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Biomedical subjects

D C Chan

Publications and source records attributed to D C Chan.

At least 73 records · Page 4Linked to original sources
PubMed1995

Polydactylous limbs in Strong's Luxoid mice result from ectopic polarizing activity.

Strong's Luxoid (1stD) is a semidominant mouse mutation in which heterozygotes show preaxial hindlimb polydactyly, and homozygotes show fore- and hindlimb polydactyly. The digit patterns of these polydactylous limbs resemble those caused by polarizing grafts, since additional digits with posterior character are present at the anterior side of the limb. Such observations suggest that 1stD limb buds might contain a genetically determined ectopic region of polarizing activity. Accordingly, we show that mutant embryos ectopically express the pattern-determining genes fibroblast growth factor 4 (fgf-4), sonic hedgehog (shh), and Hoxd-12 in the anterior region of the limb. Further, we show that anterior mesoderm from mutant limbs exhibits polarizing activity when grafted into host chicken limbs. In contrast to an experimentally derived polydactylous transgenic mouse, forelimbs of homozygotes show a normal pattern of Hoxb-8 expression, indicating that the duplication of polarizing tissue here occurs downstream or independently of Hoxb-8. We suggest that the 1st gene product is involved in anteroposterior axis formation during normal limb development.

Animals↗
PubMed1994

The relationship between preparation convergence and retention of extracoronal retainers.

PURPOSE: Although traditional ideal convergence (the sum of taper of the opposite sides) for crown preparation has been arbitrarily set at 4 degrees to 10 degrees, some believe absolute parallelism yields the highest retention. This study examined the relationship between the degree of convergence of a machined metal die and the retention of its casting. MATERIALS AND METHODS: The method used was that of cementing cast metal crowns onto full crown preparations on brass dies with varying convergence angles, and then recording the force required to remove the crowns from the dies in a vertical direction using a Tate-Emery Testing Machine and Load Indicator. RESULTS: It was found that retention (i.e., the force needed to remove the cemented castings from the die in their common long axis) increases from 0 degree convergence to peak between 6 degrees to 12 degrees convergence. It also seems that a critical film thickness does exist for optimum retention, and that film thicknesses smaller than the critical thickness may be responsible for the phenomenon that we have observed and directly related to the convergence angle itself. CONCLUSIONS: There seems to be experimental data supporting the use of traditionally taught convergence. Our study found that convergence angles between 6 degrees and 12 degrees seem to be optimum for tooth crown preparation when one plans to use zinc phosphate cement. Convergence angles of less than 6 degrees may not be desirable even if they can be clinically achieved. The results of our study indicate that a relationship exists between the convergence angle and the critical cement thickness that is necessary to realize the maximum strength properties of zinc phosphate cement.

Analysis of Variance↗
PubMed1993

Sensitive high-performance liquid chromatographic assay for the determination of chlorhexidine in saliva.

A high-performance liquid chromatographic assay was developed to determine salivary chlorhexidine concentration. Saliva sample (200 microliters) was extracted into methylene chloride. Chromatographic separation was achieved with a C18 column using a mobile phase of acetonitrile-0.05 M sodium acetate and 0.005 M heptanesulfonic acid (40:60, v/v). The standard curve was linear from 0.051 to 20.48 micrograms/ml (r2 > or = 0.997). Intra-day and inter-day coefficients of variation were < 5.5 and < or = 9%, respectively. The assay is rapid, sensitive, simple, and successfully used for quantitating salivary chlorhexidine content released from a chlorhexidine-impregnated resin worn by patients.

Chlorhexidine↗
PubMed1993

Effect of a resin lining and rebonding on the marginal leakage of amalgam restorations.

This in vitro study compared microleakage in Class V amalgam restorations with three different lining agents and with no lining agent, with and without resin rebonding of the margins. Eight groups of ten molars were divided into four pairs of groups. Class V preparations were cut in the facial surfaces of each molar so that the occlusal margin was in enamel and the gingival margin in dentine. The preparations in one pair of groups received no lining agent; the preparations in the remaining three pairs of groups were lined with one of the following: Copalite, Universal Bond 3 (UB3) Primer and Adhesive, or UB3 Primer only. The enamel walls of the preparations receiving the UB3 Primer and Adhesive were etched prior to applying the lining material. Amalgam was hand condensed into each preparation. In one of each pair of groups, the margins of restorations were treated with 37% phosphoric acid gel, rinsed and dried, and UB3 Adhesive was applied over amalgam and tooth margins and polymerized (rebonded). Specimens were thermocycled, stained and sectioned. Microleakage was graded using a stereomicroscope. Mean microleakage scores for occlusal and gingival margins were calculated and analysed. At enamel margins, non-rebonded Copalite and all rebonded specimens showed less microleakage than the other non-rebonded groups. The group lined with UB3 Primer only and rebonded showed significantly less (P < 0.01) microleakage at enamel margins than all the other groups except the group lined with Copalite and rebonded. At cementum/dentine margins, restorations lined with UB3 Primer and rebonded showed significantly less microleakage than the other groups.

Acid Etching, Dental↗
PubMed1993

A comparative study of three glass ionomer base materials.

This study compared the compressive and diametral tensile strength, compressive modulus, shear bond strength to dentin and resin composite, marginal gap between the base and tooth in Class V cavity preparation, and fluoride release of three glass ionomer base materials. In general, visible light-cured (VLC) base materials outperform the autopolymerizing glass ionomer base Ketac-Bond. The compressive modulus is significantly less for the VLC glass ionomer materials Vitrebond and Fuji LC compared to Ketac-Bond. The Fuji LC had greater diametral tensile strength, compressive strength, bond to dentin and bond to resin composite than Vitrebond or Ketac-Bond. The fluoride release of Vitrebond was greater than Ketac-Bond or Fuji LC. The marginal gap at the tooth/base interface was significantly less with the Fuji LC compared to the other two materials.

Analysis of Variance↗
PubMed1992

Elevated DT-diaphorase activity and messenger RNA content in human non-small cell lung carcinoma: relationship to the response of lung tumor xenografts to mitomycin Cł.

The enzyme DT-diaphorase (DTD; NAD(P)H:quinone oxidoreductase, EC 1.6.99.2), is an obligate two electron reductase which catalyzes reduction of a broad range of substrates, including quinones. We report here variations in DTD concentrations among different classes of lung tumors known also to vary in their responsiveness to cytotoxic agents. Small cell lung carcinomas (SCLCs) and cell lines derived from them have the low DTD activities and mRNA content characteristic of normal human lung, whereas non-small cell lung carcinomas (NSCLCs) have greatly elevated levels. DTD activity was increased up to 80-fold in NSCLC tumors relative to normal lung and 20-35-fold in NSCLC relative to SCLC cell lines. Increased DTD activity appeared to be a function of the NSCLC phenotype rather than a result of derivation from a cell type rich in DTD, since all histological classes of NSCLC showed this phenotype. In addition, where transfection of SCLC cell lines with the v-Ha-ras protooncogene caused a transition to a NSCLC phenotype, DTD activity was also elevated. Neuroendocrine-positive cells (SCLC, carcinoids, and a few NSCLC lines) typically had far lower DTD activities than did cell lines which lacked neuroendocrine markers (most NSCLC cells and mesotheliomas). High DTD activity may be exploited in the design of drugs which undergo bioreductive activation by this enzyme. Consistent with this, xenografts derived from NSCLC cell lines with high DTD that were grown in athymic nude mice were more susceptible to the antitumor quinone, mitomycin C, than were xenografts derived from SCLC cells containing low DTD. These data provide a mechanistic basis for the rational design of more effective bioreductive antitumor agents for use against NSCLC.

Animals↗
PubMed1992

The same genomic region is disrupted in two transgene-induced limb deformity alleles.

Mutations of the mouse limb deformity locus, ld, map to Chromosome (Chr) 2 and result in defects in the morphogenesis and patterning of the limb and kidney. Complementation studies have defined the existence of five recessive ld alleles. Remarkably, two of these, ldTgHd and ldTgBri, are transgene-induced mutations. Recovery of the first transgene insertional allele, ldTgHd, facilitated the molecular cloning of a large (greater than 200 kb) candidate gene at the ld locus. This gene is broadly transcribed and encodes a set of novel protein isoforms, termed formins. Here we present characterization of the ldTgBri mutation that supports the molecular identification of the ld gene. We show that the ldTgBri fails to complement both the ldTgHd and the ldOR alleles and that it has undergone a genomic deletion that disrupts the cloned ld gene and its transcripts. Curiously, the ldTgBri deletion encompasses the same 11-kb interval in which the ldTgHd insertion occurred and in which a chromosomal rearrangement has been identified in a third allele, ldIn2. These findings suggest that this region of the ld gene is a preferential site for illegitimate recombination.

Alleles↗
PubMed1992

A comparison of gingival marginal gap formation in composite inlays using three different fabrication techniques.

The purpose of this study was to evaluate the accuracy of composite inlays fabricated with three different techniques. Using 2-factor ANOVA it was found that after light curing, the Direct Method was the most accurate. After readapting and heat curing, the Indirect Method was not different from the Direct Method. The same was true after 24 hours' storage in water. The direct method is recommended if proximal contours and contact can be adequately controlled. If an Indirect Method is used, it is recommended that the laboratory use an additional cured layer of composite material in the gingival margin after initial curing to minimize the gaps.

Analysis of Variance↗
PubMed1991

Transmembrane form of the kit ligand growth factor is determined by alternative splicing and is missing in the Sld mutant.

The ligand (KL) for the c-kit receptor is a growth factor encoded at the mouse steel (Sl) locus. KL exists in both cell surface and soluble forms, though little is known of the regulation and functional significance of these forms. We show here that tissue-specific alternative splicing gives two types of KL mRNA. Both encode a transmembrane domain, but in transfected cells one produced the soluble form of KL at relatively high levels, whereas the other preferentially gave the cell surface form. Cell surface KL not only stimulated proliferation, but also mediated cell-cell adhesion. The SId allele, which impairs development of hematopoietic cells, melanocytes, and germ cells, has a deletion in the KL gene removing the transmembrane and intracellular domains. Expression of a corresponding cDNA gave a soluble protein that stimulated cellular proliferation but was not associated with the cell surface. These results provide evidence that cell surface KL has a critical role in the intact organism.

Amino Acid Sequence↗
PubMed1991

Hardening of dual-cured cements under composite resin inlays.

This study was conducted to determine the extent of hardening of three dual-cured cements under composite resin inlays and to determine the effectiveness of a light-reflecting wedge in promoting curing of the cements in the proximal margin. The exposure times needed to optimally harden the cements were determined by directly exposing the cements to the curing light. Composite resin inlays were bonded in an extracted molar with Dual cement, Dicor light-activated cement, and Duo cement. Cure-Thru reflective wedges were placed in the gingival embrasure of half of the specimens. None of the cements hardened completely by 24 hours when we used an exposure time that met or exceeded that recommended by the manufacturers. The chemical-cure component did not completely cure the cements when light was attenuated by the tooth and restoration. The light-transmitting wedge had little effect on hardening of the cements.

Analysis of Variance↗
PubMed1991

Evaluation of fluoride release from an orthodontic bonding system.

Decalcification around brackets is sometimes observed after orthodontic treatment. Fluoride-releasing orthodontic adhesives have been developed in an attempt to reduce the frequency and severity of decalcification. This study evaluated, in vitro, a light-activated, fluoride-releasing composite resin adhesive, FluorEver OBA. The findings indicate that FluorEver OBA released small concentrations of fluoride ions over time. A mean of 2.6 ppm was released on day 1, decreasing to a mean of 0.42 ppm by day 2 and to 0.04 ppm by day 43. The clinical implications of these findings are discussed.

Acrylic Resins↗
PubMed1991

Long-term fluoride release from glass ionomer-lined amalgam restorations.

This in vitro study evaluated the amount of fluoride released from glass ionomer-lined amalgam restorations over a period of 1 year. Class V cavities (2 x 2 x 7 mm) were prepared on the facial and lingual surfaces of 50 extracted human molars randomly distributed into 5 groups: Group 1: No restorations; Group 2: Dispersalloy amalgam alone; Group 3: same as Group 2 except 1 mm of Ketac-Silver was placed on the axial wall before amalgam insertion; Group 4: GC Lining/amalgam; and Group 5: Miracle Mix/amalgam. After restoration, each tooth was thermocycled (100x) at 5 degrees C and 55 degrees C with a dwell time of 30 seconds for baseline fluoride release levels. The teeth were placed in a polyethylene vial containing 4 ml of deionized water. At weekly intervals, each tooth was transferred to a fresh vial. Fluoride release was measured with a fluoride ion specific electrode for 10 consecutive weeks and then again at the end of 1 year. Calibration curves for low level measurements were prepared so the readings could be expressed in micrograms/ml F. At 1 year, fluoride released in micrograms/ml was: Group 1: less than 0.08; Group 2: less than 0.08; Group 3: 0.28; Group 4: 0.68; Group 5: 1.12. An ANOVA was used to evaluate the statistical difference between the groups. At the end of 1 year, measurable amounts of fluoride were recorded for all glass ionomer-lined groups with Miracle Mix and GC Lining releasing significantly more fluoride than Ketac-Silver (P less than 0.002).

Analysis of Variance↗
PubMed1990

Genome regulation in mammalian cells.

A theory is presented proposing that genetic regulation in mammalian cells is at least a two-tiered effect; that one level of regulation involves the transition between gene exposure and sequestration; that normal differentiation requires a different spectrum of genes to be exposed in each separate state of differentiation; that the fiber systems of the cell cytoskeleton and the nuclear matrix together control the degree of gene exposure; that specific phosphorylation of these elements causes them to assume a different organizational network and to impose a different pattern of sequestration and exposure on the elements of the genome; that the varied gene phosphorylation mechanisms in the cell are integrated in this function; that attachment of this network system to specific parts of the chromosomes brings about sequestration or exposure of the genes in their neighborhood in a fashion similar to that observed when microtubule elements attach through the kinetochore to the centromeric DNA; that one function of repetitive sequences is to serve as elements for the final attachment of this fibrous network to the specific chromosomal loci; and that at least an important part of the calcium manifestation as a metabolic trigger of different differentiation states involves its acting as a binding agent to centers of electronegativity, in particular proteins and especially phosphorylated groups, so as to change the conformation of the fiber network that ultimately controls gene exposure in the mammalian cell. It would appear essential to determine what abnormal gene exposures and sequestrations are characteristic of each type of cancer; which agonists, if any, will bring about reverse transformation; and whether these considerations can be used in therapy.

Animals↗
PubMed1990

In vitro evaluation of a fluoride-releasing orthodontic resin.

Decalcification around brackets is sometimes observed following orthodontic treatment. Fluoride-releasing orthodontic adhesives have been developed and used for reduction of the frequency and severity of decalcification. This study evaluated a light-activated, fluoride-releasing resin composite adhesive, FluorEver OBA. Our findings indicate that FluorEver OBA released small concentrations of fluoride ions over time. However, the adhesive had significantly lower tensile bond strengths than a conventional composite cement. Microhardness testing indicated that the resin underneath metal orthodontic brackets was not thoroughly polymerized, which may have contributed to cohesive failures of the resin composite. Despite its lower tensile bond strength, FluorEver OBA may be indicated for cases where decalcification is expected.

Analysis of Variance↗
PubMed1989

Microleakage of indirect composite inlays.

The purpose of this in vitro study was to evaluate the microleakage of indirect composite inlays and to compare leakage of inlays with directly placed composite restorations. Standardized MOD preparations were cut in 50 extracted human molars. One gingival margin was placed in enamel above the CEJ, and the other was placed in dentin below the CEJ. Two groups of teeth were directly filled with composites (P-30 and Heliomolar) after being etched, and dentin bonding agents were applied. Two groups of teeth were restored with composite inlays that were fabricated on stone dies. The inlays were made and luted with the same two composites. The last group of teeth was restored with Heliomolar inlays luted with Dual cement. The specimens were thermocycled 300 times between 5 and 50 degrees C. Microleakage was evaluated by use of the silver-nitrate staining technique. The depth of leakage was measured microscopically after the teeth were sectioned. Both direct restorations and inlays showed substantial leakage at gingival-dentin margins; however, there was only superficial leakage at enamel margins. P-30 inlays and Heliomolar inlays cemented with Dual leaked less than direct restorations at the gingival-dentin margins. There was no difference in leakage of enamel margins of inlays and direct restorations, except that direct Heliomolar restorations leaked more than the others. There was no difference in leakage between Heliomolar restorations luted with light-cured or dual-cured cement.

Composite Resins↗