Swelling studies of bovine corneal stroma without bounding membranes

Hydrogels as functional components in artificial cell systems

Recent years have seen substantial efforts aimed at constructing artificial cells from various molecular components with the aim of mimicking the processes, behaviours and architectures found in biological systems. Artificial cell development ultimately aims to produce model constructs that progress our understanding of biology, as well as forming the basis for functional bio-inspired devices that can be used in fields such as therapeutic delivery, biosensing, cell therapy and bioremediation. Typically, artificial cells rely on a bilayer membrane chassis and have fluid aqueous interiors to mimic biological cells. However, a desire to more accurately replicate the gel-like properties of intracellular and extracellular biological environments has driven increasing efforts to build cell mimics based on hydrogels. This has enabled researchers to exploit some of the unique functional properties of hydrogels that have seen them deployed in fields such as tissue engineering, biomaterials and drug delivery. In this Review, we explore how hydrogels can be leveraged in the context of artificial cell development. We also discuss how hydrogels can potentially be incorporated within the next generation of artificial cells to engineer improved biological mimics and functional microsystems.

Investigation of the effect of solution pH value on rabbit corneal stroma biomechanics

PURPOSE

To facilitate the protection of corneal stability during corneal epithelium defects by determining the effect of solution pH on corneal stroma biomechanics.

METHODS

Thirty rabbit corneas were extracted, and the epithelium was scraped off. The samples were immediately subjected to inflation tests with pressures ranging from 0.3 to 6 kPa at baseline and in three subsequent test cycles. During a 10-min interval between cycles, specimens were randomly divided into four groups; in three of these groups, phosphate-buffered saline (PBS) drops with pH values of 6.9, 7.4, or 7.9 were applied to the surface once per minute, whereas the fourth group did not receive drops.

RESULTS

The corneal thickness significantly increased following the administration of PBS, while the corneal tangent modulus significantly decreased. At 2.5 and 4.5 kPa, the modulus reduction was significantly smaller in the specimens treated with pH 6.9 PBS than in those treated with pH 7.4 or 7.9 PBS, adjusted for changes in corneal thickness. Linear fitting of the pressure-modulus plot revealed that the regression coefficient significantly decreased over time. The reduction in the coefficient was most prominent in the PBS-treated groups, and the administration of pH 6.9 PBS elicited the smallest reduction among those three groups, adjusted for corneal thickness changes.

CONCLUSION

The study demonstrated that the administration of PBS drops with various pH values affected corneal biomechanics independent of corneal stromal swelling, and the impact of slightly acidic PBS was minimal. The effect became more prominent as posterior pressure increased. The research provides the basis for mediating the pH value of tear film and drops to maintain biomechanical stability of epithelium defects corneal stroma.

Hydrodynamic forces influence the gene transcription of mechanosensitive intercellular junction associated genes in corneal endothelial cells

Mechanicals forces are known to influence cell behavior. In vivo, the corneal endothelium is under the influence of various mechanical forces, such as intraocular pressure (IOP) and fluid flow. In this study, we used a corneal bioreactor to understand the effect of these hydrodynamic forces on the transcription of intercellular junctions associated genes in the corneal endothelium. Native and tissue-engineered (TE) corneal endothelium were cultured in a corneal bioreactor for 7 days with 16 mmHg IOP and 5 μl/ml of medium flow. RNA was harvested, and gene expression was quantified. Cells that were used to reconstruct the TE corneal endothelia were also seeded on plastic to characterize their morphology by calculating their circularity index. For native endothelia, hydrodynamic forces increased gene expression of GJA1 (connexin 43), CDH2 (N-cadherin), TJP1 (ZO-1), ITGAV (integrin subunit αv), ITGB5 (integrin subunit β5) and CTNND1 (p120-ctn) by 1.68 ± 0.40, 1.10 ± 0.27, 3.80 ± 0.56, 1.82 ± 0.33, 1.32 ± 0.21 and 3.04 ± 0.63, respectively. For TE corneal endothelium, this fold change was 1.72 ± 0.31, 1.58 ± 0.41, 6.18 ± 1.03, 1.80 ± 0.71, 1.77 ± 0.55, 2.42 ± 0.71. Furthermore, gene transcription fold changes (hydrodynamic/control) increased linearly with TE corneal endothelium cells population morphology with r = 0.83 for TJP1 (ZO-1) and r = 0.58 for CTNND1 (p120-ctn). In fact, the more elongated the cells populations were, the greater hydrodynamic conditions increased the transcription of TJP1 (ZO-1) and CTNND1 (p120-ctn). These results suggest that hydrodynamic forces contribute to the maintenance of tight and adherens junctions of native corneal endothelial cells, as well as to the formation of tight and adherens junctions of corneal endothelial cells that are in the process of forming a functional endothelial barrier.

The Contribution of Sulfated Glycosaminoglycans to the Inflation Response of the Human Optic Nerve Head

Purpose

In this study, we measured the effect of the removal of sulfated glycosaminoglycans (sGAGs) on the pressure-induced strains of the human lamina cribrosa (LC).

Methods

We applied an ex vivo inflation method to measure the three-dimensional (3D) deformation response of six human LCs to pressure, before and after the degradation of chondroitin and dermatan sulfates. The experiment used a laser-scanning microscope (LSM) to acquire the second harmonic generation (SHG) signal of the collagen structure in the LC. Digital volume correlation (DVC) was used to calculate the deformation in the LC after a change in pressure from 5 to 45 mm Hg.

Results

The average strains between 5 and 45 mm Hg in the LC decreased significantly after sGAG degradation (P ≤ 0.03), with the greatest change occurring in regions of previously high strain (P ≤ 0.003) and the peripheral regions of the LC (P ≤ 0.02). The stiffening effect was greater in the LC of middle-aged (42–49 years) donors compared with those of older (64–88 years) donors (P < 0.0001).

Conclusions

The LC experienced less strain at the same pressures after most sGAGs were removed. These results suggest that the natural decrease in sGAGs within the LC with age may contribute to the stiffer inflation response of older LC to IOP. Likewise, the increase in the amount of sGAGs observed in the LC of glaucomatous eyes, may contribute to a more compliant LC, which may affect the susceptibility and progression of axon damage.

The structural response of the cornea to changes in stromal hydration

The primary aim of this study was to quantify the relationship between corneal structure and hydration in humans and pigs. X-ray scattering data were collected from human and porcine corneas equilibrated with polyethylene glycol (PEG) to varying levels of hydration, to obtain measurements of collagen fibril diameter, interfibrillar spacing (IFS) and intermolecular spacing. Both species showed a strong positive linear correlation between hydration and IFS² and a nonlinear, bi-phasic relationship between hydration and fibril diameter, whereby fibril diameter increased up to approximately physiological hydration, H = 3.0, with little change thereafter. Above H = 3.0, porcine corneas exhibited a larger fibril diameter than human corneas (p < 0.001). Intermolecular spacing also varied with hydration in a bi-phasic manner but reached a maximum value at a lower hydration (H = 1.5) than fibril diameter. Human corneas displayed a higher intermolecular spacing than porcine corneas at all hydrations (p < 0.0001). Human and porcine corneas required a similar PEG concentration to reach physiological hydration, suggesting that the total fixed charge that gives rise to the swelling pressure is the same. The difference in their structural responses to hydration can be explained by variations in molecular cross-linking and intra/interfibrillar water partitioning.

The contribution of glycosaminoglycans to the mechanical behaviour of the posterior human sclera

We characterized the structural and mechanical changes after experimental digestion of sulfated glycosaminoglycans (s-GAGs) in the human posterior sclera, using ultrasound thickness measurements and an inflation test with three-dimensional digital image correlation (3D-DIC). Each scleral specimen was first incubated in a buffer solution to return to full hydration, inflation tested, treated in a buffer solution with chondroitinase ABC (ChABC), then inflation tested again. After each test series, the thickness of eight locations was measured. After enzymatic treatment, the average scleral thickness decreased by 13.3% (p < 0.001) and there was a stiffer overall stress-strain response (p < 0.05). The stress-strain response showed a statistically significant increase in the low-pressure stiffness, high-pressure stiffness and hysteresis. Thus, s-GAGs play a measurable role in the mechanical behaviour of the posterior human sclera.

A new method of detecting changes in corneal health in response to toxic insults

The size and arrangement of stromal collagen fibrils (CFs) influence the optical properties of the cornea and hence its function. The spatial arrangement of the collagen is still questionable in relation to the diameter of collagen fibril. In the present study, we introduce a new parameter, edge-fibrillar distance (EFD) to measure how two collagen fibrils are spaced with respect to their closest edges and their spatial distribution through normalized standard deviation of EFD (NSDEFD) accessed through the application of two commercially available multipurpose solutions (MPS): ReNu and Hippia. The corneal buttons were soaked separately in ReNu and Hippia MPS for five hours, fixed overnight in 2.5% glutaraldehyde containing cuprolinic blue and processed for transmission electron microscopy. The electron micrographs were processed using ImageJ user-coded plugin. Statistical analysis was performed to compare the image processed equivalent diameter (ED), inter-fibrillar distance (IFD), and EFD of the CFs of treated versus normal corneas. The ReNu-soaked cornea resulted in partly degenerated epithelium with loose hemidesmosomes and Bowman's collagen. In contrast, the epithelium of the cornea soaked in Hippia was degenerated or lost but showed closely packed Bowman's collagen. Soaking the corneas in both MPS caused a statistically significant decrease in the anterior collagen fibril, ED and a significant change in IFD, and EFD than those of the untreated corneas (p<0.05, for all comparisons). The introduction of EFD measurement in the study directly provided a sense of gap between periphery of the collagen bundles, their spatial distribution; and in combination with ED, they showed how the corneal collagen bundles are spaced in relation to their diameters. The spatial distribution parameter NSDEFD indicated that ReNu treated cornea fibrils were uniformly distributed spatially, followed by normal and Hippia. The EFD measurement with relatively lower standard deviation and NSDEFD, a characteristic of uniform CFs distribution, can be an additional parameter used in evaluating collagen organization and accessing the effects of various treatments on corneal health and transparency.

From nano to macro: studying the hierarchical structure of the corneal extracellular matrix

In this review, we discuss current methods for studying ocular extracellular matrix (ECM) assembly from the 'nano' to the 'macro' levels of hierarchical organization. Since collagen is the major structural protein in the eye, providing mechanical strength and controlling ocular shape, the methods presented focus on understanding the molecular assembly of collagen at the nanometre level using X-ray scattering through to the millimetre to centimetre level using non-linear optical (NLO) imaging of second harmonic generated (SHG) signals. Three-dimensional analysis of ECM structure is also discussed, including electron tomography, serial block face scanning electron microscopy (SBF-SEM) and digital image reconstruction. Techniques to detect non-collagenous structural components of the ECM are also presented, and these include immunoelectron microscopy and staining with cationic dyes. Together, these various approaches are providing new insights into the structural blueprint of the ocular ECM, and in particular that of the cornea, which impacts upon our current understanding of the control of corneal shape, pathogenic mechanisms underlying ectatic disorders of the cornea and the potential for corneal tissue engineering.

Swelling pressure and hydration behavior of porcine corneal stroma

PURPOSE

The aim of this study was to characterize swelling pressure-thickness, swelling pressure-hydration and hydration-thickness relations of porcine cornea.

METHODS

Mechanical compression tests and free swelling experiments were performed on porcine cornea. A rheometer (DHR-2, TA Instruments) with a thermally controlled fluid chamber filled with 0.9% NaCl solution was used to measure the equilibrium swelling pressure of (n = 17) corneal stromal specimens. The samples were compressed incrementally and their swelling pressure-thickness relations were obtained. In parallel to this investigation, a transient digital imaging microscope (H800-CL, American Scope Inc.), a USB autofocus camera (UM05, ViTiny), and a precision weighing scale (AGZN100, Torbal) were simultaneously used to measure the weight-thickness relation of (n = 8) corneal specimens. This experimental study gave the thickness-hydration relationship required for expressing swelling pressure measurements as a function of hydration.

RESULTS

At the in vivo 666 ± 68 µm central corneal thickness, an average swelling pressure of 52 ± 13 mmHg and hydration of 3.36 ± 0.25 mg H2O/mg dry tissue were found. The swelling pressure was reported as functions of both tissue thickness and hydration. The average fixed charge density of ρF/F ~ 42.8 mM and dry density of 1.47±0.15 g/cm3 were found. The thickness-hydration relationship was only linear when the tissue thickness was within the range of physiological thickness.

CONCLUSION

Overall, the physiological hydration and swelling pressure of the porcine cornea were within the same range of those reported previously for other mammalian corneas such as steers, rabbits and humans. Nevertheless, the thickness-hydration behavior of the porcine cornea was only similar to that of the human cornea.

The hydrogel nature of mammalian cytoplasm contributes to osmosensing and extracellular pH sensing

Cytoplasm is thought to have many hydrogel-like characteristics, including the ability to absorb large amounts of water and change volume in response to alterations in external environment, as well as having limited leakage of ions and proteins. Some gel-like behaviors have not been rigorously confirmed in mammalian cells, and others should be examined under conditions where gel volume can be accurately monitored. Thus, possible contributions of cytoplasm hydrogel properties to cellular processes such as volume sensing and regulation remain unclear. We used three-dimensional imaging to measure volume of single substrate-attached cells after permeabilization of their plasma membrane. Permeabilized cells swelled or shrinked reversibly in response to variations of external osmolality. Volume changes were 3.7-fold greater than observed with intact cells, consistent with cytoplasm's high water-absorbing capacity. Volume was maximal at neutral pH and shrunk at acidic or alkaline pH, consistent with pH-dependent changes of protein charge density and repulsive forces within cellular matrix. Volume shrunk with increased Mg(2+) concentration, as expected for increased charge screening and ionic crosslinking effects. Findings demonstrate that mammalian cytoplasm resembles hydrogel and functions as a highly sensitive osmosensor and extracellular pH sensor. Its high water-absorbing capacity may allow rapid modulation of local fluidity, macromolecular crowding, and activity of intracellular environment.

A physiological perspective on the swelling properties of the mammalian corneal stroma

PURPOSE

The present studies were designed to assess whether measurement of corneal stroma swelling in the laboratory, especially in non-physiological solutions, was associated with a measurable effect on the keratocytes.

METHODS

Complete corneal stroma preparations were made from quality- and age-selected recent post-mortem cattle eyes. These were either assessed immediately or incubated in three different solutions, namely a balanced salts solution with glucose (BSSG), isotonic phosphate buffered saline (PBS) or pure water. Incubations were carried out at 37 degrees C for 9h, and repeated measures of wet mass made so that the rates and extent of swelling could be determined. After incubation, an aqueous extract was made of the stroma for measurements of the levels the enzymes lactate dehydrogenase (LDH), aldehyde dehydrogenase (ALDH) and N-acetyl-beta-glucosaminidase.

RESULTS

The initial rates of swelling were lowest in BSSG, marginally faster in PBS and much faster in water. The secondary rates of swelling showed the same sequence being 10.0%/h in BSSG, 14.8%/h in PBS and 34.2%/h in water. Compared to non-incubated preparations, reductions in all three enzyme activities occurred. For LDH, these were 15% with BSSG, 40% in PBS and 80% with water. Similar results were seen with ALDH activity when comparing the three incubation solutions, while incubation in BSSG also resulted in a substantial (40%) reduction in N-acetyl-glucosaminidase activity.

CONCLUSIONS

When immersed in an isotonic BSSG with added glucose at 37 degrees C, the swelling of a complete bovine corneal stroma is much less than smaller pieces of stroma, and also slightly less than if isotonic PBS was used. With the use of BSSG, little or no change in cytoplasmic enzyme activities occurred, but measurable decreases were noted with PBS and very substantial decreases when water was used, indicating a toxic effect on the keratocytes. The observation that substantial decreases in a lysosomal enzyme activity could occur even with the use of BSSG indicate substantial stress is imposed on the stroma during these types of experiments. Notwithstanding, the data collectively indicate that the keratocyte cells within the collagen matrix of the stroma can be substantially damaged and this needs to be taken into account in future experiments on the true physiology of the corneal stroma.

Neutron and X-ray scattering by ox corneal stroma differentially loaded with bound anions

Ox corneas at near physiological hydration were subjected to two variables: the amount of chloride ions bound to them and exposure of various mixtures of H(2)O/D(2)O as solvent. The preparations were then exposed to a neutron beam and the contrast match points, at which the collagen fibrils of the corneal stroma most nearly matched the scattering density of the various H(2)O/D(2)O mixtures, were measured. In both cases of high and low bound chloride, the contrast match points of the collagen fibril were equal, indicating that there were no significant changes in the water of electrostriction at the fibril surface when chloride ions bind to the stroma. The data suggest that the ligands which bind anions to corneal stroma are not located at the collagen fibril surface. When the chloride binding ligands were extracted from the corneal stroma there were significant changes in the structure of the fibrils. We suggest that the chloride binding ligands may be located within the collagen fibril.

Transparency of the bovine corneal stroma at physiological hydration and its dependence on concentration of the ambient anion

De-epithelialised and de-endothelialised bovine corneal stromas with a hydration of 3.2 equilibrated at 154 mM NaCl and buffered at pH 7.4 had their optical density (400-750 nm) measured. Stromas equilibrated against 10, 20, 30, 50 or 100 mM NaCl made isotonic to 154 mM NaCl by supplementing with sorbitol were progressively more transparent as NaCl increased. Hypertonic equilibration against 300, 600 or 1000 mM NaCl resulted in a progressive loss of transparency compared with 154 mM NaCl. Light scattering as a function of wavelength fitted a lambda(-3) function well for 10, 30, 50, 100 and 154 mM NaCl preparations between 450 and 650 nm, but not at higher wavelengths. However, hypertonic 300, 600 and 1000 mM NaCl preparations showed a lambda(-2) dependence in the 450-750 nm range. Experiments with 154 mM NaCl and either 0 or 300 mM sorbitol suggested that the changes in light scattering in hypertonic preparations are unlikely to be caused by osmotic alterations to the stromal keratocytes. Psychophysical studies of the optical transmission function of preparations indicated that corneal stromas dialysed against 154 mM NaCl had usable optical properties, but preparations dialysed against 10 mM NaCl were effectively unable to transmit an image. The results are related to the known increase of fixed negative charge in the corneal matrix when chloride ions are adsorbed onto the matrix. It is suggested that the ordering force between corneal collagen fibrils, generated in part by anion binding, may be crucial to the physiological functioning of the visual system.

The effect of temperature on the Donnan potentials in biological polyelectrolyte gels: cornea and striated muscle

The effect of temperature on the fixed electric charge in biological polyelectrolyte gels was studied between 10 and 35 degrees C using the Donnan microelectrode technique. Two tissues; cornea and striated muscle were used. In cornea, there is a gentle and uniform decrease in fixed charge over the temperature range. In rigor muscle, there is a dramatic step-function decrease in charge at around 28 degrees C. There is a charge decrease in relaxed muscle at around the same temperature, but the step function is less distinct. The significance of these different experimental relationships is discussed in relation to the Saroff model for ion binding to proteins, linked to the possible disordering effects of excess electric charge. The diverse effects in these systems are important for the physiological functions of the different tissues.

The use of X-ray scattering techniques to determine corneal ultrastructure

The manner in which X-rays are scattered or diffracted by the cornea provides us with valuable insights into the fine structure of the corneal stroma. This is because when X-rays pass through a cornea a diffraction pattern is formed due to scattering from regularly arranged collagen molecules and fibrils that comprise the bulk of the stromal matrix. Collagen provides the cornea with most of its strength, and its proper organisation is believed to be important for tissue transparency. Ever since 1978, when the first X-ray diffraction patterns were obtained from the cornea using radiation from a powerful synchrotron source, biophysicists have recorded and analysed a huge number of X-ray diffraction patterns from many different corneas. This article aims to explain the ideas that underpin our use of X-ray diffraction to investigate corneal ultrastructure, and show how the knowledge gained to date has far-reaching implications for tissue biomechanics, disease changes and transparency.

Swelling of the collagen-keratocyte matrix of the bovine corneal stroma ex vivo in various solutions and its relationship to tissue thickness

AIM

The mammalian corneal stroma, like some other connective tissues, can absorb fluid, swell and become oedematous. Since studies on the corneal stroma have been carried out with different types of preparations and solutions, inter-study comparisons are very difficult. A study was thus undertaken on a standardised preparation to assess the relative magnitude of this swelling and its relationship to thickness of the preparations.

METHODS

From selected recent post-mortem eyes of adult cattle, stroma preparations were cut from the central part of the cornea. These preparations were immersed in various solutions of known pH and osmolality, and the time-dependent changes in wet mass were assessed over 9 h at 37 degrees C. The relative rates and magnitude of the swelling of the tissue were then compared.

RESULTS

A reference value for stromal swelling was obtained by incubation in a 35 mM bicarbonate-buffered mixed salts solution equilibrated with 5% CO2-air (pH 7.60) where a 3.39-fold increase in wet mass and a 4.58-fold increase in thickness was realised in 9 h, at an initial rate of 76 +/- 3%/h. The swelling was essentially the same in an organic buffer-mixed salt solution (pH 7.5) but progressively greater in phosphate-buffered saline (pH 7.5), a range of phosphate buffers (10-67 mM, pH 7.5), NaCl solutions (0.025-1%) and with gross swelling observed in water (where a 15.9-fold increase in wet mass occurred along with a 25-fold increase in thickness, at an initial rate of 643 +/- 62%/h). Overall, the wet mass changes were strongly related to thickness (P < 0.001).

CONCLUSIONS

The results confirm that the selection of solution(s) for studies on corneal stromal swelling is critical. The swelling (oedema) is lower in a physiologically-relevant solution (similar to the aqueous humour of the eye). This indicates that the swelling tendency of the corneal stroma has been overestimated in the past, and that a similar discrepancy may also exist for studies on other connective tissues ex vivo when non-physiological experimental solutions are used.

The effect of hydration and matrix composition on solute diffusion in rabbit sclera

There is increasing interest in the possibility for drug delivery into the vitreous humor across the conjunctiva and sclera as an alternative route to the conjunctiva-cornea pathway. As a preliminary to human studies we have investigated the influence of scleral composition and hydration on solute transport in the rabbit sclera. Intermuscular sclera was excised from adult New Zealand rabbits. Tissue samples were either examined directly (controls), digested using chondroitinase ABC or crosslinked using glutaraldehyde. The effect of these treatments on the ultrastructural appearance of the sclera was assessed. Diffusion and partition coefficients for solutes of different molecular weights [sodium chloride (23 MW),(14)C sucrose (342 MW) and dextran-fluoresceins (3, 10, 40 and 70 kDa)] were measured in relation to tissue treatment. The results were used to determine the effect of tissue structure and composition on solute movement. We have found that: (1) diffusion and partition coefficients are sensitive to solute MW, decreasing as MW increases; (2) diffusion and partition coefficients are sensitive to tissue hydration, increasing as hydration increases; (3) crosslinking of the sclera by glutaraldehyde reduced the partition coefficients significantly for solutes with MW over 3 kDa; and (4) removal of glycosaminoglycans has only a small effect on either diffusion or the partition coefficient.

Re-assessment of the potential impact of physiologically relevant pH changes on the hydration properties of the isolated mammalian corneal stroma

The pH sensitivity of the swelling of the mammalian corneal stroma was reinvestigated to assess whether or not there were detectable differences in the hydration properties of this collagen-keratocyte matrix within a physiologically relevant range (as opposed to extremes of acid or alkaline pH) and at a physiologically relevant temperature. From recent post-mortem eyes of adult cows, square (8 x 8 mm) samples of corneal stroma were prepared and incubated in an isotonic, buffered (HEPES etc.), mixed salts solution with added glucose at 37 degrees C. The time-dependent changes in wet mass were assessed over 24 h. The rate and magnitude of stromal swelling were different within the range of pH 6.5-8.5. The wet mass of stromal samples increased almost 2-fold within 1 h, and then at lesser rates to realise 3.25-3.75-fold and 4-5-fold increases in wet mass by 9 h and 24 h respectively. The maximum increases were observed at pH 7.25-7.5, with most of the effect being the result of differences in the initial rate of swelling. The discontinuous swelling and the pH effect on the rates of swelling were also evident when the data were fitted to a previous kinetic model (Elliott et al., J. Physiol. (Lond.) 298 (1980) 453-470). It is concluded that pH changes in the physiological range can have a small but reproducible impact on the swelling kinetics of the isolated mammalian corneal stroma ex vivo.

Swelling studies on the cornea and sclera: the effects of pH and ionic strength

The biophysical properties of the cornea and sclera depend on the precise maintenance of tissue hydration. We have studied the swelling of the tissues as a function of pH and ionic strength of the bathing medium, using an equilibration technique that prevents the loss of proteoglycans during swelling. Synchrotron x-ray diffraction was used to measure the average intermolecular and interfibrillar spacings, the fibril diameters, and the collagen D-periodicity. We found that both tissues swelled least near pH 4, that higher hydrations were achieved at lower ionic strengths, and that sclera swelled about one-third as much as cornea under most conditions. In the corneal stroma, the interfibrillar spacing increased most with hydration at pH values near 7. Fibril diameters and D-periodicity were independent of tissue hydration and pH at hydrations above 1. Intermolecular spacings in both tissues decreased as the ionic strength was increased, and there was a significant difference between cornea and sclera. Finally, we observed that corneas swollen near pH 7 transmitted significantly more light than those swollen at lower pH levels. The results indicate that the isoelectric points of both tissues are close to pH 4. The effects of ionic strength can be explained in terms of chloride binding within the tissues. The higher light transmission achieved in corneas swollen at neutral pH may be related to the fact that the interfibrillar fluid is more evenly distributed under these conditions.

Assessment of the effects of cetylpyridium chloride on water content of the collagen-keratocyte matrix of the mammalian corneal stroma ex vivo

The effects of cationic surfactants on the time-dependent increases in hydration of the corneal stroma were investigated to assess if the contribution of the proteoglycans could be titrated and how it might relate to the maximum and minimum swelling properties of the corneal stroma. From recent post-mortem eyes from adult sheep, square (8 x 8 mm) samples of corneal stroma were prepared and incubated in isotonic neutral pH mixed salts solution with added glucose, or pure water, at 37 degrees C. The time-dependent changes in wet mass were assessed over 24 h in the absence or presence of 0. 001-2% w/v cetylpyridium chloride (CPC) or benzalkonium chloride (BAC). The rate and magnitude of stromal swelling was reduced in a concentration-dependent fashion by the surfactants. In mixed salts solution, 100% inhibition of swelling could be achieved at 2% CPC and BAC. In pure water, the relative swelling was much more substantial and could only be attenuated by CPC.

The filament lattice of striated muscle

The filament lattice of striated muscle is an overlapping hexagonal array of thick and thin filaments within which muscle contraction takes place. Its structure can be studied by electron microscopy or X-ray diffraction. With the latter technique, structural changes can be monitored during contraction and other physiological conditions. The lattice of intact muscle fibers can change size through osmotic swelling or shrinking or by changing the sarcomere length of the muscle. Similarly, muscle fibers that have been chemically or mechanically skinned can be compressed with bathing solutions containing very large inert polymeric molecules. The effects of lattice change on muscle contraction in vertebrate skeletal and cardiac muscle and in invertebrate striated muscle are reviewed. The force developed, the speed of shortening, and stiffness are compared with structural changes occurring within the lattice. Radial forces between the filaments in the lattice, which can include electrostatic, Van der Waals, entropic, structural, and cross bridge, are assessed for their contributions to lattice stability and to the contraction process.

In vitro hydration kinetics of recent post-mortem tissue versus pre-dried corneal stromal tissue

Both recent post-mortem and pre-dried corneal tissue has been used for laboratory studies of stromal swelling, but it has yet to be defined whether the same hydration (H value, mg H2O/mg dry mass) is obtained after extended re-hydration. Fresh or pre-dried pieces (8x8 mm squares) of ovine stromas were immersed in 1% NaCl for 24 hr at 37 degrees C, with the wet mass assessed regularly. Pre-drying was achieved in air for 7 days (with sulphuric acid, CaSO4 or silica gel as desiccants), or in an oven for 24 hr at 60, 70 or 80 degrees C. Fresh stroma preparations (</=2 hr post-mortem) had initial H values of 3.1, which increased to 27.8 after 24 hr in saline. After pre-drying over sulphuric acid, CaSO4 or silica gel, the H values after 24 hr in saline were 19.1, 13.1 and 7.3 respectively (all statistically different from fresh tissue (P<0.01). Following oven drying at 60, 70, and 80 degrees C, final H values of 8.5, 6.6, and 5.0 were obtained after 24 hr in saline. Recent post-mortem material showed sustained swelling over at least 24 hr. For pre-dried tissue, initial rates of hydration change over 1 hr were higher, but subsequent rates were substantially less after 2 hr. These studies indicate that fresh tissue should be used in studies concerning the swelling properties of the corneal stroma.

Chloride binding in the stroma of cultured human corneas

In the ox cornea, more than half of the non-diffusible, matrix negative charge is derived from the binding of free chloride ions. Because the magnitude of the net matrix charge is the dominant factor which determines the degree of stromal swelling, we investigated whether this phenomenon, stromal chloride binding, also occurs in human corneal stroma. Intrastromal ion concentrations were measured with radio-isotopes when human (outdated Eye Bank) corneas or (fresh) bovine corneas, physically clamped to maintain a constant hydration, were incubated in buffered 154 mM NaCl. The intrastromal chloride ion concentration was compared to the normalized concentrations of trace quantities of radio-labelled acetate and lactate ions. For human corneas, the intrastromal chloride ion concentration was found to be significantly higher (P < 0.001, t-test) than the normalized concentrations of both acetate and lactate ([Cl]i = 142.5 +/- 0.9 mM, (n = 9); [acetate]i = 131.2 +/- 1.2 mM, (n = 8); [lactate]i = 131.9 +/- 1.5 mM, (n = 5); all values are mean +/- S.E.M.). The sodium ion concentration was elevated ([Na]i = 176.0 +/- 1.8 mM, (n = 9)). These results demonstrate that chloride binding occurs to a significant extent in cultured human corneal stroma and suggest that chloride binding may be evident in the native human cornea.

In situ fluorescence spectroscopic studies on bovine cornea

The cornea is a transparent ocular tissue and its transparency is thought to be a result of intramolecular interactions and the supramolecular organization of its protein constituents. We have studied the intrinsic fluorescence properties of intact bovine corneas and compared these with that of the opaque sclera. It was observed that with increasing excitation wavelengths the emission maxima shifted toward the red edge exhibiting the phenomenon of red edge excitation shift, which is indicative of immobilization of the constituent fluorophores. The magnitude of the shift increased after photodamage by irradiation at 295 nm. Many of the spectral characteristics of the cornea are shown to be due to its proteoglycans, which show surprisingly significant red edge excitation shift in solution.

Effects of irrigation solutions on corneal endothelial function

Rabbit corneas were perfused in vitro with an irrigation solution for 90 minutes. This was followed by 6 hours of perfusion with tissue culture medium TC199 during which endothelial function was assessed by monitoring rates of swelling during a period of perfusion in the absence of bicarbonate ions, and subsequent rates of thinning when bicarbonate ions were restored to the perfusate. Corneal thickness (measured with an ultrasonic pachymeter) immediately following excision was 401 microns (SD 19, n = 23). During the 90 minute perfusion at 35 degrees C, corneas exposed to balanced salt solution (BSS), Hartmann's solution or 0.9% NaCl (all initially at room temperature) swelled, respectively, at 14 (SD 2.3, n = 4), 11 (SD 2.6, n = 4), and 70 (SD 4.3, n = 4) microns/h. Cold Hartmann's solution (initially at 4 degrees C) caused corneas to swell at 9 (SD 2.3, n = 4) microns/h. On the other hand, corneas perfused with BSS Plus thinned at 9 (SD 3.4, n = 4) microns/h and TC199 with Earle's salts had little effect on thickness. Rates of swelling and thinning during the following assessment perfusion showed no apparent effects of prior exposure to any of the irrigation solutions on the barrier properties or pump function of the endothelium. Despite this, the increased thickness of corneas exposed initially to BSS, cold Hartmann's solution, or 0.9% NaCl was not fully reversed, even by the end of the 6 hour assessment perfusion. In contrast, the swelling observed in corneas exposed to Hartmann's solution at room temperature was reversed and these corneas had returned to their normal thickness by the end of the assessment period. All corneas, even those exposed to 0.9% NaCl, had an intact endothelial mosaic with no evidence of damage or cell loss, although morphological differences in cell shape and the appearance of cell borders were evident compared with freshly isolated cornea.

Chloride is required for fluid transport by the rabbit corneal endothelium

The role of chloride in fluid transport of the rabbit corneal endothelium was examined by measuring changes in corneal thickness following ion substitutions or addition of ion transport inhibitors in media superfusing the isolated tissue. Normal fluid transport is indicated by maintenance of constant thickness in a fresh cornea or thinning (deturgescence) of a preswollen deepithelialized cornea to its initial thickness at approximately 40 microns/h. These patterns are seen when tissues are superfused with HCO(3-)-Ringer containing 114 mM Cl-. When Cl- was substituted with gluconate, glucuronate, or SO4(2-) fresh and preswollen corneas immediately thinned at greater than 150 microns/h to a value less than 300 microns and then began to swell at 30 microns/h to above their original thickness. Substitution of Cl- with NO3- or Br- had a negligible immediate thinning effect, but fresh corneas subsequently swelled and preswollen corneas failed to deturgesce fully. The rapid thinning (called a "downtransient") observed with gluconate, glucuronate, and SO4(2-) also occurred in these media when ion and fluid transport were completely inhibited with ouabain or stilbenes or by absence of HCO3-, indicating that the thinning results from osmotic gradients induced by ionic reflection coefficients different from that of Cl-. When the downstransient was avoided in deepithelialized corneas by preswelling with the same Cl(-)-free media on both sides of the cornea, corneas maintained a constant but swollen thickness in gluconate and in NO3- or Br- deturgesced slowly and incompletely; ouabain or stilbenes caused further swelling in all media. We conclude that absence of Cl- partially impairs fluid transport, most probably via its role in a Cl(-)-HCO3- exchanger which has been proposed in a recent model of endothelial fluid transport.

Transient chloride binding as a contributory factor to corneal stromal swelling in the ox

1. Investigations were made of the cation exchange capacity of fresh isolated ox corneal stroma (Q, units: mequiv fixed stromal charge/kg stromal fluid) at pH 7.4 over a variety of stomal hydrations (H, units: kg stromal fluid/kg dry tissue) both above and below the physiological hydration of 3.2, whilst the stromas were immersed in a variety of sodium chloride solutions (range 5-1000 mM). 2. At any particular salt concentration, the product QH (dry tissue exchange capacity, units: mequiv/kg dry tissue) appeared constant, over all the hydrations investigated. 3. Dry tissue exchange capacity (QH) varied, however, when the bathing salt concentration was altered. It varied between 55 mequiv/kg dry tissue (e.g. Q = 17 mequiv at H = 3.2) in 5 mM-NaCl to 240 mequiv/kg dry tissue (e.g. Q = 75 mequiv/l at H = 3.2) in 1000 mM-NaCl. 4. The variation of stromal exchange capacity in NaCl solutions of different concentrations was similar when detected by three independent procedures: stromal gel pressure measurements, intrastromal sodium ion distributions, and intrastromal electrical potentials. 5. Intrastromal chloride ion distributions were anomalous. Total chloride (measured by radio-isotopes) was consistently higher than that predicted by Donnan theory. 6. The data were consistent with Elliott's hypothesis that a fraction of intrastromal chloride ions bind to the corneal stromal matrix and in so doing contribute to the fixed negative charge of the stroma. 7. Our observations may be explained by a model of the cation exchange capacity of ox cornea which has two types of components. On is (at constant pH) invariant, and has a dry tissue exchange capacity of about 50 mequiv/kg dry tissue, and is probably generated by the sulphonic and carboxylic acid groups of the glycosaminoglycans. The other is explained by supposing it to consist of a chloride binding ligand which exhibits first order binding, is half occupied at ambient chloride concentrations of 300 mM, and has a total capacity of 240 mequiv/kg dry tissue. 8. Partial stromal extraction with 4 M-guanidine HCl indicated that the chloride binding ligand is not associated with the collagen molecules in the corneal stromal fibrils. 9. It is suggested that such a stromal chloride ion binding ligand would help to stabilize the hydration and transparency of the living cornea when it is exposed to environments of varying tonicity (such as in river or sea bathing).

Chloride concentrations in the corneal epithelium and stroma from different species

Using ion chromatography, the concentration of chloride in the corneal stroma and epithelium of ox, pig and rabbit was measured to detect possible species differences. In all three species, the chloride concentration in the stroma was found to be very similar, i.e. 363 +/- 14 mmol kg-1 dry wt in ox, 364 +/- 18 mmol kg-1 dry wt in pig and 358 +/- 24 mmol kg-1 dry wt in rabbit. In contrast, a substantially higher concentration of 232 +/- 33 mmol kg-1 dry wt was found in the corneal epithelium of rabbit in comparison to the values determined in the same tissue of ox, i.e. 137 +/- 4 mmol kg-1 dry wt and of pig, i.e. 128 +/- 24 mmol kg-1 dry wt. Using the results obtained previously for the extracellular volume (Midelfart 1988), an intracellular chloride concentration of 25 +/- 2 mmol kg-1 H2O was calculated in the bovine corneal epithelium. The results obtained in this study indicate that species differences are considered as important in studies of ion transport mechanisms in the cornea.

The measurement of ox corneal swelling pressure by osmometry

1. Ox corneal stromal swelling pressure (gel pressure) may be measured by osmometry: polyethylene glycol of nominal molecular mass 10,000 Da (PEG 10K) is a suitable non-penetrating solute. 2. Corneal hydrations equilibrate within 4 h of exposure to 154 mM-NaCl including various concentrations (2-8%) of PEG 10K, providing that the epithelium covers the anterior surface and Descemet's membrane covers the posterior surface. At equilibrium hydration, corneal gel pressure equals the external osmotic pressure contributed by PEG 10K. 3. The osmotic pressure of PEG 10K may be calibrated using Descemet's membrane as the semi-permeable membrane. 4. Corneal gel pressure decreases with increasing hydration. 5. The relationship may be adequately explained by the Donnan theory of corneal swelling with a fixed negative matrix charge of 39.5 +/- 0.8 mequiv l-1 at physiological hydration of 3.2 at this salt concentration (154 mM-NaCl).

Application of catastrophe theory to corneal swelling

Stromal swelling in human, cat, and rabbit cornea is biphasic, interpretable as an elementary cusp catastrophe proposed by Thom, with t* = log t and Q* = log Q (stromal charge Q, time t) as control parameters, and H0.5 (hydration H) as the state variable. A thermodynamic potential with two attractor regions, each with a local minimum, governs corneal stromal swelling. Transitions follow a 'saturation convention' whereby the second minimum is preferred upon availability. Corneal swelling is an example of a space-equivalent unfolding, where the transition plane moves in time. It is proposed that the transition plane coincides with the uncoupling of interfibrillary linkages or 'springs' in the corneal stroma, and is associated with a critical hydration of ca. 10 kg H2O per kilogram dry mass, and stromal charge ca. 1 x 10(-7) mol electrons.

Theoretical basis for an anomalous temperature coefficient in swelling pressure of rabbit corneal stroma

In the rabbit corneal stroma, the swelling pressure, P, has been reported to have an anomalous (negative) temperature coefficient, alpha P, contradicting traditional Donnan swelling theory. A parallel-plate, diffuse double layer Gouy-Chapman model was used to resolve this discrepancy. The present model incorporates the possibility that surface charge, sigma, is temperature dependent. It is shown that negative, zero, or positive coefficients of swelling pressure change with temperature are not mutually exclusive conditions, but can be attributed to the same underlying mechanism. For likely values of alpha P(range -7 x 10(-3) K-1 to +3.2 x 10(-3)K-1), the effective stromal charge has a negative temperature dependency, or dln sigma/dT less than 0. The present formalism is robust against variation in assumed alpha P, and is able to simultaneously satisfy the known values of swelling pressure, its thermal dependency, and stromal charge. These results implicate significant coulombic forces behind P. Predicted stromal surface charge is approximately 0.01 Cm-2. The predictions were confirmed with macrocontinuum Donnan swelling theory, suggesting that Donnan osmotic swelling is the principal macroscopic component of P.

The swelling pressure of the human corneal stroma as determined by a new method

The swelling pressure of 115 human corneas was determined using a modified electronic balance modified to simultaneously record the swelling force and the thickness of the stroma. The swelling force was found to follow a straight-line dependence on the stromal thickness when plotted in a double logarithmic scale, which means that the swelling pressure of each cornea could be expressed by a power fit of the form SP = aTb, where SP is swelling pressure, T stromal thickness, and a and b are constants of the cornea. In 45 control corneas swollen in 0.9% NaCl, pH 7.4, 0.01 M Hepes buffer, the mean value (+/- S.D.) of 'a' and 'b' were 7.09 mmHg mm-1 (+/- 2.96) and -3.48 (+/- 0.20), respectively. This corresponded to a mean swelling pressure of 84.0 mmHg at a standard stromal thickness of 0.5 mm. In paired experiments, the swelling pressure was found to be influenced insignificantly (P greater than 0.05) by a number of conditions, including lowering the pH to 4.0, increasing the temperature to 37 degrees C, and increasing the NaCl concentration to 9%. A significant correlation was found between the swelling pressure and the dry weight of the specimen (P less than 0.05), indicating a considerable biological variation of the swelling pressure. It is shown, that this variation may explain the normal variation in human corneal thickness in vivo.

Swelling pressure in bovine cornea determined by dialysis method

The swelling pressure of 62 +/- 2 mmHg and the fixed charge concentration of 15.7 +/- 1.9 mmol/kg H2O were determined in bovine corneal stroma using a dialysis method. The hydration of the dialyzed stromal tissues was independent of ambient Na concentration. The same relationship was observed by incubation of whole corneas mounted in modified Ussing chambers.

Differences in the fibril structure of corneal and tendon collagen. An electron microscopy and X-ray diffraction investigation

A detailed analysis of the D-period and axial electron density distribution of cornea and tendon collagen was carried out by means of X-ray diffraction and electron microscopy. Ultrastructural observations were made on replicas of freeze fractured and deep-etched specimens. Synchrotron radiation was used to obtain high resolution small angle X-ray diffraction patterns. The data provide evidence that D-period and intraperiod distances in cornea are shorter than in tendon collagen fibrils. The observed different banding observed is interpreted on the basis of the different morphological arrangement of the microfibrils in the two tissues: "helicoidal" in cornea and "straight" in tendon microfibrils.

Donnan equilibrium in the tectorial membrane

Basic properties of the isolated tectorial membrane were investigated. Pieces of the membrane were equilibrated in various bathing fluids and the potential difference between the matrix of the membrane and the bathing fluid was measured, using micropipette electrodes. A negative potential was recorded within the membrane, and the magnitude of the potential was dependent upon the pH and ionic strength of the bathing fluid. These observations suggest that a Donnan equilibrium is established under these experimental conditions, and demonstrate that the tectorial membrane acts as a separate phase in vitro. It may not be valid, therefore, to assume that the membrane is electrically and ionically transparent in vivo.

The axial electron density in collagen fibrils from human corneal stroma

Low angle synchrotron X-ray diffraction patterns were obtained from the demembranated human corneal stroma. These patterns showed differences in the relative intensities of the meridional reflections compared to those obtained using bovine corneas. In particular, the first order reflection, conspicuously absent in the bovine pattern, is present in the human, although it is still only one third as intense as the first order in the pattern from rat tail tendon. Using phases deduced from the electron-optical images of negatively stained corneal collagens, a difference electron density map between bovine and human corneal collagen was drawn. The features in this map are explained in terms of a different inter-relationship between collagen and proteoglycans in the two species. The function of these species differences is unknown.

Endothelial physiology and intraocular lens implantation

The endothelium is the cellular monolayer which lines the posterior surface of the cornea. This layer is important in clinical ophthalmology because it is vital to maintenance of the transparency of the cornea and vision through its pump and barrier functions which limit the ingress of fluid into the cornea from the aqueous. When the function of the corneal endothelium becomes compromised, the corneal stroma swells as it hydrates. Subsequently, epithelial bullae form with painful recurring epithelial erosions, and finally corneal scarring and blindness result. The relatively vulnerable position of the corneal endothelium renders it susceptible to iatrogenic injury during intraocular procedures, especially IOL implantation: the poor regenerative (mitotic) capacity of the human corneal endothelium limits its ability to recover normal function once it is injured.

A model of epithelial water transport. The corneal endothelium

To try to understand how an epithelial tissue can transport water between bathing solutions of equal tonicity and how intracellular solute and protein concentration are related to the structural specialization of the cell membrane at its apical, basal, and lateral margins, we have formulated and solved, using approximate analytical techniques, a new model which combines the detailed transport of local osmotic flow in extracellular channel with the multicompartment approach of thermodynamic models requiring the overall conservation of water and solute for the entire cell layer. Thus, unlike most previous models, which dealt exclusively with either the average properties of the cell layer or the local transport in the extracellular channel, we are able to solve simultaneously for the interaction of the cell with its environments across its apical, basal, and lateral cell membranes as well as the detailed transport in the extracellular channel. The model is then applied to corneal endothelium to obtain new insight into the water flow movement in this tissue under in vitro and in vivo conditions. Then in vitro solution shows that the cell at 297 mosmol/liter is slightly hypotonic to the 300-mosmol/liter external bathing solutions which drive water equally out both the aqueous (apical) and stromal (basal) cell faces. This water is replaced from the extracellular channel. There is a net flow of water because more water enters the channel through its open stromal end than through the higher resistance tight junction. In vivo, the solution predicts that the stromal swelling pressure forces water through the tight junctions towards the stroma so that there is no net flow. The interesting new features of our solution are the water recirculation pattern and the role of the osmotically active proteins in making the cell hypertonic relative to the channel.