Distal separation of chick cochlear hair cell stereocilia: analysis of contact-constraint models

One model often used in the study of hair bundle micromechanics assumes simple geometric relationships between hair displacements, constrained by contact between neighboring hairs at their distal tips. Recent observations of hair bundle motion provided the opportunity to evaluate the contact-constraint model against measured displacements for the tallest and shortest sensory hairs. A contact-constraint model was developed based on the geometry of a single column of stereocilia. The model used morphological data from chick hair bundles for which displacement data in the excitatory and inhibitory directions were also available. For each hair bundle, a unique sensory hair radius was determined so that the calculated resting bundle morphology matched the measured values. The model was then evaluated against the displacement data for each hair bundle. In each case, the model underestimated the excitatory displacement of the shortest hairs. Failure of the model to accurately predict bundle motion raises the possibility of a distal separation between the hairs at rest. It is suggested that tip links pull the hairs through this separation during excitatory deflections. Perhaps at damaging levels of displacement, the hairs suddenly come into contact, tip-link tension dramatically increases, and the tip-links are fractured.     

Kinematic analysis of shear displacement as a means for operating mechanotransduction channels in the contact region between adjacent stereocilia of mammalian cochlear hair cells

In sensory hair cells of the cochlea, deflection of the stereociliary bundle results in direct mechanical gating of mechanoelectrical transduction channels, a function generally attributed to the tip link running between the tips of short stereocilia and the sides of adjacent taller ones. However, immunocytochemical experiments indicate that the channels may not be associated with the tip link but occur just below it in a region of contact between the stereocilia. To determine whether transduction channels in this location could be operated during physiologically appropriate deflections as effectively by shear displacement as if they were associated with the tip link, a two dimensional kinematic analysis of relative motion between stereocilia has been performed assuming contact between stereocilia is maintained during deflection. Bundle geometry and dimensions were determined from transmission electron micrographs of hair cells from several frequency locations between 0.27 and 13.00 kHz in the guinea-pig cochlea. The analysis indicates that for a 10 nm deflection of the tallest stereocilia of both inner and outer hair cells, i.e. within the range of the maximum sensitivity of mammalian hair bundles, the average shear displacement in the contact region would be 1.6 nm, but that it increases systematically towards higher frequency regions for outer hair cells. This displacement is comparable in magnitude to tip-link elongation for individual stereociliary pairs.     

Expression of the smooth-muscle proteins alpha-smooth-muscle actin and calponin, and of the intermediate filament protein desmin are parameters of cardiomyocyte maturation in the prenatal rat heart

Tip growth of plant cells has been suggested to be regulated by a tip-focused gradient in cytosolic calcium concentration ([Ca2+]c). However, whether this gradient orients apical growth or follows the driving force for this process remains unknown. Using localized photoactivation of the caged calcium ionophore Br-A23187 we have been able to artificially generate an asymmetrical calcium influx across the root hair tip. This led to a change in the direction of tip growth towards the high point of the new [Ca2+]c gradient. Such reorientation of growth was transient and there was a return to the original direction within 15 min. Root hairs forced to change the direction of their growth by placing a mechanical obstacle in their path stopped, reoriented growth to the side, and grew past the mechanical blockage. However, as soon as the growing tip had cleared the obstacle, growth returned to the original direction. Confocal ratio imaging revealed that a tip-focused [Ca2+]c gradient was always centered at the site of active growth. When the root hair changed direction the gradient also reoriented, and when growth returned to the original direction, so did the [Ca2+]c gradient. This normal direction of apical growth of Arabidopsis thaliana (L.) Heynh, root hairs was found to be at a fixed angle from the root of 85 +/- 6.7 degrees. In contrast, Tradescantia virginiana (L.) pollen tubes that were induced to reorient by touch or localized activation of the caged ionophore, did not return to the original growth direction, but continued to elongate in their new orientation. These results suggest that the tip-focused [Ca2+]c gradient is an important factor in localizing growth of the elongating root hair and pollen tube to the apex. However, it is not the primary determinant of the direction of elongation in root hairs, suggesting that other information from the root is acting to continuously reset the growth direction away from the root surface.     

Evolution of anesthesiology

In a geometric model of passive elastic elements in the hair bundles of vertebrate acousticolateralis hair cells, Geisler provides evidence against a role of horizontal links in sensory transduction. One premise of this model, the tenet that horizontal links are stretched in the same way as tip links, may be invalid. We propose that the horizontal links may be rigid, implying that the attachment points of the horizontal links in the stereociliary membrane must move when the hair bundle is deflected. Since the movement is linear with the angle of deflection, a connection of transduction channels and horizontal links is conceivable.     

Myosin Ibeta is located at tip link anchors in vestibular hair bundles

Recent studies have suggested that myosin Ibeta mediates the adaptation of mechanoelectrical transduction in vestibular hair cells. An important prediction of this hypothesis is that myosin Ibeta should be found in the side insertional plaque, an osmiophilic hair bundle structure that anchors tip links and is thought to house the adaptation motor. To determine whether myosin Ibeta was situated properly to perform adaptation, we used immunofluorescence and immunoelectron microscopy with the monoclonal antibody mT2 to examine the distribution of myosin Ibeta in hair bundles of the bullfrog utricle. Although utricular hair cells differ in their rates and extent of adaptation [Baird RA (1994) Comparative transduction mechanisms of hair cells in the bullfrog utriculus. II. Sensitivity and response dynamics to hair bundle displacement. J Neurophysiol 71:685-705.], myosin Ibeta was present in all hair bundles, regardless of adaptation kinetics. Confirming that, nevertheless, it was positioned properly to mediate adaptation, myosin Ibeta was found at significantly higher levels in the side insertional plaque. Myosin Ibeta was also present at elevated levels at the second tip link anchor of a hair bundle, the tip insertional plaque, found at the tip of a stereocilium. These data support myosin Ibeta as the adaptation motor and are consistent with the suggestion that the motor serves to restore tension applied to transduction channels to an optimal level, albeit with different kinetics in different cell types.     

Evidence for opening of hair-cell transducer channels after tip-link loss

The mechanosensitive transducer channels of hair cells have long been proposed to be gated directly by tension in the tip links. These are thin, elastic extracellular elements connecting the tips of adjacent stereocilia located on the apical surface of the cell. If this hypothesis is true, the channels should close after destruction of tip links. The hypothesis was tested pharmacologically using receptor currents obtained in response to mechanical stimulation of the stereociliary bundle of outer hair cells isolated from the adult guinea pig cochlea. Application of elastase (20 U/ml) or 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetra-acetic acid (BAPTA; 5 mM), both of which are known to disrupt tip links in other hair-cell preparations, led to the expected irreversible loss of receptor currents. However, the cells then displayed a maintained inward current, implying that channels were left permanently open. This current was similar in magnitude to the receptor current before treatment and was reduced reversibly by known blockers of mechanosensitive channels, namely, dihydrostreptomycin (100 microM), amiloride (300 microM), and gadolinium ions (1 mM). These observations suggest that the maintained current flows through the mechanosensitive channels. Electron microscopical analysis of isolated hair cells, exposed to the same concentrations of elastase or BAPTA as in the electrophysiological experiments, demonstrated an almost total loss of tip links in hair bundles that showed no evidence of other mechanical damage. It is concluded that although the tip links are required for mechanoelectrical transduction, the channels are not gated directly by the tip links.     

Hair bundle morphology on surviving hair cells of the chick basilar papilla exposed to intense sound

Exposure to intense sound produces a well-defined "patch" lesion on the chick basilar papilla in which 30-35% of the short hair cells are lost. The present study compares various aspects of sensory hair bundle morphology on surviving hair cells in the patch lesion with hair bundles from matched locations on nonexposed control papilla immediately after removal from the exposure and 12-days post exposure. The height and thickness of the hairs, the total number of hairs in the bundle, the width of the bundle, and the area and perimeter of the apical surface of the hair cell were quantified from scanning electron microscope photomicrographs. An attempt was also made to determine if there was a consistent microstructure to the pattern of hair cell loss within the lesion area. Similar observations in 12-day recovered ears are also presented. The results indicated that stereocilia height increased and width decreased on surviving hair cells in the exposed ear. The width of the hair bundle, the hair cell surface area, and perimeter also decreased. However, the number of hairs per cell remained unchanged, and there was no evidence of any consistent organization to the hair cell loss within the patch across a number of specimens. These observations indicated that the hair bundles on short hair cells underwent changes as a consequence of intense sound exposure. The results after 12 days of recovery were complicated by developmental changes on the papilla and incomplete maturation of the newly regenerated hair cells. It remains to be seen whether these changes were the result of cell sampling in the sound-damaged ear or were due to true structural alterations within the sensory hairs themselves.     

The COW1 locus of arabidopsis acts after RHD2, and in parallel with RHD3 and TIP1, to determine the shape, rate of elongation, and number of root hairs produced from each site of hair formation

Two recessive mutant alleles at CAN OF WORMS1 (COW1), a new locus involved in root hair morphogenesis, have been identified in Arabidopsis thaliana L. Heynh. Root hairs on Cow1- mutants are short and wide and occasionally formed as pairs at a single site of hair formation. The COW1 locus maps to chromosome 4. Root hairs on Cow1- plants form in the usual positions, suggesting that the phenotype is not the result of abnormal positional signals. Root hairs on Cow1- roots begin hair formation normally, forming a small bulge, or root hair initiation site, of normal size and shape and in the usual position on the hair-forming cell. However, when Cow1- root hairs start to elongate by tip growth, abnormalities in the shape and elongation rate of the hairs become apparent. Genetic evidence from double-mutant analysis of cow1-1 and other loci involved in root hair development supports our conclusion that COW1 is required during root hair elongation.     

Mechanotransduction of hair bundles arising from multicellular complexes in anemones

Sea anemones are among the simplest animals to use hair bundles to detect vibrations. Although we previously found anemone bundles to be morphologically similar to vertebrate hair bundles, only indirect evidence implicated anemone bundles in mechanotransduction. Here, we test mechanotransduction of these bundles using loose-patch current recording from apical membranes of cells at the base of deflected bundles. Step bundle deflection results in graded membrane currents that are inward in some cells (positive) and outward in other cells (negative). Positive responses range from 5 to 30 pA, abruptly saturate with stronger stimuli, and increase in duration with prolonged deflections. Negative responses range from 10 to 150 pA, show a logarithmic relation to stimulus strength, and attenuate with prolonged deflections. Additionally, responses are reversibly inhibited by streptomycin. We present a model for anemone bundle mechanotransduction modified from the gating spring model for vertebrate mechanotransduction. Because anemone bundles comprise stereocilia arising from a multicellular complex, we propose that supporting cells on opposite sides of a bundle function as oppositely polarized hair cells. Thus, deflection induces ion channels to open in cells on one side of the complex, while allowing channels to close in cells on the opposite side of the complex.     

Mechanical and electromechanical properties of the stereovillar bundles of isolated and cultured hair cells of the chicken

Isolated single chicken hair cells and pieces of epithelium without the tectorial membrane, either freshly isolated or in tissue culture, were studied using water-jet stimulation of their stereovillar bundles and current injection. Responses were measured under enhanced video-microscopic observation or while using a differential photodiode technique sensitive down into the nanometer range. When stimulated with a water jet at low displacement amplitudes up to about 200 nm, the stereovillar bundle displacement was asymmetrical, indicating a lower stiffness in the excitatory direction, but the reverse was true at higher displacement amplitudes. Undamaged bundles showed no mechanical resonances below 1 kHz. In damaged bundles, however, such resonances were prominent and accompanied by splaying of the stereovilli. Hair cells in the epithelium showed small bundle movements (0.6 nm/mV) whose polarity depended on the polarity of the injected current. These movements probably resulted from activation of the bundle's adaptation motors.     

Temporal aN and aT in Earthquake Ground Motion at a Single Point

This paper describes a continued study on three-dimensional temporal characteristics of earthquake ground motions at a single point. Based on an instantaneous tangential and normal acceleration decomposition of ground acceleration trajectory, a ground motion can be partitioned into a finite sequence of staggered time intervals of acceleration and deceleration. A formulation is developed to estimate speed and angular changes over a partitioned interval in terms of rates of positive and negative tangential and normal acceleration. Based on these concepts, general ground motion properties, peak ground acceleration, peak ground velocity, and peak ground displacement are examined. Several Northridge earthquake records are studied in detail. It is found that the highest peak of ground acceleration in these records corresponds to a high peak of deceleration, and a velocity maximum often precedes the peak of acceleration.     

Representational momentum and the landmark attraction effect.

Examined the effect of a large stationary landmark on memory for the location of a smaller moving target. Os included 63 undergraduates. Forward displacement of the target was larger when the target moved toward the landmark than when it moved away from the landmark. Target size and direction of motion also influenced displacement. When the target passed close by the landmark, forward displacement of the target was larger before the target passed by the cardinal axis of the landmark than after the target passed by the cardinal axis of the landmark. Memory for a target that passed by a landmark was also displaced toward the target along the axis orthogonal to motion. Control experiments ruled out biases toward the center of the screen as causing the differences in displacement. The data support the hypotheses that representational momentum may combine with landmark attraction effects to influence the displacement of a target along the axis of motion. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Upper displacement limits for spatially broadband patterns containing bandpass noise

How is the spatial-frequency content of a moving broadband pattern analysed by the visual system? Observers were asked to discriminate the direction of motion in random-noise patterns containing equal energy in each two-dimensional octave band. Uncorrelated noise could be introduced into either low- or high-frequency bands in order to force the visual system to rely on the outputs of putative mechanisms tuned to a narrow frequency range of the stimulus. In two experiments the dependent measure was the magnitude of dmax, the largest discrete displacement whose direction could be discriminated reliably. It was found that dmax was unaffected by the presence of high-frequency noise reaching down to 0.67 c/deg, but that the task became impossible thereafter. In the case of low-frequency noise, dmax fell as the noise was moved up towards about 2 c/deg, at which point the task became impossible at any displacement. This pattern of results would be expected if the system were using information from the lowest signal frequencies in all conditions. In experiment 2, dmax was measured for stimuli in which the spectral position and quantity of high-frequency noise were manipulated. It was found that only noise spectrally-adjacent to the signal band has a detrimental effect on dmax. Three different single-filter models of motion detection each failed to provide a satisfactory account of the spatial-frequency range of good direction discrimination performance. Rather, the modelling shows that the visual system can access the outputs of a low-frequency channel when the noise is high and a high-frequency channel when the noise is low.     

Micromechanical models for the Brownian motion of hair cell stereocilia

Brownian motion of the hairs (stereocilia) of amphibian hair cells has been shown in experiments to be in the range of some nm. Our models of the Brownian motion of coupled harmonic oscillators with mechanical properties of stereocilia lead to similar displacements. Computer simulation shows that stochastic fluctuations enhance the encoding of low level acoustic signals. Stochastic resonance lowers the detection threshold of auditory signals to amplitudes one order of magnitude lower than that of the Brownian motion.     

A model for amplification of hair-bundle motion by cyclical binding of Ca2+ to mechanoelectrical-transduction channels

Amplification of auditory stimuli by hair cells augments the sensitivity of the vertebrate inner ear. Cell-body contractions of outer hair cells are thought to mediate amplification in the mammalian cochlea. In vertebrates that lack these cells, and perhaps in mammals as well, active movements of hair bundles may underlie amplification. We have evaluated a mathematical model in which amplification stems from the activity of mechanoelectrical-transduction channels. The intracellular binding of Ca2+ to channels is posited to promote their closure, which increases the tension in gating springs and exerts a negative force on the hair bundle. By enhancing bundle motion, this force partially compensates for viscous damping by cochlear fluids. Linear stability analysis of a six-state kinetic model reveals Hopf bifurcations for parameter values in the physiological range. These bifurcations signal conditions under which the system's behavior changes from a damped oscillatory response to spontaneous limit-cycle oscillation. By varying the number of stereocilia in a bundle and the rate constant for Ca2+ binding, we calculate bifurcation frequencies spanning the observed range of auditory sensitivity for a representative receptor organ, the chicken's cochlea. Simulations using prebifurcation parameter values demonstrate frequency-selective amplification with a striking compressive nonlinearity. Because transduction channels occur universally in hair cells, this active-channel model describes a mechanism of auditory amplification potentially applicable across species and hair-cell types.     

Evaluation and surveillance of blood pressure during pregnancy

BACKGROUND: Women generally regard their hair loss as socially unacceptable and go to great measures to conceal their problem. In some cases, the negative self-image brought about by hair loss may be the basis of psychiatric illness. The purpose of this study was to evaluate a 2% topical minoxidil solution (Rogaine/Regaine, The Upjohn Co, Kalamazoo, Mich) for the treatment of female androgenetic alopecia. A 32-week, double-blind, placebo-controlled trial was conducted in 11 US centers. Three hundred eight women with androgenetic alopecia were enrolled. Two hundred fifty-six of these women completed the trial. A refined photographic technique was used to objectively determine the number of nonvellus hairs regrown. RESULTS: After 32 weeks of treatment, the number of nonvellus hairs in a 1-cm2 evaluation site was increased by an average of 23 hairs in the 2% minoxidil group and by an average of 11 hairs in the placebo group. The 95% confidence interval for the difference in mean hair count change between the treatment groups was 5.9 to 17.5 hairs. The investigators determined that 13% in the minoxidil-treated group had moderate growth and 50% had minimal growth. This compared with 6% and 33%, respectively, in the placebo-treated group. Similarly, 60% of the patients in the 2% minoxidil group reported that they had new hair growth (20% moderate, 40% minimal) compared with 40% (7% moderate, 33% minimal) of the patients in the placebo group. No evaluations of dense hair growth were reported for either treatment group. No clinically significant changes in vital signs were observed and no serious or unexpected medical events were reported. CONCLUSIONS: Topical minoxidil was significantly more effective than placebo in the treatment of female androgenetic alopecia.     

Representational momentum and Michotte's "launching effect" paradigm (1946/1963).

In A. Michotte's (1946/1963) launching effect, a moving launcher contacts a stationary target, and then the launcher becomes stationary and the target begins to move. In this experiment, observers viewed modifications of a launching effect display, and displacement in memory for the location of targets was measured. Forward displacement of targets in launching effect displays was decreased relative to that of targets (a) that were presented in isolation and either moved at a constant fast or slow velocity of decelerated or (b) that moved in a direction orthogonal to previous motion of the launcher. Possible explanations involving a deceleration of motion or landmark attraction effects were ruled out. Displacement patterns were consistent with naive impetus theory and the hypothesis that observers believed impetus from the launcher was imparted to the target and then dissipated. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Motion coherence across different chromatic axes

It has been reported that equiluminant plaid patterns constructed from component gratings modulated along different axes of a cardinal colour space fail to create a coherent impression of two-dimensional motion [Krauskopf and Farell (1990). Nature, 348, 328-331]. In this paper we assess whether this lack of interaction between cardinal axes is a general finding or is instead dependent upon specific stimulus parameters. Type I and Type II plaids were made from sinusoidal components (1 cpd) each modulated along axes in a cardinal colour space and presented at equivalent perceived contrasts. The spatial angular difference between the two components was varied from 5 to 90 deg whilst keeping the Intersection of Constraints (I.O.C.) solution of the pattern constant. Observers were required to indicate the perceived direction of motion of the pattern in a single interval direction-identification task. We find that: (i) When plaids were made from components modulated along the same cardinal axis, coherent "pattern" motion was perceived at all angular differences. As the angular difference between the components decreased in a Type II plaid, the perceived direction of motion moved closer to the I.O.C. solution and away from that predicted by the vector sum. (ii) A plaid made from components modulated along red-green and blue-yellow cardinal axes (cross-cardinal axis) did not cohere at high angular differences (> 30 deg) but had a perceived direction of the fastest moving component. At lower angular differences, however, pattern motion was detected and approached the I.O.C. solution in much the same way as a same-cardinal axis Type II plaid. (iii) A plaid made from a luminance grating and a cardinal chromatic grating (red-green or blue-yellow) failed to cohere under all conditions, demonstrating that there is no interaction between luminance and chromatic cardinal axes. These results indicate that there are conditions under which red-green and blue-yellow cardinal components interact for the purposes of motion detection.     

Displacement-clamp measurement of the forces exerted by gating springs in the hair bundle

Mechanical stimuli applied to the hair bundle of a hair cell are communicated to the transduction channels by gating springs, elastic elements that are stretched when the bundle is displaced toward its tall edge. To quantify the magnitude and time dependence of the forces exerted by gating springs, we have developed a displacement-clamp system that constrains a bundle's motion while measuring the forces that the bundle produces during adaptation to mechanical stimuli, in response to channel blockage, and upon destruction of the gating springs. Our results suggest that each gating spring exerts a tension of approximately 8 pN in the resting bundle and can sustain at least 4-13 pN of additional tension. The experiments provide further evidence that the gating springs account for at least one-third of the hair bundle's dynamic stiffness and that a force of approximately 100 fN is sufficient to open a single transduction channel.     

Incorporation of adhesion peptides into nonadhesive hydrogels useful for tissue resurfacing

The recessive patchwork (pwk) mutation in mice is associated with a unique hair follicle phenotype. Mice homozygous for patchwork exhibit a variegated coat containing a mixture of white and fully pigmented hairs, but no partially pigmented hairs. We have investigated the etiology of this mutation. We report here that the white hairs result from the lack of melanocytes in the follicle. As indicated by the coat color pattern of patchwork<-->albino chimeras, the target cell for the patchwork mutation is the melanocyte and/or its precursor. Examination of these chimeras also suggested that patchwork does not act in a cell-autonomous manner. The colonization of the skin by melanoblasts in patchwork embryos was studied using a lacZ transgene. Melanoblasts die by apoptosis in hair follicles from homozygous pwk/pwk fetuses starting at embryonic day 18.5, indicating that patchwork acts from this stage. The combination of pwk and KitW-ei, a mutation responsible for a reduced number of melanoblasts in the hair follicle, suggested that pwk gene product is necessary for low numbers of melanoblasts to survive and differentiate in the hair follicle from embryonic day 18.5 onward. We conclude that the pigmented hairs on the coat of pwk/pwk mice may be attributed to a community effect among melanoblasts in the hair follicle at the end of embryogenesis.