bluuebunny/crossref_metadata_2025_split huggingface dataset:
Pax6is required to regulate the cell cycle and the rate of progression from symmetrical to asymmetrical division in mammalian cortical progenitors
Authors: Estivill-Torrus Guillermo , Pearson Helen , van Heyningen Veronica , Price David J. , Rashbass Penny 
Published: April 2021

In the proliferative zone of the developing cerebral cortex, multipotential progenitors predominate early in development and divide to increase the progenitor pool. As corticogenesis progresses, proportionately fewer progenitors are produced and, instead, cell divisions yield higher numbers of postmitotic neurones or glial cells. As the switch from the generation of progenitors to that of differentiated cells occurs, the orientation of cell division alters from predominantly symmetrical to predominantly asymmetrical. It has been hypothesised that symmetrical divisions expand the progenitor pool, whereas asymmetrical divisions generate postmitotic cells, although this remains to be proved. The molecular mechanisms regulating these processes are poorly understood.The transcription factor Pax6 is highly expressed in the cortical proliferative zone and there are morphological defects in the Pax6Sey/Sey (Pax6 null) cortex, but little is known about the principal cellular functions of Pax6 in this region. We have analysed the cell-cycle kinetics, the progenitor cleavage orientation and the onset of expression of differentiation markers in Pax6Sey/Sey cortical cells in vivo and in vitro. We showed that, early in corticogenesis at embryonic day (E) 12.5, the absence of Pax6 accelerated cortical development in vivo, shortening the cell cycle and the time taken for the onset of expression of neural-specific markers. This also occurred in dissociated culture of isolated cortical cells, indicating that the changes were intrinsic to the cortical cells. From E12.5 to E15.5, proportions of asymmetrical divisions increased more rapidly in mutant than in wild-type embryos. By E15.5, interkinetic nuclear migration during the cell cycle was disrupted and the length of the cell cycle was significantly longer than normal in the Pax6Sey/Sey cortex, with a lengthening of S phase.Together, these results show that Pax6 is required in developing cortical progenitors to control the cell-cycle duration, the rate of progression from symmetrical to asymmetrical division and the onset of expression of neural-specific markers.
her1and thenotchpathway function within the oscillator mechanism that regulates zebrafish somitogenesis
Authors: Holley Scott A. , Jülich Dörthe , Rauch Gerd-Jörg , Geisler Robert , Nüsslein-Volhard Christiane 
Published: April 2021

Somite formation is thought to be regulated by an unknown oscillator mechanism that causes the cells of the presomitic mesoderm to activate and then repress the transcription of specific genes in a cyclical fashion. These oscillations create stripes/waves of gene expression that repeatedly pass through the presomitic mesoderm in a posterior-to-anterior direction. In both the mouse and the zebrafish, it has been shown that the notch pathway is required to create the stripes/waves of gene expression. However, it is not clear if the notch pathway comprises part of the oscillator mechanism or if the notch pathway simply coordinates the activity of the oscillator among neighboring cells. In the zebrafish, oscillations in the expression of a hairy-related transcription factor, her1 and the notch ligand deltaC precede somite formation. Our study focuses on how the oscillations in the expression of these two genes is affected in the mutants aei/deltaD and des/notch1, in ‘morpholino knockdowns’ of deltaC and her1 and in double ‘mutant’ combinations. This analysis indicates that these oscillations in gene expression are created by a genetic circuit comprised of the notch pathway and the notch target gene her1. We also show that a later function of the notch pathway can create a segmental pattern even in the absence of prior oscillations in her1 and deltaC expression.Supplementary data available at http://www.eb.tuebingen.mpg.de/papers/holley_dev_2002.html
Inhibition of aggregation of the slime mould dictyostelium discoideum by a factor diffusing from escherichia coli
Authors: Shaffer B. M. 
Published: April 2021

ABSTRACT
A factor diffuses from Escherichia colt that temporarily or permanently prevents separate pre-aggregation cells of Dictyostelium discoideum from forming aggregation centres or streams. Centres are inhibited at a greater distance from the bacteria than are streams. The inhibitor does not support the slime-mould’s growth, and it has different properties from previously discovered aggregation-centre inhibitors released by the amoebae themselves. The factor, as produced, does not inhibit Polysphondylium violaceum cells from forming centres or streams. Yet although E. coli itself attracts pre-aggregation P. violaceum cells, it does not mimic a P. violaceum aggregate in inducing them to form streams.
Moiré patterns in electron micrographs of a bacterial membrane
Authors: Glauert Audrey M. 
Published: April 2021

ABSTRACT
A regular hexagonal array of morphological units is observed in negatively stained preparations of a membrane from the surface of the spherical bacterium Micrococcus radiodurans. Fragments of this membrane are frequently folded so that structural details from two or more layers are superimposed. The appearance of the resulting moiré patterns is illustrated and the need to be aware of the possibility of the formation of these patterns in similar biological specimens is stressed.
Observations on scale production in pyramimonas amylifera conrad
Authors: Manton Irene 
Published: April 2021

ABSTRACT
Scale production has been studied after glutaraldehyde fixation with the primary object of tracing the source of flagellar scales which this species is known to possess (Manton, Oates & Parke, 1963). Large numbers of all types of flagellar scales, unmixed with body-type scales, are detectable at all times, stored in an orderly arrangement within a vesicular scale reservoir of characteristic morphology which is described. These scales are liberated through a narrow duct opening to the cell exterior near the flagellar bases. This duct is thought to be temporary and subject to reformation with minor change of site, since open ducts are more frequently encountered in darkness than by day ; some putative developmental stages are illustrated. Observations on other details of scale production include demonstration of the formation of all types of body scales within the Golgi cistemae, sometimes together with a few flagella-type scales. The larger body-type scales are uniformly oriented within the Golgi cistemae in relation to the position of the subtending endoplasmic reticulum. This orientation is lost before liberation to the cell surface, which occurs from moderate-sized detached vesicles in an unknown manner. The implications of these findings for an understanding of Golgi structure and of the basic problem of scale arrangement on the receiving sites are discussed in a preliminary way.
Distribution of pyrenoids among some brown algae
Authors: Evans L. V. 
Published: April 2021

ABSTRACT
The distribution of pyrenoids among some orders of the brown algae has been investigated with the electron microscope and a report on their occurrence is given, with particular reference to the results obtained with the light microscope by Simon (1954). Some illustrated detail of the structure of the pyrenoids and of the chromatophores is included.
Pyrenoids were found to be present in the representatives examined of the Ectocarpales, Sphacelariales, Scytosiphonales and Dictyosiphonales. Excepting the Sphacelariales, this is in agreement with the results of Simon. Pyrenoids were found to be definitely absent in all members of the Dictyotales and Laminariales examined, and disagreement is expressed with Bouck (1965) who reported their presence in Chorda filum, a member of the latter order. The situation in the Fucales is the subject of another communication elsewhere.
The presence or absence of pyrenoids is regarded as an additional taxonomic character of possible phyletic use when more is known. The results are discussed from the standpoint of their possible value in assessing the relative position of the Phaeophyceae as a whole. Brown algal lamellations are composed of three, or occasionally four, parallel thylakoids which do not cohere and are not aggregated into stacks. This is thought to be more primitive than the condition in some of the other groups of the Chromophyta where there is adherence of thylakoids into stacks of two or three members as, for example, in the Haptophyceae, Xanthophyceae and Chrysophyceae. The Phaeophyceae are, however, regarded as much less primitive than the Rhodophyceae, where the widely spaced parallel thylakoids are arranged singly.
Fine structure of swarmers of Cladophora and Chaetomorpha: II. the chloroplast
Authors: Robinson D. G. 
Published: April 2021

ABSTRACT
Freeze-etched chloroplast thylakoids in swarmers of Chaetomorpha reveal the in situ arrangment of these thylakoids and also a completely new fracture face. This face, which is completely smooth, does not bear any resemblance to any freeze-etched fracture face previously recorded. The relative merits of thylakoid models are discussed in relation to this new fracture face.
Germinal vesicle breakdown in the mouse oocyte
Authors: Calarco Patricia G. , Donahue R. P. , Szollosi D. 
Published: April 2021

ABSTRACT
Germinal vesicle breakdown in mouse oocytes in vivo and in vitro has been examined by electron microscopy. In vitro oocytes were studied immediately after release from follicles and at various times (0·5–11 h) in culture. Approximately 30 min after oocyte release, chromatin condensation begins along the convoluted nuclear envelope (NE). Chromatin granules are common in all condensing chromosomes. Heterochromatin, visible from early condensation until chromosomes are of uniform density, often is observed near the kinetochores. The nucleolus breaks down after peripheral incorporation of separate nucleolus-associated bodies composed of 25-nm diameter fibrils. These bodies are later found free in the cytoplasm. As chromosome condensation progresses, the NE becomes highly convoluted, then discontinuous, finally forming NE doublets. Spindle formation begins with the appearance near the NE of small medium-dense areas from which microtubules emanate. No centrioles are present. Dark granules and mitochondria move centrally in the oocyte and surround the spindle. Peripheral cortical granules and large aggregations of multivesicular bodies are present at all stages. The Golgi apparatus is not well developed. Very little rough endoplasmic reticulum is present, although free ribosomal clusters are common. There are no significant ultrastructural differences between eggs maturing in vivo and in vitro.
Physiology and ultrastructure of phasic and tonic skeletal muscle fibres in the locust, Schistocerca gregaria
Authors: Cochrane D. G. , Elder H. Y. , Usherwood P. N. R. 
Published: April 2021

ABSTRACT
Insect muscle fibres can be classed as either phasic or tonic according to their response to potassium depolarization. The phasic fibres contract only transiently during prolonged potassium depolarization, whereas the tonic fibres give a sustained contracture.
The extensor tibiae muscle in the metathoracic leg of the locust contains both tonic (T/et) and phasic (P/et) fibres; the electrical, mechanical and ultrastructural properties of these fibres have been compared with those of phasic fibres from the retractor unguis muscle (P/ru) in the same leg. A broad correlation has been established between the mechanical response and the amount of sarcoplasmic reticulum (SR) in the fibres.
At maximal body length the rise time to peak twitch tension for the T/et fibres was found to be 790 ± 60 ms, for the P/et fibres 59 ± 2· 5 ms and for the P/ru fibres, 30 ± 1·1 ms. The half-decay times for the isometric twitch contractions were 2950 ± 88 ms for the T/et fibres, 119 ± 4·2 ms for the P/et, and 35 ± 2 ·3 ms for the P/ru. The P/et and P/ru gave brief isometric contractures during potassium depolarization; under the same treatment the T/et fibres remained contracted throughout the treatment period.
The major structural differences between the 3 types lies in the SR. Expressed as percentages of total fibre volume, the SR represents in the T/et 1·1 %, in the P/et 6·8 %, and in the P/ru 19 %. The surface area of the SR, in terms of μm2/μm3 of fibre volume is 1·0 ± 0·1 in the T/et, 2·9 ±0·2 in the P/et and 11·9 ± 1·0 in the P/ru. Microtubules, often associated with elements of the SR, are sparsely distributed amongst the contractile elements in the T/et fibres. All 3 muscle types have a well developed T-system which forms dyadic associations with the SR. Larger-diameter Z-invaginations which conduct tracheoles into the muscles also give rise to ‘longitudinal T-tubules’, particularly in the T/et fibres. Dyads arise by association of cisternae of the SR: (i) with T-tubules sensu strictu, (ii) with Z-invaginations and T-tubule-like extensions from them, and (iii) directly with the plasma membrane at the surface of the fibre.
Regulation of thymidylate synthetase activity in cultured mammalian cells
Authors: Conrad Abigail H. , Ruddle F. H. 
Published: April 2021

ABSTRACT
Changes in thymidylate synthetase specific activity in Don Chinese hamster cells grown in vitro have been examined during the culture cycle and after exposure of lag- and log-phase cultures to drugs which inhibit DNA, RNA, and protein synthesis. During the culture cycle enzyme activity was low during lag phase, rose 6- to 8-fold before log phase, fluctuated between 5-5 and 9 nmol dTMP/h/107 cells during log phase, and declined to base level during stationary phase. Puromycin prevented all increases in enzyme specific activity and caused a decrease in enzyme activity when applied to log-phase cultures. Actinomycin D prevented the initial rise in enzyme activity if applied during early lag phase but caused a pronounced increase in enzyme activity above control levels when applied during log phase.
High thymidine concentration (1 min) stopped cell division in log-phase cultures but did not alter the log-phase plateau level of thymidylate synthetase activity. Fluorodeoxyuridine stopped cell division and depressed enzyme activity to varying degrees depending upon its concentration, but at concentrations less than 10− M enzyme activity eventually returned to normal log-phase levels and cell division resumed if puromycin was not present. Methotrexate stopped cell division and caused a 3- to 4-fold increase in enzyme activity above control levels if puromycin was not present. This increase occurred in the presence of actinomycin D but was retarded by addition of thymidine when actinomycin D was not present.
These experiments suggest that the regulation of thymidylate synthetase activity in log-phase cells is complex and may involve thymidine triphosphate.
The location of blood group antigen a on cultured rabbit kidney cells as revealed by ferritin-labelled antibody
Authors: Dimmock Elizabeth , Franks D. , Glauert Audrey M. 
Published: April 2021

ABSTRACT
Fluorescein- and ferritin-labelled antibodies were used to locate blood group antigen A on the surfaces of cultured rabbit kidney cells (RK 13). Fluorescent anti-A revealed the antigen on some cells, but not on others. Controls indicated that specific staining could be obtained. Comparable observations were made with ferritin-labelled antibody, and the comparatively high resolution of this method allowed individual antigen molecules to be located. Measurements were made of the distances between the centres of the ferritin molecules and the cell surface, and the position of the blood group antigen relative to the visible components of the membrane is discussed in relation to this.
Experimental studies on the axons intrinsic to the glomerular layer of the olfactory bulb
Authors: Pinching A. J. , Powell T. P. S. 
Published: April 2021

ABSTRACT
Lesions have been made in the superficial layers of the dorsal surface of the rat olfactory bulb; the resulting axonal degeneration has been studied with light and electron microscopy, in order to determine the mode and extent of interconnexion of the intrinsic interneurons of the glomerular layer. Although the lesions necessarily involve extrinsic pathways, the axonal distribution of each of the intrinsic neurons may be deduced from this evidence and a knowledge of normal and experimental material. Tufted cell collaterals extend for 6–12 glomeruli, periglomerular cell axons for 4–5 glomeruli and superficial short-axon cell axons for 2–3 glomeruli; these figures correspond well with observations of Golgi-impregnated material. The experimental approach to the study of intrinsic connexions and its applicability to other sites is considered, and the results are discussed from the point of view of neural coding and odour specificity.
Synthesis and localization of sulphated mucopolysaccharide in megakaryocytes and platelets of the rat, an analysis by electron-microscope autoradiography
Authors: Macpherson G. G. 
Published: April 2021

ABSTRACT
Electron-microscope autoradiography has been used to investigate the synthesis and localization of sulphated mucopolysaccharide in megakaryocytes and blood platelets. Following 10-min incubation of bone marrow with 35S-sulphate in vitro the majority of the activity in megakaryocytes was associated with the Golgi apparatus, but a substantial proportion was associated with other cytoplasmic organelles, suggesting either rapid transport or sulphation of mucopolysaccharide outside the Golgi apparatus. Three hours after the intravenous injection of 35SO4 only a small proportion of the total activity was associated with the Golgi apparatus, most being associated with demarcation membranes and dense granules, while 12 h after injection almost all the activity was associated with demarcation membranes and granules. A rising proportion of activity localized solely on the demarcation membranes suggested that they may possess some activity of their own.
Autoradiographs of blood platelets prepared 72 h after the injection of 35SO4 were analysed. It was shown that most of the activity was associated with the a-granules, but there was strong evidence that the platelet membrane possessed a low level of activity.
Quantitative cytochemical observations on the control of respiration in polymorphonuclear neutrophil leucocytes of Amphiuma tridactylum
Authors: David G. B. , Mcmullen Jamie M. 
Published: April 2021

ABSTRACT
Quantitative effects of altering oxidative phosphorylation and respiration on the activity of the enzyme menadione reductase (NAD[P]H,:2-methyl-1,4-naphthoquinone oxidoreductase, E.C. 1.6.5.2), in stabilized polymorphonuclear neutrophil leucocytes of Amphiunia tridactylum, were studied by amplitude-contrast microscopy and microspectrophotometry.
In a general way, the rate of enzymic activity was proportional to ADP concentration and inversely proportional to the concentration of ATP. Terminal respiratory blocking by azide produced selective subtotal inhibition. Uncoupling of phosphorylation by dinitrophenol produced complex results.
Neutrophils of A. tridactylum, irrespective of their stage of maturation in the circulating blood, could be subdivided into 3 metabolic classes: Class I cells, of low enzymic activity’ (predominantly mitochondrial), greatly activated by ADP, somewhat activated by ATP, and only slightly inhibited by dinitrophenol; Class II cells, twice as active as Class I (in which the endoplasmic reticulum and idiozome were as active as the mitochondria), further activated by ADP used alone or with dinitrophenol, and unaffected by ATP or dinitrophenol; Class III, hyperactive cells (enzymic localization identical with that in Class II), inhibited by ATP and dinitrophenol, and not activated by ADP. Some of the mitochondria of Class III neutrophils retained nearly a third of their reductase activity when the reaction mixture contained 10−1M azide.
There is reason to believe that Class I neutrophils may form a reserve population of vegetative cells; in vitro, they can be transformed into Class II cells when a high concentration of ADP is added. Class II and Class III cells are potentially capable of amoeboid movement and phagocytosis. The metabolic mobilization of neutrophils could be interpreted as being controlled by 2 different feedback mechanisms: activation by ADP in cells of Classes I and II, and inhibition by ATP in cells of Class III.
Aggregation of burkitt lymphoma cells in stationary culture: experimental and theoretical analysis
Authors: Mayhew E. , Blumenson L. E. 
Published: April 2021

ABSTRACT
Cellular motility plays an important role in natural aggregative phenomena but previously’ has been difficult to quantitate. We here describe a general method which can be used to determine the importance of cell motility in cell aggregation behaviour.
When actively moving Burkitt lymphoma cells cultured in microtest plate wells come into contact they adhere to one another forming an aggregate. The aggregate increases in size when more cells come in contact with it. The final size and number of aggregates per well was found to be dependent on the number of cells added per well. With increasing cell numbers added per well the number of aggregates formed increased until it reached a peak of 47–57 aggregates per well 20 h after the addition of 180–710 cells per well. At higher cell concentrations the number of aggregates formed decreased. The system was analysed theoretically by programming a computer to simulate the experimental system. This simulation showed it was probable that the experimental results obtained were due to (a) random dropping of the cells at zero time and (b) the adhesion of the cells when they made contact and (c) unrestricted random movement of cells in the well when they reached the well surface. The computer simulation is such that given the experimentally determined cell concentration per well and rate of cell movement we can predict the number of aggregates formed for different probabilities that the adhesions formed after the cells come in contact are permanent. This experimental approach along with the computer simulation can be used to quantitate the role of cell motility and permanence of contact in cell aggregation.
Studies on chemically induced cell fusion
Authors: Ahkong Q. F. , Cramp F. C. , Fisher D. , Howell J. I. , Lucy J. A. 
Published: April 2021

ABSTRACT
Hen erythrocytes that were fixed after treatment with lysolecithin in aqueous solution for 30 s at 37 °C showed evidence of bridge formation between adjacent lysed cells. Generally, the homokaryons that were produced using lysolecithin in this way contained large numbers of nuclei. These giant syncytia had damaged nuclear membranes and unstable plasma membranes; complete disintegration of the syncytia occurred within 1 min of adding lysolecithin to the erythrocytes.
In order to localize the action of lysolecithin, the fusing agent was incorporated into microdroplets of lipid. Cell fusion following the addition of lysolecithin in an aqueous glyceride-lecithin emulsion was slower than with lysolecithin in aqueous solution, taking 10–30 min, and it was accompanied by considerably less damage to the plasma and nuclear membranes. The fused erythrocytes, which usually contained only two or three nuclei, lysed slowly during the 45 min following fusion, and lysis could be arrested by cooling the fused cells. The plasma membranes of lysed, multinucleated cells remained intact at 37 °C for at least 90 h.
Mouse fibroblast-hen erythrocyte heterokaryons formed with the aid of the emulsion were more stable than those produced with lysolecithin in solution, but the hybrid cells nevertheless had damaged subcellular organelles. Viable clones of hybrid mouse-hamster fibroblast cells were obtained using the emulsion although, possibly owing to reduced viability of the lysole-cithin-treated cells, only at twice the frequency of spontaneously produced hybrids.