bluuebunny/crossref_metadata_2025 huggingface dataset:
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

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

Measurements 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

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

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

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

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

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 > 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

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.
Cell size and proportional distance assessment during determination of organelle position in the cortex of the oliate Tetrahymena
Authors: Lynn D. H. , Tucker J. B.
Published: April 2021

Developing oral organelles of dividing Tetrahymena corlissi appear to be positioned by mechanisms which assess distances as a proportion of the organism’s overall dimensions. In some respects, the cortex of this protozoan obeys the ‘French flag’ rule formulated by Wolpert for describing regulation of spatial proportions during differentiation of metazoan embryos.

Dividing Tetrahymena of markedly different sizes occur when division is synchronized by starvation and refeeding. At the start of cell division, the distance between old and new mouthparts varies proportionately with respect to cell length. In addition, determination of the site where new oral organelles will develop is apparently not directly related to the number of ciliated basal bodies which separate the 2 sets of mouthparts; the greater the distance between the old and developing sets of mouthparts, the greater the number of ciliated basal bodies in the rows between them.

It is suggested that 2 distinct mechanisms are largely responsible for defining organelle position in ciliates. The new terms structural positioning and chemical signalling are defined to describe these mechanisms.
The chitin crystallite in arthropod cuticle
Authors: Neville A. C. , Parry D. A. D. , Woodhead-Galloway J.
Published: April 2021

Electron microscopy has revealed that chitin from a representative selection of insect orders (plus one crustacean and one arachnid) is localized in crystallites about 2’8 nm across. Furthermore, these crystallites are arranged on an hexagonal or pseudo-hexagonal lattice, the lateral order of which varies considerably. The lattice becomes secondarily reoriented during cuticle expansion following an ecdysis. The size of the ‘unit cell’ has been measured both by optical diffraction and direct measurements of the micrographs, permitting an estimate of the chitin and resilin content for locust rubberlike cuticle. The number of poly-N-acetyl-glucosamine chains per sheet and sheets per crystallite can be estimated from the physical dimensions of the crystallite. Each crystallite is unlikely to comprise more than 3 sheets and 6 chains per sheet. The calculated and measured density of α-chitin can be shown to be in close agreement.
A three-dimensional reconstruction of the polygonal pattern on placental coated-vesicle membranes
Authors: Ockleford C. D.
Published: April 2021

There is a surface structure on the coated vesicles of human placenta. Some features of this structure have been examined. Measurements of the polygonal network seen in surface views have been made and compared with measurements of structures projecting from vesicle walls in median sections. A 3-dimensional reconstruction of the vesicle shows the pattern to be one of raised ridges.

Use of a goniometer to tilt the specimens has confirmed the assumption that both types of image obtained are from one structure.

Although it is usually the case that vesicles are approximately spherical, some are definitely irregularly shaped. For this reason it is suggested that the walls of the polygons need not always be packed into a structure with a regular and precisely maintained pattern.

Consideration of the surface structure in the light of current understanding of the cell membrane as a dynamic system leads to a possible explanation of the process of vesicle formation in this context and of the selective nature of uptake by micropinocytosis.
Species-specific aggregation factor in sponges: VI. aggregation receptor from the cell surface
Authors: Müller W. E. G. , Müller I. , Zahn R. K. , Kurelec B.
Published: April 2021

An aggregation receptor from the siliceous sponge Geodia cydonium has been isolated and purified in an almost pure form. It sediments at about 2·6 s, has a buoyant density of 1·51 g/ml in CsCl and elutes from Sephadex G-50 at a Ve/0 value of 1·311. Chemical analysis revealed that the receptor consists of 81 % neutral carbohydrate and 7·5 % protein. The activity of the receptor is rapidly destroyed by Na-periodate.

The receptor is released from the cell surface after removal of Ca2+ from the medium or after incubation of the cells with trypsin. The depleted cells can be charged again with isolated receptor molecules. The binding of the receptor molecules on the cell surface is prevented in the presence of trypsin. For optimal binding, physiological salt concentrations with respect to NaCl (540 rπmM NaCl) and Cas+ ions are necessary.

The receptor whose isolation is described in this report, is involved in secondary aggregation processes, which are initiated by a soluble aggregation factor. The primary aggregation of the cells is not influenced by the receptor.

Time-course studies with receptor-depleted cells revealed that new aggregation receptor molecules are formed during the aggregation process.

By competition experiments it could be shown that high concentrations of soluble aggregation receptor molecules inhibit secondary aggregation. The soluble receptor molecules can compete with surface-bound receptor molecules only if these are not linked with the aggregation factor.
Spore germination promoter of dictyostelium discoideum excreted by aerobacter aerogenes
Authors: Hashimoto Yohichi , Tanaka Yoshimasa , Yamada Takuzo
Published: April 2021

The nutrient medium in which Aerobacter aerogenes was grown, contains a spore germination promoter (SGP) for the cellular slime mould Dictyostelium discoideum. SGP can cause synchronous spore germination in a short time, and triggers the germination process in just a few minutes. Germination-promoting capacity of SGP decreases as it comes in contact with increasing number of spores. When spores activated by SGP are stored at 4 °C, they gradually return to the dormant state. SGP is comparatively heat-stable, but is unstable at pH above 10 or under 3.
Apparent amitosis in the binucleate dinoflagellatePeridinium Balticum
Authors: Tippit D. H. , Pickett-Heaps J. D.
Published: April 2021

Mitosis and cytokinesis in the free-living binucleate dinoflagellate Peridinium balticum are described. P. balticum contains 2 nuclei; one is a typical dinoflagellate nucleus and the other resembles the interphase nuclei of some eucaryotic cells and is here named the supernumerary nucleus (formerly called the eucaryotic nucleus). The dinoflagellate nucleus divides in the characteristic manner already described for certain other dinoflagellates. The supernumerary nucleus does not undergo normal mitosis; its chromatin does not condense, a spindle is not differentiated for its division, nor are any microtubules present inside the nucleus during any stage of its division. Instead the supernumerary nucleus divides by simple cleavage, which is concurrent with cytoplasmic cleavage. The nucleus cleaves first on its side facing the wall, but later it cleaves circumferentially as the cytoplasmic cleavage furrow draws closer. Invariably at late cytokinesis, a portion of the dividing nucleus passes through the only remaining uncleaved area of the cell. The final separation of the supernumerary nucleus is probably accomplished by the ingrowing furrow pinching the nucleus in two. There is no apparent precise segregation of genetic material during division, nor are there any structural changes inside the dividing nucleus which distinguish it from the interphase nucleus. Certain aspects of amitosis, and previously postulated theories concerning the endosymbiont origin of the second nucleus, are discussed.
Pollen-wall proteins: quantitative cytochemistry of the origins of intine and exine enzymes in Brassica Oleracea
Authors: Vithanage H. I. M. V. , Knox R. B.
Published: April 2021

Simultaneous coupling methods for detection of acid phosphatase and non-specific esterase produce a coloured reaction product that is quantitatively related to enzyme content in freeze-sectioned Brassica pollen and tapetal cells.

The intine-located acid phosphatase has 2 periods of synthesis: the first in late vacuolate period, associated with the completion of deposition of the intine polysaccharides; the second during pollen maturation, apparently reflecting cytoplasmic synthesis. Esterase activity accumulates in the tapetal cells until dissolution at early maturation period, when there is a dramatic rise in pollen-wall esterase activity, reflecting the transfer from tapetum to exine cavities. These quantitative studies confirm the gametophytic and sporophytic origins of the intine and exine proteins.
Cell surface saccharides ofTrypanosoma lewisi.: II lectin-mediated agglutination and fine-structure cytochemical detection of lectin-binding sites
Authors: Dwyer D. M.
Published: April 2021

Bloodstream (BSF) and culture forms (CF) of Trypanosoma lewisi were specifically agglutinated with the plant lectins concanavalin A (Con A), soybean agglutinin (SBA), wheat germ agglutinin (WGA), and fucose-binding protein (FBP). Lectin-mediated cell agglutination was inhibited, and reversed in the presence of specific lectin-binding saccharides. Cells were agglutinated randomly with all lectins suggesting a uniform distribution in the trypanosome cell surface of the lectin-binding saccharide ligands. The BSF and CF were not agglutinated with phytohaemagglutinin-M, phytohaemagglutinin-P, or influenza virions. Living trypsinized BSF, which lacked a surface coat, gave agglutination results with the lectins identical to those obtained with living intact BSF. Glutaraldehyde- or formalin-fixed intact and trypsinized BSF gave results similar to those obtained with living cells and SBA, WGA, and FBP. However, intact, fixed BSF gave much lower agglutination levels with Con A than trypsinized-fixed, living intact, or living trypsinized BSF cells. Intact and trypsinized living and fixed CF gave identical agglutination results with each of the lectins. Living and fixed cells treated extensively with the glycoside hydrolases a-amylase, dextranase, and neuraminidase gave results with the lectins identical to those obtained with untreated cells.

Con A bound at the cell surface was visualized with an iron-dextran (Fe-Dex) conjugate. Dense iron marker particles were distributed randomly in the intact BSF surface coat. The Con A-bound Fe-Dex marker was present on the pellicular and flagellar membrane outer lamina of trypsinized BSF and intact CF cells. Horseradish peroxidase (HRPO)-diamino-benzidine (DAB) coupled reactions also were used to visualize surface-bound Con A. Dense Con A-HRPO-DAB deposits were present uniformly in the BSF surface coat, and on the membranes of trypsinized BSF and intact CF trypanosomes.

SBA and WGA were conjugated to HRPO and these used in DAB-coupled reactions at the ultrastructure level. Results obtained with the HRPO-conjugated lectins were similar in surface localization and distribution to those obtained with the Con A-HRPO-DAB preparations.

Treatment of BSF and CF with the several glycoside hydrolases produced no apparent enhanced or reduced reactivity for the lectins in any of the fine-structure cytochemistry experiments.

The cumulative results indicate that ligands similar or identical to α-D-mannose, N- acetylgalactosamine, and N-acetylglucosamine, and α-L-fucose are constituents in the extracellular surface coat matrix of T. lewisi BSF. Similar conclusions also pertain to the pellicular and flagellar membrane ligands of the BSF and CF cells. Moreover, results obtained with the glycoside hydrolases and influenza virions suggest that the T. lewisi cell surface ligands are not associated directly with repetitively bonded α-1,4- and α-1,6-D-glucans or sialic acid moieties.
Studies on the mechanism of circus movement in dissociated embryonic cells of a teleost,Oryzias latipes:Fine-structural observations
Authors: Fujinami Noboru
Published: April 2021

The fine structure of lobopodia in dissociated embryonic cells of the freshwater fish, Orysias talipes, was observed with the electron microscope in order to understand the mechanism of the circus movements which they display. Dense material (granular or fibrillar) is present in the zone between the lobopodium and the endoplasm, as well as in the cortical layer around the cell circumference. The direction of lobopodial movement is related to the distribution of this dense material. The band between the lobopodium and the endoplasm is conspicuous and is connected to the cortical dense layer around the cell periphery at the advancing front of the lobopodium, while the dense material is usually almost absent beneath the cell membrane in the anterior region of the lobopodium. The band between lobopodium and endoplasm is blurred or disrupted near the hind end of the lobopodium, where the peripheral dense layer is well developed.

In situ localization of actin/heavy meromyosin complexes in the cell showed that the dense material has actin-like properties. Cytochalasin B (0·5 μg/ml) induced constriction of the neck of the bleb, shrinkage of the endoplasm, and herniation of the endoplasmic contents to the enlarged hemispherical bleb, and thus arrested the circus movement.

On the basis of these results, an hypothesis concerning the mechanism of circus movement is proposed and discussed.
Sulphur in epidermal keratohyalin granules: A quantitative assay by x-ray microanalysis
Authors: Jessen H. , Peters P. D. , Hall T. A.
Published: April 2021

The elemental composition of different types of keratohyalin granules from the epidermis of newborn and adult rats was studied by means of an EMMA-4 analytical electron microscope, equipped with an energy-dispersive X-ray spectrometer. An absolute quantitation of the sulphur concentration in keratohyalin granules was performed.

The results demonstrate that epidermal keratohyalin granules are chemically heterogeneous. A type of keratohyalin granule present in the nuclei and cytoplasm of epidermal cells from both newborn and adult rats - termed single granules - is rich in sulphur, having a content of 2·5-3·6%. Other types of keratohyalin granules, which differ in newborn and adult rats, contain a sulphur-poor component; they often have a sulphur-rich component as well. The sulphur-poor keratohyalin contains 0·6-0·9% sulphur.

It is suggested that the sulphur-rich keratohyalin granules are the source of the peripheral envelope protein of cornified cells.
Heterogeneity of phospholipid synthesis in rat liver endoplasmic reticulum during proliferation of smooth membranes
Authors: Higgins Joan A.
Published: April 2021

During proliferation of smooth endoplasmic reticulum (SER) induced by phenobarbital the specific activity of acyl transferases of the smooth microsomes increases, there is a transient rise in the phospholipid/protein ratio of these membranes, and an increased incorporation of [14C]glycerol into smooth-membrane phospholipid. Microsomes separated into subfractions on 2 gradients exhibited a heterogeneous distribution of these characteristics, indicating a nonuniform distribution of the site of phospholipid synthesis in the ER under these conditions. Cytochemical localization of acyltransferases on whole liver and smooth and rough microsomes confirmed this heterogeneity, and indicated that the distribution of this- activity was not restricted to any morphologically distinct site in the ER of the intact cell. After 4 days of phenobarbital treatment the increased membrane is restricted to lighter subfractions and is similar in distribution to that of increased acyltransferase activity. These results indicate that the synthesis of membrane phospholipid and the growth of the SER in response to phenobarbital is not uniform but occurs at randomly dispersed sites in the SER while proteins may be added preferentially at these sites resulting in a final uniform distribution.
Rapid modification of the morphology of cell contact sites during the aggregation of limpet haemocytes
Authors: Jones G.E. , Gillett R. , Partridge T.
Published: April 2021

Haemocytes suspended in the blood of the limpet Patella vulgata rapidly form microspikes and aggregate in response to withdrawal from the haemocoel. It has previously been suggested that the contacts between colliding cells in shaken suspension are stabilized by the rapid spreading of participant cells over each other’s surfaces by a microspike-dependent process. In this communication we report that an electron-microscopic study of the contacts formed between cells in the early stages of aggregation suggests that intercellular contact elicits a rapid response from the organelles concerned with cell motility. This response leads to the rapid progression of each initial collision through 3 major categories of contact morphology, namely cell body/lamella, lamella/lamella and cell body/cell body. There is a concomitant increase in the area of mutually contacting surface between participating cells.

On the basis of these results, a model is constructed for the participation of sub-surface components in the process of cellular aggregation. The sequence of development of contact found between aggregating cells is envisaged to be a consequence of cellular locomotory activities resembling those by which these cells spread on to a solid substrate. Included in this process is an element of intercellular recognition. It is suggested that the involvement of motile elements in cell aggregation is a general feature of aggregating cells. This would permit a re-interpretation of aggregation experiments which show specificity of cell adhesion, for total adhesive interactions between cells would depend not only on the adhesiveness per unit area of contacting cell surface but also on the extent or rate of formation of flat contact areas. The latter element could be regulated by a recognition system between contacting cells independently of the force per unit area which holds the 2 surfaces together.
The control of mitosis in Physarum polycephalum: The effect of delaying mitosis and evidence for the operation of the control mechanism in the absence of growth
Authors: Sudbery P. E. , Grant W. D.
Published: April 2021

Possible mechanisms coordinating the control of mitosis and DNA synthesis with growth were experimentally tested in Physarum polycephalum by the response of plasmodia to 2 different kinds of perturbation of the DNA:mass ratio. Mitosis and DNA synthesis were delayed without stopping growth either by the use of fluorodeoxyuridine (FUdR) or puro- mycin, in both cases the delayed mitosis was followed by a single shortened intermitotic period, as predicted by all the mechanisms considered and substantiating a basic assumption made that the time of mitosis is homeostatically controlled to maintain a constant DNA:mass ratio. When more than 50% of the DNA was destroyed by ultraviolet irradiation 2 consecutive mitoses could occur even when growth was completely stopped by starvation. This result can only be accounted for by 2 of the 5 models considered, a finding which agrees with the results of previous experiments which were also consistent with only these 2 mechanisms.
Some ultrastructural aspects of mitosis and early differentiation in human foetal lung
Authors: Mcdougall Jennifer
Published: April 2021

In rapidly dividing human foetal lung cells there is evidence of synthesis of extra membrane prior to mitosis. During prophase the nuclear envelope breaks down and is conserved as long double or complex stacks during the following stages of mitosis. It is suggested that the membrane is retained partly for incorporation into the daughter cell nuclear envelopes and partly for the formation of RER. The relevance of the observations to the biochemical data in the literature concerning the formation of intracellular membranes is discussed briefly.
The nucleolus organizer and the synaptonemal complex inEndymion non-scriptus(L.)
Authors: Jordan E. G. , Luck B. T.
Published: April 2021

Stages of meiosis from the bluebell Endymion non-scriptus (L.) were studied by electron microscopy. The nucleolus went through the process of segregation at the beginning of meiosis with the movement to its surface of a pale-staining region. This region was shown to be the same as that called the ‘L zone’ or lacunae of nucleoli. Its chromosomal nature was strongly suggested by the presence of the synaptonemal complex within it. This demonstrated that the pale-staining region of nucleoli is the nucleolus organizer and almost certainly the chromosome region containing the ribosomal cistrons, and justifies the use of these terms to describe the structure when seen inside the nucleolus. The relationship between this zone and the heterochromatic knob called the nucleolar organizing body in maize by other workers is discussed.
Observations on intranuclear crystal and nucleolar size at different stages of cell differentiation in the midgut epithelium of several insects
Authors: Gouranton J. , Thomas D.
Published: April 2021

Based on an inverse size relationship between nuclear crystal and nucleolus in different cells it has been postulated by several authors that the crystal develops from nucleolar materials. The purpose of the present paper is to investigate the validity of this argument. Intranuclear proteinaceous crystals appear in differentiating midgut cells of Gyrinus marinus and Tenebrio molitor. In an autoradiographic study we have previously demonstrated in these two species that the crystals do not develop from nucleolar materials. However, an inverse relationship with regard to size is observed between these 2 structures during the cell differentiation: the crosssectional area of the nucleolus decreases when the cross-sectional area of the crystal increases. But a decrease in size of the nucleolus is also observed during the differentiation of the midgut cells of Gyrinus natator where the crystals are not present. Consequently an inverse size relationship cannot be a sufficient argument to postulate that intranuclear crystals and nucleoli are interconvertible structures; decrease in size of the nucleolus is not related to development of the intranuclear crystal.
Perturbation of mammalian cell division: III. the topography and kinetics of extrusion subdivision
Authors: Mullinger Ann M. , Johnson R. T.
Published: April 2021

If mitotic-arrested, cold-stored HeLa cells are incubated at 37 °C a proportion of the population divides by an aberrant process which we have called subdivision by extrusion. This process has been studied by time-lapse photography and shown to differ from normal cleavage in several respects. The cell surface becomes more generally mobile and, instead of producing the precisely localized furrowing activity of cytokinesis, gives rise to multiple surface protrusions. These protrusions enlarge at the expense of the parent cell and develop into a cluster of small daughter cells (mini segregants). The surface structure of the cell, as seen by scanning electron microscopy, also changes; the microvilli characteristic of interphase, metaphase and cleaving HeLa cells are lost during extrusion and the cell surface becomes smooth. Extrusion activity is much more variable than division by cleavage in terms of both topography and kinetics, and in general takes longer to complete.

Some cells in the cold-treated populations divide by mixtures of cleavage and extrusion or by cleavage alone. The relative numbers of cells dividing in different ways vary with the conditions of pretreatment and incubation of the mitotic cells. The greater the perturbation (e.g. longer cold storage), the greater the proportion of extruding rather than cleaving cells. Human diploid cells can also be induced to subdivide by extrusion. Possible mechanisms underlying the different types of division activity are discussed.
Intranuclear membranous inclusions in oocytes of a viviparous teleost (Xiphophorus Helleri)
Authors: Azevedo Carlos
Published: April 2021

Intranuclear inclusions were observed in oocytes of Xiphophorus helleri during prophase I. In osmium-fixed leptotene nuclei, the inclusions were made up of groups of membrane-limited vesicles or tubules with pale contents, situated near the inner nuclear membrane with which some of them exhibited apparent continuities. In zygotene nuclei, larger vesicles also appeared bounded by two or three membranes and containing tubules apparently invaginated from their walls. In pachytene-dictyate nuclei most vesicular bodies had a wall formed by stratified membranes, or were entirely made up of membranous whorls. In glutaraldehyde-osmium fixed material some of these myelin-like bodies showed a peculiar arrangement, consisting of concentric bands each containing thick inner dense lamellae 2·0–3·0 nm thick and a 5·0-nm outer lamella.

It is suggested that these inclusion bodies arise from the inner nuclear membrane of oocytes when cells start to grow intensely during prophase I. The bodies seem to become more complex at late prophase, probably by association of individual vesicles and the occurrence of multiple membrane invaginations, which may be related to active metabolic phenomena taking place at this stage in oocytes.
Regular structures in membranes: The lumenal plasma membrane of the cow urinary bladder
Authors: Knutton S. , Robertson J. D.
Published: April 2021

The ultrastructure of the lumenal plasma membrane of the cow urinary bladder has been studied in thin sections of glutaraldehyde- and glutaraldehyde-H2O2-fixed specimens, by negative staining and freeze fracture. A regular hexagonal array of particles confined to polygonal plaques 0·1–0·4-μm in diameter and separated by 0·02-μm interplaque areas is revealed by all 3 techniques. Cross-sections through particulate areas fixed with glutaraldehyde-H2O2 display a tetralaminar structure consisting of the usual ∼ 8-nm-thick trilamellar unit membrane structure, on the external dense leaflet of which is located an additional ∼ 4-nm-thick stratum which is occasionally resolved into a row of regularly spaced ∼ 4-nm-diameter particles. Non-particulate areas feature only the ∼8-nm-thick trilamellar structure. Tangential sections reveal an hexagonal array of particles with a unit cell of ∼ 16 nm. Four membrane faces can be revealed by freeze fracture and etching of membranes of the cow urinary bladder; 2 complementary split inner membrane faces (A and B) revealed by the cleaving process and the lumenal and cytoplasmic membrane surfaces exposed by etching. Face B, which belongs to the external membrane leaflet and faces the cytoplasm, displays plaques of particles arranged in a hexagonal lattice with a unit cell of ∼ 16 nm. Face A, which belongs to the cytoplasmic membrane leaflet and faces the lumen, displays a complementary array of hexagonally packed pits. The hexagonally arranged particles also protrude into the lumenal membrane surface where they can occasionally be resolved into 6 ∼5-nm-diameter subunits; the cytoplasmic surface appears smooth. Six ∼ 5-nm-diameter subunits are also revealed in negatively stained preparations. The data are consistent with a model for the membrane in which the particles forming the hexagonal structure protrude above the lumenal membrane surface and also bridge most of the thickness of the membrane.
Contacts between pigmented retina epithelial cells in culture
Authors: Middleton C. A. , Pegrum S. M.
Published: April 2021

The behaviour of primary cultures of dissociated embryonic chick pigmented retina epithelial (PRE) cells has been investigated. Isolated PRE cells have a mean speed of locomotion of 7·16 μm/h. Collisions between the cells normally result in the development of stable contacts between the cells involved. This leads to a gradual reduction in the number of isolated cells and an increase in the number of cells incorporated into islands. Ultrastructural observations of islands of cells after 24 h in culture show that junctional complexes are present between the cells. These complexes consist of 2 components: (a) an apically situated region of focal tight junctions and/or gap junctions, and (b) a more ventrally located zonula adhaerens with associated cytoplasmic filaments forming a band running completely around the periphery of each cell. The intermembrane gap in the region of the zonula is 6·0–12·0 nm. The junctional complexes become more differentiated with time and after 48 h in culture consist of an extensive region of tight junctions and/or gap junctions and a more specialized zonula adhaerens. It is suggested that the development of junctional complexes may be responsible for the stable contacts that the cells display in culture.
Evidence for a correlation between the number of marginal band microtubules and the size of vertebrate erythrocytes
Authors: Goniakowska-Witalińska Lucyna , Witaliński Wojciech
Published: April 2021

In 23 species of vertebrates the dimensions of erythrocytes and the number of their marginal band microtubules were examined. A positive correlation was found between the size of erythrocytes and the number of microtubules. The absence of microtubules in diskoid erythrocytes of mammals – Camelidae – is discussed.
Freeze-fracture studies of frog atrial fibres
Authors: Mazet Françoise , Cartaud Jean
Published: April 2021

The freeze-fracturing technique was used to characterize the junctional devices involved in the electrical coupling of frog atrial fibres. These fibres are connected by a type of junction which can be interpreted as a morphological variant of the ‘gap junction’ or ‘nexus The most characteristic features are rows of 9-nm junctional particles forming single or anastomosed circular profiles on the inner membrane face, and corresponding pits on the outer membrane face. Very seldom aggregates consisting of few geometrically disposed 9-nm particles are found. The significance of the junctional structures in the atrial fibres is discussed, with respect to present knowledge about junctional features of gap junctions in various tissues, including embryonic ones.
Motor proteins at the mitochondria–cytoskeleton interface
Authors: Kruppa Antonina J. , Buss Folma
Published: April 2021

Mitochondria are multifunctional organelles that not only produce energy for the cell, but are also important for cell signalling, apoptosis and many biosynthetic pathways. In most cell types, they form highly dynamic networks that are constantly remodelled through fission and fusion events, repositioned by motor-dependent transport and degraded when they become dysfunctional. Motor proteins and their tracks are key regulators of mitochondrial homeostasis, and in this Review, we discuss the diverse functions of the three classes of motor proteins associated with mitochondria – the actin-based myosins, as well as the microtubule-based kinesins and dynein. In addition, Miro and TRAK proteins act as adaptors that link kinesin-1 and dynein, as well as myosin of class XIX (MYO19), to mitochondria and coordinate microtubule- and actin-based motor activities. Here, we highlight the roles of motor proteins and motor-linked track dynamics in the transporting and docking of mitochondria, and emphasize their adaptations in specialized cells. Finally, we discuss how motor–cargo complexes mediate changes in mitochondrial morphology through fission and fusion, and how they modulate the turnover of damaged organelles via quality control pathways, such as mitophagy. Understanding the importance of motor proteins for mitochondrial homeostasis will help to elucidate the molecular basis of a number of human diseases.
The eggshell of Drosophila melanogaster: I. fine structure of the layers and regions of the wild-type eggshell
Authors: Margaritis Lukas H. , Kafatos Fotis C. , Petri William H.
Published: April 2021

The fine structure of the several layers and regional specializations in the Drosophila melano-gaster eggshell has been studied by a combination of shell isolation procedures and ultrastructural techniques (conventional TEM, whole-mount TEM, SEM, HVEM, freeze-fracture electron microscopy utilizing rotary replication, shadow casting, optical diffraction and stereo imaging).

The main shell consists of 5 layers: the vitelline membrane (300 nm thick), the wax layer, the innermost chorionic layer (40—50 nm), the endochorion (500-700 nm), and the exochorion (300-500 nm). The vitelline membrane consists of irregularly organized particles. The wax layer appears to contain multilayered hydrophobic plates which split tangentially upon freeze fracturing. The innermost chorionic layer is composed of a crystalline lattice. The endochorion is made of a thin (40 nm) fenestrated floor composed of 40-nm fibres and an outer solid (2oonm) roof covered with a network of 40-nm strands. Intermittently spaced pillar connect these 2 parts. Similarities in the substructure of the floor, pillars and roof suggest that they may be composed of similar or identical structural elements.

The specialized regions of the shell are the 2 respiratory appendages, the operculum area and the posterior pole. The appendages exhibit 2 sharply distinct surfaces, a dorsal side with isolated 1·5-µm plaques and a ventral side with strands of 40-50 nm connected in a network with openings of 70-80 nm. The operculum area, which includes the micropyle and the collar, is distinguished by 3 unique types of cell imprints. The posterior pole contains 2 distinctive populations of cell imprints: the central area has very thin intercellular ridges and a thin, perforated, endochorionic roof, while the peripheral area contains mixed, thick and thin, intercellular ridges and serves as a transition zone to the main shell pattern. The pillars in the central area of the posterior pole have a distinct arrangement, forming one peripheral circle within each cell imprint.

An analysis utilizing structural and developmental criteria indicates that as many as ten different populations of follicular epithelial cells may be involved in the construction of the various regions of the Drosophila eggshell.
Studies of membrane fusion vi. mechanism of the membrane fusion and cell swelling stages of sendai virus-mediated cell fusion
Authors: Knutton S.
Published: April 2021

The membrane fusion and cell swelling stages of Sendai virus-mediated cell-cell fusion have been studied by thin-section and freeze-fracture electron microscopy. Sites of membrane fusion have been detected in human erythrocytes arrested at the membrane fusion stage of cell fusion and in virtually all cases a fused viral envelope or envelope components has been identified thus providing further direct evidence that cell-viral envelope-cell bridge formation is the membrane fusion event in Sendai virus-induced cell fusion. Radial expansion of a single virus bridge connecting 2 cells is sufficient to produce a fused cell. Membrane redistribution which occurs during this cell swelling stage of the fusion process is often accompanied by the formation of a system of membrane tubules in the plane of expansion of the virus bridge. The tubules originate from points of fusion between the bridging virus envelope and the erythrocyte membrane and also expand radially as cells swell. Ultimately membrane rupture occurs and the tubules appear to break down as small vesicles. When previously observed in cross-sectioned cells these membrane tubules were interpreted as sites of direct membrane fusion. The present study indicates that this interpretation is incorrect and shows that the tubules are generated subsequent to membrane fusion when 2 cells connected by a virus bridge are induced to swell. A mechanism to explain the formation of this system of membrane tubules is proposed.