bluuebunny/crossref_metadata_2025_split huggingface dataset:
Interactions between somite cells: the formation and maintenance of segment boundaries in the chick embryo
Authors: Stern Claudio D. , Keynes Roger J. 
Published: April 2021

ABSTRACTWe have investigated the interactions between the cells of the rostral and caudal halves of the chick somite by carrying out grafting experiments. The rostral halfsclerotome was identified by its ability to support axon outgrowth and neural crest cell migration, and the caudal half by the binding of peanut agglutinin and the absence of motor axons and neural crest cells. Using the chick-quail chimaera technique we also studied the fate of each half-somite.It was found that when half-somites are placed adjacent to one another, their interactions obey a precise rule: sclerotome cells from like halves mix with each other, while those from unlike halves do not; when cells from unlike halves are adjacent to one another, a border is formed.Grafting quail half-somites into chicks showed that the fates of the rostral and caudal sclerotome halves are similar: both give rise to bone and cartilage of the vertebral column, as well as to intervertebral connective tissue. We suggest that the rostrocaudal subdivision serves to maintain the segmental arrangement when the mesenchymal sclerotome dissociates, so that the nervous system, vasculature and possibly vertebrae are patterned correctly.
Patterns of dna replication in Drosophila polytene nuclei replicating in Xenopus egg and oocyte extracts
Authors: Sleeman Andrea M. , Leno Gregory H. , Mills Anthony D. , Fairman Micaela P. , Laskey Ronald A. 
Published: April 2021

ABSTRACT
We have used Xenopus laevis cell-free extracts to study patterns of DNA replication in polytene nuclei isolated from salivary glands of Drosophila melanogaster 3rd instar larvae. Replication was visualized by supplementation with biotin-dUTP so that nascent DNA became labelled, thus allowing detection with fluorescein or Texas-Red-conjugated streptavidin.
Biotin incorporation was dependent on incubation in extracts. Transverse bands were labelled in high-speed supernatants of eggs or oocytes in which replication does not initiate de novo. These patterns corresponded to the patterns of endogenous replication forks in polytene nuclei, monitored by bromodeoxyuridine incorporation in intact salivary glands. By contrast, when nuclei were incubated in low-speed supernatants of eggs, they underwent more extensive chromatin decondensation and initiated replication. The spatial patterns of replication are strikingly different from the endogenous patterns. Instead they closely resemble patterns of clustered replication forks seen in Xenopus sperm nuclei replicating in the extract. This indicates that the egg extract can impose its pattern of replication foci even when the template is presented in the highly organized form of a polytene nucleus.
Redistribution of nuclear lamin A is an early event associated with differentiation of human promyelocytic leukemia HL-60 cells
Authors: Collard Jean-François , Senécal Jean-Luc , Raymond Yves 
Published: April 2021

ABSTRACT
The nuclear lamina of mammalian somatic cells is characterized by the constitutive presence of lamin B polypeptides while the appearance of lamins A and C generally occur during establishment of a differentiated phenotype. We have used antibodies specific to the unique carboxy-terminal domain of lamin A, i.e. distinct from the shared domains of lamins A and C, to study the individual behaviour of lamin A during establishment of a macrophage phenotype in human HL-60 cells. Lamin A was present as a nuclear cap in the majority of undifferentiated cells and it was redistributed to a full peripheral nuclear location very early after induction of differentiation by phorbol esters, even in the presence of a protein synthesis inhibitor. Induction of the cells into a reversible precommitment state by bromodeoxyuridine was accompanied by a similar redistribution of lamin A that however reverted to a cap after removal of inducer. No changes were observed in the uniform peripheral nuclear location of lamin C under all of these conditions. These results strongly suggest that lamin A plays a role in the early events of cell differentiation. Taken together with previous results on the interaction of A-type lamins with chromatin, these findings offer experimental evidence consistent with the proposed role of A-type lamins, and particularly lamin A, in the process of chromatin reorganization that accompanies the expression of a differentiated phenotype.
Nucleolus-associated telomere clustering and pairing precede meiotic chromosome synapsis in Arabidopsis thaliana
Authors: Armstrong Susan J. , Franklin F. Christopher H. , Jones Gareth H. 
Published: April 2021

The intranuclear arrangements of centromeres and telomeres during meiotic interphase and early prophase I of meiosis in Arabidopsis thaliana were analysed by fluorescent in situ hybridisation to spread pollen mother cells and embryo-sac mother cells. Meiocyte identification, staging and progression were established by spreading and sectioning techniques, including various staining procedures and bromodeoxyuridine labeling of replicating DNA.
Centromere regions of Arabidopsis are unpaired, widely dispersed and peripherally located in nuclei during meiotic interphase, and they remain unpaired and unassociated throughout leptotene. Eventually they associate pairwise during zygotene, as part of the nucleus-wide synapsis of homologous chromosomes.
Telomeres, by contrast, show a persistent association with the nucleolus throughout meiotic interphase. Variation in telomere signal number indicates that telomeres undergo pairing during this interval, preceding the onset of general chromosome synapsis. During leptotene the paired telomeres lose their association with the nucleolus and become widely dispersed. As the chromosomes synapse during zygotene, the telomeres reveal a loose clustering within one hemisphere, which may represent a degenerate or relic bouquet configuration. We propose that in Arabidopsis the classical leptotene/zygotene bouquet is absent and is replaced functionally by nucleolus-associated telomere clustering.
Kinetochore localisation and phosphorylation of the mitotic checkpoint components Bub1 and BubR1 are differentially regulated by spindle events in human cells
Authors: Taylor Stephen S. , Hussein Deema , Wang Yunmei , Elderkin Sarah , Morrow Christopher J. 
Published: April 2021

BUB1 is a budding yeast gene required to ensure that progression through mitosis is coupled to correct spindle assembly. Two related human protein kinases, Bub1 and BubR1, both localise to kinetochores during mitosis, suggesting that they play a role in delaying anaphase until all chromosomes achieve correct, bipolar attachment to the spindle. However, how the activities of Bub1 and BubR1 are regulated by spindle events and how their activities regulate downstream cell cycle events is not known.To investigate how spindle events regulate Bub1 and BubR1, we characterised their relative localisations during mitosis in the presence and absence of microtubule toxins. In prometaphase cells, both kinases colocalise to the same domain of the kinetochore. However, whereas the localisation of BubR1 at sister kinetochores is symmetrical, localisation of Bub1 is often asymmetrical. This asymmetry is dependent on microtubule attachment, and the kinetochore exhibiting weaker Bub1 staining is typically closer to the nearest spindle pole. In addition, a 30 minute nocodazole treatment dramatically increases the amount of Bub1 localising to kinetochores but has little effect on BubR1. Furthermore, Bub1 levels increase at metaphase kinetochores following loss of tension caused by taxol treatment. Thus, these observations suggest that Bub1 localisation is sensitive to changes in both tension and microtubule attachment.Consistent with this, we also show that Bub1 is rapidly phosphorylated following brief treatments with nocodazole or taxol. In contrast, BubR1 is phosphorylated in the absence of microtubule toxins, and spindle damage has little additional effect. Although these observations indicate that Bub1 and BubR1 respond differently to spindle dynamics, they are part of a common complex during mitosis. We suggest therefore that Bub1 and BubR1 may integrate different ‘spindle assembly signals’ into a single signal which can then be interpreted by downstream cell cycle regulators.
Identification of essential genes in cultured mammalian cells using small interfering RNAs
Authors: Harborth Jens , Elbashir Sayda M. , Bechert Kim , Tuschl Thomas , Weber Klaus 
Published: April 2021

We report the first RNAi-induced phenotypes in mammalian cultured cells using RNA interference mediated by duplexes of 21-nt RNAs. The 21 gene products studied have different functions and subcellular localizations. Knockdown experiments monitored by immunofluorescence and immunoblotting show that even major cellular proteins such as actin and vimentin can be silenced efficiently. Genes were classified as essential or nonessential depending on impaired cell growth after RNA silencing. Phenotypes also involved altered cell morphology and aberrant mitotic arrest. Among the essential genes identified by RNAi for which such information was previously not available are lamin B1, lamin B2, NUP153, GAS41, ARC21, cytoplasmic dynein, the protein kinase cdk1 and both β- and γ-actin. Newly defined nonessential genes are emerin and zyxin. Several genes previously characterized by other methods such as knockout of murine genes are included as internal controls and gave identical results when RNAi was used. In the case of two nonessential genes (lamin A/C and zyxin) RNAi provides a recognizable phenotype.Our results complete the characterization of the mammalian nuclear lamins. While lamins A/C appear as nonessential proteins in the mouse embryo and in RNAi treated cultured cells, the two other lamins, B1 and B2, are now identified as essential proteins. Interestingly the inner nuclear membrane protein emerin, thought to be a ligand of lamin A/C, is also a nonessential protein in tissue culture cells.
Localized activation of bending in proximal, medial and distal regions of sea-urchin sperm flagella
Authors: Brokaw C. J. , Gibbons I. R. 
Published: April 2021

ABSTRACT
Spermatozoa from the sea urchin, Colobocentrotus atratus, were partially demembranated by extraction with solutions containing Triton X-100 at a concentration which was insufficient to solubilize the membranes completely. The resulting suspension was a mixture containing some spermatozoa in which a proximal, medial, or distal portion of the flagellum was membrane covered, while the remaining portion was naked axoneme. In reactivating solutions containing 12 μ M ATP, only the naked portions of the flagellum became motile. In reactivating solutions containing 0·8 mM ADP, the membrane-covered regions became motile and beat at 6–10 beats/s, while the naked regions remained immobile, or beat very slowly at about 0·3 beat/s. Activation of membrane-covered regions in ADP solutions probably results from the membrane restricting the diffusion of ATP which is formed from ADP by the axonemal adenylate kinase. The results indicate that any region of the flagellum has the capacity for autonomous beating, and that special properties of the basal end of the flagellum are not required for bend initiation. How ever, the beating of different regions of the flagellum is not completely independent, for in a fair number of spermatozoa the beating of the distal, membrane-covered region in 0·8 mM ADP was intermittent, and was turned on and off in phase with the much slower bending cycle in the proximal region of naked axoneme.
Intermediate cells of the pancreas: III. selective autophagy and destruction of β-granules in intermediate cells of the rat pancreas induced by alloxan and streptozotocin
Authors: Melmed R. N. , Benitez Carol J. , Holt S. J. 
Published: April 2021

ABSTRACT
In addition to causing necrosis ofβcells, diabetogenic doses of alloxan or streptozotocin cause selective autophagy and destruction of β-granules in intermediate cells in the rat pancreas, thus providing evidence that these granules and those in the β-cells of the islet are identical. In acinar-β cells the autophagic vacuoles also contain small mitochondria similar to those that occur in islet endocrine cells. Apart from these effects, the cells remain structurally intact. These observations suggest that alloxan exerts a direct effect on the β-components of the β-granule-containing intermediate cells.
The destruction of the β-granules in the acinar-β cell is sometimes accompanied by the appearance of α-granules in the same cells, thus presumably reflecting the increased demand for glucagon that occurs in experimental diabetes. Although intermediate cells containing α-granules are very uncommon in the normal rat pancreas, acinar-αand α-acinar cells occurred much more frequently after alloxan treatment.
The presence of intermediate cells of the acinar-β type in normal rats and the observed change of acinar-β to acinar-α cells, or from α- to α-acinar cells after alloxan, indicates that some pancreatic cells possess a variable functional potentiality. The appearance of these cells after alloxan presumably reflects the altered metabolic state of the animal, and is not a manifestation of the transformation of one cell type into another.
Cathepsin D in cartilage: The immunohistochemical demonstration of extracellular enzyme in normal and pathological conditions
Authors: Poole A. R. , Hembry R. M. , Dingle J. T. 
Published: April 2021

ABSTRACT
Extracellular cathepsin D has been demonstrated immunohistochemically in cultured chicken and rabbit cartilage. Enzyme released from viable cells was captured by specific anti-cathepsin D antibody to form extracellular immunoprecipitates which were demonstrated with fluorescein-labelled antibodies. In unfixed frozen sections these immunoprecipitates could be dissolved in antigen excess, and closely resembled those observed in immunoprecipitin lines. Extracellular cathepsin D was detected in the perichondria of normal chick cartilaginous limb-bones and, to a lesser degree, beneath them in the epiphyseal cartilage. In cultured rabbit ear cartilage, cathepsin D was detected around chondroblasts and some peripherally situated chondrocytes. In chicken and rabbit cartilage exposed to supra-physiological levels of retinol, much extracellular enzyme was detected which had been released from chondrocytes of hypertrophic and epiphyseal chick cartilage and rabbit ear cartilage.
An Examination Into The Phenomenon Of Granulation, In Mu Particles Of Stock 540 Paramecium Aurelia
Authors: Franklin J. , Baker N. O. 
Published: April 2021

ABSTRACT
Mu particles in paramecia incubated at 37 °C granulate. The treatment causes the loss of mu and granulation is a prerequisite for loss. The granules form along the length of the mu, at intervals of about 1·50 μm. Each granule has a diameter of about 0·5 μm and consists of a number of ribosome-like particles which have the same dimensions as 70s ribosomes.
Correlation between resistance to actinomycin d, karyology, agglutination by concanavalin a and tumorigenicity in chinese hamster hybrid cells
Authors: Imbert I. , Barra Y. , Berebbi M. 
Published: April 2021

ABSTRACT
Subclones isolated from a Chinese hamster hybrid line, derived from fusion of an actinomycin D-resistant and an actinomycin D-sensitive strain, were studied with respect to their resistance to actinomycin. D, karyology, transplantability and agglutination by concanavalin A. Statistical analysis of the results allowed the establishment of a classification of the strains based on increasing resistance to actinomycin D. There appeared to be an inverse correlation between actinomycin D-resistance and tumorigenicity and a positive correlation between this resistance and the presence of a marker chromosome.
Perturbation of mammalian cell division: II studies on the isolation and characterization of human mini segregant cells
Authors: Schor S. L. , Johnson R. T. , Mullinger A. M. 
Published: April 2021

ABSTRACT
A method is described for the isolation, according to size, of mini segregants produced by the abnormal cleavage of reversibly arrested mitotic HeLa cells. Many of these mini segregants contain small amounts of DNA, as judged by Feulgen staining and chromosome analysis. After fusion with mitotic HeLa cells, the interphase chromosomes of the mini segregants are seen as either monovalent or bivalent prematurely condensed chromosomes (PCC), some of which are damaged. A proportion of isolated mini segregants synthesize DNA, RNA and protein. Fusion of mini segregants with interphase HeLa cells gives rise to cells with ‘hybrid’ karyotypes.
Trypsinized bhk21 cells aggregate in the presence of metabolic inhibitors and in the absence of divalent cations
Authors: Edwards J. G. , Campbell Janice A. , Robson R. T. , Vicker M. G. 
Published: April 2021

ABSTRACT
The rapid formation of adhesions in suspension by lightly trypsinized BHK21 cells is not dependent on protein synthesis, and only in part on cellular metabolism, although it is completely inhibited by heat- and aldehyde-fixation of the cells. A requirement for protein synthesis becomes evident only if cells are exposed to high levels of trypsin for long periods. Formation of adhesions does not require addition to the medium of divalent cations, although it is increased by divalent manganese and cobalt ions. It is promoted by cytochalasin B and by cyclic AMP and is not inhibited by p-mercuriphenylsulphonate. We discuss a possible relationship between aggregation and the formation of gap junctions.
The fine structure of oogonia and oocytes in human ovaries
Authors: Baker T. G. , Franchi L. L. 
Published: April 2021

ABSTRACT
Twenty-eight foetal and post-natal human ovaries (6 weeks post conception to 11 years post partum) have been examined with the electron microscope. Stages in the normal differentiation of germ cells (oogonia; oocytes at the pre-leptotene, leptotene, zygotene, pachytene, and diplotene stages of meiotic prophase) were identified by comparison with (a) corresponding cells in histological and squash preparations, and (b) similar cells in the rat ovary. Atretic cells (oogonia in mitosis ; oocytes at pachytene and diplotene) were also examined.
The nuclei of oogonia contain an evenly dispersed fibrillar matrix which becomes organized into irregular strands in oocytes at pre-leptotene. At leptotene these strands become the sheaths surrounding unpaired, electron-dense axial threads or ‘cores’, which become associated in pairs during the transitory zygotene stage, and tend to be polarized within the nucleus. Single and paired threads are replaced by ‘tripartite ribbons’ (synaptinemal complexes) at pachytene; longitudinal subdivision of the lateral components of the ribbon is observed in some nuclei and may represent a later stage. At diplotene, essentially unpaired cores are observed as at leptotene, although they are thicker and more intimately related to the surrounding fibrillar sheath. The latter is also more highly organized than at earlier stages. This chromosomal structure is retained in oocytes in promordial follicles in post-natal ovaries. The dictyate stage observed in rat ovaries is not found in man.
The organization of the cytoplasm does not change markedly as oogenesis advances, although organelles become more numerous, and the internal structure more complex, as the cells enlarge towards the diplotene stage.
Oogonia degenerating during mitosis differ from normal cells in that the chromosomes become fused to form an irregular mass. Abnormal membrane-bound areas are also observed in the cytoplasm. Atretic oocytes at pachytene coalesce to form ‘pools’ of cytoplasm containing several nuclei in various stages of pyknosis. Atretic cells at diplotene may contain grossly swollen cytoplasmic organelles and clumped, homogeneous chromosomes. The ‘atretic divisions’ appear to be phagocytosed by somatic cells, but the means of elimination of degenerating meiocytes was not determined.
Cytological changes in the fat body of Rhodnius during starvation, feeding and oxygen want
Authors: Wigglesworth V. B. 
Published: April 2021

ABSTRACT
Fourth-stage larvae of Rhodnius survive starvation for 7–9 months. The filamentous mitochondria in the fat-body cells become reduced to granules and short rods (say 16 weeks). By 25 weeks the granules are swollen to form spheres which still contain NAD-diaphorase. By 30–35 weeks many mitochondria break down to osmiophil spheres or cytolysomes containing acid phosphatase. The origin of these spheres agrees with that of ‘protein droplets’ in holometa-bolous insects.
After feeding, the nucleus and nucleolus enlarge, RNA begins to accumulate in the cytoplasm and both the granular and the enlarged spherical mitochondria develop into filaments which break up into rows of granules. Within 2 days the fat-body cells become distended with watery vacuoles. All these changes are arrested or delayed if the oxygen supply to the fat body is reduced by tracheal section. Mitosis is hardly affected.
Accumulation of reserves of fat and glycogen takes place later.
The fine structure of the eye of the leech, Helobdella stagnalis
Authors: Clark A. W. 
Published: April 2021

Abstract
The eye of the rhynchobdellid leech, Helobdella stagnalis, has been examined with the electron microscope. The eye is composed of a cup of pigment cells surrounding a compact mass of photoreceptor cells. In addition to pigment granules, the pigment-cell cytoplasm is characterized by mitochondria, a Golgi complex, and profiles of rough-surfaced endoplasmic reticulum. The photoreceptor cell contains a microvillous rhabdomere. The microvilli arise from the membrane of a large intracellular vesicle and obliterate much of its lumen. No connexion between the lumen of the intracellular vesicle and the extracellular space has been observed. The plasmalemma of the photoreceptor cell is folded to form thin pleats of cytoplasm which separate adjacent receptor cells from each other. No glial-like cells have been seen in the receptor cell mass. Directly subjacent to the microvilli and surrounding the intracellular vesicle is a tortuous and predominantly smooth-surfaced endoplasmic reticulum. A pair of centrioles is found near the rhabdomere. The cytoplasm around the nucleus is characterized by smooth- and rough-surfaced elements of endoplasmic reticulum, many mitochondria, and a Golgi complex. Proximally, the receptor cell narrows to form a nerve fibre which joins those from other cells to form the optic nerve.