bluuebunny/crossref_metadata_2025_split huggingface dataset:
Localization of intranucleolar dna in root meristematic cells ofAllium cepa
Authors: Chouinard L. A. 
Published: April 2021

ABSTRACTThe problem of the localization of intranucleolar DNA in root meristematic cells of Allium cepa has been investigated under both light and electron microscopy following formaldehyde fixation coupled with deoxyribonuclease extraction. The observations reveal that most, if not all, of the intranucleolar DNA is located inside small lacunar spaces confined to the fibrillar regions of the nucleolar mass. These results would be consistent with the view that the DNA-containing lacunar spaces in question correspond, in fact, to cross-sections of a meandering channel containing the extended nucleolar organizing segment of the nucleolar chromosome.
The structure of sycamore callus cells during division in a partially synchronized suspension culture
Authors: Roberts K. , Northcote D. H. 
Published: April 2021

ABSTRACT
Sycamore suspension callus cells have been partially synchronized to give a culture with a rmtotic index of 15 %. Living dividing cells of the culture have been examined with Nomarski differential interference optics and a comparable study made on fixed cells with the electron microscope.
An organized band of reticulate cytoplasm partially encircles the nucleus at mitosis. The cell divides by the formation of a phragmosome which grows across the large vacuole; this allows the organization of the cytoplasm which forms the cell plate to be examined separately from the more general cytoplasm of the cell. The cell plate grows from one side of the cell to the other and down its length a complete developmental sequence can be seen.
The Golgi bodies and the endoplasmic reticulum are probably involved in the formation of material for the construction of the cell plate and young cell wall. Microfibrils are formed within the plate in the more mature regions, while material contained within vesicles is incor porated at the young growing edge. At the edge of the plate microtubules are found and these correspond to the fibnllar appearance of the phragmoplast seen with the optical microscope.
In the living cell an active movement of organelles along the peripheral cytoplasm can be seen and with fixed cells viewed with the electron microscope microtubules are often found adjacent to the plasmalemma and lying close to mitochondria, crystal-containing bodies and plastids.
The appearance of crystal-containing bodies and plastids containing phytoferntin is described.
Temperature-induced orientation instability during meiosis: An experimental analysis
Authors: Henderson S. A. , Nicklas R. B. , Koch Carol A. 
Published: April 2021

ABSTRACT
Low-temperature (10 °C) treatments alternated with periods at room temperature (25 °C) were found to induce grossly abnormal kinetochore orientation behaviour during metaphase I in spermatocytes of the grasshopper Melanoplus differentialis. These orientation abnormalities involved multiple reorientations before a stable bipolar co-orientation was finally achieved. Similar orientation abnormalities have been induced in the past by heat-treatments in the locust Schistocerca. The unstable orientation behaviour of bivalents in eight cold-treated living spermatocytes was analysed with the aid of time-lapse cinematographic records. Analyses made included the relationship between kinetochore position within the spindle and reorientation, the frequency of reorientation and the velocity of chromosome motion after reorientation. It had been shown previously, in untreated cells, that physical tension applied by micromanipulation could stabilize experimentally induced unipolar bivalents and inhibit their reonentation. Two experiments of this type with cold-treated spermatocytes were successful in demonstrating the importance of tension in the maintenance of stability, even in unstable, cold-treated material. In discussing the interpretation of these results it is proposed that bivalents have an inherent tendency to reorient periodically. In untreated cells the time between reorientations is long enough to enable interkinetochoric tensions, and resulting stable bipolar orientation, to be achieved, but in cold-treated cells this time is reduced, thereby hindering the establishment of tension and stability.
Cell surface components and growth regulation in cultivated arterial smooth muscle cells
Authors: Nilsson Jan , Ksiazek Tomasz , Thyberg Johan , Wasteson Ake 
Published: April 2021

ABSTRACT
The surface of rat arterial smooth muscle cells was characterized with respect to some of its chemical and functional properties. The effects of selective enzymic degradations (hyaluronidase, chondroitinases, heparitinase or neuraminidase) on [35S]sulphate-prelabelled cells and on binding sites for cationized ferritin (CF) were examined to assess the presence and relative importance of individual species of macromolecules on the cell surface. The results indicate that about half of the strongly anionic sites on the cell surface (binding CF at pH 2·0) could be ascribed to sulphate groups of glycosaminoglycans and about half to carboxyl groups of sialic acid residues in glycoproteins and/or glycolipids. Weaker anionic sites (binding CF at pH 7·0) largely originated from carboxyl groups of glycosaminoglycans. Chondroitin sulphate and heparan sulphate were the main glycos aminoglycans. The surface of cells from young animals showed a higher glycosaminoglycan and a lower sialic acid content than that of cells from adult animals.
Continuous treatment of the cultures with neuraminidase stimulated serum-induced initiation of DNA synthesis, while treatment with hyaluronidase or heparitinase inhibited it. Addition of hyaluronic acid, heparin or heparan sulphate to the culture medium inhibited initiation of DNA synthesis as well as cell proliferation. The effect was more marked in cultures of cells from young animals than from adults, although the latter cells were found to grow at a higher rate and to higher densities. These results suggest a role for cell-surface and pericellular glycoconjugates in growth regulation. A possible mechanism of action is that these molecules, due to their anionic charge or by steric exclusion, interfere with the binding of platelet-derived growth factor, a highly cationic polypeptide, to its cell-surface receptor.
The interval of the cytoplasmic cycle observed in non-nucleate egg fragments is longer than that of the cleavage cycle in normal eggs of Xenopus laevis
Authors: Shinagawa Atsunori 
Published: April 2021

ABSTRACT
Non-nucleate fragments of animal eggs are known to show a cyclic change in rigidity similar to the change observed in the cleavage of normal eggs. The interval of cyclic change was compared with that of the cleavage cycle in Xenopus eggs. Procedures employed by both Hara and co-workers and Sakai & Kubota to obtain non-nucleate fragments were carefully repeated. Non-nucleate fragments produced by either of those procedures had a longer duration of cycle than the normal cleavage cycle (about 30%). By injection of colchicine or vinblastine the cycle of the nucleate fragment was lengthened to coincide with that of the non-nucleate partner. It is suggested that the basic cycle length intrinsic to egg cytoplasm is modulated by assembly-disassembly of the mitotic apparatus, as demonstrated by Sluder in the chromosome cycle of sea urchin eggs.
Units of chromosome replication and packing
Authors: Mullinger Ann M. , Johnson Robert T. 
Published: April 2021

ABSTRACT
Fusion between mitotic and S-phase cells induces the formation of prematurely condensed chromosomes (PCC) in the interphase partner. Viewed in the light microscope, S-phase PCC derived from the Indian muntjac appear to be fragmented and heterogeneous. In scanning electron micrographs prepared by an osmium impregnation technique, which avoids the need to sputter-coat the specimen, the S-phase fragments derived from an individual cell are resolved into about 1000 fibre aggregates, together with more dispersed fibres. Aggregates are roughly spherical and vary in diameter between about 0 · 25 and 1 · 6 μm. The spatial distribution of the aggregates shows some order: chains of single aggregates and, less commonly, duplicated chains occur. Regions of the PCC where the fibres are more dispersed are considered to be likely candidates for sites of replication at the time of fusion. The relationship between the condensed aggregate structure of the S-phase PCC and replication clusters is discussed, and also the assembly of aggregates to form metaphase chromosomes.
Glycoproteins of the AKR leukaemia cell surface and their relevance to leukaemiaspecific surface antigens
Authors: McClure Myra O. , Cook Geoffrey M. W. 
Published: April 2021

ABSTRACT
An attempt has been made to prepare antibodies against leukaemia-specific surface antigens by immunizing (C57 Bl/6 × C3H/He)F1 mice with formaldehyde-stabilized AKR leukaemic cells. The presence of antibodies was examined by indirect immunofluorescence microscopy (IFM) and the indirect antiglobulin resetting reaction (IARR). Galactose oxidase treatment destroyed the ability of leukaemic cells to react with antibodies prepared in the hybrid mice, an effect that was reversed by treating the enzyme-modified cells with borohydride. Analysis by immunoprecipitation and polyacrylamide gel electrophoresis of leukaemic cells, labelled by the galactose oxidase/[3H]-NaBFU technique, indicated that a group of glycoproteins of apparent molecular weight &gt;70 000 was involved. Antibodies could be raised in AKR mice to the same group of glycoproteins by immunization with irradiated leukaemic cells or irradiated neuraminidase-treated leukaemic cells. The level of antibody raised in AKR mice had no effect on the growth of leukaemic lymphoblasts introduced subcutaneously into the host.
Antibodies prepared in hybrid mice against leukaemic cells also were absorbed by lymphoid cells of pre-leukaemic 6-month-old AKR mice, indicating that contrary to previous claims in the literature antigens detected by such antisera are not related to malignancy. Hybrid mouse serum cross-reacted with antigens from purified RNA virus isolated from Abelson lymphoma, as demonstrated by the immunoelectrophoretic blotting technique. The pattern of reactivity was not appreciably altered following the absorption of antibodies directed against leukaemic cells. It is concluded that the glycoproteins detected by us may not be viral antigens but normal high molecular weight lymphoid glycoproteins with altered glycosylation patterns that are induced when the viral genomes are expressed.
Thymidylate kinase from Acetabularia ii. regulation during the life cycle
Authors: Groot Egon J. De , Schweiger Hans-Georg 
Published: April 2021

ABSTRACT
The activity of dTMP kinase (EC 2.7.4.9) was estimated during the development of Acetabularia. Enzyme activity was increased at the beginning of the generative phase. Regulation of the dTMP kinase activity was observed even in the absence of the nucleus. More than 30 days after enucleation the enzyme activity increased two- to threefold. The increase in activity was inhibited by puromycin and cycloheximide but not by rifampicin and chloramphenicol.
These results indicate that the enzyme is coded by the nuclear genome and translated on 80 S ribosomes. From mixing experiments with low-activity and high-activity cell extracts it is concluded that the regulation is due to de novo synthesis of the enzyme.
Residual cell division measurements are unreliable as indicators of the timing of events in the Saccharomyces cerevisiae cell cycle
Authors: Richmond K. M. V. , Williamson D. H. 
Published: April 2021

ABSTRACT
We report here an analysis of the execution point of the temperature-sensitive Saccharomyces cerevisiae cell cycle mutant, cdc27-47. When a logarithmically growing culture was shifted from standard growth conditions (strain 27.8B growing in YEPD at 25 °C) to the restrictive temperature cell division ceased abruptly and reproducibly within one population doubling time, the extent of cell division indicating an execution point early in the cell cycle. Approximately 50% of stationary-phase cells were able to divide when refed with fresh medium at 37 °C, showing that the execution point could be passed before ‘start’. This makes the sharp cut-off in cell division difficult to explain. This difficulty was compounded by observations of the cell cycle stage at which individual cells acquired the capacity to divide at 37 °C. Half the cells that were budded at the time of a temperature shift-up formed three division-blocked cells, and in 11 of these 13 cases, two were descended from the original mother cell and one from the original bud. Thus, mother and daughter cells pass the execution point independently; daughters usually during G1, and mothers usually in the budded phase of the previous cycle. The sharp cut-off in cell division is therefore spurious, and a mechanism is proposed to account for it, which has implications for the interpretation of the execution points of other cdc mutants.
In addition, the expression of the cdc27-47 execution point was modified by both genetic and environmental factors, being affected by carbon source, by the petite condition, and by genetic background. This illustrates the difficulties of interpreting execution point data and the dangers of extrapolation of cell cycle parameters between strains and growth conditions.
Nuclear crystalloids in sieve elements of boraginaceae: A protein digestion study
Authors: Thorsch Jennifer , Esau Katherine 
Published: April 2021

ABSTRACT
Nuclear crystalloids have been found in sieve elements of several Boraginaceae. Nuclei of differentiating sieve elements of Echium and other genera except Amsinckia contain one or more crystalloids composed of thin rods densely packed in parallel arrangement. After the nuclei disintegrate in the maturing sieve element the crystalloids are released into the cell lumen where they persist intact. In Amsinckia the crystalloid consists of two components: a dense component, similar to the crystalloid in the other genera and a loosely arranged paracrystalline component. The proteinaceous nature of the nuclear crystalloids and their possible similarity to P-protein was investigated by enzyme digestion techniques. Three proteolytic enzymes were employed in this study: protease, pepsin and trypsin. Successful digestion of the dense crystalloid in both Echium and Amsinckia was obtained with each enzyme tested. P-protein plugging the sieve plate pores was also digested. The loose component’m Amsinckia and the aggregated and dispersed P-protein were not affected by the enzyme digestion procedures. These results seemed to indicate that the density or compactness of the proteinaceous inclusions may play a role in the differential response.
Comparison of quick-frozen and chemically fixed sea-urchin eggs: Structural evidence that cortical granule exocytosis is preceded by a local increase in membrane mobility
Authors: Chandler Douglas E. 
Published: April 2021

ABSTRACT
Eggs of the purple sea-urchin, Strongylocentrotuspurpuratus, were fertilized and fixed with 2% glutaraldehyde at various stages during cortical granule exocytosis. Fixation resulted in membrane blebs being formed precisely at the point of incipient granule fusion. These blebs pinched off to form the membranous vesicles frequently seen in exocytic pockets and in the perivitelline space. In contrast, eggs that were fixed with osmium tetroxide or were quick-frozen without chemical fixation, showed no signs of bleb or vesicle formation. Rather, fusion of each granule appeared to begin at a single minute pore, 30 –50 nm in diameter, which then enlarged. We suggest that formation of blebs during glutaraldehyde fixation is an artifact that is caused by a highly localized and transient increase in membrane mobility. Normally, this increased mobility facilitates fusion of granule and plasma membranes, but in the presence of glutaraldehyde it leads to large-scale distortions of these fusing membranes.
Cloned human teratoma cells differentiate into neuron-like cells and other cell types in retinoic acid
Authors: Thompson S. , Stern P. L. , Webb M. , Walsh F. S. , Engstrom W. , Evans E. P. , Shi W. K. , Hopkins B. , Graham C. F. 
Published: April 2021

ABSTRACT
Single cell clones were isolated from the human teratoma line, Tera-2. The cells of three of these clones were studied. The progressively growing cells were shown to be tumorigenic, and they were characterized by the lack of expression of β 2β2-microglobulinand HLA-A,B,C determinants on the cell surface. The majority of the cells expressed Thy-1 antigen and a 90 × 103 molecular weight protein recognized by the monoclonal antibody F10.44.2; between a third and half of the cells expressed the sugar specificities detected by the anti-SSEA-1 monoclonal antibody. In response to 5 × 10−5 M-retinoic acid applied to cells in monolayer culture, the cells differentiated into a population of flat static cells arrested in the GI phase of the cell cycle. A substantial proportion of these differentiated cells expressed β 2-microglobulin and 43 × 103 molecular weight HLA-A,B,C polypeptides, Thy-1, SSEA-1 sugar determinants, and the 90 × 103 protein recognized by F10.44.2. The apparent molecular weight of fibronectin secreted by the cells decreased by about 5 × 103Mr to 235 × 103Mr after differentiation. The progressively growing cells lacked reactivity with reagents that mark cells in the nervous system. Following aggregation and retinoic acid treatment, neuron-like cells were formed. These cells reacted with reagents that also react with human neurons in culture: they reacted with tetanus toxin, the anti-neurofilament antibodies BF10 and RT97, the anti-ganglioside, GQlc antibody F12 A2B5, and anti-Thy-1.
The progressively growing cells of these Tera-2 clones are therefore capable of forming at least two types of cell: the flat cells in monolayer cultures and the neuron-like cells. None of the cell populations reacted with the monoclonal antibody against SSEA-3 and these cloned cells are therefore distinct from previous isolates from Tera-2.
Accumulation of smooth cisternal in the tapetal cells of Ulex europaeus l. (papilionoideae)
Authors: Misset M. T. , Gourret J. P. 
Published: April 2021

ABSTRACT
In addition to the events occurring in the tapetai cells of most angiosperm species (cell wall lysis, endoplasmic reticulum development and sporopollenin deposition on the outer face of the cells) the tapetum of Ulex europaeus L. exhibits two characteristics. No orbicules, generally associated with the locular face of the secretory tapetum of angiosperms, are found, but some intracellular macro-orbicules are formed and then released in the anther loculus.The great diversity of reticular structures is emphasized by the presence of accumulations of smooth and flattened ciβternae in the mature cells. Their similarity to those observed in the sieve elements of differentiating phloem suggests a possible functional interpretation in the light of the ‘holocrine’ behaviour of the tapetai cells.
Nuclear pores and interphase chromatin: High-resolution image analysis and freeze etching
Authors: Nicolini C. , Vernazza G. , Chiabrera A. , Maraldi I. N. , Capitani S. 
Published: April 2021

ABSTRACT
Computer-enhanced analysis of electron micrographs of thin-sectioned rat liver nuclei, combined with three-dimensional reconstruction of the same Feulgen-stained nuclei, points to a unique clustering of chromatin DNA fibres near the nuclear border.
Computer-enhanced image analysis has been applied to electron micrographs of the envelopes of the same rat liver nuclei prepared by freeze etching and a few essential geometrical parameters characterizing the pores and their distribution have been determined. During interphase, clusters of nuclear pores, closely paralleling the clustering of membrane-attached chromatin fibres, have been identified on the envelope, the number of these being similar to the number of homologus pairs of metaphase chromosomes. Furthermore, rapid changes induced in chromatin distribution appear to be associated with rapid changes in pore number, but not in the number of pore clusters.
A pathway of plasma membrane biogenesis bypassing the golgi apparatus during cell division in the green algaCylindrocapsa geminella
Authors: Sluiman Hans J. 
Published: April 2021

ABSTRACTCell division in Cylindrocapsageminella, in particular the mode of septum membrane biogenesis, has been studied with the transmission electron microscope. Septum formation takes place in a narrow layer of cytoplasm separating post-mitotic nuclei. First, each daughter nucleus develops a wide cytoplasmic pocket (invagination) containing numerous strands of rough endoplasmic reticulum (ER). Next, a proliferation of rough ER is observed in the equatorial zone of cytoplasm, which invariably contains a small number of widely scattered microtubules. The equatorially aligned cisternae of rough ER produce smooth-membraned vesicles, interpreted as smooth ER, which subsequently coalesce to form the membranous transverse septum. Thus, primary septum formation does not follow any of the two previously known basic cytokinetic patterns in green plants (i.e. plasma membrane furrowing and cell-plate formation), but instead represents a novel type of membrane flow, which effectively bypasses the Golgi apparatus. This pathway of membrane flow has remained largely ignored in current concepts of endomembrane structure and function in eukaryotes. However, it appears to be more widespread than has previously been recognized, especially in autospore-producing green algae and in red algae during the formation of tetraspores. It may represent an evolutionary intermediate type of cell division between the supposedly primitive method of plasma membrane furrowing and the more advanced cell-plate system.
Extracellular matrix interactions with the apical surface of vascular endothelial cells
Authors: Kramer Randall h. 
Published: April 2021

ABSTRACT
Cultured aortic endothelial cells, like their in vivo counterparts, form highly organized, confluent monolayers of polarized epithelioid cells that secrete, exclusively at their basal surface, an extracellular matrix to which they then attach. The influence of isolated subendothelial matrix preparations on cell polarity and monolayer organization was studied by presenting fragments of the matrix to confluent bovine aortic endothelial cell cultures. The matrix particles were immediately bound to the apical aspect of the cell monolayer and induced rapid reorganization of the monolayer into cells with a fibroblastoid morphology. To determine if fibronectin, the major glycoprotein of the subendothelial matrix, could be involved in the observed apical cell surface – matrix interactions, latex beads or small discs of Nucleopore filters were coated with the glycoprotein and presented to confluent monolayers. In a fashion similar to that observed with matrix fragments, materials coated with fibronectin caused focal reorganization of the cell layer. After contact with the coated beads, the underlying endothelial cells flowed upward and spread over the entire bead, forming a canopy of confluent cells that draped the particle. Contact of confluent monolayers with the coated filters induced similar behaviour, except that monolayer reorganization into the fibroblastoid phenotype was followed by emigration of the majority of underlying cells through the pores to the upper filter surface, where they formed a new organized cell monolayer with the typical endothelial cell mor-phology. Thus contact of the apical surface of endothelial cells with structures to which they adhere initiates a rapid disruption of the organized cell monolayer, followed immediately by a concerted effort of the local population to re-establish both cell polarity and monolayer contiguity. The expression of this behaviour may be important during tissue remodelling that occurs in neovascularization and during interactions with thromboemboli.