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R Tyson

Publications and source records attributed to R Tyson.

18 recordsLinked to original sources
PubMed2000

Fractional step methods applied to a chemotaxis model.

A fractional step numerical method is developed for the nonlinear partial differential equations arising in chemotaxis models, which include density-dependent diffusion terms for chemotaxis, as well as reaction and Fickian diffusion terms. We take the novel approach of viewing the chemotaxis term as an advection term which is possible in the context of fractional steps. This method is applied to pattern formation problems in bacterial growth and shown to give good results. High-resolution methods capable of capturing steep gradients (from CLAWPACK) are used for the advection step, while the A-stable and L-stable TR-BDF2 method is used for the diffusion step. A numerical instability that is seen with other diffusion methods is analyzed and eliminated.

Chemotaxis↗
PubMed1999

A minimal mechanism for bacterial pattern formation.

Colonies of Escherichia coli or Salmonella typhimurium form geometrically complex patterns when exposed to, or feeding on, intermediates of the tricarboxylic acid (TCA) cycle. In response to the TCA cycle intermediate, the bacteria secrete aspartate, a potent chemo-attractant. As a result, the cells form high-density aggregates arranged in striking regular patterns. The simplest are temporary spots formed in a liquid medium by both E. coli and S. typhimurium. In semi-solid medium S. typhimurium forms concentric rings arising from a low-density bacterial lawn, which are either continuous or spotted, whereas E. coli forms complex patterns arising from a dense swarm ring, including interdigitated spots (also called sunflower spirals), radial spots, radial stripes and chevrons. We present a mathematical model that captures all three of the pattern-forming processes experimentally observed in both E. coli and S. typhimurium, using a minimum of assumptions.

Bacterial Adhesion↗
PubMed1999

Model and analysis of chemotactic bacterial patterns in a liquid medium.

A variety of spatial patterns are formed chemotactically by the bacteria Escherichia coli and Salmonella typhimurium. We focus in this paper on patterns formed by E. coli and S. typhimurium in liquid medium experiments. The dynamics of the bacteria, nutrient and chemoattractant are modeled mathematically and give rise to a nonlinear partial differential equation system. We present a simple and intuitively revealing analysis of the patterns generated by our model. Patterns arise from disturbances to a spatially uniform solution state. A linear analysis gives rise to a second order ordinary differential equation for the amplitude of each mode present in the initial disturbance. An exact solution to this equation can be obtained, but a more intuitive understanding of the solutions can be obtained by considering the rate of growth of individual modes over small time intervals.

Bacterial Physiological Phenomena↗
PubMed1997

Symptomatic herniation pits of the femoral neck: anatomic and clinical study.

OBJECTIVE: The purpose of this study was to investigate the pathogenesis of herniation pits of the femoral neck, which are frequently considered a normal variant, and to describe the imaging abnormalities of such pits in three symptomatic patients. CONCLUSION: The changing relationship between the joint capsule and the iliopsoas muscle appears to be important in the pathogenesis of the herniation pit, especially in athletic persons. In some cases, these pits may enlarge, the overlying cortex may fracture, and significant clinical manifestations may be apparent.

Adult↗
PubMed1995

Spatio-temporal patterns generated by Salmonella typhimurium.

We present experimental results on the bacterium Salmonella typhimurium which show that cells of chemotactic strains aggregate in response to gradients of amino acids, attractants that they themselves excrete. Depending on the conditions under which cells are cultured, they form periodic arrays of continuous or perforated rings, which arise sequentially within a spreading bacterial lawn. Based on these experiments, we develop a biologically realistic cell-chemotaxis model to describe the self-organization of bacteria. Numerical and analytical investigations of the model mechanism show how the two types of observed geometric patterns can be generated by the interaction of the cells with chemoattractant they produce.

Amino Acids↗
PubMed1993

Pressure changes in the eye due to an injection of inert gases: a theoretical model.

In the repair of retinal tears and detachments, the vitreous humor is often completely replaced with a temporary mixture of gases, one of which is not normally found in the bloodstream. The resulting bubble can then support the healing retina. There is diffusion of gases, however, from the bubble into the bloodstream and vice versa. This alters the intraocular pressure, with possible adverse consequences, as the intraocular pressure must be maintained within a certain range for the procedure to be successful. A simple model has been developed to predict the evolution of intraocular pressure over time, given a certain initial mixture of injected gases. This model could be useful in determining what mixture to use to support effectively and safely a healing retina.

Capillaries↗
PubMed1993

Metabolic changes associated with altering blood glucose levels in short duration forebrain ischemia.

31P nuclear magnetic resonance spectroscopy was used to follow changes in cerebral pH and high-energy phosphate metabolites during forebrain ischemia in hypo-, normo- and hyperglycemic rats, and during reperfusion in animals in which the blood glucose level was altered post-ischemia. Pre-ischemia, no differences in the levels of inorganic phosphate (Pi) and adenosine triphosphate (ATP) relative to phosphocreatine (PCr) or in tissue pH between blood glucose groups were observed. During ischemia, the decrease in tissue pH was found to be dependent on the pre-ischemic blood glucose concentration, being greatest in hyperglycemic and least in hypoglycemic animals. The increase of Pi, a consequence of the hydrolysis of high-energy phosphate metabolites, also depended on the blood glucose concentration, being greatest in hypoglycemic and least in hyperglycemic animals. ATP and PCr decreased more rapidly in hypoglycemic rats compared to normo- or hyperglycemic animals, which showed no differences in the rates of depletion. Post-ischemic hyperglycemia resulted in delayed recovery of tissue pH in all groups and of PCr and ATP in animals hyperglycemic throughout the experiment. Insulin administration immediately following ischemia increased the rate of recovery of pH, ATP and PCr in hyperglycemic animals. ATP remained significantly below pre-ischemia level in all subgroups at 1 h post-ischemic, while PCr was lower than it was pre-ischemia only in those subgroups hyperglycemic prior to and/or following ischemia. In animals maintained severely hypoglycemic throughout the experiment, erratic blood pressure and cerebral energy failure during the reperfusion interval were observed.

Adenosine Triphosphate↗
PubMed1993

Patterns of central acetabular osteophytosis in osteoarthritis of the hip.

RATIONALE AND OBJECTIVES: The frequency and morphology of central acetabular osteophyte formation were evaluated in patients with osteoarthritis. METHODS: One hundred preserved acetabular specimens were evaluated for the occurrence of central osteophytes (excrescences present in the articulation of the hip joint), their relationship to marginal and femoral osteophytes of the hip, and their radiographic appearance. RESULTS: In both specimen and patient analysis, the authors found that central excrescences occur commonly and appear to be related temporally to osteophytes of the femur and the margin of the acetabulum. CONCLUSIONS: The authors demonstrated a close relationship between the degree of marginal and femoral osteophytes with those found centrally, indicating a temporal relationship. The new radiographic signs discussed are: 1) a fine linear density associated with early osteoarthritis; 2) larger, more irregular densities associated with a more advanced stage of osteoarthritis; and 3) obliteration of the acetabular fossa by excrescences as found in severe osteoarthritis.

Acetabulum↗
PubMed1992

The mushroom femoral head. A radiographic investigation.

RATIONALE AND OBJECTIVES: In 1913, anthropologist Ales Hrdlicka described a number of femora as having "mushroom heads," a deformity he attributed to arthritis deformans. The authors have attempted to define the cause of this condition. METHODS: Forty of 41 femora with "mushroom" femoral heads collected by Hrdlicka were grossly inspected and examined with plain radiographs and with computed tomography. Femoral neck angulation and degree of anteversion were determined. RESULTS: Three distinct categories were established: group I, those specimens with normal angulation and short femoral necks; group II, those with varus angulation and short femoral necks; and group III, those with normal-sized femoral necks and normal femoral neck angulation. No correlation was found between degree of anteversion and the categories. Additionally, most of the mushroom deformities were related to osteophyte formation. CONCLUSIONS: Most of the changes in the femora were the result of osteoarthritis. Two groups showed evidence of growth disturbances that occurred early in life. The changes in the second group were similar to those seen in Legg-Calvé-Perthes disease.

Epiphyses, Slipped↗
PubMed1992

Forebrain ischemia in diabetic and nondiabetic BB rats studied with 31P magnetic resonance spectroscopy.

In spontaneously diabetic BB rats, the effect of chronically maintained blood glucose levels on the degree of energy failure and brain pH change during an ischemic insult, and on subsequent recovery after reperfusion, was studied with in vivo 31P magnetic resonance spectroscopy. Short duration forebrain ischemia (10-min carotid occlusion plus hypotension of 50 mmHg) was induced in diabetic and nondiabetic male BB rats whose blood glucose levels were maintained with insulin. Spectra were obtained in 1-min blocks before, during, and for 1 h after ischemia. Before ischemia, hypoglycemic (blood glucose less than 3 mM) diabetic rats had an increased Pi peak intensity, with no significant pH change, compared with other groups. During ischemia, the rate and extent of hydrolysis of high-energy phosphate metabolites (as measured by an increase in Pi) decreased, and the severity of tissue acidosis increased as preischemia blood glucose concentration increased. Among hyperglycemic BB rats, similar ischemia-induced changes were found for subgroups with blood glucose levels of 13.7 +/- 1.2 and 20.3 +/- 0.6 mM, in keeping with the known decrease in hexose binding sites associated with chronic hyperglycemia. Decline in PCr level during ischemia was not significantly different between groups. With reperfusion, both Pi and pH values rapidly returned to preischemia values. PCr levels, however, did not recover in hyperglycemic diabetic animals, with the degree of residual impairment dependent on the preischemia glucose level. Results suggest that optimal management of diabetes may lessen the degree of injury within the ischemic penumbra in diabetic patients who suffer a stroke.

Analysis of Variance↗
PubMed1990

Osteophytosis of the knee: anatomic, radiologic, and pathologic investigation.

Although the radiologic manifestations of degenerative disease of the knee have been investigated, the distribution of marginal and central osteophytes has not been defined. This study included (a) 50 consecutive patients with osteoarthritis of the knee in whom routine and specialized projections were obtained prospectively, (b) 25 patients with calcium pyrophosphate dihydrate (CPPD) crystal deposition disease whose knee radiographs were retrospectively reviewed, and (c) four cadaveric knees that were dissected to assess pertinent anatomy. In this study the importance of the tunnel view in the evaluation of osteoarthritis and CPPD crystal deposition disease is demonstrated, the distribution of and the relationship between marginal and central osteophytes are discussed, and two new radiologic signs are described. As both marginal and central osteophytes may simulate intraarticular bodies, the recognition of these outgrowths is of clinical importance.

Calcium Pyrophosphate↗
PubMed1986

Bone reinforcement lines in chronic adult osteopenia: a hypothesis.

Although linear bands of increased radiodensity have been described in the tubular bones of children, similar but less often reported structures are frequent in the osteopenic skeleton of middle-aged and elderly patients. In this study, the radiology, pathology, and histology of osseous bands in the mature skeleton were analyzed in 20 cadavers, 29 macerated ancient and modern bone specimens, and 100 recent radiographs. These radiodense areas represent complete or incomplete, horizontally or obliquely oriented bars composed of compact lamellar bone. Although their precise pathogenesis is not clear, they are associated with long-standing (vs. acute) osteopenia. In some instances, bone bars appear to be formed during skeletal growth, remain hidden in the adult because of adjacent trabeculae, and are unmasked by osseous resorption. In other instances, bone bars appear to be formed initially in the mature skeleton as a response to chronic mechanical stress and, as such, are not identical with growth recovery lines observed in children. They are more accurately considered bone reinforcement lines.

Adult↗
PubMed1985

Age-related alterations in the vertebral spinous processes and intervening soft tissues: radiologic-pathologic correlation.

A radiologic-pathologic correlative investigation of the normal age-related alterations in the spinous processes and intervening soft tissues was performed using cadaveric spines and both ancient and modern macerated vertebral specimens. Extreme lordosis in the cervical or lumbar spine results in spinous process apposition, formation of interspinous bursae, eburnation with osteophytosis, and creation of synovial articulations. A concomitant degenerative enthesopathy involves the supraspinous or interspinous ligamentous attachments in any spinal segment. The differential diagnosis of this phenomenon, the clinical significance of which in a given patient is controversial, includes rheumatoid and other inflammatory arthritides as well as crystal deposition disease.

Aging↗
PubMed1978

Hair analysis.

Explore the source record for details and available documents.

Hair↗
PubMedDate not supplied

Acetabular alterations in untreated congenital dysplasia of the hip: computed tomography with multiplanar re-formation and three-dimensional analysis.

The alterations in acetabular morphology that occur in untreated congenital hip dysplasia (CDH) were evaluated in 29 prehistoric and four modern hemipelvis specimens using inspection, sectioning, conventional radiography, CT, and multiplanar re-formation with three-dimensional image reconstruction. Three major patterns of acetabular morphology were recognized, each reflecting a different grade of severity of dysplasia. Type I (positionally unstable or subluxatable) hips were characterized by acetabular deepening with prominent circumferential ossification within the transverse acetabular ligament and everted limbus. Type III (dislocated) hips demonstrated definite false acetabula separated from triangular remnants of the true acetabula by ossified inverted limbi. Type II (subluxated) hips revealed an intermediate pattern, with acetabular elongation and ossification of deformed limbi, but without definite pseudoacetabula. Computed tomography, particularly with three-dimensional analysis, was extremely useful in characterizing the abnormal acetabular morphology of untreated CDH. An awareness of the secondary osseous alterations of this disorder facilitates understanding the spectrum of hip instability and its soft tissue abnormalities.

Acetabulum↗