bluuebunny/crossref_metadata_2025_split huggingface dataset:
Chromatin behaviour during the mitotic cell cycle of Saccharomyces cerevisiae
Authors: Gordon C. N. 
Published: April 2021

ABSTRACT
Chromatin behaviour during the cell division cycle of the yeast Saccharomyces cerevisiae has been investigated in cells which have been depleted of 90 % of their RNA by digestion with ribonuclease. Removal of large amounts of RNA from the yeast nucleus before treatment of the cells with heavy metal fixatives and stains permits chromatin to be visualized with extreme clarity in thin sections of cells processed for electron microscopy by conventional procedures. Spindle pole bodies were also visualized by this treatment, although the associated microtubules were not.
Chromatin is dispersed during interphase and occupies the non-nucleolar region of the nucleus which is known to be Feulgen-positive from light microscopy. Because spindle micro-tubules are not visualized, direct attachment of microtubules to chromatin fibrils could not be verified. However, chromatin was not attached directly to the spindle pole bodies and kineto- chore differentiations were not observed in the nucleoplasm.
During nuclear division chromatin remains dispersed and does not condense into discrete chromatids. As the nucleus expands into the bud, chromosomal distribution to the daughter cells is thought to result from the separation of the poles of the spindle apparatus with attached chromatin fibrils. However, that such distribution is occurring as the nucleus elongates is not obvious until an advanced stage of nuclear division is reached and partition of the nucleus is nearly complete. Thus, no aggregation of chromatin into metaphase or anaphase plates occurs and the appearance of chromatin during mitosis is essentially the same as in interphase. These observations indicate that the marked changes in the topological structure of chromatin which characterize mitosis in the higher eukaryotes do not occur in S. cerevisiae.
Fusion and erosion of cell walls during conjugation in the fission yeast (Schizosaccharomyces Pombe)
Authors: Calleja G. B. , Yoo Bong Y. , Johnson Byron F. 
Published: April 2021

ABSTRACT
Conjugation in Schizosaccharomyces pombe was studied by transmission electron microscopy. Mural and nuclear events were scored from induction, the initial event, to meiosis I, the start of sporulation. These morphogenic markers were separately identifiable as flocculation, copulation, conjugation-tube formation, cross-wall formation, cross-wall erosion, conjugationtube expansion, cytoplasmic fusion, de-differentiation of site of union, nuclear migration and karyogamy. The following were identified as new structural elements: sex hairs, which presumably mediate hydrogen bonding between cells during flocculation; crimp at the site of union; dark patch, which presumably serves as a leak-proof seal at the time of cross-wall erosion; suture, an electron-dense seam formed by the union of a copulant pair; and small electron-dense particles close to the site of wall erosion. No special structures on the cell wall could be identified as indicative of specific sites for potential copulatory activity. The discontinuity of the 2 cell walls at the site of union became so de-differentiated after fusion and erosion that it was no longer possible to pinpoint the site of union.
Extracellular matrix synthesis in blastula and gastrula stages of normal and hybrid frog embryos : II. autoradiographic observations on the sites of synthesis and mode of transport of galactose- and glucosamine-labelled materials
Authors: Johnson Kurt E. 
Published: April 2021

ABSTRACT
Pulse-chase labelling experiments and light- and electron-microscopic autoradiography were used to examine the sites of synthesis, mode of transport, and sites of deposition of galactose- and glucosamine-labelled materials in different developmental stages of normal developing Rana pipiens embryos and interspecific hybrid embryos formed by fertilizing the eggs of R. pipiens with the sperm of R. catesbeiana. In both normal and hybrid embryos, after a 15-min pulse, grains are closely associated with juxtanuclear and cytoplasmic collections of membranebound vesicles which resemble the Golgi apparatus. In normal embryos following a 15– 30 min pulse and a 60-min chase, grains are largely cleared from the cytoplasmic vesicles and deposited in the extracellular spaces or along cell surfaces where the extracellular spaces are relatively large. In contrast, arrested hybrid embryos given a 15– 30-min pulse and a 60-min chase show a marked accumulation of grains over cytoplasmic structures such as the Golgi apparatus and vesicular elements in the cell cortex. Finally, early gastrula stage normal embryos are most active in the synthesis of galactose-labelled materials in cells above the dorsal lip of the blastopore, where cell migration is initiated.
Glycosyltransferase activity in developing sea-urchin embryos
Authors: Evans Irene M. , Bosmann H. Bruce 
Published: April 2021

ABSTRACTThe developmental course of activity changes of 4 endogenous membrane glycosyltransferases : UDP-glucose : glycosyltransferase, UDP-galactose : glycosyltransferase, GDP-mannose: glycosyltransferase, and UD-PN-acetylglucosamine : glycosyltransferase was determined in various stages of developing sea-urchin embryos from unfertilized eggs up to the 16-cell stage. The results show that there is a 2-fold increase in glycosyltransferase activity after fertilization. These results are discussed in relation to the postulated role of glycosyltransferases in mediating adhesion, recognition, and agglutination of cells and embryos.
Interband transcription in Drosophila
Authors: Skaer R. J. 
Published: April 2021

ABSTRACT
Most puffs contain perichromatin ribonucleoprotein granules 30-40 nm in diameter; other puffs contain ribonucleoprotein granules 25 nm in diameter or mixtures of these and perichromatin granules. All puffs contain fragments of band material possibly from several bands. By examining progressively smaller puffs the transcriptionally active region is shown to lie within an interband. Some transcriptionally active interbands are so small that there can be no significant contribution of decondensed band material to the interband. Up to 33% of all interbands contain significant evidence of transcription. These findings are discussed in relation to the use of the terms heterochromatin and euchromatin to describe the banding pattern of polytene chromosomes.
Production of mouse globin in heterokaryons of mouse erythroleukaemia cells and human fibroblasts
Authors: Alter Blanche P. , Goff Sabra C. , Hillman Diane G. , Deisseroth Albert B. , Forget Bernard G. 
Published: April 2021

ABSTRACT
In an effort to activate the globin genes of non-erythroid cells, tetrapioid murine erythroleukaemia cells (Friend cells) were fused with diploid human amniotic fibroblasts. When the Friend cells were pretreated with dimethylsulphoxide, an average of 27% heterokaryons was observed. These cells stained with benzidine, an indication that they contained haemoglobin. The cells incorporated radioactive amino acids into proteins. Electrophoresis of [3H]leucine-labelled lysates on SDS urea polyacrylamide gels indicated that up to 7% of the newly synthesized protein co-electrophoresed with globin. CM cellulose chromatography demonstrated the presence of mouse but not human globin chains. Hybridization analyses of cytoplasmic RNA also revealed only mouse globin mRNA in the heterokaryons. Although heterokaryons form readily between mouse erythroleukaemia cells and human fibroblasts, and globin synthesis does occur, only the erythroid partner in the fusion system employed here directs globin production.
Continuous dna replication in the nucleus of the dinoflagellateProrocentrum micans(ehrenberg)
Authors: Filfilan S. A. , Sigee D. C. 
Published: April 2021

ABSTRACTThe uptake of tritiated thymine into cells of a heterogeneous population of Prorocentrummicans was investigated using light-microscope and electron-microscope autoradiography. Specificity of thymine uptake into DNA was demonstrated by the specific removal of label from wax-embedded material using DNase and by the high degree of localization of nuclear label to chromosomes in the electron-microscope autoradiographs.All nuclei, including both dividing and non-dividing cells, showed a substantial uptake of label, indicating that nuclear DNA synthesis in Prorocentrum micans is a continuous process. The level of DNA synthesis does show considerable variation, however, with very high levels in some interphase nuclei.The continuous replication of nuclear DNA provides further evidence of dinoflagellate affinity to the prokaryotes, and indicates that Prorocentrummicans is a very primitive eukaryote cell.
Tissue economics of hydra: regulation of cell cycle, animal size and development by controlled feeding rates
Authors: Otto Joann J. , Campbell Richard D. 
Published: April 2021

ABSTRACT
Epithelial cell production and epithelial cell loss in 6 different size classes of Hydra attenuata were examined to understand the relationships between growth and morphogenesis. The sizes of adult hydra, the sizes of their buds, and their budding rates are all nearly proportional to the amount of food the hydra eat. Hydra fed at high rates (4-25 Artemia nauplii per day) all have the same epithelial cell cycle time (about 4 days). Budding accounts for most of their cell loss. Hydra fed 4-12 Artemia per day maintain a steady state condition in which tissue loss balances tissue growth. Animals fed 25 Artemia per day are not in a steady state growth condition and change in size. At the lowest feeding rates (0-1 Artemia per day), the epithelial cell cycle time is lengthened to about 16 days. Cell loss from the tentacles accounts for most of the cell loss, and this loss is not completely balanced by growth. As a consequence these animals cease budding and shrink in size.
Nuclear changes associated with cellular dedifferentiation in pea root cortical cells cultured in vitro
Authors: Feldman Lewis J. , Torrey John G. 
Published: April 2021

ABSTRACT
Root cortical tissues explanted from seedling roots of the garden pea, Pisum sativum L., cv. Little Marvel cultured in sterile medium containing indoleacetic acid and kinetin were fixed, sectioned and studied with the electron microscope. Nuclear changes over 60 h of culture were examined during the events of dedifferentiation of mature parenchyma cells into subdivided newly meristematic derivatives. Table 1 summarizes the events which are evident in 7 population classes designated I—VII. The initially small, round nucleus with a single, unvacuolated nucleolus and highly condensed and disperse chromatin showed marked volume increase, nucleolar enlargement of almost 20-fold, development of diffuse and then clumped chromatin and then subdivision of the nucleoli into a number (5–10) of small nucleoli immediately preceding cell division. Mitoses involved cells with diplochromosomes and daughter nuclei with doubled chromosome number, either 4n or 8n. Subdivided cortical cells showed an increased ratio of nucleus to cytoplasm although the nuclei were reduced in volume to that of the original nuclear population. Nucleoli in subdivided cells were small and unvacuolated and chromatin changed from an initially condensed state through a diffuse and disperse condition to a condensed form. Cortical cells divided at least twice in rapid succession before initiating new events leading to redifferentiation. Nuclear changes in dedifferentiation in pea were comparable in many respects to the similar process studied in animal systems including eye lens regeneration, limb regeneration and activation of division in blood lymphocytes. The cytological changes in nuclear structure could be correlated with documented changes in DNA, RNA and protein levels in these systems. By 60 h cells presumed to be ready for redifferentiation and cell specialization were observed in the subdivided population but structural evidence for commitment to a new course of cytodifferentiation was not obtained.
A microtubule—nuclear envelope complex in the spermatozoid of Pteridium
Authors: Bell Peter R. 
Published: April 2021

ABSTRACT
The microtubules on the outside of the nucleus of the mature spermatozoid of Pteridium form a complex with the envelope, and the perinuclear space is eliminated. These dissimilar components are held firmly together, possibly by hydrophobic bonding. The outer part of the chromatin is also attached to the inner boundary of the envelope, and pulls away when the envelope is detached.
Studies on pulse-labelled rna during the mitotic cycle of Physarum polycephalum by subnuclear fractionation
Authors: Davies K.E. , Walker I.O. 
Published: April 2021

ABSTRACT
A method is described for the isolation of pulse-labelled RNA from nuclei and subnuclear fractions of Physarum poly cephalum. At all times during interphase the nucleolar RNA consisted mainly of a 34-s rRNA precursor with only small amounts of 26-s and 19-s rRNAs. The nucleoplasmic RNA consisted of predominantly mature 26-S rRNA with small amounts of 19-s rRNA and a 30-s RNA species. The 30-s RNA component displayed different labelling kinetics from the rRNA precursors. A low molecular weight RNA fraction (4–8 s) appeared to accumulate in the nucleus as interphase progressed.
Histological studies on the genus Fucus: III. fine structure and possible functions of the epidermal cells of the vegetative thallus
Authors: McCully Margaret E. 
Published: April 2021

ABSTRACT
The fine structure of the epidermal cells of the vegetative Fucus thallus has been examined in material fixed with acrolein. These cells are highly polarized, with basal nuclei and chloroplasts, a hypertrophied perinuclear Golgi system, and a much convoluted wall/plasma membrane interface. Much of the intracellular volume is occupied by single membrane-bounded vesicles containing alginic acid, fucoidin and polyphenols.
The chloroplasts were examined by light and electron microscopy and shown to contain structured inclusions not previously described in Fucus plastids. It is suggested en the basis of their morphology that the epidermal cells may be specialized for the absorption of inorganic carbon and sulphate from the outside of the plant and for the secretion of alginic acid, fucoidin and polyphenols. The possible role of these cells in the prevention of desiccation and in osmoregulation is discussed.
Structural organization associated with pseudopod extension and contraction during cell locomotion in Difflugia
Authors: Wohlman A. , Allen R. D. 
Published: April 2021

ABSTRACT
In Difflugia corona, a free-living amoeboid cell, locomotion is hampered by a heavy shell or test made of sand grains and other debris. Locomotion involves pseudopod extension, attachment to the substratum, and forcible pseudopod retraction which pulls the shelled cell body forward.
When observed through a polarizing microscope, the extending pseudopodia appear isotropic or very weakly birefringent. Upon attachment to the substratum a positively birefringent fibrillar array develops rapidly at the attachment point and extends from this region back to the cell body within the test. These birefringent fibrils extend through and parallel to the long axis of the pseudopod. As the pseudopod retracts, the birefringent fibrillar array disappears, and hyaline blebs, suggestive of syneresis, appear on the pseudopodial surface. The birefringent fibrils correspond in position and approximate diameter (1 μ) to refractile fibrils visible with the Nomarski differential interference microscope.
Individual organisms were fixed for electron microscopy at a time when the pseudopodia were firmly attached to the substratum. Electron-microscopic examination of thin sections of pseudopodia revealed many 1-μ bundles of intimately associated, aligned, 55–75 Å micro-filaments. The orientation and size of the bundles indicate that they probably correspond to the birefringent, refractile fibrils observed in living cells. Microfilaments have also been observed both as randomly oriented and dispersed cytoplasmic components, and as aligned filaments in the ectoplasm adjacent to the plasmalemma.
During pseudopod extension with sporadic streaming, birefringent ‘flashes’ have been observed at the front of the pseudopod. These flashes are believed to represent a photo-elastic phenomenon.
Asymmetry of the node of ranvier
Authors: Williams P. L. , Kashef R. 
Published: April 2021

ABSTRACT
The nodal constrictions of Ranvier in normal limb nerves of mammals are bounded by asymmetric paranodal bulbs, the proximal bulb (nearer the cell body) being larger in all dimensions. This investigation was undertaken to determine whether the asymmetry results from partial damming of proximo-distally flowing axoplasm at the nodal constriction, or whether other features of local growth patterns are more relevant.
The degree of asymmetry was estimated on teased, osmicated fibres from normal immature and mature limb nerves to skin and muscle and on ventral nerve roots. Estimates were also made on the central processes of dorsal root ganglionic cells with their contrasting direction of flow, after alteration of growth patterns in regenerates of crushed immature and mature nerves, and in the recurrent laryngeal nerve which pursues an exceptional course in relation to surrounding tissues. A polarization of asymmetry with larger proximal bulbs was found in uncomplicated limb nerves and after simultaneous regeneration and limb growth following crushing of immature nerves. Mixed populations (that is, with no preferred direction of asymmetry and often symmetrical bulbs) were found in dorsal and ventral nerve roots and the apical recurved segment of the recurrent laryngeal, whilst the mature regenerates closely approached the symmetrical condition. The descending and ascending limbs of the recurrent laryngeal nerve showed a reversal of polarization with respect to the cell body, but similar with respect to the cephalo-caudal body axis. It was concluded that damming of directionally flowing axoplasm was not causally related to the formation of asymmetric bulbs. The overall interstitial growth pattern of limb nerve elongation as revealed by intemodal distance studies contrasts with the differential growth and maturation gradients shown by the various limb segments through which the nerve passes. These differences result in a relative movement between the myelinating and elongating Schwann cell and the surrounding limb tissues. The movements are considerable and unidirectional throughout most of the limb, the Schwann cells becoming relatively further removed from the limb apex, whilst the movements minimize at the limb root and apex. It is suggested that the altered mechanical conditions operating at the ‘advancing’ and ‘trailing’ ends of the cell are related to the dimensional differences of the paranodal apparatus comprising the bulb at each end of the cell, and secondarily result in asymmetric nodes of Ranvier.
Behaviour of the cell surface during movement of preaggregation cells of the slime mould dictyostelium Discoideum studied with fluorescent antibody
Authors: Garrod D. R. , Wolpert L. 
Published: April 2021

ABSTRACT
Although several theories relating to the mechanical behaviour of cell surface during movement have been put forward, a division into two groups can be made according to whether the surface is thought to turn over rapidly or to be a relatively permanent structure. In order to determine whether turnover of the surface membrane plays an important role in the locomotion of preaggregation cells of Dictyostehum discoideum, their surface was labelled with fluorescent antibody. The cell surface remains labelled during movement though there is an initial accumulation of fluorescence at the tail. There is little fluorescent labelling of the internal contents. The results are interpreted as showing that surface turnover is not an essential feature of cell movement.
Studies on the microtubules in heliozoa: IV. the effect of colchicine on the formation and maintenance of the axopodia and the redevelopment of pattern in Actinosphaerium nucleofilum (barrett)
Authors: Tilney L. G. 
Published: April 2021

ABSTRACT
When specimens of Actinosphaerium are placed in a solution of colchicine, the axopodia retract and that portion of the biréfringent core (axoneme) present in each axopodium disappears. In fixed specimens, it has been shown that the axoneme consists of a highly patterned bundle of microtubules each 220 Å in diameter. During colchicine treatment the microtubules, present in the axopodial portion of the axoneme, break down and do not reform until the cells are washed free of the colchicine and allowed to recover. In the basal portion of the axoneme or that portion confined to the cell body, most of the microtubules do not break down during colchicine treatment, although the spacing of the microbutules within the remaining axonemes is altered; in longitudinal section these tubules no longer appear straight but present an undulatory profile. When the specimens are washed free of the colchicine and allowed to recover, axopodia reform and within each numerous microtubules are present. From these observations we conclude that the microtubules are instrumental in the maintenance of the axopodia as well as in supplying the force necessary for their regrowth. Observations are also presented on the disintegration products of microtubules, on microtubules and cytoplasmic motility, and on the redevelopment of pattern. The latter is discussed at some length, for it appears that the inter-axonemal organization is somewhat modified by the colchicine treatment.
The formation of the generative cell in the pollen grain of Endymion non-scriptus (l)
Authors: Angold R. E. 
Published: April 2021

ABSTRACT
The generative cell wall in the pollen grain of Endymion non-scriptus is formed, as in somatic cells, from a cell plate between the vegetative and generative nuclei. This wall curves around the generative nucleus, and fuses with the intine to enclose the generative cell. The generative cell is subsequently freed from the intine by the constriction of the generative cell wall between the generative nucleus and the intine.
The fine structure of fertilization in the fern Marsilea vestita
Authors: Myles Diana Gold 
Published: April 2021

ABSTRACT
The ultrastructural details of fertilization in the fern Marsilea vestita, including gamete approach and fusion, the fate of the spermatozoid organelles and the development of a possible block to polyspermy are described. The spermatozoid approaches the egg through layers of mucilage that surround the megaspores. It moves down the neck of the archegonium into the cavity above the egg. In order to reach the egg, it must move through a small hole in the thick wall that lies across the top of the egg. The fusion of the plasma membranes of the gametes results in an outflow of egg cytoplasm into the clear space under the sperm plasma membrane, creating a fertilization cone. All the organelles of the fertilizing spermatozoid, including nucleus, mitochondrion, microtubule ribbon, multilayered structure, and flagellar band, with approximately 150 flagella, enter the egg cytoplasm. The nucleus enters as a condensed rod of chromatin with no nuclear envelope. The chromatin begins to disperse immediately and a new nuclear envelope is formed around the chromatin by egg endoplasmic reticulum. The mito-chondrion and the microtubules of the ribbon and flagella are broken down, but the fates of the flagellar band and the multilayered structure have not been determined. After spermatozoid penetration, a new extracellular layer appears above the surface of the egg, beginning in the region of sperm penetration and spreading across the top of the egg. This layer may be important in preventing other spermatozoids from fusing with the egg.