bluuebunny/crossref_metadata_2025 huggingface dataset:
The influence of the microtubule inhibitor, methyl benzimidazol-2-yl carbamate (MBC) on nuclear division and the cell cycle in Saccharomyces cerevisiae
Authors: Quinlan R. A. , Pogson C. I. , Gull K.
Published: April 2021

Methyl benzimidazol-2-yl-carbamate (MBC), at a concentration of 100 μM, has a pronounced effect on the growth of Saccharomyces cerevisiae, resulting in the accumulation of cells as large doublets. We have determined a specific execution point for the effect of MBC on the yeast cell cycle, and have shown that this execution point is between the cell cycle events of spindle pole body duplication and spindle pole body separation. An ultrastructural examination of the MBC-treated cells revealed the absence of cytoplasmic and spindle microtubules. MBC treatment also produced an altered spindle pole body morphology, causing the disappearance of the outer component. Nuclear size was also markedly increased in the MBC-induced doublet cells, although septa were completely absent from these doublet cells.

It is proposed that MBC inhibits microtubule polymerization, rather than causing the depolymerization of stable microtubules.
Similar size of the replicons in Chironomus polytene chromosomes at two developmental stages
Authors: Lönn Ulf
Published: April 2021

The size of the DNA replication units in salivary gland polytene chromosomes of Chironomus arvae at 2 developmental stages (red-head and late 4th instar) was investigated. The 2 developmental stages differ with respect to intensity of DNA replication with much lower values at the red-head stage. Also the level of polytenization is lower at the red-head stage. The size of the replication units was measured by determining the size of double-stranded DNA fragments released from active replication units during cell lysis. At both stages it is possible to release from the polytene chromosome a double-stranded DNA fragment of similar size. This indicates that there is no or very little difference in size of active replication units in the same tissue at the 2 development stages.
The distribution of cell-spreading activities in sera: a quantitative approach
Authors: Knox P. , Griffiths S.
Published: April 2021

Sephacryl S-300 gel filtration of animal sera is used to demonstrate that there are at least 2 components that promote the ‘spreading’ of cells in culture. A morphometric technique is described to quantitate the spreading process. For a number of cell strains and established cell lines the more quantitatively significant spreading factor is not fibronectin. Rather it is a component with a molecular weight of approximately 70000. This factor does not cross-react immunologically with fibronectin and seems to cause spreading via a different mechanism from that stimulated by fibronectin. Thus fibronectin will cause spreading in the absence of protein synthesis, whereas the smaller component requires protein synthesis. The kinetics of spreading are also different at all concentrations of the factors that are effective.

By comparing the spreading promoted by whole sera with that promoted by separate serum fractions following chromatography we conclude that under normal culture conditions plasma fibronectin plays little part in initial cell spreading. This view is supported by the fact that fibronectin-depleted serum will stimulate cell spreading.
The intranuclear relationship between centromere volume and chromosome size in Festuca scariosa × drymeja
Authors: Jenkins G. , Bennett M. D.
Published: April 2021

In the hybrid Festuca scariosa × drymeja where pairing is incomplete at pachytene, there is preferential pairing between the longer chromosomes of the complement. EM serial-section reconstruction of nuclei at zygotene and pachytene reveals that there is equally pronounced preferential pairing between larger centromeres. This evidence suggests that the longer chromosomes have large centromeres and that centromere volume is correlated with chromosome length. Confirmation of this comes from the comparison of the frequency distributions of observed centromere volumes and those predicted on the basis of chromosome length.

Although there is a positive correlation between centromere volume and chromosome length, it is not possible to identify the centromeres of each individual chromosome within the complement because (a) the differences between the lengths of each chromosome are small and (b) the estimates of relative centromere volumes vary significantly between cells.
Persistence, suppression and re-expression of pigment formation in somatic cell hybrids between mouse melanoma cells and non-melanoma cells
Authors: Junker Steffen
Published: April 2021

Pigmented subtetraploid and subhexaploid mouse melanoma cells were fused with a range of different cell types. Expression of pigment formation appeared to be dependent on the phenotype of the non-melanoma parent cell, so that hybrids with lymphoid cells or chick embryo erythrocytes produced pigment, but hybrids between fibroblasts or epithelial rat hepatoma cells did not. The results were independent of gene dosage of either parent cell, γ-irradiation of suppressing partner cells prior to fusion caused progressive increase of pigmentation with increasing dose of radiation. Cybrids between cytoplasts of suppressing fibroblasts and melanoma cells were pigmented.
Nuclear lamina assembly, synthesis and disaggregation during the cell cycle in synchronized HeLa cells
Authors: Jost Erich , Johnson Robert T.
Published: April 2021

The pattern of assembly, synthesis and disaggregation of the nuclear envelope-associated lamina in synchronized HeLa cells has been examined by means of immunofluorescence microscopy of cell preparations treated with antibody to lamina polypeptide LP67 or lamin B, using the nomenclature of Gerace and Blobel. During the cell cycle the distribution of lamina polypeptide varies dramatically. Three distinct stages can be detected with respect to its location and synthesis: Lamina polypeptide is cytoplasmically distributed during cell division particularly at the time of complete nuclear envelope breakdown. At telophase it is reassembled and becomes associated with regions of the chromosome surface. This process continues into early G1, lamina polypeptide associating progressively with the outer surface of the chromosomes; simultaneously its cytoplasmic concentration is reduced. The traffic of antigen from cytoplasm to the chromosomes is highest between telophase and G1 but continues at a reduced rate during G1. Inhibition of protein synthesis during division and G1 does not inhibit G1 lamina reformation, suggesting that the G1 lamina is constructed from depolymerized cytoplasmically-stored polypeptide derived from the previous cell cycle.Lamin B is synthesized in S-phase. The protein is rapidly accumulated at the nuclear periphery during the doubling of the nuclear surface. There is very little if any cytoplasmic location of lamina polypeptide in S-phase cells. The S-phase pattern of lamina distribution persists through early and mid G2Towards the end of G2, and particularly with the onset of prophase, the nuclear membrane-bound lamina is released into the cytoplasm over approximately 30 min. (Fusion between metaphase and either G1 or S-phase cells also results in the depolymerization of lamin B within 30 min.) Lamina polypeptides appear in the cytoplasm before the nuclear envelope has disappeared, and protein synthesis in G2, is required for complete lamina disaggregation.

Lamina reformation in late mitosis and the polarity of its appearance in the telophase cell can be perturbed by treatment with colcemid or low temperature so that lamina is induced to associate with metaphase and anaphase chromosomes. In the absence of spindle microtubules lamina reformation may proceed around the surface of whole chromosomes, leading to the formation of micronuclei in G1 cells. We propose that in the normal cell nuclear envelope formation at telophase is related to the terminal distribution of cytoskeleton structures such that lamina is laid down only on the outer surface of the telophase genome beginning at the telomeric chromosome ends adjacent to the remaining mitotic fibres of the cleavage furrow.
Studies on the rhizoplast from Naegleria gruberi
Authors: Gardiner Peter R. , Miller Robert H. , Marsh Mark C. P.
Published: April 2021

A procedure, utilizing homogenization and centrifugation in a low ionic strength buffer containing Triton X-100, has been used to facilitate the isolation of the rhizoplast from flagellates of Naegleria gruberi. This has enabled a study to be made of the physical and biochemical properties of this organelle. The rhizoplast is shown to be a proteinaceous structure with chemical properties similar to those of the molluscan gill ciliary rootlet. Polyacrylamide gel electrophoresis gives a possible subunit molecular weight of approximately 240000 Daltons. Studies with antisera raised against the rhizoplast fraction demonstrated the absence of rhizoplast antigens in amoeboid forms of Naegleria gruberi and is taken as evidence that the organelle is synthesized de novo during transformation of the amoeba to the flagellate form. Results of optical diffraction studies on isolated rhizoplasts are also presented.
Chromosomal basis of dosage compensation in Drosophila. X. Assessment of hyperactivity of the male X in situ
Authors: Chatterjee R. N. , Mukherjee A. S.
Published: April 2021

The results of examination of the template activity of the fixed polytene chromosomes of Drosophila hydei, monitored by 3H-UTP, under in situ assay conditions, upon the use of endogenous Drosophila polymerase, exogenous Escherichia coli RNA polymerase (holoenzyme) and exogenous Drosophila RNA polymerase II (or B) have been presented. Analysis of the data reveals that the transcription patterns with the 3 enzymes are not strictly comparable with the pattern obtained under in vivo conditions. Yet, with each of the 3 conditions of assay, there is a reasonable concordance between the template activity on the single X chromosome of the male and the paired Xs of the female, as observed under in vivo. There is also, in every case, a high positive correlation between the 3H-UMP incorporation into the X chromosome and that into a specific autosome. A site-wise analysis of 3H-UMP labelling under the 3 assay conditions also reveals that for most of the regions, the sites which are highly active in vivo also show high labelling in situ,’ the proportionality is maintained in both sexes. These results have been interpreted to have suggested that the hyperactivity of the male X vis-a-vis dosage compensation in Drosophila is primarily a property of the inherent organization of the X chromosome itself and is achieved through modulation in the organization, rather than exclusively through autosomal factor(s), although a secondary level of autosomal regulation has not yet been ruled out.
Membrane particle arrays in SV40-transformed 3T3 Cells
Authors: Montesano Roberto , Philippeaux Marie-Marthe
Published: April 2021

The ultrastructural organization of the plasma membrane of untransformed and simian virus (SV)40-transformed 3T3 cells in culture was studied by an in situ freeze-fracturing technique. Both 3T3 and SV3T3 cells showed randomly distributed intramembrane particles as well as gap junctions. In addition, the plasma membrane (P-face) of confluent SV40-transformed 3T3 cells was found to display a unique membrane specialization which was not observed in normal 3T3 cells. This membrane specialization consisted of irregularly-shaped arrays of loosely dispersed large (11–12 nm) intramembrane particles, located in regions of close contact between adjacent SV3T3 cells. When a P-to E-fracture face transition occurred within the boundaries of a particle array, the intercellular space appeared uniformly reduced and arrays of pits coextensive with the P-face particles could be recognized on the E-face. Particle arrays appeared particularly prominent in SV3T3 cells grown in the presence of high serum concentrations (20 % foetal calf serum or calf serum). Particle arrays resembling those found in SV3T3 cells were observed only in very rare instances in the plasma membrane of other virus-transformed cell lines. Several possible interpretations of the meaning of the particle arrays, including the hypothesis that they represent an unusual or abortive form of intercellular junction, are discussed. The absence of similar membrane specializations in the plasma membrane of untransformed 3T3 cells points to a difference in membrane organization and cell contact ultrastructure between normal and SV40-transformed 3T3 cells.
Ciliary activity of cultured rabbit tracheal epithelium: beat pattern and metachrony
Authors: Sanderson M. J. , Sleigh M. A.
Published: April 2021

The beat pattern of rabbit tracheal cilia has been investigated using high-speed cine photography and scanning electron microscopy, on cultured epithelia of known orientation.

The cilia normally rest in the position reached at the end of the effective stroke, the ciliary tips pointing towards the oropharynx. Each beat begins with a recovery (or preparative) stroke in which a bend is propagated up the cilium causing the cilium to rotate backwards in a clockwise sweep, as viewed from above. At the end of its recovery stroke the cilium progresses immediately into the effective (or power) stroke, which is almost planar and in a cephalad direction. The active cilium describes an arc of almost 110° before reaching the rest stage. This beat pattern is not significantly altered over an increase in frequency from 13 –29 Hz; the relative durations of the 2 active phases of the beat remain similar over this range.

Metachronal waves exist in the form of short erratic areas of coordinated beating which travel only short distances. Within each area, the non-planar recovery strokes initiate an antilaeoplectic wave of activity which recruits inactive cilia to extend the wave. As cilia perform their effective strokes, adjacent cilia in the plane of beating move in an antiplectic sequence. This pattern of coordination is related to the pattern of beat of the cilia and their distribution on the epithelium.
Infiltration of tumour cells into cultures of isolated hepatocytes
Authors: Roos Ed , Van De Pavert Ilja V. , Middelkoop Otto P.
Published: April 2021

MB 6A lymphosarcoma and TA3 mammary carcinoma cells have previously been shown to infiltrate from liver blood vessels, where they had been arrested, into the liver parenchyma. The same tumour cells were presently added to cultures of isolated hepatocytes, 24 h after their isolation.

Both tumour cell types adhered to both dorsal and lateral surfaces of hepatocytes. The lymphosarcoma cells rapidly infiltrated between the hepatocytes. They first extended pointed pseudopods between the liver cells, and when the tumour cell body intruded, they deeply invaginated the liver cells at an interhepatocyte boundary. The MB 6A cells accumulated between and under the hepatocytes, and after 24 h virtually all cells were contained within the cultures.

TA3 cells also invaginated hepatocytes, not only at interhepatocyte boundaries, but all over the exposed surface. They did not extend pseudopods. The process was much slower than with MB 6A cells: After 24 h a few TA3 cells were completely encircled by hepatocytes. These observations indicate that the mechanism of infiltration is different for the 2 tumour cell types.

Part of the TA3 cells did not invaginate the hepatocytes. Several of these cells spread on the hepatocyte surface, attaining a flattened shape.

TA3 cells formed extensive tight junctions with the hepatocytes, sometimes sealing an intercellular lumen that resembled both a tumour acinus and a bile canaliculus. Also desmosomes were occasionally formed.

The hepatocyte cultures appear to be a suitable model for studying the mechanism of liver infiltration, not only of tumour cells, but also of leucocytes.
Tentacle contraction in glycerinated Discophrya collini and the localization of HMM-binding filaments
Authors: Hackney Carole M. , Butler R. D.
Published: April 2021

The contractile tentacles of the suctorian Discophrya collini contain a central microtubular axoneme as well as filamentous structures in the cortical epiplasm and in a fibrous collar around the axoneme at the tentacle base. The nature and possible roles of these components has been investigated by the use of reactivatable glycerinated cells. In these a mean tentacle contraction to 70 % of the original length could be achieved by a 5-min treatment with a reaction mixture containing ATP, calcium and magnesium ions, the same treatment giving retraction to 30 % in living cells. Both the microtubules of the axoneme and the filaments of the fibrous collar and epiplasm were present in the glycerinated cells, suggesting that these components consist of large water-insoluble molecules. The addition of heavy meromyosin to whole glycerinated cells resulted in the appearance of 36 –50-nm spaced ‘tails’ or filaments attached to the epiplasmic fibres and the aggregation of 3 –6-nm filaments and electron-dense material in the region of the fibrous collar. Neither of these 2 features was apparent after treatment with ATP. It is suggested that actin-like filaments are localized in the region of the fibrous collar and in the epiplasm, and that these are involved in tentacle retraction; whilst the microtubules of the axoneme are concerned with feeding, and play only a cytoskeletal role in the contractile mechanism.
The relationship between nuclear DNA content and centromere volume in higher plants
Authors: Bennett M. D. , Smith J. B. , Ward J. , Jenkins G.
Published: April 2021

The total volume of centromeres per nucleus varies widely within Triticum aestiium cv. Chinese Spring (7-fold) and between 11 higher plant species (24-fold). Such variation is closely correlated with nuclear DNA content, nuclear volume and, to a lesser extent, the total volume of nucleoli per nucleus. Centromere volume reflects minor intraspecific developmental fluctuations in nuclear size independent of variation in nuclear DNA content, but variation in nuclear DNA plays the major role in determining centromere volume. Thus, in general a given total volume of centromeric material is apparently characteristic of an approximately constant nuclear volume and mass of nuclear DNA, but largely independent of chromosome number. The range of volumes of single centromeres in 4 taxa corresponds with the ranges of their single chromosome lengths or chromosome DNA contents. The centromere is, therefore, not a unit structure of constant size and mass but a chromosome segment whose highly variable volume closely reflects the volume and mass of the chromosome to which it belongs. The correlation between centromere size and chromosome size and DNA content is potentially useful for identifying single centromeres in unsquashed interphase and dividing nuclei; thereby facilitating studies of the intranuclear disposition of chromosomes.

The present results for centromeres provide probably the first example to indicate that variation in the total DNA content of small segments present on each chromosome sometimes varies directly in proportion to large interspecific variation in nuclear DNA C-value.

The close correlation between centromere volume, and nuclear DNA content is probably nucleotypic in origin. The functional significance of the variation in centromere volume is unknown, as is the nature of the mechanism which determines that centromere volume closely reflects nuclear and chromosome size and mass.
Surface changes during retraction-induced spreading of fibroblasts
Authors: Chen Wen-Tien
Published: April 2021

Retraction of the trailing edge of an embryonic chick heart fibroblast results in an abrupt increase in protrusive activity at the leading edge of the cell. This increase was studied with time-lapse cinemicrography and scanning electron microscopy. Increased spreading following retraction results primarily from an increase in the duration of the extension phase of lamel-lipodial spreading. Much ruffling accompanies this increased spreading, particularly during its earliest phase. Upon retraction of the trailing edge, folds appear on the surface of the retracted tail and adjacent cell body and, soon after, microvilli-like structures appear as well. Once the moving cell has fully rcspread, however, the upper surface is once again smooth and free of folds and microvilli. Artificial detachment of a spreading lamella with a microneedle, and its consequent retraction, also causes increased protrusive activity of the remaining lamellae of the cell. These results are consistent with the hypothesis that retraction of one part of the cell makes surface membrane and cvtoplasm available for forming protrusions elsewhere.
5-hydroxytrypt amine-stimulated mitochondrial movement and microvillar growth in the lower malpighian tubule of the insect, Rhodnius prolixus
Authors: Bradley J.Timothy , Satir Peter
Published: April 2021

Rapid initiation of ion transport occurs in the lower Malpighian tubule of the insect Rhodnius prolixus following feeding in vivo or stimulation with 5-hydroxytryptamine (5-HT) in vitro. Using the electron microscope, we have conducted a morphometric analysis of cells in the lowest one-third of the lower tubule, demonstrating that 5-HT also induces mitochondrial movement and microvillar growth simultaneously with, but independent of, the onset of ion transport.

Mitochondria move from a position below the cell cortex to one inside the microvilli within 10 min of stimulation with 5-HT, resulting in an 8- to 10-fold increase in the volume of mitochondria within the microvilli. Previous findings indicated that mitochondrial movement is dependent on actin-containing microfilaments, but not microtubules. As the mitochondria enter the microvillus, the core microfilaments are reorganized into a sheath of microfilaments, which extends closely parallel to the outer mitochondrial membrane down into the cell interior. This sheath of microfilaments is also observed around mitochondria in the axopods. We suggest that the core microfilaments are responsible for mitochondrial movement into the microvilli and axopods. Stimulation with 5-HT induces a shift in mitochondrial configuration from orthodox to condensed, indicating a possible increase in oxidative phosphorylation. Following stimulation, the microvilli grow about 3 × in volume and 2·5 × in surface area. These increases are more than can be accounted for by mitochondrial invasion and must involve the addition of new membrane and microfilament polymerization. The observed changes - microvillar growth, insertion of additional membrane, activation and movement of mitochondria adjacent to the ion transport membrane - are described in the light of their significance in ion transport. A simple model is proposed which unifies the observed ultrastructural changes and known ion movements in the lower tubule.
Binding of parbendazole to tubulin and its influence on microtubules in tissue-culture cells as revealed by immunofluorescence microscopy
Authors: Havercroft J. C. , Quinlan R. A. , Gull K.
Published: April 2021

We have shown that the benzimidazole carbamate, parbendazole, is a potent inhibitor of microtubule assembly in vitro and in vivo. Radiolabelled parbendazole was shown to bind to purified tubulin. Immunofluorescence studies using an antitubulin antibody showed that parbendazole effectively depolymerizes cytoplasmic microtubulues in animal cells leaving only one or two microtubules associated with one centriole. The usefulness of parbendazole and other benzimidazole carbamates as inhibitors of microtubule functions is discussed.
Binding of isovaleraldehyde, an attractant, to zoospores of the fungus Phytophthora palmivora in relation to zoospore chemotaxis
Authors: Cameron J. Neil , Carlile Michael J.
Published: April 2021

The interaction of isovaleraldehyde, an attractant, with zoospores of the fungus Phytoph-thora palmivora was investigated by using binding techniques. The amount of isovaleraldehyde bound diminished with time, an effect that may be related to sensory adaptation. In addition to non-specific binding, specific and saturable isovaleraldehyde binding was demonstrated. Cooperativity occurred at intermediate but not at low and high ligand concentrations. [3H] isovaleraldehyde was displaced by unlabelled isovaleraldehyde and by a number of chemically related ligands known to be chemotactic agents. Their dissociation constants as calculated from the displacement experiments paralleled their potency as attractants and it is suggested that these attractants act through the same receptor. Some other chemotactic agents did not displace [3H]isovaleraldehyde, and these presumably act through one or more other receptors.
Modification of fibroblast surface amines alters receptor-mediated cell spreading on protein-coated substrata but not adsorptive endocytosis
Authors: Aplin J. D. , Hughes R. C.
Published: April 2021

Fluorescein isothiocyanate (FITC) and other anionic reagents specific for amine groups have previously been shown to inhibit the adhesion and spreading of cultured fibroblasts to fibronectin-coated surfaces (Butters, Devalia, Aplin & Hughes, 1980). Here it is demonstrated that a population of FITC-labelled cells can be separated using flow cytometry into fractions displaying greater and lesser adhesivity at lower and higher fluorescence, respectively, demonstrating that the inhibition is dose-dependent. Glass coverslips covalently derivatized with the lectins ricin and concanavalin A are used to show that the inhibition also occurs in lectin-mediated cell adhesion as well as in adhesion to collagen coated with fibronectin and plastic coated with serum or antibody, suggesting that all of these responses share a common, F1TC-sensitive component.

Simple primary amine compounds inhibit adhesion to fibronectin, but specific inhibitors of transglutaminases do not affect the process. Transglutaminase activity of cell surfaces has been implicated in protein endocytosis and receptor recycling (Davies et al. 1980).

FITC modification of cells appears to affect specifically adhesive interactions, since ricin cytotoxicity and infection of cells with influenza and Sendai viruses (phenomena thought to proceed by means of receptor-mediated endocytosis) are unaffected. Evidently, receptor-mediated cell attachment, spreading on protein-coated surfaces and protein endocytosis are functionally separate events requiring different cell-surface membrane components, even when the same protein (ricin) is used to trigger these 2 processes.
Mechanisms of nutritive endocytosis i. phagocytic versatility and cellular recognition in Chlorohydra digestive cells, a scanning electron microscope study
Authors: Mcneil Paul L.
Published: April 2021

The quantity of surface membrane internalized during phagocytosis by Chlorohydra digestive cells was estimated for a range of particle types. Challenge with 2 of these particles, freshly isolated symbiotic algae (FIS) and latex spheres (LS), resulted in a greater (2·5 ×) quantity of surface membrane interiorized than with heat-treated symbiotic algae (HTS) and free living algae (FA), Chlorella vulgaris. This discriminatory process was investigated further by a scanning electron microscope (SEM) and transmission electron microscope (TEM) comparison of the surface events associated with phagocytosis of each of these 4 particles. Those particles that were avidly phagocytized, FIS and LS, were both enveloped by a tightly fitting extension of digestive-cell surface, and obtained a prominent surface coating after their injection into the gut of Chlorohydra. Phagocytic challenge with FIS resulted, furthermore, in the rapid formation of a dense microvillar cover on digestive-cell surfaces. HTS and FA, on the other hand, were enveloped by a less closely fitting extension of digestive-cell surface, did not obtain a prominent surface coating, and did not induce the formation of microvilli. In addition, SEM revealed that at least 3 morphologically distinct phagocytic modes were utilized by the versatile nutritive phagocyte of Chlorohydra’. (1) envelopment by the progressive movement of numerous, overlapping tubular protrusions (microvilli) over the particle (FIS) surface, forming first a network of tubular interlocking members, and finally a continuous but rough enclosing surface; (2) envelopment by a single, smooth-surfaced, funnel-like extension of digestive-cell surface (FIS, LS, HTS, FA); and (3) envelopment by multiple, broad folds, often of unequal size, and with overlapping margins (Artemia particles).
The centriolar complex isolated from starfish spermatozoa
Authors: Kuriyama Ryoko , Kanatani Haruo
Published: April 2021

Centrioles from spermatozoa of the starfish, Asterina pectinifera, were isolated and partially purified by solubilization of chromatin followed by sucrose density-gradient centrifugation. The ultrastructure of the isolated centriolar complex was investigated in whole mount preparations by electron microscopy. The complex unit was composed of a pair of centrioles and a pericentriolar structure, which associated with the distal end of the distal centriole by 9 spokelike satellites extending radially to a marginal ring. Each satellite bifurcated at a dense node forming 2 fan-like shapes with a periodic striated pattern. The tubular structure of the centrioles easily disintegrated, leaving the pericentriolar structure or axonemal microtubules intact.

The distal centriole in a spermatozoon served as an initiating site for flagellar microtubule assembly; that is, a number of ‘9 + 2’ axonemal tubules were observed adhering just beneath the distal end of the basal body. In experiments in vitro, polymerization of microtubule proteins purified from porcine brain was initiated by the structure at the ends of both proximal and distal centrioles, but not from the satellites or the marginal ring. Also, few if any microtubules were formed from the sides of each centriole, even in the presence of a high concentration of exogenous tubulin. On the other hand, centrioles of spermatozoa, when they were in mature ooplasm, could initiate the formation of sperm asters by microtubules. Therefore, centrioles in spermatozoa seem to be able to initiate microtubules in 2 ways. A possible explanation of the difference between the 2 types of microtubule organization in vivo, i.e. in the sperm cell itself and in the ooplasm, is discussed.
Heterocyst differentiation and cell division in the cyanobacterium Anabaena cylindrical effect of high light intensity
Authors: Adams David G. , Carr Noel G.
Published: April 2021

Heterocyst differentiation in the cyanobacterium Anabaena cylindrica is initiated by the removal of fixed nitrogen from the medium. These specialized cells occur singly at regular intervals within filaments of vegetative cells. Incubation of cultures for periods of up to 12 h immediately prior to or following removal of fixed nitrogen, at a light intensity (500 μEinsteins cm−2 s−1) approximately 10-fold higher than that required for optimum growth, resulted in the differentiation of pairs of adjacent (double) heterocysts. The frequency of double heterocysts was proportional to the length of the period of high light intensity. During growth at normal light intensity approximately 5% of cell divisions were symmetrical, but this increased more than 3-fold during 10-h incubation at high light intensity. The frequency of dividing cells remained constant during this period, but increased rapidly on return to normal light. The frequency of double heterocysts was reduced if a period of incubation at normal light intensity was interposed between the 12-h period at high light intensity and transfer to nitrogen-free medium. A period of 8 h normal light was required to reduce the frequency of double heterocysts to control values, and this corresponded to the length of time needed for the frequency of symmetrical divisions to return to control levels following 12 h at high light intensity. We confirm that cell division in Anabaena cylindrica is asymmetrical and conclude that the presence of double heterocysts results from an increase in the symmetry of cell division during incubation at high light intensity. The results also support the finding of previous workers that a cell is only susceptible to differentiation during a short period following its formation. During the period of high light the rate of doubling of the absorbance of the culture at 750 mn increased from 24 h to approximately 10 h and decreased to more than 100 h on return to normal light. The very high rate could be explained by increases in the volume and granular content of cells during incubation at high light intensity and did not represent an equivalent increase in the rate of cell division.
Nuclear cycle of Saprolegnia ferax: I. Brent Heath
Authors: Rethoret Karen
Published: April 2021

The mitotic nuclear (equivalent to cell) cycle of the oomycete fungus, Saprolegnia ferax, was analysed by quantitative serial-section electron microscopy of hyphal nuclear populations synchronized by inhibition of DNA synthesis by fluorodeoxyuridine (FdUrd). Following telophase and karyokinesis, kinetochore mitrotubules persist into G, stage as a single group of ∼ 42 per nucleus (an = 42 for this species). During G1 the centrioles replicate and kinetochore microtubules separate into 2 groups of ∼ 21, a configuration they retain through S and G2,. During metaphase a new population of kinetochore microtubules are formed, each one of an amphitelic pair connecting to the opposite pole to that associated with the persistent microtubule from the previous division. Thus, by the end of metaphase, there are ∼ 42 kinetochore microtubules per half spindle. FdUrd, applied for 2 h with uracil, completely blocks DNA synthesis yet permits centriole replication and causes nuclei to accumulate with 2 pairs of centrioles, 2 arrays (each of 21) of kinetochore microtubules, and apparently enlarged nucleoli. Removal of FdUrd permits rapid (within 30 min) DNA synthesis followed by successive rounds of decreasingly synchronous nuclear cycles. These post-FdUrd cycles are 25 times longer than normal at 2·5 h, with S plus G2 being more extended than other phases. Calculated durations of a normal nuclear cycle are: G1, 33 min; S, 7 min; G2, 10 min; metaphase, 8 min; anaphase, 0·5 min; and telophase, 4 min.
Autosomal spindle fibres influence subsequent sex-chromosome movements in crane-fly spermatocytes
Authors: Sillers Peggy J. , Forer Arthur
Published: April 2021

In meiosis-I in crane-fly spermatocytes 3 autosomal half-bivalents move to each pole in anaphase while the 2 sex-chromosomal univalents remain at the equator. The sex chromosomes move to opposite poles only after the autosomes reach the poles; the sex chromosomes start to move polewards about 25 min after the autosomal half-bivalents have begun to move.

We irradiated portions of single autosomal spindle fibres with an ultraviolet microbeam and found that these irradiations altered the subsequent sex-chromosome movements. Two effects were observed. In one, one of the sex chromosomes did not move at all; the sex chromosome that remained at the equator would normally have moved to the pole associated with the irradiated autosomal spindle fibre. In the second, both sex chromosomes moved to the same pole, always that of the non-irradiated side. These effects occurred whether or not autosomal anaphase movement was blocked by the irradiation. There was no wavelength dependence for altering sex-chromosome movements.

Sex-chromosome movements were altered only when at least one sex-chromosomal spindle fibre was adjacent to the irradiated autosomal spindle fibre; when neither sex chromosome had a spindle fibre adjacent to the irradiated autosomal spindle fibre the sex chromosomes always moved normally. Irradiation of sex-chromosomal spindle fibres during sex-chromosomal anaphase showed short blockages of movement (usually 5–8 min), and then complete recovery. Direct irradiations of sex-chromosomal spindle fibres (without irradiating autosomal spindle fibres) when the autosomes were in anaphase but the sex chromosomes were in metaphase never caused abnormal sex-chromosome movements. These results eliminate the possibility that when we irradiated autosomal spindle fibres that were adjacent to sex-chromosomal spindle fibres the sex-chromosomal spindle fibres were irradiated inadvertently and were unable to recover from the damage.

We suggest that the irradiations of autosomal spindle fibres alter a control system involved in ‘turning on’ the sex-chromosomal spindle fibre motors, rather than directly altering the motors. We suggest that interactions between spindle fibres are somehow involved in this control system.
Influence of molecular charge upon the endocytosis and intracellular fate of peroxidase activity in cultured arterial endothelium
Authors: Davies Peter F. , Rennke Helmut G , Cotran Ramzi S.
Published: April 2021

The molecular charge of the macromolecule, horseradish peroxidase (HRPase, 40000 mol. wt), was modified to yield highly anionic (pl < 3·68) and cationic (pl = 9·5–10·5) derivatives. The effects upon the interactions between HRPase and arterial endothelium were then studied in vitro. The net rate of uptake of HRPase into endocytic vesicles and vacuoles of confluent endothelium was influenced by its molecular charge, there being less internalization of the anionic HRPase than of the native (pl = 7·9–8·2) and cationic derivatives. The molecular diameter was not significantly different between the cationic (Ae= 28·8 Å), anionic (Ae= 31·2 Å) and native (Ae= 29·6 Å) HRPase. The rate of uptake of [U-14C]sucrose, a tracer of bulk fluid endocytosis, was unaffected by the presence of the differently charged HRPase, indicating that the volume of vesicles formed per cell per hour remained constant. The intracellular fate of HRPase of different charge was investigated biochemically and morphologically. The rate of loss of internalized HRPase activity in the endothelial cells approximated first-order kinetics. The rate of disappearance of intracellular HRPase activity was much greater for cationic (t 1/2 = 8 h) and native (t 1/2 = 18 h) than for anionic HRPase (t 1/2 = 80-100 h). By electron microscopy, all 3 forms of HRPase were restricted to intracellular membrane-bounded vesicles and vacuoles consistent with a vesicle-lysosomal pathway. Studies with purified lysosomal cathepsin D indicated that the differences in the intracellular half-lives of HRPase may be attributable in small part to decreased and increased rates of lysosomal proteolysis of anionic and cationic HRPase, respectively, in comparison with native HRPase. Pre-labelling of endothelial secondary lysosomes by inhibitors of phagosome-lysosome fusion (dextran sulphate, polyglutamate) lengthened the intracellular half-life of native HRPase, while introduction of cationic ferritin to cells pulsed with anionic HRPase greatly decreased its half-life. Thus an influence of molecular charge upon endosome-lysosome fusion cannot be excluded. The studies indicate that the net charge carried by exogenous HRPase influences both its internalization in endocytic vesicles and its subsequent intracellular fate, which in turn may be modified by the introduction of other differently charged macromolecules. These results are discussed in relation to macromolecular transport by vascular endothelium in vivo.
Synthesis of an enzyme determined by an erythrocyte nucleus in a hybrid cell
Authors: Harris H. , Cook P. R.
Published: April 2021

When a chick erythrocyte nucleus is introduced into the cytoplasm of a mutant mouse cell lacking the enzyme inosinic acid pyrophosphorylase, synthesis of the enzyme is induced. However, this synthesis does not begin until the erythrocyte nucleus develops a nucleolus. The kinetics of synthesis of the enzyme are essentially similar to those previously described for the synthesis of surface antigens specified by the chick nucleus. The results of both sets of experiments indicate that the nucleolus plays a critical role in the transfer of information from the genes to the cytoplasm of the cell.
Microtubules and filaments in the filopodia of the secondary mesenchyme cells of Arbacia Punctulata and Echinarachnius Parma
Authors: Tilney L. G. , Gibbins J. R.
Published: April 2021

In an attempt to understand the mechanism of contraction of the filopodia of the secondary mesenchyme cells and thus secondary invagination of the archenteron, the fine structure of these processes was examined. Whereas microtubules are commonly encountered in the cell body and at the base of the filopodia, very few (one or two) are present near the tip of the filopodia. Instead the slender processes are filled with 50-Å filaments. Colchicine and hydrostatic pressure were applied to the embryos to elucidate the action of these different fibrous elements. Both agents cause cessation of archenteron movement and the disassembly of the microtubules. Hydrostatic pressure causes the disappearance of the filaments as well. Because of the small numbers of microtubules in the slender filopodia and the fact that in no other system is there any evidence for contraction of these elements, it was concluded that they do not function in the contraction process, but are probably involved in the formation of these cell extensions: hence the effect of colchicine on archenteron movement. The 50-Å filaments, on the other hand, are likely candidates for the contraction process.
The distribution of dna within the nucleoli of the amphibian oocyte as demonstrated by tritiated actinomycin d radioautography
Authors: Ebstein Bonnie S.
Published: April 2021

The distribution of DNA within the nucleoli of the newt oocyte has been studied using tritiated actinomycin D (3H-AD) radioautography. In the course of the study further evidence was found for the specificity of this technique. In many of the cytological preparations the specificity of binding was enhanced by extraction with ethanol after exposure to 3H-AD. The actinomycin D-DNA complex is somewhat unstable in cytological preparations. Incubation in buffers causes some dissociation of the complex; divalent cations accentuate the effect. The technique of 3H-AD radioautography was again shown to be more sensitive than Feulgen staining by the detection of 3H-AD binding in the Feulgen-negative forms of the nucleoli of the amphibian oocyte. The binding of 3H-AD to all of the nucleolar forms confirms the presence of DNA in these organelles. Results from this study and from others using [3H]uridine in the same system provide evidence that the nucleolar DNA is a detached loop-chromomere complex.
Formation and loss of dna in intestinal epithelium
Authors: Appleton T. C. , Pelc S. R. , Tarbit M. H.
Published: April 2021

The hypothesis that all nuclei containing labelled DNA in the jejunum of adult mice divide before migrating into the villi and that the amount of labelled DNA is constant during the passage of cells through the villi was tested experimentally. Mice were killed 40 min, 12, 18, 24, 30 and 36 h after a single intraperitoneal injection of [3H]thymidine. Grain counts in stripping film autoradiographs showed (i) considerably higher grain counts in the villi 24 h after injection than expected from those in the crypts at 40 min, and (ii) a loss of 32 % of the grain count during passage of the cells through the villi. Consideration of various factors which might influence the grain counts show that the findings have to be accepted as correct. It is suggested that a considerable proportion of the nuclei labelled in the crypts migrate into the villi without subsequent division and that labelled DNA is lost during passage through the villi. The hypothesis of formation of metabolic DNA during differentiation and loss during function provides an explanation of the findings.
Cytogenetic and nuclear studies on A9 and B82 cells fused together by sendai virus: The early phase
Authors: Engel E. , Mcgee B. J. , Harris H.
Published: April 2021

A 9 and B82 enzyme-deficient mutants of the L line of mouse cells were hybridized artificially in vitro. After exposure of the cells to the fusing agent (inactivated Sendai virus), cultures were grown in a selective medium with aminopterin, which allows the survival only of heterokaryons which contain both A and B nuclei. The chromosome make-up of the fused cells was then studied for an initial 3-month period under various conditions. Polykaryons with 2 or more nuclei were at first observed but, as the cultures progressed, only mononucleate cells (synkaryons) persisted. Most cells in mitosis showed the chromosomal set of one A and one B cell, with an average mode and chromosome make-up very similar to, but not exactly, the sum of the average s chromosome number of one A and one B nucleus (2s cells). As a rule, the synkaryons contained three ‘dot-like’ chromosomes, two of which were probably derived from the A cell and one from the B cell.

In the course of the 3-month study the relative frequency of the 2S cells and their average modal number varied little. However, an increasing number of 2s cells with abnormally shaped chromosomes, in particular new ring chromosomes, was noted. Some of the cells had a mode intermediate between the 2s number and the s number (hypo-2S cells). They were thought to represent new segregants, but they were scarce, and their frequency did not increase when established fused cells were removed from the selective medium and then grown for up to 3 weeks.

Nuclear polymorphism was present in all cultures and certain cytological observations gave the impression that fractions of the genomes of fused cells could be rejected through nuclear fragmentation.

Synkaryons resulting from cell fusion induced by Sendai virus showed a remarkably stable chromosomal constitution for the first 3 months of culture, and were endowed with striking reproductive fitness.
The role of the nucleolus in the transfer of RNA from nucleus to cytoplasm
Authors: Sidebottom E. , Harris H.
Published: April 2021

The role of the nucleolus in the transfer of RNA from nucleus to cytoplasm was examined by means of experiments in which inactivation of the whole nucleus, or of the nucleolus, was achieved by a microbeam of ultraviolet light. In heterokaryons in which a chick erythrocyte nucleus had been reactivated, no detectable amount of RNA was transferred from the reactivated nucleus to the cytoplasm of the cell until this nucleus had developed a nucleolus. In HeLa cells, inactivation of the nucleolus alone inhibited the transfer to the cytoplasm, not only of the RNA synthesized at the nucleolar site, but also of the RNA made elsewhere in.the nucleus. It thus appears that the nucleolus governs the transport not only of its own RNA, but also of the RNA which is made on other parts of the chromosomes.
Linear synthesis of sucrase and phosphatases during the cell cycle of Schizosaccharomyces Pombe
Authors: Mitchison J. M. , Creanor J. , Williams D. A.
Published: April 2021

The synthesis of sucrase, acid phosphatase and alkaline phosphatase has been followed in synchronous cultures of the fission yeast Schizosaccharomyces pombe prepared by gradient sedimentation. These three enzymes follow a linear pattern of synthesis through the cell cycle, with a doubling in rate at a ‘critical point’ about one-fifth of the way through the cycle.

Sucrasecan be rapidly derepressed by lowering the glucose concentration in the medium. This has been used to measure the sucrase ‘potential’ or capacity to synthesize sucrase on derepression. The potential exists at all times in the cycle, and follows a stepwise pattern with a sharp rise at the critical point.

These results suggest that the functional genome doubles at the critical point. Since, however, the period of DNA synthesis is nearly one-third of a cycle before this point, there must be an appreciable delay between chemical replication and functional replication of the genome. In this respect S. pombe, a eukaryotic cell, differs markedly from bacteria.

Other physiological events take place near the critical point, and a tentative model is suggested of what may be happening at the chromosomal level.

Experiments with cycloheximide indicate that there is a delay between the synthesis and the appearance of the active enzyme in the case of sucrase and alkaline phosphatase.