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Functional Morphology of the Trabecular Meshwork*
Functional Morphology of the Trabecular Meshwork* E R Tamm, University of Regensburg, Regensburg, Germany ã 2010 Elsevier Ltd. All rights reserved.
Glossary Inner-wall region – Comprised of the inner-wall endothelium of Schlemm’s canal, its basement membrane, and the adjacent juxtacanalicular tissue. It is the site of resistance in the trabecular meshwork outflow pathways. Juxtacanalicular tissue – The outermost part of the trabecular meshwork that lies directly adjacent to Schlemm’s canal. It does not form lamellae, but represents a loose connective tissue. An alternative term is cribriform meshwork. Mechanosensor – The terminal of an afferent nerve ending that is in contact with fibrillar components of the extracellular matrix and measures stress or strain in those. An alternative term is mechanoreceptor. Myofibroblast – A resident cell in some connective tissues that shows structural and functional characteristics of both fibroblasts and smooth muscle cells, and appears to be an intermediate form of those. Schlemm’s canal – A circular vascular tube that lies in the scleral sulcus, a circular groove on the inner aspect of the corneoscleral limbus. It is part of the trabecular outflow pathways. The inner aspects of Schlemm’s canal are in contact with the juxtacanalicular tissue. Scleral spur – A wedge-shaped circular ridge protruding from the sclera posterior to Schlemm’s canal. Trabecular meshwork – A porous, filter-like tissue in the irido-corneal angle consisting of connective tissue strands (trabecular beams or lamellae) that attach to one another and are covered by cells.
Introduction The major drainage structures for aqueous humor (AH) are the conventional or trabecular outflow pathways, which are comprised of the trabecular meshwork (made up by the uveal and corneoscleral meshworks), the *An adaptation and extension of Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655. http://www.sciencedirect.com/science/ journal/00144835.
juxtacanalicular connective tissue ( JCT), the endothelial lining of Schlemm’s canal (SC), the collecting channels, and the aqueous veins. When AH has passed through the trabecular outflow pathways, it drains into the episcleral venous system. In addition, there is an unconventional or uveoscleral outflow route which is open to AH at the chamber angle in the region of the anterior insertion of the ciliary muscle, since there is no complete endothelial or epithelial layer that covers the anterior surface of the ciliary body. The trabecular meshwork (TM) outflow pathways are critical in providing resistance to AH outflow. Intraocular pressure (IOP) builds up in response to this resistance until it is high enough to allow flow of AH across the TM into SC. AH passes through the TM as bulk flow driven by the pressure gradient, and active transport is not involved as neither metabolic poisons nor temperature affects AH flow across the TM. At steady-state IOP, fluid flow rate across the trabecular outflow resistance equals the rate of aqueous production by the ciliary body. Outflow resistance in the TM outflow pathways increases with age and primary open-angle glaucoma (POAG). This article reviews the functional morphology of the TM outflow pathways.
The TM Outflow Pathways The tissue structures of the TM outflow pathways are embedded in the internal scleral sulcus, a circular groove on the inner aspect of the sclera in region of the corneoscleral limbus. The scleral sulcus extends from the peripheral edge of Descemet’s membrane to the scleral spur, a wedge-shaped circular ridge protruding from the inner sclera (Figure 1(a)). SC, a circular, vascular-like collecting channel, lies in the outer portion of the scleral sulcus, while the TM occupies most of its inner aspects (Figure 1(a)). The TM is formed by connective tissue beams or lamellae that have a core of collagenous and elastic fibers, and are covered by flat cells which rest on a basal lamina. The beams attach to one another in several layers and form a porous filter-like structure. Anteriorly, the trabecular beams are attached near the end of Descemet’s membrane (Schwalbe’s line) and extend posteriorly to the stroma of the ciliary body and iris at their junction, and to the scleral spur (Figure 1(a)). As SC is shorter in anterior–posterior direction than the TM, a filtering portion of the TM can be differentiated from a nonfiltering
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portion which has no SC next to its external aspect (Figure 1(a)). Those cells of the nonfiltering portion of the TM, which reside close to the attachment of the TM at Schwalbe’s line, differ in structure from that of the TM proper. There is some evidence that this region serves as a niche for cells with adult stem cell-/progenitor cell-like properties that are capable of dividing and repopulating the filtering part of the TM after injury. The TM consists of three regions that differ in structure: the inner uveal meshwork, the deeper corneoscleral meshwork, and the JCT or cribriform region that is localized directly adjacent to the inner-wall endothelium of SC (Figure 1(b)). The uveal meshwork, which originates from the anterior aspect of the ciliary body, consists of one to three layers of trabecular beams or lamellae (Figure 1(b)). The corneoscleral meshwork forms 8–15 trabecular layers, which are thicker than those of the uveal TM and originate from the scleral spur (Figure 1(b)). The JCT, which is localized directly to the endothelial lining of SC, is the smallest part of the TM with a thickness of only 2–20 mm. It does not form trabecular lamellae or connective tissue
beams, but rather represents a typical loose connective tissue with two to five layers of scattered cells that are embedded in a loosely arranged fibrillar extracellular matrix (ECM) (Figures 1(b) and 2(a)).
The Trabecular Lamellae Each lamella or beam of the uveal or corneoscleral TM contains densely packed collagen and elastic fibers. The collagen fibers are mostly collagens type I and III. The elastic fibers, which localize to the core of the beams (Figure 2(b)), differ in their ultrastructure from those in other parts of the body, as they contain considerable amounts of electron-dense material. TM elastic fibers are surrounded by a sheath that thickens with age and
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UTM AC (b) Figure 1 Light micrograph of a meridional section through the trabecular meshwork. (b) is a magnification of (a). Arrows in (b) point to giant vacuoles in the inner-wall endothelium of SC. Magnification bar ¼ 20 mm (a), 5 mm (b). SC, Schlemm’s canal; TM, trabecular meshwork; SS, scleral spur; CM, ciliary muscle; AC, anterior chamber; JCT, juxtacanalicular tissue; CTM, corneoscleral trabecular meshwork; UTM, uveal trabecular meshwork. From Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655.
Figure 2 Electron micrographs of the trabecular meshwork. (a) Cells and extracellular fibrils in the juxtacanalicular tissue (JCT) are arranged in an irregularly network in contrast to the more regular structure of the beams in the corneoscleral trabecular meshwork (CTM). (b) Ultrastructure of a corneoscleral trabecular meshwork beam. The core of the beam contains elastic fibers (E) that are surrounded by banded sheath material. Open arrows denote the basal lamina of the trabecular meshwork cells, while solid arrows point to aggregates of long-spacing collagen in the cortical region of the beam. The beam is completely covered by flat trabecular meshwork cells. SC, Schlemm’s canal; GV, giant vacuole. Magnification bar ¼ 2 mm. From Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655.
Functional Morphology of the Trabecular Meshwork
that may show an 80–120-nm periodicity. Clumps of similar material and periodicity have been referred to as long-spacing collagen and are found in the cortical zone of the beams, between the elastic fiber core and basal lamina of the TM cells (Figure 2(b)). Fine fibrils that enter the aggregates of long-spacing collagen have been shown to label with antibodies against collagen type VI. The cells covering the TM beams reside on a basal lamina that is rich in collagen type IV and laminin (Figure 2(b)). A three-dimensional network is established by TM cells that bridge intertrabecular spaces to cover two adjacent beams. TM cells are capable of phagocytosis and may contain pigment particles, and these particles in TM cells probably derive from the iris and their phagocytosis by TM cells may be part of an important self-cleaning mechanism of the trabecular filter. TM regions that contain a high number of pigmented cells appear to be preferentially localized adjacent to collector channels, suggesting preferential AH flow pathways in the TM.
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The Juxtacanalicular Tissue TM cells in the JCT are surrounded by fibrillar elements of the ECM to form a loose connective tissue. As JCT cells and ECM fibrils are arranged in an irregular network (in contrast to the more regular structure of the beams in the inner parts of the TM), some authors prefer the term cribriform meshwork. The cells in the JCT form long processes by which they attach to one other, to ECM fibrils or to the cells of the endothelial lining of SC (Figure 2(a)). The spaces between JCT cells and ECM fibers serve as pathway for AH and contain a ground substance consisting of various proteoglycans and hyaluronan. The extent to which this ground substance fills the spaces between cells and fibrillar ECM is not clear as proteoglycans are not readily retained during processing of tissue for conventional electron microscopy. Recent studies using quick-freeze/deep-etch methodology visualized considerably more ECM in the JCT compared to that seen by conventional electron microscopy. A characteristic structural element of the JCT is a network of elastic fibers (cribriform plexus), which is arranged tangentially to the SC endothelial lining. The elastic fibers of the cribriform plexus show ultrastructural characteristics similar to those in the trabecular beams as they consist of an electron-dense core and a sheath of banded material. The molecular nature of the sheath material has not been identified, but there is evidence that collagen type VI and fibronectin are associated with it. Fibers of the cribriform plexus connect with the SC inner-wall endothelium through either their sheath material or fine fibrils that emerge from it (Figure 3(a)). Fibronectin-based cell–matrix connections appear to be very important for the adhesion of SC endothelial cells to the fibrillar ECM
(b) Figure 3 Electron microscopy of the SC inner-wall region. (a) Connecting fibers (CF) in the juxtacanalicular tissue, which emerge from the cribriform elastic plexus, connect the plexus with the inner-wall endothelium of Schlemm’s canal (SC). The connection with the inner-wall endothelium is made through either the banded sheath material of the fibers or fine fibrils that emerge from it (solid arrows). (b) A giant vacuole in the inner wall of SC forms an intracellular pore (arrow). Magnification bar ¼ 1 mm. From Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655.
of the JCT, as treatment of perfused anterior segment organ cultures with the integrin/syndecan-binding domain of fibronectin, called the heparin II (Hep II) domain, leads to a focal detachment of SC endothelium from the JCT, and to a significant increase in outflow facility. The ECM in the JCT does not only contain structural elements, but also matricellular proteins such as thrombospondin-1 and SPARC. Matricellular proteins are secreted ECM proteins that influence cell function by modulating cell–matrix interactions. Another important component of the JCT ECM is myocilin, one of the most highly expressed molecules in the TM. Myocilin has been shown to associate with fibrillar elements of the JCT ECM, but its biochemical function is largely unclear. Synthesis and degradation of ECM molecules in the JCT is under control of autocrine and paracrine growth factors, such as transforming growth factor beta 2 (TGF-b2), connective tissue growth factor (CTGF), bone morphogenetic protein 7 (BMP7), and BMP4. In addition, glucocorticoids and prostaglandin derivatives appear to modulate
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ECM turnover in the JCT. A characteristic structural change in the eyes of patients with POAG and steroidinduced glaucoma is an increase in fibrillar ECM in the JCT. Because of their high porosity, the uveal and corneoscleral parts of the TM do not provide a significant resistance to AH outflow. Support for this comes from experimental studies, which show that cutting through the inner parts of the TM does not affect outflow facility, and from theoretical calculations using Poiseuille’s law. In contrast, there is considerable evidence that normal AH outflow resistance resides in the inner-wall region of SC, which is formed by the JCT and SC inner-wall endothelium. As of today, there is active debate and research regarding the specific role of SC inner-wall endothelium and/or JCT for the formation of resistance in the TM outflow pathways.
Schlemm’s Canal A characteristic aspect of the SC inner-wall endothelium is the formation of cellular outpouchings (the so-called giant vacuoles) in response to the pressure gradient associated with AH flow (Figure 3(b)). Endothelial cells of the SC inner wall rest on an incomplete basal lamina and considerable areas of their basal cell membrane are not supported by ECM, but are in direct contact with the open spaces of the JCT and AH flow. Giant vacuoles form when AH pushes against the basal side of SC cells, and are often associated with intracellular pores that have diameters from 0.1 to 2 mm. In addition to intracellular pores, paracellular pores have been observed, which usually have diameters comparable to that of intracellular pores, but are less common in most eyes. Paracellular pores are very likely the morphological correlate for paracellular flow through SC endothelium, which has been described in perfusion studies using cathionized ferritin. The junctional complexes between SC cells contain tight junctions and very likely restrict paracellular flow. Inner-wall pores allow passage of microparticles 200–500 nm in size. In their 1972 work, Bill and Svedbergh calculated the hydraulic conductivity and the flow resistance generated by inner-wall pores and concluded that the inner-wall endothelium could not generate more than 10% of total TM outflow resistance. More recent experiments indicate that the number of intracellular pores increases with the amount of fixative perfused through the TM outflow pathways and with the postmortem time of the enucleated donor eye, strongly indicating that the total number of pores identified by electron microscopy in fixed tissues is likely considerably smaller than that in the living eye. The number of paracellular pores was not affected by the amount of fixative or postmortem time, but increased with increasing fixation pressure. As fixation conditions can influence the apparent
pore density in the inner-wall endothelium significantly, the conclusion reached previously that pores contribute only 10% of die aqueous outflow resistance, may require reevaluation. Nevertheless, it appears to be reasonable to assume that not all intracellular pores are artifacts, since the SC inner-wall endothelium has one of the highest hydraulic conductivities in the body, comparable only to that of fenestrated endothelia. The molecular processes that contribute to or cause formation of intracellular pores in SC inner-wall endothelium are largely unknown. Small minipores that are covered by a diaphragm (Figure 4) may be involved. Diaphragmed minipores (DMPs) have similar ultrastructural characteristics as those forming the fenestrae of fenestrated capillaries, for example, in ciliary body or choroid. These are regularly found in SC inner-wall endothelium, but are considerably rarer than the fenestrae in fenestrated epithelia. In their 1995 work, Bill and Maepea hypothesized that DMPs represent an early stage in pore formation. The thin diaphragm, which in blood vessels is probably supported by the basal lamina, has no such support in the inner-wall cells and so may tend to burst.
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Figure 4 Electron microscopy of an inner-wall giant vacuole (GV) that forms two intracellular pores (boxed areas in (a)). Both boxed areas are shown in (b) and (c) at higher magnification. While the pore in (b) is largely open and only covered by very fine filamentous material, the pore in (c) is covered by a diaphragm (arrow). Magnification bar ¼ 1 mm (a), 0.5 mm (b), 0.5 mm (c). From Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655.
Functional Morphology of the Trabecular Meshwork
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Figure 5 Electron microscopy of scleral spur cells. (a) Scleral spur cells (SSC) are in close contact with banded sheath material (asterisks) of elastic fibers. The cytoplasm of the cells is filled with abundant 6–7-nm-thin (actin) filaments which run parallel to the long axis of the cells (solid arrows). The cell membrane shows numerous membrane bound caveolae (open arrows). (b) Scleral spur cells may form long processes to contact the elastic fibers (asterisk) in the scleral spur. In region of contact, dense bands (arrows) are formed at the cell membrane of the scleral spur cell. (c) Scleral spur cells are connected by gap junctions (arrow). Magnification bar ¼ 1 mm (a, b); 125 nm (c). From Tamm, E., Flu¨gel, C., Stefani, F. H., and Rohen, J. W. (1992). Contractile cells in the human scleral spur. Experimental Eye Research 54: 531–543.
Contractile Influence on Trabecular Outflow The muscle bundles of the ciliary muscle form tendons in region of their anterior insertion, which attach to the scleral spur or are continuous with the ECM of the TM. Because of the structural connections between ciliary muscle, scleral spur, and TM, contraction of the ciliary muscle pulls the spur posteriorly and/or widens the trabecular spaces, thereby inducing changes in TM geometry that lead to a reduction in outflow resistance.
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Figure 6 Meridional (a, b) and tangential sections (c, d) through ciliary muscle (CM), scleral spur (SS), and trabecular meshwork (TM) stained with antibodies against a-smooth muscle actin (a, c) or desmin (b, d). (a) Ciliary muscle cells and vascular smooth muscle cells stain positively with antibodies against a-smooth muscle actin. Arrows indicate the scleral spur, where all cells show intense immunoreactivity for a-smooth muscle actin. (b) Immunostaining with antibodies against desmin. Ciliary muscle cells stain brightly positive, whereas the scleral spur is not labeled for desmin. (c) Tangential section of scleral spur, trabecular meshwork, and ciliary muscle. Positively stained cells oriented in a circular direction are seen throughout the entire spur tissue. While ciliary muscle cells also stain positive, no staining is seen in the trabecular meshwork. (d) Tangential section of scleral spur and ciliary muscle after staining for desmin. The plane of the section is the same as in (c). Ciliary muscle cells stain brightly positive, whereas the cells of the scleral spur remain unstained. Magnification bar ¼ 30 mm. From Tamm, E., Flu¨gel, C., Stefani, F. H., and Rohen, J. W. (1992). Contractile cells in the human scleral spur. Experimental Eye Research 54: 531–543.
In addition to ciliary muscle cells, there is another contractile cell population in this area, which very likely affects the TM tone. The resident cells within the scleral spur contain numerous actin filaments (Figure 5(a)), stain with antibodies against a-smooth muscle (SM) actin (Figures 6(a) and 6(c)), and generally express a myofibroblast-like phenotype. a-SM actin is an actin isoform that is typically expressed in vascular SM cells and myofibroblasts, cells that are present in healing
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E (b) Figure 7 Mechanosensors in the scleral spur. (a) Whole mount of the scleral spur stained with antibodies against neurofilament proteins. Axons are labeled that terminate as bulb or clubshaped structures (arrows). (b) Electron micrograph of a mechanoreceptive nerve terminal in the scleral spur. The terminal is a bulb or club shaped and contains numerous neurofilaments, mitochondria, and vesicles of different sizes. The elastic fibers of the scleral spur (E, open arrows), and the scleral spur cells (solid arrows) are in close proximity to the terminal. Magnification bar ¼ 10 mm (a), 1 mm (b). From Tamm, E. R. (2009). The trabecular meshwork outflow pathways. Functional morphology and surgical aspects. In Shaarawy, T. M., Sherwood, M. B., Hitchings, R. A., and Crowston, J. G. (eds.) Glaucoma, vol. II, pp. 31–44. Philadelphia, PA: Saunders.
wounds and scars. In contrast to the ciliary muscle cells that attach to the spur, scleral spur cells are circumferentially oriented (Figure 6(c)), and do not stain for desmin, the intermediate filament that is characteristic for ciliary muscle cells (Figures 6(b) and 6(d)). Scleral spur cells are innervated, coupled to each other by gap junctions (Figure 5(c)), and form tendon-like contacts with the elastic fibers within the scleral spur and their nonelastic sheath material (Figures 5(a) and 5(b)). Since scleral spur elastic fibers are directly continuous with the elastic fibers in the TM beams and the cribriform plexus in the JCT, changes in scleral spur cell tone are likely to
modulate outflow resistance by altering the architecture of the TM outflow pathways. In addition to scleral spur cells, some scattered cells in the TM proper have also been shown to express a-SM actin. The expression of a-SM actin induced by TGF-b1 has been shown to substantially enhance cell traction force in myofibroblasts, a scenario that might be also true for TM cells where TGF-b1 has similar effects on the expression of a-SM actin. There is experimental evidence that TM cells influence the hydraulic conductivity of the inner-wall region and outflow resistance by actively changing cell shape and altering the geometry of the outflow pathways. An increase in TM cell tone is correlated with an increase in outflow resistance. Throughout the entire circumference of the scleral spur, club-shaped nerve endings are found, which derive from myelinated axons (Figure 7(a)). The ultrastructure of the nerve endings is very similar to that of mechanosensors in other parts of the body (Figure 7(b)). The cell membrane of the nerve endings is in direct contact with the elastic fibers of the scleral spur. The contact between nerve terminal and connective tissue fibers is a very characteristic feature of mechanosensors, as it is required to measure the tone of the extracellular fibers. The mechanosensors of the scleral spur may act as proprioreceptive tendon organs for the ciliary muscle, or modulate the tone of the scleral spur cells. Alternatively, scleral spur mechanosensors could perform a baroreceptor-like function in response to changes in IOP. Indeed, physiological studies indicate that such sensors might exist, as sensory discharges have been recorded in experimental animals in response to changes in IOP.
Acknowledgment This work was supported by DFG Research Unit 1075 (TP 5). See also: The Biology of Schlemm’s Canal; Biomechanics of Aqueous Humor Outflow Resistance; The Fibrillar Extracellular Matrix of the Trabecular Meshwork; Myocilin; Role of Proteoglycans in the Trabecular Meshwork; Structural Changes in the Trabecular Meshwork with Primary Open Angle Glaucoma.
Further Reading Acott, T. S. and Kelley, M. J. (2008). Extracellular matrix in the trabecular meshwork. Experimental Eye Research 86: 543–561. Bill, A. and Ma¨epea, O. (1995). Mechanisms and routes of aqueous humor drainage. In: Albert, D. M. and Jakobiec, F. A. (eds.) Principles and Practice of Ophthalmology, Vol. I: Basic Sciences, pp. 206–226. Philadelphia, PA: WB Saunders. Bron, A. J., Tripathi, R. C., and Tripathi, B. J. (1997). Anterior chamber and drainage angle. In: Bron, A. J., Tripathi, R. C., and Tripathi, B. J. (eds.) Wolff’s Anatomy of the Eye and Orbit, pp. 279–307. London: Chapman and Hall Medical.
Functional Morphology of the Trabecular Meshwork Epstein, D. L. and Rohen, J. W. (1991). Morphology of the trabecular meshwork and inner-wall endothelium after cationized ferritin perfusion in the monkey eye. Investigative Ophthalmology and Visual Science 32: 160–171. Ethier, C. R. (2002). The inner wall of Schlemm’s canal. Experimental Eye Research 74: 161–172. Ethier, C. R., Coloma, F. M., Sit, A. J., and Johnson, M. (1998). Two pore types in the inner-wall endothelium of Schlemm’s canal. Investigative Ophthalmology and Visual Science 39: 2041–2048. Fuchshofer, R. and Tamm, E. R. (2009). Modulation of extracellular matrix turnover in the trabecular meshwork. Experimental Eye Research 88: 683–688. Gong, H., Ruberti, J., Overby, D., Johnson, M., and Freddo, T. F. (2002). A new view of the human trabecular meshwork using quickfreeze, deep-etch electron microscopy. Experimental Eye Research 75: 347–358. Johnson, M. (2006). What controls aqueous humour outflow resistance? Experimental Eye Research 82: 545–557. Johnson, M., Chan, D., Read, A. T., et al. (2002). The pore density in the inner wall endothelium of Schlemm’s canal of glaucomatous eyes. Investigative Ophthalmology and Visual Science 43: 2950–2955. Lu¨tjen-Drecoll, E. (1999). Functional morphology of the trabecular meshwork in primate eyes. Progress in Retinal and Eye Research 18: 91–119. Rohen, J. W., Futa, R., and Lu¨tjen-Drecoll, E. (1981). The fine structure of the cribriform meshwork in normal and glaucomatous eyes as seen in tangential sections. Investigative Ophthalmology and Visual Science 21: 574–585. Rohen, J. W., Lu¨tjen, E., and Ba´ra´ny, E. H. (1967). The relation between the ciliary muscle and the trabecular meshwork and its importance
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for the effect of miotics on aqueous outflow resistance. A study in two contrasting monkey species, macaca irus and cercopithecus aethiops. Albrecht von Graefe’s Archive for Clinical and Experimental Ophthalmology 172: 23–47. Sit, A. J., Coloma, F. M., Ethier, C. R., and Johnson, M. (1997). Factors affecting the pores of the inner wall endothelium of Schlemm’s canal. Investigative Ophthalmology and Visual Science 38: 1517–1525. Tamm, E., Flu¨gel, C., Stefani, F. H., and Rohen, J. W. (1992). Contractile cells in the human scleral spur. Experimental Eye Research 54: 531–543. Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655. Tamm, E. R. (2009). The trabecular meshwork outflow pathways. Functional morphology and surgical aspects. In: Shaarawy, T. M., Sherwood, M. B., Hitchings, R. A., and Crowston, J. G. (eds.) Glaucoma, vol. II, pp. 31–44. Philadelphia, PA: Saunders. Tamm, E. R., Flu¨gel, C., Stefani, F. H., and Lu¨tjen-Drecoll, E. (1994). Nerve endings with structural characteristics of mechanoreceptors in the human scleral spur. Investigative Ophthalmology and Visual Science 35: 1157–1166. Tamm, E. R. and Lu¨tjen-Drecoll, E. (1996). Ciliary body. Microscopy Research and Technique 33: 390–439. Tian, B., Gabelt, B. T., Geiger, B., and Kaufman, P. L. (2008). The role of the actomyosin system in regulating trabecular fluid outflow. Experimental Eye Research 88: 713–717. Wiederholt, M., Thieme, H., and Stumpff, F. (2000). The regulation of trabecular meshwork and ciliary muscle contractility. Progress in Retinal and Eye Research 19: 271–295.
Glossary Inner-wall region – Comprised of the inner-wall endothelium of Schlemm’s canal, its basement membrane, and the adjacent juxtacanalicular tissue. It is the site of resistance in the trabecular meshwork outflow pathways. Juxtacanalicular tissue – The outermost part of the trabecular meshwork that lies directly adjacent to Schlemm’s canal. It does not form lamellae, but represents a loose connective tissue. An alternative term is cribriform meshwork. Mechanosensor – The terminal of an afferent nerve ending that is in contact with fibrillar components of the extracellular matrix and measures stress or strain in those. An alternative term is mechanoreceptor. Myofibroblast – A resident cell in some connective tissues that shows structural and functional characteristics of both fibroblasts and smooth muscle cells, and appears to be an intermediate form of those. Schlemm’s canal – A circular vascular tube that lies in the scleral sulcus, a circular groove on the inner aspect of the corneoscleral limbus. It is part of the trabecular outflow pathways. The inner aspects of Schlemm’s canal are in contact with the juxtacanalicular tissue. Scleral spur – A wedge-shaped circular ridge protruding from the sclera posterior to Schlemm’s canal. Trabecular meshwork – A porous, filter-like tissue in the irido-corneal angle consisting of connective tissue strands (trabecular beams or lamellae) that attach to one another and are covered by cells.
Introduction The major drainage structures for aqueous humor (AH) are the conventional or trabecular outflow pathways, which are comprised of the trabecular meshwork (made up by the uveal and corneoscleral meshworks), the *An adaptation and extension of Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655. http://www.sciencedirect.com/science/ journal/00144835.
juxtacanalicular connective tissue ( JCT), the endothelial lining of Schlemm’s canal (SC), the collecting channels, and the aqueous veins. When AH has passed through the trabecular outflow pathways, it drains into the episcleral venous system. In addition, there is an unconventional or uveoscleral outflow route which is open to AH at the chamber angle in the region of the anterior insertion of the ciliary muscle, since there is no complete endothelial or epithelial layer that covers the anterior surface of the ciliary body. The trabecular meshwork (TM) outflow pathways are critical in providing resistance to AH outflow. Intraocular pressure (IOP) builds up in response to this resistance until it is high enough to allow flow of AH across the TM into SC. AH passes through the TM as bulk flow driven by the pressure gradient, and active transport is not involved as neither metabolic poisons nor temperature affects AH flow across the TM. At steady-state IOP, fluid flow rate across the trabecular outflow resistance equals the rate of aqueous production by the ciliary body. Outflow resistance in the TM outflow pathways increases with age and primary open-angle glaucoma (POAG). This article reviews the functional morphology of the TM outflow pathways.
The TM Outflow Pathways The tissue structures of the TM outflow pathways are embedded in the internal scleral sulcus, a circular groove on the inner aspect of the sclera in region of the corneoscleral limbus. The scleral sulcus extends from the peripheral edge of Descemet’s membrane to the scleral spur, a wedge-shaped circular ridge protruding from the inner sclera (Figure 1(a)). SC, a circular, vascular-like collecting channel, lies in the outer portion of the scleral sulcus, while the TM occupies most of its inner aspects (Figure 1(a)). The TM is formed by connective tissue beams or lamellae that have a core of collagenous and elastic fibers, and are covered by flat cells which rest on a basal lamina. The beams attach to one another in several layers and form a porous filter-like structure. Anteriorly, the trabecular beams are attached near the end of Descemet’s membrane (Schwalbe’s line) and extend posteriorly to the stroma of the ciliary body and iris at their junction, and to the scleral spur (Figure 1(a)). As SC is shorter in anterior–posterior direction than the TM, a filtering portion of the TM can be differentiated from a nonfiltering
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portion which has no SC next to its external aspect (Figure 1(a)). Those cells of the nonfiltering portion of the TM, which reside close to the attachment of the TM at Schwalbe’s line, differ in structure from that of the TM proper. There is some evidence that this region serves as a niche for cells with adult stem cell-/progenitor cell-like properties that are capable of dividing and repopulating the filtering part of the TM after injury. The TM consists of three regions that differ in structure: the inner uveal meshwork, the deeper corneoscleral meshwork, and the JCT or cribriform region that is localized directly adjacent to the inner-wall endothelium of SC (Figure 1(b)). The uveal meshwork, which originates from the anterior aspect of the ciliary body, consists of one to three layers of trabecular beams or lamellae (Figure 1(b)). The corneoscleral meshwork forms 8–15 trabecular layers, which are thicker than those of the uveal TM and originate from the scleral spur (Figure 1(b)). The JCT, which is localized directly to the endothelial lining of SC, is the smallest part of the TM with a thickness of only 2–20 mm. It does not form trabecular lamellae or connective tissue
beams, but rather represents a typical loose connective tissue with two to five layers of scattered cells that are embedded in a loosely arranged fibrillar extracellular matrix (ECM) (Figures 1(b) and 2(a)).
The Trabecular Lamellae Each lamella or beam of the uveal or corneoscleral TM contains densely packed collagen and elastic fibers. The collagen fibers are mostly collagens type I and III. The elastic fibers, which localize to the core of the beams (Figure 2(b)), differ in their ultrastructure from those in other parts of the body, as they contain considerable amounts of electron-dense material. TM elastic fibers are surrounded by a sheath that thickens with age and
GV SC JCT
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UTM AC (b) Figure 1 Light micrograph of a meridional section through the trabecular meshwork. (b) is a magnification of (a). Arrows in (b) point to giant vacuoles in the inner-wall endothelium of SC. Magnification bar ¼ 20 mm (a), 5 mm (b). SC, Schlemm’s canal; TM, trabecular meshwork; SS, scleral spur; CM, ciliary muscle; AC, anterior chamber; JCT, juxtacanalicular tissue; CTM, corneoscleral trabecular meshwork; UTM, uveal trabecular meshwork. From Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655.
Figure 2 Electron micrographs of the trabecular meshwork. (a) Cells and extracellular fibrils in the juxtacanalicular tissue (JCT) are arranged in an irregularly network in contrast to the more regular structure of the beams in the corneoscleral trabecular meshwork (CTM). (b) Ultrastructure of a corneoscleral trabecular meshwork beam. The core of the beam contains elastic fibers (E) that are surrounded by banded sheath material. Open arrows denote the basal lamina of the trabecular meshwork cells, while solid arrows point to aggregates of long-spacing collagen in the cortical region of the beam. The beam is completely covered by flat trabecular meshwork cells. SC, Schlemm’s canal; GV, giant vacuole. Magnification bar ¼ 2 mm. From Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655.
Functional Morphology of the Trabecular Meshwork
that may show an 80–120-nm periodicity. Clumps of similar material and periodicity have been referred to as long-spacing collagen and are found in the cortical zone of the beams, between the elastic fiber core and basal lamina of the TM cells (Figure 2(b)). Fine fibrils that enter the aggregates of long-spacing collagen have been shown to label with antibodies against collagen type VI. The cells covering the TM beams reside on a basal lamina that is rich in collagen type IV and laminin (Figure 2(b)). A three-dimensional network is established by TM cells that bridge intertrabecular spaces to cover two adjacent beams. TM cells are capable of phagocytosis and may contain pigment particles, and these particles in TM cells probably derive from the iris and their phagocytosis by TM cells may be part of an important self-cleaning mechanism of the trabecular filter. TM regions that contain a high number of pigmented cells appear to be preferentially localized adjacent to collector channels, suggesting preferential AH flow pathways in the TM.
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The Juxtacanalicular Tissue TM cells in the JCT are surrounded by fibrillar elements of the ECM to form a loose connective tissue. As JCT cells and ECM fibrils are arranged in an irregular network (in contrast to the more regular structure of the beams in the inner parts of the TM), some authors prefer the term cribriform meshwork. The cells in the JCT form long processes by which they attach to one other, to ECM fibrils or to the cells of the endothelial lining of SC (Figure 2(a)). The spaces between JCT cells and ECM fibers serve as pathway for AH and contain a ground substance consisting of various proteoglycans and hyaluronan. The extent to which this ground substance fills the spaces between cells and fibrillar ECM is not clear as proteoglycans are not readily retained during processing of tissue for conventional electron microscopy. Recent studies using quick-freeze/deep-etch methodology visualized considerably more ECM in the JCT compared to that seen by conventional electron microscopy. A characteristic structural element of the JCT is a network of elastic fibers (cribriform plexus), which is arranged tangentially to the SC endothelial lining. The elastic fibers of the cribriform plexus show ultrastructural characteristics similar to those in the trabecular beams as they consist of an electron-dense core and a sheath of banded material. The molecular nature of the sheath material has not been identified, but there is evidence that collagen type VI and fibronectin are associated with it. Fibers of the cribriform plexus connect with the SC inner-wall endothelium through either their sheath material or fine fibrils that emerge from it (Figure 3(a)). Fibronectin-based cell–matrix connections appear to be very important for the adhesion of SC endothelial cells to the fibrillar ECM
(b) Figure 3 Electron microscopy of the SC inner-wall region. (a) Connecting fibers (CF) in the juxtacanalicular tissue, which emerge from the cribriform elastic plexus, connect the plexus with the inner-wall endothelium of Schlemm’s canal (SC). The connection with the inner-wall endothelium is made through either the banded sheath material of the fibers or fine fibrils that emerge from it (solid arrows). (b) A giant vacuole in the inner wall of SC forms an intracellular pore (arrow). Magnification bar ¼ 1 mm. From Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655.
of the JCT, as treatment of perfused anterior segment organ cultures with the integrin/syndecan-binding domain of fibronectin, called the heparin II (Hep II) domain, leads to a focal detachment of SC endothelium from the JCT, and to a significant increase in outflow facility. The ECM in the JCT does not only contain structural elements, but also matricellular proteins such as thrombospondin-1 and SPARC. Matricellular proteins are secreted ECM proteins that influence cell function by modulating cell–matrix interactions. Another important component of the JCT ECM is myocilin, one of the most highly expressed molecules in the TM. Myocilin has been shown to associate with fibrillar elements of the JCT ECM, but its biochemical function is largely unclear. Synthesis and degradation of ECM molecules in the JCT is under control of autocrine and paracrine growth factors, such as transforming growth factor beta 2 (TGF-b2), connective tissue growth factor (CTGF), bone morphogenetic protein 7 (BMP7), and BMP4. In addition, glucocorticoids and prostaglandin derivatives appear to modulate
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ECM turnover in the JCT. A characteristic structural change in the eyes of patients with POAG and steroidinduced glaucoma is an increase in fibrillar ECM in the JCT. Because of their high porosity, the uveal and corneoscleral parts of the TM do not provide a significant resistance to AH outflow. Support for this comes from experimental studies, which show that cutting through the inner parts of the TM does not affect outflow facility, and from theoretical calculations using Poiseuille’s law. In contrast, there is considerable evidence that normal AH outflow resistance resides in the inner-wall region of SC, which is formed by the JCT and SC inner-wall endothelium. As of today, there is active debate and research regarding the specific role of SC inner-wall endothelium and/or JCT for the formation of resistance in the TM outflow pathways.
Schlemm’s Canal A characteristic aspect of the SC inner-wall endothelium is the formation of cellular outpouchings (the so-called giant vacuoles) in response to the pressure gradient associated with AH flow (Figure 3(b)). Endothelial cells of the SC inner wall rest on an incomplete basal lamina and considerable areas of their basal cell membrane are not supported by ECM, but are in direct contact with the open spaces of the JCT and AH flow. Giant vacuoles form when AH pushes against the basal side of SC cells, and are often associated with intracellular pores that have diameters from 0.1 to 2 mm. In addition to intracellular pores, paracellular pores have been observed, which usually have diameters comparable to that of intracellular pores, but are less common in most eyes. Paracellular pores are very likely the morphological correlate for paracellular flow through SC endothelium, which has been described in perfusion studies using cathionized ferritin. The junctional complexes between SC cells contain tight junctions and very likely restrict paracellular flow. Inner-wall pores allow passage of microparticles 200–500 nm in size. In their 1972 work, Bill and Svedbergh calculated the hydraulic conductivity and the flow resistance generated by inner-wall pores and concluded that the inner-wall endothelium could not generate more than 10% of total TM outflow resistance. More recent experiments indicate that the number of intracellular pores increases with the amount of fixative perfused through the TM outflow pathways and with the postmortem time of the enucleated donor eye, strongly indicating that the total number of pores identified by electron microscopy in fixed tissues is likely considerably smaller than that in the living eye. The number of paracellular pores was not affected by the amount of fixative or postmortem time, but increased with increasing fixation pressure. As fixation conditions can influence the apparent
pore density in the inner-wall endothelium significantly, the conclusion reached previously that pores contribute only 10% of die aqueous outflow resistance, may require reevaluation. Nevertheless, it appears to be reasonable to assume that not all intracellular pores are artifacts, since the SC inner-wall endothelium has one of the highest hydraulic conductivities in the body, comparable only to that of fenestrated endothelia. The molecular processes that contribute to or cause formation of intracellular pores in SC inner-wall endothelium are largely unknown. Small minipores that are covered by a diaphragm (Figure 4) may be involved. Diaphragmed minipores (DMPs) have similar ultrastructural characteristics as those forming the fenestrae of fenestrated capillaries, for example, in ciliary body or choroid. These are regularly found in SC inner-wall endothelium, but are considerably rarer than the fenestrae in fenestrated epithelia. In their 1995 work, Bill and Maepea hypothesized that DMPs represent an early stage in pore formation. The thin diaphragm, which in blood vessels is probably supported by the basal lamina, has no such support in the inner-wall cells and so may tend to burst.
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Figure 4 Electron microscopy of an inner-wall giant vacuole (GV) that forms two intracellular pores (boxed areas in (a)). Both boxed areas are shown in (b) and (c) at higher magnification. While the pore in (b) is largely open and only covered by very fine filamentous material, the pore in (c) is covered by a diaphragm (arrow). Magnification bar ¼ 1 mm (a), 0.5 mm (b), 0.5 mm (c). From Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655.
Functional Morphology of the Trabecular Meshwork
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Figure 5 Electron microscopy of scleral spur cells. (a) Scleral spur cells (SSC) are in close contact with banded sheath material (asterisks) of elastic fibers. The cytoplasm of the cells is filled with abundant 6–7-nm-thin (actin) filaments which run parallel to the long axis of the cells (solid arrows). The cell membrane shows numerous membrane bound caveolae (open arrows). (b) Scleral spur cells may form long processes to contact the elastic fibers (asterisk) in the scleral spur. In region of contact, dense bands (arrows) are formed at the cell membrane of the scleral spur cell. (c) Scleral spur cells are connected by gap junctions (arrow). Magnification bar ¼ 1 mm (a, b); 125 nm (c). From Tamm, E., Flu¨gel, C., Stefani, F. H., and Rohen, J. W. (1992). Contractile cells in the human scleral spur. Experimental Eye Research 54: 531–543.
Contractile Influence on Trabecular Outflow The muscle bundles of the ciliary muscle form tendons in region of their anterior insertion, which attach to the scleral spur or are continuous with the ECM of the TM. Because of the structural connections between ciliary muscle, scleral spur, and TM, contraction of the ciliary muscle pulls the spur posteriorly and/or widens the trabecular spaces, thereby inducing changes in TM geometry that lead to a reduction in outflow resistance.
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Figure 6 Meridional (a, b) and tangential sections (c, d) through ciliary muscle (CM), scleral spur (SS), and trabecular meshwork (TM) stained with antibodies against a-smooth muscle actin (a, c) or desmin (b, d). (a) Ciliary muscle cells and vascular smooth muscle cells stain positively with antibodies against a-smooth muscle actin. Arrows indicate the scleral spur, where all cells show intense immunoreactivity for a-smooth muscle actin. (b) Immunostaining with antibodies against desmin. Ciliary muscle cells stain brightly positive, whereas the scleral spur is not labeled for desmin. (c) Tangential section of scleral spur, trabecular meshwork, and ciliary muscle. Positively stained cells oriented in a circular direction are seen throughout the entire spur tissue. While ciliary muscle cells also stain positive, no staining is seen in the trabecular meshwork. (d) Tangential section of scleral spur and ciliary muscle after staining for desmin. The plane of the section is the same as in (c). Ciliary muscle cells stain brightly positive, whereas the cells of the scleral spur remain unstained. Magnification bar ¼ 30 mm. From Tamm, E., Flu¨gel, C., Stefani, F. H., and Rohen, J. W. (1992). Contractile cells in the human scleral spur. Experimental Eye Research 54: 531–543.
In addition to ciliary muscle cells, there is another contractile cell population in this area, which very likely affects the TM tone. The resident cells within the scleral spur contain numerous actin filaments (Figure 5(a)), stain with antibodies against a-smooth muscle (SM) actin (Figures 6(a) and 6(c)), and generally express a myofibroblast-like phenotype. a-SM actin is an actin isoform that is typically expressed in vascular SM cells and myofibroblasts, cells that are present in healing
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(a) SS
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E (b) Figure 7 Mechanosensors in the scleral spur. (a) Whole mount of the scleral spur stained with antibodies against neurofilament proteins. Axons are labeled that terminate as bulb or clubshaped structures (arrows). (b) Electron micrograph of a mechanoreceptive nerve terminal in the scleral spur. The terminal is a bulb or club shaped and contains numerous neurofilaments, mitochondria, and vesicles of different sizes. The elastic fibers of the scleral spur (E, open arrows), and the scleral spur cells (solid arrows) are in close proximity to the terminal. Magnification bar ¼ 10 mm (a), 1 mm (b). From Tamm, E. R. (2009). The trabecular meshwork outflow pathways. Functional morphology and surgical aspects. In Shaarawy, T. M., Sherwood, M. B., Hitchings, R. A., and Crowston, J. G. (eds.) Glaucoma, vol. II, pp. 31–44. Philadelphia, PA: Saunders.
wounds and scars. In contrast to the ciliary muscle cells that attach to the spur, scleral spur cells are circumferentially oriented (Figure 6(c)), and do not stain for desmin, the intermediate filament that is characteristic for ciliary muscle cells (Figures 6(b) and 6(d)). Scleral spur cells are innervated, coupled to each other by gap junctions (Figure 5(c)), and form tendon-like contacts with the elastic fibers within the scleral spur and their nonelastic sheath material (Figures 5(a) and 5(b)). Since scleral spur elastic fibers are directly continuous with the elastic fibers in the TM beams and the cribriform plexus in the JCT, changes in scleral spur cell tone are likely to
modulate outflow resistance by altering the architecture of the TM outflow pathways. In addition to scleral spur cells, some scattered cells in the TM proper have also been shown to express a-SM actin. The expression of a-SM actin induced by TGF-b1 has been shown to substantially enhance cell traction force in myofibroblasts, a scenario that might be also true for TM cells where TGF-b1 has similar effects on the expression of a-SM actin. There is experimental evidence that TM cells influence the hydraulic conductivity of the inner-wall region and outflow resistance by actively changing cell shape and altering the geometry of the outflow pathways. An increase in TM cell tone is correlated with an increase in outflow resistance. Throughout the entire circumference of the scleral spur, club-shaped nerve endings are found, which derive from myelinated axons (Figure 7(a)). The ultrastructure of the nerve endings is very similar to that of mechanosensors in other parts of the body (Figure 7(b)). The cell membrane of the nerve endings is in direct contact with the elastic fibers of the scleral spur. The contact between nerve terminal and connective tissue fibers is a very characteristic feature of mechanosensors, as it is required to measure the tone of the extracellular fibers. The mechanosensors of the scleral spur may act as proprioreceptive tendon organs for the ciliary muscle, or modulate the tone of the scleral spur cells. Alternatively, scleral spur mechanosensors could perform a baroreceptor-like function in response to changes in IOP. Indeed, physiological studies indicate that such sensors might exist, as sensory discharges have been recorded in experimental animals in response to changes in IOP.
Acknowledgment This work was supported by DFG Research Unit 1075 (TP 5). See also: The Biology of Schlemm’s Canal; Biomechanics of Aqueous Humor Outflow Resistance; The Fibrillar Extracellular Matrix of the Trabecular Meshwork; Myocilin; Role of Proteoglycans in the Trabecular Meshwork; Structural Changes in the Trabecular Meshwork with Primary Open Angle Glaucoma.
Further Reading Acott, T. S. and Kelley, M. J. (2008). Extracellular matrix in the trabecular meshwork. Experimental Eye Research 86: 543–561. Bill, A. and Ma¨epea, O. (1995). Mechanisms and routes of aqueous humor drainage. In: Albert, D. M. and Jakobiec, F. A. (eds.) Principles and Practice of Ophthalmology, Vol. I: Basic Sciences, pp. 206–226. Philadelphia, PA: WB Saunders. Bron, A. J., Tripathi, R. C., and Tripathi, B. J. (1997). Anterior chamber and drainage angle. In: Bron, A. J., Tripathi, R. C., and Tripathi, B. J. (eds.) Wolff’s Anatomy of the Eye and Orbit, pp. 279–307. London: Chapman and Hall Medical.
Functional Morphology of the Trabecular Meshwork Epstein, D. L. and Rohen, J. W. (1991). Morphology of the trabecular meshwork and inner-wall endothelium after cationized ferritin perfusion in the monkey eye. Investigative Ophthalmology and Visual Science 32: 160–171. Ethier, C. R. (2002). The inner wall of Schlemm’s canal. Experimental Eye Research 74: 161–172. Ethier, C. R., Coloma, F. M., Sit, A. J., and Johnson, M. (1998). Two pore types in the inner-wall endothelium of Schlemm’s canal. Investigative Ophthalmology and Visual Science 39: 2041–2048. Fuchshofer, R. and Tamm, E. R. (2009). Modulation of extracellular matrix turnover in the trabecular meshwork. Experimental Eye Research 88: 683–688. Gong, H., Ruberti, J., Overby, D., Johnson, M., and Freddo, T. F. (2002). A new view of the human trabecular meshwork using quickfreeze, deep-etch electron microscopy. Experimental Eye Research 75: 347–358. Johnson, M. (2006). What controls aqueous humour outflow resistance? Experimental Eye Research 82: 545–557. Johnson, M., Chan, D., Read, A. T., et al. (2002). The pore density in the inner wall endothelium of Schlemm’s canal of glaucomatous eyes. Investigative Ophthalmology and Visual Science 43: 2950–2955. Lu¨tjen-Drecoll, E. (1999). Functional morphology of the trabecular meshwork in primate eyes. Progress in Retinal and Eye Research 18: 91–119. Rohen, J. W., Futa, R., and Lu¨tjen-Drecoll, E. (1981). The fine structure of the cribriform meshwork in normal and glaucomatous eyes as seen in tangential sections. Investigative Ophthalmology and Visual Science 21: 574–585. Rohen, J. W., Lu¨tjen, E., and Ba´ra´ny, E. H. (1967). The relation between the ciliary muscle and the trabecular meshwork and its importance
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for the effect of miotics on aqueous outflow resistance. A study in two contrasting monkey species, macaca irus and cercopithecus aethiops. Albrecht von Graefe’s Archive for Clinical and Experimental Ophthalmology 172: 23–47. Sit, A. J., Coloma, F. M., Ethier, C. R., and Johnson, M. (1997). Factors affecting the pores of the inner wall endothelium of Schlemm’s canal. Investigative Ophthalmology and Visual Science 38: 1517–1525. Tamm, E., Flu¨gel, C., Stefani, F. H., and Rohen, J. W. (1992). Contractile cells in the human scleral spur. Experimental Eye Research 54: 531–543. Tamm, E. R. (2009). The trabecular meshwork outflow pathways: Structural and functional aspects. Experimental Eye Research 88: 648–655. Tamm, E. R. (2009). The trabecular meshwork outflow pathways. Functional morphology and surgical aspects. In: Shaarawy, T. M., Sherwood, M. B., Hitchings, R. A., and Crowston, J. G. (eds.) Glaucoma, vol. II, pp. 31–44. Philadelphia, PA: Saunders. Tamm, E. R., Flu¨gel, C., Stefani, F. H., and Lu¨tjen-Drecoll, E. (1994). Nerve endings with structural characteristics of mechanoreceptors in the human scleral spur. Investigative Ophthalmology and Visual Science 35: 1157–1166. Tamm, E. R. and Lu¨tjen-Drecoll, E. (1996). Ciliary body. Microscopy Research and Technique 33: 390–439. Tian, B., Gabelt, B. T., Geiger, B., and Kaufman, P. L. (2008). The role of the actomyosin system in regulating trabecular fluid outflow. Experimental Eye Research 88: 713–717. Wiederholt, M., Thieme, H., and Stumpff, F. (2000). The regulation of trabecular meshwork and ciliary muscle contractility. Progress in Retinal and Eye Research 19: 271–295.