bluuebunny/crossref_metadata_2025_split huggingface dataset:
Downward regulation of cell size in paramecium Tetraurelia: Effects of increased cell size, with or without increased dna content, on the cell cycle
Authors: Rasmussen Colin D. , Berger James D. 
Published: April 2021

ABSTRACT
Two temperature-sensitive cell-cycle mutants were used to generate abnormally large cells (size estimated by protein content) with either normal or increased DNA contents. The first mutant, ccl, blocks DNA synthesis, but allows cell growth at the restrictive temperature. The cells do not progress through the cell cycle while at the restrictive temperature, but do recover and complete the cell cycle when returned to permissive conditions. The progeny have increased cell size and normal DNA content.
Downward regulation of cell size occurs during the ensuing cell cycle at permissive temperature. Two processes are involved. First, the G1 period is reduced or eliminated. As initial cell size increases there is a progressive shortening of the cell cycle to 75% of normal. This limit cell-cycle duration is reached when the initial mass of the cell is equal to or greater than that of normal cells at the time of DNA synthesis initiation (0·25 of a cell cycle). Cells with the limit cell cycle begin macronuclear DNA synthesis immediately after fission. The durations of the S period and fission are normal. Second, the rate of cell growth is unaffected by the increase in cell size, and results in the partitioning of excess cell mass between the daughter cells at the next fission.
The second mutant, cc2, blocks cell division, but allows DNA synthesis to occur at a reduced rate so that cells with up to about 14c% of the normal initial DNA content and twice the normal cell mass can be produced. The pattern of cell-cycle shortening is the same as in ccl. The rates of growth and both the rate and amount of DNA synthesis are proportional to the initial DNA content. This suggests that the rates of growth and DNA synthesis are limited by the transcriptional activity of the macronucleus in both ccl and cc2 cells when they begin the cell cycle with experimentally increased cell mass. Increases in both cell size and initial DNA content are required to bring about increases in the rates of growth and DNA accumulation.
Effects of vitamin A on the behaviour of migratory neural crest cells in vitro
Authors: Thorogood Peter , Smith Linda , Nicol Alastair , Mcginty Rose , Garrod David 
Published: April 2021

ABSTRACT
It has been proposed elsewhere that the teratogenic effects of retinoids on craniofacial morphogenesis are caused by a disturbance of the migration of cranial neural crest cells. The effects of 3·5 × 10−6 M and 3·5 × 10−6 M-retinol on the migration of avian neural crest cells in vitro have been investigated by monitoring cell morphology, locomotory behaviour, fibronectin distribution and actin-microfilament organization. Retinol retards migration by affecting cell-to-substratum adhesiveness. Cells exposed to medium containing retinol are less adherent to the substratum, and although the cell surface is very mobile, are unable to extend or maintain lamellipodia. As a consequence the cells do not actively translocate. Fibronectin distribution at the cell surface is sparse, possibly as a result of shedding, and actin distribution remains diffuse.
At the retinol molarities used all these effects are reversible. Thus cells allowed to recover in normal medium flatten out, display lamellipodia and commence active translocation. Fibronectin becomes organized into a fibrillar array and actin microfilaments become organized into cables. The period needed for this recovery is directly related to the molarity of retinol during the initial exposure ; after recovery the retinol-treated cells are virtually indistinguishable from control cells.
We propose that in vivo the effects of retinoids might be to impair cell-extracellular matrix interaction, thus impeding a cell’s ability to migrate through that matrix. Contrary to previous suggestions, the in vivo effects are probably not in any way ‘specific’ to neural crest cells but are more accurately considered as ‘selective’, in that any cell undergoing migration would be similarly affected.
On mitosis in the multicellular alga Ulva mutabilis føyn
Authors: Løvlie A. , Bråten T. 
Published: April 2021

ABSTRACT
Mitosis has been studied in the multicellular green alga Ulva mutabilis by light microscopy and by electron microscopy. Mitosis seems to be of the classical type with regard to spindle, centrioles and continuous and chromosomal microtubules. The nuclear membrane, however, disappears only at the poles and persists as a boundary around the mitotic apparatus. In this respect the Ulva cell seems to be in an intermediate position between those species which have intranuclear mitosis and the forms where the nuclear membrane breaks down. A hypothesis which involves localized synthesis of membranes explains partly the intracellular rearrangement of nucleus, Golgi region and chloroplast after mitosis.
Observations on the fine structure and development of the spindle at mitosis and meiosis in a marine centric diatom (Lithodesmium undulatum): III. the later stages of meiosis i in male gametogenesis
Authors: Manton Irene , Kowallik K. , Von Stosch H. A. 
Published: April 2021

ABSTRACT
Numerical information with respect to spindle microtubules in serial sections of 3 cells at metaphase of the first meiotic division has been compiled and illustrated sufficiently to supplement the previous accounts with respect to both longitudinal and transverse views. At the equator the microtubules of the spindle are united laterally into bundles, the number of bundles being of the same order as that encountered at a premeiotic mitosis, though there is slight variation from cell to cell. The number of microtubules is greater than that at mitosis in a normal spermatogonium, though the distribution in different parts of the spindle is qualitatively similar. Stages of anaphase have been illustrated for the first time and shown to involve at least 3 types of physical forces mediating chromosome movements of various kinds. The later stages of cytokinesis have been timed and are illustrated from living cells and from sections. The relation of the spindle to the cord which unites the separating protoplasts towards the end of cytokinesis has been traced in outline. The cord itself is shown to contain microtubules in limited number and it is therefore not the whole spindle. The interpretation of these findings is discussed in a preliminary way.
An estimate of the amount of microtubule protein in the isolated mitotic apparatus
Authors: Cohen W. D. , Rebhun L. I. 
Published: April 2021

ABSTRACT
The microtubule content of the isolated mitotic apparatus of sea-urchin eggs (Arbacia punctulata) has been investigated by electron microscopy. Cross-sections were made through asters or spindles of flat-embedded mitotic apparatuses of known mitotic stage and specific orientation in the block. Cross-sections between chromosomes and poles of five metaphase half-spindles revealed approximately 2000–2300 sectioned microtubules. The number was somewhat higher in three anaphase half-spindles examined, approximately 2400–2600.
A method was devised for calculating the total number of microtubules in an aster, based upon the number of microtubules appearing in cross-sections. Application of this method to selected mitotic apparatuses enabled calculation of the total number of microtubules in meta-phase mitotic apparatuses of average dimensions. Using a 13-protofilament model of the micro- tubule and existing data on possible monomer sizes and molecular weights, the total amount of microtubule protein in the isolated mitotic apparatus was calculated. The values obtained are in the range of about 1—2 × 10 −8 mg microtubule protein per isolated mitotic apparatus. These values are close to those reported for the 4–5 S protein of the isolated mitotic apparatus, but are considerably lower than the amount of 22s protein. The results are discussed with respect to cellular factors which determine microtubule number, and the possible sources and origin of mitotic microtubule protein.
Development, organization, and degeneration of the Drosophila sperm flagellum
Authors: Kiefer B. I. 
Published: April 2021

ABSTRACT
The development and degeneration of sperm from Y-mutant, sterile Drosophila melanogaster were studied with emphasis on the differences among the various flagellar microtubules and the physical linkages among the components of the axial fibre complex.
During the degenerative process the axoneme is disrupted by the breaking of connexions between adjacent peripheral groups (doublet + satellite) and between the secondary and centralfibres, and the microtubules are lost in a definite sequence: (1) inner portion of B-tubule, (2) outer portion of A-tubule, (3) inner portion of A-tubule, (4) outer portion of B-tubule, (5) central and accessory tubules. The observations support the conclusion that the various microtubules are compositionally different and, further, that the A- and B-tubules are, in turn, composed of differing outer and inner halves.
Observations on the development and subsequent breaking apart of the various flagellar structures demonstrate the presence of the following physical linkages: (1) satellite-to-satellite, (2) A-tubule to adjacent B-tubule, (3) A-tubule to adjacent satellite (accessory tubule), (4) A-tubule to secondary fibres, (5) secondary fibres to central tubules, and (6) a connexion between the two central fibres.
The influence of fixation upon the fine structure of the z-disk of rat striated muscle
Authors: Landon D. N. 
Published: April 2021

ABSTRACT
The fine structure of the Z-disk in rat striated muscle has been found to be dependent upon the manner of its fixation. Material primarily fixed in osmium tetroxide shows a square or ‘woven’ lattice in transverse sections, with a spacing of around 22 ran, and with its axes off-set by 45° from the axes of alignment of the files of I-filaments prior to their entry into the Z-disk. In longitudinal sections the disk may have a zig-zag appearance with individual I-filaments of one sarcomere apparently linked obliquely to 2 or more I-filaments of the next. Transverse sections of Z-disks in muscle primarily fixed in glutaraldehyde show a square-lattice pattern with a spacing of about 11 nm, its axes coinciding with the axes of alignment of the adjacent I-filaments. In longitudinal sections oblique linkages of the ends of the I-filaments are not seen, and they appear either to interdigitate, or to be longitudinally continuous with those of the next sarcomere. This second fine structure is interpreted as being the consequence of the superimposition of 2 basic square lattices, each of 22-nm period, 50 % out of register along each axis, and each formed from the lateral linkage of the ends of the interdigitating I-filamente with the ends of adjacent filaments from their own sarcomere. The relationship of this postulated structure to the lattice patterns previously described, the nature of the cross-linking material, and some possible functional implications are briefly discussed.
A progressive change in the electrophoretic mobility of preaggregation cells of the slime mould, Dictyostelium discoideum
Authors: Garrod D. R. , Gingell D. 
Published: April 2021

ABSTRACT
The electrophoretic mobility of slime-mould preaggregation cells has been found to decrease progressively as they near the chemotactic aggregation stage. This appears to be a spontaneous event dependent on some aspect of cell metabolism. Its possible relevance to cell adhesion and morphogenesis is discussed.
Intranuclear accumulation of rna resembling the smaller ribosomal rna component
Authors: Bramwell M. E. 
Published: April 2021

ABSTRACT
A study was made of the nuclear RNA in HeLa cells with particular reference to the rapidly labelled fractions. It was found that if cells were incubated at a high density, that is, under ‘step-down’ conditions, there was a rapid accumulation of RNA in the nucleus. The fraction of the nuclear RNA which includes rapidly labelled RNA and which binds tightly to columns of methylated albumin on kieselguhr increased in amount and reached levels which permitted enough of the material to be isolated for direct measurement of its base composition. This was found to be very similar to that of 16s ribosomal RNA.
When cells growing logarithmically were treated with low concentrations of actinomycin D and then incubated in the presence of [“[3H]uridine it was found that an RNA fraction which bound tightly to methylated albumin on kieselguhr again accumulated in the nucleus. This fraction resembled that which accumulated under ‘step-down’ conditions. It contained over 85 % of the total radioactivity in the nuclear RNA and again had a base composition very similar to 16s ribosomal RNA. Since nucleolar RNA synthesis was inhibited by the concentrations of actinomycin D used, it appeared that an RNA closely resembling 16s ribosomal RNA was synthesized outside the nucleolus.
Sedimentation patterns on sucrose density gradients and thermal denaturation profiles lent support to the view that the RNA which binds tightly to columns of methylated albumin on kieselguhr probably represents ‘nascent’ 16s ribosomal RNA.
Temperature-dependent functional structures in the polytene chromosomes of Phaseolus, with special reference to the nucleolus organizers
Authors: Nagl W. 
Published: April 2021

ABSTRACT
During the endopolyploidization of the suspensor cells of Phaseolus coccineus and P. vulgaris, polytene chromosomes and an important nucleolar apparatus develop within the nuclei. The nucleolus-associated regions of the four nucleolus-organizing chromosomes and the regions of the secondary constrictions (intranucleolar chromatin threads) show a temperature-dependent, reversible variability in their structure. They split up and are dispersed at optimal temperatures, and they are compact and partly banded at too low or too high ones. Many nucleolar extrusions, and additional nucleoli which originate on distended elements of the heterochromatic parts of different giant chromosomes, occur at temperatures higher than 15° C.
The polytene chromosomes of P. coccineus are able to form lateral loops projecting at right angles to their axes, shortly after temperature elevation. This occurs by the uncoiling of the superficial chromomeres. Several loops synthesize droplets and, moreover, they form nucleolus-like bodies in a hitherto unknown manner. The duration of the lampbrush state is about 3–4 h.
Cytochemical staining methods indicate the presence of DNA in all parts of the polytene chromosomes, including the intranucleolar secondary constriction and the lateral loops, and the presence of RNA in all types of nucleoli and nucleolus-like bodies.
The structural modifications described are compared with the structure of salivary gland chromosomes of Diptera and lampbrush chromosomes in oocytes and spermatocytes. It is suggested that the structural changes are an expression of temperature-dependent changes in gene activity.
Increased contractile strength and tightened adhesions to the substratum result from reverse transformation of cho cells by dibutyryl cyclic adenosine monophosphate
Authors: Leader W. Mark , Stopak David , Harris Albert K. 
Published: April 2021

ABSTRACTWe have examined the changes in cellular contractility and adhesive morphology that accompany the reverse transformation of CHO cells caused by dibutyryl cyclic adenosine monophosphate. The ability of these cells to distort elastic silicone rubber substrata was found to increase markedly over the 5 h period following addition of cyclic AMP and testosterone to the medium, and to relax again following hormone removal. Parallel changes were observed in the interference reflexion image of the adhesion of these cells to glass substrata; broad grey-appearing ‘close’ contacts characteristic of the transformed state were gradually replaced by numerous small black-appearing ‘focal contacts’. These changes in adhesive morphology required considerably longer (20–25 h) to be completed or to revert, however.
Endocytosis of the symbiotic dinoflagellate Symbiodinium microadriaticum freudenthal by endodermal cells of the scyphistomae of Cassiopeia xamachana and resistance of the algae to host digestion
Authors: Fitt William K. , Trench Robert K. 
Published: April 2021

ABSTRACT
The ingestion and fate of four types of particles by endodermal cells of the scyphistomae of Cassiopeia xamachana were investigated by scanning and transmission electron microscopy. Ferritin was endocytosed pinocytotically by invagination of the plasmalemma. These small pinocytotic vesicles fuse with other similar vesicles to form larger ferritin-containing vacuoles, which eventually fuse with lysosomes. Such secondary lysosomes exhibit acid phosphatase activity. The cooccurrence of acid phosphatase activity and ferritin in secondary lysosomes achieved maximum frequency within 2 h of uptake of ferritin and was evident for at least 4 h following uptake. Artemia particles, live freshly isolated symbiotic algae (Symbiodinium microadriaticum), and heat-killed S. microadriaticum are phagocytosed by endodermal cells.
Ferritin-labelled lysosomes fused with food vacuoles containing particles of Artemia. Vacuoles containing heat-killed S. microadriaticum also showed evidence of phago—lysosome fusion. S. microadriaticum in situ (i.e. in host cells) after 3 days exposure to the photosynthetic inhibitor, 3-(3-4-dichlorophenyl)-l, 1 -dimethylurea, appeared degenerate, and were found in loose-fitting host vacuoles, many in mid and apical portions of the host cell. More than 70% of these vacuoles with moribund algae contained the ferritin label, indicating that lysosome fusion had occurred. In contrast, live S. microadriaticum in control animals were almost always found at the base of the host cell in individual tight-fitting vacuoles with no evidence of lysosome fusion.
Live S. microadriaticum apparently escape host digestion by prohibiting the fusion of lysosomes with the vacuole in which they reside. Vacuoles containing defunct algal symbionts, in contrast, were subject to lysosomal attack.
Ultrastructure, organization and cytochemistry of cytoplasmic crystalline inclusions in Origanum dictamnus l. leaf chlorenchyma cells
Authors: Bosabalidis A. M. , Papadopoulos D. 
Published: April 2021

ABSTRACT
In the cytoplasm of chlorenchyma cells in primary Origanum dictamnus leaves, rectangular prism-like crystals occur, which, as shown by pepsin digestion, are proteinaceous. In sections they usually show either a square lattice or striations running parallel to the short or long axis. In both cases the interspacings are estimated to be about 100 A, suggesting that the arrangement of the crystallographic planes possesses tetragonal symmetry. High magnifications of the crystalline inclusions together with analysis of their diffraction patterns showed that the striations are composed of double helices of protein macromolecules. In leaves 3—4 mm long, eroded crystals are often observed. When leaves are larger than 5 mm, no crystalline bodies can be identified in chlorenchyma cells. It is possible that they represent a storage form of protein, which is used later by the developing cells.
Investigation of the cell cycle response of normal and fanconi’s anaemia fibroblasts to nitrogen mustard using flow cytometry
Authors: Dean S. W. , Fox Margaret 
Published: April 2021

ABSTRACT
Cell survival has been measured in normal and Fanconi’s anaemia (FA) human fibroblasts after treatment with the bifunctional alkylating agent, nitrogen mustard (HN2). Two FA cell lines exhibited 6-to 10-fold greater sensitivity than the normal cell line.
Flow cytometry was used to investigate the effects of HN2 on cell cycle progression of normal and FA cells. After 0·1 μg/ml HN2 (surviving fraction, s.f. = 0·8) normal cells exhibited an S phase accumulation within 6 h, followed by a transient G2 delay. At higher doses of HN2, the S phase delay became more pronounced and there was considerably greater accumulation of cells in G2. HN2 at 0·01 μg/ml (s.f. = 0·8) induced no detectable S or G2 delay in FA cells. A higher dose, 0·1 μg/ml (s.f. = 0·13 and 0·29), again induced no S phase delay, but a gradual accumulation of cells in G1 was observed up to 78 h after treatment.
The presence of an S phase delay in normal cells after HN2 treatment may be important in allowing time for DNA repair before completion of DNA synthesis. The absence of such a delay in FA cells suggests that an inability to delay S phase traverse in response to DNA damage from bifunctional alkylating agents may contribute to the sensitivity of FA cells to such drugs.
Effect of tunicamycin, an inhibitor of protein glycosylation, on division of tumour cells in vitro
Authors: Savage K. E. , Baur P. S. 
Published: April 2021

ABSTRACT
We determined the effect tunicamycin (TM), an inhibitor of protein glycosylation, had on cells in vitro that were derived from solid and ascites variants of a chemically induced rat hepatoma. Using flow microfluorometry (FMF), labelling index (LI), and population-doubling time assays, we monitored the progression of cells through the cell cycle after treatment with TM. Cells in monolayer culture were first incubated in 0·05 or 0·10 μg TM/ml medium for 24 h then analysed or given fresh medium without TM and allowed to recover for 6—24 h. Exposing cells to 0·05—0·50 μg TM/ml medium did not affect the percentage of viable cells as determined using the Trypan Blue exclusion procedure. However, continuous exposure to 0·05μgTM/ml medium did affect the population-doubling times of both the ascites and solid variants, and the ascites tumour cells were more sensitive than the solid tumour cells. TM reversibly inhibited hepatoma cells from entering
5 phase of the cell cycle. After exposure to TM for 24 h, the percentage of solid tumour cells in vitro in S phase decreased to 19%, as determined by autoradiography of tritiated-thymidine-labelled cells, and to 21% as determined by FMF; 49% of untreated solid tumour cells were in S phase. The percentage of ascites tumour cells in vitro decreased to 12% after exposure to TM for 24 h; 37% of untreated cells were in S phase. We concluded that TM can inhibit division of rat hepatoma cells in vitro by blocking them in G1 phase of the cell cycle.