bluuebunny/crossref_metadata_2025_split huggingface dataset:
Arrangement of subunits in microribbons from Giardia
Authors: Holberton D. V. 
Published: April 2021

ABSTRACT
Ultrasound has been used to disperse the cytoplasm of Giardia muris and Giardia duodenalis trophozoites, releasing disk cytoskeletons for negative staining and study by electron microscopy. Sonication also breaks down the cross-bridges uniting microribbons in disks. Individual ribbons, and small bundles of these structures, are found in these preparations and have been imaged both from their edges and in flat face view. The outer layers of ribbons are 2 sheets of regularly arranged globular subunits, held apart by a fibrous inner core. The axial repeat of the microribbon is 15 nm, which is also the distance separating cross-bridge sites along ribbons. Pronounced striping at this interval is a feature of ribbon faces where they are joined in bundles.
Subunits in the outer layer are arranged in vertical protofilaments that are set orthogonally to the long axis of the ribbon. Protofilaments bind tannic acid and are seen clearly in sectioned ribbons. Three protofilaments fit into the 15-nm longitudinal spacing. Optical diffraction patterns from ribbon images are dominated by orders of the 15-nm periodicity, including the third- order reflexions expected from protofilament spacings. Fourth- order reflexions indicate that the ribbon core may also be structured. Ribbon face images give rise to a strong 4-nm layer line, corresponding to the vertical spacing of subunits in protofilaments. Neighbouring protofilaments are staggered by about 0·67 nm.
The lattices found in ribbons are consistent with studies of cytoskeleton composition.
Correlation between barr body and overall chromatin decondensation in vitro
Authors: Grattarola M. , Belmont A. , Nicolini C. 
Published: April 2021

ABSTRACT
Geometric and densitometric properties of the Barr body of early and late phase II confluent human fibroblasts are analysed by the automated image analyser Quantimet 720 D. In cells with the same 2C DNA content, the state of condensation of the Barr body varies proportionally with the state of condensation of the entire nucleus yielding a correlation between nuclear and Barr body area. In light of these results, indicating the participation of the Barr body in the overall process of chromatin condensation and decondensation, a definitive ‘static’ separation between dense (‘heterochromatic’) and dispersed (‘euchromatic’) regions of chromatin seems to be arbitrary.
The implications of these results in terms of a possible attachment of interphase chromatin to the nuclear envelope are briefly discussed.
Control of cell division of the intracellular Chlorella symbionts in green hydra
Authors: McAULEY P. J. 
Published: April 2021

ABSTRACT
Green hydra maintains within its digestive cells a population of symbiotic algae which remains constant in normal culture conditions, although potentially the algae have a much higher growth rate than their animal hosts. Numbers of algae per cell vary along the body column, cells in the gastric region containing more than those of the head or peduncle.
This relationship is disturbed in excised, regenerating peduncles and heads; a transitory increase in algal numbers occurs, the decline of which may be prevented by application of the mitotic inhibitor vinblastine sulphate. No increase is seen in the already high numbers of algae in the gastric regions of regenerating animals.
A close link between host and symbiont mitosis may explain this phenomenon. Nondividing host cells with a full complement of algae inhibit symbiont mitosis; the inhibition is removed when the host cell is stimulated to divide, as in regenerating peduncles and heads. Algae divide more rapidly than host cells, so a transient increase in algal numbers occurs. During host cell division, algae are parcelled between daughter cells, which reimpose inhibition once the normal population of algae is reached.
There may be no increase in algal numbers in regenerating gastric regions because extensive mitosis already occurs.
The nature of the restriction on algal growth remains obscure, but uncoupling of animal and algal mitosis during regeneration suggests a useful experimental approach to the problem.
The formation and orientation of brush border vesicles from rat duodenal mucosa
Authors: Hearn P. R. , Russell R. G. G. , Farmer J. 
Published: April 2021

ABSTRACT
An electron-microscopic study of various fractions taken during a newly developed method for preparation of brush border vesicles from rat duodenum, has shown that vesiculation of microvilli can occur in situ on the brush border membrane. This mode of formation ensures that the vesicle membranes are oriented as in vivo and that they are ‘right-side-out. The disruption of core protein-fibre appears to be a prerequisite for vesicle formation.
Spirals and paracrystals induced by vinca alkaloids: evidence that microtubule-associated proteins act as polycations
Authors: Haskins Kathryn M. , Donoso J. Alejandro , Himes Richard H. 
Published: April 2021

ABSTRACT
Formation of the vinca-induced spirals from tubulin and microtubules is dependent upon the presence of microtubule-associated proteins. Formation of paracrystals visualized in thin-sectioned material is also shown to require the presence of microtubule-associated proteins. The polycationic behaviour of the latter in stabilizing microtubules and spirals is demonstrated in experiments where assembly of tubulin in their absence was stimulated by polylysine or high Mg2+ and addition of vincristine caused spirals to form. Spirals were stable to cold, Ca2+, and colchicine, but polyadenylic acid caused their depolymerization.
A comparison of results obtained between two methods of detecting the existence of circadian rhythmicity in cell division: an autoradiographical and a biochemical approach
Authors: Challoner D. , Simnett J. D. , Hellmann K. 
Published: April 2021

ABSTRACTIn the course of a study of circadian rhythmicity in the number of cells in the S-phase of the cell cycle in mouse kidney, a comparison was made between 2 methods of estimation of DNA synthetic activity. One of the methods used was an autoradiographical analysis of the tritiated thymidine labelling index; the other was a biochemical assay of the DNA content of the tissues coupled with liquid scintillation counting, to obtain an estimate of the incorporation of tritiated thymidine, expressed as dpm per μg of DNA. The correlation between individual estimates of DNA-synthetic activity, using the 2 methods was highly significant, as was the correlation between the 2 data sets with respect to time of zeniths. There was a suggestion that the biochemical assay procedure may be the more sensitive indicator of circadian rhythmicity in relatively homogeneous tissues with low proliferative rates. Estimated over a 3-day period, the time of maximal DNA synthetic activity lay between the hours of 22.00 and 02.00. The nadir of the curve was less well defined and occurred between 02.00 and 14.00 hours. The autoradiographical study took 5 weeks to complete exclusive of histological and autoradiographical preparation. The biochemical assay was completed in 4 days.
Functional states of rna polymerase in the macronucleus of Tetrahymena pyriformis and their dependence on culture growth
Authors: Freiburg Manfred 
Published: April 2021

ABSTRACT
RNA polymerase I in macronuclei of late log-phase cells of the ciliate Tetrahymena pyriformis is found to be present in 2 functional states, the one being actively engaged in transcription of ribosomal genes, the other one in a ‘free’ state, i.e. free to transcribe added DNA or poly d(AT).
Stimulation of RNA synthesis induced by dilution of stationary-phase cells into fresh medium is correlated with an increase in activity of template engaged and a decrease of free RNA polymerase I,
In contrast, RNA polymerase II shows no pronounced changes in activity and is not available to transcribe poly d(AT). The data favour the assumption that factors other than the amount of RNA polymerase I molecules available control transcription of the ribosomal genes.
Cooperativity of concanavalin a patching and its influence on cytoskeleton changes in fibroblast rounding and detachment
Authors: Badley R. A. , Woods Anne , Rees D. A. 
Published: April 2021

ABSTRACT
We have studied the extent of concanavalin A (Con A) patching on spread fibroblasts as a function of increasing concentration of added Con A, and also the influence of this patching on subsequent cytoskeleton changes during exposure to trypsin and EGTA. Living cells do not patch at low concentrations of Con A (around 1 μg ml-1) but progressively do so at higher levels (2–10 μg ml-1), in very close parallel with a cooperative phase of binding characterized earlier. The proportion of visibly patched cells is at a maximum when this cooperative phase is complete and the correlation remains when cells are treated with azide or cytochalasin B to shift the concentration range over which both phenomena occur. All this evidence points to a common origin for cooperative binding and visible patching which is sensitive to the state of cellular actomyosin, and involves the mediation of actin-associated structures.
As a consequence of Con A patching, cell attachment is preserved against the action of EGTA. On patched cells, this agent causes minor if any changes in cell shape and in the distributions of microtubules and 10-nm filaments, but actin stress fibres are dispersed. However, conditions appear to be retained for reassembly of the stress fibres because these can be caused to reappear within minutes after removal of EGTA when glucose, or Ca2+ or Mn2+ in the absence of glucose, is supplied in the medium. Trypsin caused only partial destruction of stress fibres in Con A-treated cells and did not promote detachment or dispersal of the microtubules or 10-nm filament systems.
Pollen tube development in Petunia hybrida following compatible and incompatible intraspecific matings
Authors: Herrero Maria , Dickinson H. G. 
Published: April 2021

ABSTRACT
Pollen tubes formed following compatible and incompatible intraspecific matings in Petunia have been examined with light and electron microscopes. Compatible and incompatible tubes develop in an identical fashion on the stigma but, on entry into the top 1 mm of the stylar transmitting tissue changes occur both to the cytology of the tubes and their rates of growth. The early cytological changes are common to tubes of both compatibilities but, although both types of tube accelerate on entry into the style, incompatible tubes grow more slowly than compatible. Cytological differences became apparent between compatible and incompatible tubes following a short period of growth in the style, the latter possessing thicker cell walls and a cytoplasm packed with both organelles and reserves. Incompatible tubes subsequently burst or simply cease growth and die. The characteristic image afforded by this cytoplasm resembles that of burst or dead compatible tubes, except in that proportions of the cell components may differ. These data are discussed in terms of current models proposed to explain pollen tube growth and the operation of the self-andincompatibility response in Petunia.
Evolutionary constraints on quantitative variation and regulation of macronuclear dna content in the genus Tetrahymena
Authors: Doerder F. P. , Ditaranto J. , Debault L. E. 
Published: April 2021

ABSTRACT
Fluorescence cytophotometry was used to examine quantitative variation and regulation of macronuclear DNA content in 44 clones representing 13 species in 3 species-complexes of the genus Tetrahymena. Mean DNA amounts for G2 macronuclei generally ranged from 20–50 pg, with extreme means of 10 and 196 pg. Both intra- and interspecific variation was usually significant at the 5% level, and in some instances DNA amounts for the same clone in repeated experiments were significantly different. Nevertheless, intraclonal ranges both within and among species frequently showed considerable overlap and, in the context of all the ciliates, the range of means is small. Calculations suggest that a biologically more meaningful measure, gene dosage, is also evolutionarily conserved.
Additional evolutionary constraints are found in the regulation of macronuclear DNA content. Analysis of G1 and G2 intraclonal variances shows that in all species the variance added by regular unequal macronuclear division is removed by modification of cell-cycle events according to Model II regulation. In Model II, macronuclei with a small amount of DNA undergo an additional 5 phase before nuclear and cell division, whereas macronuclei with a large amount of DNA omit an S phase. Chromatin extrusion is also a regular feature of macronuclear division in most species, but its role in regulation is unclear. Extrusion regulates downward the mean amount of DNA but may actually contribute to unequal division and therefore add rather than remove variance.
Circus movements in dissociated cells in normal and hybrid frog embryos
Authors: Johnson Kurt E. , Adelman Mark R. 
Published: April 2021

ABSTRACT
Circus movements, involving circumferential rotation of a hyaline cytoplasmic blister and endoplasmic flow, occur in EDTA-dissociated gastrula stage Rana pipiens embryos. Such cell movements occur in very few cells taken from pre-gastrula stage embryos. During gastrulation, there is a progressive increase in the proportion of a population of cells that is engaged in circus movements. Circus movements do not occur in dividing cells. Individual cells in culture, as well as small clusters of cells in vitro, are jostled about in an apparently aimless fashion over short distances by circus movements, although the translocation of masses of cells over long distances is substantially greater than the translocation of isolated cells. In an early gastrula stage normal embryo, cells from around the site of blastopore invagination are most active in circus movements. Cells taken from different stages of arrested hybrid embryos show variable depression in the formation of rotating hyaline blebs. Aggregates of cells from arrested hybrid embryos are also relatively immobile in culture. The morphogenetic significance of circus movements in normal embryos and gastrula-arrest hybrid embryos is discussed.
Cell types involved in the expression of foetal aldolases during rat azo-dye hepatocarcinogenesis
Authors: Guillouzo André , Weber Anne , Provost Evelyne le , Rissel Maryvonne , Schapira Fanny 
Published: April 2021

ABSTRACT
Cellular and subcellular immunolocalization of aldolase isozymes and a-foetoprotein (AFP) was performed in rat liver during the different stages of carcinogenesis induced by 3’-methyl-4-dimethylaminoazobenzene.
During the early stages, double-labelling experiments showed that oval and transitional cells that expressed foetal aldolases did not contain adult aldolase B; this isozyme was only found in small and ‘normal’ hepatocytes. AFP was present in transitional cells and in small hepatocytes. During hyperplastic nodule development, neither foetal adolases nor AFP were located in hepatocytes. These foetal proteins were still observed in transitional cells. In hepatocellular carcinomas, both foetal proteins (aldolase isozymes and AFP) and adult aldolase B were present in malignant cells. Moreover, during the different stages foetal aldolases were also found in sinusoidal cells.
These results indicate that, during azo-dye hepatocarcinogenesis, (a) several cell types synthesize foetal aldolases: oval and transitional cells, hepatoma cells and sinusoidal cells; (b) only hepatoma cells and not hepatocytes located in hyperplastic nodules can express both foetal and adult aldolases. This suggests that in primary, as in transplanted, hepatoma the resurgence of foetal isozymes is the consequence of a disturbance of control gene expression.
Control of cell shape by calcium in the euglenophyceae
Authors: Murray John M. 
Published: April 2021

ABSTRACT
The euglenoid flagellates are able to change their shape rapidly in response to a variety of stimuli, or sometimes spontaneously. Two extremes of shape can be identified: the ‘relaxed’ form is cylindrical; the contracted form is a somewhat distorted disc. These 2 forms can be interconverted by treatments that alter the Ca2+ concentration of the entire cell. The level of Ca2+ is believed to be normally controlled by a system of calcium-accumulating membranes, identified in Astasia longa by the technique of calcium oxalate precipitation. The system forms a set of parallel tubes of endoplasmic reticulum, one of which lies immediately below each of the ridges of the pellicle. The individual ridges, each with its associated reticulum, microtubules and other elements are suggested to be independent motor units. Local activation of a small number of these units by Ca2+ is made possible by the arrangement of Ca2+-sequestering reticulum, producing the characteristic squirming euglenoid movement. Uniform activation or suppression of all units produces the 2 extremes of shape.
The pellicle of A. longa with its associated microtubules has been purified and shown to contain a Ca2+-binding site and ATPase activity.
The choline and serum protein requirements of mouse fibroblast cells (Strain LS) in culture
Authors: Birch J. R. , Pirt S. J. 
Published: April 2021

ABSTRACT
The maximum cell population density of mouse fibroblast (strain LS) cells growing in static suspension culture was found to be directly proportional to the dialysed calf serum concentration. This was due to choline limitation and the fact that serum protein was a major source of choline. The growth yield (Y) was 3·2 × 105 cells/μg choline chloride.
Studies on the role of serum in the presence of excess choline showed the following. When protein was omitted from the medium, cell death occurred. Whole serum protein could be replaced by either (1) bovine serum albumin fraction V, or (2) crystalline bovine serum albumin + sodium pyruvate and α-ketoglutarate, or (3) polyvinylpyrrolidone + methylcellulose + pyruvate and a-ketoglutarate.
The population doubling time was 24 h in the presence of whole serum protein and increased considerably with the substitutes (1—3). The increase in maximum cell population density (without medium changes) exceeded 2·9 × 106 cells/ml with either whole serum protein or substitutes (1) and (2). With serum substitute (3) the maximum increase in population density was reduced to 1·6 × 106 cells/ml.
Synthesis of cortical proteins in Tetrahymena
Authors: Williams N. E. , Michelsen O. , Zeuthen E. 
Published: April 2021

ABSTRACT
The synthesis of proteins of the oral apparatus in the ciliate Tetrahymena pyriformis has been studied in relation to the development of this organelle system within the cell. Oral proteins were labelled in both logarithmic phase and synchronized cells by growing the cells in the presence of tritiated amino acids. Estimates of the relative amounts of radioactive protein within the old and developing oral structures under various experimental conditions were made by radioautography of the structures after isolation from the cells. Evidence is presented that (i) mature oral structures undergo constant protein turnover; (ii) new oral organelle systems are constructed largely from proteins which are made prior to organelle differentiation and which are contained within a cytoplasmic pool and possibly also in pre-existing oral apparatuses; (iii) there are regular features in the label incorporation and conservation data which correlate well with morphological changes known to occur in the anterior (old) oral apparatus just prior to division; (iv) incorporation of labelled amino acids into the protein of oral structures does not occur during the synchronizing heat shocks; and (v) incorporation of label into the developing oral primordium does occur during primordium development, although it is not known whether these structural proteins are those demonstrated by other studies for which synthesis during development is an absolute requirement.
On the surface coat and flagellar adhesion in trypanosomes
Authors: Vickerman K. 
Published: April 2021

ABSTRACT
Pathogenic trypanosomes in their bloodstream phase have a smooth and compact coat 12–15 nm thick enveloping the entire surface membrane of the body and flagellum. In the sleeping-sickness trypanosome Trypanosoma rhodesiense this coat is absent from the stages of development in the midgut of the tsetse-fly vector and from their counterparts obtained by cultivation of the trypanosome in vitro. In the salivary glands of the vector, however, the coat is reacquired as the trypanosomes transform from epimastigote forms into the metacyclic stage which is infective to the mammalian host. This loss and acquisition of the surface coat can be correlated with the cyclical changes in net surface charge on the trypanosome which have been observed by other workers.
The trypanosome populations of successive relapses in- the blood are known to differ in their surface antigens (agglutinogens) and the loss of antigenic identity detected when any of these populations are put into culture indicates that these variable antigens are located in the surface coat. It is suggested that the coat in bloodstream trypanosomes constitutes a replaceable surface which, after being replaced, enables the trypanosome to escape the effects of host antibodies. The coat is therefore an adaptation to life in the bloodstream. Reacquisition of the surface coat by the metacyclic trypanosome after development in the vector may reflect reversion to a ‘basic’ antigenic type at this stage, preparatory to invading the blood of the mammalian host.
The surface coat may be removed by the wide-spectrum proteolytic enzyme pronase, and this fact together with evidence from pH/mobility relationships and chemical analysis of the variable antigens suggest that the coat is basically proteinaceous.
The coat may facilitate pinocytosis by binding proteins at sites within the pocket surrounding the base of the flagellum.
In the non-pathogenic trypanosome T. lewisi a more diffuse filamentous coat is present in bloodstream forms and absent from culture forms. This trypanosome is said to carry a negative charge in both bloodstream and culture phases, so it seems likely that the nature of the coat in T. lewisi is different from that found in the pathogenic trypanosomes.
In all these trypanosomes the flagellar membrane adheres to the surface membrane of the body throughout the life-cycle. Along the zone of adhesion lies a regular row of junctional complexes of the macula adherens type which, it is argued, serve in attachment. These attachments persist regardless of changes in the intervening cell surfaces.
The egg shell of the house cricket (Acheta Domesticus): an electron-microscope study
Authors: Furneaux P. J. S. , James C. R. , Potter S. A. 
Published: April 2021

ABSTRACT
Consecutive changes in two discrete layers of the egg shell of the house cricket, Acheta domesticus, have been claimed to control the uptake of water by the eggs. The development of the shell has been re-investigated with the electron microscope by examination of eggs at different stages of embryogenesis and of ovarioles containing oocytes at various stages of maturity.
It is confirmed that fragmentation of the maternal epicuticle and deposition and resorption of the serosal cuticle are the only apparent changes in the shell during development. The existence of a serosal epicuticle is confirmed and a distinction is made between the serosal epicuticle and the vitelline membrane.
Previously unreported features of the shell are (i) an outer zone of the maternal endocuticle which seems to be the most stable part of the maternal cuticle, (ii) a microlaminar organization within the scales of the maternal epicuticle, (iii) a vitelline membrane containing specialized regions, which remains distinct from the serosal epicuticle throughout development, and (iv) the osmiophilic character of the serosal epicuticle, its complex fine structure and its origin. Observations on eggs which had just begun to absorb water allow us to suggest that fragmentation involves a shrinkage of the material of which the scales are composed.