Aqueous Humor Outflow

Elevation of the intraocular pressure (fluid pressure inside the eye) is considered as a risk factor for some ocular disorders specially glaucoma, which can eventually lead to blinding. Current treatments for this disease focus on lowering intraocular pressure (Zeng et al. 2010). Such pressure elevation within the eye is believed to be due to an increase in the resistance to aqueous humour outflow from the eye, but the exact origin of this resistance increase has not been determined yet (Johnson 2006).

The physiological aspects of aqueous humour outflow pathway have been extensively reviewed (R Tripathi 1974; RC Tripathi 1974; Bill 1975; Bill & Mäepea 1994; Gong et al. 1996; Johnson & Erickson 2000; Ethier 2002).

Aqueous humor is a colorless

2010).

Aqueous humor passively flows out of the eye using two routes. The majority of this fluid leaves the eye through the conventional outflow (Fig. 1), in which it passes the trabecular meshwork and juxtacanalicular connective tissue, crosses Schlemm's canal endothelium, enters into the Schlemm's canal and then collecting channels, aqueous vessels and eventually enters the blood stream in the episcleral vessels (Goldmann 1950; Ascher 1954; Goel et al.

In vascular system, endothelial cells are subjected to an apical-to-basal pressure gradient but they do not bear the whole forces produced by that pressure gradient as the underlying connective tissue supports such forces produced by the pressure gradient of the lumen. On the contrary, because of the basal-to-apical flow direction of the aqueous humor, the direction of resulting pressure gradient experienced by endothelial cells existing in the inner wall of Schlemm’s canal is opposite to that for the vascular endothelium. Therefore, these endothelial cells have to directly support such transcellular pressure gradient, which tends to separate them from their underlying basement membrane (Zeng et al. 2010; Overby et al. 2014) and results in the presence of pores, called “giant vacuoles”, inside the endothelial cells (Johnson 2006). Supporting such pressure gradient may imply the low resistance of this endothelium layer to aqueous humor flow (Zeng et al. 2010). However, Schlemm’s canal endothelial cells have been previously reported to play a significant role in aqueous humor outflow resistance (Underwood et al. 1999; Alvarado et al. 2005). Such uncertainties about the role of Schlemm’s canal endothelial cells in the resistance to the outflow of aqueous humor make it necessary to deeply investigate the function and properties of these