bluuebunny/crossref_metadata_2025_split huggingface dataset:
The ultrastructure and ontogeny of pollen in Helleborus foetidus l. iv. pollen grain maturation
Authors: Echlin P. 
Published: April 2021

ABSTRACT
The final stages of Helleborus pollen-grain ontogeny, which culminate in maturation and germination of the grain, have been investigated at the ultrastructural level. Following the deposition of primary and secondary exine, and during the early stages of intine formation, the microspore passes through a vacuolate phase, in which the cytoplasm appears devoid of most organelles other than the prominent nucleus. The formation of the vacuole results in the displacement of the nucleus to one side of the pollen grain. The vacuole quickly disappears and a number of organelles reappear in the cytoplasm, in particular the dictyosomes and strands of endoplasmic reticulum, with associated grey bodies. Following mitotic division of the pollen grain, the first signs of the generative cell wall appear as a pair of tightly appressed unit membranes in the narrow strand of cytoplasm separating the two newly formed generative and vegetative nuclei. As development proceeds, the space between the two membranes gradually fills with an electron-transparent material similar to the substance found in the numerous dictyosome-derived vesicles which, together with the endoplasmic reticulum, are both closely associated with the developing cell wall. The generative cell wall fuses with the cellulosic intine, which has gradually increased in amount during these stages, and the cell division is complete. The smaller generative cell contains a prominent nucleus and a small amount of cytoplasm devoid of plastids and most other organelles. The larger vegetative cell also contains a prominent nucleus and a large amount of cytoplasm containing amyloplasts, mitochondria, dictyosomes and endoplasmic reticulum, and abundant ribosomes, many of which are in a polysome configuration. The final stages in development are characterized by a progressive decrease in the amount of starch in the vegetative cell and an increase in the size of grey bodies, many of which are invested in multilayered shrouds of endoplasmic reticulum. The generative cell wall disappears and a multivesicular/granular body gradually appears at the periphery of the pollen grain. The granular-vesicular material, which is formed from the dictyosomes and/or the degenerating plastids, is thought to represent metabolic reserves necessary for pollen-tube formation. One or more pollen tubes emerge from the apertural sectors of the pollen grain, and maturation of the grain is complete.
Isolation and characterization of chromatin and dna from the sperm of the spider crab, Libinia emarg1nata
Authors: Vaughn Jack C. , Hinsch Gertrude W. 
Published: April 2021

ABSTRACT
An isolation technique is described which yields highly purified sperm from various regions of the reproductive tract of the crab Libinia emarginata. Sperm chromatin was isolated and analysed with respect to ultrastructure, ultraviolet absorption spectrum, thermal melting profile, behaviour in CsCl gradients, and chemical composition. Sperm-isolated chromatin not stabilized with uranyl acetate prior to ethanol dehydration, like prokaryotic chromatin, has aggregated fibres with a diameter in the range 30–95 nm or more. With uranyl acetate stabilization, only very fine fibres are seen. Pronase treatment of the thick fibres leads to formation of very fine fibres, showing that protein is associated with this isolated chromatin. Polyacrylamide electrophoretic analysis of the chromatin protein shows that it is not basic. Standard basic protein staining of sperm taken from testes, vas deferens, and seminal receptacles after various periods of storage there gives a negative nuclear reaction in all cases. This suggests the absence of sperm nuclear histones and protamines. Sperm chromatin protein is salt-dissociable and has a ratio of basic to acidic amino acids of 0·43–0·48. The-protein/DNA ratio is 2·3–3·2, and the ratio E280/E260, is 0·50–0·67. Thermal denaturation curves are identical for sperm chromatin and purified sperm DNA, supporting the other data showing lack of nuclear basic proteins. An (A + T)-rich satellite DNA component comprises about 7–9 % of the total sperm genome. It is suggested that the genes which specify gamete-type nuclear basic proteins are either permanently repressed or deleted in decapod Crustacea.
Isolation of homologous nuclear dnas from sea-urchin embryos
Authors: Baker R. F. 
Published: April 2021

ABSTRACT
Studies are presented on the ability of low-molecular-weight nuclear DNA fractions to hybridize to higher-molecular-weight nuclear DNAs taken from different stages of early developing sea-urchin embryos, Strongylocentrotus purpuratus. Using preparative DNA-DNA hybridization, a fraction of 60-s mid-blastula DNA was isolated for its ability to anneal to 10-s morula DNA. Approximately 80 regions on each 60-s molecule were found to be homologous to 10-s DNA. High-molecular-weight (&gt; 240-s) nuclear DNA from mesenchyme blastula stage and later stages (compared with DNA from pre-blastula nuclei) shows an increase in the number of regions homologous to fractions of 10-and 60-s DNAs.
A scanning electron-microscope study of the surface features of mammalian cells in vitro
Authors: Hodges Gisele M. , Muir Marjorie D. 
Published: April 2021

ABSTRACT
The cell surface aspects and the density, distribution and morphology of surface cytoplasmic projections (microprocesses) of HeLa, BHK and baby mouse kidney cells, maintained under different culture conditions, have been compared using scanning electron microscopy. No significant differences were noted in the surface aspects of these cell strains. Comparison of the 3 cell strains, under identical culture conditions, showed variations from one strain to another in the density and morphology of the cell surface microprocesses which may reflect basic physiological differences between the strains. For a given cell strain, variations were observed in response to changes in culture conditions indicative that the presence of surface microprocesses is related to the physiological state of the cell.
Absence of metabolic co-operation between certain types of mammalian cells in culture
Authors: Widmer-Favre Chantal 
Published: April 2021

ABSTRACT
A mutant line of BHK21 cells lacking inosinic pyrophosphorylase activity (IPP− cells) does not incorporate [3H]hypoxanthine, as opposed to wild type BHK cells. When IPP+ and IPP− cells are grown together in the presence of [3H]hypoxanthine, IPP− cells in contact with IPP+ cells incorporate the marker, as seen by autoradiography. This phenomenon is known as ‘metabolic co-operation’.
In the present work no co-operation was detected between wild type BHK cells and BHK cells lacking thymidine kinase activity (TK− cells – they do not incorporate [3H]thymidine); between wild type L cells and L-TK− cells, or between L cells and L-IPP− cells. And there is no interaction in mixed cultures of mast cells and mast-cells-IPP−.
Multiple symbiosis in a leafhopper, Helochara communis fitch (cicadellidae: Homoptera): Envelopes, nucleoids and inclusions of the symbiotes
Authors: Chang K. P. , Musgrave A. J. 
Published: April 2021

ABSTRACT
Mycetomes in Helochara communis are paired syncytia each enclosed by an epithelium. Adjacent are loose aggregates of mycetocytes. Mycetomes harboured 3 kinds of inclusions: companion, a-and t-symbiotes according to Buchner’s classification of homopteran symbiotes. In the mycetocytes, a fourth kind of inclusion was found and named g-symbiotes. All inclusions were Feulgen and Gram negative. Nucleoids were revealed in companion symbiotes by osmium-HCl-Giemsa; and, in some g-and t-, but not in a-, symbiotes by RNase-papain Giemsa. DNase digestion caused loss of basophilia in g-and t-, but not in a-, symbiotes. By electron microscopy, using Kellenberger’s fixative, companion, g-and t-symbiotes had lucid areas with reticular strands 4 nm thick; a-symbiotes were in general, homogeneously densely granular. Companion symbiotes had Gram-negative profiles with additional surface material; a-and t-symbiotes had 3, g-symbiotes 2, peripheral, trilaminar membranes. Parallel striations and regularly arranged granules occurred in companion symbiotes; crystalline inclusions in g-and t-symbiotes; and fibrillar elements and dense bodies in a-symbiotes.
Companion symbiotes may be Gram-negative bacteria or rickettsiae; g-and t-symbiotes, L-phase bacteria; but a-symbiotes are anomalous, apparently lacking DNA, and probably being derivatives of t-symbiotes, thus requiring that Muller’s hypothesis of homopteran evolutionary pathways be rephrased.
A fine-structural study of conidium initiation in Stemphylium botryosum wallroth
Authors: Carroll Fanny E. 
Published: April 2021

ABSTRACT
Conidia and conidiophores from synchronously sporulating cultures of Stemphyliiim botryosum were fixed and embedded at successive intervals for examination under the electron microscope. The spore initial arises as a small bud at the tip of the conidiophore, and no discontinuity between the conidiophore wall and the spore can be detected. The single-celled bud and the tip cell of the conidiophore have very similar cytoplasmic features.
Besides the usual cellular organelles, irregularly shaped electron-transparent areas in which membrane systems seem to perform some internal secretory activity between the leaflets them selves are present. Globules of the slightly electron-dense material originating in these specialized areas can also be found next to the plasma membrane where they are released and presumably participate in wall formation. Such a secretory system has not been described before and may be of particular importance in higher fungi which are known to lack a true Golgi apparatus.
Freeze-fracture replication of rough endoplasmic reticulum of mouse liver cells
Authors: Breathnach A. S. , Stolinski C. , Gross M. 
Published: April 2021

ABSTRACT
Fresh, chemically unfixed, glycerinated specimens of mouse liver were examined by the technique of freeze-fracture replication without sublimation (i.e. they were not ‘etched’). Where extensive areas of fractured lamellar membranes of the rough endoplasmic reticulum are revealed en face, 2 types of fracture face are distinguishable. One of these fracture faces (A) is directed towards the cytoplasm, and the other (B) towards the cisternal cavity. A characteristic mosaic, or patchwork pattern of flat areas circumscribed by particles, is evident on both faces, and more clearly so on face B, due to a greater number of more prominent particles. Similar mosaic patterns are revealed on convex faces erf the nuclear membrane, and on concave fracture faces of mitochondrial membranes, but are not evident on fracture faces of the plasma membrane.
Uncertainty in establishing the exact plane of fracture of membranes in this material, since glycerol is virtually non-sublimable, makes it difficult to assess the significance of these mosaic patterns. The fact that ribosomes are not identifiable on either face of fractured endoplasmic reticulum membranes, gives no certain indication of the plane of fracture.
An actin-like component in sperm tails of a crane fly (Nephrotoma suturalis loew)
Authors: Forer A. , Behnke O. 
Published: April 2021

ABSTRACT
In negatively stained preparations made from glycerinated crane fly spermatids and sperm, actin-like filaments are seen which bind heavy meromyosin (HMM) to form arrowhead complexes, the reaction with HMM being blocked by ATP and pyrophosphate. In preparations from young spermatids the actin-like filaments are found singly, or in small groups, while in those from mature sperm the actin-like filaments are organized into a structure which we call ‘rods’. Both rods and single filaments come from lysed sperm tails. The actin-like filaments in rods bind HMM only when frayed out on a grid.
In sections of normal or glycerinated spermatids or sperm, no actin-like filaments are seen, either because they are not preserved through the fixation and embedding procedures, or because they are present in a form which we do not recognize. In sections of glycerinated sperm-atids incubated with HMM, decorated filaments are seen in non-nuclear regions of spermatids (tails), oriented parallel to the axoneme. These probably correspond to the single filaments identified in the negatively stained preparations and not to the rods, because the actin-like filaments in rods bind HMM only after fraying out on a grid.
HMM causes polymerization of filaments: actin-like filaments are not seen in negatively stained preparations of glycerinated cells subsequently incubated in salts solution, yet are seen in such cells after a further incubation in HMM. Thus some of the decorated filaments seen in sections may have been polymerized by the HMM, raising questions about the procedure.
After rupture of living cells no single actin-like filaments are seen in negatively stained preparations, raising the question of whether glycerol as well as HMM can cause polymerization of filaments. Rods are seen in such preparations, and thus rods seem to exist as such in living cells.
Single actin-like filaments and rods are seen even when cytoplasmic microtubules are not seen (after incubation in colchicine or vinblastine). Thus the filaments do not seem to come directly from cytoplasmic microtubules.
The possible presence of actin-like filaments in cilia and in other flagella, the location of actin-like filaments in sperm tails, and the possible role of the actin-like filaments are discussed.
The basis of cell-to-cell transformation in Paramecium bursaria: II. investigation into the molecular nature of the transforming agent
Authors: Cullis C. A. 
Published: April 2021

ABSTRACT
A mating type transformation occurring in syngen 4 of Paramecium bursaria has been further investigated. The transformation event was not reversed or prevented by acridine or ionizing radiations. The transformation has not been accomplished with cell-free extracts including the microinjection of cytoplasmic extracts.
The nucleic acids of normal and transformed cells have been compared. No differences were found. Both types had 2 major ribosomal RNAs of molecular weights 1·29 × 106 and 0·70 × 106 Daltons. In both types a particular RNA species, with molecular weight 10 × 105 Daltons, was found only in sexually competent cells. The DNA had a buoyant density in caesium chloride of 1·689 8 cm−3 and a Tm in 0·1 × SSC (1 × SSC = 0·15 M sodium chloride, 0·015 M trisodium citrate) of 65 2 °C. The renaturation kinetics showed 2 types of DNA – a fast-renaturing com ponent, comprising 13% of the total DNA, with a complexity of 1·84× 109 Daltons, and a slow-renaturing component with a complexity of r6xio u Daltons, in both normal and transformed cells. Possible models for the action of the transforming agent are discussed.
The Responses in Culture of Human Tumour Astrocytes and Neuroblasts to N6,O2′-Dibutyryl Adenosine 3′,5′-Monophosphoric Acid
Authors: Macintyre E. H. , Wintersgill C. J. , Perkins J. P. , Vatter A. E. 
Published: April 2021

ABSTRACT
N6,O2′-dibutyryl cyclic AMP (DBcAMP) (3× 10-4 M) causes reproducible changes in the morphology, growth rate and terminal cell density of 3 different types of human tumour cells grown in culture.
Human tumour astrocyte lines 1181N1 and EH-118MG show generalized ablation of membrane systems when grown in the presence of this cyclic AMP analogue. This is not consistent with differentiation; it may be a toxic effect but possibly represents the response of these cell types to DBcAMP. These cells also grow more slowly and to lower terminal cell densities as a function of the concentration of DBcAMP. Although the growth kinetics demonstrate a return of these malignant cells to a state of density-dependent growth, micrographic analyses of intercellular relationships reveal that the cells still grow in overlapping patterns. Thus as judged by the commonly accepted criteria, DBcAMP does not induce contact inhibition of growth. All observed changes induced by the presence of DBcAMP are rapidly and completely reversed upon its removal from the growth medium. Human neuroblasts (NBI) grown in the presence of DBcAMP show irreversible morphological changes and the rate and extent of cell division is reduced. There is marked hyperplasia of intracytoplasmic microfilaments, which is considered consistent with differentiation.
Studies on Nucleolar RNA Synthesis in Drosophila Melanogaster: I. The Relationship between Number of Nucleolar Organizers and Rate of Synthesis
Authors: Krider H. M. , Plaut W. 
Published: April 2021

ABSTRACT
Nucleolar RNA synthesis in salivary glands from XX, XY, and XO larvae having different numbers of nucleolus organizers was examined autoradiographically following incubation of (he tissues in [3H]uridine. In addition, the presence or absence of secondary constrictions was monitored in neuroblast preparations using acridine orange staining. It was observed that: (1) The rate of nucleolar [3H]uridine incorporation is independent of the number of nucleolus organizers in the cell; (2) nucleolar incorporation in XO tissues is elevated relative to that observed in XX or XY material; and (3) where the number of organizers is in excess of the wild type (2), secondary constrictions form at only 2 of the nucleolus organizer sites.
From these and related observations we suggest that there are 2 forms of transcriptional control for nucleolar RNA synthesis. One acts as an on-off control, influencing the formation of secondary constrictions at the nucleolus organizers. The other form modulates rates of transcription at organizers where constrictions have been formed.
The Nature of Immature Avian Erythrocytes in Severe Anaemia Induced by Phenylhydrazine
Authors: Williams A. F. 
Published: April 2021

ABSTRACT
Hens were rendered severely anaemic by the use of phenylhydrazine and the characteristics of the polychromatic erythrocytes appearing in the blood examined. Even the least-dense polychromatic erythrocytes obtained after density fractionation of anaemic blood on BSA density gradients had a volume mode only 1-5 times that of mature erythrocytes. Also the immature cells always contained less haemoglobin than mature erythrocytes.
It was concluded that in severe anaemia avian erythropoiesis still consists of only one cell line, and that cells do not miss divisions as is believed to be the case in mammalian erythropoiesis.
Deoxythymidine Metabolism in Avian Erythroid Cells
Authors: Williams A. F. 
Published: April 2021

ABSTRACT
The ability of dividing and non-dividing cells of the avian erythropoietic series to phosphorylate [3H]deoxythymidine and [3H]uridine has been studied. Enzyme levels of deoxythymidine kinase were also assayed. Both dividing and immature non-dividing erythroid cells phosphorylated uridine to uridine triphosphate (UTP). However, only cells able to synthesize DNA could phosphorylate deoxythymidine to the triphosphate; also these cells were the only ones to show detectable deoxythymidine kinase activities.
DNA Polymerase in Avian Erythroid Cells
Authors: Williams A. F. 
Published: April 2021

ABSTRACT
The level and properties of DNA polymerase activity assayable in extracts of avian erythroid cells was studied. The enzyme was detectable in the dividing cells (erythroblasts) of the erythropoietic series and also the immature non-dividing erythrocytes. It could not be assayed in mature erythrocytes. Investigations showed that activity began to decline at the time of the last cell division of the erythroid series. Properties of the enzyme did change in different cell types; however, the changes did not correlate with cessation of DNA synthesis. Some preliminary results on DNA synthesis by isolated nuclei are also reported and these showed that only nuclei from erythroblasts could synthesize DNA in vitro in the absence of primer.
The Structure of Trichocysts in Paramecium Caudatum
Authors: Bannister L. H. 
Published: April 2021

ABSTRACT
The structure of undischarged and discharged trichocysts has been examined in Paramecium caudatum, and their light-microscopic appearance compared with their fine-structural organization. In living specimens undischarged trichocysts appear to be of a single type with a unimodal variation in length about a mean of 3·7 μm. When fixed for electron microscopy or compressed beneath a coverslip many of the trichocysts expand within the cell, giving rise to a variety of different forms of lower phase density.
Ultrastructurally the undischarged trichocyst consists of at least 10 different components: these include a mesh-like sheath surrounding the body of the organelle; an inner and an outer sheath enclosing the tip, the inner sheath being made up of 4 spiralling envelopes with a square net substructure, and the outer sheath being formed of a dense amorphous matrix containing longitudinal microtubules and scattered fine filaments; a boundary surface to the outer sheath; a membranous trichocyst sac the apical region of which is surrounded by a cylinder of microtubules joined to each other with dense material; and lastly, the crystalline matrix of the trichocyst body and tip. This crystalline appearance is apparently related to the presence of a loosely interwoven complex of fine filaments which form a highly regular pattern of unit structures repeating at 16-nm intervals.
In extended trichocysts the 60-nm banding pattern of the body is also composed of fine filaments arranged in a different, elongated manner in 2 distinct and alternating patterns which are taken to be 2 views of the same structure. Measurements indicate that when trichocysts extend they elongate by a factor of from 6 to 8. It is proposed that the crystalline pattern of the unextended trichocyst body transforms into the extended form by a simple rearrangement of the constituent filaments accompanied by their elongation. Possible models of the undischarged and discharged states of organization are suggested.
Prenucleolar bodies contain coilin and are assembled in Xenopus egg extract depleted of specific nucleolar proteins and U3 RNA
Authors: Bell Peter , Scheer Ulrich 
Published: April 2021

ABSTRACT
Nuclei assembled in Xenopus egg extract contain numerous spherical aggregations or nuclear bodies. Previously we have shown that they closely resemble prenucleolar bodies (PNBs), both at the compositional and ultrastructural level. Subsequently, coilin was also identified and for this reason they were called coiled bodies. Here we present morphological and immunocytochemical evidence that the in vitro nuclear bodies resemble authentic PNBs and are different from coiled bodies. In particular we show that coilin, previously considered as the defining protein constituent of coiled bodies, is also present in PNBs of cultured cells. In contrast, the PNB-associated nucleolar proteins nucleolin and B23/NO38 are not detectable in coiled bodies and may thus serve as suitable markers for PNBs. Our results suggest that PNBs are primary assembly structures which contribute to the formation of both nucleoli and coiled bodies and thus offer an explanation for the frequently observed structural association of coiled bodies with nucleoli. To gain some insight into the assembly process of PNBs in vitro, specific nucleolar proteins were removed from Xenopus egg extract. Quite surprisingly, the immunodepleted extracts still promoted the assembly of nuclear bodies which lacked either fibrillarin, nucleolin, xNopp180 or B23/NO38. Only after fibrillarin-depletion fewer PNBs were seen as compared to controls. Digestion of the extract with RNase followed by northern blot analysis revealed that U3 small nucleolar RNA is not required for the formation and structural maintenance of PNBs in vitro.