bluuebunny/crossref_metadata_2025 huggingface dataset:
Packing DNA into chromosomes
Authors: Mullinger A. M. , Johnson R. T.
Published: April 2021

Electron microscopy of HeLa metaphase nucleoids (i.e. whole metaphase cells exposed to 2 M salt and non-ionic detergent) spread by the Kleinschmidt technique, reveals a variety of protein-depleted structures (spreads) derived from chromosomes. Spreads vary in size and shape. At one extreme are oval structures with one or more cores surrounded by a network of supercoiled fibres. These fibres are probably arranged as loops and we estimate that 600-1000 may emerge from a single, large core region. At the other extreme are chromosome-shaped spreads with an elongated core which takes the form of a multifibred axis. At intervals groups of lateral fibres appear to emerge from each axis to produce the network. Spreads intermediate between these 2 extremes occur in which axial fibres can be resolved in only part of the elongated core.

Similar structures are observed in chromosomes deproteinized and spread after isolation by a procedure which preserves high molecular weight DNA. The appearance of chromosomes isolated by the Wray-Stubblefield hexylene glycol procedure agrees in general with previous findings of others, except that in some more extended spreads axial fibres are visible.

We believe our observations are consistent with the idea that the chromonema of each metaphase chromatid contains regions of multistranded DNA. We do not propose, however, that the chromatid is functionally multineme, but rather that axial fibre folding either within or between chromomere regions contributes to packing of DNA in the metaphase chromosome. These regions of constraint are also postulated as the locations of emergence of lateral loops. In spread preparations the axial fibres are seen clearly only when chromatids have been elongated beyond the contracted metaphase length. Elongation would be produced both by relaxation of chromosomal coils (gyres) and by extension of the chromonema upon deproteinization and spreading. Whereas in deproteinized nucleoids the long axis is labile and liable to elongate, fragment or collapse, the chromonema of Wray-Stubblefield chromosomes is locked at the metaphase (gyred) length and axial fibres are generally not visible. We propose that the assembly of the complex DNA axis of the metaphase chromosome from its extended interphase counterpart plays a major part in increasing the DNA packing ratio in the mitotic cell.
Ultrastructure of multiple microtubule initiation sites in mouse neuroblastoma cells
Authors: Sharp Gaynor A. , Osborn Mary , Weber Klaus
Published: April 2021

Morphologically undifferentiated and differentiated mouse neuroblastoma N115 and N18 cells were examined after serial sectioning by electron microscopy. A sizeable percentage of the cells revealed multiple centrioles, usually clustered together in the perinuclear area with 2 preferential locations, i.e. above and below the largest nuclear diameter. These results indicate that the multiple microtubule- organizing centres previously visualized by immunofluorescence microscopy with tubulin antibody in neuroblastoma cells recovering from Col-cemid poisoning are most likely in majority related to multiple centrioles. This interpretation is further strengthened by experiments in which cells are first recorded in the fluorescence microscope and then after serial sectioning in the electron microscope. The results show that under optimal conditions immunofluorescence microscopy is able to visualize single centrioles. The possible biological significance of the combined electron and immunofluorescence microscopical results is discussed.
Arrangement of subunits in microribbons from Giardia
Authors: Holberton D. V.
Published: April 2021

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

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

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

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

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

In 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

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

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

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

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

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

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

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

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

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

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

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.
Ultrastructural observations on deep-etched thylakoids
Authors: Park R. B. , Pfeifhofer A. O.
Published: April 2021

Deep etching of spinach thylakoids frozen in water exposes the inner and outer surfaces of the thylakoid. Both these surfaces differ greatly in appearance from their respective adjacent fracture planes. This finding constitutes further evidence for membrane splitting during the fracture process. Modification of the fracture face by loss of shattered material or by plastic deformation may explain the partial mismatch between the two fracture faces observed in washed thylakoids.
The effect of ethylenediaminetetra-acetate washing on the structure of spinach thylakoids
Authors: Park R. B. , Pfeifhofer A. O.
Published: April 2021

The particles removed from spinach thylakoids by washing with ethylenediaminetetraacetate were found to correspond to particles on the outside surface of the thylakoid, and not to the quantasome cores which remain within the membrane.
Cytoplasmic dna-containing bodies and the response of amoebae to dimidium bromide
Authors: Hawkins Shirley E. , Willis Lesley R.
Published: April 2021

The growth of Amoeba proteus (T1P) and Amoeba discoides (T1D) in the trypanocidal phenanthridinium, dimidium bromide was examined. At concentrations of drug between 2 and 4 μg/ ml, A. proteus divided twice before inhibition and death. A. discoides was able to undergo an additional cycle of division before death. At other concentrations there were no differences in their response. Heterotransfers between these strains resembled A. discoides, both dividing three times before death. Examination of clones derived from the micro-injection of small quantities of A. discoides cytoplasm into A. proteus showed that the ability to divide additionally in dimidium bromide could be transferred. Some other strains of A. proteus (DP, T4P) also resembled A. discoides in their response.

Cells of all the strains used were treated with acridine orange and observed under the fluorescence microscope for the presence of DNA- and RNA-containing cytoplasmic ‘bodies’. All strains able to undergo an additional division cycle also possessed cytoplasmic DNA-containing bodies. The converse was not 100%, but this may be due to a quantitative difference in the number of DNA-containing bodies in the cytoplasm. It is proposed that the difference in response to dimidium bromide observed in A. proteus and A. discoides may be associated with the presence of DNA- and RNA-containing bodies in the cytoplasm of A. discoides.
Studies on the autonomy of pellicular DNA in Paramecium
Authors: Smith-Sonneborn Joan , Plaut W.
Published: April 2021

This autoradiographic study was designed to elucidate the relationship between the macro-nucleus and pellicular DNA in Paramecium. The capacity of the cell to synthesize pellicular DNA in the absence of the macronucleus was established by demonstrating the incorporation of tritiated thymidine into DNase-sensitive material in the pellicles of amacronucleate cells. Moreover, using a technique which leads to selective labelling of the macronucleus in normal paramecia, we have looked for evidence of transfer of labelled DNA from the macronucleus to the pellicle with time. Finding none, we conclude that labelled pellicular DNA is not of macro-nuclear origin, and that labelled pellicular DNA synthesis is not directly dependent on the presence of the macronucleus.
Cell proliferation in the epithelium of the oesophagus, trachea and ureter in mice
Authors: Blenkinsopp W. K.
Published: April 2021

Over a 24-h period, groups of mice were given a single injection of colchicine (to collect blocked metaphases) and tritiated thymidine (to label nuclei synthesizing deoxyribonucleic acid). Epithelial nuclei in the oesophagus, trachea and ureter were examined and counted in paraffin sections: the duration of deoxyribonucleic acid synthesis (Ts) was calculated from the numbers of blocked metaphases and labelled nuclei, the duration of the post-synthetic gap (Tc2) was estimated from the proportion of blocked metaphases labelled, and the cell cycle time (Tc) was calculated from Ts and the proportion of nuclei labelled. In each epithelium the different layers seen by light microscopy were analysed separately.

Ts was probably the same for the basal and superficial cells in the trachea (about 8 h), and was probably the same for the basal, intermediate and superficial cells in the ureter (about 5 h). In the oesophagus Ts was 8·5 h.

T G2 was probably the same for the basal and superficial cells in the trachea (3·6 h), and probably the same for the basal, intermediate and superficial cells in the ureter (about 4·6 h). In the oesophagus Tc. was 2’8 h.

Tc was about 380 h (basal cells) and 1400 h (superficial cells) in the trachea, and about 8000 h (basal and intermediate cells) and 2700 h (superficial cells) in the ureter. In the oesophagus Tc was 41 h.
The ultrastructure and ontogeny of pollen inHelleborus FoetidusL.: III. the formation of the pollen grain wall
Authors: Echlin P. , Godwin H.
Published: April 2021

The first recognizable elements of the pollen grain wall of Helleborus foetidus are initiated in the cellulosic primexine which is formed immediately outside the microspore cytoplasm while the pollen grains are still in the tetrad configuration and enveloped in a thick layer of callose. Elements of the primexine give rise to the precursors of the rod-like bacula of the mature exine. The bacula increase in electron density due to the rapid deposition of sporopollenin, and begin to expand laterally at the outer side to form the tectum. There follows a lateral expansion on the inner side to form the foot-layer. Further deposition of sporopollenin is continued and all elements of the pollen grain wall expand outwards and laterally as the pollen grain enlarges. The enveloping callose disappears and the pollen grains are free in the thecal cavity. A secondary exine is deposited below the primary exine, particularly around the furrows. Initially this process involves a number of thin electron-transparent lines or lamellae about 4 nm thick that appear to arise from the cytoplasm and provide a locus around which sporopollenin is deposited. As the deposition proceeds, the lamellae thicken and finally merge with each other to form the secondary exine. No sign of the lamellae can be seen in the mature pollen grain wall. Towards the end of secondary exine formation the deposition of sporopollenin does not appear to be centred on thin lamellae, but appears as small granules which gradually coalesce. The secondary exine remains discontinuous in the region of the furrow, but becomes consolidated in the inter-furrow regions.

As the pollen grain matures the sporopollenin, which is electron-dense when initially deposited, becomes progressively less so.

The final stage in development is the deposition of the cellulosic intine, which forms inside the secondary exine and is associated with increased dictyosome activity and randomly oriented microtubules.
Observations on the fine structure of the eyespot and associated organelles in the dinoflagellate Glenodinium Foliaceum
Authors: Dodge J. D. , Crawford R. M.
Published: April 2021

The eyespot of the marine dinoflagellate Glenodiniumfoliaceum is a flattened orange structure, more or less trapezoid in shape with an anterior hook-like projection. It is situated on the ventral side of the organism in the vicinity of the flagellar bases at the anterior end of the sulcus. In the electron microscope the eyespot is seen to contain two layers of osmiophilic granules 80-200 nm in diameter which usually show hexagonal close-packing. The eyespot is surrounded by a triple-membraned envelope and is not connected to any other organelle. Adjacent to the eyespot is a distinctive organelle termed the ‘lamellar body’. This consists of a stack of up to 50 flattened vesicles or disks, each 16 nm thick and about 750 nm wide, the whole being orientated in an antero-posterior direction. The lamellae are continuous, at the ends of the stack, with rough endoplasmic reticulum and are joined together by occasional bridges at their edges. The bases of the two flagella lie just ventral to the lamellar body and from them roots arise which pass by the eyespot and join the subthecal microtubular system.

The eyespot of Glenodinium is unique both in structure and the presence of the associated lamellar body. It differs from eyespots which have been described from other algal groups and also from the more complex ocellus found in certain dinoflagellates belonging to the order Warnowiaceae. The method by which the eyespot functions is discussed and it is suggested that unidirectional stimuli could be perceived by shading of the lamellar body.
Synapses on the axon hillocks and initial segments of pyramidal cell axons in the cerebral cortex
Authors: Jones E. G. , Powell T. P. S.
Published: April 2021

The axon hillocks and initial segments of pyramidal cell axons can be clearly recognized in electron micrographs of the somatic sensory cortex. The initial segment is characterized by three features: bundles of neurotubules linked together by electron-dense bands, a layer of dense material attached to the inner surface of the plasmamembrane, and small membrane-bound dense bodies. All of these elements and the few ribosomes usually present disappear at the commencement of the myelin sheath. The initial segment of the axon often contains a cluster of cisternae similar to the spine apparatus, and this part of the axon sometimes gives off small branches. Axon terminals end on both the axon hillock and the initial segment, and there is an increase in number on the latter as the distance from the hillock increases. All of these terminals are relatively large, contain a high proportion of small flattened or pleomorphic synaptic vesicles and terminate in symmetrical synaptic contacts. These morphological features suggest that the synapses may be inhibitory in function.
Morphological variations in the dendritic spines of the neocortex
Authors: Jones E. G. , Powell T. P. S.
Published: April 2021

The structural variations which occur in the dendritic spines of pyramidal neurons of the somatic sensory cortex of the cat are described, particular attention being paid to spines attached to different parts of the dendritic tree. Spines may be recognized particularly by the absence of neurotubules and the common presence of a spine apparatus, and they can be considered as pednnculated or sessile, depending upon the presence or absence of a narrow pedicle. Within these 2 categories spines may be rounded, cup-like or prismatic, can be large or small, and may show various degrees of inversion of the surface receiving a synaptic contact. While spines of every size and shape may be attached to dendrites of all diameters, there is a definite tendency for the largest spines to occur on the smallest dendrites and vice versa. Furthermore, the smallest dendrites possess the spines with the longest pedicles. Every dendritic spine receives 1 axon terminal containing spherical synaptic vesicles and ending in an asymmetrical synaptic contact. In addition, 10–20 % of the spines receive a second terminal, which in some cases may contain spherical vesicles and terminate asymmetrically but in others contains small, flattened or pleomorphic vesicles and ends in a symmetrical contact. The additional terminal may end on the pedicle of the spine or on the parent dendrite near the attachment of the pedicle. In the latter site, asymmetrical synapses are commonly associated with an additional spine apparatus.
A cytochemical study of lectin receptors on isolated chick neural retina neurons in vitro
Authors: Bee James A.
Published: April 2021

The cell body, neurite and growth cone of isolated retinal neurons have been compared on the basis of their ability to bind a number of fluorescently labelled lectins, each possessing a unique carbohydrate specificity. The susceptibility of the respective binding patterns following pretreatment of these fixed cells with either neuraminidase or trypsin was also investigated. Neuronal cell bodies displayed the most intense binding of each lectin, with localization of limulin binding (specific for sialic acid) predominantly to the neurite hillock, the point on the cell body from which the neurite projects. Limulin binding was almost totally abolished by pretreatment with either neuraminidase or trypsin. In contrast to the cell body, limulin binding to the neurite or growth cone was not detected. These regions of the cell apparently possessed sialic acid, however, since pretreatment with neuraminidase reduced wheat germ agglutinin binding (to N-acetylglucosamine) and markedly enhanced Dolichos biflorus agglutinin binding (to JV-acetylgalactosamine) to both the neurite and growth cone. The initially low binding of Dolichos biflorus agglutinin to the neurite and growth cone was slightly enhanced by pretreatment with trypsin. Uniformly low levels of binding of either Ricinus communis agglutinin 60 (galactose, jV-acetylgalactosamine) or R. communis agglutinin 120 (galactose) was observed over the entire neuron. R. communis agglutinin 120 binding was not enhanced by pretreatment with neuraminidase. Receptors for either concanavalin A (mannose, glucose) or Ulex europaeus agglutinin I (fucose) were abundant over the entire nerve cell with the former exhibiting more marked trypsin sensitivity. From these data, it is apparent that the repertoire of lectin binding sites of the neurite and growth cone of these differentiating nerve cells differs markedly from that of the cell body, which itself demonstrates some degree of regionalization.
Pollination sub-systems distinguished by pollen tube arrest after incompatible interspecific crosses in Rhododendron (Ericaceae)
Authors: Williams Elizabeth G. , Knox R. Bruce , Rouse John L.
Published: April 2021

The cytology of compatible and interspecific incompatible pollinations has been followed in selected species of the genus Rhododendron (Ericaceae). Pollinated pistils were fixed, cleared, stained in decolourized aniline blue, and observed by epifluorescence microscopy. Ten different abnormalities of arrested pollen tube tips have been detected, including burst, tapered, swollen, coiled, spiralling, spiky and variable diameter syndromes. A series of five errors of callose deposition in incompatible tubes has also been defined. Six different regions in the pistil for expression of pollen tube arrest have been found, including the stigmatic exudate, the mucilage of the upper and lower style canal, the ovary loculus, the micropyle. There may also be abnormal behaviour after entry into the embryo sac. Both the site of pollen tube arrest within the pistil, and the error syndrome of tip growth and callose deposition anomalies, are characteristic of each interspecific cross. These results are discussed in relation to the genetic control of reproduction.
Changes in wool follicles of sheep following administration of dexamethasone trimethylacetate
Authors: Chapman R. E. , Panaretto B. A. , Frith P. A.
Published: April 2021

Intramuscular injection of 50 mg dexamethasone trimethylacetate produced variable effects on the fleeces of Merino sheep, ranging from an obvious discontinuity in the fleece, due to inhibition of growth of the majority of fibres, to no macroscopically observable effect. In the most affected animals, the mitotic activity and size of the follicle bulbs decreased during the first five days. Subsequently, fibre and inner root sheath cells withdrew from around the dermal papillae and were replaced by cells from the outer root sheaths. Autophagic vacuoles developed in these replacement cells. Regression of follicles proceeded until brush-ends formed on the fibres after 9–15 days. The amount of glycogen decreased inthe outer root sheath cells, which exhibited various responses at different levels in the follicles. The proportion of regressed follicles and to some extent the rate of regression were highest in the sheep with the most obvious discontinuity in the fleece. Regeneration of follicles was well advanced in all sheep 29 days after the injection.
Effect of the ionic environment on the incorporation of the intermediate-sized filament protein vimentin into residual cell structures upon treatment of ehrlich ascites tumour cells with triton x-100: II. Ultrastructural analysis
Authors: Nelson W. James , Traub Peter
Published: April 2021

Ehrlich ascites tumour cells were extracted in buffers containing Triton X-100 and mono-, di- and polyvalent cations and then analysed by phase-contrast and electron microscopy. The results of this ultrastructural analysis confirm those of the biochemical analysis in the accompanying paper that the stability of intermediate-sized filaments is dependent on the ionic environment. Furthermore, the organization of filaments in long parallel arrays is dependent on the presence of divalent cations and can be inhibited, to some extent, by the presence of monovalent cations. The stability of other detergent-resistant structures, the boundary lamina, microfilaments, microtubules, centrioles, polyribosomes and the nuclear cortex, is also affected by the ionic environment but to a lesser extent.
Dissociation of cytological and functional differential in virgin mouse mammary gland during inhibition of DNA synthesis
Authors: Vonderhaar Barbara K. , Smith Gilbert H.
Published: April 2021

Epithelial cells in mammary gland explants from mice assume a secretory appearance and synthesize the milk proteins, casein and α-lactalbumin, when cultured in the presence of insulin, hydrocortisone and prolactin. In cells from the glands of mature virgin animals such syntheses are known to require DNA synthesis. Addition of cytosine-β-D-arabinofuranoside to the explant cultures suppresses both hormonally induced DNA synthesis and enhanced production of milk protein. To determine the level at which this block in terminal differentiation occurs, epithelial cell pellets were prepared from virgin mouse mammary gland explants cultured with various combinations of insulin, hydrocortisone and prolactin, and subsequently examined by light and electron microscopy. We observed that the epithelial cells cultured in the presence of all three hormones developed fully, cytologically and ultrastructurally, even in the absence of DNA synthesis in vitro. Likewise, these cells were able to incorporate [3H]uridine into RNA efficiently and to incorporate amino acids into acid-precipitable polypeptides at levels equivalent to the untreated controls. However, immunoprecipitation of newly synthesized casein peptides showed that no new synthesis of casein occurred in cells prevented from synthesizing DNA. These data show uncoupling of cytological development and synthesis of milk protein in mammary explants from mature virgin mice inhibited from synthesizing DNA.
Temperature dependence of certain integrated membrane functions in macrophages
Authors: Shirazi Mahshid Faghihi , Aronson Nathan N. , Dean Roger T.
Published: April 2021

We have studied the effect of temperature on uptake and degradation of molecules entering mouse peritoneal macrophages by fluid-phase, adsorptive and receptor-mediated p1nocytos1s, and on degradation of their intracellular proteins. Uptake of [3H]sucrose and uptake and degradation of formaldehyde-treated 125I-labelled human serum albumin and 125I -labelled mannose-bovine serum albumin continued, but were progressively slowed as the temperature decreased from 37 °C to 20 °C. The uptake and degradation were completely abolished at approximately 15 °C. Arrhenius plots for adsorptive and fluid uptake were unilinear, whereas that for receptor-mediated endocytosis showed an inflection point at approximately 20 °C. The results did not indicate any distinction between adsorptive and fluid p1nocytos1s. An ‘intracellular turnover time’ calculated for mannose-bovine serum albumin taken up by the specific route is 19 – 24 min and this time calculated for human serum albumin is, in contrast, 99 min. Studies of the kinetics of degradation of both endocytosed and endogenous proteins showed similarity in the temperature cut-off of degradation of endocytosed and endogenous long-half-life proteins (≃ 15 °C) and continuance of endogenous short-half-life degradation at much lower temperatures.