Biomedical subjects
Publications and source records attributed to J T Walker.
As a white hourglass moves across a black background, the middle appears to lag behind its true position, resulting in the apparent bending of the axis of the hourglass. No such distortion occurs with a black hourglass moving across a white background. The results of two experiments support a visual persistence hypothesis, as opposed to a latency hypothesis.
A solid object looks larger than its outline or silhouette under many viewing conditions. This solid-superiority effect may result from the assimilation or confusion of visual contours within the projection of a three-dimensional object on the picture plane. An aspect of the Müller-Lyer illusion may also play a role.
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If a partially open Venetian blind is lighted by the sun, or by diffuse light from the sky, then the upper portion of each slat will be brightly lighted and the lower portion shaded. If an observer moves the head downward while viewing a dark object silhouetted against the sky, then the object displays a reversal of brightness, appearing brighter than the sky while the head is moving downward. Moving the head upward produces no brightness reversal. These observations in the natural visual environment are consistent with earlier laboratory demonstration under several conditions.
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In the fields of high-LET radiotherapy and space radiation safety it is important to know the relative probabilities with which a cell whose nucleus is struck by a heavy ion will be damaged or killed. Experiments were performed in which synchronous cultured human T-1 cells (presumptive HeLa) were irradiated with natural alpha particles of energy approximately 3.5 MeV at various times after mitotic selection up to the middle of S phase. Nuclear-area histograms were determined as a function of time after mitosis under conditions identical to those used for irradiation. The efficiency with which one particle passing through the nucleus killed a cell was found to be 0.14-0.20. This value was extrapolated to experimental cell survival data obtained when asynchronous cultured human cells were irradiated with He, Li, B, C, N, O, Ne, Ar ions of energy 6.58 or 5.5 MeV/amu, and the cell killing efficiency was found to be in the broad range of 0.5-1.0 under single-hit conditions. Similarly irradiated cells were examined for colony-size distribution by an image analysis technique, and it was found that the loss of large colonies was dose and LET-dependent in a systematic way. Dose-response data suggest two predominant subpopulations, resistant and sensitive cells, and it appears that the sensitive population is affected by single-hit kinetics. The single-hit coefficient for the induction of inherited slow growth varied with LET in a similar way to that for survival. The action cross section for this form of heritable damage appears to be comparable to the geometric cross section of the cell nucleus.(ABSTRACT TRUNCATED AT 250 WORDS)
High energy, high-Z (HZE) particles are present in high-altitude and high-inclination satellite orbits. Most of the HZE dose above LET = 200 keV/micrometer is due to Fe nuclei. Individual HZE particles can damage several cells adjacent to one another along the particle track in tissue. The outcome has been described as a "microlesion" by D. Grahn. The present study attempts to define conditions for microlesions in specific tissues, to seek biological evidence that microlesions are produced, and to evaluate the microlesion as a potentially useful unit of dose in assessing hazards to spaceworkers. Microlesions in individuals cells and hair follicles have been described. Microbial studies have provided some evidence for independent secondary electron action. Whether or not a few hundred microlesions would be damaging to the whole organism depends upon the nature of damage to critical tissues. For example, cancer may occur if microlesions kill several cells in a straight line and mutate other cells alongside the particle track. Fe particle irradiation of the mouse Harderian gland (Fry et al., this issue) produces tumors efficiently. Microlesions in the lens, cornea, and retina need to be considered. Further dialogue is required before a final decision can be made concerning the most appropriate way to assess the HZE hazard.
The polypentapeptide , H(L X Val1-L X Pro2-D X Ala3-L X Val4- Gly5 )n Val-OMe which is the D X Ala3 analog of the elastomeric polypentapeptide (PPP) of elastin, (L X Val1-L X Pro2-Gly3-L X Val4- Gly5 )n, has been synthesized. Its conformation is compared to that of the PPP and found to be similar with a somewhat stabilized beta-turn. The D X Ala3 analog coacervates to form a more cohesive viscoelastic material and the coacervate when cross-linked by gamma-irradiation exhibits an approximate doubling of the Young's modulus of elasticity. These results are discussed in connection with other related analogs of the polypentapeptide of elastin, which are non-elastomeric, and found to be consistent with a proposed conformationally based librational entropy mechanism of elasticity.
Dynamic ultrasonic imaging adds a time dimension to the clinical diagnosis of ocular pathology. A recent development in this area of ophthalmic sonography has been the introduction of an electronically scanned, multiple transducer system that provides improved real-time imaging of ocular tissue motion. This paper discusses clinical observations made in a variety of ophthalmic conditions with a high speed, linear array ultrasound system and discusses briefly the apparatus and special examination techniques needed to make these observations. Clinical subjects include vitreo-retinal adhesions: the clear cortical vitreous network; vitreous hemorrhage, opacities and membranes; shallow, fixed, and mobile retinal detachments; blood vessels within ocular tumors and the evaluation of intraocular foreign bodies.
The D-Ala5 analog, (L-Val1-L X Pro2-Gly3-L X Val4-D-Ala5) of the polypentapeptide (PPP) of elastin is synthesized and characterized by a series of physical methods. Carbon-13 and proton nuclear magnetic resonance spectroscopies are used to verify purity and, by means of solvent dependence of peptide C-O chemical shift and of temperature dependence of peptide NH chemical shift, to establish by comparison with the PPP of elastin the presence and increased stability of the Type II Pro2-Gly3 beta-turn. The temperature dependence of aggregation in water to form a viscoelastic phase called the coacervate is reported for several concentrations. Comparison of carbon-13 nuclear magnetic resonance spectra obtained under identical conditions for the coacervate states of the PPP of elastin and the D-Ala5 analog shows the effect of replacing the Gly5 residue by a D-Ala5 residue to be one of greatly restricting mobility of the polypeptide chain. Scanning electron micrographs, of the coacervate alone and of the coacervate cross-linked and compounded to a Dacron fabric before and after stress-strain studies, are reported which show the D-Ala5 PPP matrix to rupture during the stresses of drying and of stretching while wet. Thus, the effect of adding a methyl moiety to the Gly5 residue of the PPP of elastin is to decrease markedly the mobility of the polypeptide chain and to destroy elasticity. The results are presented as a test of the proposed librational entropy mechanism of elasticity of the PPP of elastin.
We used electron microscopy and serum blocking power tests to determine the phenotypes of 47 phage P1 amber mutants that have defects in particle morphogenesis. Eleven mutants showed head defects, 30 showed tail defects, and 6 had a defect in particle maturation (which could be either in the head or in the tail). Consideration of previous complementation test results, genetic and physical positions of the mutations, and phenotypes of the mutants allowed assignment of most of the 47 mutations to genes. Thus, a minimum of 12 tail genes, 4 head genes, and 1 particle maturation gene are now known for P1. Of the 12 tail genes, 1 (gene 19, located within the invertible C loop) codes for tail fibers, 6 (genes 3, 5, 16, 20, 21, and 26) code for baseplate components (although one of these genes could code for the tail tube), 1 (gene 22) codes for the sheath, 1 (gene 6) affects tail length, 2 (genes 7 and 25) are involved in tail stability, and 1 (gene 24) either codes for a baseplate component or is involved in tail stability. Of the four head genes, gene 9 codes for a protein required for DNA packaging. The function of head gene 4 is unclear. Head gene 8 probably codes for a minor head protein, whereas head gene 23 could code for either a minor head protein or the major head protein. Excluding the particle maturation gene (gene 1), the 12 tail genes are clustered in three regions of the P1 physical genome. The four head genes are at four separate locations. However, some P1 head genes have not yet been detected and could be located in two regions (for which there are no known genes) adjacent to genes 4 and 8. The P1 morphogenetic gene clusters are interrupted by many genes that are expressed in the prophage.
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Calcium ion is demonstrated to bind in the gramicidin A transmembrane channel at a site that is displaced outward about 1.5 A from that of the sodium ion. The calcium ion is also shown to exchange rapidly with the solution such that the impermeability of the channel to calcium ion is due to the proximity of the lipid which limits solvation energy. Also at its binding site, calcium is shown to be able to draw into binding a peptide carbonyl which is not drawn into coordination by the sodium ion. These results demonstrate aspects of calcium ion transport and interaction which are considered central to its role as a cellular messenger.
Ion-induced chemical shifts in the carbonyl carbon resonances of synthesized ad verified (1-13C)D-Val8 gramicidin A and (1-13C)D-Leu14 gramicidin A are utilized in combination with the previously determined location of the ion binding sites of the gramicidin A channel (using the carbonyls of L-residues) to determine that the helix sense of the gramicidin A channel) is left-handed. Having resolved the handedness issue, the location of the ion binding sites (which are fundamental to understanding the mechanism of ion transport) are further delineated with the results indicating two sites separated by just over 20 A. Furthermore, the demonstration that the divalent barium ion interacts at the binding site while not being transported through the channel is used to argue that the mechanism of monovalent vs. divalent cation selectivity is due to the positive image force contribution to the central barrier.
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If high ability in visual spatial perception is determined by an X-linked recessive major gene, then the following pattern of correlations is expected across the four groups of parents and children, this is, the mother-son, father-daughter, mother-daughter, and father-son groups: rMS=rFD greater than rMD greater than rFS=0. In a sample of 129 adolescents and their parents, the Spatial Relations Test of the Differential Aptitude Test and the Identical Blocks Test showed the pattern of correlations expected under the above hypothesis, but the Water Level Test did not. The present results provide but limited support for the X-linked recessive hypothesis.