Sources of structural autofluorescence in the human trabecular meshwork

Bleb-related Porcine Lymphatic Outflow is Greater from Subconjunctival compared to Subtenon Blebs

Aim

Understanding the mechanism of fluid outflow by comparing the subconjunctival and subtenon spaces can lead to improved ocular therapeutics. The purpose of the current study is to evaluate subconjunctival vs subtenon lymphatic outflow by creating tracer-filled blebs in each location.

Methods

Porcine (n = 20) eyes received subconjunctival or subtenon injection(s) of fixable and fluorescent dextrans. Blebs were angiographically imaged using a Heidelberg Spectralis ([Heidelberg Retina Angiograph] HRA + OCT; Heidelberg Engineering) and bleb-related lymphatic outflow pathways were counted. Optical coherence tomography (OCT) imaging of these pathways was used to assess structural lumens and the presence of valve-like structures. Furthermore, a comparison between tracer injection locations (superior/inferior/temporal/nasal) was made. Histologic analyses for subconjunctival and subtenon outflow pathways were performed, to confirm tracer co-localization with molecular lymphatic markers.

Results

Subconjunctival blebs demonstrated a greater number of lymphatic outflow pathways compared to subtenon blebs in every quadrant [superior: 6.10 ± 1.18 (subconjunctival) vs 0.50 ± 0.27 (subtenon); temporal: 2.30 ± 0.40 vs 0.10 ± 0.10; nasal: 5.30 ± 0.60 vs 0.30 ± 0.21; inferior: 6.00 ±1.29 vs 0.1 ± 0.1; all comparisons p < 0.001]. For subconjunctival blebs, the temporal quadrant showed fewer lymphatic outflow pathways compared to the nasal side (p = 0.005).

Discussion

Subconjunctival blebs accessed greater lymphatic outflow compared to subtenon blebs. Furthermore, regional differences existed, with fewer lymphatic vessels temporal than at the other locations.

Clinical significance

Aqueous humor drainage after glaucoma surgery is incompletely understood. The present manuscript adds to our understanding of how lymphatics might influence filtration bleb function.

How to cite this article

Lee JY, Strohmaier CA, Akiyama G, et al. Bleb-related Porcine Lymphatic Outflow Is Greater from Subconjunctival compared to Subtenon Blebs. J Curr Glaucoma Pract 2022;16(3):144-151.

Biomechanics of human trabecular meshwork in healthy and glaucoma eyes via dynamic Schlemm's canal pressurization

BACKGROUND AND OBJECTIVE

The trabecular meshwork (TM) consists of extracellular matrix (ECM) with embedded collagen and elastin fibers providing its mechanical support. TM stiffness is considerably higher in glaucoma eyes. Emerging data indicates that the TM moves dynamically with transient intraocular pressure (IOP) fluctuations, implying the viscoelastic mechanical behavior of the TM. However, little is known about TM viscoelastic behavior. We calculated the viscoelastic mechanical properties of the TM in n = 2 healthy and n = 2 glaucoma eyes.

METHODS

A quadrant of the anterior segment was submerged in a saline bath, and a cannula connected to an adjustable saline reservoir was inserted into Schlemm's canal (SC). A spectral domain-OCT (SD-OCT) provided continuous cross-sectional B-scans of the TM/JCT/SC complex during pressure oscillation from 0 to 30 mmHg at two locations. The TM/JCT/SC complex boundaries were delineated to construct a 20-µm-thick volume finite element (FE) mesh. Pre-tensioned collagen and elastin fibrils were embedded in the model using a mesh-free penalty-based cable-in-solid algorithm. SC pressure was represented by a position- and time-dependent pressure boundary; floating boundary conditions were applied to the other cut edges of the model. An FE-optimization algorithm was used to adjust the ECM/fiber mechanical properties such that the TM/JCT/SC model and SD-OCT imaging data best matched over time.

RESULTS

Significantly larger short- and long-time ECM shear moduli (p = 0.0032), and collagen (1.82x) and elastin (2.72x) fibril elastic moduli (p = 0.0001), were found in the TM of glaucoma eyes compared to healthy controls.

CONCLUSIONS

These findings provide additional clarity on the mechanical property differences in healthy and glaucomatous outflow pathway under dynamic loading. Understanding the viscoelastic properties of the TM may serve as a new biomarker in early diagnosis of glaucoma.

Aqueous humour outflow imaging: seeing is believing

Elevated intraocular pressure (IOP) is the primary risk factor for blindness in glaucoma. IOP is determined by many factors including aqueous humour production and aqueous humour outflow (AHO), where AHO disturbance represents the primary cause of increased IOP. With the recent development of new IOP lowering drugs and Minimally Invasive Glaucoma Surgeries (MIGS), renewed interest has arisen in shedding light on not only how but where AHO is occurring for the trabecular/conventional, uveoscleral/unconventional, and subconjunctival outflow pathways. Historical studies critical to understanding outflow anatomy will be presented, leading to the development of modern imaging methods. New biological behaviours uncovered by modern imaging methods will be discussed with relevance to glaucoma therapies emphasized.

Histologic Analysis of Trabecular Meshwork Obtained From Kahook Dual Blade Goniotomy

PURPOSE

To determine whether there are identifiable, reproducible findings in the trabecular meshwork (TM) of patients with primary open-angle glaucoma (POAG) who underwent Kahook Dual Blade (KDB) goniotomy.

DESIGN

Noncomparative retrospective case series.

METHODS

Tertiary academic referral center, Veterans Affairs Medical Center. Thirteen patients (14 eyes) with POAG (100%) were treated with KDB goniotomy from May to December 2017. Isolated TM tissue was collected from 9 patients (10 eyes) and submitted for histologic analysis. Hematoxylin-eosin, periodic acid-Schiff, and elastin Van Gieson stains were completed, in addition to immunohistochemistry for collagen IV.

RESULTS

Mean age of patients was 74.2 ± 6.7 years. Trabecular beams were identified in all 10 specimens, although distorted in 4 samples, of which 3 had a history of laser trabeculoplasty. Collagen IV staining was present in 10 of 10 samples, coating the trabecular beams. Elastin was present in 8 of 10 samples along the trabecular beams. Intraocular pressure and number of glaucoma medications decreased significantly in all cases postoperatively (P < .0001, P = .035, respectively).

CONCLUSIONS

This pilot study demonstrates that tissue obtained during KDB goniotomy has a high yield of containing TM compared to reported yield of TM in specimens collected from traditional ab externo trabeculectomy (71% vs 20%, respectively). These goniotomy specimens possess sufficient anatomic preservation to be studied histologically. Trabecular meshwork obtained with this procedure may provide a novel modality to study TM dysfunction in open-angle glaucomas.

Quantitative assessment of radiation-induced changes of bladder and rectum collagen structure using optical methods

The objective of the study is the quantitative analysis of the dose-time dependences of changes occurring in collagen of bladder and rectum after gamma-irradiation using optical methods [nonlinear microscopy in a second harmonic generation (SHG) detection regime and cross-polarization optical coherence tomography (CP OCT)]. For quantitative assessment of the collagen structure, regions of interest on the SHG-images of two-dimensional (2-D) distribution of SHG signal intensity of collagen were chosen in the submucosa. The mean SHG signal intensity and its standard deviation were calculated by ImageJ 1.39p (NIH). For quantitative analysis of CP OCT data, an integral depolarization factor (IDF) was calculated. Quantitative calculation of the SHG signal intensity and the IDF can provide additional information about the processes of the collagen radiation-induced degradation and subsequent remodeling. High positive correlation between the mean SHG signal intensity and the mean IDF of bladder and rectum demonstrates that CP OCT can be used as an "optical biopsy" in the grading of collagen radiation damage.

Quantification of Pulse-Dependent Trabecular Meshwork Motion in Normal Humans Using Phase-Sensitive OCT

Purpose

The purpose of this study was to characterize the pulsatile motion of trabecular meshwork (TM) in normal subjects and demonstrate its changes in accommodation with phase-sensitive optical coherence tomography (PhS-OCT).

Methods

A new PhS-OCT laboratory prototype was designed to measure pulsatile TM motion in 13 healthy humans. Two sets of images were captured in 10 subjects, first with best corrective refraction and the other with an additional 3.0 diopters of accommodation. In each image, both maximum velocity (MV) and cumulative displacement (CD) in two selected regions of TM, the internal (IMV and ICD) and external (EMV and ECD) region, were measured.

Results

For all parameters the intraclass correlation coefficient was >0.75. Neither MV nor CD was significantly different between eyes in individual subjects (PIMV = 0.967, PEMV = 0.391, PICD = 0.603, PECD = 0.482). In 26 eyes, with best corrective refraction, the EMV was higher than the IMV (23.9 ± 9.8 vs. 18.9 ± 8.08 μm/s; P = 0.0001), as was the ECD compared with the ICD (0.340 ±0.125 vs. 0.264 ± 0.111 μm; P = 0.000004). With accommodation, MV and CD significantly increased (PIMV = 0.0003, PEMV = 0.0003, PICD = 0.019, and PECD = 0.007), whereas MV and CD in the external region were still larger than those in the internal area (PEMV vs. IMV = 0.009, PECD vs. ICD = 0.023).

Conclusions

This study demonstrates the differences in TM motion between the internal and external regions of TM and displays its change with accommodation. The findings and good reproducibility suggest PhS-OCT helps to understand TM function in regulation of IOP, and, with further refinements, it may be useful in clinical management of glaucoma.

Deep tissue analysis of distal aqueous drainage structures and contractile features

Outflow resistance in the aqueous drainage tract distal to trabecular meshwork is potentially an important determinant of intraocular pressure and success of trabecular bypass glaucoma surgeries. It is unclear how distal resistance is modulated. We sought to establish: (a) multimodal 2-photon deep tissue imaging and 3-dimensional analysis of the distal aqueous drainage tract (DT) in transgenic mice in vivo and ex vivo; (b) criteria for distinguishing the DT from blood and lymphatic vessels; and (c) presence of a DT wall organization capable of contractility. DT lumen appeared as scleral collagen second harmonic generation signal voids that could be traced back to Schlemm's canal. DT endothelium was Prox1-positive, CD31-positive and LYVE-1-negative, bearing a different molecular signature from blood and true lymphatic vessels. DT walls showed prominent filamentous actin (F-actin) labeling reflecting cells in a contracted state. F-actin co-localized with mesenchymal smooth muscle epitopes of alpha-smooth muscle actin, caldesmon and calponin, which localized adjacent and external to the endothelium. Our findings support a DT wall organization resembling that of blood vessels. This reflects a capacity to contract and support dynamic alteration of DT caliber and resistance analogous to the role of blood vessel tone in regulating blood flow.

Aqueous Humor Outflow Structure and Function Imaging At the Bench and Bedside: A Review

Anterior segment glaucoma clinical care and research has recently gained new focus because of novel imaging modalities and the advent of angle-based surgical treatments. Traditional investigation drawn to the trabecular meshwork now emphasizes the entire conventional aqueous humor outflow (AHO) pathway from the anterior chamber to the episcleral vein. AHO investigation can be divided into structural and functional assessments using different methods. The historical basis for studying the anterior segment of the eye and AHO in glaucoma is discussed. Structural studies of AHO are reviewed and include traditional pathological approaches to modern tools such as multi-model two-photon microscopy and optical coherence tomography. Functional assessment focuses on visualizing AHO itself through a variety of non-real-time and real-time techniques such as aqueous angiography. Implications of distal outflow resistance and segmental AHO are discussed with an emphasis on melding bench-side research to viable clinical applications. Through the development of an improved structure: function relationship for AHO in the anterior segment of the normal and diseased eye, a better understanding of the eye with improved therapeutics may be developed.

Toward in vivo two-photon analysis of mouse aqueous outflow structure and function

The promise of revolutionary insights into intraocular pressure (IOP) and aqueous humor outflow homeostasis, IOP pathogenesis, and novel therapy offered by engineered mouse models has been hindered by a lack of appropriate tools for studying the aqueous drainage tissues in their original 3-dimensional (3D) environment. Advances in 2-photon excitation fluorescence imaging (TPEF) combined with availability of modalities such as transgenic reporter mice and intravital dyes have placed us on the cusp of unlocking the potential of the mouse model for unearthing insights into aqueous drainage structure and function. Multimodality 2-photon imaging permits high-resolution visualization not only of tissue structural organization but also cells and cellular function. It is possible to dig deeper into understanding the cellular basis of aqueous outflow regulation as the technique integrates analysis of tissue structure, cell biology and physiology in a way that could also lead to fresh insights into human glaucoma. We outline recent novel applications of two-photon imaging to analyze the mouse conventional drainage system in vivo or in whole tissues: (1) collagen second harmonic generation (SHG) identifies the locations of episcleral vessels, intrascleral plexuses, collector channels, and Schlemm's canal in the distal aqueous drainage tract; (2) the prospero homeobox protein 1-green fluorescent protein (GFP) reporter helps locate the inner wall of Schlemm's canal; (3) Calcein AM, siGLO™, the fluorescent reporters m-Tomato and GFP, and coherent anti-Stokes scattering (CARS), are adjuncts to TPEF to identify live cells by their membrane or cytosolic locations; (4) autofluorescence and sulforhodamine-B to identify elastic fibers in the living eye. These tools greatly expand our options for analyzing physiological and pathological processes in the aqueous drainage tissues of live mice as a model of the analogous human system.

Optimizing two-photon multiple fluorophore imaging of the human trabecular meshwork

PURPOSE

Advances in two-photon (2P) deep tissue imaging provide powerful options for simultaneously viewing multiple fluorophores within tissues. We determined imaging parameters for optimally visualizing three fluorophores in the human trabecular meshwork (TM) to simultaneously detect broad-spectrum autofluorescence and multiple fluorophores through a limited number of emission filters.

METHODS

2P imaging of viable human postmortem TM was conducted to detect Hoechst 33342-labeled nuclei, Alexa-568-conjugated phalloidin labeling of filamentous actin, and autofluorescence of the structural extracellular matrix (ECM). Emission detection through green (500-550 nm), near-red (565-605 nm), and far-red (590-680 nm) filters following 2P excitation at 750, 800, 850, and 900 nm was analyzed. Region-of-interest (ROI) image analysis provided fluorescence intensity values for each fluorophore.

RESULTS

Red-channel Alexa 568 fluorescence was of highest intensity with 2P 750 nm and 800 nm excitation. Alexa 568 was imperceptible with 900 nm excitation. With excitation at 750 nm and 800 nm, Hoechst 33,342 intensity swamped autofluorescence in the green channel, and marked bleed-through into red channels was seen. 850 nm excitation yielded balanced Hoechst 33342 and autofluorescence intensities, minimized their bleed-through into the far-red channel, and produced reasonable Alexa 568 intensities in the far-red channel.

CONCLUSIONS

2P excitation at 850 nm and long-wavelength emission detection in the far-red channel allowed simultaneous visualization of the specific mix of endogenous and exogenous fluorophores with reasonably balanced intensities while minimizing bleed-through when imaging the human TM.

Tissue-based multiphoton analysis of actomyosin and structural responses in human trabecular meshwork

The contractile trabecular meshwork (TM) modulates aqueous humor outflow resistance and intraocular pressure. The primary goal was to visualize and quantify human TM contractile state by analyzing actin polymerization (F-actin) by 2-photon excitation fluorescence imaging (TPEF) in situ. A secondary goal was to ascertain if structural extracellular matrix (ECM) configuration changed with contractility. Viable ex vivo human TM was incubated with latrunculin-A (Lat-A) or vehicle prior to Alexa-568-phalloidin labeling and TPEF. Quantitative image analysis was applied to 2-dimensional (2D) optical sections and 3D image reconstructions. After Lat-A exposure, (a) the F-actin network reorganized as aggregates; (b) F-actin-associated fluorescence intensity was reduced by 48.6% (mean; p = 0.007; n = 8); (c) F-actin 3D distribution was reduced by 68.9% (p = 0.040); (d) ECM pore cross-sectional area and volume were larger by 36% (p = 0.032) and 65% (p = 0.059) respectively and pores appeared more interconnected; (e) expression of type I collagen and elastin, key TM structural ECM proteins, were unaltered (p = 0.54); and (f) tissue viability was unchanged (p = 0.39) relative to vehicle controls. Thus Lat-A-induced reduction of actomyosin contractility was associated with TM porous expansion without evidence of reduced structural ECM protein expression or cellular viability. These important subcellular-level dynamics could be visualized and quantified within human tissue by TPEF.

New Details of the Human Corneal Limbus Revealed With Second Harmonic Generation Imaging

PURPOSE

To report novel findings of the human corneal limbus by using second harmonic generation (SHG) imaging.

METHODS

Corneal limbus was imaged by using an inverted two-photon excitation fluorescence microscope. Laser (Ti:Sapphire) was tuned at 850 nm for two-photon excitation. Backscatter signals of SHG and autofluorescence (AF) were collected through a 425/30-nm emission filter and a 525/45-emission filter, respectively. Multiple, consecutive, and overlapping image stacks (z-stack) were acquired for the corneal limbal area.

RESULTS

Two novel collagen structures were revealed by SHG imaging at the limbus: an anterior limbal cribriform layer and presumed anchoring fibers. Anterior limbal cribriform layer is an intertwined reticular collagen architecture just beneath the limbal epithelial niche and is located between the peripheral cornea and Tenon's/scleral tissue. Autofluorescence imaging revealed high vascularity in this structure. Central to the anterior limbal cribriform layer, radial strands of collagen were found to connect the peripheral cornea to the limbus. These presumed anchoring fibers have both collagen and elastin and were found more extensively in the superficial layers than deep layer and were absent in very deep limbus near Schlemm's canal.

CONCLUSIONS

By using SHG imaging, new details of the collagen architecture of human corneal limbal area were elucidated. High resolution images with volumetric analysis revealed two novel collagen structures.

Tissue-based imaging model of human trabecular meshwork

We have developed a tissue-based model of the human trabecular meshwork (TM) using viable postmortem corneoscleral donor tissue. Two-photon microscopy is used to optically section and image deep in the tissue to analyze cells and extracellular matrix (ECM) within the original three-dimensional (3D) environment of the TM. Multimodal techniques, including autofluorescence (AF), second harmonic generation (SHG), intravital dye fluorescence, and epifluorescence, are combined to provide unique views of the tissue at the cellular and subcellular level. SHG and AF imaging are non-invasive tissue imaging techniques with potential for clinical application, which can be modeled in the system. We describe the following in the tissue-based model: analysis of live cellularity to determine tissue viability; characteristics of live cells based on intravital labeling; features and composition of the TM's structural ECM; localization of specific ECM proteins to regions such as basement membrane; in situ induction and expression of tissue markers characteristic of cultured TM cells relevant to glaucoma; analysis of TM actin and pharmacological effects; in situ visualization of TM, inner wall endothelium, and Schlemm's canal; and application of 3D reconstruction, modeling, and quantitative analysis to the TM. The human model represents a cost-effective use of valuable and scarce yet available human tissue that allows unique cell biology, pharmacology, and translational studies of the TM.