Biomechanics of Schlemm's canal endothelium and intraocular pressure reduction

Ocular hypertension in glaucoma develops due to age-related cellular dysfunction in the conventional outflow tract, resulting in increased resistance to aqueous humor outflow. Two cell types, trabecular meshwork (TM) and Schlemm's canal (SC) endothelia, interact in the juxtacanalicular tissue (JCT) region of the conventional outflow tract to regulate outflow resistance. Unlike endothelial cells lining the systemic vasculature, endothelial cells lining the inner wall of SC support a transcellular pressure gradient in the basal to apical direction, thus acting to push the cells off their basal lamina. The resulting biomechanical strain in SC cells is quite large and is likely to be an important determinant of endothelial barrier function, outflow resistance and intraocular pressure. This review summarizes recent work demonstrating how biomechanical properties of SC cells impact glaucoma. SC cells are highly contractile, and such contraction greatly increases cell stiffness. Elevated cell stiffness in glaucoma may reduce the strain experienced by SC cells, decrease the propensity of SC cells to form pores, and thus impair the egress of aqueous humor from the eye. Furthermore, SC cells are sensitive to the stiffness of their local mechanical microenvironment, altering their own cell stiffness and modulating gene expression in response. Significantly, glaucomatous SC cells appear to be hyper-responsive to substrate stiffness. Thus, evidence suggests that targeting the material properties of SC cells will have therapeutic benefits for lowering intraocular pressure in glaucoma.

Shear stress-triggered nitric oxide release from Schlemm's canal cells

PURPOSE

Endothelial nitric oxide (NO) synthase is regulated by shear stress. At elevated intraocular pressures when the Schlemm's canal (SC) begins to collapse, shear stress is comparable with that in large arteries. We investigated the relationship between NO production and shear stress in cultured human SC cells.

METHODS

Schlemm's canal endothelial cells isolated from three normal and two glaucomatous human donors were seeded into Ibidi flow chambers at confluence, cultured for 7 days, and subjected to steady shear stress (0.1 or 10 dynes/cm(2)) for 6, 24, or 168 hours. Cell alignment with flow direction was monitored, and NO production was measured using 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) and Griess reagents. Human trabecular meshwork (TM) and umbilical vein endothelial cells (HUVECs) were used as controls.

RESULTS

Normal SC strains aligned with the direction of flow by 7 days. Comparing 0.1 vs. 10 dynes/cm(2), NO levels increased by 82% at 24 hours and 8-fold after 7 days by DAF-FM, and similar results were obtained with Griess reagent. Shear responses by SC cells at 24 hours were comparable with HUVECs, and greater than TM cells, which appeared shear-insensitive. Nitric oxide production by SC cells was detectable as early as 6 hours and was inhibited by 100 μM nitro-L-arginine methyl ester. Two glaucomatous SC cell strains were either unresponsive or lifted from the plate in the face of shear.

CONCLUSIONS

Shear stress triggers NO production in human SC cells, similar to other vascular endothelia. Increased shear stress and NO production during SC collapse at elevated intraocular pressures may in part mediate IOP homeostasis.

The application of multiple biophysical cues to engineer functional neocartilage for treatment of osteoarthritis. Part II: signal transduction

The unique mechanoelectrochemical environment of cartilage has motivated researchers to investigate the effect of multiple biophysical cues, including mechanical, magnetic, and electrical stimulation, on chondrocyte biology. It is well established that biophysical stimuli promote chondrocyte proliferation, differentiation, and maturation within "biological windows" of defined dose parameters, including mode, frequency, magnitude, and duration of stimuli (see companion review Part I: Cellular Response). However, the underlying molecular mechanisms and signal transduction pathways activated in response to multiple biophysical stimuli remain to be elucidated. Understanding the mechanisms of biophysical signal transduction will deepen knowledge of tissue organogenesis, remodeling, and regeneration and aiding in the treatment of pathologies such as osteoarthritis. Further, this knowledge will provide the tissue engineer with a potent toolset to manipulate and control cell fate and subsequently develop functional replacement cartilage. The aim of this article is to review chondrocyte signal transduction pathways in response to mechanical, magnetic, and electrical cues. Signal transduction does not occur along a single pathway; rather a number of parallel pathways appear to be activated, with calcium signaling apparently common to all three types of stimuli, though there are different modes of activation. Current tissue engineering strategies, such as the development of "smart" functionalized biomaterials that enable the delivery of growth factors or integration of conjugated nanoparticles, may further benefit from targeting known signal transduction pathways in combination with external biophysical cues.

Biomechanical strain as a trigger for pore formation in Schlemm's canal endothelial cells

The bulk of aqueous humor passing through the conventional outflow pathway must cross the inner wall endothelium of Schlemm's canal (SC), likely through micron-sized transendothelial pores. SC pore density is reduced in glaucoma, possibly contributing to obstructed aqueous humor outflow and elevated intraocular pressure (IOP). Little is known about the mechanisms of pore formation; however, pores are often observed near dome-like cellular outpouchings known as giant vacuoles (GVs) where significant biomechanical strain acts on SC cells. We hypothesize that biomechanical strain triggers pore formation in SC cells. To test this hypothesis, primary human SC cells were isolated from three non-glaucomatous donors (aged 34, 44 and 68), and seeded on collagen-coated elastic membranes held within a membrane stretching device. Membranes were then exposed to 0%, 10% or 20% equibiaxial strain, and the cells were aldehyde-fixed 5 min after the onset of strain. Each membrane contained 3-4 separate monolayers of SC cells as replicates (N = 34 total monolayers), and pores were assessed by scanning electron microscopy in 12 randomly selected regions (∼65,000 μm(2) per monolayer). Pores were identified and counted by four independent masked observers. Pore density increased with strain in all three cell lines (p < 0.010), increasing from 87 ± 36 pores/mm(2) at 0% strain to 342 ± 71 at 10% strain; two of the three cell lines showed no additional increase in pore density beyond 10% strain. Transcellular "I-pores" and paracellular "B-pores" both increased with strain (p < 0.038), however B-pores represented the majority (76%) of pores. Pore diameter, in contrast, appeared unaffected by strain (p = 0.25), having a mean diameter of 0.40 μm for I-pores (N = 79 pores) and 0.67 μm for B-pores (N = 350 pores). Pore formation appears to be a mechanosensitive process that is triggered by biomechanical strain, suggesting that SC cells have the ability to modulate local pore density and filtration characteristics of the inner wall endothelium based on local biomechanical cues. The molecular mechanisms of pore formation and how they become altered in glaucoma may be studied in vitro using stretched SC cells.

Ultrastructural changes associated with dexamethasone-induced ocular hypertension in mice

PURPOSE

To determine whether dexamethasone (DEX)-induced ocular hypertension (OHT) in mice mimics the hallmarks of steroid-induced glaucoma (SIG) in humans, including reduced conventional outflow facility (C), increased extracellular matrix (ECM), and myofibroblasts within the outflow pathway.

METHODS

Osmotic mini-pumps were implanted subcutaneously into C57BL/6J mice for systemic delivery of DEX (3-4 mg/kg/d, n = 31 mice) or vehicle (n = 28). IOP was measured weekly by rebound tonometry. After 3 to 4 weeks, mice were euthanized and eyes enucleated for ex vivo perfusion to measure C, for electron microscopy to examine the trabecular meshwork (TM) and Schlemm's canal (SC), or for immunohistochemistry to examine type IV collagen and α-smooth muscle actin. The length of basement membrane material (BMM) was measured along the anterior-posterior extent of SC by electron microscopy. Ultrastructural changes in BMM of DEX-treated mice were compared against archived human SIG specimens.

RESULTS

Dexamethasone increased IOP by 2.6 ± 1.6 mm Hg (mean ± SD) over 3 to 4 weeks and decreased C by 52% ± 17% versus controls. Intraocular pressure elevation correlated with decreased C. Dexamethasone treatment led to increased fibrillar material in the TM, plaque-like sheath material surrounding elastic fibers, and myofibroblasts along SC outer wall. The length of BMM underlying SC was significantly increased in mice with DEX and in humans with SIG, and in mice decreased C correlated with increased BMM.

CONCLUSIONS

Dexamethasone-induced OHT in mice mimics hallmarks of human SIG within 4 weeks of DEX treatment. The correlation between reduced C and newly formed ECM motivates further study using DEX-treated mice to investigate the pathogenesis of conventional outflow obstruction in glaucoma.

Stimulation of chondrogenic differentiation of adult human bone marrow-derived stromal cells by a moderate-strength static magnetic field

Tissue-engineering strategies for the treatment of osteoarthritis would benefit from the ability to induce chondrogenesis in precursor cells. One such cell source is bone marrow-derived stromal cells (BMSCs). Here, we examined the effects of moderate-strength static magnetic fields (SMFs) on chondrogenic differentiation in human BMSCs in vitro. Cells were cultured in pellet form and exposed to several strengths of SMFs for various durations. mRNA transcript levels of the early chondrogenic transcription factor SOX9 and the late marker genes ACAN and COL2A1 were determined by reverse transcription-polymerase chain reaction, and production of the cartilage-specific macromolecules sGAG, collage type 2 (Col2), and proteoglycans was determined both biochemically and histologically. The role of the transforming growth factor (TGF)-β signaling pathway was also examined. Results showed that a 0.4 T magnetic field applied for 14 days elicited a strong chondrogenic differentiation response in cultured BMSCs, so long as TGF-β3 was also present, that is, a synergistic response of a SMF and TGF-β3 on BMSC chondrogenic differentiation was observed. Further, SMF alone caused TGF-β secretion in culture, and the effects of SMF could be abrogated by the TGF-β receptor blocker SB-431542. These data show that moderate-strength magnetic fields can induce chondrogenesis in BMSCs through a TGF-β-dependent pathway. This finding has potentially important applications in cartilage tissue-engineering strategies.

Biomechanics of the Posterior Eye: A Critical Role in Health and Disease

The posterior eye is a complex biomechanical structure. Delicate neural and vascular tissues of the retina, choroid, and optic nerve head that are critical for visual function are subjected to mechanical loading from intraocular pressure, intraocular and extraorbital muscles, and external forces on the eye. The surrounding sclera serves to counteract excessive deformation from these forces and thus to create a stable biomechanical environment for the ocular tissues. Additionally, the eye is a dynamic structure with connective tissue remodeling occurring as a result of aging and pathologies such as glaucoma and myopia. The material properties of these tissues and the distribution of stresses and strains in the posterior eye is an area of active research, relying on a combination of computational modeling, imaging, and biomechanical measurement approaches. Investigators are recognizing the increasing importance of the role of the collagen microstructure in these material properties and are undertaking microstructural measurements to drive microstructurally-informed models of ocular biomechanics. Here, we review notable findings and the consensus understanding on the biomechanics and microstructure of the posterior eye. Results from computational and numerical modeling studies and mechanical testing of ocular tissue are discussed. We conclude with some speculation as to future trends in this field.

In vivo optic nerve head biomechanics: performance testing of a three-dimensional tracking algorithm

Measurement of optic nerve head (ONH) deformations could be useful in the clinical management of glaucoma. Here, we propose a novel three-dimensional tissue-tracking algorithm designed to be used in vivo. We carry out preliminary verification of the algorithm by testing its accuracy and its robustness. An algorithm based on digital volume correlation was developed to extract ONH tissue displacements from two optical coherence tomography (OCT) volumes of the ONH (undeformed and deformed). The algorithm was tested by applying artificial deformations to a baseline OCT scan while manipulating speckle noise, illumination and contrast enhancement. Tissue deformations determined by our algorithm were compared with the known (imposed) values. Errors in displacement magnitude, orientation and strain decreased with signal averaging and were 0.15 µm, 0.15° and 0.0019, respectively (for optimized algorithm parameters). Previous computational work suggests that these errors are acceptable to provide in vivo characterization of ONH biomechanics. Our algorithm is robust to OCT speckle noise as well as to changes in illumination conditions, and increasing signal averaging can produce better results. This algorithm has potential be used to quantify ONH three-dimensional strains in vivo, of benefit in the diagnosis and identification of risk factors in glaucoma.

The design and development of a high-throughput magneto-mechanostimulation device for cartilage tissue engineering

To recapitulate the in vivo environment and create neo-organoids that replace lost or damaged tissue requires the engineering of devices, which provide appropriate biophysical cues. To date, bioreactors for cartilage tissue engineering have focused primarily on biomechanical stimulation. There is a significant need for improved devices for articular cartilage tissue engineering capable of simultaneously applying multiple biophysical (electrokinetic and mechanical) stimuli. We have developed a novel high-throughput magneto-mechanostimulation bioreactor, capable of applying static and time-varying magnetic fields, as well as multiple and independently adjustable mechanical loading regimens. The device consists of an array of 18 individual stations, each of which uses contactless magnetic actuation and has an integrated Hall Effect sensing system, enabling the real-time measurements of applied field, force, and construct thickness, and hence, the indirect measurement of construct mechanical properties. Validation tests showed precise measurements of thickness, within 14 μm of gold standard calliper measurements; further, applied force was measured to be within 0.04 N of desired force over a half hour dynamic loading, which was repeatable over a 3-week test period. Finally, construct material properties measured using the bioreactor were not significantly different (p=0.97) from those measured using a standard materials testing machine. We present a new method for articular cartilage-specific bioreactor design, integrating combinatorial magneto-mechanostimulation, which is very attractive from functional and cost viewpoints.

Pharmacologic manipulation of conventional outflow facility in ex vivo mouse eyes

PURPOSE

Mouse models are useful for glaucoma research, but it is unclear whether intraocular pressure (IOP) regulation in mice operates through mechanisms similar to those in humans. Our goal was to determine whether pharmacologic compounds that affect conventional outflow facility in human eyes exert similar effects in C57BL/6 mice.

METHODS

A computerized perfusion system was used to measure conventional outflow facility in enucleated mouse eyes ex vivo. Paired eyes were perfused sequentially, either immediately after enucleation or after 3 hours storage at 4°C. Three groups of experiments examined sphingosine 1-phosphate (S1P), S1P with antagonists to S1P(1) and S1P(2) receptors, and the prostanoid EP(4) receptor agonist 3,7-dithia PGE(1). We also examined whether a 24-hour postmortem delay affected the response to 3,7-dithia prostaglandin E(1) (PGE(1)).

RESULTS

S1P decreased facility by 39%, and was blocked almost completely by an S1P(2), but not S1P(1), receptor antagonist. The S1P(2) receptor antagonist alone increased facility nearly 2-fold. 3,7-dithia PGE(1) increased facility by 106% within 3 hours postmortem. By 24 hours postmortem, the facility increase caused by 3,7-dithia PGE(1) was reduced 3-fold, yet remained statistically detectable.

CONCLUSIONS

C57BL/6 mice showed opposing effects of S1P(2) and EP(4) receptor activation on conventional outflow facility, as observed in human eyes. Pharmacologic effects on facility were detectable up to 24 hours postmortem in enucleated mouse eyes. Mice are suitable models to examine the pharmacology of S1P and EP(4) receptor stimulation on IOP regulation as occurs within the conventional outflow pathway of human eyes, and are promising for studying other aspects of aqueous outflow dynamics.

Haemodynamics in the mouse aortic arch computed from MRI-derived velocities at the aortic root

Mice are widely used to investigate atherogenesis, which is known to be influenced by stresses related to blood flow. However, numerical characterization of the haemodynamic environment in the commonly studied aortic arch has hitherto been based on idealizations of inflow into the aorta. Our purpose in this work was to numerically characterize the haemodynamic environment in the mouse aortic arch using measured inflow velocities, and to relate the resulting shear stress patterns to known locations of high- and low-lesion prevalence. Blood flow velocities were measured in the aortic root of C57/BL6 mice using phase-contrast MRI. Arterial geometries were obtained by micro-CT of corrosion casts. These data were used to compute blood flow and wall shear stress (WSS) patterns in the arch. WSS profiles computed using realistic and idealized aortic root velocities differed significantly. An unexpected finding was that average WSS in the high-lesion-probability region on the inner wall was actually higher than the WSS in the low-probability region on the outer wall. Future studies of mouse aortic arch haemodynamics should avoid the use of idealized inflow velocity profiles. Lesion formation does not seem to uniquely associate with low or oscillating WSS in this segment, suggesting that other factors may also play a role in lesion localization.

Quantitative mapping of scleral fiber orientation in normal rat eyes

PURPOSE

Previous work has suggested a major role of scleral biomechanics in the pathogenesis of glaucoma. Since fiber orientation in connective tissues is a key determinant of tissue biomechanics, experimental characterization of scleral fiber orientation is needed to fully understand scleral biomechanics. This is a report of baseline experimental measurements of fiber orientation in whole normal rat scleras.

METHODS

Twenty ostensibly normal Norway brown rat eyes were fixed in 4% paraformaldehyde. The scleras were cleaned of intra- and extraorbital tissues and dissected into five patches, and each patch was glycerol treated to maximize its transparency. Fiber orientation was measured using small-angle light scattering (SALS). Scattering patterns were analyzed to extract two microstructural parameters at each measurement location-the preferred fiber orientation and the degree of alignment-yielding a fiber orientation map for each sclera.

RESULTS

Rat sclera is structurally anisotropic with several consistent features. At the limbus, fibers were highly aligned and organized primarily into a distinct ring surrounding the cornea. In the equatorial region, the fibers were primarily meridionally aligned. In the posterior and peripapillary region, the scleral fibers were mostly circumferential but less aligned than those in the anterior and equatorial regions.

CONCLUSIONS

Circumferential scleral fibers may act as reinforcing rings to limit corneal and optic nerve head deformations, whereas equatorial meridional fibers may either provide resistance against extraocular muscle forces or limit globe axial elongation.

eNOS, a pressure-dependent regulator of intraocular pressure

PURPOSE

Pathology in the primary drainage pathway for aqueous humor in the eye is responsible for ocular hypertension, the only treatable risk factor in patients with glaucoma. Unfortunately, the mechanisms that regulate pressure-dependent drainage of aqueous humor and thus intraocular pressure (IOP) are unknown. To better understand one possible underlying molecular factor that regulates IOP, nitric oxide (NO), pressure-dependent drainage in transgenic mice overexpressing endothelial NO synthase (eNOS) was studied.

METHODS

IOP was measured by rebound tonometry in mice, and pressure versus flow data were measured by ex vivo perfusion at multiple pressures between 8 and 45 mm Hg, using mock AH ±100 μM L-NAME. A subset of eyes was examined histologically using standard techniques or was assayed for fusion protein expression by Western blot analysis.

RESULTS

IOP was lower (9.6 ± 2.7 vs. 11.4 ± 2.5 mm Hg; mean ± SD; P = 0.04) and pressure-dependent drainage was higher (0.0154 ± 0.006 vs. 0.0066 ± 0.0009 μL/min/mm Hg; P = 0.002) in the transgenic mice than in the wild-type animals; however, pressure-independent drainage was unaffected. The NOS inhibitor L-NAME normalized pressure-dependent drainage in transgenic animals. For IOP >35 mm Hg, the slope of the pressure-flow curve in wild-type mice increased to match that seen in transgenic mice. Shear stress in the pressure-dependent pathway at elevated pressures was calculated to be in a range known to affect eNOS expression and activity in vascular endothelia.

CONCLUSIONS

Endothelial NOS overexpression lowers IOP by increasing pressure-dependent drainage in the mouse eye. Data are consistent with NO's having a mechanoregulatory role in aqueous humor dynamics, with eNOS induction at elevated IOPs leading to increased pressure-dependent outflow.

In vitro models for glaucoma research: effects of hydrostatic pressure

PURPOSE

The response of cells (e.g., optic nerve head [ONH] cells) to mechanical stress is important in glaucoma. Studies have reported the biological effects of hydrostatic pressure on ONH cells cultured on a rigid substrate. An apparatus, designed to independently vary hydrostatic pressure and gas tension (including oxygen tension) in culture medium, was used to evaluate the effects of pressure and tension on cell migration, shape, and α-tubulin architecture in a transformed cell line (DITNC1 rat cortical astrocytes).

METHODS

During the assay period, cells were exposed to one of four experimental configurations: (1) control pressure and control gas tension; (2) high-pressure (7.4 mm Hg) and reduced gas tension; (3) control pressure and reduced gas tension; and (4) high-pressure and control gas tension.

RESULTS

Calculations suggested that the cells in configurations 2 and 3 were hypoxic, as confirmed by direct measurements in configuration 2. No effects of hydrostatic pressure were observed on cell migration or α-tubulin architecture. However, cells cultured under low gas tension (configurations 2 and 3) showed increased migration at 48 and 72 hours (P < 0.05).

CONCLUSIONS

A hydrostatic pressure of 7.4 mm Hg has no effect on DITNC1 astrocytes cultured on rigid coverslips, whereas hypoxia associated with a fluid column creating this pressure does. These results differ from those in a previous report, the results of which may be explained by altered gas tensions in the culture medium. Steps are recommended for control of secondary effects when testing the effect of pressure on cultured cells.

Outflow physiology of the mouse eye: pressure dependence and washout

PURPOSE

Mice are commonly used in glaucoma research, but relatively little is known about aqueous outflow dynamics in the species. To facilitate future use of the mouse as a model of aqueous humor outflow, several fundamental physiological parameters were measured in the mouse eye.

METHODS

Eyes from adult mice of either sex (C57BL/6 background) were enucleated, cannulated with a 33-gauge needle, and perfused at constant pressure while inflow was continuously measured.

RESULTS

At 8 mm Hg, total outflow facility (C(total)) was 0.022 ± 0.005 μL/min/mm Hg (all values mean ± SD; n = 21). The flow-pressure relationship was linear up to 35 mm Hg. The conventional outflow facility (C(conv)) was 0.0066 ± 0.0009 μL/min/mm Hg, and the unconventional outflow (F(u)) was 0.114 ± 0.019 μL/min, both measured at room temperature. At 8 mm Hg, 66% of the outflow was via the unconventional pathway. In a more than 2-hour-long perfusion at 8 mm Hg, the rate of facility change was 2.4% ± 5.4% (n = 11) of starting facility per hour. The ocular compliance (0.086 ± 0.017 μL/mm Hg; n = 5) was comparable to the compliance of the perfusion system (0.100 ± 0.004 μL/mm Hg).

CONCLUSIONS

Mouse eyes are similar to human eyes, in that they have no detectable washout rate and a linear pressure-flow relationship over a broad range of intraocular pressures. Because of the absence of washout and the apparent presence of a true Schlemm's canal, the mouse is a useful model for studying the physiology of the inner wall of Schlemm's canal and the conventional outflow tissues.

Biaxial mechanical testing of human sclera

The biomechanical environment of the optic nerve head (ONH), of interest in glaucoma, is strongly affected by the biomechanical properties of sclera. However, there is a paucity of information about the variation of scleral mechanical properties within eyes and between individuals. We thus used biaxial testing to measure scleral stiffness in human eyes. Ten eyes from 5 human donors (age 55.4+/-3.5 years; mean+/-SD) were obtained within 24h of death. Square scleral samples (6mm on a side) were cut from each ocular quadrant 3-9 mm from the ONH centre and were mechanically tested using a biaxial extensional tissue tester (BioTester 5000, CellScale Biomaterials Testing, Waterloo). Stress-strain data in the latitudinal (toward the poles) and longitudinal (circumferential) directions, here referred to as directions 1 and 2, were fit to the four-parameter Fung constitutive equation W=c(e(Q)-1), where Q=c(1)E(11)(2)+c(2)E(22)(2)+2c(3)E(11)E(22) and W, c's and E(ij) are the strain energy function, material parameters and Green strains, respectively. Fitted material parameters were compared between samples. The parameter c(3) ranged from 10(-7) to 10(-8), but did not contribute significantly to the accuracy of the fitting and was thus fixed at 10(-7). The products cc(1) and cc(2), measures of stiffness in the 1 and 2 directions, were 2.9+/-2.0 and 2.8+/-1.9 MPa, respectively, and were not significantly different (two-sided t-test; p=0.795). The level of anisotropy (ratio of stiffness in orthogonal directions) was 1.065+/-0.33. No statistically significant correlations between sample thickness and stiffness were found (correlation coefficients=-0.026 and -0.058 in directions 1 and 2, respectively). Human sclera showed heterogeneous, near-isotropic, nonlinear mechanical properties over the scale of our samples.

Finite element modeling of the human sclera: influence on optic nerve head biomechanics and connections with glaucoma

Scleral thickness, especially near the optic nerve head (ONH), is a potential factor of interest in the development of glaucomatous optic neuropathy. Large differences in the dimensions of the sclera, the principal load-bearing tissue of the eye, have been observed between individuals. This study aimed to characterize the effects of these differences on ONH biomechanics. Eleven enucleated human globes (7 normal and 4 ostensibly glaucomatous) were imaged using high-field microMRI and segmented to produce 3-D individual-specific corneoscleral shells. An identical, idealized ONH geometry was inserted into each shell. Finite element modeling predicted the effects of pressurizing the eyes to an IOP of 30 mmHg, with the results used to characterize the effect of inter-individual differences in scleral dimensions on the biomechanics of the ONH. Measurements of the individual-specific corneoscleral shells were used to construct a 2-D axisymmetric idealized model of the corneoscleral shell and ONH. A sensitivity analysis based on this model quantified the relative importance of different geometrical characteristics of the scleral shell on the biomechanics of the ONH. Significant variations were observed in various measures of strain in the idealized lamina cribrosa (LC) across the seven normal corneoscleral shells, implying large differences in individual biomechanics due to scleral anatomy variations alone. The sensitivity analysis revealed that scleral thickness adjacent to the ONH was responsible for the vast majority of variation. Remarkably, varying peripapilary scleral thickness over the physiologically measured range changed the peak (95th percentile) first principal strain in the LC and radial displacement of the ONH canal by an amount that was equivalent to a change in IOP of 15 mmHg. Inter-individual variations in scleral thickness, particularly peripapillary scleral thickness, can result in vastly different biomechanical responses to IOP. These differences may be significant for understanding the interactions between IOP and scleral biomechanics in the pathogenesis of glaucomatous optic neuropathy. The relationship between scleral thickness and material properties needs to be studied in human eyes.

A new method for selection of angular aqueous plexus cells from porcine eyes: a model for Schlemm's canal endothelium

PURPOSE

The authors sought to develop a technique for isolating and culturing angular aqueous plexus (AAP) cells from more plentiful porcine eyes. AAP is an analogue of Schlemm's canal.

METHODS

Cells were differentially selected with puromycin, a toxin often used to select brain microvascular endothelial cells based on the expression of P-glycoprotein (P-gp), a multidrug resistance efflux pump. Trabecular meshwork containing AAP was dissected and pooled from fresh porcine eyes, digested in collagenase I, washed, filtered, and cultured for 8 days in a gelatin-coated plastic flask. Cells were then selected by exposure to 4 μg/mL puromycin for 2 days in the culture medium. Cells were fixed and immunostained for P-gp, ICAM II, von Willebrand factor (vWF), VE-cadherin, and α-smooth muscle actin (α-SMA).

RESULTS

Histology of the limbus showed that the dissection was limited to the trabecular meshwork region, including the AAP. Before puromycin treatment, cells appeared heterogeneous and polygonal, suggestive of a mixed population. More than 90% of the cells were removed by puromycin, leaving a population that appeared uniformly cobblestone-like when grown to confluence and that was contact inhibited. Puromycin-selected cells stained positively for the endothelial markers ICAM II, vWF, and VE-cadherin but negatively for α-SMA, consistent with staining patterns in whole tissue.

CONCLUSIONS

Based on marker expression, morphology, and behavior in culture, puromycin-selected cells from porcine outflow tissues are AAP endothelial cells. Thus, porcine eyes can provide a plentiful alternative cell source for studying Schlemm's canal biology related to ocular hypertension.

Design optimization of a helical endothelial cell culture device

The specific roles of mass transfer and fluid dynamic stresses on endothelial function, important in atherogenesis, are not known. Further, the effects of mass transfer and fluid dynamic stresses are difficult to separate because areas of "abnormal" mass transfer and "abnormal" wall shear stress tend to co-localize (where abnormal is defined as any deviation from the mass transfer rate or wall shear stress present in a long straight artery with the same flow rate and diameter). Our goal was to design a cell culture device which gives maximum separation between areas of abnormal shear stress and areas of abnormal mass transfer. We used design optimization principles to design a helical cell culture device. Using shear stress and mass transfer fields predicted by solving the governing equations, the area of the device which was exposed to low rates of mass transfer and normal levels of wall shear stress was determined. The design optimization method then maximized this area by varying the design variables, resulting in the optimum design. The optimum design had Reynolds number = 50, helical radius = 3.23 and helical pitch = 3.82. The area of the device which was exposed to low rates of mass transfer and regular levels of wall shear stress was about 4.5 times the inlet cross-sectional area of the device or about 5% of the device total internal surface area. An optimum design was successfully determined and the methodology used was shown to be robust. The area of the device which was exposed to low rates of mass transfer and regular levels of wall shear stress occurred in a defined region which should aid further experimental work.

Young's modulus of elasticity of Schlemm's canal endothelial cells

Schlemm's canal (SC) endothelial cells are likely important in the physiology and pathophysiology of the aqueous drainage system of the eye, particularly in glaucoma. The mechanical stiffness of these cells determines, in part, the extent to which they can support a pressure gradient and thus can be used to place limits on the flow resistance that this layer can generate in the eye. However, little is known about the biomechanical properties of SC endothelial cells. Our goal in this study was to estimate the effective Young's modulus of elasticity of normal SC cells. To do so, we combined magnetic pulling cytometry of isolated cultured human SC cells with finite element modeling of the mechanical response of the cell to traction forces applied by adherent beads. Preliminary work showed that the immersion angles of beads attached to the SC cells had a major influence on bead response; therefore, we also measured bead immersion angle by confocal microscopy, using an empirical technique to correct for axial distortion of the confocal images. Our results showed that the upper bound for the effective Young's modulus of elasticity of the cultured SC cells examined in this study, in central, non-nuclear regions, ranged between 1,007 and 3,053 Pa, which is similar to, although somewhat larger than values that have been measured for other endothelial cell types. We compared these values to estimates of the modulus of primate SC cells in vivo, based on images of these cells under pressure loading, and found good agreement at low intraocular pressure (8-15 mm Hg). However, increasing intraocular pressure (22-30 mm Hg) appeared to cause a significant increase in the modulus of these cells. These moduli can be used to estimate the extent to which SC cells deform in response to the pressure drop across the inner wall endothelium and thereby estimate the extent to which they can generate outflow resistance.

Strain uniformity in biaxial specimens is highly sensitive to attachment details

Biaxial testing has been used widely to characterize the mechanical properties of soft tissues and other flexible materials, but fundamental issues related to specimen design and attachment have remained. Finite element models and experiments were used to investigate how specimen geometry and attachment details affect uniformity of the strain field inside the attachment points. The computational studies confirm that increasing the number of attachment points increases the size of the area that experiences sensibly uniform strain (defined here as the central sample region where the ratio of principal strains E(11)/E(22)<1.10), and that the strains experienced in this region are less than nominal strains based on attachment point movement. Uniformity of the strain field improves substantially when the attachment points span a wide zone along each edge. Subtle irregularities in attachment point positioning can significantly degrade strain field uniformity. In contrast, details of the apron, the region outside of the attachment points, have little effect on the interior strain field. When nonlinear properties consistent with those found in human sclera are used, similar results are found. Experiments were conducted on 6 x 6 mm talc-sprinkled rubber specimens loaded using wire "rakes." Points on a grid having 12 x 12 bays were tracked, and a detailed strain map was constructed. A finite element model based on the actual geometry of an experiment having an off-pattern rake tine gave strain patterns that matched to within 4.4%. Finally, simulations using nonequibiaxial strains indicated that the strain field uniformity was more sensitive to sample attachment details for the nonequibiaxial case as compared to the equibiaxial case. Specimen design and attachment were found to significantly affect the uniformity of the strain field produced in biaxial tests. Practical guidelines were offered for design and mounting of biaxial test specimens. The issues addressed here are particularly relevant as specimens become smaller in size.

3D morphometry of the human optic nerve head

Human optic nerve head (ONH) anatomy is of interest in glaucoma. Our goal was to carry out a morphometric study of the human ONH based on 3D reconstructions from histologic sections. A set of 10 human ONHs (from four pairs of eyes plus two singles) were reconstructed in an iterative procedure that required the resulting geometries to satisfy a set of quality control criteria. Five models corresponded to eyes fixed at 5 mmHg and the other five models to eyes fixed at 50 mmHg. Several aspects of ONH morphology were measured based on surface and point landmarks: the thicknesses of the lamina cribrosa (LC), the peripapillary sclera and the pre-laminar neural tissue (peripapillary and within the cup); the minimum distance between the anterior surface of the LC and the subarachnoid space; the surface area of the anterior and posterior surfaces of the LC; and the diameter of the scleral canal opening. Our results showed that about one third of the anterior LC surface was obscured from view from the front by the sclera. In all eyes the LC inserted into the pia mater, and not only into the sclera. The variations in ONH morphology between eyes of a pair exceeded, or were of the same order as, changes in morphology due to acute changes in IOP. The reconstruction and morphometry techniques introduced are suitable for application to the ONH. Comparison of measurements in eyes fixed at different pressures suggested small effects on geometry of the increase in IOP. A large variability in ONH morphology, even between contralateral eyes of different IOP, was observed. We conclude that reconstruction of human ONH anatomy from 3D histology is possible, but that large inter-individual anatomic variations make morphometric analysis of the ONH very difficult in the absence of large sample numbers. The insertion of the pia mater into the LC may have biomechanical implications and should be further investigated. Emerging clinical imaging techniques such as deep-scanning OCT will be limited to investigation of the central and mid-peripheral regions of the LC due to optical "occluding" by the peripapillary sclera.