bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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

ABSTRACT
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.
Polyteny and the functional significance of the polytene cell cycle
Authors: Pearson M. J. 
Published: April 2021

ABSTRACT
Polyteny is a special nuclear differentiation reported in larval and adult Diptera, Collembola, Protista and angiosperm ovular nuclei. In not all cases, however, do discrete giant chromosomes represent homologous pairs as in the salivary gland nuclei of Drosophila or Chironomus larvae, and in Calyptratae (e.g. Calliphora) a degree of homologue separation and dissociation of the constituents results in a reticular chromosomal organization within which banded polytene regions are prominent. The correlation previously shown between the polytene cycle of replication without mitosis and the growth habit of larval Calliphora suggests that the significance of the polytene phenomenon may lie, not in a special physiology of discrete giant chromosomes, but in the cell cycle responsible for the polytene condition; chromosome morphology may not be functionally significant. The significance of polyteny versus endopolyploidy is considered in terms of the cell cycle. Whereas the endopolyploid cycle is endonuclear and thus non-disruptive of cytoplasmic syntheses, the polytene cycle further departs from a normal mitotic cycle in that there is no M-phase: the nucleus is in permanent ‘interphase’, and clearly the cycle should not be referred to as ‘endomitotic’. The physiology of polytene chromosomes is discussed: there is no evidence that the intense activities of hypertrophied salivary cells in Diptera depend on continuous monitoring at transcription level. The aceticorcein squash technique has been used to describe the chromosomal organization in two cell types known to hypertrophy without sign of an M-phase: oenocytes of the locust Schistocerca gregaria, and the giant trophoblast cells of mouse concepti. In both cases, there is cytological evidence of polytene lateral pairing over short regions. In Schistocerca oenocytes, this organization is similar to the reticular fibrillation of polytene chromosomes seen in calyptrate epidermal and salivary nuclei. The functional significance of the polytene cell cycle is discussed in the light of its known incidence among eukaryotes, and the use of terminology examined.
The abdominal epidermis of Calliphora Erythrocephala (diptera): I. polyteny and growth in the larval cells
Authors: Pearson M. J. 
Published: April 2021

ABSTRACT
The extremely rapid growth of feeding larvae of Calliphora is achieved with a hypertrophy of larval epidermal cells and concomitant polytene specialization of their nuclei. Within the epidermis of each abdominal segment are differentiated sense organs of the campaniform and a more complexpossibly chemosensory-kind in a constant pattern; there is no post-embryonic differentiation of new sense organs before metamorphosis, while the cuticlesecreting cell of these sense organs also becomes polytene.
The polytene organization of epidermal nuclei differs from the classical giant chromosome structures of lower dipteran larval cells. Calliphora epidermal polytene chromosomes irregularly fragment into oligotene or smaller fibrils which, through apparently non-specific reassociations, form a nuclear reticulum of which the banded polytene regions are a part. This loose reticular organization is unlikely to reflect a condition of genetic activity.
Photometric densitometry shows the period of polytene DNA replication in general epidermal nuclei is closely correlated with the period of larval growth. At hatching, epidermal nuclei are diploid, with the zC DNA content. Through a series of endoreduplications, such that at any stage nuclei tend to fall into a series of DNA classes with a percentage falling in between, the largest polytene nuclei reach about 256C by the end of the feeding stage. During the feeding stage, whole mount autoradiography shows a very slow continuation of DNA synthesis in some epidermal nuclei; that is, after cessation of larval growth. DNA synthesis in cupologen nuclei of the campaniform organs ceases after secretion of the third instar cuticle at the end of the second instar.
The polytene cycle responsible for nuclear DNA increase is discussed in the context of these data, and a functional correlation drawn for the rate of polytene increase in the epidermal nuclei and the cupologen nuclei. This correlation suggests active control of the cycle, and raises the question of the significance of the polytene cell cycle.
Cell border demarcation in the scanning electron microscope by sil ver stains
Authors: Bruijn W. C. De , Mourik W. Van , Bosveld I. J. 
Published: April 2021

ABSTRACT
Silver nitrate and silver proteinate were both successfully used as electron stains for the demarcation of the aortic endothelial cell borders in the scanning electron microscope.
By energy-dispersive X-ray analysis it was demonstrated that silver was present in the demarcation lines around the endothelial cells. The presence of silver closely coincided with the places where secondary electrons were produced.
Judged by the quality of the aortic tissue in the transmission electron microscope, the method using silver proteinate was preferable to those using silver nitrate. The limited resolution of the method is demonstrated.
The role of the dextran solution in such procedures was investigated, and it was shown that the presence of such a dextran solution, or its particle size or charge, does not contribute to the formation of these silver lines around the endothelial cells. In ultrathin sections of such material the cell surface-contrasting capacity of colloidal thorium dioxide solutions was com pared to the cell border-contrasting capacity of silver proteinate. It was found that prior aldehyde fixation abolished the cell border demarcation by silver proteinate, but only reduced the cell surface demarcation by thorium dioxide particles.
Study of nucleolar vacuolation and rna synthesis in embryonic root cells of zea mays
Authors: Barsy TH. De , Deltour R. , Bronchart R. 
Published: April 2021

ABSTRACT
The process of nucleolar vacuolation has been quantitatively studied in root cells of the embryo of Zea mays. During germination, we have estimated the percentage of vacuolated nucleoli, the number of vacuoles per nucleolus and the volume density of the vacuoles, the latter by application of the stereological principles of morphometry. In the dormant embryo, no vacuoles can be detected in the nucleolus. When germination occurs at 22 or 16 °C, there is a rapid increase in the percentage of vacuolated nucleoli and in the volume density (VA), with a maximum after 4 and 8 h. At 6 °C, a temperature which does not permit emergence of the root, the percentage of vacuolated nucleoli increases regularly but never reaches the level observed at 16 and 22 °C. In parallel with the appearance of nucleolar vacuoles, the size of the nucleolus decreases statistically and shows a minimum when vacuolation is maximum. This suggests that the appearance of vacuoles within the nucleolus is the result of loss of material from this organelle. Indeed, electron micrographs show that granular components are lost from the nucleolus during the first 8 h of germination at 16 °C. According to the literature, these granular components are probably ribosomal subunits.
An autoradiographic study of tritiated uridine incorporation shows that the nucleolus does not synthesize rRNA at the early beginning of germination. Hence the nucleolus is unable for several hours to rebuild the lost granular elements.
Application of actinomycin D, 5-fluoro-uracil and 2-thio-uracil to the seeds, during the first 48 h of soaking, inhibits rRNA synthesis, the vacuolation process, the loss of granular components and the reduction in size of the nucleolus.
These results allow us to determine some of the physiological conditions characterizing the appearance of the nucleolar vacuoles.
Electron-microscope observations on the organization of the nucleus in chicken erythrocytes and a superunit thread hypothesis for chromosome structure
Authors: Davies H. G. , Murray A. B. , Walmsley M. E. 
Published: April 2021

ABSTRACTPreviously it was shown that when condensed chromatin from several different types of cell is stained with uranyl-lead and examined in thin sections in the electron microscope, the stain is distributed into a dot-dash pattern arising from threads, with lesser-staining intermediate areas. We now show that when a section through chicken erythrocyte chromatin is stained with ethanolic phosphotung3tic acid (PTA) the stain distribution is homogeneous. This shows that the lesser-staining regions after uranyl-lead, cannot be an overlap artifact. We conclude that the stains and hence the molecules in chromatin are distributed between 2 phases, an 0- and an e-phase, so called because the structural units in chromatin are arranged in an orderly way at the surface of the nucleus and give rise to oddly (o) and evenly (e) numbered bands. Measurements of electron density per unit thickness, proportional to the number of stain molecules per unit volume, are made in thin sections through erythrocytes and reticulocytes from adult hen, 4-day-old chicks and 17-day embryos. The results indicate differences in the packing of the molecules in chromatin and further show that the e-phase is quite likely to have a higher DNA to protein ratio than the o-phase. After uranyl-lead stain the visibility of the dot-dash pattern in cells from adult hen is relatively low due, we propose, to closer packing. In micrographs through condensed chromatin treated with uranyl-lead the eye selects out only the densely stained dots and dashes, width 17 nm. When erythrocyte chromatin is partially or completely disrupted in various ways, threads 25–30 nm then become visible. We propose that condensed chromatin in intact cells contains structural units which consist of a central element, width 17 nm previously referred to as the unit thread, forming the e-phase, surrounded by a cylindrical shell forming the o-phase. This socalled superunit thread is similar in width, about 25–30 nm, to that reported by other workers in preparations of chromosomes spread on water surfaces. The hypothesis therefore helps explain what appeared to be discrepancies in thread dimensions. Certain other ultrastructural features of erythrocyte nuclei are also reported which are either pertinent to the general aim of this study, namely the way in which nucleoproteins fold up in chromosomes, or to biochemical studies, to be reported shortly, in which attempts are made to locate the proteins removed from isolated erythrocyte nuclei during subsequent washing in salt solutions.
Inhibition of growth and dna synthesis in cell cultures by cyclic amp
Authors: Eker P. 
Published: April 2021

ABSTRACT
Cyclic AMP (0·1 to 1 mM) was found to inhibit the growth of human liver cells in monolayer cultures. Significant amounts of degradation products were not detected in the medium indicating that the growth-inhibiting effect was associated with the intact cyclic nucleotide. DNA synthesis in the liver cell cultures, as measured by thymidine incorporation into acid-insoluble material, was markedly inhibited by cyclic AMP. RNA and protein synthesis were not significantly affected. Cyclic AMP induced a considerable increase in the cellular uptake of thymidine and uridine from the medium.
When the liver cells were incubated in medium containing radioactive cyclic AMP, no labelled cyclic AMP could be detected in the acid-soluble cell fraction by chromatographic analysis. It is suggested that cyclic AMP does not enter the liver cells, but that its action on growth and DNA synthesis is somehow mediated through an interaction with the cell surface.