bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ultrastructure and activity of the nucleolar organizer in the mouse oocyte during meiotic prophase
Authors: Mirre C. , STAHL A. , de Lanversin A. , Moretti C. 
Published: April 2021

ABSTRACTThe mouse oocyte is the site of nucleolar synthesis during pachytene. The chromosomes containing a nucleolar organizer are attached to the nuclear envelope by their paracentromeric heterochromatin, either alone or by taking part in the formation of a chromocentre. The nucleolus appears at the junction of the paracentromeric heterochromatin with the euchromatic portion of the bivalent. In this zone, 5·0-nm-diameter fibres, thinner than those of the rest of the chromosome (10·0 nm), extend from the lateral element of the synaptonemal complex up to the nucleolar fibrillar centre in which they penetrate. At the onset of its synthesis, the nucleolus only contains the fibrillar centre and an electron-dense fibrillar component in continuity with the latter. Growth of the nucleolus often takes place in the form of a strand whose proximal end, in contact with the fibrillar centre, is formed by preribosomal fibrils and whose distal end is at first fibrillo-granular then granular.Following brief incorporation of tritiated uridine, nucleolar labelling is active in oogonia. No ribosomal RNA-synthetic activity is revealed during leptotene and zygotene. Incorporation resumes at mid-pachytene, with labelling located over the electron-dense fibrillar component adjacent to the fibrillar centre.These observations suggest that the rDNA is located in both the fibrillar centre and its associated electron-dense fibrillar component and that the rDNA transcription occurs in the latter.
The cell cycle during the vegetative stage of Dictyostelium discoideum and its response to temperature change
Authors: Zada-Hames Irene M. , Ashworth J. M. 
Published: April 2021

ABSTRACT
The cell cycle in amoebae of Dictyostelium discoideum has been analysed in cells growing asynchronously in axenic medium. For cells growing at the optimum growth temperature of 22 °C with a culture doubling time of 8 h the average times for the cell cycle phases are as follows: G1, 1 ·5 h; S, 2 ·1 h; G2, 4 ·4 h; M, 15 ·2 min. When amoebae are grown at temperatures below 22 °C, culture doubling time increases and the cell cycle phases are altered in ways characteristic for each phase. G1 is the most variable period and may occupy up to 70 % of the total cell cycle time; 5 and G1 are the least affected, increasing by only 20% when the cell generation time is doubled.
When cells which have reached the stationary phase of growth in liquid medium are washed and reinoculated into fresh medium they divide synchronously after a lag period of 5 h. By following cell number increase and nuclear DNA synthesis in these cultures we have shown that stationary phase cells are arrested in the phase of the cell cycle.
Finally, although more than 97 % of amoebae grown on a bacterial food source are uninucleate, when grown axenically up to 35 % of the cell population may become multinucleate. Our results suggest that these cells probably arise through the failure of cytokinesis to follow karyokinesis. Multinucleate cells appear to have a slightly longer G2 period than mononucleate cells.
Developmental roles of epithelial and interstitial cell lineages in hydra: Analysis of chimeras
Authors: Marcum Beverly A. , Campbell Richard D. 
Published: April 2021

ABSTRACT
Chimeric hydra were prepared by recombining epithelial and interstitial cells between 3 strains of hydra of different sizes (maxi, normal, and mini strains). The resulting chimeras generally resembled the epithelial cell parent more than the interstitial cell parent in size, budding rate, tentacle number, and form. This suggests that epithelial cells normally exert considerable influence over hydra morphogenesis. However, the chimeras show some differences ascribable to interstitial cell origin. Furthermore, the 3 original strains, when deprived of interstitial cells, lose their distinguishing size differences. Thus both epithelial and interstitial cells (or interstitial cell derivatives) mutually participate in hydra’s development.
Ultrastructure of meiosis-inducing (heterotypic) and non-inducing (homotypic) cell unions in conjugation of Blepharisma
Authors: Bedini C. , Lanfranchi A. , Nobili R. , Miyake A. 
Published: April 2021

ABSTRACT
Cells of mating types I and II of Blepharisma japonicum interact with each other and unite in heterotypic (type I-type II) or homotypic (type I-type I, type I I-type II) pairs. Heterotypic pairs undergo meiosis and other nuclear changes of conjugation, while homotypic pairs remain united for days without the nuclear changes taking place. We compared cell unions of these two kinds of pairs at the ultrastructural level. In the homotypic union, cell membranes are closely juxtaposed, separated by a distance of about 20 nm. This arrangement is interrupted in some places by vacuoles and small cytoplasmic bridges. Saccule-like structures tend to be more abundant near the united surfaces. Microtubules running at right or slightly obtuse angles with the cell surface (PACM microtubules) are characteristically present at the united region of cells. These structures are very similar to those observed in earlier stages of the heterotypic union. However, in homotypic pairs, cells unite only at the anterior half of the peristome, while in heterotypic pairs cells unite also at the posterior half of the peristome, where the cell membrane totally disappears in later stages. PACM microtubules persist for at least 18 h in homotypic unions, while they disappear within a few hours in heterotypic unions. These differences between the two kinds of cell union are discussed in relation to the initiation mechanism of meiosis and other nuclear changes of conjugation. Similarities between homotypic union and cell junctions in multicellular organisms are also discussed.
Computer simulation of aggregation in Dictyostelium discoideum
Authors: Mackay Steven A. 
Published: April 2021

ABSTRACT
The aggregation phase of development of the cellular slime mould Dictyostelium discoideum is simulated on a computer. The simulation is performed in 2 dimensions, and produces animated graphical output similar to time-lapse films. It is based on observations of cell behaviour as expressed by a set of rules for each cell, involving cell movement and release of and response to chemical signals. Following these rules, the simulation has reproduced many observed aggregation patterns: propagating waves of cell movement; formation of branching streams; entrainment of slower centres; and spiral centre formation. The simulation has proved to be an important adjunct to experimental work.
Mechanisms of invasiveness of epithelial tumours: ultrastructure of the interactions of carcinoma cells with embryonic mesenchyme and epithelium
Authors: Tickle C. , Crawley A. , Goodman M. 
Published: April 2021

ABSTRACTThree different kinds of carcinoma cell, human breast tumours, mouse lung tumour and rat bladder tumours have been implanted into the developing chick wing to assay their invasive behaviour. We found that the majority of the carcinoma cells did not invade the mesenchyme but were positioned in the ectoderm. We examined the ultrastructure of the interaction of the carcinoma cells with both the ectoderm and the mesenchyme to see if there are any differences. We found no specialized adhesive junctions between carcinoma cells and mesenchyme cells. In contrast, the carcinoma cells seem to be adhesive to the ectoderm cells; desmosomes between the two have been found and also frequent parallel alignment of membranes over long distances. This difference between the interaction of the carcinoma cells with the mesenchyme and the ectoderm may explain why the carcinoma cells can penetrate into the ectoderm but rarely do so into the mesenchyme. The carcinoma cells formed a smooth border with the mesenchyme even in the absence of a basal lamina. These results are discussed in relation to the mechanisms of invasion of epithelial tumours in vivo. In particular, the relative importance of the basal lamina, and cell adhesion within the tumour, in limiting invasiveness arc assessed.
In vivo phagocytosis by developing myocardial cells: an ultrastructural study
Authors: Hurle J. M. , Lafarga M. , Ojeda J. L. 
Published: April 2021

ABSTRACT
Phagocytosis of naturally degenerating cells, by healthy myocardial cells of the bulbus cordis of 5–7-day chick embryos, was studied by electron microscopy. Myocardial cells showed cell processes surrounding dead cell fragments. The cell fragments appeared to be internalized later and digested within phagosomes. Ruthenium red was employed to ascertain whether the cell fragments were in fact internalized or located in pockets of the cell membrane. The results are discussed in the light of present knowledge of phagocytosis.
Effect of temperature and divalent cations on the substratum attachment of rat hepatocytes in vitro
Authors: Seglen Per O. , Gjessing Rolv 
Published: April 2021

ABSTRACT
The attachment of rat hepatocytes to polystyrene-adsorbed serum protein is relatively insensitive to inhibitors such as dextran sulphate, cycloheximide, colchicine and cytochalasin B, and enzymes like trypsin and neuraminidase, but it is strongly dependent on divalent cations. Mg2+ supports attachment better than Ca2+, but a combination of both is required for maximal attachment. The attachment is very temperature-sensitive, with a biphasic Arrhenius plot indicating an activation energy of 123 kJ/mol above 34 °C and 374 kJ/mol below 34 °C.
The adsorbed attachment-promoting serum factor is inactivated by trypsin, or by Ca2+-dependent proteases which contaminate commercial preparations of collagenase. The adsorbed factor is resistant to treatment with glutaraldehyde, neuraminidase and heating to 90 °C, whereas the same factor in the unadsorbed state (in serum) is destroyed by heating to 70 °C. The factor in serum is unable to compete with the adsorbed factor for cell binding, hence it would appear that adsorption to polystyrene induces the active, heat-resistant conformation of the factor.
The mechanism of antibody-dependent, eosinophil-mediated damage to schistosomula of Schistosoma mansoni in vitro: A study by phase-contrast and electron microscopy
Authors: Glauert Audrey M. , Butterworth A. E. , Sturrock R. F. , Houba V. 
Published: April 2021

ABSTRACT
A characteristic sequence of events has been identified by phase-contrast and electron microscopy during antibody-dependent, eosinophil-mediated damage to schistosomula of Schistosoma mansoni in vitro.
Human eosinophils initially adhere to the intact schistosomulum and then, in the presence of antibody, flatten and spread very intimately over the parasite’s surface. Subsequently, dense material similar to the contents of the lysosomal granules of the eosinophils appears in the extracellular space between the eosinophil and the schistosomulum, probably following fusion of the granules with the plasma membrane of the cell. Eventually all the eosinophils adhering to the parasite are completely degranulated and large amounts of the dense material are observed on the surface of the schistosomulum. This release of granular material from the eosinophils is followed by structural changes in the schistosomulum, starting with vacuolation of the inner layer of the tegument, followed by removal of the tegument, often in the form of large sheets. Subsequently the tegument disintegrates and the fragments are phagocytosed by other eosinophils which have not degranulated. Eosinophils then attach to the exposed muscle layers of the schistosomula and participate in the further degradation of the parasites by phagocytosing fragments of muscle fibres and other cellular components.
This sequence of events is compared with published observations of the damage induced by various combinations of antibody, complement and effector cells in vitro, and of cell-mediated damage to schistosomula in vivo, and it is concluded that the observations described in the present paper may reflect a process of destruction of schistosomula in the immune host.
Regulation of interstitial cell differentiation in Hydra attenuata: V. Inability of regenerating head to support nematocyte differentiation
Authors: Yaross Marcia S. , Bode Hans R. 
Published: April 2021

ABSTRACT
Nematocyte differentiation was examined during head regeneration in Hydra attenuata. Nematocyte precursors were found to decrease in head-regenerating tissue. This decrease could not be attributed to decreased stem cell commitment or to altered cellular kinetics. The nematocyte precursors could be ‘rescued’ by regrafting a head onto the initially regenerating tissue only prior to the time at which head determination occurred. These results suggest that concurrent with head determination an irreversible change occurs in the tissue environment, resulting in decreased survival of cells committed to nematocyte differentiation.
Structure, protein composition and birefringence of the costa: a motile flagellar root fibre in the flagellate Trichomonas
Authors: Amos W. B. , Grimstone A. V. , Rothschild L. J. , Allen R. D. 
Published: April 2021

ABSTRACT
The costa is a rod-shaped intracellular organelle lying in the cytoplasm immediately below the undulating membrane. In certain large species of Trichomonas (T. gigantea, T. termopsidis and an undescribed species from the termite Porotermes adamsoni) this organelle is motile. Bending waves are transmitted along the length of the costa, in either direction. It is shown that the bending is actively produced by the costa itself. The form of the movements is described in detail. The costa is biréfringent. At the point at which bending occurs there is a sharply localized reduction in birefringence. Electron microscopy shows that the costa is composed of longitudinally running lamellae, 2-3 nm thick and spaced 12 nm apart. These are connected to transversely running elements spaced at intervals of about 37 nm. The lamellae occur in two alternative configurations: straight and zig-zag. Bending probably results from a local, transient change from the straight to the zig-zag configuration. This would account for the local change in birefringence which accompanies bending. Polyacrylamide gel electrophoresis of isolated costas shows them to contain a number of protein components, of which the principal one has a molecular weight of about 90000. Preliminary cytochemical evidence is presented for ATPase activity in the costa. The costa is a type of motile system distinct from any hitherto described.
Dyskinetoplasty in two species of trypanosomatids
Authors: Hajduk Stephen L. 
Published: April 2021

ABSTRACT
Dyskinetoplastic cells from both Crithidia fasciculata and Trypanosoma equiperdum lack detectable kinetoplast DNA (kDNA) by conventional staining techniques. Two dyskinetoplastic strains of T. equiperdum, either acriflavine-induced or spontaneously occurring, show normal amounts of kDNA (ρ = 1 · 692g/cm3) in analytical caesium chloride ultracentrifugation. Electron and fluorescence microscopy of the dyskinetoplastic strains of T. equiperdum suggest that the kDNA network is fragmented and dispersed throughout the mitochondrion. The fragmentation and dispersion of the kDNA, rather than a reduction in the amount of kDNA, is the cause of the lack of kinetoplast staining in the dyskinetoplastic strains of T. equiperdum. Acriflavine-treated cultures of C. fasciculata show a decrease in the amount of kDNA (ρ = 1·703 g/cm3) corresponding to the percentage of dyskinetoplastic cells in the cultures. Electron and fluorescence microscopy of acriflavine-treated cultures of C. fasciculata show the loss of the kDNA network in cells which lack Giemsa and Feulgen staining, confirming the hypothesis that the kDNA is lost in dyskinetoplastic trypanosomatids from insects. Possible modes of acriflavine action are considered and a proposed mechanism for acriflavine action in trypanosomes from mammals is presented.
Plasma membrane vesiculation in 3T3 and Sv3T3 cells: I. morphological and biochemical characterization
Authors: Scott Robert E. , Perkins Roger G. , Zschunke Michael A. , Hoerl Bryan J. , Maercklein Peter B. 
Published: April 2021

ABSTRACT
3T3 and SV3T3 mouse embryo cells and a variety of other monolayer cell lines can be induced to form and shed plasma membrane vesicles by exposure to sulphydryl blocking agents including formaldehyde and N-ethyl malemide. Morphological studies show that multiple vesicles are formed and released from individual cells and that the vesicle membrane is continuous with the plasma membrane of the cell. Vesicles measure from 0 ·1 to 15 μm in diameter and are free of detectable contamination with cytoplasmic membranes and organelles. Vesicles also show a 10-fold enrichment in the plasma membrane marker enzyme 5’nucleotidase and are devoid of detectable NADH-cytochrome C reductase and succinic dehydrogenase activity which are marker enzymes for endoplasmic reticulum and mitochondria, respectively. Vesicles have a high cholesterol: phospholipid ratio and show enrichment in sphingomyelin content. They contain receptors for Con A and WGA, approximately 20 size class polypeptides and intra-membranous particles. These results suggest that vesicles are derived from and have the general characteristics of plasma membranes.
Controlled cell disruption: A comparison of the forces required to disrupt different micro-organisms
Authors: Kelemen M. V. , Sharpe J. E. E. 
Published: April 2021

ABSTRACT
A cell disrupter has been developed which can measure the forces required to disrupt both eukaryotic and prokaryotic cells. It operates a continuous process and will disrupt both large and small volumes. Shear forces are set up when a suspension under laminar flow conditions is released under high pressure through a short orifice. If the applied pressure is altered, the shear forces are simultaneously changed so that the amount of cell disruption can be compared under different known and repeatable conditions. The disrupter is now manufactured and supplied by Stansted Fluid Power Limited, Stansted, England.
Phase-contrast microscopy has shown that the disrupter will break a variety of organisms including Chlorella, Aspergillus fumigatis, Fusarium sp., Saccharomyces cerevisiae, Escherichia coli, Lactobacillus casei, Bacillus subtilis, Clostridium perfringens, Streptococcus faecalis, Streptococcus zooepidermicus and Staphylococcus aureus. The cells are not all broken at one pressure but a certain pressure must be applied before disruption starts which will then increase rapidly as the applied pressure is increased. The applied pressure required to disrupt half the population in a culture is different from one species to another, rods being disrupted more easily than spheres.
The ease of disruption seems to be related to the shape and chemical composition of the cell wall. Furthermore, the disrupting process, in an unsynchronized culture is not random and may be related to the statistical size distribution of the cells.
Spermatogenesis in trichosia pubescens (diptera: Sciaridae)
Authors: Amabis J. M. , Reinach F. C. , Andrews Norma 
Published: April 2021

ABSTRACT
This paper deals with the meiotic process in males of T. pubescens. The spermatocytes of this species are associated in clusters (rosettes) of some fifteen cells which are fairly synchronized during spermatogenesis. During meiosis, which is typically achiasmatic, 2 sets of chromosomes are eliminated into a narrow neck of cytoplasm (bud) which protrudes from the cell pole facing the centre of the rosette. The bud formation starts prior to the onset of the meiosis. The spindle that appears during the first meiotic division is unipolar with the fibres running from the pole at the end away from the bud up to the equatorial region of the cell. The chromosomes which move toward the spindle pole display a V-shape while the others, moving budward, do not show any consistent shape. We suggest that a special mechanism of chromosome motion is involved in the migration of these chromosomes. The second meiotic division is fairly similar to the ordinary type observed in other organisms except for the migration of both chromatids of one chromosome precociously toward the pole opposite the bud. At this division a typical bipolar spindle is present and all migrating chromosomes display a V-shape. The chromosomes eliminated during the first meiotic division clump together remaining in a pycnotic state at the distal portion of the bud until the end of spermatogenesis. The chromosomal set eliminated at the second meiotic division, as compared to the first, decondense simultaneously with the group located in the cell body forming a typical interphase nucleus.
T. pubescens has 3 chromosomes limited to the germ-line cells. These chromosomes are typically heterochromatic, replicating their DNA out of phase with the S-chromosomes, probably at a later stage in interphase. Our results suggest that they are transcriptionally active in the interphase between the 2 meiotic divisions and just after the meiotic process.
Gap and septate junctions in the excitable endoderm of Polyorchis penicillatus (hydrozoa, anthomedusae)
Authors: King Michael G. , Spencer Andrew N. 
Published: April 2021

ABSTRACT
The morphological basis of impulse conduction in a jellyfish epithelium was investigated. Lanthanum impregnation of endodermal canal and endodermal lamella verified the existence of true gap junctions in Polyorchis. In both transverse and en face sections of gap junctions, electron-lucent globules, with a width of 7–8 run and a spacing of about 11 run, are evident. Gap-junctions are concentrated at the peripheral canal margin and septate junctions are localized around the canal lumen. Epithelial cells of the endodermal canals are capable of conducting a non-decrementing action potential. It is suggested that endodermal spike propagation, which can mediate ‘crumpling’ behaviour, is dependent upon low-resistence ionic pathways provided by gap-junctions and upon sealing of the intercellular space from saline extracellular fluids by septate junctions.
Studies on scale morphogenesis in the golgi apparatus of Pyramimonas tetrarhynchus (prasinophyceae)
Authors: Moestrup Øjvind , Walne Patricia L. 
Published: April 2021

ABSTRACT
Ultrastructural investigations of scale formation in the cisternae of the Golgi apparatus have been carried out on the prasinophycean flagellate Pyramimonas tetrarhynchus, whose cell surfaces are covered with 6 different scale types, 3 on the flagella and 3 on the cell body. Our results suggest that at least 4 and probably all 6 scale types can be formed together within the same cisterna and that there is some degree of intracisternal differentiation, since the formation of 2 scale types (the small underlayer scales on cell body and flagella) is restricted to the cisternal peripheries, whereas the remaining scale types are formed in the more central portions. Detailed studies of morphogenesis of the larger body scales reveal the earliest identifiable stages as 8-armed figures, with 8 thin arms in the intermediate body scales (IBS), and 4 thick and 4 thin arms in the outer body scales (OBS). From these incipient structures that bear little resemblance to the finished products, the complex, 3-dimensi0nal mature body scales are elaborated in each of the cell’s 4 dictyosomes, and maintain throughout their different developmental sequences a close relationship to the inner surfaces of the cisternal membranes, as well as a particular orientation within the dictyosomes. Preliminary calculations of total numbers of scales that cover cell and flagellar surfaces are included. The small, undeilayer scales, which on the flagella are shown to be arranged in 2 4 rows, number about 350000; larger scales of more complex construction number about 20000.