bluuebunny/crossref_metadata_2025 huggingface dataset:
Sex chromatin and X chromosome replication patterns and incidence of sex chromatin in canine cell cultures
Authors: Brown Robert C. , Cole K. W.
Published: February 2005

In order to provide evidence as to whether sex chromatin (SC) of interphase cells is equivalent to the late replicating X chromosome in female mammalian cells, time‐lapse cinephotometric and autoradiographic methods were used to give precise data for comparison of the DNA replication patterns of SC with that of each of the X chromosomes throughout the S period. Canine kidney epithelial cells were selected because they have distinct large metacentric X chromosomes and typical SC. Time‐lapse cinephotometry was used to avoid possible alteration of DNA synthesis by chemical cell synchronization agents. Determination of the incidence of SC during the stages of the cell life cycle of proliferating cells of the same origin was performed in order hopefully to clarify conflicting reports on the subject. Our results clearly show that time and intensity of the SC replication throughout S period is like that of the late replicating X chromosome and unlike that of the early replicating X chromosome. The incidence of SC in proliferating cells in culture was found to vary with the stage of the cell life cycle, increasing with increasing postmitotic interval — least in G1, greater in S, and greatest in G2. The SC incidence increased strikingly from G1 to S and a less marked increase was observed between S and G2.
Modification of cell surface antigenicity as a function of culture conditions
Authors: Pfeiffer S. E. , Herschman H. R. , Lightbody J. E. , Sato G. , Levine L.
Published: February 2005

Adaptation of monolayer cultures of a clonal line of rat glial cells to suspension culture resulted in the nearly complete loss of certain surface antigens. This change in surface antigenicity was paralleled by the loss of the ability of the cells to accumulate in vitro a protein specific to the nervous system (“S100‐protein”). In contrast, when glial cells were co‐cultivated in monolayer culture with another cell line apparently lacking these surface antigens, the number of these antigens was markedly increased. The possibility of a causal relationship between the changes in the surface antigenicity and the expression of differentiated function is considered.
Inhibition of cellular adhesiveness by sulfhydryl blocking agents
Authors: Grinnell Frederick , Srere Paul A.
Published: February 2005

Rat hepatoma cells rapidly adhered to the polystyrene surfaces of culture bottles. The adhesiveness of the cells was inhibited by sulfhydryl blocking reagents including mercurials, arsenicals, and alkylating agents. Inhibition was prevented, and in some cases reversed, by the addition of dithiothreitol to the adhesive medium. The current model explaining cellular adhesion is discussed in the light of these observations.
Surface charge and cell volume of synchronized mouse lymphoblasts (L5178‐Y)
Authors: Shank Brenda Buckhold , Burki H. John
Published: February 2005

Cellular electrophoretic mobility, and therefore, surface charge/ unit area, was found to be constant throughout the cell cycle of synchronized L5178‐Y mouse lymphoblasts, remaining at – 1.21 μ sec−1 V−1 cm. Measured cell volumes of these synchronized cells increased linearly over the cell cycle and at mitosis each cell divided into two cells, each cell having half of the parent volume. An hypothesis is presented which explains the presence of a mobility peak at mitosis in cells which normally are grown attaching to a glass surface on the basis of differential morphological changes during mitosis; cells such as the L5178‐Ys, which do not attach to glass normally, do not undergo such morphological changes at mitosis and, therefore, would not demonstrate a mobility peak at that time.
Influence of culture pH on photo‐inhibition of division in Euglena gracilis
Authors: Cook J. R.
Published: February 2005

In the dark, growth rates of Euglena gracilis were independent of culture pH between the limits of three to eight. Visible light of moderate intensity inhibited growth rates, with the degree of inhibition being markedly pH dependent. The most severe inhibition was observed at pH 4 to 5, with little or no inhibition at pH 3 or above pH 6.8 at the light intensity used (400 foot‐candles).
Control of mating type expression in Paramecium multimicronucleatum, syngen 2
Authors: Clark Mary Anne
Published: February 2005

Two isogenic acyclic strains of Paramecium multimicronucleatum, syngen 2, expressing mating types III and IV, and a cycler strain, which displays circadian alternation of the two mating types, were treated with proteolytic enzymes, or the inhibitors actinomycin D or puromycin at various stages of the mating reactive period induced by food depletion. Reactivity of acyclic animals was sensitive to proteolytic enzymes, and to both inhibitors before or after the onset of reactivity, suggesting that reactivity is attained and maintained via continuous RNA and protein synthesis, and that protein integrity is essential to the activity of the type‐specific substances. A new cell cycle was not necessary for recovery from a block in reactivity.

Type III animals were consistently more sensitive to inhibition by puromyin, and required longer recovery periods after all treatments except actinomycin D given after the development of reactivity. Possible explanations of these observations are considered.

The daily type changes seen in cycler animals seemed to occur by replacement of one mating type substance with the other. Type‐switching was neither prevented nor displaced by exposure to actinomycin D or puromycin during a period before or including a type change, even though reactivity was reduced by more than 95%, suggesting that type‐switching is not directly associated with transcription or translation of the type‐specific genes.
Measurement of the size distribution of human erythrocytes by a sedimentation method
Authors: Groom A. C. , Anderson J. C.
Published: February 2005

The distribution of sedimentation velocities was determined, by a photoelectric method, for human erythrocytes at low concentrations in Ringer solution. The light absorption at 414 nm was measured, as a function of time, 10 mm below the top of the column. From the frequency distribution of cell velocities that of Rs √ρ‐σ was found; Rs being the Stokes' radius, ρ the cell density and σ the density of the solution. Cell density was measured by the phthalate method and the mean Stokes' radius was found to be 2.58 μm. The size distributions showed some skewness but were in good general agreement with those measured by Celloscope counter, and with reported measurements from photomicrographs of cells in hanging drop suspensions. The skewness was much less than that encountered with electric sensing zone instruments (e.g. Celloscope) and the sedimentation method, being based on entirely different premises, provides an important check on such data. The skewness is due to a bias in the orientation of human erythrocytes during sedimentation. This bias may be a characteristic of biconcave cells; it could be absent in many species and reliable measurements of size distribution would then be obtained.
The role of cell‐cell contact in “contact” inhibition of cell division: A review and new evidence
Authors: Martz Eric , Steinberg Malcolm S.
Published: February 2005

The term “contact inhibition of cell division” was borrowed from “contact inhibition of cell movement.” We prefer the term “postconfluence inhibition of cell division” as being more operational and less mechanistically biased; it is operationally defined as a pronounced depression of the mitotic rate in a postconfluent culture which displays a stationary density despite periodic nutrient renewal, the inhibition being locally reversibly by removal of the adjacent cells.

The mechanism of postconfluence inhibition is of considerable interest because of the inverse correlation between postconfluence inhibition and the tumorigenicity of a number of cell lines. Several hypotheses, involving direct cell‐to‐cell contacts or locally restricted diffusion gradiens, could explain postconfluence inhibition.

With the goal of discriminating among these hypotheses, time‐lapse films were taken of carefully regulated, perfused cultures of 3T3 mouse cells, in which the transition from rapid growth to the stationary phase was recorded. Measurements of cell‐to‐cell contact, local cell density, and generation times were made on an individual cell level and analyzed with the aid of a computer.

We observed that all‐around cell‐cell contact or a high local cell density present throughout G1 often did not produce immediate inhibition of cell division. We conclude that either (i) simple visible cell‐cell contacts or a high local cell density are not the direct cause of postconfluence inhibition of cell division, or (ii) their effects often do not inhibit cell division until after a delay of about one cell generation time. Such a delay may be partly responsible for the 50% overshoot past the stationary density that we observed in 3T3 cultures.
Studies on the surface of chick blastoderm cells. II. Electron microscopy of surface binding characteristics
Authors: Sanders Esmond J. , Zalik Sara E.
Published: February 2005

The surfaces of cells from the early embryo of the chick were examined with the electron microscope using histochemical techniques in order to determine the distribution of cell surface material. The use of lanthanum nitrate as a stain for protein‐polysaccharide showed that in whole embryos the surface layer was present in areas of close membrane apposition, but relatively sparse where the membranes bounded large intercellular spaces. On cells freshly dissociated with EDTA, this technique demonstrated a very uniform layer over the surface, possibly indicating some redistribution. Close examination of the lanthanum stained material in whole embryos revealed the presence of periodicities about 180 Å in diameter.

In correlation with a progressive reduction of electrophoretic mobility, demonstrated previously, from embryonic stage 1 to stage 5, some decrease in the affinity of the surface for lanthanum and colloidal iron was observed.
Induction of erythroblast increase in rabbit marrow cells by extracts of leukocytes and certain metabolic compounds. IV. Studies on erythropoiesis
Authors: Borsook Henry , Jiggins Susan , Wilson Rosina T.
Published: February 2005

Two kinds of extracts of peripheral blood leukocytes induced erythroblast increase and maturation in vitro in rabbit marrow cells. One extract was a secretion or exudate into plasma from intact leukocytes; the other was the supernatant after homogenizing the cells in ethanol. The extracts were less active than the intact leukocytes. The response with the two extracts together was a little greater than with either alone, but less than additive.

11‐Ketopregnanolone and 3′5′ cyclic AMP were active inducers. Neither increased the effects of leukocytes or their extracts.
The mechanism of regulation of fibroblastic cell replication. I. Properties of the system
Authors: Ellem K. A. O. , Mironescu Stefan
Published: February 2005

Sixty to eighty per cent of the cells in a culture of human diploid fibroblasts may be stimulated from the state of density dependent inhibition of replication to active DNA synthesis and division. The maximum response is effected by 50% serum within the pH range 7.2–8.0. The proportion of cells responding depends on the concentration of serum protein in the medium which may be effectively substituted by crystalling serum albumin. There is a differential sensitivity to the stimulus of cells in the densely packed centers of whorls and in the less dense areas between the whorls. The cell response is parasynchronous and the median durations of the various phases of the cell cycle are: G1I6β Œæ® ¿ ∞ 8 hours, G2 = 6 hours and doubling time = 30 hours.

The stimulatory effect of fresh medium is lost during contact with dense cultures so that it has only 50% of its initial capacity after 14 hours. It can be restored by dialysis against serum‐free medium. The stimulus must be applied for at least ten hours to be effective in inducing DNA synthesis. During the latter half of ten hour induction period subsequent DNA synthesis becomes exquisitely sensitive to actinomycin D. After this time an increasing number of cells become irreversibly committed to replicate. The data are interpreted to indicate that during contact with serum proteins (including albumin) changes in the cell surface, if continued long enough, trigger a mechanism which involves the synthesis of a unique RNA species during the fifth to tenth hours. After this RNA has been synthesized the cells are then committed to DNA synthesis.
Lesch‐Nyhan mutation: The influence of population density on purine phosphoribosyltransferase activities and exogenous purine utilization in monolayer cultures of skin fibroblasts
Authors: Wood Stephen , Pinsky Leonard
Published: February 2005

In extracts of normal and Lesch‐Nyhan (LN) heterozygous skin fibroblast monolayer cultures, hypoxanthine‐guanine (H‐G) and adenine (A) phosphoribosyltransferase (PRT) activities are correlated. These activities vary concomitantly during the life cycle of a culture: Peak activities occur during exponential growth. The ratio of H‐GPRT to APRT activity can manifest heterozygosity at the LN locus when H‐GPRT activity, per se, is close to and unreliable different from the normal range.

If 2 or 8 × 104 cells are planted per 35 mm petri dish, a strain with clearly heterozygous levels of H‐GPRT activity in its cell extract may not reveal its genotype by simultaneous measurement of adenine and hypoxanthine incorporation into its acid‐insoluble fraction one day after subculture. In contrast, a 1:1 coculture of normal and mutant cells yields the ratio of adenine to hypoxanthine incorporation expected from the behavior of each cell type separately. In heterozygous cultures at the higher population density, the incorporation of hypoxanthine relative to adenine is 28% greater than in those at the lower population density. A 1:1 mixture of normal and mutant cells exhibited a 20% increase in the relative incorporation of hypoxanthine over the same four fold increase in population density. These observations appear to provide a quantitative expression of the phenomenon of metabolic cooperation between contacting H‐GPRT‐negative and H‐GPRT‐positive cells.
Incorporation and intraparticulate hydrolysis of 131 I‐albumin by liver and kidney of an amphibian (Bufo arenarum)
Authors: Bertini Francisco , Bari Daniel R. , Mazzei Hector
Published: February 2005

After a single injection of formaldehyde‐treated 131 I‐albumin into the heart, the incorporation of the labelled protein by liver (% of total injected radioactivity/% of body weight of the organ) was far greater than in other organs. In kidney and spleen it was respectively six and three times greater than in lungs, intestine, testis and fatty body. No radioactivity was found in brain. The radioactivity in liver and kidney reached a peak 30 minutes after the injection, and quickly decreased during the following four hours. In the 27,000 g × ten minute particles recovered from liver homogenates of animals sacrificed at various times after injection, the rate of 131 I‐albumin hydrolysis in vitro and the percentage of trichloroacetic acid soluble radioactivity at zero time of incubation showed different stages of intraparticulate hydrolytic activity. The incorporation and intraparticulate hydrolysis in toad kidney was very low if compared with that of toad liver or mouse kidney; however the catheptic specific activity in toad kidney doubles that of mouse kidney. Isolated toad liver was perfused with total blood, containing 131 I‐albumin, for five hours at 22°C in a special chamber. In this conditions, 16% of the labelled albumin was hydrolyzed by the liver.
Regulation of mitochondrial respiration in senescence
Authors: Chen Jenn C. , Warshaw Joseph B. , Sanadi D. Rao
Published: February 2005

The ADP‐stimulated (State 3) respiration of myocardial mitochondria with glutamate‐malate, glutamate‐pyruvate, palmitylcarnitine and β‐hydroxybutyrate as substrates declined in rats after the age of 20 months. There was no significant decline in pyruvate‐malate, α‐oxoglutarate, palmityl‐CoA, succinate and ascorbate cytochrome c oxidation. Skeletal muscle mitochondria from senescent animals showed a similar decline in glutamate‐malate oxidation but not in palmityl‐CoA, palmitylcarnitine, succinate and ascorbate‐cytochrome c oxidation. The controlled oxidation with ADP‐limiting (State 4) and the ADP/O ratio were not affected. The results indicate an alteration in the subtle regulatory capacity for mitochondrial oxidation in senescent rats. It is suggested that the alteration may be in certain anion transport and associated functions across the mitochondrial membrane or dehydrogenase activity.
Effects of prednisolone on the distribution and activity of two phosphoesterases in membrane preparations from HeLa cells
Authors: Costlow Mark E. , Melnykovych George
Published: February 2005

Activities of two membrane associated enzymes, viz. 5′‐nucleo‐tidase (EC 3.1.3.5) and alkaline phosphatase (EC 3.1.3.1) are known to be altered by glucocorticoid treatment of cells in culture. In certain HeLa cell lines, their activity is elevated, while in others, it is decreased. In order to confirm the data obtained previously with cell homogenates, cells were fractionated essentially as described by Bosmann, Hagopian and Eylar ('68) after 96 hours exposure to 0.5 μg/ml prednisolone. This treatment resulted in changes both in the specific activity and in the distribution of the two enzymes among the isolated membrane fractions. The alterations in specific activity in the individual membrane fractions reflected changes previously observed in preparations from whole cells. A shift in distribution of total enzymatic activity to fractions of higher density occurred in both cell lines in response to the steroid. These results may be indicative of cellular membrane alterations as a consequence of steroid treatment.
Formation in agar of fibroblast‐like colonies by cells from the mouse pleural cavity and other sources
Authors: Metcalf Donald
Published: February 2005

Colonies of elongated fibroblast‐like cells (stellate colonies) developed in agar cultures of mouse pleural cavity cells mixed with whole blood. Cultures of pleural cells alone developed only abortive clusters of round cells. The frequency of colony‐forming cells in the pleural cavity was highest in neonatal mice (200/105 cells) and fell progressively with aging. Stellate colony‐forming cells were not in cell cycle but were radiosensitive. In adult mice, only occasional colony‐forming cells were detected in peritoneal cavity, thymic, spleen, lymph node or bone marrow cell populations. Stellate colony formation was not stimulated by the granulopoietic regulator, colony stimulating factor. The active component in whole blood required for stellate colony formation was present in plasma but not serum or washed red or white cells.
Multiplication‐stimulating activity for chicken embryo fibroblasts from rat liver cell conditioned medium: A family of small polypeptides
Authors: Dulak Norman C. , Temin Howard M.
Published: February 2005

A partially purified multiplication‐stimulating activity for chicken embryo fibroblasts in cell culture was isolated from rat liver cell conditioned medium (see preceding paper, Dulak and Temin, 1973). It has been analyzed by isoelectric focusing and by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Multiplication‐stimulating activity resided in a family of at least four polypeptides which were similar in apparent molecular size, but different in electrical charge. These polypeptides have a specific activity of about 50,000 with respect to serum. One of them has been purified on a small scale to apparent homogeneity in a sodium dodecyl sulfate polyacrylamide gel.
On the kinetics of erythroid cell differentiation in fetal mice. I. Microspectrophotometric determination of the hemoglobin content in erythroid cells during gestation
Authors: Steiner Rudolf , Vogel Helmut
Published: February 2005

In a microspectrophotometric study, photographic emulsions and a computer are used for measuring the hemoglobin content of a large number (about 50,000) of erythroid cells in fetal mice. Histograms of the hemoglobin content in erythroid cells illustrate the kinetics of erythropoiesis in yolk sac derived nucleated cells in the fetal peripheral blood, in fetal liver, and in fetal spleen. After the occasional extrusion of their nucleus, yolk sac derived erythrocytes remain as “macrocytes” in fetal circulation two or three days longer than the nucleated yolk sac derived erythrocytes do. Erythrocytes in fetal liver have a constant hemoglobin content of 28 pg 2 until day 17 of gestation. During further erythropoiesis in liver and then in the spleen, this amount is gradually adapted to the normal hemoglobin content in red blood cells of 16 pg.
Erythropoietic progenitors capable of colony formation in culture: State of differentiation
Authors: Gregory C. J. , McCulloch E. A. , Till J. K.
Published: February 2005

The differentiated state of mouse erythropoietic progenitor cells (CFU‐E), detected by their ability to form erythropoietin‐dependent colonies in vitro, has been investigated. Transfusion‐induced plethora was found to reduce the population size of CFU‐E in both spleen and femoral marrow, which indicates that a significant number of CFU‐E arise by differentiation processes that are themselves erythropoietin‐dependent. Individual spleen colonies were found to be heterogeneous in their content of CFU‐E, and the numbers of CFU‐E per colony were not correlated either positively or negatively with the numbers of granulocyte‐macrophage progenitors (CFU‐C) present in the same colonies. The absence of a negative correlation between CFU‐E and CFU‐C indicates that the erythropoietic and granulopoietic pathways of differentiation are not mutually exclusive within individual spleen colonies. The numbers of CFU‐E per spleen colony were also found to vary independently of the numbers of pluripotent stem cells (CFU‐S) per colony; in contrast, as found previously, the numbers of CFU‐C and CFU‐S per colony were positively correlated. These results indicate that more randomizing events separate CFU‐E from CFU‐S than separate CFU‐C from CFU‐S, and are consistent with the view that CFU‐E occupy a position on the erythropoietic pathway of differentiation that is more remote from the pluripotent stem cells than is the corresponding position of CFU‐C on the granulopoietic pathway.
Characteristics of calcium accumulation by lymphocytes and alterations in the process induced by phytohemagglutinin
Authors: Whitney Richard B. , Sutherland Robert M.
Published: February 2005

Calcium uptake by normal human lymphocytes was found to be a saturable process which was competitively inhibited by manganese indicating the existence of a carrier‐mediated mechanism for calcium uptake. Exchange diffusion was not observed, Phytohemagglutinin (PHA) significantly stimulated calcium uptake within minutes after treatment. The increased uptake was attributed to a decreased Km for the proposed membrane carrier rather than to an increased Vmax. Also PHA did not stimulate a normally unused exchange diffusion process, nor did it affect calcium efflux. Uptake by both unstimulated and PHA‐treated lymphocytes was not influenced by magnesium or by cycloheximide or actinomycin D.
Cell division, ciliary regeneration and cyclic amp in a unicellular system
Authors: Wolff Jason
Published: February 2005

It is possible, by direct measurements, to demonstrate the presence of cyclic AMP in Tetrahymena. Its presence in a unicellular eukaryote suggests a fundamental role in cellular metabolism independent of its function as a “second messenger.” Caffeine, an inhibitor of phosphodiesterase, raises the intracellular concentration of cyclic AMP and causes an inhibition of cellular division. Dibutyryl cyclic AMP also causes a partial inhibition of proliferation and is potentiated by caffeine. Inhibited cells do not recover spontaneously during continued exposure to caffeine but the inhibition is reversible resulting in a synchronous wave of division peaking one generation time after return to fresh medium.

Synchronous cells subjected to pulses of caffeine at various intervals of the cell cycle are set back in time as measured by the excess division delay. There is a linear relationship between the age of the cells at the pulse and the duration of the excess division delay such that the older the cells are the greater is the setback.

Caffeine and dibutyryl cyclic AMP inhibit ciliary regeneration as does also deuterium oxide.
Effect of fluoride and other metobolic inhibitors on intracellular sodium and potassium concentrations in l cells
Authors: Quissell D. O. , Suttie J. W.
Published: February 2005

Fluoride, iodoacetate, oxamate, 2,4‐dinitrophenol, cycloheximide, and ouabain were studied to determine if any of these inhibitors affected the intracellular concentration of sodium and potassium in an L cell strain of mouse fibroblasts and to determine if the changes observed in these parameters could be correlated with growth rate. The results indicated that (1) the intracellular concentration of sodium and potassium could not be correlated with growth rate, (2) fluoride, iodoacetate, oxamate, 2,4‐dinitrophenol, and cycloheximide at concentrations having an equal effect on growth had a similar effect on the intracellular sodium and potassium concentration These changes were not as great as those seen with ouabain, which at a concentration which did not inhibit growth, had an equal or greater effect on the intracellular sodium and potassium concentration.
Proteins from melanosomes of mouse and chick pigment cells
Authors: Doezema Philip
Published: February 2005

Purified subcellular fractions containing melanosomes from B‐16 mouse melanoma were treated with 2% sodium dodecyl sulfate or 0.5 M sodium hydroxide to dissolve protein. Quantitative measurements indicate that each melanosome contains 0.065×10−10 of protein or about 19% by weight. SDS acrylamide gel electrophoresis of proteins from purified melanosomes resolved six polypeptide bands of major density and about 15 minor bands. These results indicate that the melanosome may be more complex than previous genetic, biochemical or morphological evidence had suggested.
Sedimentation velocity characterisation of the cell cycle of granulocytic progenitor cells in monkey hemopoietic tissue
Authors: Williams Neil , Moore Malcolm
Published: February 2005

Sedimentation velocity separation of Rhesus monkey bone marrow cells has demonstrated a reproducible but heterogeneous size distribution of cells capable of forming granulocytic colonies in agar culture (CFC's). This heterogeneity is shown to be due to the cell cycle status of the progenitor cell population. In vitro exposure of bone marrow cells to lethal doses of tritiated thymidine (H3TdR) either before or after separation restricts the size distribution of CFC's, greatly reducing the proportion of rapidly sedimenting cells. The calculation of the volume distribution of such cells before and after H3TdR exposure indicates that 55% of total CFC's in adult marrow are in G0 or G1 with a volume of 410 μ3, 42% are in S phase and of volume 450–950 μ3, and the remainder are in G2 and mitosis with a volume of between 600–950 μ3. CFC's in mid gestation fetal liver were larger than their adult counterparts and were of homogeneous volume indicative of a single non cycling population with no evidence of an S or G2 component. H3TdR exposure confirmed the non‐cycling status of these fetal progenitor cells.
Synthesis of immunoglobulin and nuclear protein in synchronized mouse myeloma cells
Authors: Liberti P. , Baglioni C.
Published: February 2005

The synthesis of immunoglobulin and of nuclear proteins has been studied in synchronized mouse myeloma cells of the C1 line. Synchronization has been obtained by a double thymidine block. C1 cells synthesize immunoglobulin at a relatively constant rate throughout the cell cycle except for mitosis, when a decrease in the rate of synthesis of total protein and of immunoglobulin is observed. Cell synchrony around mitosis is not sufficiently good to determine whether immunoglobulin is synthesized at all. Nuclear protein and in particular histones appear to be synthesized synchronously with DNA during the S phase of the cell cycle.
Origin of thymidine kinase deficient (TK−) haploid frog cells via an intermediate thymidine transport deficient (TT−) phenotype
Authors: Lacko L. , Wittke B. , Geck P.
Published: February 2005

Wild‐type cultured cells of the frog cell line ICR 2A give rise to 5‐bromodeoxyridine (BUdR)‐resistant colonies only when the selecting concentration of the drug is 5 × 10−5 M or lower. The progeny of these colonies multiply in 10−4 M BUdR; resistance is correlated with the absence of a thymidine (TdR)‐specific transport reaction with a Km in the range of 2–7 × 10−4 M. All of the TdR transport‐deficient (TT‐) isolates examined (25) had TdR kinase activity (4% to 100% of wild‐type). Variants deficient in TdR kinase activity (5% of wild‐type) were obtained by exposing TT‐cultures to 10−3 M BUdR. The TK ‐ variants multply continuously in 10−3 M BUdR and retain the phenotype after prolonged culture in the absence of the drug. The frequency with which they occur is increased 20 to 50 fold by prior treatment of the culture with ICR 191, an acridine mustard mutagen.

In haploid cells, it would be expected that TK‐ variants would arise in equal numbers from wild‐type and TT‐ cultures if loss TdR kinase occurred independently of loss of the transport reaction. However, wild‐type cells give no colonies resistant to 10−3 M BUdR under conditions the give 1 to 50 colonies per million TT‐ cells. The TT‐ phenotype seems to be a required intermediate state in the origin of the TK‐ phenotype. Therefore, the TK‐ clones described above are unlikely to be products of mutation at a single genetic locus.
DNA synthesis and mitosis in a temperature sensitive Chinese hamster cell line
Authors: Roscoe D. H. , Robinson Hildred , Carbonell A. W.
Published: February 2005

Viability, DNA synthesis and mitosis have been followed in the temperature sensitive Chinese hamster cell mutant K12 under permissive and non‐permissive conditions. On incubation at 40°C cells retained their ability to form colonies at 33°C for 15 to 20 hours, but viability was lost gradually during the following 20 hours. When random cultures of K12 were shifted to 40°C the rate of DNA synthesis was normal for three to four hours but then decreased markedly, reaching 95% inhibition after 24 hours. Under the same conditions mitosis was inhibited after 15 hours. If cultures which had been incubated at 40°C for 16 hours were placed at 33°C the rate of DNA synthesis increased five hours after the shift down and mitosis 18 hours after. These results can be interpreted on the assumption that K12 at 40°C is unable to complete a step in the cell cycle which is essential for DNA synthesis and which occurs three to four hours before the start of S at 33°C.
Changes in membrane transport function in G0 and G1 cells
Authors: Costlow Mark , Baserga Renato
Published: February 2005

Confluent quiescent monolayers of aneuploid and euploid cells in culture can be stimulated to proliferate by appropriate nutritional changes. In confluent monolayers of WI‐38 human diploid fibroblasts the uptake of cycloleucine is increased three hours after these cells are stimulated to proliferate by a change of medium plus 10% serum. No changes in the uptake of cycloleucine are observed in logarithmically‐growing WI‐38 cells exposed to fresh medium plus 10% serum, or in WI‐38 confluent monolayers in which the conditioned medium has been replaced by fresh medium with 0.3% serum (a change that does not cause stimulation of cellular proliferation in WI‐38 cells). In 3T6 cells in the stationary phase stimulated to proliferate by nutritional changes, there is a prompt increase in the uptake of cycloleucine, within one hour after stimulation of cell proliferation. Similar results were obtained with stationary 2RA cells which are SV‐40 transformed WI‐38 fibroblasts. In addition, chromatin template activity which is known to increase in the early stages after stimulation of confluent WI‐38 cells, was unchanged in confluent 3T6 or 2RA cells stimulated to proliferate.

These results show that at least two of the very early biochemical events occurring in response to stimulation of cell proliferation are different in WI‐38 diploid cells and in aneuploid 2RA or 3T6 cells. It is proposed that WI‐38 cells in the stationary phase are arrested in the G0 phase of the cell cycle, while 2RA and 3T6 cells are arrested in the G1 phase.
The effect of cytochalasin B on pigment dispersion and aggregation in perfused Xenopus laevis tailfin melanophores
Authors: Fisher Marilyn , Lyerla Timothy A.
Published: February 2005

A perfusion technique is described for the study of melanosome response in ventral tailfin melanophores of Xenopus laevis tadpoles. The melanosomes remain aggregated (punctate melanophores) in Ringer's. Theophylline (15 mM) and caffeine (30 mM) cause a reversible dispersion (stellate melanophores) of melanosomes which is partly blocked by cytochalasin B (10 μg/ml). When added with theophylline or caffeine to stellate cells, cytochalasin B causes a disrupted distribution of pigment granules, characterized by a melanosome free central region. C‐AMP (20 mM) and dibutyryl c‐AMP (1 mM) cause a reversible dispersion of melanosomes which is partly inhibited by cytochalasin. When cytochalasin plus a nucleotide are added to stellate cells, some show the disrupted distribution of melanosomes. Colchicine (5 mM) causes irreversible, while griseofulvin (0.2 mM) causes a slight, but reversible dispersion of melanosomes, and cytochalasin has little effect on these reactions. Perfused tailfin melanophores remain capable of responding to reversible reagents for at least 12 hours and are unresponsive to changes in illumination.
Conditional lethal mutants of Chinese hamster cells: Mutants requiring exogenous carbondioxide for growth
Authors: Scheffler I. E.
Published: February 2005

A series of Chinese hamster cell lines were tested and found to be able to proliferate in the absence of added bicarbonate and carbondioxide if hypoxanthine and uridine were present in the medium. Conversely, cells incapable of salvaging one of these precursors, such as hypoxanthine‐guanine phosphoribosyltransferase (HGPRT−) deficient cells did not multiply under these conditions.

We describe another variant capable of utilizing hypoxanthine and uridine which has an absolute requirement for exogenous CO2/NaHCO3 for growth. These cells appear to be defective in the complete oxidation of pyruvate to carbondioxide, and indications are that the entry of pyruvate into the Krebs cycle is affected.
The separation of different cell classes from lymphoid organs. X. Preparative electrophoretic separation of lymphocyte sub‐populations from mouse spleen and thoracic duct lymph
Authors: v. Boehmer H. , Shortman K. , Nossal G. J. V.
Published: February 2005

Conditions have been established for the separation of viable mouse lymphoid cells by continuous free‐buffer film preparative electrophoresis. The detailed electrophoretic distribution profiles of T and B lymphocytes from mouse spleen and thoracic duct have been determined. Cell surface θ‐antigen was used as a marker for T cells, and high surface‐density of immunoglobulin as a marker for B cells. Spleen cells from athymic “nude” mice were also studied.

In the unselected normal spleen cell populations B lymphocytes are heterogeneous, about 60% being of low mobility with the remainder distributing broadly, and extending into the highest mobility fractions. T lymphocytes are predominantly of high mobility. Lymphoid cells lacking markers of either the B or T lineage are of intermediate mobility. There is only partial separation of T and B cells because of the extensive overlap between the populations.

The high mobility B cells, which separate along with T cells, include a substantial proportion of large cells, and include cells with high surface density of immunoglobulin. The majority of these large B cells can be selectively eliminated by their adherence on passage through a glass‐bead column.

By pre‐selecting the 50% non‐adherent lymphocytes from spleen as the starting material, a very sharp and more extensive separation of B and T cells can be achieved, with 100% pure B cells and 90% pure T cells in many fractions. However these samples are not representative of the total T and B cell populations of spleen.

In thoracic duct lymph high mobility B‐cells are absent, there is little overlap between T and B cell mobility. 100% pure T and B cells can be isolated.
Intracellular conversions of deoxyribonucleosides by Novikoff rat hepatoma cells and effects of hydroxyurea
Authors: Plagemann Peter G. W. , Erbe John
Published: February 2005

The incorporation of 3H‐labeled deoxyadenosine and deoxyguanosine into nucleic acids by cultured Novikoff rat hepatoma cells is about 80% into RNA and 20% into DNA. The pathways of incorporation have been elucidated in studies with whole cells and cell‐free extracts. Deoxyadenosine is very rapidly deaminated to deoxyinosine. Most of the deoxyinosine formed by whole cells is transported out of the cells and accumulates in the medium. A portion of the deoxyinosine, and deoxyguanosine are phosphorolyzed by purine nucleoside phosphorylase to hypoxanthine and guanine, respectively. The latter are subsequently converted by hypoxanthine‐guanine phosphoribosyl transferase to IMP and GMP, respectively. Incorporation of the purine deoxyribonucleosides into DNA is mainly via this pathway and the subsequent reduction of ADP and GDP by ribonucleoside reductase, although a small proportion of the deoxyadenosine and deoxyguanosine taken up by the cells seems to be directly phosphorylated to dAMP and dGMP, respectively. Deoxyguanosine is incorporated only into guanine residues of RNA and DNA. Deoxyadenosine is also mainly incorporated into guanine residues of RNA and DNA, although the radioactivity of deoxyadenosine in the acid‐soluble pool is almost exclusively associated with ATP. A similar labeling pattern is observed with labeled deoxyinosine, inosine or hypoxanthine. The pyrimidine deoxyribonucleosides, on the other hand, are specific precursors for their respective bases in DNA.

Hydroxyurea inhibits the incorporation of all deoxyribonucleosides into DNA. Results from pulse‐chase experiments indicate that the inhibition of DNA synthesis is prevented by the presence of high concentrations of deoxyadenosine plus deoxyguanosine in the medium. Either purine deoxyribonucleoside alone or deoxycytidine, hypoxanthine or inosine alone or in combination with deoxyadenosine or deoxyguanosine are ineffective. The results are consistent with the conclusion that the inhibition of DNA synthesis is due to a depletion of the dATP and dGTP pools as a result of the hydroxyurea treatment. On the other hand, hydroxyurea causes an increased incorporation of thymidine and deoxycytidine into the dTTP and dCTP pools, respectively. Evidence is presented to indicate that this effect of hydroxyurea is due to an increased synthesis of dTTP and dCTP rather than to an inhibition of their turnover.
Effect of growth conditions on the activity of ornithine decarboxylase in cultured hepatoma cells. II. Effect of serum and insulin
Authors: Hogan Brigid L. M. , McIlhinney Ann , Murden Susan
Published: February 2005

HTC cells incubated in the absence of serum for more than 14 hours have very low levels of ornithine decarboxylase, the first enzyme on the pathway of polyamine synthesis. Readdition of serum causes an increase in the activity of ODC, reaching a maximum on average 17 times above the basal level after five hours. This increase is due in part to a decrease in the apparent rate of degradation of ODC, and also to a stimulation of its synthesis. Within the first two hours the serum induction of ODC is resistant to Actinomycin D. Insulin at 5 μm/ml alsocauses an increase in ODC activity but only after a delay of two hours, in contrast to its more rapid stimulation of tyrosine transaminase activity.
Differences in the subcellular localization of the stimulatory effects of glucose and pyruvate on protein synthesis in rat thymus cells in vitro
Authors: Giddings Stephen J. , Young Donald A.
Published: February 2005

Thymocytes incubated as cell suspensions in vitro are known to be markedly dependent upon added glucose for maintenance of maximal rates of incorporation of radiolabelled amino acids into protein. This requirement is only partially satisfied by other added substrates, such as pyruvate. Evidence is presented that incorporation of amino acids into protein associated with the nuclear fraction isolated from these cells is more dependent upon added glucose than is labelling of protein found in the rest of the cell.

The dependence of the labelling of nuclear protein upon glucose is shown by comparing the ability of glucose and pyruvate to stimulate the incorporation of [14C‐L] valine into the protein of nuclear and cytoplasmic fractions of thymus cells. The fractions are isolated on sucrose gradients after incubating suspensions of cells in substrate‐free medium for two hours, adding carbohydrates and labelled L‐valine for 30 min and then stopping the incubation by breaking the cells with hypotonic shock. When the protein‐synthetic stimulatory effects of glucose and pyruvate are compared, glucose is almost equally capable (90%) at stimulating rates of protein synthesis in nuclear compared to cytoplasmic fractions. Pyruvate is much less effective in nuclear than in cytoplasmic fractions (30%).

Evidence is also presented from pulse‐chase experiments that the glucosedependent labelling of protein associated with the nuclear fraction occurs within that fraction, as opposed to migration to the nuclear fraction after being synthesized elsewhere.

It is suggested from these and other data that a unique ability of glucose to provide non‐mitochondrial ATP to the nucleus may be central to the dependence of the labelling of nuclear protein on this substrate.
A comparison of ligand‐induced redistribution of surface immunoglobulins, alloantigens, and concanavalin A receptors on mouse lymphoid cells
Authors: Stackpole Christopher W. , DeMilio Lawrence T. , Jacobson Janet B. , Hämmerling Ulrich , Lardis Michael P.
Published: February 2005

Redistribution of surface immunoglobulins (Ig), H‐2b, Thy‐1.2 and TL. 1,2,3 alloantigens, and concanavalin A (Con A) receptors on mouse thymus, lymph node and spleen cells into “caps” induced by bivalent antibodies or ligands was compared by immunofluorescence. Surface Ig was capped rapidly following attachment of anti‐Ig antibody at 37°. Capping of alloantigens and Con A receptors occurred very slowly following attachment of alloantibody or Con A, but much more rapidly after addition of a secondary bivalent antibody. An inverse relationship between the number of surface component sites per cell and the extent of capping of that component was observed. Capping of alloantigens sparsely represented on the cell surface was not inhibited by high concentrations of alloantibody, in contrast to capping of alloantigens present in greater quantities. These results suggest that factors in addition to molecular cross‐linking may be involved in ligand‐induced redistribution of cell surface components.
Neoplastic differentiation: Characteristics of cell lines derived from a murine teratocarcinoma
Authors: Lehman J. M. , Speers W. C. , Swartzendruber D. E. , Pierce G. B.
Published: February 2005

Monolayer cultures of a mouse teratocarcinoma were established in vitro. These cultures contained embryonal carcinoma, the malignant stem cell, and its differentiated progeny: parietal yolk sac, neuroepithelial, and mesenchymal cells. Tissues such as squamous epithelium, cartilage, striated muscle, neuroepithelium, and glands were produced from embryonal carcinoma that was maintained under conditions of long term culture. Frequent subcultivation with pancreatin allowed the establishment of cell lines of embryonal carcinoma which have been maintained for more than 18 months in vitro and continue to produce differentiated cells under specific culture conditions. Chromosomally these lines of embryonal carcinoma have a stem line of 39 chromosomes. Two lines of parietal yolk sac cells have been established which produce basement membrane, are not tumorigenic, and chromosomally are hypotetraploid. This system may yield information concerning neoplastic differentiation and its possible use in therapy for cancer.
Modification of the proliferative capacity of transplanted bone marrow colony forming units by changes in the host environment
Authors: Beran M. , Tribukait B.
Published: February 2005

Regulation of the proliferation of transplanted colony forming units (CFUs) was investigated in lethally irradiated mice, pretreated by methods known to accelerate hemopoietic recovery after sublethal irradiation. Prospective recipients were exposed to either hypoxia, vinblastine or priming irradiation and at different intervals thereafter lethally irradiated and transplanted with bone marrow. Repopulation of CFUs was determined by counting the number of splenic colonies in primary recipients or by retransplantation.

Regeneration of grafted CFUs was greatly accelerated and their self‐renewal capacity increased in mice grafted within two days after hypoxia. Also the number of splenic colonies formed by grafted syngeneic CFUs as well as by C57BL parent CFUs growing in BC3F1 hosts was significantly increased. The effect was not dependent on the seeding efficiency of CFUs and apparently resulted from hypoxia induced changes in the hosts physiological environment. Proliferative capacity of grafted CFUs increased remarkably in hosts receiving vinblastine two or four days prior to irradiation. Priming irradiation given six days before main irradiation accelerated, given two days before impaired regeneration of CFUs. The increased rate of regeneration was not related to the cellularity of hemopoietic organs at the time of transplantation. The growth of CFUs in diffusion chambers implanted into posthypoxic mice was only slightly improved which does indicate that the accelerated regeneration of CFUs in posthypoxic mice is mainly due to the changes in the hemopoietic microenvironment. A short conditioning of transplanted CFUs by host factor(s) was sufficient to improve regeneration. The results might suggest that the speed of hemopoietic regeneration depends on the number of CFUs being induced to proliferate shordy after irradiation, rather than on the absolute numbers of CFUs available to the organism.
Cyclic AMP levels in temperature sensitive SV40 transformed cell lines
Authors: Burstin Stuart J. , Renger Hartmut C. , Basilico Claudio
Published: February 2005

The cAMP levels of the 3T3 cell line and its transformed derivatives were determined. The level was found to be lower in transformed cell lines than in parental 3T3 and to decrease at confluence in all cell lines tested. SV40 transformed 3T3 cell lines which are temperature sensitive in growth control were found to have lower cAMP levels than 3T3, but these levels were not temperature dependent. Retransformation of these cell lines by murine sarcoma virus did not markedly affect their cAMP levels.
Some characteristics of myotubes cultured from slow and fast chick muscles
Authors: Purves R. D. , Vrbová Gerta
Published: February 2005

Explant cultures were prepared from the slow anterior latissimus dorsi muscle and the fast posterior latissimus dorsi muscle of 15 day chick embryos. The morphology and growth pattern of myotubes from the two types of muscle were very similar. Intracellular microelectrode studies did not reveal consistent differences between the myotube types in regard to resting potential, input resistance, input time constant, or ability to produce active electrogenic responses. It is suggested that specific differentiation of the two muscles is determined by their innervation.