bluuebunny/crossref_metadata_2025_split huggingface dataset:
A model of pattern formation in insect embryogenesis
Authors: Meinhardt H. 
Published: April 2021

ABSTRACT
A model is proposed in which the interaction of an autocatalytic substance with a short diffusion range - the activator - and its more diffusible antagonist - the inhibitor - leads to a local high concentration of activator at the posterior pole of the egg. The inhibitor, which is then produced mainly in this activated region, diffuses into the rest of the egg, where it acts as a ‘morphogen forming a concentration gradient which supplies positional information.
This model can account quantitatively for the patterns resulting from a large number of different experiments performed during early insect development, including ligation, u.v.- irradiation and microsurgical manipulations. The formation of additional posterior structures is interpreted as the result of the appearance of a new activator peak. Omission of segments after ligation of the egg is explained as the result of accumulation of morphogen (the inhibitor) at the posterior side of the ligation and a decrease of morphogen on the anterior side. In order to account for certain quantitative features of the ligation experiments it is necessary to assume that determination in response to the morphogen gradient is a slow, stepwise process, in which the nuclei or cells first pass through determination stages characteristic for more anterior structures until they ultimately form a given structure.
The mutual interactions of activator and inhibitor are expressed as a set of partial differential equations. The individual experiments are simulated by solving these equations by use of a computer.
An ultrastructural study of the relationship between the plasma membrane and the cell wall of the coenocytic alga Hydrodictyon africanum
Authors: Bailey D. S. , Northcote D. H. 
Published: April 2021

ABSTRACT
A wall/plasma membrane unit was prepared from Hydrodictyon africanum by microdissection. A replica of the inner surface of the membrane was made and by freeze-fracture of the whole cell, 2 corresponding internal fracture faces were obtained. The large coenocytes of the alga were plasmolysed and the wall separated by cutting it away. Its inner surface was directly viewed in the electron microscope after shadowing with Pt/C. Particles were found on the outer half of the internal fracture face of the membrane which were oriented in the same 2 directions as the microfibrils laid down at the inner surface of the wall. No structures were found at the inner surface of the membrane. Some evidence was obtained for a structural connexion between the innermost layers of the wall and the plasma membrane.
Mitotic spindles of Drosophila melanogaster: A phase-contrast and scanning electron-microscope study
Authors: Milsted Amy , Cohen William D. , Lampen Nina 
Published: April 2021

ABSTRACT
Mitotic spindles have been isolated from the blastema stage of Drosophila melanogaster embryos using modified tubulin-polymerizing medium. ‘Clean’ spindles, relatively free of contaminating cytoplasmic material, are obtained. Under phase contrast, mitotic stages appear remarkably similar to those seen in situ, as reported in early literature. This preservation of morphological integrity, coupled with relative structural simplicity due to low chromosome number (2n = 8), makes these spindles ideal subjects for study. Use of the scanning electron microscope provides excellent visulization of their general structural organization, changes in whole spindle structure during the course of mitosis, and higher resolution viewing of surface detail than is permitted with light microscopy.
Gibberellic acid-induced germination of spores of Anemia phyllitidis; autoradiographic study of the timing and regulation of nucleic acid and protein synthesis in relation to cell morphogenesis
Authors: Raghavan V. 
Published: April 2021

ABSTRACT
The timing and regulation of nucleic acid and protein synthesis during gibberellic acid-induced germination of spores of the fern, Anemia phyllitidis (L.) Swartz were investigated using metabolic inhibitors combined with cytological and autoradiographic techniques. Incubation of spores in gibberellic acid for at least 24 h was necessary for their subsequent germination in the absence of the hormone. Based on the action of cycloheximide, it appeared that continuous synthesis of proteins was essential for the hydrolysis of storage reserves, enlargement of the protoplast, division of the spore nucleus and formation of the protonemal cell associated with germination. Addition of actinomycin D or 5-fluorouracil along with gibberellic acid did not visibly affect germination, although the latter inhibited division of the spore nucleus. Both compounds also inhibited RNA synthesis in germinating spores without any noticeable effect on DNA or protein synthesis. Cordycepin was most effective in inhibiting germination when it was added during the first 24 h of gibberellic acid action. Spores visibly germinated in the absence of nuclear division when DNA synthesis was inhibited by hydroxyurea. On the other hand, incubation of spores in an osmoticum such as mannitol led to no discernible signs of germination, although division of the nucleus and formation of the protonemal cell occurred in the absence of cell enlargement. It is tentatively proposed that the dry spore contains a stable mRNA which is activated in the presence of gibberellic acid. This mRNA provides templates for the synthesis of proteins necessary for the early events of germination.
The role of nuclear Ca2+ in maintaining neuronal homeostasis and brain health
Authors: Mozolewski Pawel , Jeziorek Maciej , Schuster Christoph M. , Bading Hilmar , Frost Bess , Dobrowolski Radek 
Published: April 2021

ABSTRACT
Nuclear Ca2+ has emerged as one of the most potent mediators of the dialogue between neuronal synapses and the nucleus that regulates heterochromatin states, transcription factor activity, nuclear morphology and neuronal gene expression induced by synaptic activity. Recent studies underline the importance of nuclear Ca2+ signaling in long-lasting, activity-induced adaptation and maintenance of proper brain function. Diverse forms of neuroadaptation require transient nuclear Ca2+ signaling and cyclic AMP-responsive element-binding protein (CREB1, referred to here as CREB) as its prime target, which works as a tunable switch to drive and modulate specific gene expression profiles associated with memory, pain, addiction and neuroprotection. Furthermore, a reduction of nuclear Ca2+ levels has been shown to be neurotoxic and a causal factor driving the progression of neurodegenerative disorders, as well as affecting neuronal autophagy. Because of its central role in the brain, deficits in nuclear Ca2+ signaling may underlie a continuous loss of neuroprotection in the aging brain, contributing to the pathophysiology of Alzheimer's disease. In this Review, we discuss the principles of the ‘nuclear calcium hypothesis’ in the context of human brain function and its role in controlling diverse forms of neuroadaptation and neuroprotection. Furthermore, we present the most relevant and promising perspectives for future studies.
Calcium exchangeability in subcellular fractions of pancreatic islet cells
Authors: Borowitz J. L. , Matthews E. K. 
Published: April 2021

ABSTRACT
The distribution and exchangeability of calcium within subcellular compartments of rat pancreatic islet cells was determined in pulse-chase experiments following isotopic labelling with 45Ca2+. In unstimulated islets the mitochondrial fraction showed the greatest, and the insulin secretory granule fraction the least, uptake of 45Ca2+; subsequently the rate of loss from each compartment during 120 min was in the order mitochondrial &gt; microsomal &gt; secretory granule fraction. On exposure to D-glucose, 16·8 mM, the 45Ca labelling of all 3 subcellular compartments was increased after 5 and 30 min stimulation. In contrast, theophylline 10 mM caused a significant decrease in mitochondrial 45Ca content with little effect on other compartments. These results are discussed in relation to the importance of calcium disposition, exchangeability, and mobility for the initiation and control of insulin secretion.
The effect of chloramphenicol, ethidium bromide and cycloheximide on mortality and mitochondrial protein synthesis of adult blowflies
Authors: Ashour Badr , Tribe Michael , Whittaker Peter 
Published: April 2021

ABSTRACT
The effects of cycloheximide, chloramphenicol and ethidium bromide on the blowfly Calliphora erythrocephala were studied. In the first set of experiments, toxic levels were determined by examining activity and mortality of flies after injection of various doses of each drug. In the second set of experiments, the effect of drug treatment on flight muscle mitochondrial protein synthesis was determined in relation to age by following the incorporation of radioactively labelled amino acid, [3H]leucine, into mitochondrial protein in vivo. To confirm the developmental changes in flight muscle mitochondria, mitochondrial protein content per fly was estimated from emergence to 30 days of age; the highest protein level was recorded between 6 and 10 days of age.
Maximum incorporation of labelled amino acid was found in newly emerged flies, and this age group was also the most sensitive to drug treatment. By the time flies had reached 6 –10 days of age, amino acid incorporation had declined to about two-thirds of the rate obtained with newly emerged flies. With 6-10-day-old flies, however, the highest value for flight muscle mitochondrial protein per fly was recorded, and these flies also displayed the greatest resistance to drug treatment of any age group investigated. For example, inhibition of protein synthesis following injection of 300 μg/fly of chloramphenicol was only about 15 % below the untreated control in 6-10-day-old flies, whereas in all other age groups investigated, inhibition ranged between 30 and 50 % of the controls.
At 15 –20 days of age, protein synthesis decreased to a third of the newly emerged flies’ rate and continued to decrease further in the 30 –35-day-old group, where it was less than one sixth of the youngest age group. The effect of drug treatment on these older flies was also less than that observed with newly emerged flies, especially after chloramphenicol and ethidium bromide injections. The effect of cycloheximide however, was much the same in all age groups, with inhibition of protein synthesis being 80—90 % of controls.
Surprisingly, cycloheximide (1 –10 μg/fly) had little initial effect on mortality of young flies, despite almost complete blockage in the synthesis of mitochondrial proteins at these concentrations. 95 % mortality occurred only when doses of 20 μg/fly were given. In contrast, high doses of chloramphenicol (400 μg/fly) and ethidium bromide (15 μg/fly) caused almost total mortality a few hours after injection, although such doses never induced more than about 50 % inhibition of mitochondrial protein synthesis.
Each drug therefore has a different site of inhibition and induces different mortality effects. Possible explanations for these differences in mortality are discussed.
Ultrastructure of polytene chromosomes of Drosophila isolated by microdissection
Authors: Mott Margaret R. , Burnett E. J. , Hill R. J. 
Published: April 2021

ABSTRACT
Drosophila polytene chromosomes prepared by a new micromanipulative procedure, which avoids acid squashing, have been examined at the ultrastructural level in the electron microscope. Puffs at 2B, 68C, 74EF, 75B and 85EF, have been examined in some detail, along with the chromocentre and various interbands. The ultrastructure of these chromosomes, which have never been exposed to acid protein denaturants, compares favourably with that of classical acid-fixed specimens. Ribonucleoprotein particles in puffs are seen to be organized in linear arrays and evidence is adduced for looped transcription units. Particles with characteristic sizes and morphologies are observed near the chromocentre, in puffs and in interbands. In interbands RNP particles and ‘superbead’-like chromatin particles may be distinguished. Drosophila polytene chromosomes isolated by micro-manipulation should prove useful for the localization of native chromosomal proteins at an ultrastructural level.
Measurement and characteristics of fusion of isolated membrane fractions from maize root tips
Authors: Baydoun Elias A-H. , Northcote D. H. 
Published: April 2021

ABSTRACT
Maize root tips were incubated in vivo with radioactive glucose, choline or diazotized sulphanilic acid. Membrane fractions were prepared from radioactive and non-radioactive roots. The transfer of radioactivity between mixed membrane fractions has enabled a quantitative system to be developed to study in vitro membrane fusion between Golgi apparatus and plasma membrane-rich fractions. Membrane fusion was found to be dependent on time, temperature, Mn2+ and Ca2+. Mn2+ was as effective as Ca2+, other divalent cations had a moderate or no effect. The effect of various substances, including inhibitors of microtubular and microfilament assembly and blocking reagents for sulphydryl group on membrane fusion has been investigated. The process appears to be dependent on the membrane proteins or glycoproteins; trypsinization of mixed membranes prior to the addition of Ca2+ inhibited significantly the fusion process. SDS-polyacrylamide gel electrophoresis of trypsin-treated membranes revealed the selective loss of one particular polypeptide which could play a role in membrane fusion.
Hierarchy of fibrillar organization levels in the polytene interphase chromosomes of Chironomus
Authors: Sass Heinz 
Published: April 2021

ABSTRACT
The present study shows a hierarchy of fibrillar organization levels in polytene somatic interphase chromosomes. The chromosomes of several Chironomus species have been examined by electron microscopy in situ, i.e., in the physiological environment of intact salivary glands, and in vitro as spread preparations. In situ, the basic structural element of bands, interbands and constitutive heterochromatin is a DNP fibre (chromatid) which varies in thickness from 11 to 25 nm. The fibre displays a varying degree of coiling in bands and interbands. It runs more or less straight through the interbands but forms aggregates (chromomeres), which have a densely packed fibrillar texture, in the band regions. On the whole there is always an abrupt transition between the fibre and the chromomeres which it connects. Constitutive heterochromatin is also made up of a fibre of the same thickness as in bands and interbands. However, in the heterochromatin the fibre generally measures 20–25 nm. In telomeric heterochromatin the fibre is extremely densely arranged, whereas the centric heterochromatin (chromocentre) of C. melanotus is made up of very loosely bundled fibres. Neither type of heterochromatin exhibits any visible subdivision into chromomeres and interchromomeres.
In spread preparations of native isolated polytene chromosomes of different Chironomus tissues seen in vitro the bands and interbands are also composed of a DNP fibre which varies in thickness from 11 to 25 nm. Fibres in spread preparations are at most 20–25 nm thick. The spreading of band regions enables one to recognize more or less tangled, sometimes transcribed 0 · 8–8 · 4 μm loops, which are regarded as unfolded chromomeres. The shape of the 20–25-nm fibre in band (loop) and interband regions is irregular and clearly reflects orderly packing of globular particles (superbeads) embedded in a diffuse matrix. The latter can be removed by spreading the fibres in low salt medium or distilled water, which also makes individual superbeads visible. Arranged in a row, 4 superbeads account for 0·1 μm. Superbeads are structures consisting of subunits which correspond in size (n 0± 10 nm) to nucleosomes. Tentatively, there are 7–12 nucleosomes in one superbead. A row of nucleosomes constitutes the 11–13-nm fibre. This is the basic fibre of polytene chromosomes. The 20–25-nm fibre appears to be a higher order of the basic fibre. Digestion of the 20–25-nm fibre by trypsin and DNase reveals that the fibre is unineme as far as its DNA is concerned. The fact that polytene interphase chromosomes analysed in situ and in vitro both show a 20-25-nm fibre, leads to the conclusion that this fibre represents the highest level of fibrillar organization of the DNP fibre. Presumably the distinct levels of organization revealed by the 20–25-nm fibre in vitro also exist in the chromosomes in situ, but are invisible because the fibres prepared at physiological ionic strength are densely packed.
The role of negative charge in spontaneous aggregation of transformed and untransformed cell lines
Authors: Wright Thomas C. , Smith Barton , Ware Ben R. , Karnovsky Morris J. 
Published: April 2021

ABSTRACT
No correlation between net negative surface charge as determined by electrophoretic light-scattering techniques and the rates of spontaneous aggregation of 3T3MIT and SVPy 3T3MIT cells has been found. Neuraminidase treatment of both 3T3MIT cells and SVPy 3T3MIT cells causes a significant decrease in electrophoretic mobility but only the 3T3MIT cells show an increase in spontaneous aggregation. An increase in spontaneous aggregation of 3T3MIT cells is seen after growth in 200 mM urea for 18 h but no change in net surface charge occurs.
The distribution of anionic sites on the membranes of cells was determined using the ultrastructural marker polycationized ferritin. The distribution of polycationized ferritinbinding sites was essentially identical for both cell lines under all conditions when they were labelled at 4 °C. When the cells were labelled with polycationized ferritin at 37 °C it was found that cells which have a high net rate of spontaneous aggregation also show rearrangement of anionic sites on their surface membrane. Clustering and rearrangement of anionic sites at 37 °C correlate with high rates of spontaneous aggregation.
Immunofluorescent histological studies of the role of fibronectin in the expression of the associative preferences of embryonic tissues
Authors: Armstrong Peter B. , Armstrong Margaret T. 
Published: April 2021

ABSTRACT
The identity of the chemical factors controlling the spreading behaviour of sheets of cells was examined in organ culture. When aggregates of two dissimilar tissues are apposed in organ culture, one tissue spreads reproducibly over the surface of the second. The present study employed indirect immunofluorescent localization techniques to evaluate the hypothesis that the spreading behaviour of chick embryonic heart tissue in culture is dominated by the presence or absence of the cell-surface and extracellular matrix protein fibronectin in the surface layers of the aggregates. Specifically, the hypothesis proposes that aggregates that display surface fibronectin earlier after culturing and/or in higher quantities segregate internally to aggregates that are slower to develop a surface layer of fibronectin or in which this layer contains reduced amounts of fibronectin. The hypothesis has been supported for 3 categories of behaviour of chick embryo heart tissue: (1) myocyte aggregates spread over myocyte aggregates containing a 20% admixture of heart fibroblasts, which in turn spread over heart fibroblast aggregates; (2) S-day embryonic ventricle-tissue fragments maintained in culture for 0μ·5 days spread over ventricle fragments cultured for 2·5 days; and (3) 2-day embryonic ventricle spreads over 5-day ventricle. In all these situations, the aggregate type that segregates to an internal position displays more fibronectin at its surface than aggregate types that spread to occupy an external position. Evidence is presented that the fibronectin in heart tissue aggregates is elaborated by heart fibroblasts.
The proportion of ribosomes engaged in rat pancreatic polysomal structures as influenced by pilocarpine injection
Authors: Poort C. , Kramer M. F. 
Published: April 2021

ABSTRACT
Rats intravenously injected with pilocarpine at a concentration of 4 mg per 100 g body weight were killed after various time intervals and the relative numbers of ribosomes per cell active in protein synthesis were determined. It was found that until 1 h after pilocarpine injection there is a significant decrease in the number of ribosomes engaged in protein synthesis, whereas 2–3 h after pilocarpine injection, the number of active ribosomes equals that of control animals. After a short incubation in vitro, the same relative number of active ribosomes is always found, independent of the value determined in the pancreas directly after the death of the animal.
A search for synaptonemal complexes in Ustilago maydis
Authors: Fletcher H. L. 
Published: April 2021

ABSTRACT
Germinating spores, germ-tubes (promycelia), gall tissue and spores developing in the gall were examined. No synaptonemal complexes (SCs) were found in any of these cell types. There are 3 possible explanations for this: (1) Ustilago maydis does not have SCs. This is the case in Schizosaccharomyces pombe (Olson, Eden, Egel-Mitani & Egel, 1978) and in many strains of Saccharomyces cerevisiae (e.g. see Byers & Goetch, 1975). (2) The SC might occur in spores either in the gall or before germination when the spore wall was too solid to allow examination of the contents by the methods used. (3) The SC was present, but for a very short time, and most of the cells examined were in any case not undergoing meiosis, so the SCs were not seen. (Large numbers of spores were examined, but few spores germinate and undergo normal meiotic division; also a minority of the cells in the gall are forming spores at any one time.) The gall was found to contain uninucleate single cells (i.e. it is a yeast, as it is in artificial culture medium) and virtually all these cells were haploid, 1 in 3000 recovered cells was diploid. It appears that the haploids fuse to form a heterokaryotic or diploid cell immediately before spore formation. A heterokaryotic phase is presumed to exist to establish and maintain the infection.
Intracellular distribution of nadph-cytochrome c reductase in rat hepatocytes studied by direct ferritin-immunoelectron microscopy
Authors: Aoi Kazuo , Fujii-Kuriyama Yoshiaki , Tashiro Yutaka 
Published: April 2021

ABSTRACT
NADPH-cytochrome c reductase was purified from rat liver microsomes and the monospecific antibodies to the reductase were prepared from the antiserum by affinity chromatography using immunoadsorbent gel. Ferritin was coupled to the specific antibodies and the approximately equimolar conjugates were isolated by gel filtration. By direct ferritin-immuno-electron microscopy, using these conjugates, it was revealed that the ferritin particles are localized exclusively on the microsomal vesicles and the outer nuclear envelope. In contrast, binding of ferritin particles to Golgi membranes, outer mitochondrial membranes and plasma membranes was slight and at control level. On each microsomal vesicle, the ferritin particles were distributed heterogeneously, sometimes forming clusters. An assay of the binding of equimolar conjugates with microsomes showed that microsomes bind ∼ 1 mol of antibody per mol of reductase. From these data the maximum number of ferritin particles that can bind with microsomes was calculated. This number is in agreement with the average number of ferritin particles bound per microsome, as determined experimentally by observing a number of crosssectional profiles of microsomal vesicles previously incubated with the conjugates at saturation level. This showed that the distribution of the reductase could be analysed semi-quantitatively by the present ferritin-immunoelectron-microscopical analyses. It was also shown that smooth microsomes can bind more conjugates than rough microsomes. The average number of ferritin particles on each microsomal vesicle increased in proportion to the increase in the amount of reductase in the microsomes after treatment with phenobarbital. Finally, the non-random distribution of ferritin particles on microsomal vesicles was confirmed by statistical analysis of electron micrographs of a number of the labelled microsomes.