bluuebunny/crossref_metadata_2025_split huggingface dataset:
Suppression of myoblast fusion by concanavalin A: Possible involvement of membrane fluidity
Authors: Sandra Alexander , Leon Myron A. , Przybylski Ronald J. 
Published: April 2021

ABSTRACT
Experimental evidence is presented which is consistent with the involvement of membrane fluidity during myoblast fusion. Treatment of pretrypsinized myoblasts with tetrameric Con A, but not with the dimeric succinyl derívate, inhibits fusion. Inhibition is reversed by treatment with a-methyl-D-mannoside or subsequent trypsinization. No inhibition is observed when the lectin is incubated with cells at 4 °C unless the incubation is followed by treatment with glycogen, a multivalent Con A cross-linking agent. This effect of glycogen is reversed by subsequent treatment with a-amylase. Direct observation of Con A-binding site topography by transmission electron microscopy of membrane replicas of cells labelled with Con A and haemocyanin reveals that inhibition of fusion correlates with a clustered distribution of Con A-binding sites, whereas normal fusion correlates with a dispersed distribution.
Tubular elements-a new structure in blue-green algal cells
Authors: Khan Z. N. Tahmida , Godward M. B. E. 
Published: April 2021

ABSTRACT
New cell inclusions, ‘tubular elements’ are described from the blue-green alga Calothrix marchica. These structures are found in about 20% of all cells observed. They are 18–22 run in diameter, apparently composed of sub-units of which the estimated number, as seen in transverse section, is 8–10.
Transcriptional reactivation of isolated xenopus erythrocyte nuclei: Patterns of rna synthesis
Authors: Gregory S. P. , Hilder V. A. , Maclean N. 
Published: April 2021

ABSTRACT
Nuclei isolated from Xenopus erythrocytes can be transcriptionally reactivated by exposure to certain cytoplasmic proteins. The types of RNA synthesized during this reactivation have been studied and compared with those present in, or synthesized by, isolated nuclei not so reactivated or in entire Xenopus erythrocytes. In all cases, the pattern of transcription indicates the synthesis of a broad range of low molecular weight RNAs. Competitive hybridization demonstrates that the reactivated nuclei synthesize some transcripts not normally produced by the isolated nuclei and we have shown that a proportion of these possess amino acid-accepting activity. The significance of these results is discussed in relation to the control of gene activity in these cells.
Pattern formation in early insect embryogenesis-data calling for modification of a recent model
Authors: Kalthoff Klaus 
Published: April 2021

ABSTRACT
A mathematical model of biological pattern formation based upon lateral inhibition has recently been applied by Meinhardt to insect embryogenesis. This model has stimulated a re-evaluation of previous results, and new experiments designed to test the validity of the model. Split u.v. dose experiments with eggs of the chironomid midge Smittia show that the effective targets for the production of the aberrant pattern ‘double abdomen’ are not subject to the rapid turnover which is required by the model in its currently published version. Certain types of segment pattern, and differences in the length of segments as predicted by the model could not be observed. Other data conflict with the rather unusual type of photoreversal and the particular view of determination associated with the model. The model can be reconciled with part of the conflicting data if the effective targets for double abdomen induction are regarded as morphogen-producing structures, rather than the morphogen itself which specifies the segment pattern (Meinhardt, personal communication). This version of the model, however, is still at variance with some of the data discussed here.
A complementary explanation is proposed taking into account relevant aspects of homoeotic transformations.
Cytoplasmic changes accompanying the female meiosis in Lilium longiflorum thunb
Authors: Dickinson H. G. , Potter Ursula 
Published: April 2021

ABSTRACT
The events that accompany female meiosis in Lilium parallel almost exactly those characteristic of male cells. The megaspore mother-cell is first cut off from investing tissue by a callosic wall, and this isolated cytoplasm undergoes a phase of comprehensive dedifferentiation. Not only does the major part of the ribosome population appear to be removed from both the cytoplasm and plastids, but also the plastids themselves dedifferentiate to such a degree that they represent almost the sole expression of their genome. Following meiotic prophase the cytoplasm redifferentiates. The ribosomes return by means of the disintegration of cytoplasmic nucleoloids derived from material originally produced at the nucleolar organizer in pachytene, and the plastids redifferentiate into amyloplasts. The mitochondria also exhibit a similar, but less extreme, cycle of differentiation during which a proportion of their population appears to be eliminated. As in the male cells, the megaspore mother-cell energy reserves are converted from starch to lipid in early meiosis, and a low level of protein synthesis appears to be maintained within encapsulated regions of cytoplasm, protected by membranous envelopes from the hydrolytic enzymes present.
Since these cycles of differentiation appear common to both sexes of a higher plant, they must be regarded as forming a significant component of the plant life-cycle. Their precise roles have yet to be clarified, but most certainly they must include the elimination of those informationcarrying molecules characteristic of the diploid phase of growth, and the provision of a fresh and efficient cytoplasm for the expression of the new haploid genome.
Development of hydra lacking nerve and interstitial cells
Authors: Marcum Beverly A. , Campbell Richard D. 
Published: April 2021

ABSTRACTHydra attenuata were rendered free of interstitial cells (I cells) and interstitial cell derivatives by colchicine treatment. These hydra were then cloned and cultivated for 18 months and their developmental capacities were studied. Some experimental hydra possessed a few (about 1% of the normal numbers) interstitial cells and retained this low level during prolonged culture and active growth without the differentiation of I-cells into specialized cells. Other hydra were completely freed of interstitial cells by the colchicine treatment. Maceration and histological analyses showed that once a hydra is freed of all interstitial cells it does not recover them, nor do its buds contain interstitial cells. I cell-free hydra also lack nerve cells, nematocytes, gametes and endodermal gland cells, and the tissue consists solely of ectodermal and endodermal epithelial cells. Hydra completely lacking interstitial cells grow, bud, exhibit tissue renewal patterns, regenerate and preserve polarity generally typical of normal hydra. I cell-free hypostomal tissue has inductive capacity, as does normal hypostomal tissue, when implanted in I cell-free or normal gastric tissue. Regenerating I cell-free tissue undergoes precocious determination as does normal tissue. Only in some quantitative aspects do I cell-free hydra develop abnormally. We conclude that hydra consisting only of epithelial cells are capable of essentially normal development.
The cellular origin of chemically induced tumours
Authors: Iannaccone P. M. , Gardner R. L. , Harris H. 
Published: April 2021

ABSTRACT
Tumours were induced by chemical carcinogens in chimaeric mice made by the amalgamation of two embryos each producing a different electrophoretic variant of the enzyme glucose phosphate isomerase. Although, in these chimaeras, the smallest samples of normal tissue that could be analysed almost invariably contained both isoenzymes, almost all the tumours contained only one. An analysis of the size of the clones that formed the chimaeric epidermis permitted the conclusion that the epidermal tumours produced could not have risen from more than 8 cells; and the most probable interpretation of the data is that these tumours were clonal growths.
Lectin-binding sites are found in rat liver cell plasma membrane only on its extracellular surface
Authors: Virtanen Ismo , Miettinen Aaro , Wartiovaara Jorma 
Published: April 2021

ABSTRACT
In the present study ultrastructural localization of binding sites for 5 lectins was studied in rat liver cell surface membrane fractions. For this purpose ferritin-coupled Concanavalin A, wheat germ agglutinin, soybean agglutinin, Ricinus communis agglutinin 120 and Lotus tetragonolobus agglutinin I were used as probes for mannose, N-acetyl glucosamine, N-acetyl galactosamine, galactose and fucose moieties in glycoproteins and glycolipids.
Although recent reports suggest presence of glycogroups on the cytoplasmic surface of cellular membranes ultrastructural identification of membrane surfaces in the present study indicated an asymmetric localization of lectin-binding sites exclusively on the extracellular side of the membranes.
Some mitochondrial changes in denervated muscle
Authors: Miledi R. , Slater C. R. 
Published: April 2021

ABSTRACT
An electron-microscopic study was made of the mitochondria in normal and denervated rat diaphragm muscles. Two types of structurally different muscle fibres were found: one type had a well-defined M-line, while the other did not.
The mitochondria in normally innervated muscle are regularly arranged at both sides of the Z-line. The mitochondria are very long and branched, and surround the myofibrils at the level of the Z-line. In transverse sections of the muscle fibres the worm-like mitochondria give a very characteristic picture.
After denervation the mitochondria are smaller and less regularly arranged. In transverse sections of the muscle fibres the mitochondria have small circular profiles. This contrasts sharply with the normal appearance and makes it possible to distinguish normal from denervated fibres.
The mitochondrial changes can be detected less than 24 h after denervation.
Membranes associated with the disintegration of mycetomal micro-organisms in Sitophilus Zea-Mais (Mots.) (Coleoptera)
Authors: Musgrave A. J. , Grinyer I. 
Published: April 2021

ABSTRACT
The mycetomal micro-organisms of Sitophilus zea-mais give evidence in electron micrographs of having two peripheral unit membranes, an intermediate layer, intracytoplasmic membranes and DNA-like material. They appear to disintegrate, at times, with the concomitant formation of membrane masses. It is suggested that these are visible with the phasemicroscope and that they are probably myelin figures.
Autoradiographic evidence for the In Situ synthesis of chloroplast and mitochondrial rna
Authors: Gibbs Sarah P. 
Published: April 2021

ABSTRACT
The rate of appearance of labelled RNA in the chloroplast and mitochondria as compared with the rate in the remaining cytoplasm was studied in the unicellular flagellate, Ochromonas danica, by electron-microscope autoradiography. Greening cells were labelled with uridine-5,6[3H] for a short (30 min) and a long (2 h) interval and the concentration of label, expressed as grains/unit area, determined for each cell component. The data demonstrate that there is the expected lag in the labelling of the cytoplasm proper, but no apparent lag in the labelling of the chloroplast and mitochondria. This observation, combined with the fact that after the short labelling time the chloroplast and mitochondria have a much heavier concentration of labelled RNA than the surrounding cytoplasm, indicates that most, if not all, chloroplast and mitochondrial RNA is synthesized in situ.
The three kinds of ribosomes present in the cell are distinctly different in size. The mitochondrial ribosomes measure 150–170 Å in diameter, the chloroplast ribosomes average 170–200 Å in diameter, whereas the cytoplasmic ribosomes are 210–230 Å in diameter in glutaraldehyde-osmium-fixed cells. During chloroplast development in the light, the number of chloroplast ribosomes increases approximately tenfold.
The fine structural localization of acid phosphatase in the prolactin cell of the eel pituitary
Authors: Hopkins C. R. , Baker Bridget I. 
Published: April 2021

ABSTRACT
In the prolactin cell of the eel adenohypophysis acid phosphatase activity occurs within the majority of the Golgi cisternae and developing secretory granules. Acid phosphatase is also present within larger membrane-bound bodies, most of which are similar to the lytic dense bodies described in other cell types. In discussing the functional significance of this enzyme distribution particular attention is paid to its association with the secretory mechanisms of the prolactin cell.
Nucleolar dna in oocytes of Xenopus Laevis
Authors: Macgregor H. C. 
Published: April 2021

ABSTRACT
The ovaries of newly metamorphosed Xenopus females contain oocytes in all stages of early meiotic prophase. In pachytene nuclei extrachromosomal nucleolar DNA appears in the form of a thin cap covering one side of the nucleus. During pachytene this cap of DNA enlarges to occupy half the nucleus. After pachytene the nucleus grows rapidly and the cap of DNA disperses into numerous tiny granules which become scattered throughout the nucleoplasm. Autoradiographs of cells which have been incubated with [3H]thymidine, and microspectro-photometric measurements of the Feulgen dye contents of nuclei in various stages of meiosis, show that the extrachromosomal nucleolar DNA is synthesized during the pachytene and immediate post-pachytene stages. Microspectrophotometric comparison of the Feulgen dye contents of post-pachytene nuclei in which the DNA cap has dispersed, and similarly prepared mouse liver nuclei, show that the post-pachytene nuclei have about 30 μ μg of extrachromosomal pachytene nucleolar DNA. In [3H]thymidine autoradiographs early pachytene nuclei are less heavily labelled than late pachytene and early diplotene nuclei. Consequently it is proposed that primary replicas of the chromosomal nucleolar organizer undergo a series of post-detachment replications.
Fine structure and function of the cytopharyngeal basket in the ciliate Nassula
Authors: Tucker J. B. 
Published: April 2021

ABSTRACT
The fine structure of the cytopharyngeal basket is described in detail. A circular palisade of cytopharyngeal rods is encircled at certain levels by a sheath and two annuli; crest-shaped structures project outwards from the basket. The rods, crests and sheath are largely composed of cytoplasmic microtubules of approximate diameter 240 Å. One of the annuli is mostly composed of densely staining inter-tubular material; the other consists mainly of fine fibres ranging in diameter from 40 to 90 Å.
Ingestion of algal filaments by Nassula and the sequence of movements of the rods which occurs during feeding are described. Evidence is presented which indicates that the crests and fibrous annulus both contract during feeding and act antagonistically to displace the apparently rigid rods. The initial displacement of the rods is spatially related to the orientation of the algal filament which is to be ingested. It is suggested that certain cilia respond to contact with the filament by transmitting some form of excitation to some of the crests along small bundles of microtubules which interconnect them. The suction force drawing the filament into the organism apparently acts only in the lumen of the basket. The basket seems to function as a fairly rigid structure which, acting in conjunction with the suction force, guides the filament into the organism and manipulates it to some extent. The basket also apparently grips the partly ingested filament to prevent it from slipping out of the organism during pauses in the action of the suction force.
Perfusion fixation with glutaraldehyde and post-fixation with osmium tetroxide for electron microscopy
Authors: Van Harreveld A. , Khattab F. I. 
Published: April 2021

ABSTRACT
The conductivity of cerebral cortex drops during perfusion with glutaraldehyde in 5 min to about 60% of the original value, to remain unchanged during the subsequent 10–15 min of perfusion. Circulatory arrest causes a similar drop in the tissue conductivity. Perfusion of asphyxiated tissue with glutaraldehyde does not produce additional major changes in the conductivity. Perfusion of the cortex with an osmium tetroxide solution causes an initial drop in conductivity. However, after about 3 min this trend is reversed and the conductivity increases again to close to the pre-perfusion value. Perfusion of asphyxiated cortex with OsO4 causes a marked increase of the conductivity. So does perfusion with an OsO4 solution of tissue previously treated with glutaraldehyde.
One interpretation of these impedance changes is that glutaraldehyde perfusion causes, like asphyxiation, a transport of extracellular material into the intracellular compartment and that during OsO4 perfusion an extracellular space is again created. This possibility is supported by electron micrographs made of this material. Cerebral cortex perfused with glutaraldehyde and post-fixed with OsO4 shows electron-transparent dendritic elements and to a lesser extent pre-synaptic terminals, which seem to be swollen. When the cortex is flooded with a salt solution during glutaraldehyde perfusion the dendrites exhibit ballooning in the surface layer of the cortex, suggesting that the fluid on the cortex participates in the swelling. The tissue elements in the glutaraldehyde-perfused and OsO4 post-fixed cortex are separated by narrow extracellular spaces. The latter may have been produced by the OsO4 perfusion as is suggested by a comparison of micrographs prepared by freeze substitution (which tends to preserve the water distribution) of glutaraldehyde-perfused but not post-fixed cortex with micrographs of cortex treated with OsO4 after the glutaraldehyde perfusion. In accordance with the conductivity changes, the former micrographs showed very little extracellular space, and in many places tight junctions, whereas the latter showed clefts between the tissue elements.
The atp-dependent concentration of calcium by a golgi apparatus-rich fraction isolated from rat liver
Authors: Hodson Stuart 
Published: April 2021

ABSTRACT
Fractions rich in Golgi apparatus were isolated from female rat liver and incubated with different concentrations of MgATP and calcium. Calcium was sequestered by the fraction in the presence but not in the absence of ATP. The kinetics of the uptake process showed an exponential type accumulation to a maximum value which could be explained by a ‘pump-leak’ mechanism where Vmax was estimated at 6·7 nmol calcium/mg protein/min. Apparent Km for ATP was 1 mM and for calcium was 85μ. The uptake is not inhibited in 5 mM azide, nor is it enhanced in 5 mM oxalate, suggesting that the sequestration is not caused by contamination from mitochondria or microsomes. Parallel experiments on a fraction rich in plasma membrane show at least a 10-fold reduction of activity over the Golgi apparatus-rich fraction, which is interpreted as evidence that plasma membrane contamination in the Golgi apparatus-rich fraction is not primarily responsible for the calcium sequestration activity. Morphometric analysis showed that about 85% of the recognizable membrane profiles in the fraction was associated with elements of the Golgi apparatus. Further evidence is given which suggests that at least 90% of the fraction was uniform in its ability to sequester calcium. In preliminary experiments it was not found possible to separate galactosyltransferase activity (a specific enzyme associated with the Golgi apparatus) from the calcium-sequestration mechanism. The stoichiometry of the hydrolysis of ATP associated with the uptake of calcium is the same as that shown by other isolated membrane systems.
The role of spindle pole bodies and modified microtubule ends in the initiation of microtubule assembly in Saccharomyces cerevisiae
Authors: Byers Breck , Shriver Kathleen , Goetsch Loretta 
Published: April 2021

ABSTRACT
The spindle poles of the budding yeast, Saccharomyces cerevisiae, have been removed from mitotic and meiotic cells by osmotic lysis of spheroplasts. The spindle pole bodies (SPBs) -diskoidal structures also termed ‘spindle plaques’ -have been analysed for their ability to potentiate the polymerization of microtubules in vitro.
Free SPBs were completely deprived of any detectable native microtubules by incubation in the absence of added tubulin and were then challenged with chick neurotubulin, which had been rendered partially defective in self-initiation of repolymerization. Electron microscopy revealed that these SPBs served as foci for the initiation of microtubule polymerization in vitro. Because the attached microtubules elongated linearly with time but did not increase in numbers after the first stage of the reaction, it is apparent that there are a limited number of sites for initiation. The initiating potential of the SPBs was found to be inhibited by enzymic hydrolysis of protein but not of DNA. The microtubule end proximal to the site of initiation on the SPB is distinguished by a ‘closed’ appearance because of a terminal component which is continuous with the microtubule wall, whereas the distal end has the ‘open’ appearance characteristic of freely repolymerized neurotubules. SPBs which were partially purified on sucrose gradients retained their ability to initiate the assembly of microtubules with the same structural differentiation of their ends. The occurrence of closed proximal ends on native yeast microtubules suggests that closed ends may play a role in the initiation of microtubule polymerization in vivo, as well as in vitro.
Formation and growth of gap junctions in mouse myocardium during ontogenesis: A freeze-cleave study
Authors: Gros D. , Mocquard J. P. , Challice C. E. , Schrevel J. 
Published: April 2021

ABSTRACT
The freeze-cleave technique demonstrates the presence of gap junctions at early stages of mouse cardiac muscle ontogenesis. The formation and growth of these junctions were studied at 4 stages of development: 10, 14, 18 days post-coitum (dpc) and at the adult stage. The diverse aspects of the gap junctions are interpreted as different steps in their formation. The first indication of this formation seems to be the presence of linear arrays of 9-nm particles on PF faces. At one end of these arrays a small aggregate of particles appears which acts as nucleation site and grows by incorporating individual gap particles and/or linear arrays. Nexuses with arms and/or central particle-free zones would represent intermediate steps in the formation of junctions. The largest nexuses could be formed by fusion of smaller ones and/or by accretion of gap particles. Analysis of the size distribution of gap junctions shows their growth during their development. At 10 dpc the surface area (.S) of nexuses ranges from 01 to 3 × 10−2μm2, at 14 dpc from 01 to 15 × 10−2μm2, at 18 dpc from 0·1 to 26 ×10−2μm2, and at the adult stage from 01 to 54 ×1o−2μm2. The percentage of large nexuses (S&gt;0·5× 10−2μm2) steadily increases from 10 dpc to the adult stage.
Fixation by glutaraldehyde before glycerol infiltration does not induce any modification in the size distribution of adult heart gap junctions.