Computational Modeling of Developing Cartilage Using Experimentally Derived Geometries and Compressive Moduli.

During chondrogenesis, tissue organization changes dramatically. We previously showed that the compressive moduli of chondrocytes increase concomitantly with extracellular matrix (ECM) stiffness, suggesting cells were remodeling to adapt to the surrounding environment. Due to the difficulty in analyzing the mechanical response of cells in situ, we sought to create an in silico model that would enable us to investigate why cell and ECM stiffness increased in tandem. The goal of this study was to establish a methodology to segment, quantify, and generate mechanical models of developing cartilage to explore how variations in geometry and material properties affect strain distributions. Multicellular geometries from embryonic day E16.5 and postnatal day P3 murine cartilage were imaged in three-dimensional (3D) using confocal microscopy. Image stacks were processed using matlab to create geometries for finite element analysis using ANSYS. The geometries based on confocal images and isolated, single cell models were compressed 5% and the equivalent von Mises strain of cells and ECM were compared. Our simulations indicated that cells had similar strains at both time points, suggesting that the stiffness and organization of cartilage changes during development to maintain a constant strain profile within cells. In contrast, the ECM at P3 took on more strain than at E16.5. The isolated, single-cell geometries underestimated both cell and ECM strain and were not able to capture the similarity in cell strain at both time points. We expect this experimental and computational pipeline will facilitate studies investigating other model systems to implement physiologically derived geometries.

Suppression Measured from Chinchilla Auditory-Nerve-Fiber Responses Following Noise-Induced Hearing Loss: Adaptive-Tracking and Systems-Identification Approaches.

The compressive nonlinearity of cochlear signal transduction, reflecting outer-hair-cell function, manifests as suppressive spectral interactions; e.g., two-tone suppression. Moreover, for broadband sounds, there are multiple interactions between frequency components. These frequency-dependent nonlinearities are important for neural coding of complex sounds, such as speech. Acoustic-trauma-induced outer-hair-cell damage is associated with loss of nonlinearity, which auditory prostheses attempt to restore with, e.g., "multi-channel dynamic compression" algorithms.Neurophysiological data on suppression in hearing-impaired (HI) mammals are limited. We present data on firing-rate suppression measured in auditory-nerve-fiber responses in a chinchilla model of noise-induced hearing loss, and in normal-hearing (NH) controls at equal sensation level. Hearing-impaired (HI) animals had elevated single-fiber excitatory thresholds (by ~ 20-40 dB), broadened frequency tuning, and reduced-magnitude distortion-product otoacoustic emissions; consistent with mixed inner- and outer-hair-cell pathology. We characterized suppression using two approaches: adaptive tracking of two-tone-suppression threshold (62 NH, and 35 HI fibers), and Wiener-kernel analyses of responses to broadband noise (91 NH, and 148 HI fibers). Suppression-threshold tuning curves showed sensitive low-side suppression for NH and HI animals. High-side suppression thresholds were elevated in HI animals, to the same extent as excitatory thresholds. We factored second-order Wiener-kernels into excitatory and suppressive sub-kernels to quantify the relative strength of suppression. We found a small decrease in suppression in HI fibers, which correlated with broadened tuning. These data will help guide novel amplification strategies, particularly for complex listening situations (e.g., speech in noise), in which current hearing aids struggle to restore intelligibility.

Structural and biochemical abnormalities in the absence of acute deficits in mild primary blast-induced head trauma.

OBJECTIVE: Blast-induced neurotrauma (BINT), if not fatal, is nonetheless potentially crippling. It can produce a wide array of acute symptoms in moderate-to-severe exposures, but mild BINT (mBINT) is characterized by the distinct absence of acute clinical abnormalities. The lack of observable indications for mBINT is particularly alarming, as these injuries have been linked to severe long-term psychiatric and degenerative neurological dysfunction. Although the long-term sequelae of BINT are extensively documented, the underlying mechanisms of injury remain poorly understood, impeding the development of diagnostic and treatment strategies. The primary goal of this research was to recapitulate primary mBINT in rodents in order to facilitate well-controlled, long-term investigations of blast-induced pathological neurological sequelae and identify potential mechanisms by which ongoing damage may occur postinjury. METHODS: A validated, open-ended shock tube model was used to deliver blast overpressure (150 kPa) to anesthetized rats with body shielding and head fixation, simulating the protective effects of military-grade body armor and isolating a shock wave injury from confounding systemic injury responses, head acceleration, and other elements of explosive events. Evans Blue-labeled albumin was used to visualize blood-brain barrier (BBB) compromise at 4 hours postinjury. Iba1 staining was used to visualize activated microglia and infiltrating macrophages in areas of peak BBB compromise. Acrolein, a potent posttraumatic neurotoxin, was quantified in brain tissue by immunoblotting and in urine through liquid chromatography with tandem mass spectrometry at 1, 2, 3, and 5 days postinjury. Locomotor behavior, motor performance, and short-term memory were assessed with open field, rotarod, and novel object recognition (NOR) paradigms at 24 and 48 hours after the blast. RESULTS: Average speed, maximum speed, and distance traveled in an open-field exploration paradigm did not show significant differences in performance between sham-injured and mBINT rats. Likewise, rats with mBINT did not exhibit deficits in maximum revolutions per minute or total run time in a rotarod paradigm. Short-term memory was also unaffected by mBINT in an NOR paradigm. Despite lacking observable motor or cognitive deficits in the acute term, blast-injured rats displayed brain acrolein levels that were significantly elevated for at least 5 days, and acrolein's glutathione-reduced metabolite, 3-HPMA, was present in urine for 2 days after injury. Additionally, mBINT brain tissue demonstrated BBB damage 4 hours postinjury and colocalized neuroinflammatory changes 24 hours postinjury. CONCLUSIONS: This model highlights mBINT's potential for underlying detrimental physical and biochemical alterations despite the lack of apparent acute symptoms and, by recapitulating the human condition, represents an avenue for further examining the pathophysiology of mBINT. The sustained upregulation of acrolein for days after injury suggests that acrolein may be an upstream player potentiating ongoing postinjury damage and neuroinflammation. Ultimately, continued research with this model may lead to diagnostic and treatment mechanisms capable of preventing or reducing the severity of long-term neurological dysfunction following mBINT.