Intracochlear pressure and organ of corti impedance from a linear active three-dimensional model.

Intracochlear pressure and basilar membrane (BM) velocity are calculated from a physiologically based chinchilla cochlea model . The model includes three-dimensional viscous fluid and the pectinate zone of the elastic BM with dimensional and material property variation along its length. The passive response shows excellent agreement with measurement at high sound pressure levels. The active process is represented by adding the motility of the outer hair cells (OHCs) to the passive model with the feed-forward approximation of the organ of Corti (OC), as was done previously. The current model explains recent observations including: (1) agreement with characteristic frequency (CF)-to-place map, (2) CF shift in the active model, (3) BM displacement gain from OHC motility, (4) lower intracochlear pressure gain than BM displacement gain, and (5) OC impedance (Z(OC)).

Regulation of cutaneous malignancy by gammadelta T cells.

The localization of gammadelta T cells within epithelia suggests that these cells may contribute to the down-regulation of epithelial malignancies. We report that mice lacking gammadelta cells are highly susceptible to multiple regimens of cutaneous carcinogenesis. After exposure to carcinogens, skin cells expressed Rae-1 and H60, major histocompatibility complex-related molecules structurally resembling human MICA. Each of these is a ligand for NKG2d, a receptor expressed by cytolytic T cells and natural killer (NK) cells. In vitro, skin-associated NKG2d+ gammadelta cells killed skin carcinoma cells by a mechanism that was sensitive to blocking NKG2d engagement. Thus, local T cells may use evolutionarily conserved proteins to negatively regulate malignancy.

Effects of chair restraint on the strength of the tibia in rhesus monkeys.

To determine the effects of the relative inactivity and unloading on the strength of the tibias of monkeys, Macaca mulatta, we used a non-invasive test to measure bending stiffness, or EI (Nm2), a mechanical property. The technique was validated by comparisons of in vivo measurements with standard measures of EI in the same bones post-mortem (r2 = 0.95, P < 0.0001). Inter-test precision was 4.28+/-1.4%. Normative data in 24 monkeys, 3.0+/-0.7 years and 3.6+/-0.6 kg, revealed EI to be 16% higher in the right than left tibia (4.4+/-1.6 vs. 3.7+/-1.6 Nm2, P < 0.05). Five monkeys, restrained in chairs for 14 days, showed decreases in EI. There were no changes in EI in two chaired monkeys that lost weight during a 2-week space flight. The factors that account for both the decreases in bone mechanical properties after chair restraint at 1 g and lack of change after microgravity remain to be identified. Metabolic factors associated with body weight changes are suggested by our results.

Efficacy of monitoring long-bone fracture healing by measurement of either bone stiffness or resonant frequency: numerical simulation.

Development of noninvasive mechanical tests to monitor fracture healing has been hindered because relationships between bone geometry, measurement conditions, and fracture callus strength are not well understood. Beam theory was used to analyze the effects of fracture length, fracture location, end conditions, and fracture callus stiffness on mechanical properties (resonant frequency, bending stiffness, and torsional stiffness) of healing bone. Actual bone mineral geometry from a human tibia, quantified every 1 mm, was used in the beam analysis. Geometry of the fracture callus segment was uniformly scaled from the values for intact bone. Experimental tests on multisegmented machined rods were used to verify analytical methods. Mechanical properties of the healing bone initially increased very rapidly to 30-70% of the stiffness of intact bone, depending on the configuration. The increases then tapered off dramatically. Lateral bending stiffness was sensitive to changes in callus properties for a larger portion of the healing process than was either torsional stiffness or resonant frequency. Because callus strength increases at half the rate of callus stiffness, measures of whole-bone mechanical properties can provide insight into changes in callus strength until a maximum of less than one-half the strength of intact bone is regained. The analytical method presented is proposed for clinical use to develop individualized models of bone, fracture, and fixation conditions to identify early stages of healing. Because increases in whole-bone mechanical properties are small in the later stages of fracture healing, however, such measures must be used prudently beyond the initial stages.

Gamma(delta) T cells: non-classical ligands for non-classical cells.

A recent study describes direct binding between a gammadelta T-cell receptor and its ligand, T22, a non-classical class I major histocompatibility complex (MHC) molecule. A companion study, solving the crystal structure of T22, highlights the differences between this interaction and those of classical MHC molecules and alphabeta T cells.

Toward three-dimensional analysis of cochlear structure.

Recent results from a three-dimensional model of the cochlea are summarized. The features include physically realistic values of basilar membrane stiffness, mass, and fluid viscosity. The simple 'feed-forward' principle for the active process yields results in qualitative agreement with recent measurements in the cochlea. The limitation is a simplified representation of the organ of Corti, with two degrees of freedom representing the motion of the pectinate and arcuate zones of the basilar membrane. However, the inner sulcus fluid flow is included. The new feature presented in this paper is an approach to treat all the structural detail of the organ of Corti, with the sole input to the calculation in a form easily understood by anyone familiar with the cochlea. Specific results are shown for the Pakistani water buffalo, since a fairly complete anatomical description of this cochlea is available. The static stiffness from the calculation, based on only the anatomy and known values for the protein elastic moduli, are in remarkable agreement with recent measurements in the gerbil cochlea. Only preliminary results for the dynamic response with inviscid fluid are reported. Of interest, however, are the propagation modes related to significant fluid displacement and pressure in the different compartments of the organ of Corti.

MHC class II antigen processing in B cells: accelerated intracellular targeting of antigens.

Processing and presentation by Ag-specific B cells is initiated by Ag binding to the B cell Ag receptor (BCR). Cross-linking of the BCR by Ag results in a rapid targeting of the BCR and bound Ag to the MHC class II peptide loading compartment (IIPLC). This accelerated delivery of Ag may be essential in vivo during periods of rapid Ag-driven B cell expansion and T cell-dependent selection. Here, we use both immunoelectron microscopy and a nondisruptive protein chemical polymerization method to define the intracellular pathway of the targeting of Ags by the BCR. We show that following cross-linking, the BCR is rapidly transported through transferrin receptor-containing early endosomes to a LAMP-1+, beta-hexosaminadase+, multivesicular compartment that is an active site of peptide-class II complex assembly, containing both class II-invariant chain complexes in the process of invariant chain proteolytic removal as well as mature peptide-class II complexes. The BCR enters the class II-containing compartment as an intact mIg/Igalpha/Igbeta complex bound to Ag. The pathway by which the BCR targets Ag to the IIPLC appears not to be identical to that by which Ags taken up by fluid phase pinocytosis traffick, suggesting that the accelerated BCR pathway may be specialized and potentially independently regulated.

Cochlear model with three-dimensional fluid, inner sulcus and feed-forward mechanism.

A three-dimensional model of the guinea pig cochlea using the phase-integral method is presented. This model incorporates the viscous fluid effects in the cochlea, dimensional and material property variation along the cochlear duct and the active feed-forward mechanism of the outer hair cells. Two degrees of freedom of the basilar membrane are considered, which results in two traveling waves propagating along the duct for a given frequency. Basilar membrane response with the active feed-forward mechanism compares favorably with published experimental measurements.

Vibration of reflective beads placed on the basilar membrane.

Most investigators place reflective beads on the basilar membrane to measure its vibration with optical methods. It is therefore important to find out if the beads faithfully follow the motion of the structures on which they are placed. Vibration of the beads on the basilar membrane and basilar membrane adjacent to the beads are measured in the third turn of the guinea pig cochlea in a temporal bone preparation. It is shown that the beads do not follow the motion of the organ. The mechanism by which this departure may occur is investigated by modeling the motion of the beads on the Claudius' cells.

A dynamic model of outer hair cell motility including intracellular and extracellular fluid viscosity.

The deformation response of a guinea pig outer hair cell is modeled for mechanical and electrical stimulation up to 25 kHz. The analysis uses a Fourier series technique for a finite length cell surrounded internally and externally by a much larger continuum of viscous fluid. The analytical solution predicts that outer hair cell length changes occur due to applied mechanical or electrical stimulation without significant resonance, characteristic of a highly damped system. The deformation is found to have little attenuation up to a corner frequency of about 2 kHz for long cells and 10 kHz for short cells, in agreement with published experimental results. For electrical loading of 1 mV across the lateral cell wall, deformation for short cells is calculated to be greater than 1 nm for frequencies up to 20 kHz. These results support the proposition that in vivo the outer hair cell modifies the character of basilar membrane deformation on a cycle-by-cycle basis. An estimate of the capability of the cell to supply energy to the basilar membrane is given based on published values of outer hair cell material properties.

Fluid-structure interaction of the stereocilia bundle in relation to mechanotransduction.

Current hypotheses regarding mechanotransduction rely upon motion of the stereocilia relative to the apical surface of the hair cell. The viscosity of the surrounding endolymphatic fluid will, however, attenuate stereocilia motion at higher frequencies of excitation. To investigate stereocilia motion for physiologically reasonable deflections and frequencies of excitation, the fluid-structure interaction of the stereocilia bundle is considered analytically. Solutions in the frequency domain are determined for stereocilia bundle dimensions at several locations along the cochlear duct of the chinchilla. Results indicate that motion of the stereocilia is analogous to that of a low-pass filter. Comparison of these solutions with Greenwood's frequency-place map demonstrates that motion of the stereocilia bundle exists without substantial attenuation at least up to frequencies appropriate for the location of the corresponding hair cell along the cochlear duct. The variation in stereocilia morphology within the mammalian cochlea thus appears to provide a collection of low-pass mechanoreceptors, arranged in order of increasing corner frequency across the auditory spectrum.

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.

Mechanical properties of the lateral cortex of mammalian auditory outer hair cells.

Mammalian auditory outer hair cells generate high-frequency mechanical forces that enhance sound-induced displacements of the basilar membrane within the inner ear. It has been proposed that the resulting cell deformation is directed along the longitudinal axis of the cell by the cortical cytoskeleton. We have tested this proposal by making direct mechanical measurements on outer hair cells. The resultant stiffness modulus along the axis of whole dissociated cells was 3 x 10(-3) N/m, consistent with previously published values. The resultant axial and circumferential stiffness moduli for the cortical lattice were 5 x 10(-4) N/m and 3 x 10(-3) N/m, respectively. Thus the cortical lattice is a highly orthotropic structure. Its axial stiffness is small compared with that of the intact cell, but its circumferential stiffness is within the same order of magnitude. These measurements support the theory that the cortical cytoskeleton directs electrically driven length changes along the longitudinal axis of the cell. The Young's modulus of the circumferential filamentous components of the lattice were calculated to be 1 x 10(7) N/m2. The axial cross-links, believed to be a form of spectrin, were calculated to have a Young's modulus of 3 x 10(6) N/m2. Based on the measured values for the lattice and intact cell cortex, an estimate for the resultant stiffness modulus of the plasma membrane was estimated to be on the order of 10(-3) N/m. Thus, the plasma membrane appears to be relatively stiff and may be the dominant contributor to the axial stiffness of the intact cell.

Noninvasive determination of bone mechanical properties using vibration response: a refined model and validation in vivo.

Accurate non-invasive mechanical measurement of long bones is made difficult by the masking effect of surrounding soft tissues. Mechanical response tissue analysis (MRTA) offers a method for separating the effects of the soft tissue and bone; however, a direct validation has been lacking. A theoretical analysis of wave propagation through the compressed tissue revealed a strong mass effect dependent on the relative accelerations of the probe and bone. The previous mathematical model of the bone and overlying tissue system was reconfigured to incorporate the theoretical finding. This newer model (six-parameter) was used to interpret results using MRTA to determine bone cross-sectional bending stiffness, EIMRTA. The relationship between EIMRTA and theoretical EI values for padded aluminum rods was R2 = 0.999. A biological validation followed using monkey tibias. Each bone was tested in vivo with the MRTA instrument. Postmortem, the same tibias were excised and tested to failure in three-point bending to determine EI3-PT and maximum load. Diaphyseal bone mineral density (BMD) measurements were also made. The relationship between EI3-PT and in vivo EIMRTA using the six-parameter model is strong (R2 = 0.947) and better than that using the older model (R2 = 0.645). EIMRTA and BMD are also highly correlated (R2 = 0.853). MRTA measurements in vivo and BMD ex vivo are both good predictors of scaled maximum strength (R2 = 0.915 and R2 = 0.894, respectively). This is the first biological validation of a non-invasive mechanical measurement of bone by comparison to actual values. The MRTA technique has potential clinical value for assessing long-bone mechanical properties.

Orthotropic piezoelectric properties of the cochlear outer hair cell wall.

The mammalian outer hair cell has been shown to possess significant coupling between mechanical and electrical properties. This electromotile property may play a key role in cochlear tuning. In order to characterize quantitatively the electrical and mechanical behavior, the cell wall is modeled as a thin linear elastic piezoelectric material. Experimental findings from several investigators are used to determine the mechanical and electrical generalized stiffness coefficients described by the model. The model analysis indicates that orthotropic mechanical properties in the plane of the cell wall are required to match experimental behavior. The calculated orthotropic coefficients predict that the outer hair cell deforms due to cilia deflection with a force gain of 0.5 for perfectly constrained end conditions and a displacement gain of 3.6 for free end conditions. These values reflect the potential role of the OHC as a feedback mechanism to the basilar membrane. Results are for small deformation and quasi-static conditions with viscosity and inertial effects neglected. It is further assumed that cell permeability is negligible at the time scale of the fast deformation considered here.

Influence of recreational activity and muscle strength on ulnar bending stiffness in men.

Bone bending stiffness (modulus of elasticity [E] x moment of inertia [I]), a measure of bone strength, is related to its mineral content (BMC) and geometry and may be influenced by exercise. We evaluated the relationship of habitual recreational exercise and muscle strength to ulnar EI, width, and BMC in 51 healthy men, 28-61 yr of age. BMC and width were measured by single photon absorptiometry and EI by mechanical resistance tissue analysis. Maximum biceps strength was determined dynamically (1-RM) and grip strength isometrically. Subjects were classified as sedentary (S) (N = 13), moderately (M) (N = 18), or highly active (H) (N = 20) and exercised 0.2 +/- 0.2; 2.2 +/- 1.3; and 6.8 +/- 2.3 h.wk-1 (P < 0.001). H had greater biceps (P < 0.0005) and grip strength (P < 0.05), ulnar BMC (P < 0.05), and ulnar EI (P = 0.01) than M or S, who were similar. Amount of activity correlated with grip and biceps strength (r = 0.47 and 0.49; P < 0.001), but not with bone measurements, whereas muscle strength correlated with both EI and BMC (r = 0.40-0.52, P < 0.005). EI also correlated significantly with both BMC and ulnar width (P < 0.0001). Ulnar width and biceps strength were the only independent predictors of EI (r2 = 0.67, P < 0.0001). We conclude that levels of physical activity sufficient to increase arm strength influence ulnar bending stiffness.

In vivo assessment of forearm bone mass and ulnar bending stiffness in healthy men.

The cross-sectional bending stiffness EI of the ulna was measured in vivo by mechanical resistance tissue analysis (MRTA) in 90 men aged 19-89 years. MRTA measures the impedance response of low-frequency vibrations to determine EI, which is a reflection of elastic modulus E and moment of inertia I for the whole ulna. EI was compared to conventional estimates of bone mineral content (BMC), bone width (BW), and BMC/BW, which were all measured by single-photon absorptiometry. Results obtained from the nondominant ulna indicate that BW increases (r = 0.27, p = 0.01) and ulnar BMC/BW decreases (r = -0.31, p < or = 0.005) with age. Neither BMC nor EI declined with age. The single best predictor of EI was BW (r2 = 0.47, p = 0.0001), and further small but significant contributions were made by BMC (r2 = 0.53, p = 0.0001) and grip strength (r2 = 0.55, p = 0.0001). These results suggest that the resistance of older men to forearm fracture is related to age-associated changes in the moment of inertia achieved by redistributing bone mineral farther from the bending axis. We conclude that the in vivo assessment of bone geometry offers important insights to the comprehensive evaluation of bone strength.

Noninvasive assessment of ulnar bending stiffness in women.

The load-carrying capacity of cortical bone is closely related to its geometry and to its fundamental material properties, including mineral content (BMC). Together these determine the bending stiffness EI, where I is the cross-sectional moment of inertia and E is Young's modulus of elasticity. To assess the relationship of BMC and bone width (BW) to EI in healthy women, we used mechanical response tissue analysis (MRTA), a noninvasive method that involves analysis of tissue responses to ulnar vibration. A total of 48 healthy women were enrolled into an older (64 +/- 1y, n = 25) and a younger (25 +/- 0.6y, n = 23) group. BMC and BW of the dominant ulna were measured by single-photon absorptiometry (SPA). EI was determined by MRTA. BMC (0.75 +/- 0.02 versus 0.63 +/- 0.02 g/cm), BMC/BW (0.75 +/- 0.02 versus 0.63 +/- 0.02 g/cm2), and EI (27.7 +/- 1.3 versus 21.3 +/- 1.1 N.m2) were significantly greater (p less than 0.005) in the young subjects. BW did not change with age (1.00 +/- 0.01 versus 1.01 +/- 0.01 cm). In young women, simple correlations of BMC and BW with EI were both significant. By multiple regression analysis only BW independently predicted EI (EI = -0.35 + 39.1 x BMC, R2 = 0.52). In older women BMC and BW correlated with EI, but in multiple regression only BMC was significant (EI = -34.5 + 62.1 x BW; R2 = 0.45). When this analysis of older women included only those whose BMC values were within 2 SD of the young mean, BMC remained the only significant predictor of EI.(ABSTRACT TRUNCATED AT 250 WORDS)

Noninvasive determination of ulnar stiffness from mechanical response--in vivo comparison of stiffness and bone mineral content in humans.

An approach referred to as Mechanical Response Tissue Analysis (MRTA) has been developed for the noninvasive determination of mechanical properties of the constituents of the intact limb. Of specific interest in the present study is the bending stiffness of the ulna. The point mechanical impedance properties in the low frequency regime, between 60 and 1,600 Hz are used. The procedure requires a proper design of the probe for good contact of the skin at midshaft and proper support of the proximal and distal ends of the forearm to obtain an approximation to "simple support" of the ulna. A seven-parameter model for the mechanical response is then valid, which includes the first mode of anterior-posterior beam bending of the ulna, the damping and spring effect of the soft tissue between probe and bone, and the damping of musculature. A dynamic analyzer (HP3562A) provides in seconds the impedance curve and the pole-zero curve fit. The physical parameters are obtained from a closed-form solution in terms of the curve-fit parameters. The procedure is automated and is robust and analytically reliable at about the five percent level. Some 80 human subjects have been evaluated by this mechanical response system and by the Norland single photon absorptiometer, providing for the first time in vivo, a comparison of elastic bending stiffness (ulna) and bone mineral content (radius). Three functional parameters of potential clinical value are the cross-sectional bending stiffness EI, the axial load capability Pcr (Euler buckling load) and the bone "sufficiency" S, defined as the ratio of Pcr to body weight. The correlation between EI and bone mineral (r = 0.81) is only slightly less than previous in vitro results with both measurements on the same bone (r = 0.89). When sufficiency is taken into consideration, the correlation of Pcr and bone mineral content is improved (r = 0.89). An implication is that "quality" of bone is a factor which is not indicated by bone mineral content but which is indicated by stiffness. Bone mineral is necessary for proper stiffness but not sufficient. Therefore mechanical measurement should provide a new dimension to be used toward a better understanding of the factors related to bone health and disease.

Influence of physical activity on the regulation of bone density.

Using a mathematical model which relates bone density to daily stress histories, the influence of physical activities on the apparent density of the calcaneal cancellous bone was investigated. Assuming that the mechanical bone maintenance stimulus is constant for all bone tissue, bone apparent density was calculated by a linear superposition of the mechanical stimulus provided by different daily physical activities. An empirical weighting factor, m, accounted for possible differences in the relative importance of load magnitude and number of cycles in each activity. By considering hypothetical variations in body weight and occupational activity levels, the range of probable m values was established. The model was then applied to the results of two previous running studies in which calcaneal density was measured to obtain an estimate of the stress exponent parameter, m. The results indicate that stress magnitudes (or joint forces) have a greater influence on bone mass than the number of loading cycles. We demonstrate that by carefully considering the magnitudes of imposed skeletal forces and the number of loading cycles, it may be possible to design exercise programs to achieve predictable changes in bone mass.