bluuebunny/crossref_metadata_2025 huggingface dataset:
Altered patterns of ribonucleic acid synthesis during the cell cycle: a mechanism compensating for variation in gene concentration
Authors: Fraser R. S. S. , Nurse Paul
Published: April 2021

In the fission yeast Schizosaccharomyces pombe, a series of diploid mutants divides at smaller cell sizes than wild type. In these smaller strains, the mean gene concentration (defined by previous authors as the DNA to protein ratio) is higher than in wild type. Such an increase in gene concentration should also increase the concentration of those components such as messenger and ribosomal RNA, whose rate of synthesis is determined by gene dosage. We show that the mean concentrations of these 2 RNA species in the small cells are not increased, but are the same as in wild type. The small mutant cells are thus able to compensate for changes in gene concentration. This compensation is shown to operate through differences in the patterns of synthesis of RNA during the cell cycle. In all the strains of the diploid series, the rates of synthesis of messenger and ribosomal RNA double as steps once in each cell cycle. The timings of the steps in the cell cycle appear to be cell-size related, since the smaller the cell at division, the later are the steps in the cell cycle. In contrast, there is comparatively little variation in the timing of DNA replication in the cycles of cells of different sizes. We propose that after DNA replication, there is a delay before doubling in the rate of transcription. Such a cell mass-related delay is all that is required to compensate for increased gene concentration, and results in the same mean functional DNA concentration in all strains. This mechanism will maintain the same mean messenger and ribosomal RNA concentrations in cells dividing at different sizes. Ways in which the cell size-related control over transcription may operate are discussed.
Complementation of G1-phase variants of a mammalian cell cycle
Authors: Naha P.M.
Published: April 2021

Complementation between temperature-sensitive (ts) variants of Balb/c-3T3 defective in the G1 phase of its cell cycle was measured in the [3H]thymidine-labeling indices of the multinucleated cells during incubation at the restricted temperature (38 degrees C) following cell fusion. One ts variant from each group along the length of the G1 phase was tested for complementation. Varying degrees of complementation were observed between the 4 ts variants tested, judging by the time of entry into S-phase and the degree of synchrony attained. At least 3 complementation groups were discernible.
Pollen-pistil incompatibility in Petunia hybrida: changes in the pistil following compatible and incompatible intraspecific crosses
Authors: Herrero M. , Dickinson H. G.
Published: April 2021

The structural events in the stigma and transmitting tissue of Petunia hybrida pistils that accompany compatible and incompatible intraspecific pollinations have been investigated in detail, together with the changes in reserve levels that also take place at this time. Many of these phenomena may be explained in terms of 3 phases of secretion by the cells in the upper regions of the transmitting tissue. The first, independent of pollination, results in the deposition of an intercellular matrix, rich in protein and carbohydrate. The second, triggered by pollination, although independent of the compatibility of the pollen grain, involves synthesis of molecules believed to be specific to the S(incompatibility)-gene system. The third phase of secretion occurs only following a compatible mating, and involves the transfer of stylar reserves to support the growth of the pollen tubes. These observations are discussed in terms of current models of the incompatibility mechanism operating in Petunia.
Ultrastructure of the endoplasmic factor responsible for cytoplasmic streaming in Chara internodal cells
Authors: Nagai Reiko , Hayama Tomio
Published: April 2021

Previous investigators have proposed that cytoplasmic streaming in Chara intemodal cells results from the interaction between an endoplasmic factor and fibrils composed of microfilaments in the stationary cortex. Using the internal perfusion technique, we confirmed the observation that organelles which had been attached to the fibrils by decreasing the internal concentration of ATP moved along the fibrils after ATP was introduced. Thin-sectioned specimens revealed that endoplasmic organelles of various shapes were linked to microfilament bundles in the absence of ATP. Linkage was effected by regularly arranged electron-dense materials with a spacing of 100 – 130 nm at definite regions on each organelle. The organelles in question were studied in negatively stained preparations of endoplasm. The organelles had some common features. (1) They were all membrane-limited. (2) Their sizes and configurations varied largely. (3) One or more protuberances were present on them. (4) The protuberances were usually rod- or hom-like. (5) Small globular bodies 20 – 30 nm in diameter were found in ordered array with the same spacing as those in thin sections at the surface of the protuberances. (6) Many fine filaments were always attached to the surface of the protuberances. These fine filaments differed from F-actin in diameter (less than 4 nm) and inability to react with heavy meromyosin from rabbit skeletal muscle. The role of such components of the organelles in cytoplasmic streaming is discussed.

A paracrystalline array of microfilaments with a transverse periodicity of about 38 nm is presented, together with its optical diffraction pattern.
A control acting over the initiation of dna replication in the yeast Schizosaccharomyces pombe
Authors: Nasmyth Kim A.
Published: April 2021

The control of cell division in the yeast Schizosaccharomyces pombe appears to be quite different to that of any other eukaryotic organisms for it is usually exerted not at the initiation of S-phase but at that of mitosis. However, it has been suggested that a control over the initiation of S-phase does also exist but that its action is redundant whilst the mitotic control is operating. This study has chosen conditions in which the latter appears to be largely absent in order to study the cryptic S-phase control. The timing of S-phase has been studied in cells grown at varying rates under nitrogen limitation in a chemostat. It is found that under these conditions the control of cell division resembles that of other eukaryotes. As the dilution rate of the chemostat is reduced, all increase in the generation time can be accounted for by a lengthened G1 period. In contrast, the length of S + G2 remains invariant Thus, there must indeed be a control acting in G1 in S. pombe. An analysis of the size of cells at different growth rates shows that the initiation of S-phase is correlated with a particular cell size.
Replication in Drosophila chromosomes: I. replication of intranucleolar dna in polytene cells of D. nasuta
Authors: Lakhotia S. C. , Roy Sabita
Published: April 2021

The organization and replication of intranucleolar DNA in polytene cells of Drosophila nasuta have been examined. Normal Giemsa and fluorescent (Hoechst 33258 and quinacrine mustard) staining reveal that in D. nasuta polytene cells, the intranucleolar DNA is organized into condensed and disperse forms and very often shows connexion to the alpha heterochromatin of the chromocentre. Results of [3H]thymidine autoradiography indicate that the known underreplication of rDNA sequences in polytene cells is due to the slower replication of the intranucleolar DNA. Some aspects of the organization of condensed and disperse intranucleolar DNA are discussed.
Fertilization in brown algae: II. evidence for lectin-sensitive complementary receptors involved in gamete recognition in Fucus serratus
Authors: Bolwell G. P. , Callow J. A. , Callow Maureen E. , Evans L. V.
Published: April 2021

Fertilization in Fucus serratus is directly proportional to the number of sperm added, saturating at approximately 250 sperm per egg with an apparent Km of 120 sperm per egg. The effect of a range of lectins on fertilization has been tested. Preincubation of gametes with Con A and fucose-binding protein (FBP) inhibited fertilization. At low concentrations this was by specifically binding to eggs; at high concentrations pretreatment of either gametes inhibited fertilization probably due to cytotoxicity. Fertilization was not inhibited by simple sugar haptens, but polysaccharides containing fucosyl or mannosyl residues (yeast mannan, fucoidan, ascophyllan) inhibited fertilization by binding to sperm. Pretreatment of eggs with α-fucosidase or α-mannosidase was effective in inhibiting fertilization. All the results indirectly demonstrate that fertilization in Fucus serratus is based on an association between fucosyl- and mannosyl-containing ligands on the egg surface and specific carbohydrate-binding receptors on the sperm surface.
Transport of glial cell acid phosphatase by endoplasmic reticulum into damaged axons
Authors: Griffiths Gareth W.
Published: April 2021

Earlier observations indicated that cutting the photoreceptor (R) cell perikarya in the retina of the flies Musca domestica and Calliphora erythrocephala resulted in an extremely rapid degeneration of the R-cell axons. The process manifested itself within minutes and secondary lysosomes appeared early on in the degenerating axons. In this study, biochemical assays of the classical lysosomal marker enzyme acid phosphatase (AcPase) showed a rapid increase in activity upon cutting the retina, reaching a peak around 5 min, followed by a drop and a subsequent slow steady rise in activity between about 4 and 24 h after the cut. Inhibitor studies indicated that atleast 2 different acid phosphatases were involved, which showed identical activity changes. EM cytochemical studies with either β-glycerophosphate or cytidine monophosphate as substrates indicated that the degeneration-dependent AcPase activity originated exclusively in one specific glial cell type in the distal lamina, the satellite glia. These cells are contiguous with R-cell axons in the pseudocartridges, directly proximal to the basement membrane of the retina. In normal uncut flies, reaction product indicative of AcPase activity was found in satellite glial cells in both rough ER (and perinuclear cistemae) and in smooth ER, the 2 types of cistemae being clearly in continuity. The electron-dense reaction product resulting from AcPase activity often showed continuity between the smooth ER of satellite glial cells and extracellular sites between the satellite glia and R-axons. This may result from the low levels of AcPase being synthesized in rough ER, and then exported to extra-cellular sites. Within minutes of cutting the retina the frequency of satellite glia smooth ER staining for AcPase increases significantly. Increases in AcPase activity were subsequently found sequentially in extracellular sites, in axons in the distal lamina, and finally in the axon terminals. The time course for these increases in AcPase activity correlated well with the time course of the fine-structural pathological changes in the axons.

A significant feature of the satellite glia cells is the massive packing of unidirectional microtubules (MTs), running perpendicular to the projections of the R-axons -in the same direction as AcPase transport. The MTs terminate at the glial membrane directly adjacent to the axon. The smooth ER cistemae interdigitate among the MTs and may be structurally connected to them by thin fibrils.

Thick-section cytochemistry and stereology were used to reveal the 3-dimensional extent of the AcPase-reactive smooth ER system. Evidence is presented which suggests that this system represents GERL, a specialized region of smooth ER involved in lysosome formation, that in this glial cell appears to function also to export hydrolytic enzymes.
Mechanical tension produced by nerve cells in tissue culture
Authors: Bray D
Published: April 2021

Evidence is presented that (a) the growth cone of cultured neurons can exert mechanical tension, and (b) that the direction of advance of the growth cone is determined by the tension existing between it and the rest of the cell.

(a) The evidence that growth cones can pull comes from a vectorial analysis of the outlines of individually isolated sensory neurons. The angles formed in these outgrowths are very close to those of tension-generated networks anchored at their free ends and these values are restored shortly after an experimental displacement. The relative mechanical tension on each segment of an outgrowth can be calculated by standard methods and is found to decrease at each branch point. It appears to be correlated with the diameter of the fibre so that thicker fibres maintain more tension than thinner ones.

(b) The influence of tension on the direction of advance of the growth cone is shown by 2 kinds of experiment. If a growing neurite is pulled to one side with a microelectrode then the direction of its advance is changed immediately according to the new stress. If the mechanical tension on the growth cone of a neurite is released by amputation or displacement the growth cone is found to have a high probability of branching shortly afterwards.

The ability of the growth cone to exert tension is discussed in relation to evidence that microspikes have contractile properties and in terms of the distribution of microfilaments within the neurite. It is suggested that the exertion of tension by a growth cone could serve to guide the neurite along paths of high adhesivity both in vitro and in vivo.
Interpolation of microtubules into cortical arrays during cell elongation and differentiation in roots of Azolla Finn Ata
Authors: Hardham A. R. , Gunning B. E. S.
Published: April 2021

Longitudinal sections of roots of Azolla pirmata R. Br. were prepared for electron microscopy so that cortical microtubules could be counted along the longitudinal walls in cell files in the endodermis, pericycle, and inner and outer cortex, and in sieve and xylem elements. With the exception of the xylem, where there are no transverse cell divisions, each file of cells commences with its initial cell and then possesses a zone of concomitant cell expansion and transverse cell division, followed, after completion of the divisions, by a zone of terminal cell differentiation.

The cells augment their population of cortical microtubules as they elongate and divide, showing a net increase of up to 0.6 μm of polymerized microtubule length per min. Two main sub-processes were found: (i) When a longitudinal wall is first formed it is supplied with a higher number of microtubules per unit length of wall than it will have later, when it is being expanded. This initial quota becomes diluted as the second sub-process commences, (ii) The cells interpolate new microtubules at a rate which is characteristic of the cell, and, in the endodermis, of the face of the cell, while the cell elongates. Most cell types thus maintain a set density of cortical microtubules while they elongate and divide. Comparisons of endodermal cells in untreated controls, and roots that had been treated with colchicine, low temperature, or high pressure indicate that the initial quota of microtubules, and the later interpolations, and differentially sensitive to microtubule perturbations.

Three types of behaviour, all related to changes in the cell walls, were noted as cortex, xylem and sieve element cells entered their respective phases of cell differentiation. The cortical cells expanded in all dimensions, and the interpolation of microtubules diminished or ceased. The sieve elements continued to elongate, and interpolated at a high rate, reaching unusually high densities of microtubules when the cell walls were being thickened. During this period a net increase of 2.0 μm of polymerized microtubule length per min was calculated. Thereafter interpolation ceased and the density of microtubules declined. The same applied to developing xylem except that, because wall-thickening is localized rather than widespread, the rise and subsequent fall in the density of microtubules was less marked.

The data are discussed in relation to the participation of microtubules in wall deposition and to the hypothesis that cortical microtubules arise in discrete zones along the edges of cells.
Different mechanisms in the attachment of cells to native and denatured collagen
Authors: Schor Seth L. , Court Janet
Published: April 2021

The attachment of cells to collagen has been reported previously to require the presence of serum and the particular serum protein involved in this process, variously known as CIG, CAP or fibronectin, has been isolated. This conclusion that cell attachment to collagen requires serum (or more precisely, fibronectin) is based on experiments measuring the kinetics of cell attachment to films of collagen. We have measured the kinetics of attachment of HeLa and PyBHK cells to different collagen-containing substrata under a variety of experimental conditions and present evidence that the serum-dependent mechanism of cell attachment described by others is actually only the case for films of denatured collagen, while cell attachment to native collagen fibres occurs by a different, serum-independent, mechanism. The possible relevance of these findings to cell-matrix interactions in vivo is discussed.
Phenylglyoxal is not a selective inhibitor of phagocytosis
Authors: Schaik Margriet L. J. Van , Weening Rons. , Roos Dirk
Published: April 2021

In 1976, it has been reported that phenylglyoxal (C0H5COCHO) selectively inhibits endo eytosis in phagocytes of rabbit and mouse. We have tested the specificity of this compound by measuring its effect on human neutrophil chemotaxis, respiration and release of lysosomal enzymes. Pretreatment of human neutrophils with 100 µg phenylglyoxal/ml for 30 min at 37 °C resulted in almost complete inhibition of phagocytosis of opsonized zymosan. However, after treatment with phenylglyoxal, spontaneous mobility as well as chemotaxis of these cells towards casein, rosette formation with opsonized zymosan, stimulation of the oxidative metabolism and release of lysosomal enzymes were also severely decreased. Most of these functions were only partially restored by resuspension of the cells in a medium without phenylglyoxal. The intracellular level of ATP was not affected by phenylglyoxal, but the level of reduced glutathione was decreased. We conclude from the inhibitory action of phenylglyoxal on the stimulated oxygen consumption and its reaction with intracellular glutathione that phenylglyoxal does not necessarily act exclusively on the outside of the plasma membrane.

From our studies, it follows that phenylglyoxal is not a specific inhibitor of endo- or exocytosis in human neutrophils. Phenylglyoxal can be used effectively in the bacterial-killing test of phagocytes to inhibit intracellular killing after an initial period of ingestion.
Specific contact-dependent cell-to-cell communication during preconjugant interaction of the ciliate Euplotes crassus
Authors: Luporini Pierangelo , Bracchi Paolo , Esposito Fulvio
Published: April 2021

A system has been developed to study cellular interactions between cells of complementary mating types prior to mating in the ciliate, Euplotes crassus. The presumptive mates were distinguished by using singlet and doublet cells of appropriate mating types in the mixtures. Cells of a given mating type were prelabelled with [3H] leucine and mixed with unlabelled complementary cells. Exchange of [3H]leucine-labelled material from donor to recipient cells was monitored through the various stages of the preconjugant interaction. A label transfer between the mating type complementary cells was detected from the beginning of the visible mating reaction, which occurs after a waiting period from the time of cell mixing and involves ciliary agglutination prior to cell body fusion. Complementary cells which were prevented from physically contacting each other and cells which were not competent to mate appeared unable to take up the labelled material. It is suggested that this material consists of some sub-stance(s) playing an important role in the preconjugant cell-to-cell interactions of E. crassus.
Expression of the transformed phenotype and tumorigenicity in somatic cell hybrids
Authors: Marshall C. J.
Published: April 2021

We have previously shown by examining the anchorage dependence, density-dependent inhibition of growth, LETS protein and microfilament bundles that the transformed phenotype of the parental tumours are suppressed in hybrids between rat embryo fibroblasts (REF) and mouse tumour cells (TA3B). Hybrids between TA3B and Syrian hamster sarcoma cells (BHK-Bi) also show suppression. We now demonstrate that tumorigenicity in nude mice is also suppressed in TA3B × REF and B1 × TA3B hybrids. Tumours arise from the suppressed hybrids by the selective outgrowth of variants with properties different from the majority of cells inoculated. These tumour variants always had an altered cytoskeleton but 1 out of 14 cases retained anchorage-dependent growth. Selection in culture for anchorage-independent growth selected for tumorigenicity. The coordinate suppression of the transformed phenotype and tumorigenicity could be explained by postulating a pleiotropic control mechanism affecting both. However the occasional dissociation of tumorigenicity from anchorage dependence in the variants suggests that the targets for the control mechanism must be different.
Ultraviolet-induced DNA excision repair in human B and T lymphocytes: III. Repair in lymphocytes from chronic lymphocytic leukaemia
Authors: Yew F.-H. , Johnson R. T.
Published: April 2021

There have been conflicting reports about the capacity of lymphocytes from individuals with chronic lymphocytic leukaemia (CLL) to undertake u.v.-induced DNA repair. We have examined repair in CLL cells and in control and age-matched purified B and T lymphocytes by a technique independent of incorporation of radioactive precursor, i.e. by the recovery of normal sedimentation behaviour of nucleoid bodies obtained from these cells by lysis in high salt and non-ionic detergent. Recovery of normal sedimentation is associated with restoration of DNA supercoiling. CLL cells were found to be as sensitive to u.v. and to repair at similar rates as age-matched B controls. They are considerably more sensitive than young B cells and repair less efficiently. Reasons for the reported discrepancies in CLL repair are discussed.
Insulin-induced receptor loss in the cultured human lymphocyte: quantitative morphological perturbations in the cell and plasma membrane
Authors: Gorden Phillip , Carpentier Jean-Louis , Van Obberghen Emmanuel , Barazzone Philippe , Roth Jesse , Orci Lelio
Published: April 2021

When cultured human lymphocytes (IM-9) are exposed to 10−6 M porcine insulin for 6 h, washed, and incubated with 125I-insulin, the ability of the cell to bind the labelled hormone is reduced by a mean of 78 %. Under these experimental conditions that induce insulin-receptor loss in this cell there is a mean 95 % increase in microinvaginations in the plasma membrane revealed by electron microscopy on freeze-fractured replicas of the cell. At the same time, horseradish peroxidase uptake, a marker of endocytosis, is increased in the cells incubated with insulin.

Coupled with our recent EM autoradiographic evidence that labelled insulin is acutely internalized by this cell, these studies are consistent with the possibility that endocytosis represents a mechanism by which receptor is removed from the cell surface.
Freeze-drying methods for the scanning electron-microscopical study of the protozoon Spirostomum ambiguum and the statocyst of the cephalopod mollusc loligo vulgaris
Authors: Boyde A. , Barber V. C.
Published: April 2021

The present study concerns methods of preparing ciliated surfaces for direct examination in the scanning electron microscope.

Air-drying methods provide good results with some ciliated structures but do not always preserve the cilia of ciliated protozoons, although the pellicle is well preserved. Air-drying is not suitable for certain epithelia because considerable shrinkage and tearing occur.

Freeze-drying methods, with or without pre-fixing, are described. These preserve the cilia in the protozoon Spirostomum in a fairly life-like position. There are some differences in the appearance of unfixed and fixed freeze-dried material—for example, the peristomial membranelles are not seen in the unfixed material. Freeze-drying again proved to be a better method of preparing the sensory epithelium lining the statocyst of the cephalopod mollusc Loligo, because it was successful in preventing the distortion due to shrinkage. The number of hair cells, their orientation, and the area covered by the cells was determined for the macula. The crista was found to be asymmetrical.
Extra-chromosomal dna in early stages of oogenesis in Acheta Domesticus
Authors: Cave M. D. , Allen E. R.
Published: April 2021

A large extra-chromosomal DNA body is found in gonial and oocyte nuclei of Acheta domesticus. Somatic cells within the ovary do not contain the DNA body which is limited to the nuclei of gametogenic cells. During early prophase of meiosis this bodyisspherically shaped and intensely Feulgen positive. Electron microscopy shows it to be composed of tightly packed fibrogranular material. The body is formed in the nuclei of premeiotic interphase cells where it first appears as a mass of dense chromatin material located within the nucleolus. In nuclei of early prophase cells the body is closely associated with the nuclear membrane. It increases in size, reaching a maximum in mid-pachytene nuclei. During late pachytene-early diplotene stage of meiosis the tight fibrillar material within the body loosens and takes on a less compact appearance. At the same time large fascicles of RNA-containing material accumulate within and around the DNA body. The amount of RNA material surrounding the body increases as the oocytes proceed into an arrested diplotene stage of development.
Effects of chloramphenicol on growth, size distribution, chlorophyll synthesis and ultrastructure of Euglena Gracilis
Authors: Ben-Shaul Y. , Markus Y.
Published: April 2021

Multiplication of Euglena cells treated by 0-5-1-0 mg/ml chloramphenicol was not disturbed for the first 36 h and inhibition appeared only at later stages.

The mean cell volume of treated dividing cells was decreased, although the initial rise in cell volume, which normally occurred during the first 12 h of incubation, was not prevented. The antibiotic also lowered the chlorophyll content of green dividing cells. In dark-grown cells transferred to light, inhibition of chlorophyll synthesis was immediate but not complete, and was followed by a decreased rate of plastid elongation and thylakoid formation. Our findings suggest that chloramphenicol does not cause the loss of existing pigment and that impaired chlorophyll synthesis is a secondary effect of inhibition of protein synthesis. The results also indicate that the greening process is more sensitive than cell division to the antibiotic.
Mitochondrial extensions associated with microtubules in outgrowing processes from chick spinal cord In Vitro
Authors: Grainger Felicity , James D.W.
Published: April 2021

Electron-microscopic examination of processes growing out from chick spinal cord cultivated in vitro raises the possibility that mitochondria may be continuous with a system of membrane-bound profiles ramifying within the cytoplasm. These profiles are distinctive in appearance, and appear to establish a particular spatial relationship with microtubules. In this, dense material extends from the profiles to constitute a meshwork within whose interstices tubules lie. The suggestion is made that these appearances may reflect the utilization of the products of mitochondrial activity for transport by microtubules.
Membrane alterations and other morphological features associated with polyethylene glycol-induced cell fusion
Authors: Robinson J. M. , Roos D. S. , Davidson R. L. , Karnovsky M. J.
Published: April 2021

Polyethylene glycol (PEG) induces rapid fusion of LM cells. Membrane fusion, as detected by formation of pentalaminar membrane arrays, occurs as early as 1 min after PEG treatment. The entire cell surface appears to be capable of fusion since fusion occurs in regions where pseudopodia make contact with each other or with a neighbouring cell body and also in areas where cells are in contact along their entire periphery. Cytoskeletal components showed no apparent deleterious effect from PEG treatment or subsequent cell fusion as determined by thin-section EM. Freeze-fracture of monolayer cultures reveals a thermotropic rearrangement of intramembranous particles following PEG treatment.
A reliable method for obtaining matched replicas of freeze-fractured cell suspensions
Authors: Wilkinson M. J. , Northcote D. H.
Published: April 2021

A method that overcomes the majority of practical problems involved in performing matched freeze-fracture replication of cell suspensions is described. Specimens are rapidly frozen between copper support plates. The prior attachment of gold grids to the plates using Formvar ensures the exact alignment of corresponding grid squares and the production of a central fissure running through the specimen upon fracturing. Grids, with replicas permanently attached, are removed from the support plates by dissolving the Formvar. Sample digestion and rinsing of replicas is by a gentle procedure employing glass capillaries as a means of handling the grids. Matched replication of plant and animal cells has been achieved by this method. Apart from the excellent preservation of the membrane ultrastructure of unfixed cells and the capacity for matching both membrane halves this technique also enhances the fracture characteristics of certain specimens, notably isolated protoplasts, providing larger fracture faces than are obtained by the knife-splintering method.
Plasma membrane ultrastructure during plant protoplast plasmolysis, isolation and wall regeneration: A freeze-fracture study
Authors: Wilkinson M. J. , Northcote D. H.
Published: April 2021

The freeze-fracture morphology of the plasma membrane of cells and isolated protoplasts of plant callus suspensions has been investigated. Plasmolysis of suspension cells leads to the formation of 2 types of hexagonal arrays of intramembrane particles situated on the inner fracture face (PF). These arrays are interpreted as proteins that have ‘crystallized ‘in the plane of the membrane as the area of surrounding lipid bilayer is reduced during protoplast retraction from the cell wall. Time-course studies have revealed no positive relationship between the distribution of hexagonal arrays and the occurrence of microfibrils regenerated around isolated protoplasts during periods of culture. No evidence for the specialized transport functions attributed to hexagonal arrays of plant cells by previous workers has been found.
Two new septate junctions in the phylum coelenterata
Authors: Green C. R. , Flower N. E.
Published: April 2021

Freeze-fracture of fixed and unfixed tissue, lanthanum tracer and conventional thin-section studies have revealed 2 new types of septate junction in the class Anthozoa, phylum Coelenterata. These new junctions have the 15−18-nm intercellular spacing of all other described septate junctions and are found around the apical circumference of cells lining a lumen or outside edge. However, in freeze-fracture replicas and tangential views of lanthanum-impregnated tissue, they are seen to be quite different from other known septate junction types. One of the new junctions is found in endothelial tissue such as that lining the gut or the inside of the tentacles. In tangential view it is seen to consist of relatively short, straight, double septa, again with lateral projections. Tn freeze-fracture of unfixed tissue, the junction consists of double rows of particles on the P face, the particles of one row being rounded, those of the other being elongated at right angles to the line of the septum. This dichotomy in particle size is unexpected, as the 2 halves of the septa as seen in tangential view are symmetrical. In freeze-fracture of fixed material the particle arrays remain on the P face and appear similar to those of unfixed material, but never as clear. In fixed tissue, some distortion has occurred and in extreme cases septa appear as a single broad jumbled row of particles. In this double septa junction, the rows of particles seen in freeze-fracture are occasionally seen to anastomose with a septum dividing into 2 and a third row of particles aligning with the 2 new septa to form their double particle rows. In both fixed and unfixed tissues, the E face of the junction consists of wide, shallow grooves.

The second of the new junctions occurs in epithelial tissue, such as around the outer edge of sea-anemone tentacles, and consists of long wavy septa with lateral projections. In views where these projections appear longest, they arise predominantly from one side of the septa. In freeze-fracture of both fixed and unfixed tissue, this junction appears as rows of closely spaced particles on the P face. Occasionally rows of particles are seen on the E face, but usually this face is characterized by shallow grooves. In some aspects these 2 new junctions have features in common with the Hydra type junction also found in the Coelenterata. In all 3 types septa are relatively straight, rather than pleated, and there are lateral projections on the septa.
Characteristics of pollen diffusates and pollen wall cytochemistry in poplars
Authors: Ashford A. E. , Knox R. B.
Published: April 2021

Pollen diffusates, known to be important in pollen-stigma interactions controlling inter-specific incompatibility between Populus deltoides and Populus alba, have been partly characterized and shown to contain more than 20 protein bands by polyacrylamide gel electro-phoresis, at least 4 of these being glycoproteins. Seven fractions had antigenic activity in rabbits and several enzyme activities were also present. Peroxidase and leucine aminopeptidase isoenzymes were detected in the diffusates, demonstrating the extracellular location of these 2 enzymes. Isoenzyme patterns of peroxidase, esterase and acid phosphatase were complex, with some bands common to both species. Localization of acid phosphatase in the intine and esterase in the exine was demonstrated after brief aldehyde fixation and low-temperature embedding in glycol methacrylate. The in tine and exine sites were distinguished by their chemical and structural features. Calcofluor white M2R new proved to be an excellent stain for differentiating the intine. Aniline blue-positive material, probably β-1,3-glucan, is present associated with the intine of many ungerminated as well as germinating grains: production of this material may be a response to damage.
Adhesive and non-adhesive membrane domains of amphibian embryo cells
Authors: Roberson Marie , Armstrong Jon , Armstrong Peter
Published: April 2021

Superficial blastomeres of mid-cleavage stage amphibian embryos (32- and 64-cell stage) display regional cell-surface differences in adhesiveness. The cells are adhesive on the lateral and basal cell surfaces and non-adhesive on the apical surface. These adhesive differences are maintained by single cells which have been dissociated from the intact embryo. Pigmentation differences afford a means of distinguishing apical surface from basal/lateral surface in dissociated cells. The apical surface is underlain by melanin granules and so appears dark, whereas the lateral and basal surfaces lack pigment and are white. Scanning electron-micrograph observations of isolated superficial cells reveal striking morphological differences between the adhesive and non-adhesive regions. The apical (non-adhesive) portion of the cell surface has a convoluted appearance and is almost entirely devoid of microvilli. The lateral and basal (adhesive) surfaces lack the convolutions of the apical surface and bear scattered microvilli.

The isolated superficial cells continue to divide in culture. The cells display a unilateral or one-sided furrow which begins in the basal surface and progresses toward the apical surface. The plane of cleavage of most of the cells is oriented such that the pigmented cap is bisected, with very few cells having the plane of the furrow pass parallel to the apical surface. The direction of furrowing in the isolated cells from the 32-to 64-cell embryo represents a direction of furrow progression in the intact embryo from the interior to the surface. Since the early cleavages are initiated at the surface and progress into the interior, it is suggested that a reversal in direction of furrowing occurs by mid-cleavage stages.
Human β-globulin factors affecting growth of human lymphocytes and mouse fibroblasts
Authors: Thomou H. , Koussoulakos S. , Stathakos D.
Published: April 2021

Evidence is presented to show the existence in human β-globulin (Cohn fraction III) of 2 growth-promoting factors that stimulate DNA synthesis and cell division in human lympho-cytes and 3T3 B mouse fibroblasts.

After extraction of human β-globulins at pH 3·0 in 0·1 M NaCl followed by sieve chromatography on Sephadex, 2 distinct fractions were obtained containing the biological activity; one of polypeptide nature and molecular weight of approx. 10000 Daltons and the other consisting of a homogeneous ribonucleic acid, as revealed by polyacrylamide gel electrophoresis.

The above-mentioned factors appear to lack both cell and species specificity, since they are mitogenic agents for cells as diverse as mouse fibroblasts and human lymphocytes.
Investigations on ploidy levels of haploid and diploid callus and cell suspension cultures of Datura innoxia mill
Authors: Kibler R. , Blaschke J. , Forche E. , Neumann K.-H.
Published: April 2021

Using haploid and diploid Datura innoxia Mill, callus cultures and cell suspensions it was shown that meristematic material usually attains the lowest possible C-value of a given ploidy level. Parenchyma material of callus cultures, however, indicates a broad scattering of C-values up to 16 C, which is paralleled by C-value distribution of the free cell fraction in actively dividing cell suspensions. In these suspensions, cell division activity seems to be restricted to small meristem-like clusters.
The development of lampbrush chromosome-type transcription in the early diplotene oocytes of Xenopus laevis: an electron-microscope analysis
Authors: Hill R. S. , Macgregor H. C.
Published: April 2021

Oocytes of Xenopus laevis in pachytene and early diplotene of meiosis have been studied using the Miller spreading technique. Transcription first appears in germinal vesicles 25–40μm in diameter, when the oocyte is in early diplotene. Transcription at this stage consists of arrays of short RNF transcripts, irregularly distributed along the DNP axis. Linear regression analysis has shown that many of these arrays are transcription units (Type 1) with the transcripts having a common point of origin. The lengths of these early transcription units (mean = 7·06 ± 5 06/tm), calculated from the linear regression data, are comparable to the lengths of transcription units from later stages, including Dumont stage 3. The polymerase granules of Type 1 transcription units are widely and irregularly spaced, having a mean spacing of 5051748 nm. More advanced transcription units (Type II, mean length = 8·72 ± 3·77μm) are usually found in the same chromosome set as the Type I units. Type II transcription units have closer and more regularly spaced polymerase granules than Type I transcription units (mean spacing = 92±49nm). Both Type I and II transcription units have comparatively short RNP transcripts, the mean values for the slopes of their regression lines being 0-1336 and 0·1440 respectively. By the time the germinal vesicles are about 50–60μm in diameter the transcription units have a quite different morphology (Type III). The lengths of the Type III transcription units are comparable to the Type I and II units, the mean length being 6-3414-03 /tm. The spacing of the polymerase granules in the Type III units is closer and more regular than the earlier stages (70±40nm). Another significant difference between Type III and Types I and II transcription units is a decrease in the foreshortening of the Type III RNP transcripts. The mean slope of the regression lines for Type III transcription units is 0 2439. The morphological appearance of the Type III transcription unit is virtually identical to that of the transcription units from Dumont stage 3 oocytes, both with respect to the length and the spacing of the polymerase granules. However, the transcripts in Type III transcription units are still more foreshortened than those of Dumont stage 3 oocytes, having mean regression slopes of 0–4728. From the data obtained in the present study, it has been concluded that the pattern of lampbrush-type transcription is virtually fully established by the time most germinal vesicles are about 50 μm in diameter.
Motile apparatus in Vallisneria leaf cells. I. organization of microfilaments
Authors: Yamaguchi Yasuo , Nagai Reiko
Published: April 2021

The organization of the microfilaments in epidermal cells of Vallisneria leaves was investigated with respect to the induction of cytoplasmic streaming (secondary streaming).

In many of the epidermal cells, cytoplasm exhibited rotational streaming along the anticlinal wall of the cell after exposure to intense light. In these cells, many bundles of 6–7 nm microfilaments were observed in the cortex around the anticlinal wall. The bundles were arrayed in parallel to the streaming direction. They were recognized usually as 10–40 closely packed dense dots in cross-section. The spacing between bundles was not even. Bundles tended to form groups of 4 to 5 in which the spacing between bundles was usually 0·3 and 0·5 μm.

The microfilaments were identified as F-actin. Together with the fact that rotational streaming in Vallisneria cells was inhibited by cytochalasin B, the motile mechanism of secondary streaming was concluded to be similar in its essential features to the cytoplasmic streaming seen in Characean cells (primary streaming).

In epidermal cells that had been kept under low-intensity light the cytoplasm and the chloroplasts were mostly localized along the wall facing the exterior (the periclinal wall). No cytoplasmic streaming occurred in these cells. The bundles of microfilaments remained in the very thin layer of cytoplasm lining the anticlinal wall, although they were fewer and somewhat loosely packed.

EGTA at appropriate concentration could induce cytoplasmic streaming in these cells. The mechanism of the induction is discussed on the basis of the effectiveness of EGTA and the requirement of a low concentration of free Ca2+ for cytoplasmic streaming in Characean cells.
Commitment during nematocyte differentiation in Hydra
Authors: Fujisawa Toshitaka , David Charles N.
Published: April 2021

Nematocytes in Hydra differentiate from interstitial stem cells. Desmonemes differentiate mainly in the distal half of the body column while stenoteles differentiate predominantly in the proximal half. This difference was used to determine the timing of nematocyte-type commitment in the differentiation pathway. Cells were transferred from distal or proximal regions to all positions in the body column to test when the proportion of stenotele and desmoneme differentiation changed to reflect the new environment. In the first experiment, the distal region of the body column was isolated and permitted to regenerate a whole Hydra. In the second experiment, dissociated cells from distal or proximal regions were transplanted into regenerating aggregates of Hydra tissue. Both experiments effectively transferred cells from distal or proximal positions to positions throughout the body column. By comparing the kinetics of stenotele and desmoneme differentiation with the time required for distal or proximal cells to differentiate stenoteles and desmonemes in accord with their new environment, it was possible to conclude that stenotele and desmoneme commitment occurs during the terminal cell cycle prior to nematocyte differentiation and not at the stem cell. Additional experiments indicated that the number of rounds of cell division preceding differentiation is fixed at the time stem cells enter the nematocyte pathway.
Effects of steroids and different culture media on cell cycle of the androgen-sensitive human cell line NHIK3025
Authors: Østgaard Kjetill , Wibe Einar , Lindmo Tore , Eik-Nes Kristen B.
Published: April 2021

The median cell cycle of synchronized NHIK3025 cells grown in Eagle’s MEM with 10 % foetal calf serum was 23·6 h, compared to 18 h observed earlier in Puck’s Eza medium with 30 % serum (20 % human and 10 % horse). This difference is due to a prolongation of both G1 and G2 in the MEM type medium.

Testosterone and 4-androstene-3β,17β-diol reduced the cell cycle of synchronized populations of NHIK3025 cultured in MEM type medium. Dexamethasone gave cell cycle prolongation while estradiol had no effect. These results are in accordance with steroid-induced changes in growth of asynchronous cell populations observed earlier. The androgen growth stimulation was partly due to a shortening of G2 but was not exclusively located in one particular phase of the cell cycle.
Ultrastructural analysis of hyphal tip cell growth in fungi: Spitzenkörper, cytoskeleton and endomembranes after freeze substitution
Authors: Howard Richard J.
Published: April 2021

The ultrastructure of freeze-substituted tip cells of Fusarium acuminatum was analysed by conventional and high-voltage transmission electron microscopy (HVEM). At least 2 morphologically distinct types of Golgi-like endomembrane cisternae were observed, each existing as single, fenestrated sheets and tubular elements that were often, very closely associated with mitochondria. From HVEM observations of thick (0 · 25 and 0 · 5 µ m) sections, the Spitzenkdrper appeared to correspond to an apical mass of vesicles. A network of microfilamenis was identified among component vesicles of the Spitzenkörper and adjacent to developing septa. Microtubules were oriented primarily parallel to the direction of hyphal growth and were located in all areas of the cytoplasm, including the tip cell apex. Cytoplasmic vesicles were closely associated with these microtubules. From these observations it is suggested that cytoskeletal elements play important roles in localized cell wall formation. The filasome, a previously unreported type of coated vesicle in fungi, might also be involved in wall synthesis.