Photoreceptor function in unilateral amblyopia

We investigated whether photoreceptor function in amblyopic eyes differed from that in non-amblyopic eyes. Photoreceptor function was assessed with the optical Stiles-Crawford effect (SCE), psychophysical SCE, and foveal visual pigment density in both eyes of ten unilateral amblyopic subjects. Optical SCE and density measurements were carried out with a custom-built scanning laser ophthalmoscope (SLO). Amblyopic and normal eyes did not differ in Stiles-Crawford effect, nor in foveal visual pigment density. Contrary to suggestions in the literature, we found no indication of retinal dysfunction at the level of the cone photoreceptors in amblyopic eyes.     

Photoreceptor loss in age-related macular degeneration

PURPOSE: The authors showed previously that parafoveal rods, but not cones, decrease during the course of adulthood in donor eyes that were screened to exclude the grossly visible macular drusen and pigmentary disturbances typical of age-related macular degeneration (AMD). Because AMD begins in the parafovea, this selective loss of rods actually may be subclinical AMD not yet visible in the fundus. If so, AMD must have a predilection for rods over cones. The authors tested this hypothesis by determining the relative numbers of cones and rods in donor eyes with mid-to late-stage AMD and in age-matched controls. METHODS: Thirteen eyes (from seven donors) with grossly visible macular drusen and pigmentary disturbances were either wholemounted for photoreceptor counts or sectioned through the fovea for histopathology and carbonic anhydrase histochemistry to label red-green cones. Eyes were assigned to AMD or control groups on the basis of histopathology and clinical history. RESULTS: Five nonexudative AMD (NE-AMD) eyes from three donors showed sparing of foveal cones and loss of rods and cones in the parafovea. In two donors, rod loss exceeded cone loss at most parafoveal locations, and in one donor, rod density was normal and cone density was reduced. In eight exudative AMD (EX-AMD) eyes from five donors, photoreceptors surviving along the margins of and overlying disciform scars were largely cones. CONCLUSIONS: Photoreceptors are lost in NE-AMD as well as in the more severe exudative form, consistent with functional and clinical studies. The authors propose that rods die in older eyes without evidence of overt retinal pigment epithelial disease. In persons susceptible to AMD, the retinal pigment epithelium becomes dysfunctional. Secondarily, rod loss continues and cones begin to degenerate. Eventually, only degenerate cones remain; ultimately, all photoreceptors may disappear.     

Influence of high-pressure torsion on the structure of an FeCoSiBNb alloy with an aperiodic phase in the initial state

The transformation of the initial structure (cubic quasicrystal) of the Fe₆₀Co₁₅Nb₆Si₁₅B₄ (at %) alloy during severe torsion deformation under a high quasi-hydrostatic pressure is studied. It is found that, at the first stage, a substantially misoriented and fragmented structure forms without changes in the phase composition; at the final stage, the structure consists of a mixture of an amorphous phase and bcc α-Fe-based nanocrystals. The results are compared to the changes in the alloy structure under action of severe deformation of another type, namely, milling. The role of compressive stresses in structure formation is discussed.     

Sterols stabilize the ripple phase structure in dihexadecylphosphatidylcholine

The presence of various sterols in mixtures with dihexadecylphosphatidylcholine (DHPC) was studied using static X-ray diffraction of temperature equilibrated samples, and real-time X-ray diffraction of samples undergoing temperature scans. It was found that these sterols eliminate the interdigitation of the alkyl chains in the DHPC sub-gel and gel-state bilayers while stabilizing the ripple gel-state at the expense of the gel-state bilayer phase. The ripple-ripple phase transition previously observed for dipalmitoylphosphatidylcholine in the presence of low molar concentrations of sterols (Wolfe et al. (1992) Phys. Rev. Lett. 68, 1085-1088) was also observed for similar DHPC-sterol mixtures. In addition, we show the first evidence that the presence of 5 alpha-cholestane-3 beta,5,6 beta-triol will cause the lipid mixtures to continue to adopt a ripple mesophase structure even after the DHPC alkyl chain becomes disordered.     

Existence of a flat phase in red cell membrane skeletons

Biomolecular membranes display rich statistical mechanical behavior. They are classified as liquid in the absence of shear elasticity in the plane of the membrane and tethered (solid) when the neighboring molecules or subunits are connected and the membranes exhibit solid-like elastic behavior in the plane of the membrane. The spectrin skeleton of red blood cells was studied as a model tethered membrane. The static structure factor of the skeletons, measured by small-angle x-ray and light scattering, was fitted with a structure factor predicted with a model calculation. The model describes tethered membrane sheets with free edges in a flat phase, which is a locally rough but globally flat membrane configuration. The fit was good for large scattering vectors. The membrane roughness exponent, zeta, defined through h alpha L zeta, where h is the average amplitude of out-of-plane fluctuations and L is the linear membrane dimension, was determined to be 0.65 +/- 0.10. Computer simulations of model red blood cell skeletons also showed this flat phase. The value for the roughness exponent, which was determined from the scaling properties of membranes of different sizes, was consistent with that from the experiments.     

WC-Co硬质合金的Co相结构

采用X射线衍射分析WC-Co硬质合金的Co相结构时,WC相对Co相的衍射峰有强烈的掩盖作用,因此本文采取先经电解腐蚀去除合金表面适量的WC后再用X射线衍射,分析常见牌号硬质合金的Co相组成。结果表明:电解腐蚀可以有效地去除硬质合金表面的WC晶粒,从而减少X射线衍射时WC相对Co相衍射峰的掩盖作用;与相关文献报道不同,常见牌号硬质合金的Co相结构基本上全部为面心立方结构;常规的金相样品制备过程会引起Co相晶型结构由fcc结构向hcp结构转变。     

稳定核激发态的奇特结构

在相对论平均场理论框架下,利用壳模型方法处理对关联,研究17N和18O基态和激发态性质,计算17N和18O基态和激发态的中子和质子方均根半径,给出中子和质子的密度分布.结果表明:这些核的激发态半径比基态大,中子密度具有更大的空间分布,形成了类晕或皮的奇特结构.     

Photoreceptor allografts in a feline model of retinal degeneration

BACKGROUND: Photoreceptor transplants provide a potential means to restore function in a degenerate retina and/or rescue degenerating host photoreceptors by trophic influences. We have examined photoreceptor allografts in the Abyssinian cat model of hereditary photoreceptor degeneration to determine the viability and influence of such transplants on the host retina. METHODS: Small pieces of 3- to 5-day-old normal kitten retina containing undifferentiated photoreceptors were injected into the subretinal space of adult Abyssinian cats at an early stage of retinal degeneration using standard vitreo-retinal surgical techniques. The retinas were examined by ophthalmoscopy and fundus photography, then by light and electron microscopy at different times after surgery. RESULTS: Such allografts survive for at least 6 months after surgery. The photoreceptors develop outer segments, invariably in rosettes. The transplants gradually integrate with the host retina but detach the host photoreceptor layer from the retinal pigment epithelium (RPE), which tends to reduce the number of host photoreceptors over the transplant. There is no slowing of the photoreceptor degeneration in neighboring non-detached retina. Inflammation or rejection was not detected. CONCLUSION: Undifferentiated, neonatal photoreceptor allografts survive and develop outer segments in the subretinal space of the Abyssinian mutant feline retina. The allografts gradually integrate with the host neural retina without inducing rejection. In the vicinity of the transplant there is increased loss of host photoreceptors, considered to be due to their detachment from the RPE layer. There is no evidence of any rescue of host photoreceptors elsewhere in this mutant retina.     

Photoreceptor recovery in retinoid-deprived rats after vitamin A replenishment

Dietary deficiency in the retinoid precursors of the visual pigment chromophore 11-cis retinal results in the synthesis of photoreceptor outer segments containing opsin in excess of the vitamin A available for rhodopsin regeneration. This suggests that vitamin A-free opsin may be incorporated into newly synthesized outer segment disc membranes. If this opsin is functionally intact, it should be possible convert it to rhodopsin in vivo by providing the appropriate retinoids, and the resulting rhodopsin should should be able to mediate visual transduction. Experiments were conducted to evaluate this possibility and to identify the rate-limiting steps in photoreceptor recovery from retinoid depletion. Rates were maintained on diets either containing or lacking retinoid precursors of 11-cis retinal for 23 weeks, at which time outer segment opsin content greatly exceeded the availability of visual cycle retinoids in the retina. The retinoid-deprived animals were then each given a single intramuscular injection of all-trans retinol. At various time intervals after retinol administration, electroretinograms (ERGs) were recorded on some rats, and retinal rhodopsin contents were determined in others. At similar time intervals, blood and retinal pigment epithelial (RPE) retinoid levels and photoreceptor outer segment size were also determined. No significant increase in retinal rhodopsin content was observed up to 8 hr after injection, despite the fact that by 3 hr, blood retinol levels had recovered to more than 30% of normal. By 1 day after injection, however, rhodopsin levels had recovered to 30% of normal and ERG responses showed increases in visual sensitivity commensurate with the recovery of rhodopsin. The lag in rhodopsin recovery was apparently due to delayed uptake of retinol from the blood by the RPE. Photoreceptor outer segment size was reduced by over 50% in the retinoid- deprived rats and did not begin to recover by 1 day. By 1 week, however, outer segment size had returned to an average of 65% of normal. Commensurate with this regrowth of the outer segments, both rhodopsin levels and visual sensitivity increased between 1 and 7 days after vitamin A administration. Because the rates of recovery in rhodopsin levels and visual sensitivity greatly exceeded the normal rate of new opsin synthesis at short time intervals after vitamin A repletion, it appears that the opsin incorporated into the disc membranes of retinoid-deprived rats is able to form functional rhodopsin in vivo when the chromophore is supplied. Regrowth of the outer segments back to their normal size is required for full recovery of visual sensitivity.     

Duchenne dystrophy: alteration in muscle plasma membrane structure

Freeze fracture studies of skeletal muscle from eight patients with Duchenne dystrophy showed nonuniform distribution and depletion of particles on both protoplasmic and extracellular faces of the muscle plasma membrane. The findings support the view that the muscle surface membrane is abnormal in this disorder and indicate that alterations are present in the internal molecular architecture of the membrane.     

The solid state portion of the Hf-Ta phase diagram

The solid state reactions in the Hf-Ta system were investigated by high-temperature X-ray diffraction, differential thermal analysis, and electron probe microanalysis. The results support a phase diagram with a monotectoid at about 40 at. pct Ta and an isotherm at 1083°C. A small miscibility gap was observed between about 40 and 80 at. pct Ta with a maximum temperature of about 1150°C.     

The solid state portion of the Hf-Ta phase diagram

The solid state reactions in the Hf-Ta system were investigated by high-temperature X-ray diffraction, differential thermal analysis, and electron probe microanalysis. The results support a phase diagram with a monotectoid at about 40 at. pct Ta and an isotherm at 1083°C. A small miscibility gap was observed between about 40 and 80 at. pct Ta with a maximum temperature of about 1150°C.