bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface features of striated muscle: I. guinea-pig cardiac muscle
Authors: Rayns D. G. , Simpson F. O. , Bertaud W. S.
Published: April 2021

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

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

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

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

For 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

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

According 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

Tracer 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.
Ultrastructure and activity of the nucleolar organizer in the mouse oocyte during meiotic prophase
Authors: Mirre C. , STAHL A. , de Lanversin A. , Moretti C.
Published: April 2021

The mouse oocyte is the site of nucleolar synthesis during pachytene. The chromosomes containing a nucleolar organizer are attached to the nuclear envelope by their paracentromeric heterochromatin, either alone or by taking part in the formation of a chromocentre. The nucleolus appears at the junction of the paracentromeric heterochromatin with the euchromatic portion of the bivalent. In this zone, 5·0-nm-diameter fibres, thinner than those of the rest of the chromosome (10·0 nm), extend from the lateral element of the synaptonemal complex up to the nucleolar fibrillar centre in which they penetrate. At the onset of its synthesis, the nucleolus only contains the fibrillar centre and an electron-dense fibrillar component in continuity with the latter. Growth of the nucleolus often takes place in the form of a strand whose proximal end, in contact with the fibrillar centre, is formed by preribosomal fibrils and whose distal end is at first fibrillo-granular then granular.

Following brief incorporation of tritiated uridine, nucleolar labelling is active in oogonia. No ribosomal RNA-synthetic activity is revealed during leptotene and zygotene. Incorporation resumes at mid-pachytene, with labelling located over the electron-dense fibrillar component adjacent to the fibrillar centre.

These observations suggest that the rDNA is located in both the fibrillar centre and its associated electron-dense fibrillar component and that the rDNA transcription occurs in the latter.
The cell cycle during the vegetative stage of Dictyostelium discoideum and its response to temperature change
Authors: Zada-Hames Irene M. , Ashworth J. M.
Published: April 2021

The cell cycle in amoebae of Dictyostelium discoideum has been analysed in cells growing asynchronously in axenic medium. For cells growing at the optimum growth temperature of 22 °C with a culture doubling time of 8 h the average times for the cell cycle phases are as follows: G1, 1 ·5 h; S, 2 ·1 h; G2, 4 ·4 h; M, 15 ·2 min. When amoebae are grown at temperatures below 22 °C, culture doubling time increases and the cell cycle phases are altered in ways characteristic for each phase. G1 is the most variable period and may occupy up to 70 % of the total cell cycle time; 5 and G1 are the least affected, increasing by only 20% when the cell generation time is doubled.

When cells which have reached the stationary phase of growth in liquid medium are washed and reinoculated into fresh medium they divide synchronously after a lag period of 5 h. By following cell number increase and nuclear DNA synthesis in these cultures we have shown that stationary phase cells are arrested in the phase of the cell cycle.

Finally, although more than 97 % of amoebae grown on a bacterial food source are uninucleate, when grown axenically up to 35 % of the cell population may become multinucleate. Our results suggest that these cells probably arise through the failure of cytokinesis to follow karyokinesis. Multinucleate cells appear to have a slightly longer G2 period than mononucleate cells.
Developmental roles of epithelial and interstitial cell lineages in hydra: Analysis of chimeras
Authors: Marcum Beverly A. , Campbell Richard D.
Published: April 2021

Chimeric hydra were prepared by recombining epithelial and interstitial cells between 3 strains of hydra of different sizes (maxi, normal, and mini strains). The resulting chimeras generally resembled the epithelial cell parent more than the interstitial cell parent in size, budding rate, tentacle number, and form. This suggests that epithelial cells normally exert considerable influence over hydra morphogenesis. However, the chimeras show some differences ascribable to interstitial cell origin. Furthermore, the 3 original strains, when deprived of interstitial cells, lose their distinguishing size differences. Thus both epithelial and interstitial cells (or interstitial cell derivatives) mutually participate in hydra’s development.
Ultrastructure of meiosis-inducing (heterotypic) and non-inducing (homotypic) cell unions in conjugation of Blepharisma
Authors: Bedini C. , Lanfranchi A. , Nobili R. , Miyake A.
Published: April 2021

Cells of mating types I and II of Blepharisma japonicum interact with each other and unite in heterotypic (type I-type II) or homotypic (type I-type I, type I I-type II) pairs. Heterotypic pairs undergo meiosis and other nuclear changes of conjugation, while homotypic pairs remain united for days without the nuclear changes taking place. We compared cell unions of these two kinds of pairs at the ultrastructural level. In the homotypic union, cell membranes are closely juxtaposed, separated by a distance of about 20 nm. This arrangement is interrupted in some places by vacuoles and small cytoplasmic bridges. Saccule-like structures tend to be more abundant near the united surfaces. Microtubules running at right or slightly obtuse angles with the cell surface (PACM microtubules) are characteristically present at the united region of cells. These structures are very similar to those observed in earlier stages of the heterotypic union. However, in homotypic pairs, cells unite only at the anterior half of the peristome, while in heterotypic pairs cells unite also at the posterior half of the peristome, where the cell membrane totally disappears in later stages. PACM microtubules persist for at least 18 h in homotypic unions, while they disappear within a few hours in heterotypic unions. These differences between the two kinds of cell union are discussed in relation to the initiation mechanism of meiosis and other nuclear changes of conjugation. Similarities between homotypic union and cell junctions in multicellular organisms are also discussed.
Computer simulation of aggregation in Dictyostelium discoideum
Authors: Mackay Steven A.
Published: April 2021

The aggregation phase of development of the cellular slime mould Dictyostelium discoideum is simulated on a computer. The simulation is performed in 2 dimensions, and produces animated graphical output similar to time-lapse films. It is based on observations of cell behaviour as expressed by a set of rules for each cell, involving cell movement and release of and response to chemical signals. Following these rules, the simulation has reproduced many observed aggregation patterns: propagating waves of cell movement; formation of branching streams; entrainment of slower centres; and spiral centre formation. The simulation has proved to be an important adjunct to experimental work.
Mechanisms of invasiveness of epithelial tumours: ultrastructure of the interactions of carcinoma cells with embryonic mesenchyme and epithelium
Authors: Tickle C. , Crawley A. , Goodman M.
Published: April 2021

Three different kinds of carcinoma cell, human breast tumours, mouse lung tumour and rat bladder tumours have been implanted into the developing chick wing to assay their invasive behaviour. We found that the majority of the carcinoma cells did not invade the mesenchyme but were positioned in the ectoderm. We examined the ultrastructure of the interaction of the carcinoma cells with both the ectoderm and the mesenchyme to see if there are any differences. We found no specialized adhesive junctions between carcinoma cells and mesenchyme cells. In contrast, the carcinoma cells seem to be adhesive to the ectoderm cells; desmosomes between the two have been found and also frequent parallel alignment of membranes over long distances. This difference between the interaction of the carcinoma cells with the mesenchyme and the ectoderm may explain why the carcinoma cells can penetrate into the ectoderm but rarely do so into the mesenchyme. The carcinoma cells formed a smooth border with the mesenchyme even in the absence of a basal lamina. These results are discussed in relation to the mechanisms of invasion of epithelial tumours in vivo. In particular, the relative importance of the basal lamina, and cell adhesion within the tumour, in limiting invasiveness arc assessed.
In vivo phagocytosis by developing myocardial cells: an ultrastructural study
Authors: Hurle J. M. , Lafarga M. , Ojeda J. L.
Published: April 2021

Phagocytosis of naturally degenerating cells, by healthy myocardial cells of the bulbus cordis of 5–7-day chick embryos, was studied by electron microscopy. Myocardial cells showed cell processes surrounding dead cell fragments. The cell fragments appeared to be internalized later and digested within phagosomes. Ruthenium red was employed to ascertain whether the cell fragments were in fact internalized or located in pockets of the cell membrane. The results are discussed in the light of present knowledge of phagocytosis.
Effect of temperature and divalent cations on the substratum attachment of rat hepatocytes in vitro
Authors: Seglen Per O. , Gjessing Rolv
Published: April 2021

The attachment of rat hepatocytes to polystyrene-adsorbed serum protein is relatively insensitive to inhibitors such as dextran sulphate, cycloheximide, colchicine and cytochalasin B, and enzymes like trypsin and neuraminidase, but it is strongly dependent on divalent cations. Mg2+ supports attachment better than Ca2+, but a combination of both is required for maximal attachment. The attachment is very temperature-sensitive, with a biphasic Arrhenius plot indicating an activation energy of 123 kJ/mol above 34 °C and 374 kJ/mol below 34 °C.

The adsorbed attachment-promoting serum factor is inactivated by trypsin, or by Ca2+-dependent proteases which contaminate commercial preparations of collagenase. The adsorbed factor is resistant to treatment with glutaraldehyde, neuraminidase and heating to 90 °C, whereas the same factor in the unadsorbed state (in serum) is destroyed by heating to 70 °C. The factor in serum is unable to compete with the adsorbed factor for cell binding, hence it would appear that adsorption to polystyrene induces the active, heat-resistant conformation of the factor.
The mechanism of antibody-dependent, eosinophil-mediated damage to schistosomula of Schistosoma mansoni in vitro: A study by phase-contrast and electron microscopy
Authors: Glauert Audrey M. , Butterworth A. E. , Sturrock R. F. , Houba V.
Published: April 2021

A characteristic sequence of events has been identified by phase-contrast and electron microscopy during antibody-dependent, eosinophil-mediated damage to schistosomula of Schistosoma mansoni in vitro.

Human eosinophils initially adhere to the intact schistosomulum and then, in the presence of antibody, flatten and spread very intimately over the parasite’s surface. Subsequently, dense material similar to the contents of the lysosomal granules of the eosinophils appears in the extracellular space between the eosinophil and the schistosomulum, probably following fusion of the granules with the plasma membrane of the cell. Eventually all the eosinophils adhering to the parasite are completely degranulated and large amounts of the dense material are observed on the surface of the schistosomulum. This release of granular material from the eosinophils is followed by structural changes in the schistosomulum, starting with vacuolation of the inner layer of the tegument, followed by removal of the tegument, often in the form of large sheets. Subsequently the tegument disintegrates and the fragments are phagocytosed by other eosinophils which have not degranulated. Eosinophils then attach to the exposed muscle layers of the schistosomula and participate in the further degradation of the parasites by phagocytosing fragments of muscle fibres and other cellular components.

This sequence of events is compared with published observations of the damage induced by various combinations of antibody, complement and effector cells in vitro, and of cell-mediated damage to schistosomula in vivo, and it is concluded that the observations described in the present paper may reflect a process of destruction of schistosomula in the immune host.
Regulation of interstitial cell differentiation in Hydra attenuata: V. Inability of regenerating head to support nematocyte differentiation
Authors: Yaross Marcia S. , Bode Hans R.
Published: April 2021

Nematocyte differentiation was examined during head regeneration in Hydra attenuata. Nematocyte precursors were found to decrease in head-regenerating tissue. This decrease could not be attributed to decreased stem cell commitment or to altered cellular kinetics. The nematocyte precursors could be ‘rescued’ by regrafting a head onto the initially regenerating tissue only prior to the time at which head determination occurred. These results suggest that concurrent with head determination an irreversible change occurs in the tissue environment, resulting in decreased survival of cells committed to nematocyte differentiation.
Structure, protein composition and birefringence of the costa: a motile flagellar root fibre in the flagellate Trichomonas
Authors: Amos W. B. , Grimstone A. V. , Rothschild L. J. , Allen R. D.
Published: April 2021

The costa is a rod-shaped intracellular organelle lying in the cytoplasm immediately below the undulating membrane. In certain large species of Trichomonas (T. gigantea, T. termopsidis and an undescribed species from the termite Porotermes adamsoni) this organelle is motile. Bending waves are transmitted along the length of the costa, in either direction. It is shown that the bending is actively produced by the costa itself. The form of the movements is described in detail. The costa is biréfringent. At the point at which bending occurs there is a sharply localized reduction in birefringence. Electron microscopy shows that the costa is composed of longitudinally running lamellae, 2-3 nm thick and spaced 12 nm apart. These are connected to transversely running elements spaced at intervals of about 37 nm. The lamellae occur in two alternative configurations: straight and zig-zag. Bending probably results from a local, transient change from the straight to the zig-zag configuration. This would account for the local change in birefringence which accompanies bending. Polyacrylamide gel electrophoresis of isolated costas shows them to contain a number of protein components, of which the principal one has a molecular weight of about 90000. Preliminary cytochemical evidence is presented for ATPase activity in the costa. The costa is a type of motile system distinct from any hitherto described.
Dyskinetoplasty in two species of trypanosomatids
Authors: Hajduk Stephen L.
Published: April 2021

Dyskinetoplastic cells from both Crithidia fasciculata and Trypanosoma equiperdum lack detectable kinetoplast DNA (kDNA) by conventional staining techniques. Two dyskinetoplastic strains of T. equiperdum, either acriflavine-induced or spontaneously occurring, show normal amounts of kDNA (ρ = 1 · 692g/cm3) in analytical caesium chloride ultracentrifugation. Electron and fluorescence microscopy of the dyskinetoplastic strains of T. equiperdum suggest that the kDNA network is fragmented and dispersed throughout the mitochondrion. The fragmentation and dispersion of the kDNA, rather than a reduction in the amount of kDNA, is the cause of the lack of kinetoplast staining in the dyskinetoplastic strains of T. equiperdum. Acriflavine-treated cultures of C. fasciculata show a decrease in the amount of kDNA (ρ = 1·703 g/cm3) corresponding to the percentage of dyskinetoplastic cells in the cultures. Electron and fluorescence microscopy of acriflavine-treated cultures of C. fasciculata show the loss of the kDNA network in cells which lack Giemsa and Feulgen staining, confirming the hypothesis that the kDNA is lost in dyskinetoplastic trypanosomatids from insects. Possible modes of acriflavine action are considered and a proposed mechanism for acriflavine action in trypanosomes from mammals is presented.
Plasma membrane vesiculation in 3T3 and Sv3T3 cells: I. morphological and biochemical characterization
Authors: Scott Robert E. , Perkins Roger G. , Zschunke Michael A. , Hoerl Bryan J. , Maercklein Peter B.
Published: April 2021

3T3 and SV3T3 mouse embryo cells and a variety of other monolayer cell lines can be induced to form and shed plasma membrane vesicles by exposure to sulphydryl blocking agents including formaldehyde and N-ethyl malemide. Morphological studies show that multiple vesicles are formed and released from individual cells and that the vesicle membrane is continuous with the plasma membrane of the cell. Vesicles measure from 0 ·1 to 15 μm in diameter and are free of detectable contamination with cytoplasmic membranes and organelles. Vesicles also show a 10-fold enrichment in the plasma membrane marker enzyme 5’nucleotidase and are devoid of detectable NADH-cytochrome C reductase and succinic dehydrogenase activity which are marker enzymes for endoplasmic reticulum and mitochondria, respectively. Vesicles have a high cholesterol: phospholipid ratio and show enrichment in sphingomyelin content. They contain receptors for Con A and WGA, approximately 20 size class polypeptides and intra-membranous particles. These results suggest that vesicles are derived from and have the general characteristics of plasma membranes.
Controlled cell disruption: A comparison of the forces required to disrupt different micro-organisms
Authors: Kelemen M. V. , Sharpe J. E. E.
Published: April 2021

A cell disrupter has been developed which can measure the forces required to disrupt both eukaryotic and prokaryotic cells. It operates a continuous process and will disrupt both large and small volumes. Shear forces are set up when a suspension under laminar flow conditions is released under high pressure through a short orifice. If the applied pressure is altered, the shear forces are simultaneously changed so that the amount of cell disruption can be compared under different known and repeatable conditions. The disrupter is now manufactured and supplied by Stansted Fluid Power Limited, Stansted, England.

Phase-contrast microscopy has shown that the disrupter will break a variety of organisms including Chlorella, Aspergillus fumigatis, Fusarium sp., Saccharomyces cerevisiae, Escherichia coli, Lactobacillus casei, Bacillus subtilis, Clostridium perfringens, Streptococcus faecalis, Streptococcus zooepidermicus and Staphylococcus aureus. The cells are not all broken at one pressure but a certain pressure must be applied before disruption starts which will then increase rapidly as the applied pressure is increased. The applied pressure required to disrupt half the population in a culture is different from one species to another, rods being disrupted more easily than spheres.

The ease of disruption seems to be related to the shape and chemical composition of the cell wall. Furthermore, the disrupting process, in an unsynchronized culture is not random and may be related to the statistical size distribution of the cells.
Spermatogenesis in trichosia pubescens (diptera: Sciaridae)
Authors: Amabis J. M. , Reinach F. C. , Andrews Norma
Published: April 2021

This paper deals with the meiotic process in males of T. pubescens. The spermatocytes of this species are associated in clusters (rosettes) of some fifteen cells which are fairly synchronized during spermatogenesis. During meiosis, which is typically achiasmatic, 2 sets of chromosomes are eliminated into a narrow neck of cytoplasm (bud) which protrudes from the cell pole facing the centre of the rosette. The bud formation starts prior to the onset of the meiosis. The spindle that appears during the first meiotic division is unipolar with the fibres running from the pole at the end away from the bud up to the equatorial region of the cell. The chromosomes which move toward the spindle pole display a V-shape while the others, moving budward, do not show any consistent shape. We suggest that a special mechanism of chromosome motion is involved in the migration of these chromosomes. The second meiotic division is fairly similar to the ordinary type observed in other organisms except for the migration of both chromatids of one chromosome precociously toward the pole opposite the bud. At this division a typical bipolar spindle is present and all migrating chromosomes display a V-shape. The chromosomes eliminated during the first meiotic division clump together remaining in a pycnotic state at the distal portion of the bud until the end of spermatogenesis. The chromosomal set eliminated at the second meiotic division, as compared to the first, decondense simultaneously with the group located in the cell body forming a typical interphase nucleus.

T. pubescens has 3 chromosomes limited to the germ-line cells. These chromosomes are typically heterochromatic, replicating their DNA out of phase with the S-chromosomes, probably at a later stage in interphase. Our results suggest that they are transcriptionally active in the interphase between the 2 meiotic divisions and just after the meiotic process.
Gap and septate junctions in the excitable endoderm of Polyorchis penicillatus (hydrozoa, anthomedusae)
Authors: King Michael G. , Spencer Andrew N.
Published: April 2021

The morphological basis of impulse conduction in a jellyfish epithelium was investigated. Lanthanum impregnation of endodermal canal and endodermal lamella verified the existence of true gap junctions in Polyorchis. In both transverse and en face sections of gap junctions, electron-lucent globules, with a width of 7–8 run and a spacing of about 11 run, are evident. Gap-junctions are concentrated at the peripheral canal margin and septate junctions are localized around the canal lumen. Epithelial cells of the endodermal canals are capable of conducting a non-decrementing action potential. It is suggested that endodermal spike propagation, which can mediate ‘crumpling’ behaviour, is dependent upon low-resistence ionic pathways provided by gap-junctions and upon sealing of the intercellular space from saline extracellular fluids by septate junctions.
Studies on scale morphogenesis in the golgi apparatus of Pyramimonas tetrarhynchus (prasinophyceae)
Authors: Moestrup Øjvind , Walne Patricia L.
Published: April 2021

Ultrastructural investigations of scale formation in the cisternae of the Golgi apparatus have been carried out on the prasinophycean flagellate Pyramimonas tetrarhynchus, whose cell surfaces are covered with 6 different scale types, 3 on the flagella and 3 on the cell body. Our results suggest that at least 4 and probably all 6 scale types can be formed together within the same cisterna and that there is some degree of intracisternal differentiation, since the formation of 2 scale types (the small underlayer scales on cell body and flagella) is restricted to the cisternal peripheries, whereas the remaining scale types are formed in the more central portions. Detailed studies of morphogenesis of the larger body scales reveal the earliest identifiable stages as 8-armed figures, with 8 thin arms in the intermediate body scales (IBS), and 4 thick and 4 thin arms in the outer body scales (OBS). From these incipient structures that bear little resemblance to the finished products, the complex, 3-dimensi0nal mature body scales are elaborated in each of the cell’s 4 dictyosomes, and maintain throughout their different developmental sequences a close relationship to the inner surfaces of the cisternal membranes, as well as a particular orientation within the dictyosomes. Preliminary calculations of total numbers of scales that cover cell and flagellar surfaces are included. The small, undeilayer scales, which on the flagella are shown to be arranged in 2 4 rows, number about 350000; larger scales of more complex construction number about 20000.
Topographical variations in the structure of the smooth septate junction
Authors: Skaer Helen Leb. , Harrison J. Barrie , Lee William M
Published: April 2021

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

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

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

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

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

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

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

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

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.