The trabecular outflow pathway is the main drainage system of aqueous humor in the eye. The core structures in the pathway are the trabecular meshwork (TM), the endothelial lining of Schlemm’s canal (SC), SC collector channels, and aqueous veins. The TM has 3 distinct structural areas: the inner uveal meshwork, the corneoscleral meshwork, and the juxtacanalicular (JCT) or cribriform region. JCT is immediately adjacent to the inner wall of SC (Fig. 1). TM structure and experimental flow studies indicate that flow resistance is highest in the region of the JCT and the inner wall of SC, although the exact location/proportions of resistance are unclear.1–5 The JCT contains an elastic-like network that connects to the inner wall endothelium of SC. The outer tendons of the ciliary muscle (CM), the corneoscleral TM, also insert into the network.6,7 Contraction of the CM spreads the lamellated portion of the meshwork, expanding the area of filtration so that resistance is reduced.7–9 The JCT region also contains electron microscopically optically empty spaces next to the inner wall endothelium, where giant vacuoles or pores are formed that open into SC.10 It is thought that these spaces are aqueous humor pathways.
Nerve endings have been identified in the scleral spur region that may be part of a mechanoreceptive system for responding to stress or strain in the connective tissue elements of the scleral spur, perhaps induced by CM contraction or changes in intraocular pressure (IOP).11 Morphologically distinct types, or proprioreceptors, are found in the CM. Receptors at the posterior muscle tips might measure stretch of the tendons, whereas the large mechanoreceptor-like endings located between the muscle tips and in the scleral spur region may respond to shear stress. With multiple types of intrinsic nerve cells, the contraction/relaxation of the CM might be able to respond locally to changes in the immediate environment.12 Similarly cholinergic and nitrergic nerve terminals in contact with the elastic-like network of the TM and scleral spur could induce contraction and relaxation of TM and SS cells and also indicate some self-regulatory ability.13
This configuration of structural elements illustrates how TM and CM contraction and relaxation function together to provide control of outflow and regulation of IOP.14 Recent investigations into the role of endothelial nitric oxide (NO) synthase suggest that TM mechanosensitivity may be a part of the CM/TM homeostatic mechanism mediated in part by NO.15 The active role that the TM plays in the regulation of IOP is also mediated by cytoskeleton and contractility mechanisms—the efferent arm of the reflexive and regulatory mechanism; their arrangement governs the final outflow facility. The endothelial NO synthase/NO system may be a signal/transduction arm that mediates response to the stressors, responses that are modulated afferently by the various sensors in the CM tendons, the CM apex, and the TM itself.16
The synthesis and degradation of the TM extracellular matrix (ECM) also plays a role in outflow resistance and the glaucomatous disease process.17–21 The ECM is comprised of a complex arrangement of fibronectin, collagen, laminin, proteoglycans, glycosaminoglycans, and matricellular proteins, which are mediated/modulated by elements of the transforming growth factor-β/bone morphogenetic protein/SMAD signaling pathways, connective tissue growth factor, matrix metalloproteinase-2, a tissue inhibitor of metalloproteinases also known as TIMP-2, and endothelin-1 (ET-1).21–30 In addition, glucocorticoids and prostaglandin derivatives appear to modulate ECM turnover in the JCT.31–35
This complex system is altered with age and the glaucomas, and it can be difficult to differentiate between the various etiological effects/agents. With age, the number of TM cells decreases, pigmentation in TM cells increases, as does the incidence of cells detaching from trabeculae and adhesions between adjacent trabeculae.36–38 An increase in accumulation of extracellular material in both the TM and CM is seen with increasing age, including in sheath-derived (SD) plaques and fibrillar material in the JCT, increasing outflow resistance in the TM outflow pathways.6,39 These changes may be the result of imbalances in responses to age-related stresses such as oxidative damage to long-lived molecules, protein cross-linking, and loss of elasticity that could in turn trigger an increase in production of factors such as transforming growth factor-β, interleukin-1, and CD44S, leading to increases in fibronectin and a decrease in the breakdown in ECM.18,20,25,35,40,41 Changes in the extracellular environment may result in decreases in outflow facility and uveoscleral outflow, which can contribute to an increase in IOP.40,42,43
Specimens of glaucomatous TM show fusion and thickening of trabecular lamellae, increased amounts SD plaque material in the JCT, and an abundance of long-spacing (lattice) collagen.39,40,43–45 Quantitation of the area filled by the SD plaques reveals a significant increase of SD plaque material in glaucomatous eyes compared with normal eyes of a similar age range. The increase in SD plaque material is also inversely correlated with axon counts in the optic nerve, a measure of glaucomatous damage.39,44 Eyes with less damage (more axons) have smaller amounts of TM SD plaques than eyes with more damage (fewer axons). The decreased thickness of the JCT, thickened beams, and shortening of the connecting fibrils found in the JCT elastic fiber network of glaucomatous eyes could reduce the ability of the tissue between the JCT and the inner wall of SC to expand. This reduces the amount of space for fluid flow and may diminish the influence of CM and TM contraction on outflow resistance. The resulting underperfusion of the TM could lead to an even greater increase in ECM buildup and tissue rigidity further increasing outflow resistance.2,40 Thus a self-perpetuating cycle could develop, which might be a factor in the pathogenesis of glaucoma.
The TM of glaucomatous eyes shows changes in glycosaminoglycan composition with a decrease in hyaluronic acid and an increase in chondroitin sulfate (CS), both of which contribute to flow resistance and influence flow rate in vitro.9,45–49 Factors such as a compromised antioxidant defense system and altered ECM metabolism are known to contribute to impaired outflow and may be common to primary open-angle glaucoma (POAG), exfoliation syndrome (XFS), and exfoliation glaucoma (XFG).50–52
XFS is considered an age-related, systemic disorder that includes ocular indications.53,54 In both XFS and XFG fibrillar, proteinaceous extracellular material (PEX) is produced in excess and accumulates in both outflow pathways.50,53,55 PEX is found in isolated aggregates beneath the inner wall endothelium of SC, as well as the uveal and JCT regions of the meshwork, which may limit aqueous access to the inner wall and SC55–57 (Fig. 2). The dimensions of SC in POAG eyes are significantly smaller than those in normal eyes.58 An inverse correlation between the amount of PEX material in the JCT and the length of the filtration region of SC indicates that eyes with larger amounts of PEX material also tend to have a smaller SC.56 As noted above, increased SD plaques are correlated with fewer axons in the optic nerve in POAG.44 In donor eyes with XFS, the amount of PEX material in the subendothelial region of SC was also inversely correlated with the number of axons in the optic nerve and was positively correlated with an increase in IOP, suggesting that the material contributes to an increase in outflow resistance.55,56 Both uveoscleral and trabecular outflow are reduced in XFG, but only uveoscleral outflow is reduced in XFS, indicating that XFS might involve changes in the CM and its surrounding ECM.59
The presence of patches of PEX material in the uveal meshwork region suggests that PEX material washes into the meshwork from the aqueous.56 PEX deposits may also be locally produced and accumulate in the intertrabecular spaces, JCT, and the inner wall of SC.55 The buildup of PEX is likely due to a combination of both excessive synthesis and insufficient degradation.60 An increase in JCT plaques and decreased cellularity in the TM are thought to contribute to decreased outflow facility in glaucoma patients, but in TM specimens from XFG patients there is a reduction of extracellular plaque material in the JCT, the structural integrity of trabecular endothelial cells is mostly retained and TM cellularity remains unchanged.8,50,55–57,60,61
Understanding similarities and differences between aging, glaucoma, and XFS/XFG will be critical for developing effective diagnostic and treatment strategies.
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