bluuebunny/crossref_metadata_2025_split huggingface dataset:
The structure and development of the spermatozoon of Aspiculuris tetraptera (nematoda)
Authors: Lee D. L. , Anya A. O. 
Published: April 2021

ABSTRACT
The mature spermatozoon of Aspiculuris tetraptera has a distinct head-like structure and a tail-like structure. The tail contains a long mitochondrion which extends from within the head almost to the posterior end of the tail. Two bundles of microtubules oriented parallel to the longitudinal axis also extend the length of the tail on either side of the mitochondrion. The head of the spermatozoon contains abundant cytoplasm but no nucleus. The DNA of the spermatozoon is in the tail-like structure where it is associated with an electron-dense sheath and the two bundles of microtubules. The head can put out pseudopodia and it is suggested that they may be involved in locomotion or may be acrosome-like. The tail, which structurally resembles a motile appendage, has not been seen to move. This spermatozoon differs markedly in its structure from all others which have been described.
Cytoplasmic microtubules in the leaf glands of Phaseolus vulgaris
Authors: O’Brien T. P. 
Published: April 2021

ABSTRACT
Preliminary observations on the fine structure of the club-shaped glands on Phaseolus vulgaris leaves are reported. The perinuclear cytoplasm of the apical cells of these glands contains an abundance of microtubules. These occur either as aggregates of 2–8 or more tubules, or they may be organized around a central core of material to form a fibre-like structure. The cells also contain cortical microtubules and are rich in rough endoplasmic reticulum and dictyosomes. The nuclei of these cells also contain a proteinaceous fibre, visible in the light microscope. The possible significance of these structures is discussed in relation to cytoplasmic streaming, maintenance of cellular asymmetry, and reaction to virus infection.
An electron-microscope study of subcellular fractions of Octopus brain
Authors: Jones D.G. 
Published: April 2021

ABSTRACT
Using subcellular fractionation techniques the primary fractions (P1 P2 and P3) have been prepared from homogenates of the supraoesophageal lobes (brain) of Octopus vulgaris. A morphological study of the fractions was made, particular emphasis being placed upon the synapto-somes (nerve-ending particles), and the agranular and granular vesicles they contain. The acetylcholine levels in the fractions were determined by bioassay.
Synaptosomes and isolated mitochondria are present in all the fractions examined. In addition, the remains of blood vessels are found in P1 nuclei, vesicular structures and lamellated bodies in Px and P2, and microsomes in P3 and the final supernatant. The synaptosomes are similar to those isolated from mammalian nervous tissue. The remains of the post-synaptic endings are seen as recognizable processes around their periphery. They are more extensive than the post-synaptic membranes of mammals, and thickenings on them are seen in tissue stained with phosphotungstic acid. The number of mitochondria outside synaptosomes varies with the fixative used, being far greater with formalin than with permanganate. The agranular vesicles vary in diameter from 150 to 1200 Å; over 70 % of them are between 250 and 500 Å. In formalin-fixed preparations, the synaptosomes can be separated into two types: those containing mainly rounded vesicles, and those with many ellipsoidal vesicles. About 4 % of all the vesicles are granular; they have an average diameter of 720 Å, with few of them measuring over 1000 Å.
Acetylcholine is distributed throughout the fractions, the highest level being in P2, with appreciable levels in the others. The total recovered for the combined fractions of the supra-oesophageal lobes was 104·6 mμmole/g tissue. For the combined fractions of the optic lobes, which are also rich in synaptosomes, the total was 32·3 mμmole/g. tissue.
The ultrastructure of the olfactory epithelium of the lamprey Lampetra fluviatilis
Authors: Thornhill R. A. 
Published: April 2021

ABSTRACT
In common with those on other vertebrate olfactory receptors, the short cilia on the sensory cells are arranged in a ring on the tip of the dendrite. The cilia have two distinct regions—a proximal region, having a typical ciliary structure, and a distal region in which only single peripheral fibres are present. One or more basal feet are present, projecting from the basal body of the olfactory cilium. A 40–50 Å subunit structure in the peripheral fibres of the olfactory cilium is described. Numerous microtubules are present in the dendrite and in the basal cells, but are absent from the supporting cells. The supporting cells contain an extensive system of tonofilaments. Junctional complexes are situated between the plasma membranes at the apex of supporting cells and between supporting cells and neurons. Degenerating neurons and supporting cells are present as well as possible regenerating cells. On the basis of this evidence it is postulated that limited cell replacement may occur. It is also postulated that the arrangement of olfactory nerve fibres within the Schwann cell may allow some processing of information to take place at this level by means of electrotonic field effects.
Similarities of the golgi apparatus membrane and the plasma membrane in rat liver cells
Authors: Hodson S. , Brenchley G. 
Published: April 2021

ABSTRACT
A Golgi apparatus-rich fraction and a plasma membrane-rich fraction were isolated from a common homogenate of rat liver. Their respective buoyant densities, appearances in the electron microscope and 5′-nucleotidase and UDP-galactose ovalbumin galactosyltransferase activities were in accord with published data on separately isolated Golgi apparatus-rich and plasma membrane-rich fractions. Contamination by endoplasmic reticulum and mitochondria was low.
Gel electrophoresis of the membrane proteins of the Golgi apparatus-rich and plasma membrane-rich fractions (separately and mixed) showed a close similarity. After Neville’s demonstration that electrophoretic patterns of membrane protein subunits from different subcellular fractions are easily distinguishable, the present work demonstrates an unusually close relationship between the Golgi apparatus membrane and the cell membrane. It is possible that membrane similarity may be mediated by the transfer of membrane-bound vesicles from the Golgi apparatus to the cell membrane.
Template activity of unfixed metaphase chromosomes
Authors: Cohen B. B. , Deane D. L. 
Published: April 2021

ABSTRACT
Unfixed metaphase and non-metaphase cells were tested for their template activity with RNA polymerase. A device was used which disrupts the cell membrane by centrifugation, and which also ensures that the cells do not continue with their mitotic cycle during the transcription process. The template activity of acid/methanol fixed cells was also tested.
None of the unfixed metaphase cells transcribed RNA whereas most of the non-metaphase cells did. In contrast, using fixed cells both classes of cells were transcribed equally well and to a much greater extent. It was concluded that metaphase chromosomes in vivo cannot act as templates for RNA synthesis.
Regulation of interstitial cell differentiation in Hydra attenuata: I. homeostatic control of interstitial cell population size
Authors: Bode H. R. , Flick Kristine M. , Smith G. S. 
Published: April 2021

ABSTRACT
Mechanisms regulating the population size of the multipotent interstitial cell (i-cell) in Hydra attenuata were investigated. Treatment of animals with 3 cycles of a regime of 24 h in 10−2 M hydroxyurea (HU) alternated with 12 h in culture medium selectively killed 95—99% of the i-cells, but had little effect on the epithelial cells. The i-cell population recovered to the normal i-cell: epithelial cell ratio of 1:1 within 35 days. Continuous labelling experiments with [3H]thymidine indicate that the recovery of the i-cell population is not due to a change in the length of the cell cycle of either the epithelial cells or the interstitial cells.
In control animals 60% of the i-cell population undergo division daily while 40% undergo differentiation. Quantification of the cell types of HU-treated animals indicates that a greater fraction of the i-cells were dividing and fewer differentiating into nematocytes during the first 2 weeks of the recovery after HU treatment. Therefore, the mechanism for recovery involves a shift of the 60:40 division: differentiation ratio of i-cells towards a higher fraction in division until the normal population size of the i-cells is regained. This homeostatic mechanism represents one of the influences affecting i-cell differentiation.
The influence of gibberellic acid and abscisic acid on cell and tissue differentiation of bean callus
Authors: Haddon Lindsay , Northcote D. H. 
Published: April 2021

ABSTRACT
Bean callus was induced to form roots (tissue differentiation) and vascular nodules (cell differentiation) by lowering the ratio of auxin to cytokinin in the growth medium. Both types of differentiation were inhibited by the addition of abscisic acid (at concentrations greater than 1 μM) to induction medium. Initiation of differentiation was inhibited, but its subsequent development was not, and the inhibition was not affected by the addition of gibberellic acid. Addition of gibberellic acid (GA) alone to induction medium stimulated tissue differentiation, although cell differentiation was unaffected (30 μM GA) or inhibited (45 μM GA) and its onset was delayed at both concentrations. Root initiation was also stimulated by gibberellic acid (0·1-45 μM) at an auxin-to-kinin ratio 10 times that normally optimal for cell differentiation.
The phenylalanine ammonia lyase (PAL) activity of the calluses was closely correlated with the amount of cell differentiation which had occurred, and measurement of this confirmed that gibberellic acid delayed the initiation of cell differentiation. The increase and subsequent decline of PAL and βI → 3 glucan synthetase activities, normally induced by transfer to induction medium, was abolished by abscisic acid. Addition of gibberellic acid did not affect the βI → 3 glucan synthetase activity.
Two ultrastructurally distinct tubulin paracrystals induced in sea-urchin eggs by vinblastine sulphate
Authors: Starling D. 
Published: April 2021

ABSTRACT
Two types of ultrastructurally distinct tubulin paracrystals have been induced in sea-urchin eggs with vinblastine sulphate (VLB) under different sets of conditions. One type of paracrystal appears to consist of hexagonally-close-packed microtubules and closely resembles paracrystals present in mammalian cells treated with vinblastine or vincristine sulphate, but not previously reported in sea-urchin eggs. The other type is also made up of tubulin subunits, but these do not seem to have polymerized into microtubules.
Both types of paracrystal are induced in sea-urchin eggs in the presence of VLB at a time when tubulin subunits would not normally polymerize. Possible mechanisms for tubulin activation and the induction of paracrystal formation are discussed in respect to the available information on the binding sites of the tubulin subunits.
The effects of mitotic inhibitors on the structure of vinblastine-induced tubulin paracrystals from sea-urchin eggs
Authors: Starling D. 
Published: April 2021

ABSTRACT
Vinblastine sulphate (VLB) is known to induce in vivo formation of tubulin paracrystals in sea-urchin eggs. Corresponding paracrystals have been prepared in the presence of both vinblastine sulphate and other mitoclasic agents. Careful standardization of conditions was required to restrict the formation of alternative forms of the paracrystals induced by vinblastine alone. Comparisons were made between preparations in terms of paracrystal shape, size, proportion of eggs containing paracrystals, number per egg and their relative times of first appearance. A correlation between such properties was established. Comparison of paracrystals at the ultrastructural level showed them all to be similar regardless of the drugs present during their formation. The implications of tubulin polymerization in the presence of mitoclasic agents are discussed and mechanisms for paracrystal enhancement by combinations of such drugs are suggested. Some similarities of paracrystal and microtubule seeding are discussed together with the activation of tubulin in the pool.
The dynamics of antibody-induced redistribution of viral envelope antigens in the plasma membranes of avian tumour virus-infected chick embryo fibroblasts
Authors: Phillips E. R. , Perdue J. F. 
Published: April 2021

ABSTRACT
Avian tumour virus-infected chick embryo fibroblasts express new antigens, identical with the viral envelope antigens, in their plasma membranes. Electron-microscopic examination of carbon-platinum replicas of cells labelled with haemocyanin-marked antibody has shown the distribution of these antigens to be diffuse over the cell surface with an increased concentration on peripheral cell processes. However, antigen–antibody complexes (AAC), resulting from reaction with specific antibody, may be redistributed into a variety of patterns.
Observation of the time course of antibody-induced antigen mobility revealed a rapid and a delayed phase of redistribution. During the rapid phase (10 min or less) some of the antigenbearing cells reorganized AAC into patches, while the remainder maintained a diffuse distribution. A fraction of the cells with either diffuse or patchy distribution also redistributed AAC into a pattern of ‘marginal redistribution (MR)’, consisting of linear aggregation of AAC, at the cell edge. During the ‘late’ phase of redistribution (after about 20 min), AAC began to condense into one or more foci of coalescence (FC) on each cell. As the number of cells with FC increased with time, the fraction of cells which were labelled decreased. Electron-microscopic observation of thin sections of ferritin-labelled specimens indicated that AAC were lost by endocytosis and that this process was probably related to FC formation.
Inhibitors of oxidative phosphorylation, protein synthesis, RNA synthesis, or microtubule assembly had no significant effect on the patterns or the course of redistribution. lodoacetic acid (IAA), which depletes cellular ATP, and cytochalasin B (CB), which is believed to depolymerize microfilaments, partially inhibited MR and completely prevented FC formation and endocytosis. Paradoxically, IAA or CB-treated cells lost AAC very rapidly by some alternate mechanism not involving FC formation but which may entail a centrifugal migration of complexes to the cell extremities during the process of AAC disposal.
Analysis of the chemotactic response during aggregation inDictyostelium minutum
Authors: Raman Rajagopalan K. 
Published: April 2021

ABSTRACTMeasurements on the chemotactic aggregation of the amoebae of the cellular slime mould Dictyostelium minutum are reported. With the aid of previous theoretical results, the observed patterns of early aggregation on 2- and 3-dimensional substrates are interpreted. There is no statistically significant difference in the patterns in the 2 cases. The results suggest that the simplest plausible chemotactic law for this system is the relative gradient law in which the response velocity of the amoebae is proportional to the relative acrasin gradient. The results also suggest that the differentiation of the founder cell, which initiates aggregation, occurs before all cells have acquired chemotactic sensitivity. A diffusible chemical may be involved in the development of the latter.
The cell cycle during amphibian limb regeneration
Authors: Wallace H. , Maden M. 
Published: April 2021

ABSTRACT
The duration of the cell cycle in the blastema of regenerating limbs of axolotls has been measured by means of [3H]thymidine pulse labelling and autoradiography. A chase was required to define the pulse period. An average cell cycle at 20 °C takes 53 h, S-phase takes 38 h; including parts of mitosis, G1 is 10 h and G2 is 5 h long. The protracted cycle and S-phase are consonant with the large genome in axolotls and other urodeles. The rapidly growing blastema probably contains a steady population of about 5000 proliferating cells, as there is a regular withdrawal of differentiating cells from the population. The kinds of determination which exist in this population of cells, or are exerted on it, are briefly considered.
Meiotic maturation of mouse oocytes in vitro: association of newly synthesized proteins with condensing chromosomes
Authors: Wassarman P. M. , Letourneau G. E. 
Published: April 2021

ABSTRACT
The nature, intracellular distribution, and role of proteins synthesized during meiotic maturation of mouse oocytes in vitro have been examined. Proteins synthesized during the initial stages of maturation are concentrated within the nucleus (germinal vesicle) and become intimately associated with the condensing chromosomes. Inhibition of protein synthesis during this period does not prevent germinal vesicle dissolution or chromosome condensation, but meiotic progression is blocked reversibly at the circular bivalent stage. A protein is synthesized during meiotic maturation of the mouse oocyte which exhibits several of the characteristics of the very lysine-rich histone, F1; this and other histones are phosphorylated during the initial stages of maturation. These results are discussed in relation to studies of meiotic maturation of oocytes from non-mammalian species and chromosome condensation in both oocytes and mitotic cells.
Ultracryotomy of biological tissues to preserve membrane structure
Authors: Hodson Stuart , Williams Leslie 
Published: April 2021

ABSTRACT
A vacuum transfer stage is described which permits visualization of ultracryotome sections without the considerable distortions found in sections which have either thawed or rehydrated after freeze drying. Membrane structure - nucleus, nucleolus, mitochondria with their cristae and plasma membrane - was observed only in cells at the surface of the tissue which had undergone the fastest freezing rates. Inspection of knife damage in sections through these superficial cells showed that glass or diamond knives which are perfectly adequate for sectioning resin-embedded tissues are less suited to sectioning frozen biological tissue. Deeper in the tissue, where the freezing rates were slower, ice crystal cavities destroyed all membranous structures.
Elimination of Hydra interstitial and nerve cells by means of colchicine
Authors: Campbell Richard D. 
Published: April 2021

ABSTRACT
Hydra treated with colchicine or Colcemid become depleted of 95–99% of their interstitial cells and derivatives of this stem cell: nematoblasts, nematocytes and nerve cells. A second treatment removes most or all remaining interstitial cells. The most effective treatment is an 8-h immersion of whole Hydra attenuata in 0 0·4% Colcemid or 0·4% colchicine. Interstitial cells are eliminated through phagocytosis by both ectodermal and endodermal epithelial cells. The endodermal digestive cells send processes through the mesoglea which engulf interstitial cells and retract them into the endoderm. The resultant hydra, though devoid of nematocysts, can be artificially fed: these animals grow and bud and can be used to study the behaviour and development of tissue lacking nerve and interstitial cells.
Cell-to-substrate contacts in living fibroblasts: an interference reflexion study with an evaluation of the technique
Authors: Izzard C. S. , Lochner Linda R. 
Published: April 2021

ABSTRACT
The closeness of contact between cultured chick heart fibroblasts and glass substrates has been examined by interference reflexion microscopy. Evaluation of the optical technique demonstrated that both the film of medium between the cell and substrate and the cell itself act as two superimposed thin films which generate a complex interference pattern in reflected light at low illuminating numerical aperture (I.N.A.). The interference pattern generated by the cell consists of first and higher orders of interference and can be eliminated from the image by increasing the I.N.A. to &gt; 1·0. The residual zero-order interference pattern at high I.N.A. originates from the thin film of medium between cell and substrate and corresponds to the closeness of contact between cell and substrate. Based on the zero-order interference pattern and a comparison of the same cells with differential interference optics, the following features were recognized in living chick heart fibroblasts. Focal contacts, 0·25–0·5 μm wide, 2–10 μm long, separated by 10–15 nm from the substrate, are located under the peripheral regions of the leading lamellae and near the edge of extended non-spreading regions of the cell margin in moving and stationary cells. Individual focal contacts are coincident with cytoplasmic fibres of the same dimensions as the contact or with the peripheral ends of longer fibres that extend from the focal contact toward the centre of the cell. In spreading cells a second type of contact is present under the peripheral regions of the leading lamellae. This close contact consists of a broad area of uniform cell-to-substrate separation distance (ca. 30 nm). Focal contacts are distributed within the close contact under the leading lamellae. Lamellipodia extend ahead of the close contact and are separated by too nm or more from the substrate. Depending on the previous motile history of the cell, the close contact extends to varying degrees under the centre of the cell but it is typically dissected here by discrete areas of distinctly greater cell-to-substrate separation (100–140 nm). Evidence for the adhesive nature of the focal contacts is considered. The common identity is discussed of the focal contacts and associated cytoplasmic fibres described here in living cells with the regions of closest apposition to the substrate and associated cytoplasmic plaques and bundles of microfilaments seen in EM studies.
An ultrastructural and radioautographic study of the chromocentric interphase nucleus in plant meristematic cells (Raphanus Sativus)
Authors: Lord A. , Lafontaine J. G. 
Published: April 2021

ABSTRACT
In Raphanus sativus, the mitotic chromosomes are quite short and, on reaching the cell poles, soon undergo extensive unravelling. By late telophase and early interphase, only a few chromosome segments, believed to correspond to the centromeric regions, are still visible in the form of chromocentres closely associated with the nuclear envelope.
Although interphase nuclei show little internal structural differentiation, high-resolution radioautography has permitted us to establish which of them have reached the early, mid and late S periods. In early S nuclei, only the nucleolus and the euchromatin which pervades the nuclear cavity become labelled. By the mid S-period, the diffuse chromatin and nucleolus incorporate less thymidine and DNA synthesis is initiated within the peripheral chromocentres. Subsequently, the radioautographic grains become restricted to the chromocentres. The finding that certain late S nuclei exhibit loosely organized chromocentres strongly suggests that these heterochromatic chromosome segments undergo important conformational modifications during DNA replication. Finally, the presence of radioautographic grains over the lacunar regions of the nucleolus in early and mid S nuclei demonstrates that intranucleolar DNA replicates during the earlier portion of the S-period.