Mathematical models of dorsal closure.

Dorsal closure is a model cell sheet movement that occurs midway through Drosophila embryogenesis. A dorsal hole, filled with amnioserosa, closes through the dorsalward elongation of lateral epidermal cell sheets. Closure requires contributions from 5 distinct tissues and well over 140 genes (see Mortensen et al., 2018, reviewed in Kiehart et al., 2017 and Hayes and Solon, 2017). In spite of this biological complexity, the movements (kinematics) of closure are geometrically simple at tissue, and in certain cases, at cellular scales. This simplicity has made closure the target of a number of mathematical models that seek to explain and quantify the processes that underlie closure's kinematics. The first (purely kinematic) modeling approach recapitulated well the time-evolving geometry of closure even though the underlying physical principles were not known. Almost all subsequent models delve into the forces of closure (i.e. the dynamics of closure). Models assign elastic, contractile and viscous forces which impact tissue and/or cell mechanics. They write rate equations which relate the forces to one another and to other variables, including those which represent geometric, kinematic, and or signaling characteristics. The time evolution of the variables is obtained by computing the solution of the model's system of equations, with optimized model parameters. The basis of the equations range from the phenomenological to biophysical first principles. We review various models and present their contribution to our understanding of the molecular mechanisms and biophysics of closure. Models of closure will contribute to our understanding of similar movements that characterize vertebrate morphogenesis.

Emergent properties during dorsal closure in Drosophila morphogenesis.

Dorsal closure is an essential stage of Drosophila development that is a model system for research in morphogenesis and biological physics. Dorsal closure involves an orchestrated interplay between gene expression and cell activities that produce shape changes, exert forces and mediate tissue dynamics. We investigate the dynamics of dorsal closure based on confocal microscopic measurements of cell shortening in living embryos. During the mid-stages of dorsal closure we find that there are fluctuations in the width of the leading edge cells but the time-averaged analysis of measurements indicate that there is essentially no net shortening of cells in the bulk of the leading edge, that contraction predominantly occurs at the canthi as part of the process for zipping together the two leading edges of epidermis and that the rate constant for zipping correlates with the rate of movement of the leading edges. We characterize emergent properties that regulate dorsal closure, i.e., a velocity governor and the coordination and synchronization of tissue dynamics.

6450 nm wavelength tissue ablation using a nanosecond laser based on difference frequency mixing and stimulated Raman scattering.

A four-stage laser system was developed, emitting at a wavelength of 6450 nm with a 3-5 ns pulse duration, < or = 2 mJ pulse energy, and 1/2 Hz pulse repetition rate. The laser system successfully ablated rat brain tissue, where both the collateral damage and the ablation rate compare favorably with that previously observed with a Mark-III Free-Electron Laser.

Upregulation of forces and morphogenic asymmetries in dorsal closure during Drosophila development.

Tissue dynamics during dorsal closure, a stage of Drosophila development, provide a model system for cell sheet morphogenesis and wound healing. Dorsal closure is characterized by complex cell sheet movements, driven by multiple tissue specific forces, which are coordinated in space, synchronized in time, and resilient to UV-laser perturbations. The mechanisms responsible for these attributes are not fully understood. We measured spatial, kinematic, and dynamic antero-posterior asymmetries to biophysically characterize both resiliency to laser perturbations and failure of closure in mutant embryos and compared them to natural asymmetries in unperturbed, wild-type closure. We quantified and mathematically modeled two processes that are upregulated to provide resiliency--contractility of the amnioserosa and formation of a seam between advancing epidermal sheets, i.e., zipping. Both processes are spatially removed from the laser-targeted site, indicating they are not a local response to laser-induced wounding and suggesting mechanosensitive and/or chemosensitive mechanisms for upregulation. In mutant embryos, tissue junctions initially fail at the anterior end indicating inhomogeneous mechanical stresses attributable to head involution, another developmental process that occurs concomitant with the end stages of closure. Asymmetries in these mutants are reversed compared to wild-type, and inhomogeneous stresses may cause asymmetries in wild-type closure.

Applications of free-electron lasers in the biological and material sciences.

Free-Electron Lasers (FELs) collectively operate from the terahertz through the ultraviolet range and via intracavity Compton backscattering into the X-ray and gamma-ray regimes. FELs are continuously tunable and can provide optical powers, pulse structures and polarizations that are not matched by conventional lasers. Representative research in the biological and biomedical sciences and condensed matter and material research are described to illustrate the breadth and impact of FEL applications. These include terahertz dynamics in materials far from equilibrium, infrared nonlinear vibrational spectroscopy to investigate dynamical processes in condensed-phase systems, infrared resonant-enhanced multiphoton ionization for gas-phase spectroscopy and spectrometry, infrared matrix-assisted laser-desorption-ionization and infrared matrix-assisted pulsed laser evaporation for analysis and processing of organic materials, human neurosurgery and ophthalmic surgery using a medical infrared FEL and ultraviolet photoemission electron microscopy for nanoscale characterization of materials and nanoscale phenomena. The ongoing development of ultraviolet and X-ray FELs are discussed in terms of future opportunities for applications research.

Free-electron laser etching of dental enamel.

OBJECTIVE: The purpose of this study was to evaluate the Mark-III free-electron laser as a means of etching enamel surfaces, with potential application to resin bonding. METHODS: The FEL was tuned to wavelengths ranging from 3.0 to 9.2 microm. Specific wavelengths that are resonantly absorbed by phosphates, proteins, and water were used. First, bovine enamel was polished and exposed to static FEL exposures. Lased enamel was examined using scanning electron microscopy (SEM). Additional bovine enamel specimens were exposed to FEL at similar wavelengths, but with rastering to create treated rectangular areas on each specimen. Surface roughness was evaluated using profilometry and atomic force microscopy (AFM). Composite was bonded to the lased enamel, and shear bond strengths were determined using an Instron universal testing machine. As a control, the surface roughness of, and shear bond strengths to, acid-etched enamel were determined. RESULTS: Static FEL exposures caused changes in the enamel ranging from an etched appearance to pits, cracks, and frank cratering. The surface roughness of lased enamel was much greater than that of acid-etched enamel, and was qualitatively different as well. Shear bond strengths of resin to acid-etched enamel were significantly higher than bond strengths to lased enamel. CONCLUSIONS: Under the conditions used in this study, the FEL did not offer a practical and effective method of etching enamel for resin bonding. However, the ability of the FEL to deliver many specific wavelengths makes it an interesting tool for further research of laser effects on tooth structure.

Hollow-glass waveguide delivery of an infrared free-electron laser for microsurgical applications.

The purpose of this research is to deliver free-electron-laser (FEL) pulses for intraocular microsurgery. The FEL at Vanderbilt University is tunable from 1.8 to 10.8 microm. To deliver the FEL beam we used a metallic-coated hollow-glass waveguide of 530-mum inner diameter. A 20-gauge cannula with a miniature CaF2 window shielded the waveguide from water. Open-sky retinotomy was performed on cadaver eyes. The system delivered as much as 6 x 10(5) W of FEL peak power to the intraocular tissues without damage to the waveguide or to the surgical probe.

Right versus left symmetry of ulnar variance. A radiographic assessment.

One hundred skeletally mature healthy volunteers underwent standardized bilateral posteroanterior radiographs in unloaded (static) and loaded (dynamic) conditions to determine the symmetry of ulnar variance. The mean age was 32 +/- 9 years (range, 19-61 years), with 58 women and 42 men. Ulnar variance was measured to the closest 0.5 mm using the method of perpendiculars. Three separate measurements were made of each radiograph in a blinded fashion by the same investigator. An intraobserver standard deviation of 0.21 was used to calculate a 95% tolerance interval of 0.7 mm (rounded up to 1 mm) as a measure of significance. The average static ulnar variance was -0.13 +/- 1.5 mm on the left and -0.29 +/- 1.6 mm on the right. The average dynamic ulnar variance was 0.93 +/- 1.5 mm on the left and 0.82 +/- 1.5 mm on the right. When compared individually, there was a greater than or equal to 1 mm side to side difference in 37% of volunteers under static and 38% under dynamic conditions. There were no significant correlations between ulnar variance measurements and patient age, gender, race, or handedness. Use of the normal wrist radiograph as a baseline for static radial length measurements is valid in only 63% of cases.

Growth and nutrition of Quercus rubra L. seedlings and mature trees after three seasons of ozone exposure.

Seedling growth and nutritional status have been shown to be sensitive to ozone, but the influence of multi-season ozone exposure on mature tree growth and nutrition has not been examined. To determine if seedlings and mature trees were similarly affected by ozone exposure, growth and nutrient concentrations in northern red oak (Quercus rubra L.) 4-year-old seedlings and 32-year-old mature trees were examined after treatment with subambient, ambient and twice ambient concentrations of ozone for three growing seasons. SUM00 values summed over the three growing seasons were 147, 255 and 507 ppm-h, respectively, for the subambient, ambient and twice ambient exposures. For mature trees, no influence of ozone treatment on lower stem diameter growth, stem growth within the mid-canopy and foliar biomass was observed. Seedling height was increased by ozone, but biomass and diameter were unaffected. A reduction in the specific leaf weight of leaves in response to ozone coincident with the loss of recurrent flushing was observed in seedlings. Ozone exposure reduced foliar nitrogen concentrations and increased woody tissue nutrient concentrations in seedlings and mature trees at the end of the third growing season. These results suggest an influence of ozone on retranslocation processes in seedlings and mature trees.

Seasonal patterns of light-saturated photosynthesis and leaf conductance for mature and seedling Quercus rubra L. foliage: differential sensitivity to ozone exposure.

Extrapolation of the effects of ozone on seedlings to large trees and forest stands is a common objective of current assessment activities, but few studies have examined whether seedlings are useful surrogates for understanding how mature trees respond to ozone. This two-year study utilized a replicated open-top chamber facility to test the effects of subambient, ambient and twice ambient ozone concentrations on light-saturated net photosynthesis (P(max)) and leaf conductance (g(l)) of leaves from mature trees and genetically related seedlings of northern red oak (Quercus rubra L.). Gas exchange measurements were collected four times during the 1992 and 1993 growing seasons. Both P(max) and g(l) of all foliage followed normal seasonal patterns of ontogeny, but mature tree foliage had greater P(max) and g(l) than seedling foliage at physiological maturity. At the end of the growing season, P(max) and g(l) of the mature tree foliage exposed to ambient ( approximately 80-100 ppm-h) and twice ambient ( approximately 150-190 ppm-h) exposures of ozone were reduced 25 and 50%, respectively, compared with the values for foliage in the subambient ozone treatment ( approximately 35 ppm-h). In seedling leaves, P(max) and g(l) were less affected by ozone exposure than in mature leaves. Extrapolations of the results of seedling exposure studies to foliar responses of mature forests without considering differences in foliar anatomy and stomatal response between juvenile and mature foliage may introduce large errors into projections of the response of mature trees to ozone.

Growth and physiology of Northern Red Oak: preliminary comparisons of mature tree and seedling responses to ozone.

Considerable progress has been made during the past decade in the development of mechanistic models that allow complex chemical, physical, and biological processes to be evaluated in the global change context. However, quantitative predictions of the response of individual trees, stands, and forest ecosystems to pollutants and climatic variables require extrapolation of existing data sets, derived largely from seedling studies, to increasing levels of complexity with little or no understanding of the uncertainties associated with these extrapolations. Consequently, a project designed to address concerns associated with scaling from seedling to mature tree responses was initiated. During the 1990 and 1991 growing seasons, mature northern red oak (Quercus rubra L.) trees and seedlings were exposed to subambient, ambient, and twice ambient ozone (O(3)) concentrations. The initial focus of the study was to identify possible trends and obvious differences between mature trees and seedlings, both in terms of growth and physiology and in response to O(3). Generally, mature trees exhibited a greater decrease in photosynthesis rates over the growing season than did the seedlings. Ozone treatments had no consistent effect on gas exchange rates of seedlings, but the twice ambient O(3) treatment resulted in reduced photosynthesis rates in the mature tree. Despite no effect of O(3) on seedling gas exchange rates, total seedling biomass was significantly less at the end of the 1991 growing season for those seedlings exposed to twice ambient O(3) levels. Disproportionate reductions in root biomass also resulted in reduced root to shoot ratios at elevated O(3) concentrations.

A comparison of sensitivity to ozone in seedlings and trees of Quercus rubra L.

Assessments of ozone sensitivity in tree species have been based primarily on studies of Seedlings without any knowledge of the similarities between seedling and mature tree responses. To determine if seedlings and trees responded similarly to ozone, 2-yr-old seedlings and 30-yr-old trees of Quercus rubra L. were fumigated with subambient, ambient and twice-ambient ozone for one growing season (cumulative doses of 18, 45 and 87 ppm h-1 , respectively). Carboxylation efficiency, apparent quantum yield and light-saturated net photosynthesis of leaves from the lower canopy of the 30-yr-old trees were reduced by ambient and twice-ambient ozone treatment. However, physiological characteristics of leaves from the upper canopy of trees showed no signs of ozone injury. Canopy leaf dry weight of trees was reduced by the twice-ambient ozone treatment through reductions in leaf area rather than specific leaf weight, but bole diameter and branch growth of trees were unaffected by ozone treatment. In contrast, ozone did not influence leaf physiology of second flush leaves, stem diameter, and height and biomass growth of 2-yr-old seedlings. These results indicate that for deciduous hardwood species, studies of seedlings may underestimate the ozone sensitivity of larger and more physiologically mature trees. For large trees with developed canopies, leaf position within the canopy may influence the sensitivity of physiological processes to ozone.

Ectomycorrhizal colonization of loblolly pine seedlings during three growing seasons in response to ozone, acidic precipitation, and soil Mg status.

A three-year study was initiated in 1987 to evaluate the impact of O3, acidic precipitation, and soil Mg on ectomycorrhizal colonization of loblolly pine (Pinus taeda L.) seedlings. Thirty-six open-top chambers equipped with a rainfall exclusion/addition system were utilized to administer three levels of O3 (subambient, ambient, or twice ambient) and two precipitation acidity levels (pH 3.8 or 5.2) to seedlings growing in 24-liter plastic pots containing soil having either 35 or 15 mg kg(-1) of exchangeable Mg. Seedlings exposed to the twice ambient O3 treatment exhibited smaller percentages of total ectomycorrhizal short roots at the end of each year of the study, but trends were statistically significant in 1989 only. Changes in number of specific ectomycorrhizal morphotypes in response to O3 were not consistent from year to year. Acidic precipitation treatments had no effect on number or percent of mycorrhizal short roots, and responses of two morphotypes to soil Mg treatments were probably due to differences in the soil environment rather than a result of changes in aboveground processes. Temporal shifts in morphotype frequencies were observed for seedlings in all treatments and indicate that mycorrhizal succession occurred during the study period.

Intra-articular fractures of the distal part of the radius treated with the small AO external fixator.

Thirty adults who had a severely comminuted intra-articular closed fracture of the distal part of the radius were treated by closed reduction and AO external fixation consisting of a converging-pin configuration with a double row of connecting bars. The patients were followed for an average of 2.6 years (range, two to four years). Twenty-seven patients had an excellent result; two, a good result; and one, a poor result, on the basis of pain, motion, strength, and radiographic appearance. Complications were rare, and there was no loss of fixation of the pins. The average grip strength was 92 per cent of normal. Motion of the wrist and rotation of the forearm averaged more than 90 per cent of that of the normal side. Carpal height was used as an indicator of distraction force produced by the fixator. There was an average increase in carpal height of four millimeters initially and 3.7 millimeters immediately before removal of the fixator, indicating near-constant distraction throughout the treatment. Radial length was well maintained, with shortening averaging less than one millimeter. The converging pins of the AO fixator prevent loosening, thereby diminishing the risks of infection, loss of reduction of the fracture, and breakage of the pins. This geometry of the pins allows the use of smaller-diameter (2.5-millimeter) pins and provides rigid fixation, even in osteoporotic bone.

Radial head fractures with acute distal radioulnar dislocation. Essex-Lopresti revisited.

Seven adults with displaced radial head fractures had concurrent dislocation of the distal radioulnar joint. Because support of the radius was lost at both the elbow and wrist, proximal migration of the radius from 5 to 10 mm occurred. Different types of fractures were classified to designate the best method of restoring radial length to prevent chronic wrist pain and stiffness. Type I fractures had large displaced radial head fragments with minimal or no comminution and amenable to interfragmentary fixation. Type II fractures had severe comminution requiring radial head excision and prosthetic replacement. Type III were old injuries with irreducible proximal migration of the radius managed by ulnar shortening and radial head prosthetic replacement. There were three Type I, two Type II, and two Type III fractures. Results of treatment were graded as 3, excellent; 2, good; 1, fair; and 1, poor. The three excellent results were in patients in which restoration of radial length was achieved within one week of injury. Suboptimal results occurred in the remaining four patients when definitive surgery was delayed four to ten weeks. The poor result was in a patient treated only by radial head excision and who refused further surgery. Recommendations include meticulous clinical and roentgenographic examination of the distal radioulnar joint in all patients with displaced radial head fractures. Preservation of the radial head with anatomic reduction and rigid internal fixation is preferred, but radial head replacement may be necessary in cases with extensive comminution. Radial head excision alone, though contraindicated, may be restructured by ulnar shortening and radial head prosthetic replacement.

Intercarpal arthrodesis for static and dynamic volar intercalated segment instability.

Since 1982 seven patients with volar intercalary segment instability (VISI) have been operated on at the Massachusetts General Hospital. All had preoperative wrist pain and described a painful "clunk" with ulnar deviation. In each case there was palpable evidence of instability when the wrist was deviated ulnarly that produced a "buckling" sensation as the distal and proximal rows rotated with ulnar deviation. Arthrograms in six patients and a cineradiography in one patient confirmed that this buckling correlated with volar rotation of the lunate and triquetrum and dorsal rotation of the capitate and hamate. All the patients had some type of intercarpal arthrodesis including four capitate-lunate-triquetrum hamate (CLTH), one lunate-triquetrum (LT), one lunate-triquetrum-hamate (LTH), and one triquetrum-hamate (TH). Surgical findings included the position of the lunate that had rotated on the capitate so that it was tilting volarly and the major ligament instability was between the proximal and distal rows although ligament tears were also present between lunate and triquetrum. Arthrodesis of the proximal and distal rows provided relief of wrist pain in five of six patients. The one patient with the arthrodesis limited to the proximal row had a poor result. Of the five successful cases, the postoperative range of wrist motion was 81 degrees of extension/flexion arc (63% of the normal contralateral) and 35 degrees of radial and ulnar deviation arc (57% of the normal contralateral wrist). The grip strength postoperatively averaged 58 pounds (74% of the normal contralateral side).

Irreducible trans-scaphoid perilunate dislocation.

Two cases of irreducible trans-scaphoid perilunate dislocation are described. In each case there was soft tissue interposition between the proximal and distal elements of the fracture-dislocation and this prevented capitolunate and scaphoid reduction. The dorsal capsule was impaled on the distal scaphoid fragment in one case, and a flap of dorsal capsule was entrapped between the capitate and the lunate in the other. In light of these findings, we recommend a dorsal approach for open reduction of trans-scaphoid perilunate dislocation when the capitolunate joint cannot be reduced by closed manipulation.

Microwave-field-driven acoustic modes in DNA.

The direct coupling of a microwave field to selected DNA molecules is demonstrated using standard dielectrometry. The absorption is resonant with a typical lifetime of 300 ps. Such a long lifetime is unexpected for DNA in aqueous solution at room temperature. Resonant absorption at fundamental and harmonic frequencies for both supercoiled circular and linear DNA agrees with an acoustic mode model. Our associated acoustic velocities for linear DNA are very close to the acoustic velocity of the longitudinal acoustic mode independently observed on DNA fibers using Brillouin spectroscopy. The difference in acoustic velocities for supercoiled circular and linear DNA is discussed in terms of solvent shielding of the nonbonded potentials in DNA.