Age-related changes in the canal of Schlemm

Comparison of Schlemm's Canal Morphology Parameters Between Propensity Score-Matched Primary Open-Angle Glaucoma and Exfoliation Glaucoma

Purpose

This study aimed to histologically compare the status of Schlemm's canal (SC) and Schlemm's canal endothelial (SCE) cells between trabeculectomy specimens from patients with primary open-angle glaucoma (POAG) and exfoliation glaucoma (EXG).

Methods

A total of 182 eyes from 152 patients with POAG and 138 eyes from 116 patients with EXG underwent immunohistochemical staining for thrombomodulin. Equal numbers of cases were selected from both groups using propensity score matching. The following parameters were evaluated: total SC length, staining positive and negative SC length (PSC and NSC, respectively), opened and closed SC length, staining positive and opened SC length, staining positive and closed SC length, staining negative and opened SC length (NOSC), and staining negative and closed SC length.

Results

After matching for age and gender, 87 cases were selected in each group. The EXG group had significantly higher preoperative IOP and medication scores. PSC was significantly longer in the POAG group, while NSC and NOSC were longer in the EXG group. Multiple regression analysis of these 174 cases revealed that PSC was significantly shorter in the EXG group. After matching for age, gender, preoperative IOP, and medication score, 64 cases were selected in each group, and NOSC was significantly longer in the EXG group.

Conclusions

These findings suggest that in EXG, SCE loss occurs independently of background factors such as aging and medication use. The loss of SCE may have a more critical impact on IOP elevation in EXG compared to POAG.

Topographic distribution and phenotypic heterogeneity of Schlemm's canal endothelium in human donor eyes

Endothelium phenotype is known to be closely associated with flow shear stress. This study is to determine the topographic distribution of endothelial cells and the phenotype of different quadrants and regions of Schlemm's canal using human donor eyes. This study infers differences in flow dynamics based on cell shape and intracellular structure. The Schlemm's canal from 15 human donor eyes were either perfusion labelled using silver stain or dissected for float labeling with Phalloidin to enable visualization of endothelial cell border and intracellular structure. Data were acquired for endothelial cells from the outer and inner wall of Schlemm's canal and grouped according to quadrant of origin. Measurements included endothelial cell length, width, area, and aspect ratio and compared between quadrants. Endothelial cells are mostly spindle-shape and the cell size on the outer wall are larger and longer than those from the inner wall. Significant differences in endothelial cell size and shape were seen in different quadrants. The endothelial cells have varied shapes and orientations close to large ostia in the outer wall and remarkably long endothelial cells were found in the walls of collector channels. F-actin aggregation was found at all endothelial cell borders, and inside some of the endothelial cytoplasm. The presence of various spindle shapes, significant phenotype heterogeneity and F-actin aggregation of endothelial cells indicates aqueous humor flow likely creates variations in shear stress within Schlemm's canal. Further investigation of the relationship between the phenotype heterogeneity and hydrodynamics of aqueous flow may help us understand the mechanisms of outflow resistance changes in glaucoma.

Effect of age on the morphologies of the human Schlemm's canal and trabecular meshwork measured with swept‑source optical coherence tomography

Purpose

We aimed to measure the sizes of Schlemm’s canal (SC) and the trabecular meshwork (TM) in healthy individuals and to evaluate variations with age from childhood to old age by using swept-source optical coherence tomography (OCT).

Methods

Anterior chamber angle imaging of the superior, inferior, nasal, and temporal regions of the right and left eyes was performed with swept-source OCT. The diameter and area of SC and TM width and thickness were measured manually from OCT images.

Results

A total of 114 healthy individuals were enrolled and included 48 male subjects and 66 female subjects; their ages ranged from 7 to 83 years. Both the SC diameter and area in the four quadrants decreased significantly with aging (P < 0.001) and were wider in the nasal and temporal quadrants compared with the superior and inferior quadrants. Changes in SC size showed significant positive correlations with axial length (AL) and anterior chamber depth (P < 0.001). There was a significant positive association between age and TM thickness in the nasal and temporal quadrants (P < 0.05). The inferior quadrant TM width was the widest among the quadrants. The superior quadrant TM thickness was the thinnest among the quadrants. Changes in TM thickness in the nasal and temporal quadrants showed a significant negative correlation with AL (P < 0.05). There was no statistically significant correlation in SC and TM parameters with central corneal thickness, intraocular pressure, sex, or right or left eye (P > 0.05).

Conclusions

With aging, the SC diameter and area became smaller, TM thickness increased, and TM width seemed to remain constant. Measurements of the sizes of SC and the TM with swept-source OCT could assist in clinical assessments and treatment planning for glaucoma.

Profiling of Cytokines Secreted by Conventional Aqueous Outflow Pathway Endothelial Cells Activated In Vitro and Ex Vivo With Laser Irradiation

PURPOSE

To profile which cytokine genes are differentially expressed (DE) as up- or downregulated by cultured human trabecular meshwork (TMEs) and Schlemm's canal endothelial cells (SCEs) after three experimental treatments consisting of selective laser trabeculoplasty (SLT) irradiation, exposure to media conditioned either by SLT-irradiated TMEs (TME-cm) or by SCEs (SCE-cm). Also, to profile which cytokines are upregulated ex vivo in SLT-irradiated human conventional aqueous outflow pathway (CAOP) tissues.

METHODS

After each treatment, Affymetrix microarray assays were used to detect upregulated and downregulated genes for cytokines and their receptors in TMEs and SCEs. ELISA and protein antibody arrays were used to detect upregulated cytokines secreted in SLT-irradiated CAOP tissues ex vivo.

RESULTS

The SLT irradiation upregulated numerous cytokine genes in TMEs, but only a few in SCEs. Exposure to TME- and SCE-cm induced SCEs to upregulate many more cytokine genes than TMEs. Selective laser trabeculoplasty irradiation and exposure to TME-cm downregulated several cytokine genes in TMEs but none in SCEs. Selective laser trabeculoplasty irradiation induced one upregulated and three downregulated cytokine-receptor genes in TMEs but none in SCEs. Exposure to TME-cm induced upregulation of one and downregulation of another receptor gene in TMEs, whereas two unique cytokine-receptor genes were upregulated in SCEs. Cytokine protein expression analysis showed that at least eight cytokines were upregulated in SLT-irradiated human CAOP tissues in situ/ex vivo.

CONCLUSIONS

This study has helped us identify a cytokine signaling pathway and to consider newly identified mechanisms regulating aqueous outflow that may lay the foundation for the future development of cytokine-based glaucoma therapies.

Lipid peroxidation: pathophysiological and pharmacological implications in the eye

Oxygen-derived free radicals such as hydroxyl and hydroperoxyl species have been shown to oxidize phospholipids and other membrane lipid components leading to lipid peroxidation. In the eye, lipid peroxidation has been reported to play an important role in degenerative ocular diseases (age-related macular degeneration, cataract, glaucoma, diabetic retinopathy). Indeed, ocular tissues are prone to damage from reactive oxygen species due to stress from constant exposure of the eye to sunlight, atmospheric oxygen and environmental chemicals. Furthermore, free radical catalyzed peroxidation of long chain polyunsaturated acids (LCPUFAs) such as arachidonic acid and docosahexaenoic acid leads to generation of LCPUFA metabolites including isoprostanes and neuroprostanes that may further exert pharmacological/toxicological actions in ocular tissues. Evidence from literature supports the presence of endogenous defense mechanisms against reactive oxygen species in the eye, thereby presenting new avenues for the prevention and treatment of ocular degeneration. Hydrogen peroxide (H2O2) and synthetic peroxides can exert pharmacological and toxicological effects on tissues of the anterior uvea of several mammalian species. There is evidence suggesting that the retina, especially retinal ganglion cells can exhibit unique characteristics of antioxidant defense mechanisms. In the posterior segment of the eye, H2O2 and synthetic peroxides produce an inhibitory action on glutamate release (using [(3)H]-D-aspartate as a marker), in vitro and on the endogenous glutamate and glycine concentrations in vivo. In addition to peroxides, isoprostanes can elicit both excitatory and inhibitory effects on norepinephrine (NE) release from sympathetic nerves in isolated mammalian iris ciliary bodies. Whereas isoprostanes attenuate dopamine release from mammalian neural retina, in vitro, these novel arachidonic acid metabolites exhibit a biphasic regulatory effect on glutamate release from retina and can regulate amino acid neurotransmitter metabolism without inducing cell death in the retina. Furthermore, there appears to be an inhibitory role for neuroprostanes in the release of excitatory amino acid neurotransmitters in mammalian retina. The ability of peroxides and metabolites of LCPUFA to alter the integrity of neurotransmitter pools provides new potential target sites and pathways for the treatment of degenerative ocular diseases.

Oxidative stress impact on barrier function of porcine angular aqueous plexus cell monolayers

PURPOSE

Our goal was to investigate the effect of chronic oxidative stress on angular aqueous plexus (AAP, functional equivalent to human Schlemm's canal) endothelial cells from porcine eyes.

METHODS

AAP cells were differentially isolated from porcine outflow tissues using puromycin selection. Confluent cultures of porcine AAP cells were grown for 2 weeks in physiological (5% O2) or hyperoxic conditions (40% O2) to model elevated oxidative stress associated with ageing. Cell growth rate, size, transendothelial electrical resistance (TEER), and hydraulic conductivity (HC) were measured. The expression of senescence-associated β-galactosidase and DNA damage marker 8-hydroxy-2'-deoxyguanosine (8-OHdG) was monitored, and the levels of cytoskeletal and cell-cell adhesion proteins such as F-actin, phospho-myosin light chain (phosphor-MLC), occludin, claudin-5, ZO-1, β-catenin, and VE-cadherin were measured by immunofluorescence staining and Western blot analysis.

RESULTS

Data showed that chronic hyperoxia inhibited cell growth rate from day 3 onward, the cell size increased by 18.2%±5.1%, and cells stained positive for β-galactosidase and 8-OHdG. Hyperoxia resulted in a significant 30% increase in TEER compared with the control group (P<0.05, n=6). When perfused in the basal-to-apical direction at 4 mm Hg, HC of AAP cells was 1.97±0.12 and 1.54±0.13 μL/mm Hg/min/cm2 in control and hyperoxia groups, respectively (P<0.05, n=6). Stressed cells expressed a significantly greater abundance of F-actin, phospho-MLC, occludin, claudin-5, β-catenin, and VE-cadherin compared to the control group by both immunofluorescence and Western blot analyses.

CONCLUSIONS

Chronic exposure of AAP cells to oxidative stress decreased cell monolayer permeability and up-regulated cytoskeletal and cell-cell adhesion protein expression; suggesting that, with age and increased oxidative stress, resistance at the level of Schlemm's canal increases.

Abnormalities of the optic disc

The optic disc represents the anterior end of the optic nerve, the most forward extension of the central nervous system (CNS). The optic disc gives a rare glimpse into the CNS. Hence, diseases of the CNS are often manifested on fundus examination. Abnormalities of the optic disc may reflect eye disease (such as glaucoma), problems in development (as in various syndromes), or CNS disease (such as increased intracranial pressure). Each optic nerve is composed of about 1.2 million axons deriving from the retinal ganglion cells of one eye. Optic atrophy is a morphological sequela reflecting the loss of many or all of these axons. Myriad diseases such as hereditary, metabolic, tumor, and increased intracranial pressure can lead to optic atrophy. Some diseases, such as optic disc drusen, intracranial masses, orbital tumors, ischemic optic neuropathies, inflammations, and infiltrations, can produce optic disc edema before leading to optic atrophy. A number of new imaging modalities, such as optical coherence tomography (OCT), quantitate the thickness of the peripapillary retinal nerve fiber layer as an indirect measure of axonal loss or swelling. OCT can therefore be used to quantitate pathology or the response to therapy in various generalized CNS conditions, such as multiple sclerosis.

A model of giant vacuole dynamics in human Schlemm's canal endothelial cells

Aqueous humour transport across the inner wall endothelium of Schlemm's canal likely involves flow through giant vacuoles and pores, but the mechanics of how these structures form and how they influence the regulation of intraocular pressure (IOP) are not well understood. In this study, we developed an in vitro model of giant vacuole formation in human Schlemm's canal endothelial cells (HSCECs) perfused in the basal-to-apical direction (i.e., the direction that flow crosses the inner wall in vivo) under controlled pressure drops (2 or 6 mmHg). The system was mounted on a confocal microscope for time-lapse en face imaging, and cells were stained with calcein, a fluorescent vital dye. At the onset of perfusion, elliptical void regions appeared within an otherwise uniformly stained cytoplasm, and 3-dimensional reconstructions revealed that these voids were dome-like outpouchings of the cell to form giant vacuole-like structures or GVLs that reproduced the classic "signet ring" appearance of true giant vacuoles. Increasing pressure drop from 2 to 6 mmHg increased GVL height (14 ± 4 vs. 21 ± 7 μm, p < 0.0001) and endothelial hydraulic conductivity (1.15 ± 0.04 vs. 2.11 ± 0.49 μl min⁻¹ mmHg⁻¹ cm⁻²; p < 0.001), but there was significant variability in the GVL response to pressure between cell lines isolated from different donors. During perfusion, GVLs were observed "migrating" and agglomerating about the cell layer and often collapsed despite maintaining the same pressure drop. GVL formation was also observed in human umbilical vein and porcine aortic endothelial cells, suggesting that giant vacuole formation is not a unique property of Schlemm's canal cells. However, in these other cell types, GVLs were rarely observed "migrating" or contracting during perfusion, suggesting that Schlemm's canal endothelial cells may be better adapted to withstand basal-to-apical directed pressure gradients. In conclusion, we have established an in vitro model system to study giant vacuole dynamics, and we have demonstrated that this system reproduces key aspects of giant vacuole morphology and behaviour. This model offers promising opportunities to investigate the role of endothelial cell biomechanics in the regulation of intraocular pressure in normal and glaucomatous eyes.

Ageing of Schlemm's canal in nonglaucomatous subjects

AIM

to study age-related Schlemm's canal endothelial changes and evaluate consequences on the filtration function.

MATERIAL AND METHODS

the inner wall endothelium of Schlemm's canal was examined in 9 non-glaucomatous subjects aged between 32 and 75 years, by a combined technique of light and electron microscopy (scanning and transmission). Quantitative analysis included counts of bulges, pores, nuclei, giant vacuoles and other protruding structures, as well as measures of pores, giant vacuoles and Schlemm's canal size parameters (diameter and inner wall width). Outflow facility calculations were realised using a modified previously described mathematical model.

RESULTS

The main structures affected by ageing in Schlemm's canal appeared to be giant vacuoles. Their density but also their size is significantly reduced with increasing age. The intracellular pore population is also found to diminish with age and is correlated to that of giant vacuoles, suggesting that those pores are luminal openings of vacuoles. Outflow facility calculations revealed a global decrease of endothelial outflow facility of about 60% between the 3rd and 7th decades. The study also showed a different age-related pattern for the two subtypes of endothelial pores.

CONCLUSIONS

Our study demonstrated that Schlemm's canal filtration function is significantly influenced by age, as the endothelial inner wall outflow facility is found to be widely reduced. This is partly the result of an age-related reduction in counts of giant vacuoles and intracellular pores. The second pore population (border or intercellular) doesn't follow the same evolution, but may have a more significant regulator role in transendothelial permeability.

Atropine reduces but does not eliminate the age-related decline in perfusion outflow facility in monkeys

Accommodative amplitude and outflow facility decline with age in rhesus monkeys and humans. In monkeys, there is an age-related reduction in ciliary muscle (CM) mobility due to stiffening of its posterior and outer attachments. Since the CM inserts into the trabecular meshwork (TM) and CM contraction deforms the TM so as to increase outflow facility, we asked whether the age-related decline in outflow facility observed in anesthetized monkeys is consequent to the age-related reduction in CM mobility. One eye of 19 pentobarbital-anesthetized rhesus monkeys aged 4-24 years underwent 2-level constant pressure perfusion of the anterior chamber (AC) for 40 min to measure baseline total outflow facility. Both eyes then received 100 micrograms of atropine, given topically to the central cornea. The AC of the second eye was cannulated 40 min after atropine treatment. Facility was measured simultaneously in both eyes for 40 min beginning 45 min after atropine treatment. Baseline facility declined with age by -0.0160 +/- 0.0059 microliters min-1 mmHg-1 yr-1 (P = 0.009). The average facility at baseline was approximately 50% higher in the youngest (ages 4-10 years) compared to the oldest (ages 21-25 years) animals (P < or = 0.03). Following atropine facility decreased by approximately 25% in the youngest monkeys (N.S.), but not at all in the oldest animals when compared to baseline. Thus, the age-related facility decline persisted after atropine in both eyes, but was not as dramatic as prior to atropine (-0.0092 +/- 0.0050 [P < or = 0.09] and -0.0078 +/- 0.0044 [P < or = 0.10] microliter min-1 mmHg-1 yr-1 respectively). Apparently, atropine inhibits facility-relevant anesthetic-induced ciliary muscle tone to a greater extent in younger than in older monkeys, presumably because stiffening of the posterior and outer attachments of the CM in the older animals has already reduced its ability to move and thereby deform the TM. The inability of atropine to completely eliminate the age-related facility decline indicates the presence of atropine-independent, facility-relevant age-dependent changes in the TM itself, such as loss of cells or build-up of extracellular material.

Age-related depletion of the cell population in the human trabecular meshwork

The number of cells in the human trabecular meshwork was assessed for counting meshwork cell nuclei in semithin Araldite sections cut in the meridonal plane. The counts of nuclei decreased with increasing age and the decrease was associated with all regions of the meshwork. The appropriate regression equation and a mean length for meshwork nuclei was used to estimate cell numbers. At 20 years of age the estimated cell number for the whole meshwork was 763,000 which decreased to 403,000 cells by 80 years with a loss rate of 6,000 cells per year.