purposes. In any case, endothelial cells of TM seem to have a leading role in outflow: probably, their tridimensional architecture and allocation on the trabecular beams considerably increases the filtration surface whose degeneration, resulting in the decay of HTM cellularity, causes IOP increase and triggers glaucoma. Important alterations in the cellular component and in the TM in toto, during POAG or in aging, occur in the inner layers near the anterior chamber (Alvarado et al., 1984a, b). Furthermore, in the juxtacanalicular– Schlemm’s canal region, a marker of cell senescence stains the endothelia cells more than controls (Liton et al., 2005).

The analysis of POAG patients’ eyes revealed thickened trabeculae, an increased amount of plaque-material deposits, and a collagen abundance. The uveal meshwork is partly deprived of cells. The cribriform layer often contained numerous enlarged, light cells with many small mitochondria and lysosomes (Gabelt and Kaufman, 2005).

Human outflow facility decreases with age. The age-related decrease is about 30% from the youngest (o40 years) to the oldest (W60 years) (Gaasterland et al., 1978). This phenomenon could be explained by the decrease in pseudofacility alone which could, in part, be due to the increase in ocular rigidity with age (Toris et al., 1999). The aqueous production shows an age-related decline that approximately amounts to 15–35% over the age range of 20–80 years (Gaasterland et al., 1978; Brubaker et al., 1981; Toris et al., 1999).

IOP increase and free radicals

Anterior chamber endothelial cells are always in contact with free radicals and are involved in antioxidant activities that counteract the toxic effects of oxidative stress. Aging is an inevitable biological process characterized by a general decline in physiological function that manifests itself in the anterior chamber with an endothelial cellular loss in TM (Alvarado et al., 1981, 1984a, b) involving, above all, the filtering area (Alvarado et al., 1981). This phenomenon can explain why IOP slightly increases with age in many Western populations (Brubaker et al., 1981).

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In POAG, TM cell loss corresponds to a decrease of the AH flow: 65% of inner-layer trabecular cells die. In addition, in the cribriform layer, there is also an increase of the amount of extracellular material and of thickness in the elastic fiber sheaths (Gabelt et al., 2003).

Increased incidence of cells detaching from trabeculae and a progressively greater incidence of fusion between adjacent trabeculae occur with age (Grierson et al., 1982).

HTM is a heterogeneous tissue where the endothelial cells drive a mechanism that controls the permeability of the TM. These endothelial cells release vasoactive cytokines and other factors able to increase the permeability of the endothelial barrier of the Schlemm’s canal. Therefore, cytokines alterations may cause outflow shift and, subsequently, IOP may rise to abnormal levels (Alvarado et al., 2005).

It is possible that toxic substances present in AH could in some way contribute to the appearance of these burden pathogenetic alterations of the TM. For example, the AH is capable of producing levels of H2O2 (Spector et al., 1998), yet HTM cells are in contact with relatively high concentrations of H2O2 and the exposure to H2O2 causes a decrease in facility in eyes with the GSH-depleted TM (Kahn et al., 1983).

Free radicals are the triggering agents in cellular decay, and AH antioxidant activity in patients with POAG is reduced (Ferreira et al., 2004). Indeed, the H2O2 effect on the adhesion of HTM cells to ECM proteins results in rearrangement of cytoskeletal structures that may induce a decrease of TM cell adhesion, cell loss, and compromise HTM integrity (Zhou et al., 1999).

In AH, there are many factors that have a protective role for endothelial cells, as GSH, which protects anterior segment tissues from high levels of H2O2 (Costarides et al., 1991). Likewise, a 28-amino acid neurotrophic factor present in human AH protects corneal endothelial cells from H2O2 and from other oxidative insults (Koh and Waschek, 2000).

Both GSH and ascorbate have been detected in AH (Rose et al., 1998). In humans, ascorbic acid is the major water-soluble antioxidant (Reiss, 1986), and is 15 times higher in concentration in the AH

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than in plasma (Becker, 1957), even if its concentration is closely correlated to the level of ascorbic acid in serum (Haung et al., 1997). Both vitamin C and GSH operate in fluid outside the cell, and GSH participates directly in the neutralization of free radicals and reactive oxygen compounds, being essential to protect protein thiol groups in order to maintain enzyme activity; moreover, GSH maintains exogenous antioxidants such as vitamins C and E in their reduced (active) forms (Cardoso et al., 1998). The GSH redox system is believed to protect ocular tissues from H2O2 low concentrations, whereas catalase is thought to protect ocular tissues from the damage induced by higher H2O2 concentrations (Riley, 1990; Costarides et al., 1991).

These antioxidant agents seem to play a particularly important role in glaucomatous disease. Indeed, patients with glaucoma exhibit low levels of circulating GSH, suggesting a general impairment of antioxidative defenses (Gherghel et al., 2005). In particular, in AH, oxidative stress may lead to an induction of antioxidant enzymes and contribute to decreasing the activities of the antioxidant enzymes SOD, catalase, and glutathione peroxidase (Ferreira et al., 2004). Indeed, POAG patients’ AH contains lipid peroxides in higher concentrations than in normal eyes (Babizhayev and Bunin, 1989).

When mitochondrial GSH and vitamin E levels are reduced to 20% of the normal level, lipid peroxidation occurs (Augustin et al., 1997). In any case, GSH decrease and H2O2 increase in AH may contribute to collagen deposition in POAG TM (Veach, 2004). Yet, DNA damage of TM cells in POAG patients shows statistically significant correlations with IOP peaks (Sacca`et al., 2005). Besides, a recent experimental study shows that oxidative damage and IOP increase contribute to glaucomatous optic neuropathy (Liu et al., 2007). Therefore, insufficient GSH combined with exogenous H2O2 may induce collagen matrix remodeling and trabecular cell apoptosis independently of mitochondria (Sacca`et al., 2007).

H2O2 activates both trabecular cells and astrocytes to produce heat shock proteins (HSP) (Tamm et al., 1996; Takuma et al., 2002). HSP are produced in response to stress from reactive

oxygen species (ROS), and in glaucomatous eyes, there is an increased concentration of HSP, both in the lamina cribrosa and TM (Tamm et al., 1996; Lutjen-Drecoll et al., 1998; Tezel et al., 2000). This phenomenon appeared to be the consequence of oxidative stress in ECM tissues (Tamm et al., 1996; Takuma et al., 2002). From a biomolecular point of view, mitochondria are selective targets for the protective effects of HSP against oxidative injury, still, HSP prevented lipid peroxidation of the mitochondrial membrane and H2O2-induced cell death (Polla et al., 1996) (Fig. 1).

Trabecular meshwork-inducible glucocorticoid response (TIGR), also known as myocilin (MYOC), may have a role in the regulation of aqueous outflow and, therefore, may have an extracellular function. The enhanced matrix deposition mediated by ascorbate in the AH could lead to the increased deposition of TIGR/MYOC from the AH into the ECM of TM in vivo (Filla et al., 2002). How extracellular TIGR/MYOC may function in the TM is unknown; moreover, the specificity of a TIGR/MYOC interaction with matrix proteins is confirmed and specifically interacts with the Hep II domain of fibronectin (Filla et al., 2002). This fibronectin domain is well known for its biological role in cellular adhesion, cytoskeleton organization, signal transduction, and phagocytosis (Gong et al., 1996; Tian et al., 2000).

Furthermore, in glaucoma patients’ TM, the expression of the endothelial-leukocyte adhesion molecule (ELAM-1) is abundant and it is mediated by an autocrine feedback mechanism of activation by interleukin-1 through the nuclear transcription factor kappa B (NF-kB) (Wang et al., 2001).

The influence of genetic polymorphism

Moreover, genetic polymorphisms have been detected for GSH transferase (GST) isoenzyme, and the GSTM1-null genotype has been found to be significantly more common in patients with POAG than in controls (Izzotti et al., 2003). This genetic risk factor for glaucoma development is actually controversial even if other authors have confirmed this data (Izzotti and Sacca`, 2004; Yildirim et al., 2005). For example, Unal et al. (2007) have reported