or during fertilization were completely absent at pH 7·4. Although changes in intracellular pH or Ca2+ may be involved in these phenomena, direct action by the weak base itself cannot be ruled out.
Temperature-induced modifications in size and pattern of microtubular organelles in a ciliate, Dileptus: I. supernumerary microtubules in axonemes of sensory cilia
Authors: Golinska Krystyna 
Published: April 2021

ABSTRACT
Supernumerary microtubules were found in the so-called sensory cilia, in addition to a sensory axoneme. The supernumerary microtubules were not structurally connected to a basal body, but were probably anchored to clusters of dense material inside the ciliary shaft. The frequency of appearance of the supernumerary micro-tubules was found to be temperature-dependent: the higher the temperature during formation of sensory cilia, the greater was the number of supernumerary microtubules in cross-sections, and the more cross-sections contained them.
The possibility is discussed that the formation of the supernumerary microtubules is not due to formation of new nucleating sites inside the cilium. Instead, the microtubules may be remnants of a previously existing axoneme, separated from the basal body during the formation of a sensory cilium. Some of the microtubules of the released axoneme may persist as the super-numerary microtubules, if capped with dense material or some other structure within the ciliary shaft.
Onset of expression of the variant surface glycoproteins of Trypanosoma brucei in the tsetse fly studied using immunoelectron microscopy
Authors: Tetley L. , Turner C. M. R. , Barry J. D. , Crowe J. S. , Vickerman K. 
Published: April 2021

ABSTRACT
The acquisition of the variant surface glycoprotein (variable antigen) coat by metacyclic stage Trypanosoma brucei in the salivary glands of the tsetse fly, Glossina mor si tans, has been studied in situ by transmission and scanning electron microscopy using monoclonal anti-bodies raised against metacyclic variable antigen types and complexed with horseradish peroxidase or colloidal gold. The coat is acquired after binary fission has ceased but while the parasite is still attached to the gland epithelium, i.e. before the mature metacyclic is released into the gland lumen. The variable antigen type heterogeneity previously observed in discharged mature metacyclics is here demonstrated in the nascent (attached) metacyclic population. The variant surface glycoprotein genes are thus not expressed in a fixed sequence since different metacyclic variable antigen types are present ab initio. The distribution of immunogold-marked nascent metacyclics of a particular variable antigen type, as shown by quadrat analysis of a scanning electron micrograph montage of the infected salivary gland epithelium, conforms to a Poisson series. This provides evidence that individual variant surface glycoprotein genes are stochastically activated and suggests that selective activation occurs after trypanosome division has ceased.
Order out of chaos
Authors: Downes C. S. 
Published: April 2021

ABSTRACT
Those who puzzle over the origins of life are perplexed by the improbability of several independent functions arising together, in the absence of either a directing intelligence or a complete, selectable organism. As if to illustrate the inherent limitations of random assembly in producing a coherent whole, the Swedish Academy of Sciences have published, with minimal editing, almost all the papers presented at what must have been a stimulating but perhaps unsatisfactory conference.
Role of calcium in triggering rapid ultrastructural damage in muscle: a study with chemically skinned fibres
Authors: Duncan C. J. 
Published: April 2021

ABSTRACT
Agents (A23187, caffeine) believed to raise [Ca]i in vertebrate cardiac and skeletal muscles cause rapid and characteristic subcellular damage in vitro and in vivo. By using saponin-skinned amphibian pectoris cutaneous muscle and Ca-EGTA-buffered solutions it is shown that low [Ca] consistently triggers the same rapid (2–20 min), ultrastructural damage.
Electron micrographs reveal a close similarity between the damaged intact and skinned preparations, namely loss of myofilament organization, specific Z-line damage, dissolution and hypercontraction bands, characteristic mitochondrial swelling and division. Where both actin and myosin filaments were lost, an underlying cytoskeletal network frequently remained, still attached to the Z-line framework.
Ca was effective in skinned preparations from 5×10−7M to 8×10−6M, within the concentration range experienced by a contracting muscle. Damage was [Ca]- and time-dependent and it is suggested that it is probably the active movement of Ca ions across key membrane sites that is critical in triggering damage of the myofilament apparatus. Strontium can substitute for Ca at higher concentrations. The action of saponin suggests that the chemically skinned cell is partially activated. Ca-triggering can be bypassed experimentally by membrane-active agents or by sulphydryl agents.
Ruthenium Red and trifluoperazine indirectly cause damage in the intact cell by raising [Ca] i. Studies with saponin-skinned cells and protease inhibitors show that changes in pHi, loss of ATP, Ca-activated neutral protease, or release of lysosomal enzymes (cathepsins B, D, L or H), are not involved in characteristic rapid myofilament damage.
Regulation of G
2 by cell size contributes to maintaining cell size variability within certain limits in higher plants
Authors: Navarrete Matilde H. , Cuadrado Antonio , Escalera Mariano , Cánovas Josél 
Published: April 2021

ABSTRACT
The variability of (1) surface area projection (size) at which cells terminate DNA replication, (2) the area at which they initiate mitosis, (3) the area at which they divide, (4) the duration of G2, and (5) the duration of G2 plus mitosis (in fact, prophase + metaphase + anaphase) has been estimated in steady-state cell populations of Allium cepa root meristems. The coefficient of variation of cell area at termination of DNA synthesis was found to be 14% while the coefficient of variation of cell area at mitosis initiation was 13%. As there is also a substantial variability of G2 (the coefficient of variation was estimated to be 38%), the combination of these data indicates that cell size regulation of G2 contributes to maintaining cell size variability (and therefore DNA concentration) within certain limits. Mitosis also varies but less than G2 (the coefficient of variation of G2 + mitosis was found to be 31%). As the coefficient of variation of cell area at division (14%) is hardly larger than the coefficient of variation of cell area at initiation of mitosis, it can be suggested that coordination between cell size and mitosis duration helps to avoid a significant increase in the variability of cell size at the end of the division cycle.
Inhibition of cell adhesion by a synthetic polymer adsorbed to glass shown under defined hydrodynamic stress
Authors: Owens Norman F. , Gingell David , Rutter Paul R. 
Published: April 2021

ABSTRACT
A co-polymer with hydrophobic and hydrophilic segments was allowed to adsorb from aqueous solution onto glass previously made hydrophobic by derivatization with octadecyl dimethylchlorosilane. The polymer is thought to adsorb via its hydrophobic segments, leaving the hydrophilic segments free to extend into the water. After allowing cells to settle on the treated surface, the shear stress at the chamber wall required to remove red blood cells, Dictyosteliiun discoideum amoebae and Escherichia coli was determined in a calibrated laminar flow chamber. On octadecyl glass a shear stress of 2·3 N m−2 evicts 50% of adherent red cells and E. coli. No D. discoideum amoebae could be removed at 5Nm−2. In striking contrast, the lowest experimentally obtainable shear stress of 0·03 N m−2 removes 97·0–99·5% of cells of all three types from the polymer-treated surface, even after a cell residence time of 1 h without flow in the absence of free polymer. The minimum shear stress of 0·03 Nm−2 corresponds to only ≈20 times the force of gravity on a red cell. The mechanism of action of the polymer and the implications of the results are discussed.
Spike-shaped oscillations in the absence of measurable changes in cyclic AMP concentration in a mutant of Dictyostelium discoideum*
Authors: Wurster Bernd , Mohn Renate 
Published: April 2021

ABSTRACT
Periodic activities of Dictyostelium discoideum cells involve two types of oscillations, spikeshaped and sinusoidal. Spike-shaped oscillations are accompanied by the periodic synthesis and release of cyclic AMP, and cyclic AMP-activated cyclic AMP synthesis is believed to control these oscillations. Experiments described here call into question the importance of cyclic AMP in spikeshaped oscillations. Cell suspensions of strain agip43, an aggregation-deficient mutant of D. discoideum, displayed spike-shaped oscillations in light scattering with period lengths about 1-5 times larger than those of the parent strain. These oscillations were not accompanied by measurable oscillations of cyclic AMP and cyclic GMP. Applied cyclic AMP pulses elicited increases of two- to threefold in the cyclic AMP level and increases of seven-to ninefold in the cyclic GMP concentration. Cyclic AMP additions caused phase shifts in the oscillations of agip43 cells, suggesting that cyclic AMP receptors at the cell surface communicate with the oscillator. We interpret these results in terms of an oscillator not based on cyclic AMP. This oscillator should be coupled to the reaction system involving cyclic AMP synthesis and release. The latter can operate in an oscillatory manner in the parent strain Ax2 but not in mutant agip43.
Characterization of a heat shock-induced insoluble complex in the nuclei of cells
Authors: Littlewood T. D. , Hancock D. C. , Evan G. I. 
Published: April 2021

ABSTRACT
The formation of an insoluble complex in isolated nuclei incubated at physiological temperature (37°C) is demonstrated. A similar complex is shown to form in the nuclei of intact cells subjected to temperatures that induce the classical heat-shock response. The formation of this complex occurs rapidly in response to hyperthermia and is induced by small increases in temperature both in vitro and in vivo. We have characterized the formation of the complex in isolated nuclei and the nuclei of intact cells. A small number of the subset of nuclear proteins involved in the complex have been identified. The significance of the loss of solubility of these proteins in the nucleus following hyperthermia is discussed.
Isolation of cytoskeletons from synchronized plant cells: the interphase microtubule array utilizes multiple tubulin isotypes
Authors: Hussey Patrick J. , Traas Jan A. , Gull Keith , Lloyd Clive W. 
Published: April 2021

ABSTRACT
Biochemical analysis of the plant cytoskeleton has been hampered by an inability to isolate anything more than cytoskeletal fragments. Methods for isolating entire detergent-resistant cytoskeletons from carrot suspension cells are now reported. This enables the expression of tubulin isotypes to be studied in functional microtubular arrays, freed of the soluble pool by detergent extraction.
When osmotically cushioned with 0·4M-sorbitol, microtubule-stabilizing buffer and dimethylsulphoxide, carrot protoplasts can be extracted by mild detergent, without fragmenting. Cytoskeletons isolated by sucrose density centrifugation are shown by electron and fluorescence microscopy to contain a complex meshwork of three major fibrous sytems: F-actin, microtubules and bundles of 7 nm fibrils.
Plant cells can assemble tubulin into one of four microtubule arrays depending upon the phase of the cell cycle. Previous work had established that interphase cells contained multiple tubulin isotypes. However, the tubulins had been isolated by taxol assembly in vitro, which need not reflect patterns of usage by the interphase microtubule array in vivo. To address this problem, cells blocked in interphase were converted to cytoskeletons and their usage of tubulin isotypes determined by immunoblotting two-dimensional gels. This confirmed that all of the alpha and beta isotypes that can be identified on two-dimensional gels of carrot suspension cells are utilized by the interphase microtubule array.
intermediate filament formation after transfection with modified hamster vimentin and desmin genes
Authors: van den Heuvel Richard M. M. , van EYS Guillaume J. J. M. , Ramaekers Frans C. S. , Quax Wim J. , Vree Egberts Wilma T. M. , Schaart Gert , Cuypers H. Theo M. , Bloemendal Hans 
Published: April 2021

ABSTRACT
Previously we cloned and characterized the hamster intermediate filament genes coding for vimentin and desmin. It was demonstrated that the cloned desmin gene was expressed after gene transfer and that the newly synthesized protein assembles into intermediate filaments. Here we present data on the transfection of modified vimentin and desmin genes onto simian virus 40-transformed hamster lens cells and HeLa cells. Modifications included: (1) removal of exons encoding the desmin COOH-terminal domain; (2) exchange of exons encoding the COOH-terminal domain of vimentin and desmin; and (3) deletion of part of exon I of desmin, coding for the NH2-terminal amino acids 4–148.
In transient transfection assays it was shown that the modifications in the COOH region had no detectable effects on the filament forming potential of the encoded proteins as demonstrated with desmin antibodies in the indirect immunofluorescence test. On the other hand, deletion of a considerable part of the first exon of the desmin gene results in a lack of bona fide intermediate filament formation. Immunoblotting with desmin antibodies of cell populations enriched for the transfected modified genes showed that the presence of the modified genes results in the synthesis of the corresponding proteins with the expected molecular weights. From our results we conclude that in vivo: (1) the presence of the COOH terminus is not essential for filament formation; (2) that an exchange of COOH-terminal parts of vimentin and desmin does not prevent assembly into intermediate filaments; and (3) that removal of the NH2 terminus of desmin affects intermediate filament formation.
Growth of endosymbiotic algae in the green hydra, Hydra Viridissima
Authors: Dunn Kenneth 
Published: April 2021

ABSTRACT
Stable endosymbiosis depends upon balanced growth of the symbionts. In green hydra, coincident patterns of host and algal mitotic index suggest that coordinated reproduction provides for balanced growth. However, when hydra shrink during food shortage, the population of endosymbiotic algae in a green hydra must likewise decline in size. Thus far, no mechanism of reducing the size of the endosymbiont population has been described. Algal mitosis was found here to be stimulated by host feeding and clumped in its distribution among host cells, supporting the notion of some degree of control of algal mitosis exercised at the level of the host cell. However, comparisons of the rates of algal mitosis with the realized rates of algal population growth show that substantial numbers of algae disappear from hydra, in numbers in excess even of those necessary to accommodate host shrinkage. Only a small proportion of these lost algae was found to be expelled by hydra. Microscopic observations of the cells of macerated hydra show evidence of algal disintegration in nearly 50% of the digestive epithelial cells of regularly fed hydra. Coincidence of remnants of algal cells and food-derived materials within the same vacuoles suggests that algae are digested by host cells.
Development of macrociliary cells in Beroë I. Actin bundles and centriole migration
Authors: Tamm Signhild , Tamm Sidney L. 
Published: April 2021

ABSTRACT
Differentiation of macrociliary cells on regener-ating lips of the ctenophore Beroë was studied by transmission electron microscopy. In this study of early development, we found that basal bodies for macrocilia arise by an acentriolar pathway near the nucleus and Golgi apparatus, in close association with plaques of dense fibrogranular bodies. Procentrioles are often aligned side-by-side in double layers with the cartwheel ends facing outward toward the surrounding plaques of dense granules. Newly formed basal bodies then dis-band from groups and develop a long striated rootlet at one end. At the same time, an array of microfilaments arises in the basal cytoplasm. The microfilaments are arranged in parallel strands oriented toward the cell surface. The basal body-rootlet units are transported to the apical surface in close association with the assembling actin filament bundle. Microfilaments run parallel to and alongside the striated rootlets, to which they often appear attached. Basal body–rootlet units migrate at the heads of trails of microfilaments, as if they are pushed upwards by elongation of their attached actin filaments. Near the apical surface the actin bundle curves and runs below the cell membrane. Newly arrived basal body–rootlets tilt upwards out of the microfilament bundle to contact the cell membrane and initiate ciliogen-esis. The basal bodies tilt parallel to the flat sides of the rootlets, and away from the direction in which the basal feet point. The actin bundle continues to enlarge during ciliogenesis. These results suggest that basal body migration may be driven by the directed assembly of attached actin filaments.
Heterogeneity and cell cycle analyses from time-lapse studies of human keratinocytes in vitro
Authors: Dover R. , Potten C. S. 
Published: April 2021

ABSTRACT
We have analysed the behaviour of cultured epidermal Keratinocytes using time-lapse video recordings. We have found evidence for heterogeneity in the behaviour of the cells. Some lines underwent extensive self-renewal, thus expanding the population, while others produced daughter cells that migrated suprabasally and are presumed to have undergone terminal differentiation. We also present kinetic data on the cell cycle times, mitotic durations and post-mitoticresidence times. The latter is the time between a cell’s birth and eventual suprabasal migration. The data suggest that the ‘decision’ to migrate is in some way ‘programmed’ but that the actual migration is a stochastic process. Time-lapse analysis is a very powerful technique for lineage and cell kinetic analysis.
The site of ferricyanide photo-reduction in the lamellae of isolated spinach chloroplasts: a cytochemical study
Authors: Hall D. O. , Edge H. , Kalina M. 
Published: April 2021

ABSTRACT
Ferricyanide was used as a Hill oxidant to localize the site of photoreduction in chloroplast lamellae. The ferrocyanide formed on illumination was complexed with copper ions to form insoluble, electron-dense precipitates of Cu ferrocyanide, which can be easily seen in unstained preparations in the electron microscope. Control experiments showed no precipitates in the dark, at zero time, in the absence of ferricyanide, or on addition of sodium ascorbate. Discrete precipitates of Cu ferrocyanide were seen on both stroma and grana lamellae. It is concluded that both stroma and grana lamellae have photosystem II activity necessary for reduction of ferricyanide.
Freeze-etch studies on the innervation of mesenteric arteries and vas deferens
Authors: Devine C. E. , Simpson F. O. , Bertaud W. S. 
Published: April 2021

ABSTRACT
The innervation of mesenteric arteries and vas deferens of guinea-pig and vas deferens of mouse was examined by freeze-etching. Axons in bundles at large distances from the smooth muscle cells, were invested by Schwann cells and contained mainly neurotubules, while axons close to the smooth muscle cells had varicosities up to 1·6 μm in diameter and 20 μm long containing mainly small (approximately 50 nm) and large (approximately 100 nm) synaptic vesicles. Vascular axons differed from those in the vas deferens in that the former were at the medial adventitial border with an observed closest neuromuscular distance of approximately 200 nm and the latter were between smooth muscle cells at distances of 20–50 nm. Depressions of the axonal surface were seen and particles up to 15 nm were found on the axonal membrane.