Retinal photoreceptor density decreases with age

PURPOSE: To evaluate the effect of age on the density of retinal photoreceptors in humans. METHODS: Fifty-five normal eyes from human donors, with a mean age of 58.7 +/- 19.1 years and an axial length of less than 27 mm, were examined. After opening of the globes by a 16-mm corneoscleral trephination, 25 retinal tissue samples were obtained in four meridians. The photoreceptors were counted on photographs taken from the photoreceptor inner segments after sonographic removal of the outer segments. The cones in the foveal center could not be evaluated. RESULTS: Outside the foveal center, the photoreceptor density decreased significantly with increasing age. In absolute and relative terms, the decline was more marked for the rods (mean rod loss, 236 +/- 63 cells or 0.37% +/- 0.10%/mm2 and year of a mean density of 62.987 rods/mm2) than for the cones (mean cone loss, 5.90 +/- 0.68 cells or 0.18% +/- 0.02%/mm2 and year of a mean density of 3320 +/- 578 cones/mm2). For both cones and rods, the cell loss was more marked at an eccentricity of approximately 5-8 mm than in the retinal periphery at an eccentricity of 14 to 20 mm. There were no significant differences between the superior, inferior, temporal, or nasal meridian nor between women and men. CONCLUSIONS: The photoreceptors decrease in density with increasing age. The annual cell loss of approximately 0.2% to 0.4% is similar to the age-related loss of retinal ganglion cells and pigment epithelium cells. The decline in photoreceptor count affects more rods than cones. It does not prefer special age groups. It is more pronounced at an eccentricity of 5 to 8 mm than in the retinal periphery at an eccentricity of more than 14 mm. It may be important for an age-related decrease in visual functions and for diseases affecting the photoreceptors in elderly patients.     

Low prevalence of antibody to hepatitis C virus in an urban adolescent population

We tested 869 subjects attending either an urban adolescent medicine clinic or a school-based clinic for antibody to hepatitis C virus. Demographic data indicated a representative study sample, and behaviors associated with hepatitis C virus transmission. One subject (0.1%) was seropositive for antibody to hepatitis C virus, indicating a very low prevalence of hepatitis C virus exposure in adolescents.     

Strain-induced nucleation of MnS in electrical steels

The nucleation of MnS was investigated during the creep of electrical steels. Precipitation start(P _s) times were measured in the temperature range from 800 °C to 1100 °C. Direct evidence regarding the locations of the nucleation sites was obtained by means of electron microscopy. The results show that both dislocations and grain boundaries act as nucleation sites for such strain-induced precipitation. The experimental data were analyzed using classical nucleation theory, on the basis of which it is demonstrated that nucleation at grain boundaries is dominant at the higher testing temperatures. TheP _s values in this temperature range are determined by the corresponding nucleation rate. As the temperature is decreased, however, nucleation on dislocations becomes more important. This is due to the additional driving force contributed by deformation-induced vacancies, as well as because the higher dislocation densities at the lower temperatures provide a higher density of potential nucleation sites. In addition, the influence of the growth of these particles following nucleation is considered in the analysis pertaining to theP _s curves.     

Effect of deformation parameters on the

The recrystallization behavior of three Nb-bearing high-strength low-alloy (HSLA) steels was investigated during multipass deformation under continuous cooling conditions. The niobium concentrations of these steels varied from 0.05 to 0.09 wt pct. The specimens were tested on a computerized torsion machine using a simulation schedule of 17 passes. Deformation tem-peratures of 1180 °C to 700 °C were employed, together with pass strains of 0.1 to 0.7, strain rates of 0.2 to 10 s^-1, and interpass times of 5 to 200 seconds. By means of mean flow stressvs 1000/T diagrams, the effect of the deformation conditions on the no-recrystallization tem-perature (T _nr ), the temperature at which recrystallization is no longer complete, was determined. It decreases with increasing strain and also decreases slightly with increasing strain rate. There is aT _r minimum at times of about 12∼15 seconds, and both increases and decreases from this value raise this characteristic temperature. When the interpass times are short, solute atoms control the rate of recrystallization, the extent of which decreases as the time is decreased. When the interpass times are long, precipitation takes place and retards recrystallization, so that the extent of softening decreases. Formerly Research Associate, McGill University, Department of Metallurgical Engineering.     

Capacitance monitoring of the infiltration of ceramic Particulate Preforms with Liquid Metals at 750 to 850 °C

The authors previously developed a capacitance technique to determine the position of the infiltration front as a function of time during the pressure infiltration of ceramic particulate preforms with liquid metals. In the present work, the technique was extended to higher temperatures and was used to monitor the infiltration of alumina particulate preforms with Al-Mg alloys and silicon carbide particulate pre-forms with Sn and an Al-Si alloy. The infiltration front position could be determined in the alumina pre-forms. However, it was not possible to clearly interpret the results obtained from the infiltration of semiconducting silicon carbide preforms. The observed range of capacitance circuit voltage to infiltra-tion distance ratios agreed with the predicted range for both simulated and alumina preform infiltration experiments. Formation of unstable infiltration fronts could also be detected.     

Sulphonylureas do not increase insulin secretion by a mechanism other than a rise in cytoplasmic Ca2+ in pancreatic B-cells

The following sequence of events is thought to underlie the stimulation of insulin release by hypoglycaemic sulphonylureas. Interaction of the drugs with a high-affinity binding site (sulphonylurea receptor) in the B-cell membrane leads to closure of ATP-sensitive K+ channels, depolarization, opening of voltage-dependent Ca2+ channels, Ca2+ influx and rise in cytoplasmic [Ca2+]i. Recent experiments using permeabilized islet cells or measuring changes in B-cell membrane capacitance have suggested that sulphonylureas can increase insulin release by a mechanism independent of a change in [Ca2+]i. This provocative hypothesis was tested here with intact mouse islets. When B-cells were strongly depolarized by 60 mM K+, [Ca2+]i was increased and insulin secretion stimulated. Under these conditions, tolbutamide did not further increase [Ca2+]i or insulin release, whether it was applied before or after high K+, and whether the concentration of glucose was 3 or 15 mM. This contrasts with the ability of forskolin and phorbol 12-myristate 13-acetate (PMA) to increase release in the presence of high K+. Tolbutamide also failed to increase insulin release from islets depolarized with barium (substituted for extracellular Ca2+) or with arginine in the presence of high glucose. Glibenclamide and its non-sulphonylurea moiety meglitinide were also without effect on insulin release from already depolarized B-cells. In the absence of extracellular Ca2+, acetylcholine induced monophasic peaks of [Ca2+]i and insulin secretion which were both unaffected by tolbutamide. Insulin release from permeabilized islet cells was stimulated by raising free Ca2+ (between 0.1 and 23 microM). This effect was not affected by tolbutamide and inconsistently increased by glibenclamide. In conclusion, the present study does not support the proposal that hypoglycaemic sulphonylureas can increase insulin release even when they do not also raise [Ca2+]i in B-cells.     

27Al and 29Si NMR study of sol-gel derived aluminosilicates and sodium aluminosilicates

Solid state ²⁷Al and ²⁹Si NMR was used to examine the structures of aluminosilicates and sodium aluminosilicates prepared by the sol-gel method from metal alkoxides. In contrast to the borosilicate system, where B-O-Si bonds are not formed until heat treatment above 150° C, Al-O-Si formation appears complete upon gelation. Aluminium occupies tetrahedral [AlO₄]^– sites in the polymer network and octahedral [Al(H₂O)₆]³⁺ (or similar) sites in the intersticies for charge balance. When sodium is added as a counter ion the octahedral aluminium is converted to tetrahedral aluminium in the oxide network. In gels of high aluminium content prepared from (Bu^sO)₂Al-O-Si(OEt)₃, some aluminium in five coordinate environments is also observed. All gels remain amorphous on heating to 800° C.