bluuebunny/crossref_metadata_2025_split huggingface dataset:
Topographical variations in the structure of the smooth septate junction
Authors: Skaer Helen Leb. , Harrison J. Barrie , Lee William M 
Published: April 2021

ABSTRACT
Smooth septate junctions in the midgut of Musca domestica and in Malpighian tubules of both Musca and Rhodnius prolixus are described. Details of the structures revealed after standard fixation, fixation in the presence of the stain, lanthanum hydroxide, and after freeze-fracture are discussed in the light of models previously put forward to explain the interrelations of the images obtained by these different methods. The organization of the junction between cells of the midgut varies in the apical-to-basal axis. At the apical border the septa (or ridges in freeze-fracture replicas) are packed tightly and follow an undulating but strictly parallel course. This packing loosens towards the middle of the junction until, at its basal extremity, the septa (ridges in replicas) are widely separated and follow independent meandering courses. That these features are found both in lanthanum-infiltrated specimens and freeze-fracture replicas allows a correlation to be made between the septa and the freeze-fracture ridges. The functional significance of these smooth septate junctions is discussed.
Mechanism of cleavage of newt eggs
Authors: Kubota Tomoyuki 
Published: April 2021

ABSTRACT
Cytokinesis by contraction of the base of the cleavage furrow and the dependence of the position of the future furrow on the arrangement of cytoplasm under it have been demonstrated at relatively earlier stages of cleavage in amphibian eggs. The following experiments and observations on dividing eggs of the newt Cynops pyrrhogaster indicated the presence of the contracting force at the base of the furrow and revealed that the above cleavage mechanisms occurred throughout cleavage. (1) Deformation of the animal surface induced conversion of an early furrow to a ridge, the base of the furrow forming the top of the ridge. (2) A needle placed in the path of the furrow blocked its deepening, not only at the point of contact but also in neighbouring regions. Any part of the plane of cleavage could be blocked in this way. (3) A microfilamentous band was found under the plasma membrane at the base of the furrow at a late stage of cleavage. (4) Alteration in the position of the furrow base could be induced in the middle of cleavage by outflow of egg cytoplasm.
A crystalline inclusion in sieve element nuclei of Amsinckia: II. The inclusion in maturing cells
Authors: Esau Katherine , Magyarosy Andrew C. 
Published: April 2021

ABSTRACT
The compound crystalloids formed in sieve element nuclei of Amsinckia douglasiana A. DC. (Boraginaceae) during differentation of the cell become disaggregated during the nuclear breakdown characteristic of a maturing sieve element. The phenomenon occurs in both healthy and virus-infected plants. The crystalloid component termed cy, which is loosely aggregated, separates from the densely aggregated component termed ex and disperses. The cx component may become fragmented, or broken into large pieces, or remain intact after the cell matures. After their release from the nucleus both crystalloid components become spatially associated with the dispersed P-protein originating in the cytoplasm, but remain distinguishable from it. The component tubules of P-protein are hexagonal in transections and are somewhat wider than the 6-sided cy tubules. The ex tubules are much narrower than the P-protein or the cy tubules and have square transections. Both the P-protein and the products of distintegrated crystalloids accumulate at sieve plates in sieve elements subjected to sudden release of hydro static pressure by cutting the phloem. The question of categorizing the tubular components of the nuclear crystalloid of a sieve element with reference to the concept of P-protein is discussed.
Mitochondria and mitochondria tonofilament-desmosomal associa tions in the mammary gland secretory epithelium of lactating cows
Authors: Lee C. S. , Morgan G. , Wooding F. B. P. 
Published: April 2021

ABSTRACT
The lactating cow mammary secretory epithelial cell is very active synthetically and contains numerous very pleomorphic mitochondria. Cup- and ring-shaped mitochondria are frequent and many are extremely elongated. Preferential localization of mitochondria in the basal region, or at the lateral margins of the secretory cell adjacent to intraepithelial monocytes have been observed. Occasional mitochondria show one or several very densely staining cristae with a lattice pattern. Mitochondria are frequently seen closely associated with the tonofilament-desmosomal complex. These structures and associations are present after immersion or perfusion fixation of mammary gland from several breeds of cow; their possible significance is discussed.
Synthesis of intracellular membrane proteins in vitro. Relation between rough endoplasmic reticulum and mitochondrial outer membrane
Authors: Shore Gordon C. 
Published: April 2021

ABSTRACT
Hepatic rough microsomes were incubated in a messenger-dependent protein-synthesizing system from rabbit reticulocytes. Up to 30 % of the total product labelled with [1S]methionine, and subsequently recovered with the microsomes, was located in an intrinsic protein fraction associated with these membranes, i.e. was retained by the membrane following extensive sonication in the presence of 1·5 M KC1, 0·1 % deoxycholate, and 5 mM ethylenediaminetetra-acetate (EDTA). When products synthesized with the use of membrane-free mRNA from rough microsomes and free polysome were post-incubated with rough microsomes, ribosome-stripped rough microsomes, or outer mitochondrial membrane, low amounts of intrinsic-type polypeptide product were recovered with these membranes. Higher recovery was achieved, however, when ribosome-stripped rough microsomes were added at the beginning of polypeptide synthesis in a reticulocyte lysate supplemented with additional ribosomal-wash factors. Analysis of these products by polyacrylamide gel electrophoresis showed that a numberco-migratedwith intrinsic proteins located in both rough microsomes and mitochondrial outer membrane. In addition, a prominent in vitro product co-migrated with a major protein which is located in outer mitochondrial membrane fractions, but is barely detectable in rough microsomal fractions. The present experiments were unable to detect a unique set of intrinsic polypeptides which were synthesized and assembled in vitro under the direction of mRNA from free polysomes, and not from rough microsomes. The results suggest that synthesis of at least some intrinsic membrane proteins which are destined for the outer mitochondrial membrane occurs on rough ER in rat liver.
Stem cell growth and differentiation in Hydra attenuata I. Regulation of the self-renewal probability in multiclone aggregates
Authors: Sproull Frederick , David Charles N. 
Published: April 2021

ABSTRACT
Interstitial stem cells in Hydra are rapidly proliferating multipotent stem cells which continuously give rise to precursors for nerve and nematocyte differentiation. Growth of the stem cell population is controlled by the cell cycle time of the stem cells and the self-renewal probability, Ps (the fraction of stem cells in each generation which divide to yield more stem cells). In normal Hydra the stem cell generation time is 24 h and Ps = o·6; under these conditions the stem cell population doubles in 3·5 days.
In the present experiments we have systematically investigated the dependence of Ps on stem cell density. We culture stem cells in a feeder layer system consisting of aggregates of nitrogen-mustard (NM)-inactivated Hydra cells. In this system stem cell density can be varied over a wide range by changing the number of clone-forming units (CFU) added to the aggregates. We have measured the growth rate of the stem cell population and the cell cycle of stem cells in NM aggregates after 4-7 days of culture. From these data we calculate the value of Ps. The results indicate that the growth rate decreases 4-fold as the number of CFU seeded per aggregate increases from 10 to 400. Under these same conditions the cell cycle remains constant. The values of Ps calculated from these results indicate the Ps decreases from 0-75 in aggregates seeded with 10-30 CFU to 0·55 in aggregates seeded with 200-400 CFU. These results support a model in which Ps is controlled by negative feedback from neighbouring stem cells.
In addition, our experiments indicate that Ps decreases during the growth of stem cell clones. When only a few stem cells are seeded in aggregates, they give rise to isolated clones distributed throughout the aggregate. Ps decreases markedly within such clones as they grow in size presumably due to increasing stem cell content of the clones. Since Ps in such isolated clones declines with growth, we infer that the local stem cell concentration is what controls Ps and that the spatial range of the negative feedback signal is short compared to the dimensions of NM aggregates.
Observation of cultured embryonic epithelial cells in side view
Authors: Sanders E. J. , Prasad S. 
Published: April 2021

ABSTRACT
Epithelial cells from the early chick embryo were explanted and grown on a strip of coverglass which was positioned in a culture chamber in such a way that the cells could be filmed in side view. Ruffling of the cells at the leading edge of the sheet (marginal cells) appeared similar to the ruffles at the leading lamella of fibroblasts. The ruffles of the sub-marginal cells showed different characteristics, being longer and not propagated successively as were the marginal ruffles. Superimposition of cells was always the result of the active movement of a non-yolky cytoplasmic process underneath a yolky process (i.e. underlapping). The non-yolky processes appeared to be strongly adherent to the substrate in comparison with the yolky processes, which were easily displaced. Active movement of cells or lamellae on the dorsal surface of the sheet was never observed. Preliminary ultrastructural evidence is presented which suggests that the strongly adherent non-yolky processes possess well-defined microfilament tracts associated with the ventral plasma membrane, while the poorly adherent processes do not possess such tracts.
The effect of colchicine on fibrillar material in wheat meiocytes
Authors: Bennett M. D. , Smith J. B. 
Published: April 2021

ABSTRACT
Wheat meiocytes contain intranuclear bundles of microfibres at stages from pre-meiotic mitosis until zygotene. This includes early pre-meiotic interphase, the stage when colchicine induces univalents at subsequent metaphase I. This paper reports the effect of colchicine on the structure and distribution of such fibrillar material. Many normal-looking bundles of intranuclear fibrillar material formed after prolonged colchicine treatment (up to 5 days) which totally suppressed microtubule assembly in meiocytes. Thus, fibrillar material is unlikely to comprise tubulin. However, the formation of intranuclear fibrillar material was retarded in colchicine-trcated meiocytes, and fibrillar material was absent during the colchicinesensitive stage for univalency. The occurrence of cytoplasmic fibrillar material was delayed and greatly increased in colchicine-treated meiocytes compared with controls. A novel curved form of fibrillar material was seen in colchicine-treated meiocytes. It is concluded that colchicine had significant effects on the assembly and distribution of fibrillar material. These results are, therefore, compatible with the possibility that colchicine causes univalency in wheat via its effects on fibrillar material.
The organization of supercoiled DNA from human chromosomes
Authors: Mullinger A. M. , Johnson R. T. 
Published: April 2021

ABSTRACT
Nucleoids obtained from metaphase-arrested HeLa cells by gentle lysis in non-ionic detergent and 2 M salt were prepared for electron microscopy by Kleinschmidt spreading techniques. From biophysical studies nucleoids have been shown to contain high molecular weight DNA which is supercoiled and topologically constrained.
The contents of nucleoids disperse and collapse into 2 dimensions on the water surface as discrete particles (spreads), which are provisionally equated with metaphase chromosomes. In some cases separate spreads are linked.
The structure of the spreads is complex. A preliminary description is presented, although we believe that some of the structures reported may be products of preparation and spreading rather than significant features of the organization of intact nucleoids. There is considerable variation in the appearance of different spreads which may be related to the degree of unfolding, spreading or damage of the different preparations.
Each spread consists of one or more core areas surrounded by a network of fibres. Cores are composed, at least in part, of compressed fibres and superficially appear to constrain fibres of the network. In the network the inter-twining of pairs of fibres and the occurrence of flat fibre spirals (disks) are interpreted as evidence of DNA supercoiling, but other fibres of similar thickness are not visibly supercoiled. Higher-order structures including groupings of disks are observed: these may be the structural correlates of the folded domains of supercoiled DNA established biophysically. The relative proportion of disks and other network structures varies between spreads.
Oocyte maturation: aberrant post-fusion responses of the rabbit primary oocyte to penetrating spermatozoa
Authors: Berrios M. , Bedford J. M. 
Published: April 2021

ABSTRACT
Primary oocytes cannot be fertilized normally; they begin to develop this capacity as meiosis resumes. To elucidate the changes involved in acquisition of their fertilizability, rabbit primary oocytes displaying a germinal vesicle (GV oocytes) were placed in Fallopian tubes inseminated previously with spermatozoa, recovered 2–5 h later and examined by light and electron microscopy. At least 4 aspects of GV oocyte/sperm interaction were abnormal. Although the vestments and oolemma seem normally receptive to spermatozoa, fusion with the oolemma of the primary oocyte did not elicit exocytosis of cortical granules, and consequently multiple entry of spermatozoa into the ooplasm was common. Secondly, the GV oocyte cortex failed to achieve a normal engulfment of the anterior part of the sperm head. It sank into the ooplasm capped by only a small rostral vesicle or left the stable inner acrosomal membrane as a patch in the oolemma. Only rarely then was there significant dispersion of the sperm chromatin, and this remained surrounded by nuclear envelope. The persistence of this envelope constitutes a further aberrant feature, for it disappears immediately in secondary oocytes and was absent in primary oocytes in which germinal vesicle breakdown had occurred. The results are discussed with particular reference to current ideas about male pronucleus formation.
Lymphocyte surface and cytoplasmic changes associated with translational motility and spontaneous capping of Ig
Authors: Bhalla D. K. , Braun J. , Karnovsky M. J. 
Published: April 2021

ABSTRACT
Murine B-lymphocytes during translatory motion undergo a series of changes with respect to their morphology and distribution of surface immunoglobulins (Ig). The sequence of events comprising these changes was followed by fluorescence microscopy and a correlated detection of surface features by scanning microscopy on exactly the same cell. A round, presumably non-motile lymphocyte exhibited a random distribution of Ig and its surface displayed evenly distributed microvilli. Formation of a ruffled edge at one pole, accompanied by a decreased fluorescence at this pole marked the initial events of lymphocyte motility. In the subsequent stages, the ruffled edge became progressively prominent and displayed a constriction at its base, while the microvilli were displaced to the opposite pole. Ig in such lymphocytes was localized at the trailing, microvilli-rich pole. Thin sections of motile lymphocytes revealed Ig, microtubules, microfilaments and coated vesicles as the characteristic features of the trailing end. These observations may have bearing on the mechanism of lymphocyte motility and spontaneous capping of Ig.
Scanning electron-microscopic study of the uptake of Leishmania parasites by macrophages
Authors: Zenian Asadour , Rowles Peter , Gingell David 
Published: April 2021

ABSTRACT
The interaction of promastigotes of the protozoan parasite Leishmania tropica with mouse peritoneal macrophages in vitro was studied by scanning electron microscopy. Motile promastigotes attached to host cells by their flagellar tips to which the macrophages responded by producing rather closely fitting lamellar sheaths and progressively enveloping first the flagellum and then the body of the parasite. Lamellar advance during engulfment was rapid in the first 10 min but much slower later on. Fully engulfed parasites could be seen after i h but most parasites associated with host cells remained extracellular even after 4 h. On the other hand, parasites immobilized by fixation adhered by either their flagellar or somatic ends. Engulfment proceeded at a steady rate, and by 4 h most of them were completely engulfed. Both the attachment and engulfment stages of parasite uptake were inhibited by low temperature, cytochalasin D and mild fixation of macrophages. The rheological features of the host cells’ response to parasite adherence indicate that invasion by parasites is through phagocytosis rather than penetration.
Temperature effects on anaphase chromosome movement in the spermatocytes of two species of crane flies (Nephrotoma suturalis loew and Nephrotoma ferruginea fabricius)
Authors: Schaap Catherine J. , Forer Arthur 
Published: April 2021

ABSTRACT
Using phase-contrast cinemicrography on living crane fly (Nephrotoma suturalis Loew and Nephrotoma ferruginea Fabricius) spermatocytes, we have studied the effects of a range of temperatures (6–30 °C) on the anaphase I chromosome-to-pole movements of both autosomes and sex chromosomes. In contrast to previous work we have been able to study chromosome-to-pole velocities of autosomes without concurrent pole-to-pole elongation. In these cells we found that the higher the temperature, the faster was the autosomal chromosome movement. From reviewing the literature we find that the general pattern of the effects of temperature on chromosome movement is similar whether or not pole-to-pole elongation occurs simultaneously with the chromosome-to-pole movement. Changes in cellular viscosities calculated from measurements of particulate Brownian movement do not seem to be able to account for the observed velocity differences due to temperature.
Temperature effects on muscle contraction speed, flagellar beat frequency, ciliary beat frequency, granule flow in nerves, and chromosome movement have been compared, as have the activation energies for the rate-limiting steps in these motile systems: no distinction between possible mechanisms of force production is possible using these comparisons. The data show that even the different autosomes within single spermatocytes usually move at different speeds. These velocity differences cannot simply be related to chromosome size as the autosomes are visually indistinguishable.
The sex chromosomes start their anaphase poleward movement after that of the autosomes, and move more slowly (by a factor of about 4), but their velocities appear to be affected by temperature in the same fashion as those of the autosomes. The interval between the onset of autosome anaphase and sex chromosome anaphase is also affected by temperature: the higher the temperature, the shorter the interval between the 2 stages. We have observed abnormalities in sex chromosome segregation, which may be due to temperature, but have not determined what the exact temperature shift conditions are that cause these abnormalities.
Mitochondrial associations with specific microtubular components of the cortex of Tetrahymena thermophile: I. Cortical patterning of mitochondria
Authors: Aufderheide Karl 
Published: April 2021

ABSTRACT
Many of the mitochondria of Tetrahymena thermophila are localized in the cell cortex in regular, identifiable patterns. Two different mitochondrial patterns are seen; whether a cell expresses one or the other type apparently depends upon nutrient conditions in the culture and not upon other factors tested. Consistent associations between cortical mitochondria and certain of the cortical microtubular bands are seen at the light-microscopic level. Electron-microscopic observations confirm this and, furthermore, identify ultrastructural associations between cortical microtubules and the cortically located mitochondria. It appears that the cortical microtubular arrays serve as a guide for the localization (and thus patterning) of the cortical mitochondria.
Intracellular distribution of lead in Tetrahymena during continuous exposure to the metal
Authors: Nilsson Jytte R. 
Published: April 2021

ABSTRACT
Lead acetate (0·1–0·2%) forms a precipitate with the organic growth medium. The Tetrahymena cells ingest this lead-containing precipitate and cell growth is resumed after a variable lag period. Ingested lead is observed as electron-dense material in food vacuoles. Soon after exposure, cytoplasmic lead (preserved with certain fixation only) is revealed as electron-dense particles in cilia and in a halo around digestive vacuoles. Later the lead particles pervade the entire cell but after the lag period they are confined to membrane-bound spaces. In dilute growth medium, high concentrations of lead inhibit food-vacuole formation and cell growth. Under these conditions lead is deposited in alveoli of the pellicle and is also found in autophagic vacuoles and other membrane-limited structures. The study has revealed that lead enters Tetrahymena through the membrane of digestive vacuoles and through the cell surface. The change in distribution of lead during the lag period indicates that a mechanism is activated for removal of lead into membrane-bound spaces. The final storage of lead seems to be in lysosomes.
Some observations on the nucleolus inSpirogyra
Authors: Jordan E. G. , Godward M. B. E. 
Published: April 2021

ABSTRACTSeveral species of Spirogyra have been collected and fixed for electron microscopy in either 2 % osmium tetroxide or 5 % glutaraldehyde. Mitotic cells were selected with the light microscope from filaments of Spirogyra embedded in Epon.Evidence is given for an interphase cycle in the nucleolus of Spirogyra. A clear association between nucleolar material and chromosomes occurs at mitosis. The breakdown of the nucleolus is shown to occur in two stages, one as a consequence of the withdrawal of the nucleolar chromosomes, and the other after telophase in the new nucleus. The significance of these findings is discussed.