NaCl osmotic perturbation can modulate hydration control in rabbit cornea

Reduced humidity experienced by mice in vivo coincides with reduced outflow facility measured ex vivo

Mice are routinely used to study aqueous humour dynamics. However, physical factors such as temperature and hydration affect outflow facility in enucleated eyes. This retrospective study examined whether differences in temperature and relative humidity experienced by living mice within their housing environment in vivo coincide with differences in outflow facility measured ex vivo. Facility data and environmental records were collected for one enucleated eye from 116 mice (C57BL/6J males, 9-15 weeks old) at two institutions. Outflow facility was reduced when relative humidity was below the lower limit of 45% recommended by the UK Code of Practice, but there was no detectable effect of temperature on outflow facility. Even when accounting for effects of humidity, there were differences in outflow facility measured between institutions and between individual researchers at the same institution. These data indicate that humidity, as well as additional environmental factors experienced by living mice within their housing environment, may significantly affect outflow facility measured ex vivo.

A structural model for the in vivo human cornea including collagen-swelling interaction

A structural model of the in vivo cornea, which accounts for tissue swelling behaviour, for the three-dimensional organization of stromal fibres and for collagen-swelling interaction, is proposed. Modelled as a binary electrolyte gel in thermodynamic equilibrium, the stromal electrostatic free energy is based on the mean-field approximation. To account for active endothelial ionic transport in the in vivo cornea, which modulates osmotic pressure and hydration, stromal mobile ions are shown to satisfy a modified Boltzmann distribution. The elasticity of the stromal collagen network is modelled based on three-dimensional collagen orientation probability distributions for every point in the stroma obtained by synthesizing X-ray diffraction data for azimuthal angle distributions and second harmonic-generated image processing for inclination angle distributions. The model is implemented in a finite-element framework and employed to predict free and confined swelling of stroma in an ionic bath. For the in vivo cornea, the model is used to predict corneal swelling due to increasing intraocular pressure (IOP) and is adapted to model swelling in Fuchs' corneal dystrophy. The biomechanical response of the in vivo cornea to a typical LASIK surgery for myopia is analysed, including tissue fluid pressure and swelling responses. The model provides a new interpretation of the corneal active hydration control (pump-leak) mechanism based on osmotic pressure modulation. The results also illustrate the structural necessity of fibre inclination in stabilizing the corneal refractive surface with respect to changes in tissue hydration and IOP.

Assessment of Full-Eye Response to Osmotic Stress in Mouse Model In Vivo Using Optical Coherence Tomography

NaCl based solutions were applied as osmotic stress agents to alter the hydration state of the mouse eye. Full-eye responses to these osmotic challenges were monitored in vivo using a custom-built optical coherence tomography (OCT) with an extended imaging range of 12.38 mm. Dynamic changes in the mouse eye were quantified based on the OCT images using several parameters, including the central corneal thickness (CCT), the anterior chamber depth (ACD), the crystalline lens thickness (LT), the cornea-retina distance (CRD), the iris curvature (IC), and the lens scattering intensity (LSI). Apparent but reversible changes in the morphology of almost all the ocular components and the light transparency of the lens are exhibited. Particularly, the ocular dehydration induced by the hypertonic challenges resulted in a closing of the iridocorneal angle and an opacification of the lens. Our results indicated that the ocular hydration is an important physiological process which might be correlated with various ocular disorders, such as dry eye, cataract, and angle-closure glaucoma, and would affect the biometry and imaging of the eye. OCT uniquely enables the comprehensive study of the dynamic full-eye responses to the ocular hydration in vivo.

THz and mm-Wave Sensing of Corneal Tissue Water Content: In Vivo Sensing and Imaging Results

A pulsed terahertz (THz) imaging system and millimeter-wave reflectometer were used to acquire images and point measurements, respectively, of five rabbit cornea in vivo. These imaging results are the first ever produced of in vivo cornea. A modified version of a standard protocol using a gentle stream of air and a Mylar window was employed to slightly dehydrate healthy cornea. The sensor data and companion central corneal thickness (CCT) measurements were acquired every 10-15 min over the course of two hours using ultrasound pachymmetry.. Statistically significant positive correlations were established between CCT measurements and millimeter wave reflectivity. Local shifts in reflectivity contrast were observed in the THz imagery; however, the THz reflectivity did not display a significant correlation with thickness in the region probed by the 100 GHz and CCT measurements. This is explained in part by a thickness sensitivity at least 10× higher in the mm-wave than the THz systems. Stratified media and effective media modeling suggest that the protocol perturbed the thickness and not the corneal tissue water content (CTWC). To further explore possible etalon effects, an additional rabbit was euthanized and millimeter wave measurements were obtained during death induced edema. These observations represent the first time that the uncoupled sensing of CTWC and CCT have been achieved in vivo.

Numerical simulation of corneal transport processes

This paper presents a numerical study on the transport of ions and ionic solution in human corneas and the corresponding influences on corneal hydration. The transport equations for each ionic species and ionic solution within the corneal stroma are derived based on the transport processes developed for electrolytic solutions, whereas the transport across epithelial and endothelial membranes is modelled by using phenomenological equations derived from the thermodynamics of irreversible processes. Numerical examples are provided for both human and rabbit corneas, from which some important features are highlighted.

Ocular biomechanics and biotransport

The eye transduces light, and we usually do not think of it as a biomechanical structure. Yet it is actually a pressurized, thick-walled shell that has an internal and external musculature, a remarkably complex internal vascular system, dedicated fluid production and drainage tissues, and a variety of specialized fluid and solute transport systems. Biomechanics is particularly involved in accommodation (focusing near and far), as well as in common disorders such as glaucoma, macular degeneration, myopia, and presbyopia. In this review, we give a (necessarily brief) overview of many of the interesting biomechanical aspects of the eye, concluding with a list of open problems.

Mathematical modelling of corneal swelling

This paper presents a differential model of the corneal transport system capable of modelling thickness changes in response to osmotic perturbations applied to either limiting membrane. The work is directed towards understanding corneal behaviour in vivo. The model considers the coupled viscous flows within the corneal stroma and across the epithelial and endothelial membranes. The flows within the stroma are established based on transport theory in porous media, while the flows across the membranes are described using the phenomenological equations of irreversible thermodynamics. The ability of the numerical model to reproduce corneal thickness changes in response to endothelial perturbations was tested against available experimental data. The sensitivity of the model to changes in stromal and membrane transport coefficients was examined.

Fluid transport across cultured layers of corneal endothelium from aquaporin-1 null mice

We explored the role of AQP1, the only known aquaporin in corneal endothelium, on active fluid transport and passive osmotic water movements across corneal endothelial layers cultured from AQP1 null mice and wildtype mice. AQP1 null mice had grossly transparent corneas, just as wildtype mice. Endothelial cell layers grown on permeable supports transported fluid at rates of (in microl h(-1) cm(-2), n = 9 mean+/-s.e.): 4.3+/-0.6, wildtype mice (MCE); 3.5+/-0.6, AQP1 null mice (KMCE; difference not significant). The osmotic water flow (also in microl h(-1) cm(-2)) induced by a 100 mOsm sucrose gradient across MCE cell layers (8.7+/-0.6, n = 8) was significantly greater than that across KMCE (5.7+/-0.7, n = 6, p = 0.007). When plated on glass coverslips, plasma membrane osmotic water permeability determined by light scattering was significantly higher for cells from wildtype vs. AQP1 null mice (in microm sec(-1): 74+/-4, n = 19 vs. 44+/-4 microm sec(-1), n = 11, p < 0.001). Unexpectedly, after 10% hypo-osmotic challenge, the extent of the regulatory volume recovery was significantly reduced for AQP1 null mice cells (in%: MCE controls, 99+/-1, n = 19 vs. KMCE: 64+/-5, n = 11, p < 0.001). Thus, as in other 'low rate' fluid transporting epithelia, deletion of AQP1 in mice corneal endothelium reduces osmotic water permeability but not active transendothelial fluid transport. However, that deletion impaired the extent of regulatory volume decrease after a hypo-osmotic challenge, suggesting a novel role for AQP1 in corneal endothelium.