bluuebunny/crossref_metadata_2025_split huggingface dataset:
The effect of retinol and of retinol binding protein on embryonic skeletal tissue in organ culture
Authors: Dingle J. T. , Fell Honor B. , Goodman D. S. 
Published: April 2021

ABSTRACT
Retinol, bound non-specifically to serum proteins in organ culture medium, caused the degradation of the extracellular matrix of chick limb-bone rudiments. The loss of polymeric material was demonstrated histologically and measured chemically.
Retinol, at the same concentration, bound specifically to retinol-binding protein, produced no significant effect. The addition of prealbumin to this system was also without discernible effect.
The addition of apo-RBP to medium containing free retinol prevented the normal hyper-vitaminous A effects. The results indicate that the retinol-RBP complex has a high stability, sufficient to prevent the uptake, by the plasma or lysosomal membranes, of the amount of retinol needed for physical changes to occur.
It is suggested that retinol-binding protein may serve to transport retinol to selected sites, whilst protecting biological membranes against a general non-specific effect of the vitamin’s surface active properties.
Studies on the development of ameloblasts: I. fine structure
Authors: Katchburian E. , Holt S. J. 
Published: April 2021

ABSTRACT
The cellular and fine-structural changes which take place during differentiation of ameloblasts into highly secretory cells involve growth and elongation of the cytoplasm, a change in polarity, and sequential development of organelles. The undifferentiated cells of the ameloblastic layer seem to be mainly concerned with the production of material for internal use, as they are rich in free ribosomes and have only few profiles of rough-surfaced endoplasmic reticulum. First signs of differentiation are detected when the cells begin to elongate and large numbers of microtubules oriented parallel to the cell long axis appear in the distal cytoplasm. This suggests that the microtubules provide directional guidance for the elongation of the cells. The next stage of differentiation is characterized by proliferation of the rough-surfaced endoplasmic reticulum in the distal cytoplasm and the migration of the small Golgi complex, initially at the base of the cell, to the distal cytoplasm. Intracisternal accumulation of enamel matrix-like material in the rough-surfaced endoplasmic reticulum at this stage suggests that the Golgi complex is unable to process for secretion the materials synthesized in the rough-surfaced endoplasmic reticulum. However, the presence of enamel matrix-like material in the inter-cellular spaces may indicate an attempt to secrete matrix by a mechanism which by-passes the Golgi complex. With further differentiation the Golgi complex reaches the distal cytoplasm where it develops and becomes very extensive. The appearance of enamel matrix-like material in the cisternae of the mature Golgi complex and in large secretion granules in secretory ameloblasts suggests that secretory material is processed in a manner similar to that of other exocrine cells. As soon as enamel matrix begins to be deposited at the distal surface, ameloblasts withdraw, leaving the cytoplasmic extension (Tomes process) in the early calcifying enamel. The presence of microtubules and filaments in both the early and late developing Tomes process suggests that they play a role in the directional and localized growth of this constricted cell extension. Based on previous evidence that the Tomes process has an extensive lysosomal system and on the observation that its surface membrane possesses numerous invaginations and coated pits, it is concluded that the present findings support the concept that the Tomes process – via its lysosomal vacuolar apparatus – plays a role in the changes which occur in the enamel matrix in early amelogenesis.
Roles of cortical and subcortical components in cleavage furrow formation in amphibia
Authors: Sawai Tsuyoshi 
Published: April 2021

ABSTRACT
In the eggs of the newt, Triturus pyrrhogaster, 2 separate factors are recognized which take part in cleavage furrow formation. (1) The inductive capacity for the furrow formation by the cytoplasm lying under the cortex along the cleavage furrow (FIC); and (2) the reactivity of the overlying cortex to form a furrow in response to FIC.
(1) FIC. The inductive capacity is shown by the fact that FIC induces a furrow on which-ever part of the surface under which FIC is transplanted. FIC is distributed along the cleavage furrow and even extends along the future furrow plane ahead of the furrow tip. The distance FIC precedes the furrow tip is about 10 mm in the animal hemisphere and is less in the vegetal hemisphere. In the direction at right angles to the furrow plane, FIC does not spread more than 0·1 mm.
FIC is also present in the eggs of Xenopus laevis. Species specificity of FIC for induction is not found between Triturus and Xenopus.
(2) Surface layer. At the onset of the first cleavage, the reactivity of the cortex to form the furrow in answer to FIC induction is localized on the animal pole region. The reactivity of the cortex propagates medially as a belt along the surface towards the vegetal pole with the advancing tip of the cleavage furrow. After the furrow is completed, the reactivity begins to be lost from the animal pole region, and eventually over the entire surface. The reactivity, however, reappears on the animal pole region simultaneously with the second cleavage.
Cell cycle kinetics and development of Hydra attenuata: I. epithelial cells
Authors: David C. N. , Campbell R. D. 
Published: April 2021

ABSTRACT
The cell cycle parameters of epithelial cells of Hydra attenuata are described. Specifically the rate of proliferation and the fraction of proliferating cells have been determined under conditions of defined growth rate. Techniques involved standard methods of cell cycle analysis using histological and tissue maceration preparations; pulse-chase and continuous labelling with [3H]thymidine followed by autoradiographic analysis, and microspectrophotometric determination of nuclear DNA content in single cells. The results indicate that more than 90% of hydra epithelial cells are actively proliferating with a cell cycle duration about equal to the tissue doubling time. In well fed hydra the average cell cycle is about 3 days long. S period is 12–15 h, G1 0–1 h, and mitosis 1·5 h. Most of the cell cycle consists of a long G2 period of variable duration (24–72 h). The results provide no evidence for a subpopulation of rapidly proliferating cells as predicted by ‘growth zone’ models of hydra morphogenesis. The results also indicate that the population of epithelial cells is self-sustaining requiring no input by differentiation from other cell types. The long and variable G2 period means that DNA synthesis and the following cell division are effectively uncoupled such that inhibitors of DNA synthesis may not stop epithelial cell division. The variable nature of the G2 period suggests it as a possible point of control of hydra growth.
Turnover of plasma membrane during phagocytosis
Authors: Goodall R. J. , Lai Y. F. , Thompson J. E. 
Published: April 2021

ABSTRACT
Levels of radioactive glycine and glycerol incorporated into the plasma membrane of Acanthamoeba castellanii during phagocytosis were determined in order to elucidate how surface membrane expended during this process is replaced. The amoebae were allowed to ingest latex beads in the presence of the labelled membrane precursors and plasma membrane was then isolated and analysed for the presence of radioactivity. The isolated membrane fragments were found to be quite highly labelled. In order to ascertain whether this represented preferential synthesis of plasma membrane in response to phagocytosis, the specific radio-activities of the isolated membrane fractions were compared with those of corresponding particulate homogenates, which were composites of all cellular membranes. Enrichment values calculated in this manner proved to be essentially similar for both phagocytosing and non-phagocytosing amoebae. This indicates that de novo synthesis of plasma membrane is not essential for phagocytosis and in turn suggests that pre-existing cytoplasmic membrane is used to replace surface membrane consumed during ingestion. Presumably the incorporation of membrane precursors that was observed represents molecular turnover that occurs irrespective of phagocytosis.
Sodium-Induced Changes in the Nuclei of Monolayer HeLa Cultures
Authors: Jobst K. , Kellermayer N. 
Published: April 2021

ABSTRACT
In HeLa cells cultured in a medium containing 142 mequiv./l. sodium ions only the mitotic forms are biréfringent (anisotropic index, 27/1000). In a culture medium containing 155 – 160 mequiv./l. sodium ions, large numbers of nuclear forms reminiscent of the prophase were found. In such a medium the anisotropic index was 86/1000. At low sodium ion concentration (90 – 110 mequiv./l.) these anomalous, prophase-like forms were not seen and the anisotropic index was 18/1000. The appearance of the biréfringent, prophase-like forms is related to a structural rearrangement and condensation of DNP in a hypertonic medium.
Studies on Nucleolar RNA Synthesis in Drosophila Melanogaster: II. The Influence of Conditions Resulting in a Bobbed Phenotype on Rate of Synthesis and Secondary Constriction Formation
Authors: Krider H. M. , Plaut W. 
Published: April 2021

ABSTRACT
The influence of conditions resulting in bobbed phenotypes on nucleolar RNA synthesis and the formation of constrictions at nucleolus organizers was examined in larval tissues of Drosophila melanogaster. By means of [3H]uridine incorporation and autoradiographic analysis, a mutation at the bobbed locus was shown to limit the rate of nucleolar RNA synthesis in salivary glands of XO larvae. The formation of constrictions at the organizer sites of a 4-nucleolus-organizer stock was monitored in dividing neuroblast cells stained with acridine orange. Loss of the ribosomal cistrons had been reported by other workers when such stocks were maintained for several generations. In the first generation in our work, constrictions were visible at only 2 of the 4 nucleolus organizers. This situation persisted until the fifth generation, when constrictions appeared at all 4 of the organizer sites. An increase in the rate of nucleolar RNA synthesis in the salivary glands was temporally correlated with the appearance of the extra constrictions. We interpret these observations to mean that 2 of the organizers of the 4-nucleolus-organizer stock were caused to function through the loss of ribosomal RNA cistrons; thus the functional status of an organizer would appear to be subject to control.
Nucleocytoplasmic Interaction in the Eggs of Pteridium Aquilinum Maturing in the Presence of Thiouracil
Authors: Bell P. R. 
Published: April 2021

ABSTRACT
In the eggs of Pteridium aquilinum maturing in the presence of a small quantity of thiouracil (0·6 × 10-3 M in the medium), known to reduce the rate of maturation by a half or more, the nuclear evaginations are particularly conspicuous. On the grounds of their structural features, their regularly narrow connexion with the nucleus, and the absence of nuclear pores from their envelopes, the evaginations are regarded as specialized formations, and not merely extensions of an amoeboid nucleus. An aggregate of electron-opaque material, probably of nucleolar origin, and believed on the basis of enzyme extractions to be largely protein, but with some ribonucleic acid, is a characteristic feature of the evaginations.
The fate of the evaginations, which disappear shortly after fertilization, remains obscure. Refined methods of fixation and staining have, however, revealed striking resemblances between the evaginations, bodies which may be detached evaginations, and mitochondria. The hypothesis that the evaginations become detached, and that some at least differentiate into mitochondria, the simplest interpretation of the events observed, has not been invalidated by this investigation.
The Long-Term Organ Culture of Tissues from Adult Amphiuma, The Congo Eel
Authors: Monnickendam Marjorie A. , Balls M. 
Published: April 2021

ABSTRACT
Fragments of pancreas, liver, spleen, kidney and lung from Amphiuma means, the Congo eel, were maintained in organ culture for up to 35 days in a modified Leibovitz L15 medium. These Amphiuma tissues retained their typical histological features throughout the culture period, surviving better and for longer than tissues from other amphibian species. Liver and spleen survived better in hypotonic or isotonic media (125–230 mosmol/kg) than in hypertonic media (255–305 mosmol/kg). The mitotic incidences of these tissues in vivo were very low, and while there were no significant increases in liver, spleen and lung fragments in vitro, there were large increases in kidney and pancreas fragments. Amylase activity was found in pancreas fragments and in medium from pancreas cultures after 28 days in vitro. Amphiuma organ cultures may be useful in the in vitro study of the control of cell proliferation and cell function while normal cell interactions are maintained.
Biogenesis of Plasma Membranes in Salt Glands of Salt-Stressed Domestic Ducklings: Localization of Acyltransferase Activity
Authors: Levine A. M. , Higgins Joan A. , Barrnett R. J. 
Published: April 2021

ABSTRACT
In response to salt water stress there is a marked increase in the plasma membranes of the epithelial secretory cells of the salt glands of domestic ducklings. In the present study, the fine-structural localization of the acyltransferases involved in synthesis of phospholipids has been investigated in this tissue during this increased biogenesis of plasma membranes. The specific activity of the acyltransferases of the salt gland rose in response to salt stress, and this preceded the rapid increase in weight and cellular differentiation. After the weight increase of the gland became established, the specific activity of the acyltransferases declined, but the total activity remained constant. Salt gland tissue fixed in a mixture of glutaraldehyde and formaldehyde retained 35 % of the acyltransferase activity of unfixed tissue. Cytochemical studies of the localization of acyltransferase activity in fixed and unfixed salt gland showed reaction product associated only with the lamellar membranes of the Golgi complex. This localization occurred in partially differentiated cells from salt-stressed glands to the greatest extent; and to only a small extent in cells of control tissue from unstressed salt glands. Omission of substrates resulted in absence of reaction product in association with the Golgi membranes. In addition, vesicles having limiting membranes morphologically similar to the plasma membrane occurred between the Golgi region and the plasma membrane in the partially differentiated cells. The phospholipid component of the plasma membrane appears therefore to be synthesized in association with the Golgi membranes and the membrane packaged at this site from which it moves in the form of vesicles to fuse with the pre-existing plasma membrane.
Helical Structures in the Cytoplasm of the Male Germ Cells of the Desert Locust
Authors: Gawadi Nadya 
Published: April 2021

ABSTRACT
A helical structure which had previously been reported in the cytoplasm of many cell types in the desert locust, Schistocerca gregaria, was found in large quantities in the developing male germ cells. The helices were isolated from these cells and negatively stained. The helices, which lie free in the cytoplasm of spermatogonia, primary spermatocytes and secondary spermatocytes (sometimes associated with a second type of helix, a double helix), closely invest the outer surface of the Nebenkern membrane in spermatids. As the Nebenkem elongates, the helices gradually disappear and are replaced by microtubules. The mitochondrial derivatives of the Nebenkem, which form part of the sperm tail, are in late spermatids completely surrounded by a sheath of parallel microtubules.
The role of laminin-5 and its receptors in mammary epithelial cell branching morphogenesis
Authors: Stahl Stephanie , Weitzman Sigmund , Jones Jonathan C. R. 
Published: April 2021

ABSTRACT
In vivo, normal mammary epithelial cells utilize hemidesmosome attachment devices to adhere to stroma. However, analyses of a potential role for hemidesmosomes and their components in mammary epithelial tissue morphogenesis have never been attempted. MCF-10A cells are a spontaneously immortalized line derived from mammary epithelium and possess a number of characteristics of normal mammary epithelial cells including expression of hemidesmosomal associated proteins such as the two bullous pemphigoid antigens, α6β4 integrin and its ligand laminin-5. More importantly, MCF-10A cells readily assemble mature hemidesmosomes when plated onto uncoated substrates. When maintained on matrigel, like their normal breast epithelial cell counterparts, MCF-10A cells undergo a branching morphogenesis and assemble hemidesmosomes at sites of cell-matrigel interaction. Function blocking antibodies specific for human laminin-5 and the α subunits of its two known receptors (α3β1 and α6β4 integrin) not only inhibit hemidesmosome assembly by MCF-10A cells but also impede branching morphogenesis induced by matrigel. Our results imply that the hemidesmosome, in particular those subunits comprising its laminin-5/integrin ‘backbone’, play an important role in morphogenetic events. We discuss these results in light of recent evidence that hemidesmosomes are sites involved in signal transduction.
Studies on the eel sperm flagellum: I. The structure of the inner dynein arm complex
Authors: Woolley D. M. 
Published: April 2021

ABSTRACT
The highly motile, 9 + 0 sperm axoneme of Anguilla has inner dynein arms (IDAs) but not outer dynein arms. The in situ morphology of these IDAs is shown here to be essentially identical to the IDA morphology already seen in the axonemes of Chlamydomonas, Tetrahymena and Beroë, and in the sperm tails of echinoderms and several vertebrate species. In addition, this study demonstrates: (1) that the nexin (circumferential) links are present in Anguilla and are typical; (2) that IDA1 incorporates an archway, supported by a pillarstructure; (3) that images from thin sections and whole mounts are consistent with those from replicas of rapidly-frozen specimens; and (4) that the IDA and nexin link morphology is apparently unaffected by whether the axoneme is depleted of ATP, relaxed with ATP and vanadate, or inhibited by high ATP. An attempt has been made to reconcile the emergent morphology of the IDA complex with all earlier descriptions in the literature. From a detailed comparison of the results with published information on Chlamydomanas mutants, it is concluded that the nexin (circumferential) link is a major part of the ‘dynein regulatory complex’.
Histidine kinases in signal transduction pathways of eukaryotes
Authors: Loomis William F. , Shaulsky Gad , Wang Nancy 
Published: April 2021

ABSTRACT
Autophosphorylating histidine kinases are an ancient conserved family of enzymes that are found in eubacteria, archaebacteria and eukaryotes. They are activated by a wide range of extracellular signals and transfer phosphate moieties to aspartates found in response regulators. Recent studies have shown that such two-component signal transduction pathways mediate osmoregulation in Saccharomyces cerevisiae, Dictyostelium discoideum and Neurospora crassa. Moreover, they play pivotal roles in responses of Arabidopsis thaliana to ethylene and cytokinin. A transmembrane histidine kinase encoded by dhkA accumulates when Dictyostelium cells aggregate during development. Activation of DhkA results in the inhibition of its response regulator, RegA, which is a cAMP phosphodiesterase that regulates the cAMP dependent protein kinase PKA. When PKA is activated late in the differentiation of prespore cells, they encapsulate into spores. There is evidence that this two-component system participates in a feedback loop linked to PKA in prestalk cells such that the signal to initiate encapsulation is rapidly amplified. Such signal transduction pathways can be expected to be found in a variety of eukaryotic differentiations since they are rapidly reversible and can integrate disparate signals.
Changes in histone synthesis and modification at the beginning of mouse development correlate with the establishment of chromatin mediated repression of transcription
Authors: Wiekowski Maria , Miranda Miriam , Nothias Jean-Yves , DePamphilis Melvin L. 
Published: April 2021

ABSTRACT
The transition from a late 1-cell mouse embryo to a 4-cell embryo, the period when zygotic gene expression begins, is accompanied by an increasing ability to repress the activities of promoters and replication origins. Since this repression can be relieved by either butyrate or enhancers, it appears to be mediated through chromatin structure. Here we identify changes in the synthesis and modification of chromatin bound histones that are consistent with this hypothesis. Oocytes, which can repress promoter activity, synthesized a full complement of histones, and histone synthesis up to the early 2-cell stage originated from mRNA inherited from the oocyte. However, while histones H3 and H4 continued to be synthesized in early 1-cell embryos, synthesis of histones H2A, H2B and H1 (proteins required for chromatin condensation) was delayed until the late 1-cell stage, reaching their maximum rate in early 2-cell embryos. Moreover, histone H4 in both 1-cell and 2-cell embryos was pre-dominantly diacetylated (a modification that facilitates transcription). Deacetylation towards the unacetylated and monoacetylated H4 population in fibroblasts began at the late 2-cell to 4-cell stage. Arresting development at the beginning of S-phase in 1-cell embryos prevented both the appearance of chromatin-mediated repression of transcription in paternal pronuclei and synthesis of new histones. These changes correlated with the establishment of chromatin-mediated repression during formation of a 2-cell embryo, and the increase in repression from the 2-cell to 4-cell stage as linker histone H1 accumulates and core histones are deacetylated.