bluuebunny/crossref_metadata_2025 huggingface dataset:
Specific sequences in p120ctn determine subcellular distribution of its multiple isoforms involved in cellular adhesion of normal and malignant epithelial cells
Authors: Aho Sirpa , Levänsuo Laura , Montonen Outi , Kari Csaba , Rodeck Ulrich , Uitto Jouni
Published: April 2021

P120 catenin (p120ctn) belongs to the Armadillo family of proteins, which is implicated in cell-cell adhesion and signal transduction. Owing to alternative splicing and multiple translation initiation codons, several p120ctn isoforms can be expressed from a single gene. All p120ctn isoforms share the central Armadillo repeat domain but have divergent N- and C-termini. Little is known about the biological functions of the different isoforms. In this study, we examined the distribution of various p120ctn isoforms and the consequences of their expression in cultured cells of epidermal origin. Immunohistochemical analysis and western blotting revealed that melanocytes and melanoma cells primarily express the long isoform 1A, whereas keratinocytes express shorter isoforms, especially 3A, which localize to cell-cell adhesion junctions in a calcium-dependent manner. The shortest isoform 4A, which was detected in normal keratinocytes and melanocytes, was generally lost from cells derived from squamous cell carcinomas or melanomas. The C-terminal alternatively spliced exon B was present in the p120ctn transcripts in the colon, intestine and prostate, but was lost in several tumor tissues derived from these organs. To test whether p120ctn isoforms serve in distinct biological functions, we transiently transfected the expression constructs into melanoma cells (1205-Lu) and immortalized keratinocytes (HaCaT). Indeed, distinct domains of p120ctn are responsible for its different biological functions. The prominent branching phenotype was induced equally by isoforms 1A, 2A and 3A, whereas the shortest isoform 4A,which was devoid of the N-terminal domain, completely lacked this ability. Also, the exon-B-encoded sequences, as in the isoform 1AB, were sufficient to abolish the branching phenotype as induced by the isoform 1A. The induction of the branching phenotype cosegregated with the nuclear localization of the p120ctn isoforms 1A, 2A and 3A, whereas the isoforms 4A and 1AB, which were excluded from the nucleus, did not induce the branching phenotype. The N-terminal sequences that contain seven out of eight tyrosine residues,recently characterized as potential candidates for phosphorylation by Src kinase, are required for the nuclear localization and for the formation of the branching phenotype. Finally, expression of the p120ctn isoforms, which caused the branching phenotype, was associated with cellular relocalization of E-cadherin in HaCaT cells. Collectively, we have identified sequences within the p120ctn N-terminus that are prerequisites for both nuclear localization and the p120ctn-induced branching phenotype. Loss of the cytoplasmic pool of p120ctn from tumor cells suggests an important function for such isoforms in normal cells and tissues.
Cytoplasmic dynein-associated structures move bidirectionallyin vivo
Authors: Ma Shuo , Chisholm Rex L.
Published: April 2021

Intracellular organelle transport is driven by motors that act upon microtubules or microfilaments. The microtubulebased motors, cytoplasmic dynein and kinesin, are believed to be responsible for retrograde and anterograde transport of intracellular cargo along microtubules. Many vesicles display bidirectional movement; however, the mechanism regulating directionality is unresolved. Directional movement might be accomplished by alternative binding of different motility factors to the cargo. Alternatively,different motors could associate with the same cargo and have their motor activity regulated. Although several studies have focused on the behavior of specific types of cargoes, little is known about the traffic of the motors themselves and how it correlates with cargo movement. To address this question, we studied cytoplasmic dynein dynamics in living Dictyostelium cells expressing dynein intermediate chain-green fluorescent protein (IC-GFP) fusion in an IC-null background. Dynein-associated structures display fast linear movement along microtubules in both minus-end and plus-end directions, with velocities similar to that of dynein and kinesin-like motors. In addition, dynein puncta often rapidly reverse their direction. Dynein stably associates with cargo moving in both directions as well as with those that rapidly reverse their direction of movement, suggesting that directional movement is not regulated by altering motor-cargo association but rather by switching activity of motors associated with the cargo. These observations suggest that both plus- and minus-end-directed motors associate with a given cargo and that coordinated regulation of motor activities controls vesicle directionality.
The projection domain of MAP2b regulates microtubule protrusion and process formation in Sf9 cells
Authors: Bélanger Dave , Farah Carole Abi , Nguyen Minh Dang , Lauzon Michel , Cornibert Sylvie , Leclerc Nicole
Published: April 2021

The expression of microtubule-associated protein 2 (MAP2), developmentally regulated by alternative splicing, coincides with neurite outgrowth. MAP2 proteins contain a microtubule-binding domain (C-terminal) that promotes microtubule assembly and a poorly characterized domain, the projection domain(N-terminal), extending at the surface of microtubules. MAP2b differs from MAP2c by an additional sequence of 1372 amino acids in the projection domain. In this study, we examined the role of the projection domain in the protrusion of microtubules from the cell surface and the subsequent process formation in Sf9 cells. In this system, MAP2b has a lower capacity to induce process formation than MAP2c. To investigate the role of the projection domain in this event, we expressed truncated forms of MAP2b and MAP2c that have partial or complete deletion of their projection domain in Sf9 cells. Our results indicate that process formation is induced by the microtubule-binding domain of these MAP2 proteins and is regulated by their projection domain. Furthermore, the microtubule-binding activity of MAP2b and MAP2c truncated forms as well as the structural properties of the microtubule bundles induced by them do not seem to be the only determinants that control the protrusion of microtubules from the cell surface in Sf9 cells. Rather, our data suggest that microtubule protrusion and process formation are regulated by intramolecular interactions between the projection domain and its microtubule-binding domain in MAP2b.
BH3-only proteins — evolutionarily conserved proapoptotic Bcl-2 family members essential for initiating programmed cell death
Authors: Bouillet Philippe , Strasser Andreas
Published: April 2021

The BH3-only members of the Bcl-2 protein family are essential initiators of programmed cell death and are required for apoptosis induced by cytotoxic stimuli. These proteins have evolved to recognise distinct forms of cell stress. In response, they unleash the apoptotic cascade by inactivating the protective function of the pro-survival members of the Bcl-2 family and by activating the Bax/Bax-like pro-apoptotic family members.
Delineation by lanthanum staining of filamentous elements associated with the surfaces of axonal microtubules
Authors: Burton P. R. , Fernandez H. L.
Published: April 2021

The major formed elements of axons of the ventral nerve cord of the crayfish are longitudinally oriented microtubules, filamentous matrix material, and mitochondria. Lanthanum staining delineates both microtubules and filamentous material to great advantage, and it is demonstrated that filamentous elements continuous with matrix material are associated with the surfaces of microtubules. Microtubules are interconnected by filamentous elements and connexions also exist between microtubules and the plasma membrane to form a 3-dimensional latticework. This latticework is hypothesized to play a role in translocation of materials down the axon: the filamentous material probably provides the gelatinous integrity of the axoplasm and may provide the motive force for axonal transport, with the microtubules serving as attachment sites and guideways extending the length of the axon. In pieces of nerve cord treated with cytochalasin B, the filamentous material of the axoplasm is structurally unchanged. Treatment of pieces of nerve cord with hyaluronidase, however, induces the formation of large tubular elements (macrotubules), 40–50 ran in diameter, with which lateral filamentous material is also associated.
The effects of adapting human diploid cells to grow in glutamic acid media on cell morphology, growth and metabolism
Authors: Griffiths J. B.
Published: April 2021

Two lines of human diploid cells, the W1-38 and MRC-5, were adapted to utilize glutamic acid in place of glutamine. This adaptation resulted in (a) more cells per unit culture area, (b) an alteration in cell size and protein content, (c) a morphological change of the cells from fibroblastic to epithelial-like, and (d) increased metabolic activity. Changes in the agglutin-ability of adapted cells by 3 lectins together with the results from polyacrylamide gel electrophoresis could be interpreted as a change in plasma membrane structure. Comparative studies of unadapted, adapted and transformed cells showed that adaptation to glutamic acid produced cells with a metabolism and amino acid uptake similar to those of transformed cells. These changes were reversible and were not accompanied by any apparent karyological change. The significance of these results for the study of density-dependent inhibition of growth is discussed.
Evidence for the importance of puromycyl peptides in the inhibition by ptjromycin of cell aggregation In Vitro
Authors: Dunn M. J. , Owen E. , Kemp R. B.
Published: April 2021

Trypsin-dissociated cells from the muscle tissue of 9-day-old chick embryos were employed to investigate the effects of cycloheximide and a puromycin-cycloheximide mixture on cell aggregation, protein synthesis and respiratory metabolism.

Cycloheximide when introduced at a concentration of 10 μg/ml into a suspension of cells in Eagle’s MEM inhibited aggregation by 25 % at 24 h. At this time an inhibition of 40 % was apparent in the presence of a mixture of cycloheximide and puromycin both at a concentration of 10 μg/ml.

Both cycloheximide and the cycloheximide-puromycin mixture arrested protein synthesis of rotated cells by 90 % within 15 min of introducing the antibiotics into cell suspensions. The antibiotics retained their inhibitory effects on protein synthesis for the 24-h period of rotation.

Cycloheximide inhibited cellular oxygen uptake and carbon dioxide evolution of rotated cells by 25 % at the end of the 24-h experimental period. At this time an inhibition of 30 % was observed in the presence of the cycloheximide-puromycin mixture. The release of radioactive carbon dioxide by cycloheximide-treated cells was inhibited by 46 % at 24 h. In the presence of the antibiotic mixture, 14CO2 release was inhibited by 30 % at 4 h, but after 8 h very little further 14CO2 was evolved.

As a control, puromycin (10μg/ml) inhibited cell aggregation and respiration to an extent similar to that previously reported.

The results are discussed in terms of puromycyl peptides producing a metabolic effect on cell aggregation. It is considered that this is additional to the effect of puromycin inhibiting aggregation through the arrest of protein synthesis.
Intermediate cells of the pancreas: II. the effects of dietary soybean trypsin inhibitor on acinar-β cell structure and function in the rat
Authors: Melmed R. N. , Turner R. C. , Holt S. J.
Published: April 2021

The well known hypertrophy of the rat exocrine pancreas caused by dietary soybean trypsin inhibitor (STI) has now been shown to be accompanied by an increased prominence of acinar-β intermediate cells and a reduction in the size of the islets of Langerhans. The acinar-β cells are situated in the exocrine tissue surrounding the islets where the acinar cells in STI-fed rats show a greater number of zymogen granules than in acinar cells elsewhere in the pancreas or in peri-islet acinar cells of control rats.

Consistent with the reduction in islet size in STI-fed rats is a lowered insulin content of the pancreas and a lowered insulin secretion following the intravenous administration of glucose. However, the release of insulin into the blood and pancreatic juice after intravenous administration of the hormone secretin is similar in both STI-fed and control rats.

The results of these morphological and functional studies are discussed in relation to the possible contribution of the acinar-β cell to the functional insulin pool.
Origin and development of kineto somes in Oxytricha fallax
Authors: Grimes G.W.
Published: April 2021

Two significant features of the origin and development of kinetosomes in the ciliate, Oxytricha fallax, are indicated by electron micrography of serial sections. (1) Some kinetosomes arise in varied orientations to adjacent kinetosomes instead of in the single orientation regularly found in other material; and some kinetosomes arise in positions that are not adjacent to any kineto some. Hence, even in ciliates, some kinetosomes arise without using existing kinetosomes or their micromilieu as nucleation sites. (2) Growth of kinetosomal microtubules appears to occur at their proximal as well as distal ends. This is indicated by proximal cross-sections with only one or two microtubules of a triplet, while more distal sections of the same incompletely grown kinetosome show the complete triplet pattern.
Ultrastructure of meiosis and centriole behaviour in Ulva mutabilis fØyn
Authors: Bråten T. , Nordby Ø.
Published: April 2021

The present work deals with the ultrastructure of the meiotic processes leading to the formation of zoospores. The formation of an exit pore on the outer surface of the sporangium is the first visible sign that the cell will undergo meiosis and not just a somatic division. Prophase I nuclei differ from nuclei in mitotic prophase by having one or several invaginations of the nuclear membrane. Electron-dense lines probably representing the lateral elements of the synaptinemal complex are observed during prophase I. The central element of the complex has, however, never been observed.

The fate of the centrioles during meiosis is described in detail. The number of duplications of the centrioles is found to be the same at meiotic and mitotic zooid formation. The number of cell divisions is, however, different in the 2 cases.
The synaptonemal complex and the achiasmatic condition
Authors: Stack S.
Published: April 2021

The onion species AUium amplectans includes both a triploid and a tetraploid variety. By light microscopy both varieties appear to have normal synapsis during pachytene of meiosis. However, the triploid does not form chiasmata and exhibits almost total asynapsis following pachytene. The tetraploid forms at least one chiasma per homologue and retains pairing through metaphase I. Electron-microscopic examination of pachytene nuclei in these 2 varieties reveals apparently identical synaptonemal complexes. Three-dimensional reconstructions of chromosome arrangements in triploid pachytene nuclei confirm that synapsis is as complete as could be expected in an autotriploid. These observations give firm support to the hypothesis that the presence of apparently structurally normal synaptonemal complexes is not a sufficient prerequisite to ensure chiasma formation. It is suggested that a faulty or missing endonuclease which is normally involved in crossing over is responsible for the achiasmatic condition in triploid A. amplectans.
An ultrastructural and radio autographic study of the effect of ethidium bromide on the interphase nucleus of meristematic plant cells
Authors: Lord A. , Lafontaine J. G.
Published: April 2021

In the present work the effects of an intermediate dose (20 μg/ml) of ethidium bromide on nuclear structures of meristematic plant cells (Raphanus sativtis) have been studied under both light and electron microscopy. Radioautography was also exploited to assess the action of this drug on the incorporation activity of nucleic acid precursors.

After a 30-min treatment with ethidium bromide, no noticeable ultrastructural modifications are detected but the incorporation of both thymidine and uridine is already markedly impaired. As treatment is extended to 1 h, the numerous light lacunae characterizing the fibrillar nucleolar zones in normal cells transform into elongated areas which are often found to extend to the surface of this organelle and are continuous with perinucleolar chromatin masses. After 2 h, many small bodies consisting of granular material appear in the nucleoplasm. The nucleolus gradually loses most of its granular component and, eventually, becomes predominantly fibrillar in texture; in many cells it breaks up into a number of irregular masses. Throughout various stages of the experiment, certain cells show small spherical nucleoli characterized by the presence on their surface of one or two conspicuous reticulate masses which are heavily labelled with thymidine and, consequently, consist mostly of chromatin. In view of the fact that ethidium bromide has been observed in this study to induce a certain degree of condensation of chromatin within the nuclear cavity, it is surmised that this drug also brings about migration of intranucleolar DNA to the surface of this organelle.
The spermatozoon of arthropoda. XXII. the 12 + 0’, ‘14 + 0’ or aflagellate sperm of protura
Authors: Baccetti B. , Dallai R. , Fratello B.
Published: April 2021

The spermatozoa of three genera of Protura are unconventional. Two of them, Acerentulus and Acerentomon, display hitherto undescribed axoneme patterns: ‘12 + 0 ‘in the former, ‘14 + 0’ in the latter. All the doublets are devoid of arms. The third genus, Eosentomon has a disk-shaped spermatozoon, devoid of tail. The presence of such unusual patterns is related to the non-motility of the spermatozoa.
The effect of partial extraction of dynein arms on the movement of reactivated sea-urchin sperm
Authors: Gibbons Barbara H. , Gibbons I. R.
Published: April 2021

Sea-urchin sperm were extracted with 0 · 5 M KC1 for 45 s at room temperature in the presence of Triton X-100, and then transferred to reactivating solution containing 1 mM ATP. The flagellar beat frequency of these KCl-extracted sperm (16 beats/s) was only about half that of control Triton-extracted sperm that had not been exposed to 0 · 5 M KC1 (31 beats/salthoughthe form of their bending waves was not significantly altered. Examination by electron microscopy showed that the extraction with 0 · 5 M KC1 removed the majority of the outer arms from the doublet tubules, leaving the inner arms apparently intact. By varying the duration of the KCl-extraction, it was shown that the rate of decrease in beat frequency paralleled the rate of disappearance of the arms. Prolonging the extraction time beyond 45 3 at room temperature, or 4 min at o°C, had little further effect on beat frequency. ATPase measurements suggested that 60—65 % of the dynein in the original axonemes had been solubilized when the extraction with KC1 was permitted to go to completion. These results indicate that the generation and propagation of flagellar bending waves of essentially typical form are not prevented by the removal of the outer row of dynein arms from the doublet tubules. In terms of the sliding filament model of flagellar bending, the results suggest that the rate of sliding between tubules under these conditions is proportional to the number of dynein arms present. The lack of significant change in wave form implies that the total amount of sliding that occurs during each bending cycle is not affected by the reduced number of dynein arms, but is regulated independently in some manner by the elastic forces generated by other structures in the bent axoneme.
Plasma membrane fluidity and surface motility of mouse C-1300 neuroblastoma cells
Authors: Comoglio P. M. , Filogamo G.
Published: April 2021

Under suitable conditions, topographical redistribution of plasma membrane molecules with oligosaccharide residues specifically bound by Concanavalin-A can be induced on neuroblastoma C-1300 cells. This shows that transformed nerve cell membrane is fluid at 37 °C and provides further support for the suggestion that such fluidity may be a common property of all animal cells. As previously reported for mesenchymal cells (lymphocytes and fibroblasts), clustering of Concanavalin-A binding sites on the cell surface is dependent on temperature and on reagent concentration. The formation of larger aggregates, known as patches or polar caps, requires metabolic energy provided by the cellIn fibroblasts caps are formed in as little as 15–30 min. C-1300 neuroblastoma cells, however, need several hours of incubation before patches or, more rarely, single caps appear. The mechanism of formation of patches and caps is discussed with reference to cell membrane fluidity and the possibility that there exists a membrane motility characteristic of each cell type.
Lethality of adenosine for cultured mammalian cells by interference with pyrimidine biosynthesis
Authors: Ishii K. , Green H.
Published: April 2021

Adenosine at low concentration is toxic to mammalian cells in culture. This may escape notice because some sera (such as calf or human) commonly used in culture media, contain adenosine deaminase. In the absence of serum deaminase, adenosine produced inhibition of growth of a number of established cell lines at concentrations as low as 5 × 10 − 6 M, and killed at 2 × 10 −5 M. This effect required the presence of cellular adenosine kinase, since a mutant line deficient in this enzyme was 70-fold less sensitive to adenosine. The toxic substance is therefore derived from adenosine by phosphorylation, and is probably one of the adenosine nucleotides.

The toxic effect of adenosine in concentrations up to 2 × 10 −4 M was completely prevented by the addition of uridine or of pyrimidines potentially convertible to uridine, suggesting that the adenosine was interfering with endogenous synthesis of uridylate. In the presence of adenosine, the conversion of labelled aspartate to uridine nucleotides was reduced by 80–85 % and labelled orotate accumulated in both the cells and in the culture medium. The lethality of adenosine results from inhibition by one of its nucleotide products of the synthesis of uridylate at the stage of phosphoribosylation of orotate.
Pollen-wall proteins: release of the allergen antigen e from intine and exine sites in pollen grains of ragweed and Cosmos
Authors: Howlett B. J. , Knox R. B. , Heslop-Harrison J.
Published: April 2021

Cytochemical staining methods and immunofluorescence techniques have been used to follow the release of proteins, including the allergen Antigen E, from whole pollen of ragweeds (Ambrosia spp.) and Cosmos bipinnatus (Compositae). These proteins have been detected in 2 sites in the wall: in the inner cellulosic intine layer especially at the germinal apertures, and in the cavea and the cavities of the outer exine layer, the sexine. It was found that the release of exine-associated proteins, including the allergenic fraction, begins within 2 8 of contact with aqueous media, the loss being mainly through the micropores of the tectum at the bases of the sexine spines. The intine proteins were emitted more slowly, discharge beginning at the colpi after 1 min of moistening. So far as can be judged from the methods used, most of the mobile protein lost from the pollen of Cosmos on hydration comes from the exine and is therefore sporophvtic in origin, while in the ragweeds the bulk comes from the intine in the neighbour-hood of the germinal apertures, and so is gametophytic.
Axonal microtubules and associated filaments stained by alcian blue
Authors: Hinkley R. E.
Published: April 2021

Delicate filamentous projections 3–8 nm in thickness radiate laterally from the surface of microtubules in crayfish ventral axons and anastomose with similar filaments projecting from neighbouring microtubules and with unattached filaments in the axoplasmic matrix. Following Alcian blue treatment, a layer of alcianophilic material 5–30 nm thick was revealed on the axoplasmic surface of all intra-axonal structures, including microtubules, and the filamentous projections possessed significantly enhanced contrast. In Alcian blue-penetrated axons these prominent filamentous projections were organized into a conspicuous axoplasmic network connecting microtubules up to 350 nm apart. Intra-axonal alcianophilia was nearly abolished when connectives were stained in the presence of 0·1–0·2 M MgCl2 or when pretreated with hyaluronidase. The filamentous elements and the exterior surface of microtubules retained some degree of alcianophilia when stained at pH 5·5 and 4·1 but few deposits were associated with intra-axonal membrane systems. These observations are consistent with the hypothesis that the projecting filamentous elements and perhaps the exterior surface of the microtubule walls have, at least in part, a carboxylated polyanionic character.
Further observations on the fine structure of freeze-cleaved tight junctions
Authors: Staehelin L. A.
Published: April 2021

The fine structure of freeze-cleaved tight junctions has been examined in high-resolution replicas of rat small intestine. By carefully comparing the changes in the surface topography of the cleavage faces of specimens, which, before freezing, were either prefixed with glutaraldehyde and then infiltrated with glycerol, infiltrated with glycerol alone, or infiltrated with glycerol and then fixed with glutaraldehyde, a more precise definition of the tight-junction architecture at the supramolecular level has become possible. The results of this analysis suggest that the bilayer membranes contributing to a tight junction are held together along interconnected fines of attachment that are arranged in the form of a continuous band-like meshwork. Each line consists of 2 parallel rows, one in each membrane, of closely spaced adhesion particles. The sensitivity of these particles to glutaraldehyde and their cleaving behaviour under different experimental conditions indicate that they represent globular proteins which bridge the width of the adjoining membranes and are linked together in the plane of the intercellular space. Thus, the morphology of a tight-junction seal resembles a modified zipper with the locking units making head to head contact. Similarly, the presence of many open-ended sealing elements between crypt cells has been interpreted as suggesting that the formation of tight-junction seals could resemble a ‘zippering-up’ process.

At the juncture of 3 cells the tight-junction meshwork is both modified and extended basally to produce a characteristic pattern. In the central area of such a triple junction, 3 parallel and very closely spaced vertical seals have been resolved, each joining a pair of adjacent plasma membranes. Small, regularly spaced, cross-bridging elements interlink pairs of central seals within the plane of each membrane. At different levels each individual central seal may further give rise to horizontal sealing elements which connect in a ladder-like fashion either to the major network or to vertical elements positioned at greater distances from the central axis.

Evidence is presented suggesting that fragments of tight junctions can be internalized and broken down in lysosome-like vesicles.
An ultrastructural autoradiographic study of the incorporation of [35S]cystine in the wool fibre cortex
Authors: Chapman R. E. , Gemmell R. T.
Published: April 2021

Previous autoradiographic and chemical studies gave incompatible results as regards the incorporation of cystine into and cystine content of the 2 cortical segments of the wool fibre. In this study the incorporation of [35S]cystine into the wool fibre cortex was therefore re-examined by electron-microscope autoradiography. Skin samples were taken from a Merino sheep 1 h and 5 h after intradermal injections of L-[35 S]cystine. At both times there was very little incorporation of 35S in the follicle bulbs. By 5 h incorporation occurred distally from the suprabulbar region throughout the zone of macrofibril (filament bundle) formation. More 35S was incorporated per unit area in the paracortex than in the orthocortex, and at the level of maximal uptake near the middle of this zone there was about a 2-fold difference per unit area. However, when the relative cross-sectional areas of the cortical segments were also considered, the actual amount of 35S incorporated at this level was slightly greater in the orthocortex than in the paracortex. These differences in the incorporation of [35S]cystine by the cortical segments agreed with previous results from chemical studies on cortical fractions separated from wool.
Studies of the effect of temperature shocks on preparation for cell division in mouse fibroblast cells (l cells)
Authors: Miyamoto Hiroshi , Rasmussen L. , Zeuthen E.
Published: April 2021

As L cells go through their growth-division cycle they acquire the capacity to respond progressively more strongly to certain standard changes in the temperature of the environment. Using techniques described earlier, we found that chilling to 1, 6 or 10 °C for 1 h had little effect on the timing of the forthcoming division. Conversely, heating for 1 h to temperatures between 41 and 42 °C had a strong effect. Generally, the older the cell when heated, the more extended is its generation time; in other words, the longer is the forthcoming division postponed. We found evidence that late in the cycle the cells undergo transition from a state in which they are maximally delayed with respect to the performance of a division to one in which they are less delayed.

We attempted to synchronize cell divisions with single and with series of heat shocks (41·6 °C for 1 h). Like our predecessors in the field, we obtained only partial synchrony. However, because L cells appear to prepare for division between shocks, and because heat shocks tend to reverse such preparations for division, we find reason to continue these experiments, using previous experience with Tetrahymena and Schizosaccharomyces as a guide. Both the latter cells respond to proper temperature treatment with synchronous cell division.
Transcription and translation of the sex message in the smut fungus, Ustilago violacea: I. the effect of ultraviolet light
Authors: Day A. W. , Cummins J. E.
Published: April 2021

Cells of opposite mating type, a1 and a2, of the anther smut, Ustilago violácea assemble a conjugation tube after about 3–4 h of mating on nutrient-free media. Low doses of ultraviolet light (u.v.) delay but do not prevent conjugation in wild-type strains if given in the first 2–3 h of mating. However, irradiation later than this period has little effect and conjugation proceeds normally. The u.v. effect is photoreactivable and we conclude that u.v. induces dimers which affect transcription of specific messenger RNA molecules needed for conjugation (‘sex message ‘). Our evidence suggests that the dimers may cause mistranscription rather than the complete prevention of transcription. The effect of u.v. on conjugation in reciprocal crosses of u.v.-sensitive and wild-type strains indicates clearly that both partners must complete transcription of sex messages in order to conjugate. Inactivation of either partner before transcription prevents conjugation, but conjugation proceeds when either cell is inactivated after transcription of the sex messages has occurred. These results suggest that a mutual and reciprocal exchange of information between the 2 mating-types occurs prior to the assembly of the conjugation tube.
The process of selection of erythromycin-resistant mitochondria by erythromycin in Paramecium
Authors: Perasso R. , Adoutte A.
Published: April 2021

Mitochondrial mutations conferring erythromycin resistance (ER) are available in Paramecium and it is possible to obtain (by conjugation and cytoplasmic exchange) exconjugant cells containing a majority of wild-type erythromycin-sensitive (ES) mitochondria and a minority of E? ones. In the presence of erythromycin, such ‘mixed’ cells progressively become resistant.

This process of acquisition of resistance has been studied cytologically (on thin sections of single cells) and genetically (by evaluating, on the basis of previous data, the proportion of ER/ES mitochondrial genomes at various times).

While at early stages of the process of transformation the whole mitochondrial population appears rather homogeneous, at later stages, (i.e. when the cell has resumed growth in the antibiotic-containing medium) one finds, side by side, both resistant-looking mitochondria (structurally normal) and sensitive-looking ones, showing the typical alterations induced in ES cells by erythromycin. Conversely, a progressive decrease in the number of Es genomes can be demonstrated.

The complete genetical and cytological transformation from erythromycin sensitivity to erythromycin resistance can occur after less than three fissions in erythromycin-containing medium.

The results indicate that intensive selective multiplication of ER mitochondria occurs, under the pressure of erythromycin, virtually in the absence of cellular division. The possibility of dissociating mitochondrial division from cell division emphasizes the extent of mitochondrial autonomy.
Different kinds of heterochromatin in higher plant chromosomes
Authors: Stack S. M. , Clarke C. R. , Cary W. E. , Muffly J. T.
Published: April 2021

After the use of different Giemsa staining techniques, variations in chromosome banding patterns have often been observed in animal chromosomes. Such staining differences are usually interpreted to indicate that there is more than one type of heterochromatin in many animal chromosomes. Using two differential Giemsa staining techniques we have found different staining patterns in the chromosomes of two higher plants, Allium cepa and Ornithogalum virens. Furthermore, pericentric heterochromatin that occurs so commonly in animal chromosomes was specifically Giemsa stained in O. virens. These results suggest the basic similarity of higher plant and animal chromosomes.
Cell renewal in the gill of the freshwater mussel, Margaritifera margaritifera: An autoradiographic study using high specific activity tritiated thymidine
Authors: Tomasovic S. P. , Mix M. C.
Published: April 2021

High specific activity tritiated thymidine (50·3 Ci/mM and 56 Ci/mM) and autoradiographic techniques were used to study cell renewal in the gill epithelium of the freshwater mussel, Margaritifera margaritifera.

The cell renewal system in the gill epithelium of M. margaritifera appears to consist of a stem-type population in the gill furrow and gill furrow edges which supplies, through division, cells for a maturing, dividing transient transitional population along the proximal gill ridge sides which, in turn, supplies cells to a simple transient, differentiated, functional population on the distal gill ridge sides and tip. Loss of cells from the cell renewal system appears to be through cell death and/or extrusion from the gill ridge tip. No emigration or immigration of labelled nuclei out of, or into, the gill epithelium was observed. The minimum transit time from the dividing transient population to the functional population in the gill ridge tip may be no more than 24 h.

We were unable to detect any radiobiological effects or the presence of cytoplasmic labelling due to the use of high specific activities. However, such possibilities cannot be eliminated from consideration in further studies.
The cytoproct of Paramecium caudatum: Sxstructure and function, microtubules, and fate of food vacuole membranes
Authors: Allen Richard D. , Wolf R. W.
Published: April 2021

The cytoproct or cell anus of Paramecium caudatum was studied, using light optics and electron microscopy, at known times before, during and following food vacuole egestion. This was accomplished by microscopically observing single cells, fixing these cells at specific times and finally serial sectioning these individually processed cells. The cytoproct, at rest, is a long narrow ridge along the posterior suture. It contains 2 uniquely positioned components which identify this structure as the cytoproct: piles of fibres along the inside surfaces of the ridge, and microtubules passing from the epiplasm at the summit of the ridge down into the cytoplasm. The plasma membrane is continuous over the top of the ridge. The cortical basal bodies adjacent to the ridge have bundles of microtubules passing into the cytoplasm from an opaque plaque at their proximal ends. These 2 sets of microtubules may function in guiding the food vacuoles to the cytoproct. A model is presented in which motive forces generated between the microtubules and the food vacuole membrane bring the food vacuole to the cytoproct and, in addition, pull the cytoproct lips apart so that the food vacuole membrane and plasma membrane come into contact and fuse together, thus opening the food vacuole to the outside. The plasma membrane increases in area between the parting lips, possibly, as the result of intercalation of membrane vesicles into the plasma membrane at the top of the ridge. Immediately after this opening is formed the food vacuole membrane changes from a smooth topography to a highly convoluted one. The membrane is engulfed through a process of endocytosis resulting in an accumulation of membranous fragments in the cytoplasm below the cytoproct. The endocytic forces probably bring about the restitution of the cytoproct ridge by pulling the lips back together as the membrane is engulfed. A filamentous meshwork underlying the food vacuole membrane may be active in this endocytic process.
Cell surfaces and fibre relationships in sympathetic ganglion cultures: a scanning electron-microscopic study
Authors: Hill Caryl E. , Chamley Julie H. , Burnstock G.
Published: April 2021

Sympathetic ganglia from newborn rats and guinea-pigs were grown in modified Rose chambers and examined with scanning electron microscopy after 5–7 days. The cell types seen were macrophages, neurons, glial cells and connective tissue cells. They presented a wide range of surface morphologies and 3-dimensional configurations, from spheroid with an irregular surface to flattened with a smooth surface. The arrangement of the nerve fibres and cells in the outgrowth was essentially 2-layered with connective tissue cells nearest the substrate and nerve fibres, glial cells and macrophages lying over them. The relationships of sympathetic nerve fibres to the different cell types were also investigated. In all cases nerve fibres closely followed the cellular surface contours although the nature of the relationships varied. Fine finger-like cytoplasmic projections were sometimes seen from connective tissue cells and macrophages. The possible role of these structures in adhesion and motility is discussed.
Filament formation by slime mould myosin isolated at low ionic strength
Authors: Hinssen H. , D’Haese J.
Published: April 2021

Myosin was isolated and purified from plasmodia of the slime mould Physarum polycephalum by a new method. This method is based on actomyosin extraction at low ionic strength after extensive washing, followed by the selective precipitation of myosin at pH 6·1 under relaxing conditions. The yield of myosin was 3–5 times higher than reported for other methods.

In contrast to earlier studies a remarkably strong tendency to filament formation was found for slime mould myosin, probably due to a better preservation of some structural properties during preparation. Conditions were worked out under which numerous filaments up to 4 μm in length can be produced. It was established that not only a gradual decrease of ionic strength may influence filament formation, but also pH, ATP concentration and the presence of divalent cations. Compared to the current filament models a difference exists in the structure of the filaments. No central bare zone can be found, and thus, they lack an apparent bipolarity. Along the entire filament there are lateral projections representing the head portion of myosin molecules. A clear periodicity with an axial repeat of about 14 nm was observed, indicating a highly ordered arrangement of these projections.

In this paper it is shown for the first time that myosin from one of the primitive motile systems is able to form aggregates of high structural order, indicating that the contraction of non-muscular actomyosin systems is not necessarily effected with oligomeric or randomly aggregated myosin. The possible role of myosin aggregation in vivo and the similarity of filament structure to that recently reported for myosin from vertebrate smooth muscle and striated muscle are discussed.
The analysis of malignancy by cell fusion: V. further evidence of the ability of normal diploid cells to suppress malignancy
Authors: Wiener F. , Klein G. , Harris H.
Published: April 2021

When cells of a highly malignant fibroblastic cell line, A9HT, are fused with normal diploid lymphocytes, malignancy is suppressed; but it is not suppressed when A9TIT cells are fused with cells of a malignant lymphoma.
Some effects of antimetabolites on cell detachment
Authors: Weiss L. , Huber D.
Published: April 2021

Actinomycin D, cycloheximide, puromycin and ouabain facilitated the detachment of Ehrlich ascites tumour cells from glass surfaces. The results suggest that the altered detachment pattern is due to impaired synthesis of constituents of the cell periphery, which results in a decrease in its mechanical strength.

These reagents have been shown by others to reduce the rate of cell aggregation in various shaker-incubators, and observations of this type have generally been accepted as evidence of diminished cell adhesion. The present results support the alternative interpretation that the reagents act through their effects on cell separation, which is independent of adhesion rate.
Hypertonicity and the arrest of mammalian cells in metaphase: A synchrony technique for hela cells
Authors: Wheatley D. N.
Published: April 2021

Raising the strength of culture medium from 149 or 165 mM by the addition of concentrated saline solutions to about 225–235 mM arrests HeLa cells for 3 h in metaphase without preventing the entry of G2 cells into mitosis. Apart from an initial period of 30 min, metaphases have well developed equatorial plates and chromosomes are discrete from one another.

By employing a standardized continuous wash procedure for detachment of dividing cells during exposure of HeLa monolayers to hypertonicity, excellent yields of mitotic cells consisting almost entirely of metaphases were obtained. They compared favourably with a col-cemid control and were superior in yield and synchrony to isotonic collection of mitotic cells. This method is compared with a manual shake-off procedure. Reversibility of the hypertonically collected metaphases by dilution allowed excellent synchronous progression of cells out of mitosis and into their next division 24 h later. Recovery was also studied by conventional plating and growth-kinetic studies to establish the loss of viability after exposure of cells to varying degrees of hypertonicity.

It is concluded that the arrest of cells in metaphase by hypertonicity and the ease of detachment of arrested cells from monolayers offers an excellent synchronizing procedure for HeLa cells.
Morphogenesis and the fine structure of cultured carrot cells
Authors: Wilson H. J. , Israeli H. W. , Steward C.
Published: April 2021

A study has been made of cultured cells from the root of Daucus carota at defined stages along the route from quiescent cells to the globular embryos known to produce plants. The salient stages were: (i) the quiescent cells in tissue explants exposed only to a basal medium; (ii) cells variously induced to grow by prescribed growth-promoting supplements; (iii) clusters of cells whether attached to a proliferating explant or free in a liquid medium ; (iv) stock cultures routinely maintained in an incipiently embryonic condition in the form of small cell clusters; and (v) cultures in which, as pro-embryonic globules, embryogenesis has been released at will. The fine structure of the cells along this morphogenetic route has been investigated and related to the causal and controlled conditions by which the stages in question were achieved. A range of distinctive cytoplasmic inclusions that occur in cells at these specified stages has been described; these inclusions are not virus particles per se and their occurrence and form is an obvious function of the cultural conditions. The morphogenetic propensity of the cells that give rise to embryos is associated with cytoplasm that is notably free of identified inclusions, that are otherwise conspicuous, but this cytoplasm is rich in microtubules and has some other distinguishing characteristics that are described.
Regulation of polyribosome formation and cell division in cultured soybean cells by cytokinin
Authors: Short K. C. , Tepfer D. A. , Fosket D. E.
Published: April 2021

Cytokinin was shown to be required for cell division in cultured cells of Glycine max var. Sodifury. This cytokinin-induced mitotic activity was correlated with a high cellular content of polysomes. Within 24 h after transfer to a cytokinin-containing medium there was a 4”5-fold increase in the percentage of ribosomes bound as polyribosomes, as determined by sucrose density gradient centrifugation of extracted ribosomal material. Relatively high levels of polysomes and mitotic activity were maintained through the first 6 days of the culture period on cytokinin-containing medium. Thereafter, both cell division activity and the percentage of polyribosomes declined progressively with increasing time in culture. A comparatively small increase in polyribosomes occurred within 24 h of transfer to medium lacking cytokinins, followed by the progressive decline of the level of polyribosomes. The time course of cytokinin-induced polyribosome formation was determined by treating cells with cytokinin after they had been cultured for 24 h on a medium lacking cytokinin. Under these conditions there was a rapid increase in polyribosomes over the next 3 h with no detectable lag period, and near maximal levels of polyribosomes after 6 h of treatment. The initial stimulation of polyribosome formation by cytokinin was not blocked by actinomycin D. Cytokinin was shown to have a comparatively small effect on the ribonuclease activity in extracts of these cells.