bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cell organelles at uncoated cryofractured surfaces as viewed with the scanning electron microscope
Authors: Woods P. S. , Ledbetter Myron C. 
Published: April 2021

ABSTRACT
A method of direct visualization of cell organelles by scanning electron microscopy (SEM) is described. Plant and animal tissues fixed in glutaraldehyde and osmium tetroxide are treated with the ligand thiocarbohydrazide and a second osmium tetroxide solution, to increase their osmium content. Tissues are then dehydrated, infiltrated with an epoxy monomer, and together solidified with dry ice and fractured. The pieces are transferred to pure acetone, critical-point dried, attached to stubs with silver paint and viewed by SEM. The ligating procedure increases the osmium concentration at its original bonding site sufficiently to render the tissue electrically conductive, thus obviating the need for metallic coating. The organelles at the fractured surface are revealed in relation to their osmium incorporation rather than by surface irregularities as with coating methods. The image derived from the uncoated surface approaches in resolution that of transmission electron micrographs of thin sections. A portion of the image arising from a small distance below the surface, while at progressively lower resolution, provides some 3-dimensional information about cell fine structure.
The crystalline glycoprotein cell wall of the green alga Chlorogonium Elongatum: a structural analysis
Authors: Roberts K. , Hills G. J. 
Published: April 2021

ABSTRACT
Members of the Chlamydomonaceae, mostly single-celled green algae, have been shown to contain a crystalline glycoprotein cell wall component. Most of the species examined fall into a class of algae whose walls have an identical crystalline unit cell. Chlorogonium elongatum has been chosen as a representative of this class in order to investigate in more detail its cell wall structure. The alga has a spindleshaped cell wall which retains its asymmetric shape on isolation. Sections from walls fixed in the presence of tannic acid clearly reveal a regular subunit monolayer, about 20 nm thick, within the wall. Sodium dodecylsulphate (SDS) polyacrylamide gel electrophoresis shows the presence of at least 2 major glycoprotein species in the wall. Negatively stained purified cell walls demonstrate the crystalline nature of the cell wall. Optical diffraction of bright-field images and direct electron diffraction both give clear diffraction patterns whose spacings extend out to 3 nm and fall on a reciprocal lattice whose vectors describe a 2-dimensional unit cell within the wall 2 · 15 nm × 7 · 0 nm and an included angle of 80°. Lattice defects within the cell wall are revealed by both negative staining and surface replication. Through-focal series were used to choose images with the optimal degree of underfocus for image processing. Linear integration and optical filtering of such images gave essentially the same result. A similar image was also obtained by computing the autocorrelation function of the amplitudes in the electron-diffraction pattern and the optical-diffraction pattern of the in-focus image. On the basis of these data a 2-dimensional model of the crystalline cell wall layer is presented.
Some effects of positively charged surface groups on cell aggregation
Authors: Maslow D. E. , Weiss L. 
Published: April 2021

ABSTRACT
A study was made of the effects of 2,3-dimethylmaleic anhydride (DMA), a reagent removing positive charges, on the aggregation and surface charge of embryonic chick neural retina cells. Neural retina cells, recovered from the dissociation procedure, were cultured on a gyratory shaker and the aggregate diameters formed in the presence of DMA or DMA-serum dialysate, following DMA-pretreatment, or in appropriate control cultures measured. The electrophoretic mobilities of similarly treated cells were also determined. In addition, cellulose acetate electrophoresis was carried out on samples of serum containing DMA, and the incorporation of 14C-amino acids into DMA-treated cells studied.
Aggregates formed in the presence of DMA, or following DMA-pretreatment, were significantly smaller than aggregates from control cultures. The electrophoretic mobility of DMA-treated cells was significantly increased in serum-containing medium, but not serum-free Hanks’ solution. At 24 h after removal of DMA-containing medium, the mobilities of pretreated cells were similar to those of controls.
The electrophoretic pattern of DMA-treated serum was changed only with concentrations of DMA many times that affecting cell aggregation or mobility. DMA-serum dialysate did not significantly reduce aggregate size. The incorporation of 14C-amino acids in DMA-treated cells and the structure of aggregates were unchanged from controls.
It is concluded that positively charged constituents of the cell periphery play a demonstrable, but not limiting, role in cell aggregation, while a minor role for positive charges on serum protein cannot be totally excluded.
The actions of restriction endonucleases on lampbrush chromosomes
Authors: Gould D. C. , Callan H. G. , Thomas C. A. 
Published: April 2021

ABSTRACT
Lampbrush chromosomes from oocytes of Notophthalmus viridescens were dispersed in media containing restriction endonucleases isolated from Haemophilus and E. coli. These endonucleases cleave duplex DNAs at specific palindromic sequences of nucleotides, and several sensitive sites occur per micron of DNA. The overwhelming majority of the lateral loops of lampbrush chromosomes are extensively fragmented by these endonucleases, but an occasional pair of loops is refractory. A notable example of loops showing this refractory property are the giant loops on chromosome II in the presence of Hae. These loops, whose DNA-containing axes are several hundred micra long, are sensitive to other nucleases such as EcoB, endonuclease I and pancreatic DNase I; their refractory behaviour towards Hae therefore indicates that the sequence sensitive to this particular endonuclease is systematically absent. This anomalous property can be comprehended if it be assumed that the axial DNA of the giant loops consists of tandem repeats of a sequence which happens not to include the sensitive site.
Synthesis of the inner mitochondrial membrane and the intercalation of respiratory enzymes during the cell cycle of Chlorella
Authors: Forde B. G. , Gunning B. E. S. , John P. C. L. 
Published: April 2021

ABSTRACT
The ratio of inner to outer mitochondrial membrane area remains close to 1·8 throughout the cell cycle in synchronized cells of Chlorella fusca var. vacuolata 211-8p. Using estimates of this ratio, together with our previous estimates of mitochondrial surface area, to calculate the absolute area of inner mitochondrial membrane, it is demonstrated that growth of the inner mitochondrial membrane during the cell cycle occupies an extended period and parallels the growth of the whole cell. In contrast, the synthesis of succinate dehydrogenase and cytochrome oxidase is restricted to the last third of the cell cycle. It is concluded that mitochondrial growth involves the intercalation of periodically synthesized respiratory enzymes into membranes made earlier in the cycle, with consequent 5-fold changes in the density of active enzyme molecules in the membrane. These observations are discussed in relation to the control of mitochondrial membrane synthesis, membrane assembly and respiration rate during the cell cycle.
45Calcium localization in islets of langerhans, a study by electron-microscopic autoradiography
Authors: Howell S. L. , Tyhurst Margaret 
Published: April 2021

ABSTRACT
Attempts were made to localize the sites of uptake of 45calcium in B cells of islets of Langerhans by electron-microscopic autoradiography. Despite the potential sources of error inherent in the partial loss of radioactivity during fixation, and in the relatively high energy of emission of this isotope, distribution of silver grains differed significantly from random in all experiments. Grains were concentrated over mitochondria and to a lesser extent over storage granules. Incubation of islets in the presence of 10 mM glucose and isobutylmethyl-xanthine before fixation and autoradiography resulted in a small but not statistically significant reduction in silver grains associated with the mitochondria. These results further indicate a dominant role of mitochondria in the regulation of cytosolic calcium concentrations in pancreatic B cells.
Spindle birefringence of isolated mitotic apparatus: further evidence for two birefringent spindle components
Authors: Forer Arthur , Kalnins V. I. , Zimmerman A. M. 
Published: April 2021

ABSTRACT
We studied sea-urchin zygote mitotic apparatus (MA) isolated in hexylene glycol, transferred immediately to a glycerol-dimethylsulphoxide medium, and stored for 2 weeks at room temperature. Treatment with 0·5 M KC1 caused loss of 45% of the birefringence, but microtubules remained intact (as seen electron microscopically in glutaraldehyde-fixed MA), and tubulin was not extracted (as determined by polyacrylamide gel electrophoresis). These results suggest that a non-tubulin component which is extracted by the KC1 contributes 45% of the MA birefringence.
Further evidence for this conclusion came from indirect immunofluorescence experiments. Non-extracted (control) MA were fixed with formaldehyde and reacted with antibody against tubulin: there was intense staining of the spindle fibres and astral rays. Electron microscopically, however, microtubules were not present in formaldehyde-fixed MA. Since formaldehyde fixation caused breakdown of microtubules but the tubulin remained in the MA (as judged by reaction with antibodies) we suggest that after microtubule breakdown the tubulin remains in the MA because it is bound to a peri-microtubule spindle component (which we call ‘substance γ’). When KCl-extracted MA were fixed with formaldehyde and reacted with antibody against tubulin there was very little staining of spindle fibres and astral rays. Electron microscopically, formaldehyde caused microtubule breakdown, and since the tubulin is lost from formaldehyde- fixed, KCl-extracted MA (as judged by reaction with antibodies), we suggest that the tubulin- binding component, substance γ, is extracted by the 0·5 M KCl.
Pressure treatment caused the asters not to stain with antibody against tubulin, suggesting that the stability of substance γ is different in different regions of the mitotic apparatus.
The ordered columnar structure of mouse filiform papillae
Authors: Hume William J. , Potten Christopher S. 
Published: April 2021

ABSTRACT
Examination of carefully oriented 1-μm and 5-μm sections of mouse dorsal tongue together with scanning electron microscope observations indicates a high degree of cellular organization in the papillae. It has been suggested that each filiform papilla consists of 2 dominant and 2 minor columns of cells. Labelling patterns of the basal cells have been investigated in relation to these columns.
Synthesis of a liver enzyme in hybrid cells
Authors: Riddle V. G. H. , Harris H. 
Published: April 2021

ABSTRACT
Rat hepatoma cells were fused with cells of an established mouse lymphoma line, with normal diploid mouse macrophages, lymphocytes and fibroblasts and with normal diploid rat macrophages and lymphocytes. The liver-specific enzyme tyrosine aminotransferase was produced by almost all the hybrid cells, but usually at a lower level than in the parental hepatoma cells. Most of the hybrids also showed increased levels of this enzyme after exposure to dexamethasone. In the rat x mouse hybrids, the electrophoretic mobility of the enzyme indicated that only the rat hepatoma enzyme was produced. The findings are difficult to explain in terms of simple models involving a single diffusible repressor or activator of tyrosine aminotransferase synthesis.
Cyclic adenosine monophosphate and the development of polysphondylium
Authors: Jones M. E. , Robertson A. 
Published: April 2021

ABSTRACT
Centre formation in Polysphondylium violaceum is delayed for 2 h on buffered agar containing 10−3 M c-AMP, and for up to 22 h on unbuffered agar with the same c-AMP concentration. With ambient c-AMP concentrations as low as 10−8 M, P. pallidum forms numerous, small, atypical aggregates which do not fruit. This effect is independent of whether the agar is buffered.
P. violaceum amoebae are weakly attracted to the tip of a microelectrode containing 10−3 or 10−4 M c-AMP, but the electrode cannot compete when natural centres form nearby. P. pallidum amoebae are not attracted.
Aggregates of P. violaceum and of P. pallidum are strongly attracted to a microelectrode releasing c-AMP. The observation of Shaffer that Polysphondylium grex switch over from secreting an acrasin that attracts homologous amoebae to one that attracts the larger Dictyo- stelium species suggests that the second acrasin might be c-AMP. The above results strengthen this conjecture.
As c-AMP inhibits centre formation, the secretion of c-AMP by older aggregates may explain an inhibition of centre formation in the ‘ overlay’ experiments of Shaffer.
A theoretical study of the effects of cyclic amp phosphodiesterases during aggregation in Dictyostelium
Authors: Nanjundiah Vidyanand , Malchow Dieter 
Published: April 2021

ABSTRACT
During aggregation the larger Dictyostelium species use cAMP as a chemoattractant and possibly also as a transmitter. In passage from cell to cell, cAMP levels are modulated by diffusion and by enzyme hydrolysis. It appears that the important cAMP-hydrolysing enzyme is a phosphodiesterase bound to the cell membrane, the main roles of which are (1) very fast hydrolysis of cAMP and (2) steepening of spatial cAMP gradients. An extracellular phosphodiesterase has no function, so far as can be conjectured from present data.
Differentiation and positioning of nuclei in eggs of the cecidomyid Heteropeza pygmaea
Authors: Meats Marian , Tucker J. B. 
Published: April 2021

ABSTRACT
During the first three cleavage divisions of the egg nuclei a precise sequence of spindle orientation and elongation parallel to the longitudinal axis of the egg is apparently involved in positioning one nucleus among the polar granules at the posterior pole of the egg. The size of this nucleus, and the position at which the egg cleaves when pole cell formation occurs, appear to constitute part of the mechanism which ensures that only one nucleus is included in the first pole cell.
Blastoderm formation occurs without a well-defined migration of nuclei to the egg surface. Nuclei are so large in relation to the size of the egg that uniform spacing and distribution of nuclei ensures that a large proportion are situated near the egg surface. Those nuclei which are near the egg surface divide synchronously to form a layer of blastoderm nuclei, while membranous cleavage furrows invaginate from the egg surface between them. Nuclei in the central region of the egg chamber condense to form yolk nuclei before blastoderm nuclei have been separated from the rest of the egg by the completion of the cleavage membranes.
Polar granules provide the only evidence of fine-structural differences in different regions of the egg chamber cytoplasm. They are found near the posterior pole of the egg from an early stage of oogenesis. They undergo a specific sequence of structural changes and increase in size as the egg grows. No microtubular or microfibrillar arrays have been found in the egg chamber which might form a cytoskeletal basis for spindle orientation or for the spatial differences which develop during differentiation of the uncleaved egg cytoplasm.
Conformational constraints in nuclear dna
Authors: Cook P. R. , Brazell I. A. 
Published: April 2021

ABSTRACT
We have investigated DNA superstructure in a wide range of nuclei of higher cells by gently lysing cells to release structures that resemble nuclei but are depleted of nuclear proteins. The sedimentation properties of these structures, which we call nucleoids, have been examined in sucrose gradients containing the intercalating agent, ethidium. The sedimentation rate of nucleoids derived from the growing cells of mammals, birds, amphibians and insects varies in the manner characteristic of circular and superhelical molecules of DNA. These characteristic changes in sedimentation rate are abolished by irradiating the nucleoids with low doses of γ-rays, a procedure known to introduce single-strand scissions into DNA. We have also investi gated by similar means DNA superstructure in nucleoids derived from a variety of different chick cells. Nucleoids derived from adult hen erythrocytes differ from the other nucleoids studied in that their sedimentation rate does not vary in the manner characteristic of super-coiled DNA.
Characterization of nuclear structures containing superhelical DNA
Authors: Cook P. R. , Brazell I. A. , JOst E. 
Published: April 2021

ABSTRACT
Structures resembling nuclei but depleted of protein may be released by gently lysing cells in solutions containing non-ionic detergents and high concentrations of salt. These nucleoids sediment in gradients containing intercalating agents in a manner characteristic of DNA that is intact, supercoiled and circular. The concentration of salt present during isolation of human nucleoids affects their protein content. When made in 1·95 M NaCl they lack histones and most of the proteins characteristic of chromatin; in 1·0 M NaCl they contain variable amounts of histones. The effects of various treatments on nucleoid integrity were investigated.
Purification of gibberellic acid-induced lysosomes from wheat aleurone cells
Authors: Gibson R. A. , Paleg L. G. 
Published: April 2021

ABSTRACT
Using isopycnic density gradient centrifugation, lysosomes were concentrated in a single region of a sucrose-Ficoll gradient (ρ = 1·10 g cm−3), well separated from most other cell organelles. Gibberellic acid-induced lysosomes were found to be rich in α-amylase and protease but not ribonuclease. The lysosomal band also contained a majority of the NADH2-cytochrome c reductase, a marker enzyme for endoplasmic reticulum, found in the gradient. Examination of electron micrographs revealed that a purified band of lyosomes contained at least 3 vesicle types, ranging in size from 0·1 to 0·5 μm. The significance of these findings to proposed mechanisms of action of gibberellic acid is discussed.
Barium accumulation in rat pancreatic B cells
Authors: Howell S.L. , Tyhurst Margaret 
Published: April 2021

ABSTRACT
Barium has been used as an electron-opaque substitute for calcium in a study of the distribution of divalent cations between organelles in homogenates or intact rat islets of Langerhans. These were incubated in the presence of barium acetate. Accumulation of electron-opaque deposits was stimulated during incubation of islets in the presence of high glucose concentrations and was diminished in conditions in which intracellular cyclic AMP levels were raised. Mitochondria were found to be the principal sites of accumulation of electron-opaque deposits. Addition of dinitrophenol to homogenates or intact islets abolished mitochondrial barium accumulation. X-ray microanalysis of the deposits in frozen sections showed them to consist predominantly of barium and phosphate.
These experiments serve to emphasize further the critical role of mitochondria in the regulation of divalent cation accumulation in B cells, and to confirm that a direct effect on intra-cellular distribution of divalent cations may represent one important mechanism of action of cyclic AMP in regulating insulin secretion.
Molecular regulation of Snai2 in development and disease
Authors: Zhou Wenhui , Gross Kayla M. , Kuperwasser Charlotte 
Published: April 2021

ABSTRACT
The transcription factor Snai2, encoded by the SNAI2 gene, is an evolutionarily conserved C2H2 zinc finger protein that orchestrates biological processes critical to tissue development and tumorigenesis. Initially characterized as a prototypical epithelial-to-mesenchymal transition (EMT) transcription factor, Snai2 has been shown more recently to participate in a wider variety of biological processes, including tumor metastasis, stem and/or progenitor cell biology, cellular differentiation, vascular remodeling and DNA damage repair. The main role of Snai2 in controlling such processes involves facilitating the epigenetic regulation of transcriptional programs, and, as such, its dysregulation manifests in developmental defects, disruption of tissue homeostasis, and other disease conditions. Here, we discuss our current understanding of the molecular mechanisms regulating Snai2 expression, abundance and activity. In addition, we outline how these mechanisms contribute to disease phenotypes or how they may impact rational therapeutic targeting of Snai2 dysregulation in human disease.
On the asymmetric partitioning of nucleocytoplasmic transport – recent insights and open questions
Authors: Kalita Joanna , Kapinos Larisa E. , Lim Roderick Y. H. 
Published: April 2021

ABSTRACT
Macromolecular cargoes are asymmetrically partitioned in the nucleus or cytoplasm by nucleocytoplasmic transport (NCT). At the center of this activity lies the nuclear pore complex (NPC), through which soluble factors circulate to orchestrate NCT. These include cargo-carrying importin and exportin receptors from the β-karyopherin (Kapβ) family and the small GTPase Ran, which switches between guanosine triphosphate (GTP)- and guanosine diphosphate (GDP)-bound forms to regulate cargo delivery and compartmentalization. Ongoing efforts have shed considerable light on how these soluble factors traverse the NPC permeability barrier to sustain NCT. However, this does not explain how importins and exportins are partitioned in the cytoplasm and nucleus, respectively, nor how a steep RanGTP–RanGDP gradient is maintained across the nuclear envelope. In this Review, we peel away the multiple layers of control that regulate NCT and juxtapose unresolved features against known aspects of NPC function. Finally, we discuss how NPCs might function synergistically with Kapβs, cargoes and Ran to establish the asymmetry of NCT.
RACK1 plays a critical role in mast cell secretion and Ca2+ mobilization by modulating F-actin dynamics
Authors: Filho Edismauro G. Freitas , da Silva Elaine Z. M. , Ong Hwei Ling , Swaim William D. , Ambudkar Indu S. , Oliver Constance , Jamur Maria Célia 
Published: April 2021

ABSTRACT
Although RACK1 is known to act as a signaling hub in immune cells, its presence and role in mast cells (MCs) is undetermined. MC activation via antigen stimulation results in mediator release and is preceded by cytoskeleton reorganization and Ca2+ mobilization. In this study, we found that RACK1 was distributed throughout the MC cytoplasm both in vivo and in vitro. After RACK1 knockdown (KD), MCs were rounded, and the cortical F-actin was fragmented. Following antigen stimulation, in RACK1 KD MCs, there was a reduction in cortical F-actin, an increase in monomeric G-actin and a failure to organize F-actin. RACK1 KD also increased and accelerated degranulation. CD63+ secretory granules were localized in F-actin-free cortical regions in non-stimulated RACK1 KD MCs. Additionally, RACK1 KD increased antigen-stimulated Ca2+ mobilization, but attenuated antigen-stimulated depletion of ER Ca2+ stores and thapsigargin-induced Ca2+ entry. Following MC activation there was also an increase in interaction of RACK1 with Orai1 Ca2+-channels, β-actin and the actin-binding proteins vinculin and MyoVa. These results show that RACK1 is a critical regulator of actin dynamics, affecting mediator secretion and Ca2+ signaling in MCs.
This article has an associated First Person interview with the first author of the paper.