bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies of membrane fusion.: V. fusion of erythrocytes with non-haemolytic sendai virus
Authors: Knutton S.
Published: April 2021

The fusion of human erythrocytes with non-haemolytic ‘1 -day’ Sendai virus has been studied by electron microscopy. The mechanism of viral envelope-cell fusion is the same as that described previously for haemolytic ‘3-day’ Sendai virus except that fusion is frequently arrested at an initial stage when 2 segments of smooth linear viral membrane fuse and become incorporated into the erythrocyte membrane. After longer periods of incubation at 37 °C, in addition to many partly fused virus particles, long (up to 4 μm) lengths of smooth linear viral membrane are seen within the erythrocyte membrane which arise by linear aggregation of shorter (∼ 0·25 μm long) segments of smooth linear membrane derived from individual fused viral envelopes. Cell-Cell fusion, as a result of the fusion of a viral envelope with 2 adjacent erythrocytes also occurs but, in the absence of cell swelling, fusion is arrested at this stage with cells joined by one (or more) small cytoplasmic bridges. Typical fused cells are produced if such cells are swollen with hypotonic buffer. These observations provide further evidence that membrane fusion and cell swelling are distinct events in cell fusion and that cell swelling is the driving force both for completing the incorporation of the viral envelope into the cell membrane and for expanding cells connected by small cytoplasmic bridges to form spherical fused cells. Little lateral diffusion of viral envelope components occurs in the absence of cell swelling; in fact, some aggregation of components occurs. Comparison with previous studies using haemolytic ‘3-day’ Sendai virus suggests that virally induced cell swelling perturbs membrane structure so as to allow the rapid lateral diffusion of integrated viral envelope components.
Arrest of cell membrane movements by In Vitro incubation with polycation reversed by poly anion
Authors: Ebbesen Peter , Güttler Flemming
Published: April 2021

A total arrest of cell membrane movement occurred after incubation for 2 h of cultured fibroblasts with the polycation DEAE-dextran. The immobilized cells resumed normal membrane undulations after exposure for 5 min to the polyanion dextran-sulphate. The influence of DEAE-dextran on the electrophoretic mobility, which was seen after 10 min of incubation was also erased by a subsequent incubation with dextran-sulphate. No influence on growth of the culture was seen.
Kinetics of the regeneration of sea-urchin cilia : II regeneration of animalized cilia
Authors: Burns Roy G.
Published: April 2021

The kinetics of the regeneration of cilia of Arbacia punctulata animalized by treatment with trypsin have been examined. The cilia regenerate with biphasic kinetics: an initial linear phase which changes abruptly to a second, slower linear rate. Inhibition of protein synthesis with 10 μM emetine has little effect on the first phase but totally abolishes the second, suggesting that the first phase represents the utilization of a pre-existing precursor pool while the second reflects de novo protein synthesis. However, cilia will regenerate again following a second deciliation in the presence of emetine, to half the plateau value of the first regeneration, indicating that only a fraction of the pool of the limiting precursor can be assembled into the regenerating cilium. It is proposed that a residual pool of this precursor is required to maintain the assembly-disassembly equilibrium in favour of the assembled organelle.
Morphological alterations in the mitochondria of Amoeba Proteus induced by uncoupling agents
Authors: Smith Robert A. , Ord M. J.
Published: April 2021

Ultrastructural changes in mitochondrial morphology were observed in amoebae exposed to the uncoupling agents dinitrophenol (DNP), pentachlorophenol (PCP), and m-chlorocarbonyl cyanide phenylhydrazone (CCCP). These alterations occurred with rapidity and were present before whole-cell activity changes could be detected. They included changes in profile shape and overall dimensions, matrix density changes, and alterations to the cristal membranes, so that distinction between control Type I and Type II conformations was eliminated and a form with intermediate characters generated. It is proposed that, in producing these altered forms, the uncoupling agents are acting by suppressing the control functional states of the mitochondria.

At the end of the uncoupler treatment all alterations were shown to be reversible, i.e. in cells transferred back to normal culture medium, mitochondrial profiles identical to those in control cells were again evident with time. These effects on the mitochondria could be produced whether the uncoupler was introduced externally by total cell exposure, or to the inside of the cell by microinjection. The importance of precisely defining conditions for treatments aimed at disrupting cell activity was evident. Thus, an uncoupler, which is a weak acid, proved to be more effective in whole cell treatments if applied at a pH near to its pK2 - so ensuring its penetration across the outer cell membrane.
The effect of local anaesthetics and barbiturates on myogenesis and myotube integrity in rat skeletal muscle cultures
Authors: Stygall Katie , Mirsky Rhona , Mowbray John
Published: April 2021

Several cationic anaesthetics and barbiturates including bupivacaine, lignocaine and pentobarbital reversibly inhibit myoblast fusion in rat primary muscle cultures. The same drugs also cause disruption of myotube structure, which is normally complete within 2 h; fusion of myoblasts and reformation of myotubes takes place on removal of the drugs. The disruption of myotubes caused by tertiary amine anaesthetics observed in vitro appears to mimic muscle fibre disruption caused by the same drugs in vivo. The effects of temperature, culture age, medium constitution and calcium flux on anaesthetic-induced myotube breakdown have been examined.
Capping of variable antigen on Trypanosoma Brucei, and its immunological and biological significance
Authors: Barry J. David
Published: April 2021

Pathogenic trypanosomes undergo antigenic variation, whereby the glycoprotein molecules constituting the cell surface coat are changed, the parasite thus evading the host’s immune response. On application of homologous antiserum in indirect immunofluorescence to a given variable antigen type of Trypanosoma brucei, the surface variable antigen moves to the flagellar pocket region, which overlies the Golgi apparatus. This redistribution, or capping, is temperature-dependent, occurring at 37 °C but not at 0–4 °C. Patching does not occur at either temperature. Immediately after capping no homologous or heterologous variable antigen, or host plasma or blood cell antigens, can be detected by immunofluorescence on the cell surface outside the cap; only trypanosome membrane common antigens can be found.

It seems unlikely for two reasons that this antibody-induced redistribution is relevant to antigenic variation. Capping of the coat requires the indirect, rather than the direct, irnmunofluorescent method; a single layer of antibody, as available in nature, would appear to be ineffective. Also, capping of variable antigen of one type is followed within 3 h by appearance of antigen of the same, and not another, type.

The necessity for 2 antibody layers is usually thought of as meaning that the individual molecules of the cell surface antigen are spaced further apart than the binding sites of an individual antibody molecule, so that the necessary cross-linked lattice cannot be formed, but on T. brucei the surface variable antigen molecules are very closely packed. It is proposed that one layer of antibody is ineffective for steric reasons; the dimensions of the exposed face of each variable antigen molecule may not permit the binding of more than one molecule of immunoglobulin, or perhaps the antigen molecules are so closely packed that most of the antigenic determinants are hidden from antibodies. To test this hypothesis, an attempt was made to cap variable antigen on trypanosomes transforming in vitro from the bloodstream to the procyclic (insect midgut) stage; such forms have a much less densely packed surface coat. Patching was observed, indicative of lattice formation, but these trypanosomes did not survive the in vitro manipulation long enough to permit any possible capping.

T. brucei differs structurally from most other eukaryotic cells. It has no detectable microfilaments under the plasma membrane, except at the desmosomes in the region of flagellar binding, and it also has a pellicular cortex of microtubules. Capping of its surface antigen would appear then to differ from that on mammalian cells, either in the cellular components involved or in that specialized areas of the plasma membrane are involved.
Actin in spindles of Haemanthus Katherinae endosperm :IX. general results using various glycerination methods
Authors: Forer Arthur , Jackson W. T.
Published: April 2021

We have studied actin-containing filaments in spindles in Haemanthus endosperm cells glycerinated by various methods; the actin-containing filaments were identified by their reaction with rabbit skeletal muscle heavy meromyosin (HMM) to form ‘decorated’ filaments. Actin-containing filaments in the spindle were seen in amongst microtubules in bundles (both non-kinetochore microtubule bundles and kinetochore microtubule bundles) and were also seen not associated with microtubules. There were very few extra-spindle actin-containing filaments in these cells. Actin-containing filaments seemed to interact with microtubules, because the filaments remained close to and parallel to microtubules even when the microtubules were sharply curved. Because of the close association between microtubules and actin-containing filaments we could not identify all the actin-containing filaments present in microtubule bundles: microtubules obscured actin-containing filaments.

We studied Haemanthus endosperm cells as they were glycerinated. For some of these observations we used phase-contrast microscopy. Glycerination caused the cells to shrink, initially, and this was followed by rapid expansion, but the cells did not expand to as large a volume as before glycerination. Spindle structure was maintained despite these changes in cell size. Evidences for this are that relative chromosome positions were maintained during glycerination, that spindle birefringence was maintained during glycerination, and that individual chromosomal spindle fibres remained biréfringent during glycerination. Electron-microscopic observations supported this in that kinetochore microtubule bundles and non-kinetochore microtubule bundles were maintained during glycerination, as was the helical arrangement of spindle ribosomes into polyribosomes.

One-step glycerination procedures were used (cells were treated with mixtures containing 25% glycerol, Triton-X-100, and HMM), and such procedures might be of general use. Living cells were embedded in fibrin clots in making light-microscopic observations; this procedure, too, might be of general use.
An ultrastructural study of the effects of wheat germ agglutinin (WGA) on cell cortex organization during the first cleavage of Xenopus Laevis eggs II. cortical wound healing
Authors: Geuskens M. , Tencer R.
Published: April 2021

Uncleaved fertilized eggs of Xenopus laevis treated with wheat germ agglutinin (WGA) have been pricked at the animal pole both inside and outside the regressed furrow region. The wounded cortex of both regions has been studied with the electron microscope and compared with the same region of wounded, untreated eggs. In all 3 cases, filaments are organized in an annular zone in the damaged cortex. When the surface is pricked outside the regressed furrow of WGA-treated embryos, bundles of microfilaments radiate from the ring and extend in deep folds which form a ‘star’ around the wound at the surface of the embryo. However, when the surface is pricked in the new membrane of the regressed furrow, filaments are intermingled with internalized portions of the plasma membrane. It is suggested that, when the surface is pricked outside the furrow region, more filaments are mobilized to counteract the tangential retraction of the membrane which has acquired more rigidity after WGA binding.
Probing the free space within rat and chicken chromatin with active and passive probes
Authors: Burgoyne Leigh A. , Skinner Jennifer D.
Published: April 2021

The cavity systems within chicken erythrocyte nuclei and rat liver nuclei were compared using passive probes of radioactive glycogen and active probes of nuclease-armed-glycogen. The passive probe curves have a form that indicates that they are due to passive occupation of spaces and not due to the effects of a limiting membrane.

The technique of probing nuclei with glycogen armed with a small enzyme is described.

The chicken erythrocytes appeared to have 11–15-nm and 4–5-nm cavity systems similar to those we have previously reported in rat nuclei. Evidence is presented to show that the bridge DNA is enclosed within the 4-5-nm cavities in both rat liver and chicken erythrocyte nuclei.

Minor differences between the rat liver and chicken erythrocyte nuclear cavity systems are noted in the region of 5–8 nm. A relatively mild procedure is described for preparing chicken erythrocyte nuclei.
A crystalline inclusion in sieve element nuclei of Amsinckia: I. The inclusion in differentiating cells
Authors: Esau Katherine , Magyarosy Andrew C.
Published: April 2021

The presence of usually single, elongated, compound crystalloids in nuclei of sieve elements is an outstanding characteristic of the phloem of Amsinckia douglasiana A. DC. (Boraginaceae). The crystalloid consists of two components forming alternating panels, or blocks, that extend through the entire length of the crystalloid and radiate from the centre where one of the components predominates. Three to seven panels for each component were recorded. One component consists of 4-sided tubules closely packed in highly ordered aggregates, the other of wider 6-sided tubules rather loosely arranged in paracrystalline aggregates. The crystalloid arises at the beginning of sieve clement differentiation. Aggregates of 4-sided tubules appear first. In plants infected with the curly top virus, the crystalloids do not differ from those in non infected controls in structure and conformation. But because the phloem in infected plants is hypcrplastic, with most of the cells differentiating as sieve elements, the crystalloids are far more abundant in diseased than in healthy plants.
Granulocyte migration through endothelium in culture
Authors: Beesley Julian E. , Pearson Jeremy D. , Hutchings Amanda , Carleton J. Susan , Gordon John L.
Published: April 2021

Using a model system with which we have previously investigated the adhesion of granulocytes to cultured endothelium, we have now shown that adherent granulocytes migrate through cultured endothelium in a manner closely resembling that found during the acute inflammatory response in vivo. The migration of granulocytes was markedly enhanced in the presence of erythrocytes, whereas blood platelets did not affect migration. Several test agents, including Paroven and some prostaglandins, had different effects upon migration and adhesion. We conclude that the adhesion of granulocytes to endothelium and their migration through it are responses that are, at least in part, separately controlled, and that migration does not depend exclusively upon extravascular stimuli.
The interaction of lectins with the surface of differentiating erythroleukaemic cells
Authors: Hunt Richard C. , Marshall Linda M.
Published: April 2021

Friend erythroleukacmic cells can be induced to mature along the erythroid differentiation pathway when an inducing agent such as dimethyl sulphoxide is included in the medium. In the absence of the inducing agent, the 707B line of Friend erythroleukaemic cells is highly agglutinable by the lectins concanavalin A or wheat germ agglutinin. However, 48 h after the induction of differentiation, there is a marked decrease in the agglutination of the cells in the presence of either lectin. This suggests that early in differentiation a change occurs in the cell membrane preceding the onset of globin synthesis which starts approximately 72 h after induction. The change in agglutination by concanavalin A also occurs in the presence of reagents which do not induce haemoglobin synthesis in the 707B line of Friend erythroleukaemic cells but which are able to stimulate the synthesis of this protein in other erythroleukaemic cell lines.

The reduction in the agglutinability of the differentiating cells does not seem to result from a reduction in the number of concanavalin A receptors on the cells, nor does it reflect a change in the clustered distribution of concanavalin A receptors in the differentiating cells. Both the control and dimethyl sulphoxide-induced cells show a similar patchy distribution of ferritin-labelled concanavalin A when examined by electron microscopy.

Polyacrylamide gel electrophoresis shows little change in the total pattern of protein synthesis by control and differentiating cells when pulse-labelled with [35S] methionine. However, use of 125I-labelled concanavalin A to stain polyacrylamide gels, on which the total proteins of control and differentiating cells had been separated, revealed a profound change in the composition of the concanavalin A-binding proteins. The control, undifferentiated cells contained eleven or more classes of concanavalin A-binding glycoproteins, many of which stained to a lesser degree as the cell density increased. After the onset of differentiation, 2 new concanavalin A-binding glycoproteins appeared within 48 h. One of these proteins has a molecular weight in excess of 180000 while the other migrated with an apparent molecular weight of approximately 100000. After erythroid differentiation had progressed for 120 h, these newly synthesized glycoproteins became the major concanavalin A-binding proteins of the erythroleukaemic cells.
Co-operativity of lectin binding to fibroblasts and its relation to cellular actomyosin
Authors: Thom David , Cox Diana S. , Safford Richard , Rees David A.
Published: April 2021

We have investigated the binding of 125 I-concanavalin A (125 I-Con A) and 125 I-succinyl concanavalin A (125 I -s-Con A) to rat fibroblasts (16C line) as a function of the concentration of added lectin, and the alterations to this binding behaviour caused by drugs which modify the cytoskeleton. The changes in cell behaviour which occur at different levels of binding have also been studied. As shown previously for some other systems, the binding of Con A is complex and partly co-operative. Three phases can be distinguished in our system: (i) prenucleation binding, (ii) binding which shows a small positive slope in a Scatchard plot and a Hill coefficient greater than unity, and which therefore is incipiently co-operative, and (iii) post-co-operative binding.

The co-operative phase of binding is paralleled by progressive inhibition of EGTA-mediated cell detachment from substrata, with inhibition being complete when this phase of binding is complete. Likewise, the phagocytosis of latex spheres is progressively inhibited up to a threshold which coincides with the completion of co-operative binding. Thirdly, cells pretreated with Con A round up with colchicine (10− 5 M) if co-operative binding is complete, but adopt broad epithelial shapes if it is not. r-Con A does not show co-operative binding, and correspondingly does not inhibit EGTA-mediated cell detachment, or show a distinct threshold in the inhibition of phagocytosis, or promote the 2 types of shape change with colchicine.

The pattern of Con A binding is drastically altered by pretreatment of cells with cytochalasin B or azide. The Scatchard and Hill plots show that the co-operative phase remains and is complete at about the same level of binding, but that it is more readily nucleated and takes place against a changed number and/or distribution of receptors. Pretreatment of cells with colchicine causes changes in the pattern of binding which are different from those observed with cytochalasin B or azide and are more difficult to interpret.

We conclude that a reciprocal relationship exists between the cellular actomyosin and the state of cell surface receptors. Perturbation of actomyosin by cytochalasin B or azide can enhance the freedom of some receptors to participate in a co-operative rearrangement which facilitates the binding of further molecules of lectin. Vice versa, the co-operative event has a feedback influence on the cellular actomyosin to cause alterations of cellular response.
The behaviour of fibroblasts migrating from chick heart explants: changes in adhesion, locomotion and growth, and in the distribution of actomyosin and fibronectin
Authors: Couchman John R. , Rees David A.
Published: April 2021

Fibroblasts migrating from heart explants of chick embryos at first have a high rate of locomotion but lack focal contacts or adhesions and also lack substantial actin-containing bundles. A meshwork of 7-nm filaments is present particularly in submembranous regions and is proposed to be directed towards efficient locomotion whilst maintaining a high degree of spreading. Also during the first 48 h there is little production of extracellular fibronectin and the growth rate is low. Later, these fibroblasts develop focal contacts and focal adhesions together with actomyosin bundles, with a parallel increase in fibronectin expression. We propose that progressive immobilization by the development of focal adhesions and actomyosin structures occurs to set these cells up for growth.
The effective site of the lesion resulting from a trichocyst non-discharge stable differentiation of somatic nuclei in Paramecium tetraurelia
Authors: Aufderheide Karl J. , Sonneborn T. M.
Published: April 2021

Previous work has shown in Paramecium tetraurelia that recessive genic mutants with the phenotype N, non-discharge of trichocysts from intact cells in response to a standard test (exposure to tannic or picric acid), are in some mutants due to lesions in trichocyst development and in others due to lesions in the cytoplasm; and that in stock dII3 the same homozygous germinal genotype can yield by stable differentiation of somatic nuclei both N clones and D (discharge) clones. The site of the lesion in the N clones of dII3 was investigated by the same microinjection protocol used earlier for analysis of the genic mutants: cytoplasm containing trichocysts was removed from wild-type (D) and from dII3 (N) cells and injected into cells of both dII3 (N) and the genic mutant ftA (normal cytoplasm, but morphologically abnormal trichocysts incapable of insertion in the cell cortex or of discharge from intact cells). Trichocysts from each donor behaved the same in both recipients: trichocysts from D cells could discharge; those from dII3 N cells could not. Thus, the cytoplasm of dII3 (N) cells does not prevent the discharge of trichocysts from D cells and the trichocysts from dII3 (N) cells cannot discharge in ftA cells in which trichocysts from wild-type (D) cells can. It appears, therefore, that the lesion in dII3 (N) cells, due to a stable differentiation of somatic nuclei, is in the trichocysts themselves, as it is in certain genic mutants.
The loss of morphogenetic potential and induction of phenylalanine ammonia-lyase in suspension cultures of Phaseolus vulgaris
Authors: Bevan Michael , Northcote D. H.
Published: April 2021

The loss of morphogenetic potential in bean suspension cultures has been investigated by measuring the amounts of phenylalanine ammonia-lyase activity induced in the cells when they are transferred from a medium in which they are grown and maintained to an induction medium.

The tissue has been grown in 2 types of medium: (1) supplemented with 2,4-dichlorophenoxy-acetic acid as the only growth hormone, and (2) supplemented with 2,4-dichlorophenoxyacetic acid and coconut milk. When cells were grown in medium with only 2,4-dichlorophenoxyacetic acid for a period of 5–10 subcultures and samples were transferred to the induction medium at intervals during the subcultures, the amounts of phenylalanine ammonia-lyase activity and the number of xylem elements induced progressively declined. Cells grown in the presence of coconut milk did not lose the ability to induce phenylalanine ammonia-lyase or xylem elements.

Cells grown in the presence of coconut milk were cloned and clones capable of producing different amounts of phenylalanine ammonia-lyase when transferred to induction medium were obtained. However, clones producing low amounts of activity did not grow faster in the medium lacking coconut milk and no evidence was obtained to show that selective growth of non-inducible cells was responsible for the loss of morphogenetic potential.

In addition to the induction brought about by the presence of naphthylacetic acid and kinetin in the induction medium the cells could also be stimulated to produce phenylalanine ammonialyase activity by dilution at subculture.

This increase in activity occurred within 10 h of the dilution, whereas that produced by the hormones in the induction medium occurred after 120 h. The induction produced by dilution also occurred in these cells which had lost their ability to respond to the hormonal induction. Thus the mechanism that produced the increase in phenylalanine ammonia-lyase activity was intact but had lost its ability to respond to the hormones of the induction medium. The loss of inducibility was therefore probably not due to a genetic change in the cells bought about by continuous growth in a medium lacking coconut milk, but to reversible changes in the hormonal requirements of the cells necessary for induction.
An unusual cell surface modification: A double plasma membrane
Authors: Lane Nancy J. , Harrison J. Barrie
Published: April 2021

The occurrence of an unusual double plasma membrane structure is reported; it has been studied in conventional thin sections, after lanthanum-impregnation and with freeze-fracturing. This modification of the plasmalemma is found where the luminal cell membrane (I membrane) of gut microvilli in the haematophagous insect, Rhodnius prolixus, is surrounded by a second, outer membrane (O membrane), the 2 separated from one another by a highly regular I–O space of about 10 nm. Lanthanum impregnation reveals the presence of columns inclined at an angle, within this I–O space; as in the continuous junctions which link the lateral borders of these cells, these columns may maintain the very precise I–O distance. From the outer microvillar membranes radiate short spoke-like fibrils or sheets which encounter another more extensive system of myelin-like sheets. Freeze-fracturing reveals that the spoke-like sheets and the other ones which lie like a tube, around and parallel to the microvilli, contain linear ridges composed of particles, lying at random within layers of the myelin-like material which also extends into the lumen of the gut. The microvillar membranes, both O and I, fracture into faces containing rows of either PF particles or EF pits arranged as spiral ridges or grooves around the sides and across the tip of each microvillus. These could be the insertion sites of one or both of the I–O columns and spoke-like sheets while the sheets could represent a variant of peritrophic membrane. The double membrane may be a cellular device to increase the strength of the microvillar layer in these blood-sucking animals, since the cell layer must withstand great pressure owing to the sudden massive extension of the gut during a blood meal.
Synthesis and deposition of oocyte envelopes (vitelline membrane, chorion) and the uptake of yolk in the dragonfly (odonata: aeschnidae)
Authors: Beams H. W. , Kessel R. G.
Published: April 2021

Light and electron-microscope studies on dragonfly ovarioles reveal evidence that the precursor vitelline membrane and chorion secretions are synthesized within the follicle cells. It is suggested that the sequence of synthesis and deposition of the vitelline membrane occurs as follows. The vitelline membrane presecretion appears to be synthesized by the rough-surfaced endoplasmic reticulum, giving rise to intracisternal granules. These appear to migrate in the cisternae to the region of the Golgi complex where the endoplasmic reticulum loses most of its ribosomes and the intracisternal granules move into the Golgi region where they appear within small vesicles. These seem to find their way into the Golgi cisternae where they may be incorporated with the secretions from the Golgi cisternae to produce the definitive previtelline secretion. The previtelline secretion bodies are eventually discharged into the space between the oocyte and follicle cells, forming rows of secretion bodies between the microvilli. These fuse into progressively larger bodies until a complete membrane is established. Follicle cells actively secreting precursor vitelline membrane substance show many disk-shaped, relatively clear vesicles in the cytoplasm.

After the vitelline membrane is laid down, the follicle cells take on an entirely different function; namely, the synthesis and deposition of the chorion. The first visible chorion secretion appears in profile as elongate dense bodies within the Golgi cisternae which tend to coil, and in so doing, expand the cisternae. As this occurs, the enlarged cisterna, loaded with concentric coiled secretion material, separates from the remainder of the Golgi cisternae and becomes free in the cytoplasm as a prechorion secretion body. These migrate to, and collect below, the surface of the cell where they are eventually ejected between the surface folds and become incorporated into the developing chorion.

Uptake of yolk in the dragonfly seems to be predominantly by micropinocytosis. The oocyte surface during active vitellogenesis bears many pits which contain an extracellular material closely applied to the outer surface of the plasma membrane. Thin, radially oriented bristles are continuous with the inner surface of the plasma membrane in this region. The pits continue to invaginate until they are cut off from the plasma membrane and come to lie in the oocyte cortex as coated vesicles. These appear to lose their coats gradually and fuse with one another to produce definitive yolk spheres.
Metabolic co-operation between biochemically marked mammalian cells in tissue culture
Authors: Subak-Sharpe H. , Bürk R. R. , Pitts J. D.
Published: April 2021

Cells of a genetic variant of the hamster fibroblast line BHK 21 which lack inosinic pyrophosphorylase activity (IPP− cells) and therefore cannot normally incorporate [3]hypo-xanthine were grown in mixed culture with cells of BHK 21 sublines which have inosinic pyrophosphorylase activity (IPP+ cells). If not in contact with IPP+ cells, IPP− cells do not incorporate added [3H]hypoxanthine into nucleic acid. IPP+ cells always do incorporate [3H]hypoxanthine and IPP− cells when in direct or indirect contact with IPP+ cells also incorporate the isotope. Cell to cell contact appears to be essential for this gain of a metabolic function by IPP− cells.

The possible molecular basis and general implications of the phenomenon are discussed.
The cellular basis of movement of the migrating grex of the slime mould Dictyostelium Discoideum
Authors: Garrod D. R.
Published: April 2021

Previous work has failed to suggest a mechanism for grex movement, which takes account of both the movement and contact interaction of grex cells, and properties of the whole grex. Here, some observations on the movement of grex cells under different conditions are reported. From these, it seems that the production of pseudopods by grex cells depends upon mechanical factors of their environment and that movement depends on pseudopods being able to form stable adhesion with the surfaces of other cells. Intercellular adhesions are stable to forces applied at right angles to the plane of adhesion, but a force parallel to the plane of adhesion causes the cell surfaces to slide past each other. It is suggested from observations on grex placed in aqueous media and paraffin oil that the slime sheath may play an important role in controlling the polarity of grex movement by restricting pseudopod formation in all directions except forwards. These suggestions are discussed briefly in relation to previous observations on grex movement.
A molecular basis for indiscriminate and selective cell adhesion
Authors: Jones B.M. , Morrison G. A.
Published: April 2021

When cells of different type are intermixed they will adhere to one another indiscriminately. In mixed aggregates of o.th-incubation-day fibroblasts and 12th-day liver cells from chick embryos, the liver cells accumulated as groups which moved at random relative to the surrounding fibroblasts; groups of liver cells were liable to migrate from the interior to occupy the surface.

Quantitative estimations of the effect of temperature on the rate of reaggregation of dissociated fibroblast cells, using a turbidimetric method, indicated that the rate was controlled by a chemical reaction. The energy of activation calculated for this reaction was 21·1 kcal/mole. The significance of this value is discussed in relation to values associated with other kinds of reactions.

The indiscriminate adhesions formed between the unlike cells allowed them to move relative to one another, whereas the adhesions (selective) formed between like cells, when by chance they made contact, did not. Thus the aggregates of like cells increased in extent and the inherent ability of homogeneous aggregates to consolidate the intercellular bonding ensured that the process became effectively irreversible.

If, as is evident, there is a basic similarity in the adhesive mechanisms of the various types of cells that exist, the question that required answering was what kind of differences could occur in the bonds to account for differential adhesiveness upon which the sorting out process depends. It is suggested that the linkage sites at the surfaces of cells move (oscillate) between two positions, the one unfavourable and the other favourable to adhesion, and the theory put forward is that differential adhesiveness is the expression of differences in the frequency of the oscillations of the otherwise equivalent linkage sites. The theory provides an explanation for the temporary nature of indiscriminate adhesion allowing the movement of unlike cells in contact relative to one another and the more permanent nature of selective adhesions responsible for the consolidation of like cells in their appointed territories.
Injected histone antibodies interfere with transcription of lampbrush chromosome loops in oocytes of Pleurodeles
Authors: Scheer U. , Sommerville J. , Bustin M.
Published: April 2021

Antibodies to calf thymus histone H2B were purified by chromatography on DEAE-cellulose and injected into oocyte nuclei of Pleurodeles waltlii. As shown by indirect immunofluorescence these antibodies cross-reacted strongly with corresponding histones associated with lampbrush chromosomes. Shortly after injection the lateral loops of the chromosomes retracted into the chromomeres and by 3 h postinjection the ‘lampbrush’ appearance was completely lost and the chromosomes appeared in light-microscopic preparations as rod-like structures consisting of longitudinally coalesced chromomeres. In control oocytes injected with non-immune immunoglobulins or antibodies against a ubiquitous transcript-associated protein no morphological alterations of the lampbrush chromosomes could be observed. Electron microscopic spreads of chromosomes prepared at various times after injection of anti-H2B revealed a progressive loss of transcriptional complexes from the loop axes. Finally, higher-order chromatin configurations, like supranucleosomal globules (‘superbeads’) or cable-like chromatin strands 50–60 nm thick predominated, indicating complete transcriptional inactivation of all chromosomal regions. The results indicate that H2B antibodies react specifically with histones associated with the transcribed DNA of lateral loops in their native state. The resulting antigenantibody complexes seem to inhibit progression of the RNA polymerases along the template, thus causing the premature release of transcripts, a process analogous to the stripping effect of actinomycin D. The demonstration of histones associated with heavily transcribed regions, which are not compacted into nucleosomes but largely extended, supports the current concept that unfolding of nucleosomes to allow transcription of the DNA does not involve dissociation of histones. In contrast, amplified ribosomal RNA genes are unaffected by injected H2B antibodies. This does not necessarily indicate absence of histones from nucleolar chromatin, since we do not know whether it is accessible in vivo to antibodies or whether the histone antigenic determinants are masked by the presence of other proteins. The technique of injecting specific antibodies should be widely applicable when analysing the in vivo distribution of chromosomal components at the electron-microscopic level and when studying complex metabolic processes, like the cleavage and modification of RNA, by selective inhibition of defined enzymic steps.
Location of fucosyl transferases in the membrane system of maize root cells
Authors: Green John R. , Northcote D. H.
Published: April 2021

There are two fucosyl transferase activities present within, the endomembranes of the cells of maize root-tips. One transfers fucose to polyprenyl phosphate and occurs in the endoplasmic reticulum, the second transfers fucose probably to polysaccharide or glycoprotein. In order to show an association of this second fucoayl transferase activity with the endoplasmic reticulum as well as the Golgi apparatus, a method of fractionating the membranes in a discontinuous sucrose gradient was used. Membranes were prepared in the presence of Mg3+, which maintained the attachment of ribosomes to the endoplasmic reticulum, and also in the presence of EDTA, which removed most of the ribosome complex. This caused a shift in density of these membranes. Two types of experiments were carried out; either maize roots were incubated in L-[1-3H]fucose and then membranes prepared and the amount of polymer synthesized in vivo determined or isolated membranes were incubated with GDP-L-[U-14C]fucose in vitro and the amount of polymer synthesized was found. The results showed that the Golgi apparatus had the highest amount of this fucosyl transferase activity, but there was a significant amount of activity associated with the endoplasmic reticulum and the latter was shifted in the sucrose gradient depending on the conditions used.
Induction of G
1
− cells at high frequency
Authors: Naha P. M.
Published: April 2021

Experimental conditions are reported on the induction of G1- cells at high frequency by ultraviolet irradiation of temperature-sensitive cells arrested in G1 phase at the restricted temperature. Neither the wild type cells of Balb/C-3T3 nor the temperature-sensitive derivative of t8 A83 grown at the permissive temperature could be ‘mutated’ under similar conditions. One such ‘mutant’ (R18) was ‘inducible’ from a G1+ phenotype at 33 °C to G1- state at 38 °C within one cell generation time. The ‘inducible’ property of R18 from G1+ to G1_ lends support to the theory that the duration of the G1+phenotype is determined by the level of precursors for DNA synthesis. Since the G1- variants were isolated from revertants, the frequency of ‘mutagenesis’ appeared to be artificially high.
Analysis of sister-chromatid exchanges and tumorigenicity in cell hybrids
Authors: Jonasson J. , Santesson B. , Ström A.
Published: April 2021

The relationship between the ‘spontaneous’ frequency of sister-chromatid exchanges (SCE) and tumorigenicity was studied in a series of hybrids between a C57BL melanoma cell line and diploid cells, but no correlation was found between the 2 variables. Hybrids in which malignancy was suppressed and malignant segregants derived from them showed virtually identical SCE frequencies.

Variation of SCE frequencies was observed, however, between the different hybrid clones, and most hybrids showed consistently less SCE per chromosome than the corresponding parental cell types did under similar growth conditions. The lower SCE frequencies could neither be related to a higher number of chromosomes in the hybrid nor could they be related to the method of hybrid selection. These findings suggest that cell fusion might have induced epigenetic SCE frequency changes possibly in the same way as modulation of SCE frequencies is known to occur in the human leukocyte series.
The role of cell swelling and haemolysis in sendai virus-induced cell fusion and in the diffusion of incorporated viral antigens
Authors: Knutton S. , Bächi T.
Published: April 2021

The role of the haemolytic activity of Sendai virus in cell–cell fusion has been examined in monolayers of human erythrocytes and erythrocyte ghosts fused with either haemolytic or non-haemolytic virus. Morphological observations indicate that cell swelling and haemolysis is a distinct event in cell-cell fusion irrespective of whether it is vitally induced or, in the case of non-haemolytic virus, experimentally induced. Osmotic swelling appears to be the driving force by which cells which have established sites of membrane fusion expand such sites to form poly-erythrocytes. Immunofluorescent labelling of viral antigens incorporated into the erythrocyte membrane as a result of viral envelope–cell fusion indicates that diffusion of antigens in the plane of the membrane is restricted in intact erythrocytes and resealed erythrocyte ghosts but not in haemolysed erythrocytes or unsealed ghosts. A perturbation of the erythrocyte membrane resulting from osmotic lysis appears to form a prerequisite for the lateral diffusion of viral elements.
Structures linking the tips of ciliary and flagellar microtubules to the membrane
Authors: Dentler William L.
Published: April 2021

The distal tips of both central pair and outer doublet microtubules are linked to flagellar membranes of Chlamydomonas and to ciliary membranes of both Tetrahyniena and Aequipecten by the central microtubule cap and by distal filaments, respectively. The distal filaments are linked to ciliary membranes at their distal ends and, at their proximal ends, form a carrot-shaped plug which inserts into the lumen of the A-tubule of each outer doublet. Both types of tip structures are present in regenerating cilia and flagella. At least 2 different types of distal filaments are present in Tetrahyniena. Certain cilia in Tetrahymena oral apparatuses and in certain molluscan gills contain distal filaments which join with the central microtubule cap and with the ciliary membrane to form a cap-filament-membrane complex.
Organization of chromosomes in hela cells: Isolation of histone-depleted nuclei and nuclear scaffolds
Authors: Adolph Kenneth W.
Published: April 2021

Histone-depleted nuclei were prepared from, isolated HeLa nuclei by extracting the histones and other proteins with polyanions (dextran sulphate and heparin) or with high salt concentrations as used previously. The particles were characterized by sucrose density gradient sedimentation, thin sectioning and electron microscopy, and by polyacrylamide gel electrophoresis. The general result of the experiments is that the DNA in the histone-depleted nuclei is highly organized, and that this residual, higher-order structure is maintained by a reproducible subset of nuclear proteins, and perhaps by RNA. Furthermore, the residual proteins remain associated, in some conditions, as rapidly sedimenting structures even when the DNA is digested with nucleases. These nuclear scaffolds can resemble extracted nuclei.

Histone-depleted HeLa nuclei sediment in sucrose density gradients as well defined peaks with sedimentation coefficients of around 12000 s, when 2 M NaCl is used to extract the histones, or 6000 s, when dextran sulphate is used. The rate of sedimentation is drastically decreased by treating the particles with trypsin, and reduced to a lesser extent with RNase A. Thin sectioning and electron microscopy show that histone-depleted nuclei possess the nuclear periphery and that internal material is also present. These general features are also seen in thin sections of nuclear scaffolds, which are prepared by treating the nuclei with micrococcal nuclease or DNase I in addition to extracting the histones. Two groups of major proteins are associated with histone-depleted HeLa nuclei and the nuclear scaffolds: One group has molecular weights of around 65000–70000 Daltons, while the second group has molecular weights of 50000–55000 Daltons. The major species of this latter group of proteins have mobilities that are similar to the proteins of the metaphase chromosomal scaffold.
New aspects of intranuclear structures following partial decondensation of chromatin: A cytochemical and high-resolution autoradiographical study
Authors: Puvion-Dutilleul Francine , Puvion Edmond
Published: April 2021

Exposure of cultured CV1 monkey kidney cells to a hypotonic medium containing a nonionic detergent induces a partial decondensation of nuclear components which can be studied advantageously in ultrathin sections. In addition to previously described 20–25 nm, DNA-containing chromatin fibres arrayed perpendicularly to the nuclear lamina, a new nuclear configuration was found which consists of abundant clusters of 6–7 nm fibrils. These ‘filamentous masses’ are the most heavily labelled component of the nucleus in autoradiographs after brief (2 min) exposure to [5-3H]uridine, and they are greatly diminished in number after treatment of the cells with a low dose of actinomycin D. Therefore, we interpret the filamentous masses to be nascent RNP and to represent the fibrillar regions of the nucleolus which have become decondensed into partially spread ‘Christmas-tree’-like figures detectable in situ in ultrathin sections. Similar structures occur in all cell types examined to date. This paper outlines the preparative procedure.
Double-replica topography of disk membrane structures in the carp (Cyprinus carpio)
Authors: Tonosaki A. , Washioka H. , Yamasaki M. , Mizoguchi J.
Published: April 2021

By an improved freeze-fracture device, a number of double replicas were obtained from the rod disk membranes of the carp retina for investigation of complementary, continuing, and transitional relationships between fracture faces. Some separate particles found along the smooth edge bordering on the adjacent interdisk cytoplasm can be occasionally matched with gaps found between particles forming the complementary rough face edge. Consequently the particles on both faces can be derived from a single continuous layer, corresponding to the ad-cytoplasmic component of the disk membrane. This observation seems to agree with a prevailing disk-membrane model in which the visual pigment molecules are localized in the ad-cytoplasmic lipid lamella, but not to conform with a recent hypothesis, based upon single replicas, that the fracture plane might pass along either the inner or outer true surface of the membrane and the visual pigment molecules be attached to the latter.