bluuebunny/crossref_metadata_2025 huggingface dataset:
Supercoils in human dna
Authors: Cook P. R. , Brazell I. A.
Published: April 2021

The three-dimensional structure of a double-stranded DNA molecule may be described by distinguishing the helical turns of the DNA duplex from any superhelical turns that might be superimposed upon the duplex turns. There are characteristic changes in the hydrodynamic properties of superhelical DNA molecules when they interact with intercalating agents. The hydrodynamic properties of nuclear structures released by gently lysing human cells are changed by intercalating agents in this characteristic manner. The characteristic changes are abolished by irradiating the cells with γ-rays but may be restored by incubating the cells at 37 °C after irradiation. These results are interpreted as showing that human DNA is supercoiled. A model for the structure of the chromosome is suggested.
Effects of adenosine 5′-monophosphate and adenosine 3′,5′-cyclic monophosphate on l cells
Authors: Taylor-Papadimitriou J. , Karemfyllis T. , Eukarpidou A. , Karamanlidou G.
Published: April 2021

The adenine nucleotides, 5′-AMP and 3′5′-cyclic AMP block L cells in the S-phase of the cell cycle. The intracellular level of cyclic AMP is reduced after incubation of cells with 5′-AMP, and rates of uridine transport are increased after incubation with either 5′-AMP or cyclic AMP. On the contrary, cyclic AMP levels are increased and uridine transport decreased in cells treated with an inhibitor of the cyclic AMP phosphodiesterase. This inhibitor partially reverses the growth-inhibitory effect of cyclic AMP, indicating that a breakdown product is the effective inhibitor of growth. The inhibition of cell growth induced by the adenine nucleotides is prevented by uridine, suggesting that the block in S is due to a lack of availability of pyrimidines.
Immaturity substances: Material basis for immaturity in Paramecium
Authors: Miwa Isoji , Haga Nobuyuki , Hiwatashi Koichi
Published: April 2021

Cells of Paramecium caudatum in the immaturity period of the life cycle contain substances which repress the expression of mating ability when introduced into mature cells. When 5 × 103μm3 of immature cytoplasm were injected into mature cells, loss of mating ability occurred and it continued for more than 15 fissions after the injection. The effective substances are contained in the soluble fraction of immature cells and have the properties of heat-labile macromolecules.
Studies on intercellular invasion in vitro using rabbit peritoneal neutrophil granulocytes.: II. Adhesive interaction between cells
Authors: Lackie J. M. , Armstrong P. B.
Published: April 2021

A possible mechanism for intercellular invasion is that the strength of adhesion between host and invading cells is greater than the average of the strengths of homotypic adhesions. This hypothesis has been examined by a study of the kinetics of aggregation of dispersed populations of an invasive cell type (the rabbit peritoneal neutrophil granulocyte) and a host cell type (the chick embryo heart fibroblast) in shaken suspension culture. Since aggregation in mixed populations of the 2 cell types demonstrated tissue specificity, the hypothesis is not supported by these studies, heterotypic adhesions seem in fact to be weaker than homotypic adhesions.
An electron-microscope study of capsule formation by insect blood cells
Authors: Grimstone A. V. , Rotheram Susan , Salt G.
Published: April 2021

Haemocytes of Ephestia kuehmella adhere to foreign objects introduced into the haemocoel and form capsules around them. The fine structure of these capsules, and the blood cells which form them, has been studied by electron microscopy. By 72 h the capsule is a compact tissue, about 50–60 cells thick. The cells retain their integrity and do not form a syncytium. Cells nearest the implant develop large numbers of cytolysomes and appear necrotic. Peripheral to this the cells, which are initially more or less spherical and about 6 μ across, are flattened into thin sheets, on average about o·3 μ thick. Capsule cells adhere closely to each other, and a conspicuous electron-dense material may be present between them. The structure of developing capsules 24 h old and of capsules retrieved from adult insects following metamorphosis is described. The formation of capsules is discussed in relation to general problems of cell recognition and adhesion.
Quantitative investigation on the effect of puromycin on the aggregation of trypsin- and versene-dissociated chick fibroblast cells
Authors: Kemp R. B. , Jones B. M. , Cunningham I. , James M. C. M.
Published: April 2021

Embryonic chick fibroblast cells were used to test the effects of puromycin on cell adhesion and aggregation. Single cell suspensions were prepared by dissociating the muscle tissues of 5- and 9-day-old chick embryos with either trypsin or versene according to standard cell dissociation procedures.’ Cell aggregation was quantitatively estimated by a turbidimetric method.

Quantitative analyses of the components in cell-free supernatants revealed that both trypsin and versene when used as cell dissociating agents removed substantial amounts of protein from the surfaces of the cells and the intercellular regions. Trypsin removed larger quantities of protein and of N-acetylneuraminic acid than did versene. Nucleic acids were also released during trypsin and versene dissociation, and estimations of these nucleic acids gave a measure of the extent of cell lysis. The proteins and peptides were separated by gel filtration. The solubilized proteins removed by versene treatment were separated into three peaks on Sephadex G.200. The tryptic digest gave one main peptide peak on Sephadex G.50.

Both trypsin- and versene-dissociated cells, on rotation in Hanks’s balanced salts solution with or without serum, began to adhere to one another at the start of the test period, indicating that the cell dissociation procedures had not impaired the adhesive properties of the separated cells.

When puromycin at a final concentration of 10 μg/ml was introduced at zero time into a cell suspension with or without serum, it did not delay the adhesion of cells to one another, and did not prevent the formation in the first hour of initial aggregates ; it succeeded only in preventing progress in aggregation beyond the initial stages. Adding puromycin at the 2nd or 3rd hour of the 4-h test period, when aggregation had already taken place, produced results of a similar pattern, progress in aggregation being brought to a halt an hour later. Puromycin at the above concentration effectively blocked protein synthesis by more than 90 % as measured by the incorporation of L-[α-14C]leucine into the proteins of the rotated cells. These results implied that aggregation was not dependent on the synthesis of ‘cell aggregating ‘material to replenish the loss of such material from the surfaces of the cells owing to the trypsin treatment. It is significant that 2,4-dinitrophenol, which is known to suppress cellular metabolism by inhibiting the production of ATP, also brought cell aggregation to a halt only after a lag period during which initial aggregates were formed.

It is suggested, as evidenced in the results of our investigations, that puromycin may well be exerting its inhibitory effect by ultimately depressing cellular metabolism below the level at which certain basic adhesive mechanisms at the cell surface can operate.
The Localization of [3H]Thymidine Incorporation in the DNA of Replicating Spinach Chloroplasts by Electron-Microscope Autoradiography
Authors: Rose R. J. , Possingham J. V.
Published: April 2021

Electron-microscope autoradiography has been used to obtain information on the localization of DNA labelled with [®H]thymidine in chloroplasts known to be replicating and concomitantly synthesizing and segregating DNA, in cultured leaf disks. The studies were made using both Microdol-X developer and a ‘compact’ developer which gave a smaller grain size. About 80 % of the grains were associated with the granal membranes and with presumptive DNA regions (3-nm fibril material in clear areas). Few grains occurred in association with the chloroplast envelope. We suggest that the DNA of chloroplasts is associated with the grana lamellae and extends into the stroma. Some light-microscope autoradiographs of whole chloroplasts show spiral or helical-like labelling patterns. We interpret these patterns as demonstration of the possibility that DNA occurs along the length of a continuous lamellar membrane system. Chloroplast fractionation experiments provided data consistent with the electron-microscope autoradiographic studies as most of the label was associated with chlorophyll-containing lamellae.

We consider an association of chloroplast DNA molecules along the length of a continuous lamellar system would ensure an orderly segregation of DNA to daughter chloroplasts, during the binary fission of spinach chloroplasts by constriction division.
Goblet Cell Membrane Differentiations in the Midgut of a Lepidopteran Larva
Authors: Flower N. E. , Filshie B. K.
Published: April 2021

So-called goblet cells are present in the midgut of lepidopteran larvae. They are thought to be involved in the active transport of potassium out of the haemolymph and into the gut lumen. A number of plasma membrane differentiations within the goblet cell cavity has been investigated using conventional staining, lanthanum tracer and freeze-etch techniques. Of particular interest are junction-like inter- and intra-membrane differentiations found on the villus-like cytoplasmic projections present at the apical tip of the goblet cell cavities. These cytoplasmic projections appear to act as a valve; in some cases they seem to close off the top of the goblet cell cavity, so isolating it from the gut lumen, while in other cases they are spread apart leaving a wide channel from the cavity into the lumen. The junction-like structures on these cytoplasmic projections are different in structure from the septate-type junctions which seal the midgut cells together at their apical borders, and the 2 types are present on the same plasma membrane, often within one micron of each other. The need for a different type of junction may possibly be related to the fact that it occurs between 2 areas of the same plasma membrane. The morphology of this unusual junction-like structure is discussed and 2 diagrams are presented to illustrate our interpretation of its structure.
The Secretion of Collagen by Insects: Uptake of [3H] Proline by Collagen-Synthesizing Cells in Locusta Migratoria and Galleria Mellonella
Authors: Ashhurst Doreen E. , Costin Natalie M.
Published: April 2021

The uptake of [3H]proline by collagen-secreting cells of the locust, Locusta migratoria, and wax-moth, Galleria mellonella, has been investigated by electron autoradiography. The locust cells are around the ejaculatory duct and they secrete collagen in the young adult male, while the wax-moth cells are those which produce the dorsal mass of connective tissue on the abdominal nerve cord during the late pupal stage. The cells were exposed to [3H]proline either by injection of the [3H]proline into the insect, or as a pulse while the tissue was maintained in a culture medium. The tissues were fixed at differing experimental times after exposure to the [3H]proline. The resulting electron autoradiographs were subjected to quantitative analysis, and the silver grain distribution was determined as the relative number of grains per unit area over a series of tissue compartments. When the results of this analysis for the matrix, rough endoplasmic reticulum and Golgi complexes of the two tissues were plotted against experimental time, it was seen that the relative number of grains per unit area over the rough endoplasmic reticulum decreases while that over the matrix increases; statistical analysis has shown that these changes are significant. For the Golgi complexes, however, the theoretical variances are much greater, due to the small relative area occupied by this organelle. There is little evidence for anything other than random sampling fluctuations in the relative numbers of grains per unit area, and hence it is unlikely that the time course of the label over the Golgi complexes follows that over the rough endoplasmic reticulum.

The conclusions drawn from these experiments are firstly that a large portion of the labelled protein passes straight from the rough endoplasmic reticulum to the matrix, but that a smaller portion of the labelled material might pass from the rough endoplasmic reticulum to the Golgi complexes and thence to the matrix. It is assumed that collagen comprises most of the protein which passes straight from the rough endoplasmic reticulum to the matrix, and while there is no evidence to exclude collagen from the material passing through the Golgi complexes, it is probable that other proteins and glycosaminoglycans are also present in this labelled material. These conclusions about the intracellular pathway for collagen secretion are similar to those derived from recent studies of some vertebrate fibroblasts. There is, however, conflicting opinion about the intracellular pathway of collagen and it is pointed out that there is diversity in collagen-synthesizing cells, which may account for the differences in the intracellular pathways for collagen secretion which have been proposed.
Phagocytosis of concanavalin a in normal and enucleated cultures of mammalian cells
Authors: Wise Gary E.
Published: April 2021

The fate of concanavalin A (Con A) bound to normal and enucleated L cells was followed at the ultrastructural level over a 20-h period. In both enucleates and normal cells the Con A is seen to be distributed in a uniform manner over the entire cell surface following a 30-min pulse with a low concentration of Con A. In the subsequent chase period the cells then aggregate the Con A and Con A sites into large clusters on the cell membrane. The cells then phagocytoze the Con A and large phagocytic vacuoles containing it are observed. Thus, enucleated cells are capable of phagocytozing Con A and its sites in the same manner as normal cells.
Signal emission and signal propagation during early aggregation in Dictyostelium discoideum
Authors: Gross J. D. , Peacey M. J. , Trevan D. J.
Published: April 2021

Waves of chemotactic movement during the early phase of aggregation in Dictyostelium discoideum are of 2 kinds, concentric waves produced by cells that emit cyclic AMP signals spontaneously, and spirals generated by excitations relayed continuously around loops of excitable cells. The period of a spiral wave is the time taken for the excitation to make one complete circuit of the pacemaker loop. We have compared signal emission from the 2 types of source in time-lapse films made at a variety of temperatures. Our results show that spiral waves have a characteristic period length throughout most if not all of the early phase of aggregation, and that the period of concentric waves is generally longer and more variable. Temperature has a pronounced effect on period length and a lesser effect on propagation velocity. We find that each individual wave is propagated at constant velocity over distances of 1–2 cm but that the velocity of successive waves declines. This decline probably reflects some cumulative effect of the chemotactic excitations on the excitable properties of the aggregating cells.
The analysis of malignancy by cell fusion. VIII. evidence for the intervention of an extra-chromosomal element
Authors: Jonasson J. , Harris H.
Published: April 2021

Diploid human, fibroblasts and lymphocytes were fused with the cells of a malignant mouse melanoma and a range of hybrid clones selected for study. The ability of these clones to produce progressive tumours was assayed in nude mice. Although human chromosomes were preferentially eliminated in all the hybrid clones, the human diploid cells were as effective as mouse diploid cells in suppressing the malignancy of the mouse melanoma cells. The suppression produced by fibroblasts was again more profound than that produced by lymphocytes. Malignancy was also found to be suppressed in a hybrid clone in which a single X was the only human chromosome present; and this clone continued to give a very low take incidence even after the human X had been eliminated by back selection. Hybrids were made between the melanoma cells and diploid human fibroblasts that had been given 100 J kg−1 of gamma radiation before cell fusion. These hybrids contained no recognizable human chromosomes, but their ability to produce progressive tumours was greatly reduced compared to that of the melanoma parent cells. The take incidences given by the crosses between the melanoma cells and the irradiated human fibroblasts were, however, substantially higher than those given by the crosses between the melanoma cells and unirradiated fibroblasts. These findings suggest that the suppression of malignancy involves the activity of some extra-chromosomal element and that this element is radio-sensitive.
The isolation of plasma membrane from protoplasts of soybean suspension cultures
Authors: Galbraith David W. , Northcote D. H.
Published: April 2021

A procedure for the isolation of plasma membranes from protoplasts of suspension-cultured soybean is described. Protoplasts were prepared by enzymic digestion of the cell wall and the plasma membrane was labelled with radioactive diazotized sulphanilic acid. The membrane systems from broken protoplasts were separated by continuous isopycnic sucrose gradient centrifugation. Radioactivity was localized in a band possessing a buoyant density of 1·14 g ml−1. The activities of NADPH- and NADH-cytochrome c reductase, fumarase, Mg2+-ATPase, IDPase and acid phosphodiesterase in the various regions of the density gradient were determined. A plasma membrane fraction was selected which was relatively uncontaminated with membranes derived from endoplasmic reticulum, tonoplasts and mitochondria. The results indicated that Mg2+-ATPase and possibly acid phosphodiesterase were associated with the plasma membrane.
Chromatin behaviour during the mitotic cell cycle of Saccharomyces cerevisiae
Authors: Gordon C. N.
Published: April 2021

Chromatin behaviour during the cell division cycle of the yeast Saccharomyces cerevisiae has been investigated in cells which have been depleted of 90 % of their RNA by digestion with ribonuclease. Removal of large amounts of RNA from the yeast nucleus before treatment of the cells with heavy metal fixatives and stains permits chromatin to be visualized with extreme clarity in thin sections of cells processed for electron microscopy by conventional procedures. Spindle pole bodies were also visualized by this treatment, although the associated microtubules were not.

Chromatin is dispersed during interphase and occupies the non-nucleolar region of the nucleus which is known to be Feulgen-positive from light microscopy. Because spindle micro-tubules are not visualized, direct attachment of microtubules to chromatin fibrils could not be verified. However, chromatin was not attached directly to the spindle pole bodies and kineto- chore differentiations were not observed in the nucleoplasm.

During nuclear division chromatin remains dispersed and does not condense into discrete chromatids. As the nucleus expands into the bud, chromosomal distribution to the daughter cells is thought to result from the separation of the poles of the spindle apparatus with attached chromatin fibrils. However, that such distribution is occurring as the nucleus elongates is not obvious until an advanced stage of nuclear division is reached and partition of the nucleus is nearly complete. Thus, no aggregation of chromatin into metaphase or anaphase plates occurs and the appearance of chromatin during mitosis is essentially the same as in interphase. These observations indicate that the marked changes in the topological structure of chromatin which characterize mitosis in the higher eukaryotes do not occur in S. cerevisiae.
Fusion and erosion of cell walls during conjugation in the fission yeast (Schizosaccharomyces Pombe)
Authors: Calleja G. B. , Yoo Bong Y. , Johnson Byron F.
Published: April 2021

Conjugation in Schizosaccharomyces pombe was studied by transmission electron microscopy. Mural and nuclear events were scored from induction, the initial event, to meiosis I, the start of sporulation. These morphogenic markers were separately identifiable as flocculation, copulation, conjugation-tube formation, cross-wall formation, cross-wall erosion, conjugationtube expansion, cytoplasmic fusion, de-differentiation of site of union, nuclear migration and karyogamy. The following were identified as new structural elements: sex hairs, which presumably mediate hydrogen bonding between cells during flocculation; crimp at the site of union; dark patch, which presumably serves as a leak-proof seal at the time of cross-wall erosion; suture, an electron-dense seam formed by the union of a copulant pair; and small electron-dense particles close to the site of wall erosion. No special structures on the cell wall could be identified as indicative of specific sites for potential copulatory activity. The discontinuity of the 2 cell walls at the site of union became so de-differentiated after fusion and erosion that it was no longer possible to pinpoint the site of union.
Extracellular matrix synthesis in blastula and gastrula stages of normal and hybrid frog embryos : II. autoradiographic observations on the sites of synthesis and mode of transport of galactose- and glucosamine-labelled materials
Authors: Johnson Kurt E.
Published: April 2021

Pulse-chase labelling experiments and light- and electron-microscopic autoradiography were used to examine the sites of synthesis, mode of transport, and sites of deposition of galactose- and glucosamine-labelled materials in different developmental stages of normal developing Rana pipiens embryos and interspecific hybrid embryos formed by fertilizing the eggs of R. pipiens with the sperm of R. catesbeiana. In both normal and hybrid embryos, after a 15-min pulse, grains are closely associated with juxtanuclear and cytoplasmic collections of membranebound vesicles which resemble the Golgi apparatus. In normal embryos following a 15– 30 min pulse and a 60-min chase, grains are largely cleared from the cytoplasmic vesicles and deposited in the extracellular spaces or along cell surfaces where the extracellular spaces are relatively large. In contrast, arrested hybrid embryos given a 15– 30-min pulse and a 60-min chase show a marked accumulation of grains over cytoplasmic structures such as the Golgi apparatus and vesicular elements in the cell cortex. Finally, early gastrula stage normal embryos are most active in the synthesis of galactose-labelled materials in cells above the dorsal lip of the blastopore, where cell migration is initiated.
Glycosyltransferase activity in developing sea-urchin embryos
Authors: Evans Irene M. , Bosmann H. Bruce
Published: April 2021

The developmental course of activity changes of 4 endogenous membrane glycosyltransferases : UDP-glucose : glycosyltransferase, UDP-galactose : glycosyltransferase, GDP-mannose: glycosyltransferase, and UD-PN-acetylglucosamine : glycosyltransferase was determined in various stages of developing sea-urchin embryos from unfertilized eggs up to the 16-cell stage. The results show that there is a 2-fold increase in glycosyltransferase activity after fertilization. These results are discussed in relation to the postulated role of glycosyltransferases in mediating adhesion, recognition, and agglutination of cells and embryos.
Interband transcription in Drosophila
Authors: Skaer R. J.
Published: April 2021

Most puffs contain perichromatin ribonucleoprotein granules 30-40 nm in diameter; other puffs contain ribonucleoprotein granules 25 nm in diameter or mixtures of these and perichromatin granules. All puffs contain fragments of band material possibly from several bands. By examining progressively smaller puffs the transcriptionally active region is shown to lie within an interband. Some transcriptionally active interbands are so small that there can be no significant contribution of decondensed band material to the interband. Up to 33% of all interbands contain significant evidence of transcription. These findings are discussed in relation to the use of the terms heterochromatin and euchromatin to describe the banding pattern of polytene chromosomes.
Production of mouse globin in heterokaryons of mouse erythroleukaemia cells and human fibroblasts
Authors: Alter Blanche P. , Goff Sabra C. , Hillman Diane G. , Deisseroth Albert B. , Forget Bernard G.
Published: April 2021

In an effort to activate the globin genes of non-erythroid cells, tetrapioid murine erythroleukaemia cells (Friend cells) were fused with diploid human amniotic fibroblasts. When the Friend cells were pretreated with dimethylsulphoxide, an average of 27% heterokaryons was observed. These cells stained with benzidine, an indication that they contained haemoglobin. The cells incorporated radioactive amino acids into proteins. Electrophoresis of [3H]leucine-labelled lysates on SDS urea polyacrylamide gels indicated that up to 7% of the newly synthesized protein co-electrophoresed with globin. CM cellulose chromatography demonstrated the presence of mouse but not human globin chains. Hybridization analyses of cytoplasmic RNA also revealed only mouse globin mRNA in the heterokaryons. Although heterokaryons form readily between mouse erythroleukaemia cells and human fibroblasts, and globin synthesis does occur, only the erythroid partner in the fusion system employed here directs globin production.
Continuous dna replication in the nucleus of the dinoflagellateProrocentrum micans(ehrenberg)
Authors: Filfilan S. A. , Sigee D. C.
Published: April 2021

The uptake of tritiated thymine into cells of a heterogeneous population of Prorocentrummicans was investigated using light-microscope and electron-microscope autoradiography. Specificity of thymine uptake into DNA was demonstrated by the specific removal of label from wax-embedded material using DNase and by the high degree of localization of nuclear label to chromosomes in the electron-microscope autoradiographs.

All nuclei, including both dividing and non-dividing cells, showed a substantial uptake of label, indicating that nuclear DNA synthesis in Prorocentrum micans is a continuous process. The level of DNA synthesis does show considerable variation, however, with very high levels in some interphase nuclei.

The continuous replication of nuclear DNA provides further evidence of dinoflagellate affinity to the prokaryotes, and indicates that Prorocentrummicans is a very primitive eukaryote cell.
Tissue economics of hydra: regulation of cell cycle, animal size and development by controlled feeding rates
Authors: Otto Joann J. , Campbell Richard D.
Published: April 2021

Epithelial cell production and epithelial cell loss in 6 different size classes of Hydra attenuata were examined to understand the relationships between growth and morphogenesis. The sizes of adult hydra, the sizes of their buds, and their budding rates are all nearly proportional to the amount of food the hydra eat. Hydra fed at high rates (4-25 Artemia nauplii per day) all have the same epithelial cell cycle time (about 4 days). Budding accounts for most of their cell loss. Hydra fed 4-12 Artemia per day maintain a steady state condition in which tissue loss balances tissue growth. Animals fed 25 Artemia per day are not in a steady state growth condition and change in size. At the lowest feeding rates (0-1 Artemia per day), the epithelial cell cycle time is lengthened to about 16 days. Cell loss from the tentacles accounts for most of the cell loss, and this loss is not completely balanced by growth. As a consequence these animals cease budding and shrink in size.
Nuclear changes associated with cellular dedifferentiation in pea root cortical cells cultured in vitro
Authors: Feldman Lewis J. , Torrey John G.
Published: April 2021

Root cortical tissues explanted from seedling roots of the garden pea, Pisum sativum L., cv. Little Marvel cultured in sterile medium containing indoleacetic acid and kinetin were fixed, sectioned and studied with the electron microscope. Nuclear changes over 60 h of culture were examined during the events of dedifferentiation of mature parenchyma cells into subdivided newly meristematic derivatives. Table 1 summarizes the events which are evident in 7 population classes designated I—VII. The initially small, round nucleus with a single, unvacuolated nucleolus and highly condensed and disperse chromatin showed marked volume increase, nucleolar enlargement of almost 20-fold, development of diffuse and then clumped chromatin and then subdivision of the nucleoli into a number (5–10) of small nucleoli immediately preceding cell division. Mitoses involved cells with diplochromosomes and daughter nuclei with doubled chromosome number, either 4n or 8n. Subdivided cortical cells showed an increased ratio of nucleus to cytoplasm although the nuclei were reduced in volume to that of the original nuclear population. Nucleoli in subdivided cells were small and unvacuolated and chromatin changed from an initially condensed state through a diffuse and disperse condition to a condensed form. Cortical cells divided at least twice in rapid succession before initiating new events leading to redifferentiation. Nuclear changes in dedifferentiation in pea were comparable in many respects to the similar process studied in animal systems including eye lens regeneration, limb regeneration and activation of division in blood lymphocytes. The cytological changes in nuclear structure could be correlated with documented changes in DNA, RNA and protein levels in these systems. By 60 h cells presumed to be ready for redifferentiation and cell specialization were observed in the subdivided population but structural evidence for commitment to a new course of cytodifferentiation was not obtained.
A microtubule—nuclear envelope complex in the spermatozoid of Pteridium
Authors: Bell Peter R.
Published: April 2021

The microtubules on the outside of the nucleus of the mature spermatozoid of Pteridium form a complex with the envelope, and the perinuclear space is eliminated. These dissimilar components are held firmly together, possibly by hydrophobic bonding. The outer part of the chromatin is also attached to the inner boundary of the envelope, and pulls away when the envelope is detached.
Studies on pulse-labelled rna during the mitotic cycle of Physarum polycephalum by subnuclear fractionation
Authors: Davies K.E. , Walker I.O.
Published: April 2021

A method is described for the isolation of pulse-labelled RNA from nuclei and subnuclear fractions of Physarum poly cephalum. At all times during interphase the nucleolar RNA consisted mainly of a 34-s rRNA precursor with only small amounts of 26-s and 19-s rRNAs. The nucleoplasmic RNA consisted of predominantly mature 26-S rRNA with small amounts of 19-s rRNA and a 30-s RNA species. The 30-s RNA component displayed different labelling kinetics from the rRNA precursors. A low molecular weight RNA fraction (4–8 s) appeared to accumulate in the nucleus as interphase progressed.
Histological studies on the genus Fucus: III. fine structure and possible functions of the epidermal cells of the vegetative thallus
Authors: McCully Margaret E.
Published: April 2021

The fine structure of the epidermal cells of the vegetative Fucus thallus has been examined in material fixed with acrolein. These cells are highly polarized, with basal nuclei and chloroplasts, a hypertrophied perinuclear Golgi system, and a much convoluted wall/plasma membrane interface. Much of the intracellular volume is occupied by single membrane-bounded vesicles containing alginic acid, fucoidin and polyphenols.

The chloroplasts were examined by light and electron microscopy and shown to contain structured inclusions not previously described in Fucus plastids. It is suggested en the basis of their morphology that the epidermal cells may be specialized for the absorption of inorganic carbon and sulphate from the outside of the plant and for the secretion of alginic acid, fucoidin and polyphenols. The possible role of these cells in the prevention of desiccation and in osmoregulation is discussed.
Structural organization associated with pseudopod extension and contraction during cell locomotion in Difflugia
Authors: Wohlman A. , Allen R. D.
Published: April 2021

In Difflugia corona, a free-living amoeboid cell, locomotion is hampered by a heavy shell or test made of sand grains and other debris. Locomotion involves pseudopod extension, attachment to the substratum, and forcible pseudopod retraction which pulls the shelled cell body forward.

When observed through a polarizing microscope, the extending pseudopodia appear isotropic or very weakly birefringent. Upon attachment to the substratum a positively birefringent fibrillar array develops rapidly at the attachment point and extends from this region back to the cell body within the test. These birefringent fibrils extend through and parallel to the long axis of the pseudopod. As the pseudopod retracts, the birefringent fibrillar array disappears, and hyaline blebs, suggestive of syneresis, appear on the pseudopodial surface. The birefringent fibrils correspond in position and approximate diameter (1 μ) to refractile fibrils visible with the Nomarski differential interference microscope.

Individual organisms were fixed for electron microscopy at a time when the pseudopodia were firmly attached to the substratum. Electron-microscopic examination of thin sections of pseudopodia revealed many 1-μ bundles of intimately associated, aligned, 55–75 Å micro-filaments. The orientation and size of the bundles indicate that they probably correspond to the birefringent, refractile fibrils observed in living cells. Microfilaments have also been observed both as randomly oriented and dispersed cytoplasmic components, and as aligned filaments in the ectoplasm adjacent to the plasmalemma.

During pseudopod extension with sporadic streaming, birefringent ‘flashes’ have been observed at the front of the pseudopod. These flashes are believed to represent a photo-elastic phenomenon.
Asymmetry of the node of ranvier
Authors: Williams P. L. , Kashef R.
Published: April 2021

The nodal constrictions of Ranvier in normal limb nerves of mammals are bounded by asymmetric paranodal bulbs, the proximal bulb (nearer the cell body) being larger in all dimensions. This investigation was undertaken to determine whether the asymmetry results from partial damming of proximo-distally flowing axoplasm at the nodal constriction, or whether other features of local growth patterns are more relevant.

The degree of asymmetry was estimated on teased, osmicated fibres from normal immature and mature limb nerves to skin and muscle and on ventral nerve roots. Estimates were also made on the central processes of dorsal root ganglionic cells with their contrasting direction of flow, after alteration of growth patterns in regenerates of crushed immature and mature nerves, and in the recurrent laryngeal nerve which pursues an exceptional course in relation to surrounding tissues. A polarization of asymmetry with larger proximal bulbs was found in uncomplicated limb nerves and after simultaneous regeneration and limb growth following crushing of immature nerves. Mixed populations (that is, with no preferred direction of asymmetry and often symmetrical bulbs) were found in dorsal and ventral nerve roots and the apical recurved segment of the recurrent laryngeal, whilst the mature regenerates closely approached the symmetrical condition. The descending and ascending limbs of the recurrent laryngeal nerve showed a reversal of polarization with respect to the cell body, but similar with respect to the cephalo-caudal body axis. It was concluded that damming of directionally flowing axoplasm was not causally related to the formation of asymmetric bulbs. The overall interstitial growth pattern of limb nerve elongation as revealed by intemodal distance studies contrasts with the differential growth and maturation gradients shown by the various limb segments through which the nerve passes. These differences result in a relative movement between the myelinating and elongating Schwann cell and the surrounding limb tissues. The movements are considerable and unidirectional throughout most of the limb, the Schwann cells becoming relatively further removed from the limb apex, whilst the movements minimize at the limb root and apex. It is suggested that the altered mechanical conditions operating at the ‘advancing’ and ‘trailing’ ends of the cell are related to the dimensional differences of the paranodal apparatus comprising the bulb at each end of the cell, and secondarily result in asymmetric nodes of Ranvier.
Behaviour of the cell surface during movement of preaggregation cells of the slime mould dictyostelium Discoideum studied with fluorescent antibody
Authors: Garrod D. R. , Wolpert L.
Published: April 2021

Although several theories relating to the mechanical behaviour of cell surface during movement have been put forward, a division into two groups can be made according to whether the surface is thought to turn over rapidly or to be a relatively permanent structure. In order to determine whether turnover of the surface membrane plays an important role in the locomotion of preaggregation cells of Dictyostehum discoideum, their surface was labelled with fluorescent antibody. The cell surface remains labelled during movement though there is an initial accumulation of fluorescence at the tail. There is little fluorescent labelling of the internal contents. The results are interpreted as showing that surface turnover is not an essential feature of cell movement.
Studies on the microtubules in heliozoa: IV. the effect of colchicine on the formation and maintenance of the axopodia and the redevelopment of pattern in Actinosphaerium nucleofilum (barrett)
Authors: Tilney L. G.
Published: April 2021

When specimens of Actinosphaerium are placed in a solution of colchicine, the axopodia retract and that portion of the biréfringent core (axoneme) present in each axopodium disappears. In fixed specimens, it has been shown that the axoneme consists of a highly patterned bundle of microtubules each 220 Å in diameter. During colchicine treatment the microtubules, present in the axopodial portion of the axoneme, break down and do not reform until the cells are washed free of the colchicine and allowed to recover. In the basal portion of the axoneme or that portion confined to the cell body, most of the microtubules do not break down during colchicine treatment, although the spacing of the microbutules within the remaining axonemes is altered; in longitudinal section these tubules no longer appear straight but present an undulatory profile. When the specimens are washed free of the colchicine and allowed to recover, axopodia reform and within each numerous microtubules are present. From these observations we conclude that the microtubules are instrumental in the maintenance of the axopodia as well as in supplying the force necessary for their regrowth. Observations are also presented on the disintegration products of microtubules, on microtubules and cytoplasmic motility, and on the redevelopment of pattern. The latter is discussed at some length, for it appears that the inter-axonemal organization is somewhat modified by the colchicine treatment.
The formation of the generative cell in the pollen grain of Endymion non-scriptus (l)
Authors: Angold R. E.
Published: April 2021

The generative cell wall in the pollen grain of Endymion non-scriptus is formed, as in somatic cells, from a cell plate between the vegetative and generative nuclei. This wall curves around the generative nucleus, and fuses with the intine to enclose the generative cell. The generative cell is subsequently freed from the intine by the constriction of the generative cell wall between the generative nucleus and the intine.
The fine structure of fertilization in the fern Marsilea vestita
Authors: Myles Diana Gold
Published: April 2021

The ultrastructural details of fertilization in the fern Marsilea vestita, including gamete approach and fusion, the fate of the spermatozoid organelles and the development of a possible block to polyspermy are described. The spermatozoid approaches the egg through layers of mucilage that surround the megaspores. It moves down the neck of the archegonium into the cavity above the egg. In order to reach the egg, it must move through a small hole in the thick wall that lies across the top of the egg. The fusion of the plasma membranes of the gametes results in an outflow of egg cytoplasm into the clear space under the sperm plasma membrane, creating a fertilization cone. All the organelles of the fertilizing spermatozoid, including nucleus, mitochondrion, microtubule ribbon, multilayered structure, and flagellar band, with approximately 150 flagella, enter the egg cytoplasm. The nucleus enters as a condensed rod of chromatin with no nuclear envelope. The chromatin begins to disperse immediately and a new nuclear envelope is formed around the chromatin by egg endoplasmic reticulum. The mito-chondrion and the microtubules of the ribbon and flagella are broken down, but the fates of the flagellar band and the multilayered structure have not been determined. After spermatozoid penetration, a new extracellular layer appears above the surface of the egg, beginning in the region of sperm penetration and spreading across the top of the egg. This layer may be important in preventing other spermatozoids from fusing with the egg.
Protease secretion during onset of development in Dictyostelium discoideum
Authors: Rossomando Edward F. , Maldonado Barbara , Crean Edmund V. , Kollar Edward J.
Published: April 2021

At the onset of development, the single cells of the eukaryotic micro- organism Dictyostelium discoideum secrete proteolytic activity which can be assayed using the insoluble substrate remazolbrilliant blue hide. The activity is not secreted by exponentially growing cells, but does appear extracellularly at the onset of the stationary growth phase. When growth phase cells are resuspended in non-nutrient buffer, proteolytic activity begins to appear outside the cells. It accumulates in the buffer at a rate similar to that observed for 2 glycosidases of lysosomal origin and reaches a maximum after about 2 h of incubation. After 3–4 h incubation, centrifugation of the non-nutrient buffer removes the cells, producing a supernatant which we refer to as conditioned medium.

Subsequent experiments with conditioned medium showed: (a) its incubation with purified plasma membranes results in the release of polypeptides which can be recovered and, when displayed on polyacrylamide gels, can be shown to be stage specific; and (b) that conditioned medium can decrease the rate of detachment of cells from a collagen substratum. Both effects can be prevented by the addition of remazolbrilliant blue hide suggesting that they are due to proteolytic activity present in the conditioned medium.

Finally, we were able to show that conditioned medium contains components which, when spread over the bottom of plastic Petri dishes, enhance the rate of multicellular structure formation. Additional studies showed that this effect of conditioned medium could also be brought about by components which remained behind on uncoated plastic dishes after the removal of a D. discoideum cell layer. These data may be accommodated to a model in which the protease secreted during the onset of development acts on the cell membrane releasing components which coat the substratum and facilitate migration and multicellular structure formation.
pH oscillations in cell suspensions of Dictyostelium discoideum: Their relation to cyclic-amp signals
Authors: Malchow D. , Nanjundiah V. , Gerisch G.
Published: April 2021

Cells of Dictyostelium discoideum known to release cyclic AMP (cAMP) rhythmically in the form of pulses, change with the same period of about 8 min the pH of their medium. The pH is used here as an indicator to investigate the effect of externally added cAMP pulses on the oscillations. Both a temporary increase in amplitude and a permanent phase shift can be induced. The phase-response curve indicates that the period can be increased and decreased by rhythmic stimulation with cAMP pulses.
Intracellular motility of mitochondria: Role of the inner compartment in migration and shape changes of mitochondria in xth-cells
Authors: Bereiter-Hahn J.
Published: April 2021

Mitochondrial movements have been followed by phase-contrast microscopy in living XTH-cells (Xenopus laevis tadpole-heart cells) in tissue culture. The same organelles have been viewed subsequently in electron micrographs. Locomotion of mitochondria proceeds at velocities up to 100 μm/min. Formation of branches of mitochondria and other shape changes may occur with the same speed. Mitochondrial motility can be classified into 4 types: (1) Alternating extension and contraction at the two ends of rod-shaped mitochondria. (2) Lateral branching. (3) Alternate stretching and contraction of arbitrary parts of a mitochondrion amounting to a kind of peristaltic action. (4) Transverse wave propagation along the organelle. Types 1 to 3 can be reduced to the same underlying principle; they cause locomotion. Formation of mitochondrial extensions is due to elongation of cristae.

The observations are discussed in terms of 4 specific proposals. (1) Intracellular locomotion of mitochondria is caused by local enlargements and contractions of the organelles. (2) The shape changes are correlated with alterations in the arrangement of the cristae. (3) Such arrangements are not associated with overall swelling or shrinkage of the mitochondrion; they are local features. (4) Estimates of the time required for rearrangement of the inner compartment amount to less than 0·3 s for single crista arrangements during the fastest shape changes, and less than 1–3 s during slower alterations. This high velocity is in good accord with the hypothesis of energy conservation by conformational events during oxidative phosphorylation.
The ultrastructure of cell division inEuglena gracilis
Authors: Gillott Marcelle A. , Triemer Richard E.
Published: April 2021

The ultrastructure of mitosis in Euglena gracilis was investigated. At preprophase the nucleus migrates anteriorly and associates with the basal bodies. Flagella and basal bodies replicate at preprophase. Cells retain motility throughout division. The reservoir and the prophase nucleus elongate perpendicular to the incipient cleavage furrow. One basal body pair surrounded by a ribosome-free zone is found at each of the nuclear poles. The spindle forms within the intact nuclear envelope. Polar fenestrae are absent. At metaphase, the endosome is elongated from pole to pole, and chromosomes are loosely arranged in the equatorial region. Distinct, trilayered kinetochores are present. Spindle elongates as chromosomes migrate to the poles forming a dumb-bell shaped nucleus by telophase. Daughter nuclei are formed by constriction of the nuclear envelope. Cytokinesis is accomplished by furrowing. Cell division in Euglena is compared with that of certain other algae.