Topography and mechanics of the cupula in the fish lateral line. I. Variation of cupular structure and composition in three dimensions

Functional Consequences of Keratan Sulfate Sulfation in Electrosensory Tissues and in Neuronal Regulation

Keratan sulfate (KS) is a functional electrosensory and neuro-instructive molecule. Recent studies have identified novel low sulfation KS in auditory and sensory tissues such as the tectorial membrane of the organ of Corti and the Ampullae of Lorenzini in elasmobranch fish. These are extremely sensitive proton gradient detection systems that send signals to neural interfaces to facilitate audition and electrolocation. High and low sulfation KS have differential functional roles in song learning in the immature male zebra song-finch with high charge density KS in song nuclei promoting brain development and cognitive learning. The conductive properties of KS are relevant to the excitable neural phenotype. High sulfation KS interacts with a large number of guidance and neuroregulatory proteins. The KS proteoglycan microtubule associated protein-1B (MAP1B) stabilizes actin and tubulin cytoskeletal development during neuritogenesis. A second 12 span transmembrane synaptic vesicle associated KS proteoglycan (SV2) provides a smart gel storage matrix for the storage of neurotransmitters. MAP1B and SV2 have prominent roles to play in neuroregulation. Aggrecan and phosphacan have roles in perineuronal net formation and in neuroregulation. A greater understanding of the biology of KS may be insightful as to how neural repair might be improved.

Sensing the flow beneath the fins

Flow sensing, maneuverability, energy efficiency and vigilance of surroundings are the key factors that dictate the performance of marine animals. Be it swimming at high speeds, attack or escape maneuvers, sensing and survival hydrodynamics are a constant feature of life in the ocean. Fishes are capable of performing energy efficient maneuvers, including capturing energy from vortical structures in water. These impressive capabilities are made possible by the uncanny ability of fish to sense minute pressure and flow variations on their body. This is achieved by arrays of biological neuromast sensors on their bodies that 'feel' the surroundings through 'touch at a distance' sensing. The main focus of this paper is to review the various biomimetic material approaches in developing superficial neuromast inspired ultrasensitive MEMS sensors. Principals and methods that translate biomechanical filtering properties of canal neuromasts to benefit artificial MEMS sensors have also been discussed. MEMS sensors with ultrahigh flow sensitivity and accuracy have been developed mainly through inspiration from the hair cell and cupula structures in the neuromast. Canal-inspired packages have proven beneficial in hydrodynamic flow filtering in artificial sensors enabling signal amplification and noise attenuation. A special emphasis has been placed on the recent innovations that closely mimic the structural and material designs of stereocilia of neuromasts by exploring soft polymers.

Cupula-Inspired Hyaluronic Acid-Based Hydrogel Encapsulation to Form Biomimetic MEMS Flow Sensors

Blind cavefishes are known to detect objects through hydrodynamic vision enabled by arrays of biological flow sensors called neuromasts. This work demonstrates the development of a MEMS artificial neuromast sensor that features a 3D polymer hair cell that extends into the ambient flow. The hair cell is monolithically fabricated at the center of a 2 μm thick silicon membrane that is photo-patterned with a full-bridge bias circuit. Ambient flow variations exert a drag force on the hair cell, which causes a displacement of the sensing membrane. This in turn leads to the resistance imbalance in the bridge circuit generating a voltage output. Inspired by the biological neuromast, a biomimetic synthetic hydrogel cupula is incorporated on the hair cell. The morphology, swelling behavior, porosity and mechanical properties of the hyaluronic acid hydrogel are characterized through rheology and nanoindentation techniques. The sensitivity enhancement in the sensor output due to the material and mechanical contributions of the micro-porous hydrogel cupula is investigated through experiments.

Novel structure in sciaenid fish skulls indicates continuous production of the cephalic neuromast cupula

The presence of a conspicuous and frequent but never-described structure in the skull cavities of sciaenid fish was noted during population studies in an urbanized bay. The ultrastructure closely resembles the cupula of neuromasts, an organ associated with the perception of the environment in teleost fish. The bodies were recorded detached in both preserved and freshly sampled individuals and without associated cilia. Prominent characteristics are acellularity, the elliptic-conic shape composed of stack-like protein lamellas, and a mesh-like appearance in cross section. These acellular lamellar cephalic bodies (ALCBs) were more abundant in larger individuals and showed temporal peaks of abundance independently of the fish size. The conic and lamellar features suggest that the deposition of protein layers follows fish growth, and the bimodality of the size of these structures in individuals indicates temporal peaks of production. These results indicate that these ALCBs are a consequence of the accretion of the cupula of neuromasts at a faster rate than they degrade. Given the novelty of this structure and the increasing records of diseases of marine organisms worldwide, an important question is whether these bodies occur subsequently to some environmental change and whether their accumulation in the skull cavities has consequences to fish health.

Nanofibril scaffold assisted MEMS artificial hydrogel neuromasts for enhanced sensitivity flow sensing

We present the development and testing of superficial neuromast-inspired flow sensors that also attain high sensitivity and resolution through a biomimetic hyaulronic acid-based hydrogel cupula dressing. The inspiration comes from the spatially distributed neuromasts of the blind cavefish that live in completely dark undersea caves; the sensors enable the fish to form three-dimensional flow and object maps, enabling them to maneuver efficiently in cluttered environments. A canopy shaped electrospun nanofibril scaffold, inspired by the cupular fibrils, assists the drop-casting process allowing the formation of a prolate spheroid-shaped artificial cupula. Rheological and nanoindentation characterizations showed that the Young's modulus of the artificial cupula closely matches the biological cupula (10-100 Pa). A comparative experimental study conducted to evaluate the sensitivities of the naked hair cell sensor and the cupula-dressed sensor in sensing steady-state flows demonstrated a sensitivity enhancement by 3.5-5 times due to the presence of hydrogel cupula. The novel strategies of sensor development presented in this report are applicable to the design and fabrication of other biomimetic sensors as well. The developed sensors can be used in the navigation and maneuvering of underwater robots, but can also find applications in biomedical and microfluidic devices.

Role of the lateral line mechanosensory system in directionality of goldfish auditory evoked escape response

Goldfish (Carassius auratus) escape responses to sudden auditory stimuli are mediated by a pair of reticulospinal neurons, the Mauthner (M-) cells, which integrate mechanosensory inputs from the inner ear and the lateral line (LL) to initiate a fast directional response away from the aversive stimulus. This behavior is context dependent; when near an obstruction the fish may rather turn towards the sound to avoid hitting the object. Mechanisms underlying this directionality remain unknown. Here we investigate the contribution of the LL system to auditory evoked escapes and provide behavioral evidence that it transmits stimulus - and environmental-dependent information that determines the initial response direction of the escape. We quantified escape latency, probability and directionality following abrupt sound stimuli before and after removal of the entire LL with 0.03 mmol l(-1) cobalt chloride (CoCl(2)), 0.002% gentamicin or selective posterior LL nerve (pLLn) transection. CoCl(2) significantly increased escape onset latency without affecting probability and reduced open field directionality from 77% to chance, 52%. This effect on directionality was also observed with gentamicin. Transection of the pLLn had no effect on directionality, indicating the anterior LL nerve (aLLn) afferents are more likely to transmit directional information to the M-cell. When the fish were near a wall, the error rate was quadrupled by both CoCl(2) and pLLn transection. Visual elimination had no influence on directionality unless combined with LL elimination.

Hair cell mechano-transduction: its influence on the gross mechanical characteristics of a hair cell sense organ

The complex mechanical behaviour of a hair cell bundle appears to be a direct consequence of the gating forces on the individual transduction channels. The mechanical molecular interactions involved in transduction channel gating, therefore, also bear a reciprocal influence, via the hair bundles, on the mechanical properties of accessory structures driving them. This allows for the possibility to investigate, under in vivo conditions, the mechanical gating machinery of ion channels via the dynamics of accessory structures. We have performed such studies on the lateral line organ of fish and were thus able to relate the mechanics of elementary molecular events to the macroscopical dynamics of an intact organ.

Visualisation of domains in the avian tectorial and otolithic membranes with monoclonal antibodies

The staining patterns observed with six monoclonal antibodies (mAbs) raised in vitro against a fraction derived from the utricular macula were examined with cryosections of the auditory and vestibular organs of the avian inner ear. These antibodies revealed several distinct domains within the gelatinous membranes. Three different labelling patterns were observed in the tectorial membrane. Staining was seen either throughout the entire tectorial membrane, restricted to its core, or in a narrow zone lying close to the surface of the basilar papilla. In the maculae, the mAbs stained either the striolar region of the otolithic membrane or the entire structure. One monoclonal which labelled otoconia, stained small otoconia in their entirely, whilst larger otoconia were only labelled around their periphery. Only one of the mAbs stained the cupulae of the semi-circular canal ampullae and this antibody stained neither the tectorial nor the otolithic membranes. These results suggest that there are biochemically distinct regions in the gelatinous membranes of the inner ear and indicate that these matrices are not simply homogeneous extracellular structures.

Effects of quinine on the mechanical frequency response of the cupula in the fish lateral line

Quinine induces changes in the motion of the cupula in the lateral line canal of the African knife-fish in response to sinusoidal water movements. Two different phases in the action of quinine on the cupular frequency response can be discerned. In the first phase the best frequency, i.e., the frequency at which the cupular vibratory displacement is maximal in response to constant-amplitude sinusoidal canal fluid displacement, shifts toward higher frequencies. During this phase, lasting about 70-100 min, the best frequency increases by a factor between 1.3 and 1.5. In the second phase, during roughly the following 90 min, the best frequency decreases gradually to a value 0.3-0.5 times that observed before the application of quinine.

Stiffness changes of the cupula associated with the mechanics of hair cells in the fish lateral line

Cupular vibration in the lateral-line canal of fish was measured in response to motion of the fluid in the canal by laser-heterodyne interferometry. The results show that the mechanical output/input ratio of the cupula depends on the stimulus amplitude; the cupula thus behaves nonlinearly. The nonlinearity is due to the hair bundles, since it disappears when the cupula is uncoupled from the underlying hair cells. A model of cupular dynamics in which the behavior of the gating springs of the hair cells is incorporated predicts nonlinear responses that are similar to the measurements, suggesting that the nonlinear behavior of the cupula may be attributed to the opening and closing of the transduction channels of the hair cells.

Mechanical demodulation of hydrodynamic stimuli performed by the lateral line organ

Tonic displacements of the fish lateral line cupula were observed during stimulation of the organ with amplitude-modulated water motion. The modulation frequency was fixed at 2.4 Hz and the carrier frequency was varied from 25 to 500 Hz. The time waveforms of the cupular displacement at carrier frequencies below 280 Hz and above 470 Hz were essentially amplitude-modulated waves. Between 350 Hz and 410 Hz the magnitude at the modulation frequency increased sharply and the predominant shape of the displacement waveform changed to that of the modulating frequency. The mechanism for extraction of the modulation component may play a key role in the decoding of sensory information.