bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ultrastructure of the adrenal cortex in frozen thin sections
Authors: Magalhães M. M. 
Published: April 2021

ABSTRACT
Pieces of both unfixed and fixed rat adrenal cortex were frozen and cut with a cryoultramicrotome. In unstained frozen sections of unfixed adrenals patches of heterochromatin were seen in the nucleus, while numerous round uniformly dense structures, the mitochondria, could be identified in the cytoplasm. Lipid droplets appear as small light round areas. Staining the sections with potassium permanganate, phosphotungstic acid, or uranyl acetate, or immersing them in glutaraldehyde, increased the contrast of cell organelles. In frozen sections of fixed material, membranes became visible in negative contrast. Mitochondria display vesicular cristae, whereas the Golgi complex, lysosomes and peroxisomes are easily recognizable. The possibility of cutting frozen sections of unfixed material with reasonable morphological preservation allows the fine-structural localization of steroids in adrenal cells after applying autoradiographic emulsion to the sections.
Effects of ultraviolet light on synchronized chinese hamster ovary cells; potentiation by hydroxyurea
Authors: Burg Kornel , Collins Andrew R. S. , Johnson Robert T. 
Published: April 2021

ABSTRACT
We have examined the effects of hydroxyurea on u.v.-irradiated Chinese hamster CHO-Ki cells. Ultraviolet irradiation followed by incubation with hydroxyurea causes only slight disruption of DNA and chromosome structure in CHO-Ki cells compared with HeLa cells. There is, however, a clear potentiation by hydroxyurea of the u.v. killing of CHO-Ki cells, which is most pronounced at those points in the cycle which are reported to have small DNA precursor pools. This potentiation is reduced when DNA precursors are provided together with hydroxyurea. These data are discussed in terms of an uncoupling of excision and repair DNA synthesis.
Differentiation of the tapetum in A vena II. the endoplasmic reticulum and golgi apparatus
Authors: Steer M. W. 
Published: April 2021

ABSTRACT
During the differentiation of the tapetai cell the endoplasmic reticulum develops into an extensive, regionally differentiated system, consisting of 2 types of smooth cisternae interconnected by rough cisternae. Smooth cisternae with narrow lumens encircle the plastids and mitochondria and lead into rough cisternae with a wider lumen containing a moderately densely staining material. This material is stored in an unusual system of distended smooth cisternae, rather than being transferred to the Golgi apparatus. This system is believed to represent a holocrine secretion system, since it disappears at cell senescence at the same time as the pollen grains accumulate metabolite reserves. No morphological evidence could be found for the contribution of products of the reticulum to orbicule or sporopollenin synthesis.
The dictyosomes and vesicles undergo a continuous series of changes as the tapetai cells differentiate. Development of flat dictyosome cisternae with dark amorphous contents is followed by a further change to more undulating, or curved cisternae. These different dictyosomes produce vesicles with morphologically distinct contents, some of which are observed fused with the plasma membrane. At maturity the vesicles form unique cylindrical channels, sometimes branched, which occur throughout the cytoplasm. Examination of thick sections by HVEM and STEM suggests that they are developed to facilitate movement of the dictyosome product through the masses of endoplasmic reticulum cisternae to the cell surface.
Control of flagellar motion in Chlamydomonas and Euglena by mechanical microinjection of Mg2+ and Ca2+ and by electric current injection
Authors: Nichols M.Kathleen , Rikmenspoel Robert 
Published: April 2021

ABSTRACT
Upon impalement with a microelectrode in a Caa+-free medium containing 5 mM ATP, a Chlamydomonas cell lost its flagellar activity within 45 s, and the injection of either positive or negative direct current did not stimulate the flagella to beat after that time. When 3 mM Mg2+was added to the external medium, the impaled cell exhibited a flagellar frequency of 22 ± 7 Hz. With 5 mM ATP and 3 mM Mg2+ in a Ca2+-free medium, negative direct electric current inhibited flagellar frequency and positive direct electric current enhanced flagellar activity. The flagella recovered to approximately their characteristic frequency (20 ± 6 Hz) upon the cessation of current. Euglena and Chlamydomonas cells were mechanically microinjected with Ca2+ or Mg2+ ions contained in 1·0 M KCl-filled microelectrodes. In both cells, the injection of 0·02 M Ca2+ resulted in a decrease in flagellar frequency dependent on the amount of Ca2+ injected. The frequency decreased to zero Hz upon the injection of 16 × 10−14 1. in Euglena and 3·5× 10−141. in Chlamydomonas. The microinjection of 10 × 10−14 1. of o·2 M Mg2+ into Euglena cells resulted in an approximately 2-fold increase in flagellar frequency. Chlamydomonas flagella, which stop beating upon impalement in a Mg2+-free medium, began to beat when the cell was injected with Mg2+ ions. The flagella exhibited an average frequency of 16 ± 3 Hz when injected with 1·5 × 10−141. of 0·2 M Mg2+. The data indicate that an increase in internal Mg2+stimulates flagellar frequency. The microinjection of Ca2+ inhibits flagellar motility.
Cross-sectional myofibre and myofibril growth in immobilized developing skeletal muscle
Authors: Shear Charles R. 
Published: April 2021

ABSTRACT
The effect of immobilization on skeletal muscle fibre and myofibril growth in the developing chick latissimus dorsi posterior muscle was investigated. Immobilization, immediately upon hatching, results in a 25% decrease in mean myofibre cross-sectional area within 24 h. Longer periods of immobilization further retard normal myofibre growth until by day 27 after hatching the mean myofibre cross-sectional area was 70% less than normal. The variation in individual myofibre cross-sectional areas (measured as the coefficient of variation, CV = standard deviation/mean × 100) was greater in immobilized muscles. The larger immobilized myofibres showed atypical myofibril and sarcotubular complex organization at the ultra- structural level of observation whereas the smaller immobilized myofibres appeared normal, except for their size. In 1- to 27-day-old chicks, freeing the immobilized muscles for 24 h resulted in myofibril reorganization and sarcotubular proliferation but little myofibre growth. Freeing the immobilized muscles for 72 h resulted in a nearly complete recovery, in chicks up to 18 days post-hatching, of both cross-sectional myofibre area and ultrastructural morphology. Myofibres immobilized for longer than 18 days and subsequently freed for 72 h develop a normal myofibrillar and sarcotubular morphology but remain 23 to 29% smaller in crosssectional area than myofibres from control muscles.
Normal myofibril growth and organization are severely retarded in the immobilized myofibres. The evidence presented here suggests that sarcomere shortening causes growing myofibrils to rupture. The ruptured myofibrils are filled with proliferating elements of the sarcotubular complex which, in turn, completely subdivide (split) the myofibril.
Dna replication studies in genus Vicia through fibre radioautography
Authors: Gaddipati J. P. , Sen S. K. 
Published: April 2021

ABSTRACT
An experiment has been conducted to study the mode of replication in 3 species of Vicia having different C-values by means of fibre radioautography. Characteristics of the units involved in DNA replication of these species have been compared. It has been observed that replication occurs predominantly through bidirectional chain growth, replicating units are arranged in tandem, and replication proceeds from the point of origin in both directions. It has also been shown that there is a positive, though not proportionate, correlation between replication rates, the sizes of replication units and C-values.
Fine-structural localization of adenosine triphosphatase in the rectum ofCalliphora
Authors: Berridge M. J. , Gupta B. L. 
Published: April 2021

ABSTRACTAdenosine triphosphatase (ATPase) activity in the rectal papillae of Calliphora has been studied by biochemical and histochemical techniques. The microsomal fraction contained a Mg2+-activated ATPase with a pH optimum of 8·0. The enzyme was not stimulated by the addition of Na+ plus K+ and was insensitive to ouabain. Histochemical studies using modifications of the Wachstein–Meisel method showed that at pH 7·2 this Mg2+-activated ATPase was specifically localized on the intracellular surface of the lateral plasma membranes. A similar though less intense reaction was obtained with adenosine diphosphate and inosine triphosphate, but not with guanosine triphosphate, uridine triphosphate or β-glycerophosphate as substrates. At an acid pH (6·6–6·8), very little reaction occurred on the lateral plasma membrane but some reaction product was present in mitochondria and nuclei. Very little enzyme activity was found in the flattened rectal epithelium. These results are discussed in relation to the available data on transport ATPases and on the structural basis of fluid transport by rectal papillae. It is proposed that the ATPase localized on the stacks of lateral plasma membrane may be involved with ion secretion into the intercellular spaces to create the osmotic gradient necessary to extract water from the lumen.
The ultrastructure and ontogeny ofpollen in Helleborus Foetidus l. I. the development of the tapetum and ubisch bodies
Authors: Echlin P. , Godwin H. 
Published: April 2021

ABSTRACT
The ontogeny of the tapetum and Ubisch bodies in Helleborus foetidus L. has been examined at the ultrastructural level, and their development has been closely linked with that of the sporo-genous cell and pollen grains. During development the tapetum passes through successive phases of synthesis, maturity and senescence, ending in complete dissolution. During the anabolic phase of growth, precursors of the Ubisch bodies are formed as spheroidal vesicles of medium electron density within the tapetal cytoplasm; they are associated with a zone of radiating ribosomes, which, as development proceeds, can clearly be seen to be situated on strands of endoplasmic reticulum. The callose special wall round the microspores and the tapetal cell wall now disintegrate and the pro-Ubisch bodies are extruded through the cell membrance of the tapetal cells, where they remain on the surface of the anther cavity and soon become irregularly coated with sporopollenin. Deposition of sporopollenin continues on the Ubisch bodies at the same time as upon the exines of the developing pollen grains. In both cases, the later stages of sporopollenin deposition are associated with electron-transparent layers of unit-membrane dimensions appearing in section as white lines of uniform thickness. Continuing deposition of sporopollenin leads to the formation of compound or aggregate Ubisch bodies. It is conjectured that the sporopollenin is synthesized from the compounds of low molecular weight released into the anther loculus by the breakdown of the callose special wall and the tapetal cell wall. The final stages of tapetal autolysis involve the disappearance of all the cell organelles. An attempt is made to relate the findings to those described in other recent studies on Ubisch body formation and to combine them in a common ontogenetic pattern.
An association between viral transformation and forssman antigen detected by immune adherence in cultured BHK21 cells
Authors: O’neill C. H. 
Published: April 2021

ABSTRACT
A method of immune adherence applicable to tissue culture cells in monolayers is described. It is some twenty times more sensitive than cytotoxicity (dye-exclusion) as a quantitative immunoassay, applicable to the antigens of the cell surface. It has been found that adherence does not occur to confluent cell sheets, and this can be explained if immune adherence to tissue culture cells occurs only at the tips of pseudopods. This source of variation can be reduced to a minimum by the use of sparse cell layers, and under suitable conditions the antiserum titres obtained do not vary by more than one doubling dilution. With the aid of this method, the antigens of BHK21 hamster kidney cell surfaces have been studied after transformation by polyoma and other viruses. Using rabbit antisera, it was found that Forssman antigen, which is absent from BHK21 cells, appeared on the surfaces of all the transformed lines studied. Rous virus-transformed BHK21 cells also showed this antigen, and reversion to the normal cell morphology, which occurs in these lines, was accompanied by a loss of this antigen.
Surface features of striated muscle: I. guinea-pig cardiac muscle
Authors: Rayns D. G. , Simpson F. O. , Bertaud W. S. 
Published: April 2021

ABSTRACT
A general survey of guinea-pig myocardium was undertaken using the freeze-etch technique. Replicas of myocardial cell membranes were obtained. These showed an ordered array of pits or stumps situated at Z levels. The pits are interpreted as the apertures of the transverse tubules (T-tubules) seen from outside the cell, and the stumps as the remnants of the T-tubules remaining attached to the cell membrane after the cell contents have been removed. Pinocytotic vesicles were also present. T-tubules, mitochondria and myofilaments could be seen in replicas of the interior of myocardial cells.
Capillary endothelial cells were seen from various aspects; pinocytotic vesicles were their most prominent feature.
The appearances of the cell membranes in the present study suggest that the fracture plane tends to pass along either the outer or the inner surface of the membrane rather than to split the membrane.
The Synthesis Of Dna Through The Cell Cycle Of Amoeba Proteus
Authors: Ord M. J. 
Published: April 2021

ABSTRACT
DNA synthesis has been investigated in Amoeba proteus by pulse-labelling cells of known age with [3H]thymidine. Ninety per cent of the DNA so labelled was synthesized during the first quarter of the cell cycle. Synthesis was in two peaks: the first occurring between 0·5 and 4 h after division and the second at about 10 h. All cells labelled at both peaks. The authenticity of the second peak was proved statistically.
Considerable variation was observed among amoebae of similar age. In experiments in which daughter amoebae underwent different treatments, differences in the rates of incorporation of [3H]thymidine due to differences in the nutrition of the cells were found to be a predominant cause of variation. Heavy feeding reduced labelling; starvation increased labelling while at the same time reducing variability.
X-ray emission microanalysis of proteins and sulphur in rat plantar epidermis
Authors: Sims R. T. , Hall T. 
Published: April 2021

ABSTRACT
The distributions of proteins and protein-bound sulphur have been studied by X-ray emission microanalysis in the plantar epidermis of six albino rats. Plantar skin was fixed in 10% formaldehyde for 24 b and 5-μ paraffin sections obtained. Discrete and continuous measurements of the relative concentrations (mass/unit area) of proteins and sulphur were made along lines of scan perpendicular to the skin surface. The concentrations of proteins and sulphur almost double from the stratum basale across the stratum spinosum. There is a sharp twofold increase in their concentrations across the stratum granulosum to the stratum corneum where they become constant. The sulphur: protein ratio is constant across all layers of the epidermis.
Rough estimates of cell volumes give a ratio of 1:9:7-5 for the cells of the strata basale, granulosum and corneum respectively. Cell volume changes cannot be responsible for the changes in concentration so it is concluded that epidermal cells synthesize and accumulate proteins throughout their migration to the stratum corneum. The observations are regarded as circumstantial evidence for the incorporation of keratohyalin granules into cornified cells.
Membranes in lupin root nodules: II. preparation and properties of peribacteroid membranes and bacteroid envelope inner membranes from developing lupin nodules
Authors: Robertson J. G. , Warburton M. P. , Lyttleton P. , Fordyce A. M. , Bullivant S. 
Published: April 2021

ABSTRACT
Peribacteroid membranes and bacteroid envelope inner membranes have been isolated from developing lupin nodules. Isolation of the peribacteroid membranes was achieved by first preparing membrane-enclosed bacteroids free from other plant organelles or membranes. The peribacteroid membranes were then released by osmotic shock and purified by centrifugation to equilibrium on sucrose gradients. The bacteroids were broken in a pressure cell and the bacteroid envelope inner membranes were isolated using sucrose gradient fractionation of the bacteroid total envelope preparation. The density of the peribacteroid membranes decreased during the period of development of N2-fixation in lupin nodules from 1·148 g/ml for nodules from 12-day plants to 1·137 g/ml for nodules from 18-day plants. The density of the bacteroid envelope inner membranes from nodules from 18-day plants was 1·153 g/ml. The identity and homogeneity of the isolated membranes was established, by comparison with membranes in intact nodules, using phosphotungstic acid and silver staining of thin sections and particle densities on faces of freeze-fracture replicas of the membranes.
Analyses for NADH oxidase and succinate dehydrogenase, spectral analyses and gel-electro-phoretic analysis of proteins were also used to characterize the membrane and soluble protein fractions from the nodules. The ratio of lipid to protein was 6·1 for the peribacteroid membranes and 2·5 for the bacteroid envelope inner membranes. Leghaemoglobin was localized in the plant cytoplasm in lupin nodules and not in the peribacteroid space.
Small nuclear rnas: An association with condensed chromatin in amoebae
Authors: Goldstein Lester , Wise Gary E. , Stephenson Christopher , Ko Christine 
Published: April 2021

ABSTRACTFor the purpose of studying the relationship of small nuclear RNAs (snRNAs) to chromatin, we studied the effect of the chromatin-binding agent actinomycin D (AMD) on the nuclei of amoebae. By electron microscopy we noted that when mitotic cells were placed in AMD their chromatin was highly condensed 3 h later. This contrasts strikingly with the situation in untreated cells, in which the chromatin at the same time is largely decondensed. After arranging, by nuclear transplantation of labelled nuclei into unlabelled cytoplasm, for the only labelled molecules in the cell to be JH-snRNAs, we noted that in such cells similarly treated with AMD the labelled material was clustered in a form quite similar to that of the chromatin in 3-h post-division, AMD-treated cells. In untreated cells the 3H-snRNAs were distributed essentially uniformly throughout the nucleus.These observations, together with earlier findings that snRNAs associate with chromosomes during mitosis, lead us to conclude that snRNAs have a particular affinity for condensed chromatin.
Meiotic maturation of mouse oocytes in vitro: Protein synthesis in nucleate and anucleate oocyte fragments
Authors: Schultz Richard M. , Letourneau Gail E. , Wassarman Paul M. 
Published: April 2021

ABSTRACT
Nucleate and anucleate fragments of mouse oocytes have been isolated following treatment of fully grown oocytes with cytochalasin B. The nucleate oocyte fragments resume meiosis in vitro, progressing from dictyate of the first meiotic prophase to metaphase II (‘meiotic maturation’), and exhibit all of the changes in protein synthesis normally associated with meiotic maturation of mouse oocytes. The anucleate oocyte fragments also undergo certain of the changes in protein synthesis associated with meiotic maturation, despite the absence of nuclear progression. These results suggest that the acquisition of meiotic competence (i.e. the ability to undergo meiotic maturation) during growth of the mammalian oocyte is due to changes in the quality, rather than the quantity, of cytoplasm and that the reprogramming of protein synthesis during meiotic maturation is directed by RNA templates already present in the cytoplasm. The behaviour of anucleate oocyte fragments is discussed in terms of the proposed role for nucleoplasm in the initiation of changes in protein synthesis during meiotic maturation of mouse oocytes.
Cell movements in a confluent monolayer are not caused by gaps: evidence for direct contact inhibition of overlapping
Authors: Timpe L. , Martz E. , Steinberg M. S. 
Published: April 2021

ABSTRACTAccording to the hypothesis of contact inhibition of movement, cells in a confluent mono-layer are restrained from major overlapping by a directional inhibition of locomotion. This explanation of monolayering proposes that contact between 2 cells locally paralyses the locomotory function, preventing movement in the direction that would lead to overlapping. Consequently, a cell in contact on all sides with neighbouring cells should be immobilized. Yet in strictly monolayered cultures of confluent chick liver or mouse 3T3 cells, we have previously observed both translational cell movements and re-shufflings of relative cell positions. The ‘confluence’ was not perfect, however, and it seemed possible that the movements observed were due to release from contact inhibition by occasional transitory gaps seen to open up between cells.In the present study, detailed gap experiences and cell movements were recorded for 31 cells over a total of 1637 cell-hours. There was no significant correlation between frequency of gaps experienced and the extent of cell movement measured as neighbour-exchanges. We conclude that gaps are not a major cause of the movements observed. The hypothesis based on contact inhibition of motion, which attempts to explain monolayering indirectly by imposing a restraint on cell locomotion, cannot explain the substantial cell movements seen in the confluent cell monolayer studied here. To explain contact inhibition of overlapping, the evidence favours a more direct hypothesis which places no restriction on cell movement other than that overlapping be avoided. Such direct avoidance of overlapping could result from differences in the strengths with which cells adhere to one another and to the substratum.
The blood/germ cell barrier in maleschistocerca gregaria:the time of its establishment and factors affecting its formation
Authors: Jones R. Theresa 
Published: April 2021

ABSTRACTTracer studies carried out on the testes of the insect Schistocerca gregaria demonstrate a blood/germ cell barrier which is established prior to the onset of meiosis in the 3rd instar. Ecdysterone treatment in vitro prematurely stimulates the formation of the barrier in the testicular follicles of 2nd instar hoppers; this effect is antagonized by juvenile hormone. The follicles may be a store for the barrier-stimulating factor because testis extract also induces premature production of the barrier.