Cleavage Initiation Research Articles

The plasticity and cleavage of polycrystalline beryllium II. The cleavage strength and ductility transition temperature

For a grain diameter d, the cleavage strength is proportional to d-1/2, but intercepts the stress axis. Initiation of cleavage in second phase particles of a size that varies suitably with d could produce this relation. More likely, the cleavage strength is determined by the condition for propagation of a microcrack across a grain boundary. An explanation of the stress intercept is given in terms of the probability that the critical microcrack size is an increasing multiple of d as the grains become finer. Directly measured ductility transition temperatures agree with those deduced from the intersection of a temperature dependent flow stress with a temperature independent cleavage strength.

Crack propagation and cleavage initiation in Fe-2.6%-Si single crystals under controlled plastic crack tip opening rate in various gaseous environments

Crack propagation and crack initiation measurements were carried out in Fe-2.6%-Si single crystals under a new type of control: The elastic strain rate is controlled before yielding. Beyond the yield point, however, the rate of plastic elongation of the specimens is non-linearily controlled. Since plasticity is exclusively concentrated onto the crack tip. the plastic crack tip opening rate is prescribed with reasonable accuracy in this type of experiment. In b.c.c. FeSi single crystals at temperatures below 100°C the crack tip opening rate determines whether the crack advance is ductile or brittle. The critical crack tip opening rate, at which the fracture mode changes, is strongly temperature dependent and yields an activation energy of 0.68 eV for the initiation of cleavage. This value is close to the activation energy reported in the literature for dislocation glide in FeSi. In a small range of the crack tip opening rate, below the critical rate, stable brittle fracture (‘quasi-brittle fracture’) is found. Evidence is reported that plasticity may be responsible in a yet unknown way for the stable quasi-brittle crack growth.

Differential modes of processing and decay for the major N-dependent RNA transcript of coliphage λ

The major leftward, N-dependent RNA synthesized from the p L (leftward promoter)- att operon of bacteriophage λ in wild-type and rivonuclease III (RNase III)-deficient hosts has been investigated by transcriptional mapping analysis. These studies reveal a sequence-specific program of post-transcriptional events leading to differential modes of RNA processing and decay of the N-dependent ( l′ 3) RNA transcript. The initial events, which comprise the RNA processing (rapid) phase, include RNase III-mediated cleavage of the p L-proximal l° i (containing the 5′-ppp terminus) and the next adjacent l 1 RNA species (both about 4.5 S in size) and a third cleavage event in the vicinity of the leftward transcriptional termination signal ( t L1) just to the left of gene N. This latter cleavage appears to be responsible for the differentially rapid decay of the gene N RNA segment derived from the 3′-end of l 1l -to- lt L1. This series of events is completed by the time the N-dependent transcription is terminated at the att site. The events of the succeeding secondary (relatively slow) phase are associated with the remaining t L1- att ( l 3 RNA) segment and comprise the initiation of endonucleolytic cleavage and subsequent chemical decay events which result in the uniform decay of the entire l 3 RNA segment of the l′ 3 RNA transcript.

Ultrastructural studies of vegetative cells, mitosis and cell division inEnteromorpha Intestinalis(L.) Link

Using light and transmission electron microscopy, the ultrastructure of vegetative and dividing cells is described. Vegetative cells were found to be very similar to those described from closely-related genera. They contained a prominent chloroplast with a single pyrenoid. The doubling of the pyrenoid was the first indication of imminent cell division. The process of mitosis, from the re-arrangement of organelles to the initiation of cytoplasmic cleavage, and the initial stages of wall deposition are also similar to those described from species of related genera.

Induced formation of asters and cleavage furrows in oocytes of Xenopus laevis during in vitro maturation.

We have previously reported that injection of purified basal bodies or sperm into unfertilized eggs of Xenopus laevis induced the formation of asters and irregular cleavage furrows. Fully grown oocytes were found to be unable to form asters or cleavage furrows. In this paper we show that the oocyte acquires the ability to form asters upon basal body injection at the time of germinal vesicle breakdown during in vitro maturation. Our evidence indicates that aster formation requires progesterone-stimulated changes in the oocyte and mixing of cytoplasm and germinal vesicle plasm. The ability of the oocyte to form cleavage furrows arises six to eight hours after germinal vesicle breakdown. We infer that some maturational change in the cell cortex occurs to enable the egg surface to furrow. Experiments on the relationship of aster formation to furrow initiation indicates that asters stimulate furrow formation. However, some furrowing could be induced without aster formation in mature oocytes and unfertilized eggs by an activation stimulus, showing that asters are not essential for cleavage initiation. The significance of these observations are discussed in the light of our current understanding of meiotic maturation, cell cleavage and aster growth.

Tubulin and Microtubules in the Early Development of the Axolotl and Other Amphibia

SYNOPSIS. Unfertilized eggs of the axolotl, Ambystoma mexicanum , contain a pool of soluble tubulin accumulated during oogenesis. After initiation of cleavage the tubulin pool decreases somewhat and then remains constant through early development. Some properties of tubulin alter during development, but at least some of these changes are not due to changes in tubulin perse. However, the tubulin in axolotl oocytes, eggs, and embryos differs in some electrophoretic properties from tubulin in adult axolotl brain and testis. Equivalent differences were observed in Necturus maculosus tubulins. Heterogeneity of axolotl tubulins was confirmed by peptide mapping: Different patterns of peptides were formed by specific limited proteolysis of soluble tubulin from eggs and testis. The heterogeneity was more marked in the a than in the β subunit. Mobilization of soluble tubulin into the mitotic apparatus depends on the functioning of microtubule organizing centers after activation of the egg at fertilization. In eggs of the nc mutant axolotl there is a lesion in some step of activation, one effect of which is that even though the eggs contain an essentially normal pool of tubulin, microtubules fail to assemble, no mitotic apparatus forms, and embryonic development does not begin. These eggs can be partially corrected by injection of heterologous microtubule fragments, which elicit the mobilization of nc tubulin into arrays of microtubules, followed by initiation of cleavage and development to a partial blastula stage. The results of these experiments are discussed in comparison with other reports in the literature about the function of microtubule organizing centers during amphibian egg development.

The establishment of the oral-aboral axis in the ctenophore embryo

ABSTRACTIn a small percentage of normal embryos and in a higher percentage of embryos centrifuged prior to the first cleavage the positions of the polar bodies and the site of the first cleavage furrow do not coincide. These cases have been used to establish whether polar body formation sites or first cleavage initiation sites correlate best with the oral-aboral axis of the embryo. In all cases when the first cleavage is initiated at a site different from the site where the polar bodies were given off, the pattern of the first four cleavages is normal, the segregation of comb plate potential at these stages is normal, and the larvae that form are normal. The extent to which comb plate potential is localized along the oral-aboral axis of the embryo prior to the first cleavage, during the first cleavage and at the 2-cell stage was also examined. These experiments demonstrate that the oral-aboral axis is established at the time of the first cleavage, that cleavage plays a causal role in setting up the axis, and that comb plate-forming potential begins to be localized in the aboral region of the embryo at this time.

Microtubule proteins in axolotl eggs and developing embryos

Tubulin from eggs and embryos of the Mexican axolotl was characterized by electrophoresis and colchicine binding. In urea-polyacrylamide gel electrophoresis, soluble axolotl egg tubulin migrated as two bands, identical to tubulins from sea urchin sperm and Drosophila eggs. However, in SDS-containing gels, on which the α and β subunits of standard tubulins were well resolved, axolotl egg tubulin migrated as a single band with an apparent molecular weight of 53,500. The method of disruption of the eggs affected both yield of tubulin from vinblastine sulfate precipitates and stability of the colchicine binding activity. The colchicine binding activity of soluble tubulin from gently disrupted eggs was specific and of high affinity, with properties similar to those reported for other tubulins. The tubulin pool in unfertilized eggs was determined to be approximately 2 μg/egg; the level decreased 20% after initiation of cleavage and then remained constant through development to postneurula stages. The colchicine binding activity of soluble tubulin from embryos was much less stable than that of unfertilized eggs and decreased further during development. No differences were found in properties of tubulin from eggs of several strains of normally pigmented axolotls; however, tubulin from albino eggs showed slightly different properties in both electrophoresis and colchicine binding. The colchicine binding activity of soluble tubulin accounts for only half the total activity in axolotl eggs; they possess, in addition, a particulate nontubulin colchicine binding activity.

The effect of myosin antibody on the division of starfish blastomeres.

Antiserum against starfish egg myosin was produced in rabbits. Antibody specificity to myosin was demonstrated by Ouchterlony's immunodiffusion test and by immunoelectrophoresis in the presence of sodium dodecylsulfate (SDS). The latter technique showed that the antibody binds to both heavy and light chains of egg myosin. Furthermore, the antibody reacted with starfish sperm mysosin and starfish adult muscle myosin at both the heavy and light chains. It did not react with bovine platelet mysosin or rabbit skeletal muscle myosin in Ouchterlony's test; however, a weak reaction was observed in the presence of SDS between the antibody and these myosin heavy chains. Ca- and Mg-ATPase activities of egg myosin were not affected by the antibody, but it did inhibit actin-activated ATPase activity of egg myosin. Microinjection of the antibody into blastomeres of starfish eggs at the two-cell stage was carried out. Anti-egg myosin gamma-globulin inhibited the subsequent cleavages at an amount of more than 0.3 ng when injected at interphase. The inhibition was reduced when the injection was carried out near the initiation of cleavage. At the onset of the second cleavage the antibody was not inhibitory; however, an appropriate amount inhibited the third cleavage. Although the disappearance of the nuclear membrane was observed in the presence of the antibody, the formation of the mitotic apparatus was more or less disturbed. However the formation of daughter nuclei seemed to be scarcely affected by the antibody except that the distance between the nuclei was significantly smaller than normal.

A possible bio-assay for chromosome movement in isolated mitotic apparatus

Abstract We isolated mitotic apparatus (MA) from sea urchin zygotes using several methods. The isolated MA were injected into nucleated frog eggs or into enucleated frog eggs, and 24 h later we determined whether or not the eggs had undergone normal cleavage. Normal cleavage occurred in about 50% of the cases when MA isolated in glycerol-dimethyl sulphoxide were injected into either nucleated or enucleated frog eggs. Likewise for MA isolated initially in hexylene glycol and transferred immediately into glycerol-dimethyl sulphoxide. No cleavage occurred when MA isolated in hexylene glycol (and stored in hexylene glycol) were injected into frog eggs. We discuss two possible interpretations of the results. In one interpretation cleavage of the frog eggs is a bioassay, measuring the ability of the isolated MA to support chromosome movement. In the other interpretation the isolated MA contribute only nuclear material and cleavage initiation factors, and there is no chromosome movement in the isolated MA per se.

Some microstructures of experimentally deformed argillaceous sediments

Soft argillaceous sediments have been experimentally deformed and the resulting microstructures examined in thin section. Specimens of various natural clays, ball clay, kaolinite, etc., with saline water contents up to 40% have been subjected to (i) triaxial compression at strain rates between 10 −3/sec. and 10 −7/sec. and confining pressures up to 300 bars, (ii) direct and distributed shear at similar strain rates and with normal stress up to 14 bars. In general the materials are weak and ductile but slip is involved in most of the wide range of microstructures that is produced. None of the effects is wholly pervasive. Deformation at failure is chiefly accomplished by ductile shear along inclined, discrete, planar zones. ‘Layer boundary crenulations’ consist of broad, tapering crenulations which form at high angles to the junction with a layer of contrasting lithology. The structure has been recognized in a number of rocks which are thought to have undergone some movement while still soft. Crenulations of the primary settling fabric are common but vary greatly in spacing, width and sharpness. At one extreme the crenulations grade into discrete kink-bands and at the other produce closely-spaced highly-asymmetric ‘creases’ which may have important implications for the initiation of slaty cleavage. The microstructures may provide new guides for recognizing pre-lithification movements in sedimentary rocks.

The permeability to cytochalasin B of the new unpigmented surface in the first cleavage furrow of the newt’s egg

Two of 10 mug/ml cytochalasin B (CB) caused retraction of the first cleavage furrow in Triturus eggs, a spreading of the unpigmented surface from the furrow region and a flattening of the whole egg. CB appears to act against the contractility of the microfilamentous band at mid-cleavage so as to relax the furrow and also to weaken unpigmented surface to allow the egg to flatten. Uncleaved eggs and the initial formation of the cleavage groove were unaffected by CB. A fully-retracted first cleavage furrow reformed itself on transfer of the egg to normal medium but only at the time of second cleavage. Initiation of second cleavage depended upon there being sufficient of the original pigmented surface on the animal hemisphere. Tritium-labelled CB of high specific activty was prepared and used to study its ability to penetrate the surface of newt eggs during cleavage. Scintillation couting of whole eggs showed that CB was not taken into the newt egg until mid-cleavage (about 17 min after the double stripe stage) when new surface began to spread in the cleavage furrow. Fixation in glutaraldehyde and osmium tetroxide retained radioactivity in the egg, but more CB was retained after fixation in osmium tetroxide alone than after double fixation. Most of the retained radioactivity was in yolk platelets. Autoradiographs were prepared to sectioned eggs which had been fixed at late cleavage after [3H]CB had flattend the furrow. These showed that CB entered the egg through the unpigmented surface which formed in the furrow but it could not enter through the pigmented surface. The impermeability of the pigmented surface explains the observations that CB does not prevent initial furrowing at cleavage. Once inside the egg CB is transported slowly. CB penetrates to a limited extent beneath the pigmented surface from its border with the unpigmented surface in the first cleavage furrow and this seems insufficient in some circumstances to suppress the contractile phase of second cleavage.

Microtubule assembly mutant.

WE report here a new maternal effect mutation (nc, for no cleavage) in the Mexican axolotl, Ambystoma mexicanum. This mutation involves a defect in the assembly of microtubules in the fertilised egg. There is a large literature describing the molecular and biochemical events which accompany the in vitro self-assembly of microtubules; the nc mutant is the first known system with a genetic lesion in the regulation of these events in vivo. This mutant will provide a means to examine directly the control of microtubule assembly, and may also be useful in the identification of the events which trigger initiation of cleavage after activation of the egg at fertilisation.

Cleavage initiation activities of microtubules and in vitro reassembled tubulins of sperm flagella.

Cleavage of fish (Oryzias latipes) eggs was induced by injection of heterologous (sea urchin Hemicentrotus pulcherrimus and oyster Crassostrea gigas) sperm microtubules. Cleavage initiating CI activity of microtubules was higher in 3% PVP suspension than in 6% BSA, and not affected significantly by the concentration of microtubules themselves. The CI activity of microtubules suspended in 3% PVP was comparatively stable in the frozen state. Heat-treatment at more than 55 degrees C resulted in the loss of most or all of their CI activity. Such activity was observed in side-by-side aggregates of tubulin linear polymers of sea urchin (Hemicentrotus pulcherrimus) spermatozoa but not in dispersed linear polymers or tubulin dimers. Microtubules with CI activity seem to participate in initiating cleavage as astral centers, or a "seed" for polymerization of ooplasmic tubulins in activated eggs.

Development of the discharge apparatus in the fungus Entophlyctis

Correlative light and electron microscopic observations were used to reconstruct the morphological events involved in the development of the discharge apparatus of Entophlyctis zoosporangia. A discharge plug formed as vesicles containing fibrillar material fused with the plasma membrane and deposited their matrices between the plasma membrane and zoosporangial wall. At the apex of the enlarging plug, the zoosporangial wall lost its microfibrillar appearance, became diffuse, and left an inoperculate discharge pore. The discharge plug exuded through this pore and then expanded into a sphere which rested at the tip of the discharge papilla or tube. After the release of the discharge plug, the number of fibrilla containing vesicles decreased and abundant endoplasmic reticulum appeared in the cytoplasm below the plug. Granular material then accumulated at the interface of the discharge plug and the plasma membrane. This was the endo-operculum. A single layer of endoplasmic reticulum subtended the area of plasma membrane which the endo-operculum covered. Later, dictyosomes appeared in the cytoplasm below the endo-operculum. Fusion of Golgi vesicles with the plasma membrane below the endo-operculum coincided with the initiation of cytoplasmic cleavage. This sequence of events indicates that, unlike the discharge plug, the endo-operculum does not originate by vesicular addition of preformed material.

Initiation of C3 Cleavage in the Alternative Complement Pathway

The capacity of C3 and 125I-C3 to interact with B and D to generate a C3 convertase was recognized by identification of C3b and Bb on immunoelectrophoresis and 125I-C3b on disc gel electrophoresis. Since the C3 was devoid of C3b as assessed by immunoelectrophoresis, alkaline disc gel electrophoresis, and isoelectric focusing, the initial C3 cleavage was not by the C3b-dependent C3 convertase. In addition, on isoelectric focusing C3 hemolytic activity and the capacity to permit B cleavage by D were isoelectric at pH 6.4, WHEREAS THE CAPACITY OF C3b to permit cleavage by D was isoelectric at pH 5.65. Comparison of the dose-response effects of C3b and C3 for D revealed linear and sigmoidal relationships, respectively, consistent with formation of an initial C3 convertase independent of C3b followed by generation of the C3b-dependent convertase in the reaction initiated with native C3. Further, preincubation of C3 with C3bINA did not diminish its subsequent capacity to permit B inactivation by D as compared to the introduction of C3bINA during the assay, thus supporting the view that native C3, B, and D can form a convertase capable of generating initial C3b.

Acadian Slaty Cleavage Noncoplanar with Fold Axial Surfaces in the Northern Appalachians

The Acadian orogeny though polyphase produced a single generation of folds and a single generation of slaty cleavage over wide areas of the northern Appalachians. Acadian slaty cleavage noncoplanar with fold axial surfaces is more widespread than has hitherto been recognized-Published examples from several areas are quoted. Cleavage coplanar with the axial surface in one part of a fold may transect the axial surface and both limbs elsewhere in the fold, and the time relations to folding are implicitly the same. Geometric relationships of regional Acadian slaty cleavage to axial surfaces of associated folds in Silurian sediments in northeast New Brunswick are described. Slaty cleavage is superimposed upon the folds whether coplanar or noncoplanar with axial surfaces. Convergent fans and refraction of the noncoplanar cleavage indicate compression later than the cleavage. Total shortening of Silurian strata is a combination of folding and of superimposed compression accompanying and succeeding initiation of regional slaty cleavage. A possible origin of the noncoplanar Acadian structures resulting from décollement of Silurian sediments upon Ordovician basement is discussed.

Effect of ferrous ions on microsomal phospholipid peroxidation and related light emission

Purified NADPH cytochrome c reductase catalyzes the oxidation of NADPH in the presence of Fe 3+, ADP and EDTA. EDTA in this system appears to elevate the redox potential of ferric ion and of its iron complex thereby facilitating the transfer of one electron from NADPH to tri-valent iron (more rapidly than superoxide is formed) through a flavin moiety in the reductase, but it diminishes the concentration of free iron to be required for phospholipid peroxidation. The reduction of Fe 3+ by the xanthine-xanthine oxidase system is different from that manifested by the NADPH-NADPH cytochrome c reductase system in the manner in which the former is carried out in the main by O staggered2 staggered− · generated by the substrate - O 2 - enzyme interaction. Reduced iron, which is free in the solution, plays an important role for the initiation and propagation of the phospholipid peroxidation, monitored by malondialdehyde assay and light emission. In the xanthine-induced lipoxygenation system, the ·OH radical, probably produced from hydrogen peroxide by the action of Fe 2+, is not involved in the initiation of the peroxidative cleavage of phospholipid in microsomal lipoprotein.

The activation of sea urchin eggs by the divalent ionophores A23187 and X-537A

The divalent ionophores A23187 and X-537A induce parthenogenesis in sea urchin eggs. This results from their ability to mobilize intracellular Ca 2+, which is implicated in both artificial parthenogenesis as well as the natural fertilization process. A23187 causes expulsion of cortical granules and elevation of the fertilization membrane within 0.5–9 min followed by an initiation of cell cleavage. The broader spectrum ionophore X-537A is less potent, but the production of cytoplasmic aberrations are more apparent. In contrast to the sperm-activated egg, the initial phase of ionophore induced activation is accompanied either by relatively insignificant changes in membrane resistance, or an increase.

Timing of Slaty Cleavage During Folding of Precambrian Rocks, Northwest Tasmania

Detailed analysis of folds and cleavage in five small areas in the Precambrian Burnie Quartzite and Slate near Sulphur Creek, in northwest Tasmania, reveals that slary cleavage can be interpreted as an essentially planar structure initiated in a short interval during protracted folding. Cleavage initiated late during folding appears to be largely superimposed on fold profiles, and where rotation of early folds occurs during folding, cleavage may cut across the profile from one limb to the other. Cleavage initiated partway through folding is refracted in sandy beds forming cleavage fans. The largest cleavage-fan angles develop in folds where cleavage is initiated early in folding. Geometrical relations show that, for the area as a whole, either the onset of folding or the initiation of slaty cleavage was diachronous.