The changing paradigm of outflow resistance generation: towards synergistic models of the JCT and inner wall endothelium

Abstract

Aqueous humor outflow resistance is the primary determinant of intraocular pressure (IOP), and increased outflow resistance is the basis for elevated IOP associated with glaucoma. Experimental evidence suggests that the bulk of outflow resistance is generated in the vicinity of the inner wall endothelium of Schlemm's canal, its basement membrane and the juxtacanalicular connective tissue (JCT). However, attempts to sort out the contribution of each of these tissues to total outflow resistance have not been successful. Conventional understanding of outflow resistance assumes that the resistance of each tissue strata (i.e., the inner wall endothelium, its basement membrane and JCT) in the outflow pathway adds in series to contribute to total outflow resistance generation. However, this perspective leads to a paradox where the apparent resistances of all tissues in the outflow pathway are much lower than the measured total resistance. To resolve this paradox, we explore synergistic models of outflow resistance generation where hydrodynamic interactions between different tissue strata lead to a total resistance that is greater than the sum of the individual tissue resistances. We closely examine the "funneling" hypothesis that has emerged as a leading synergistic model, and we review the basis of funneling, mechanical and biological requirements for funneling and evidence in support of this hypothesis. We also propose refinements to the funneling model and describe how funneling may relate to segmental variability of aqueous humor outflow patterns observed within the trabecular meshwork. Pressure gradients across the JCT and inner wall endothelium will generate mechanical loads that influence the morphology of these tissues. Because tissue morphology may in turn affect outflow resistance, there exists the potential for a two-way coupling or a "fluid-solid interaction" between outflow hydrodynamics and the mechanical behavior of the inner wall and JCT. Furthermore, the adhesions and tethers between the inner wall and JCT must be physically capable of supporting such loads. We examine the structure and mechanical strength of these adhesions, and provide evidence that these adhesions and tethers are unable to support the full load imposed by the bulk of outflow resistance generation unless a substantial fraction of outflow resistance is generated within the JCT, consistent with the funneling model. This indicates that these attachments between the inner wall and JCT have considerable physiological importance for outflow resistance regulation, by maintaining the proximity between these two tissues to facilitate funneling. Further study is greatly needed to better characterize these important interactions.

Figure 1

A scanning electron micrograph of the conventional outflow pathway, showing the corneoscleral meshwork (CM), Schlemm’s canal (SC), juxtacanalicular connective tissue (JCT), and a collector channel (asterisks). The inner wall of SC is shown (arrowheads), along with the direction of aqueous humor flow (arrow). Scale bar is 50 μm. Adapted from Freddo (1993).

Figure 2

Juxtacanalicular connective tissue adjacent to Schlemm’s canal (SC), showing a giant vacuole (GV) at the inner wall. Image graciously contributed by Haiyan Gong, Boston University School of Medicine.

Figure 3

Scanning electron micrograph of the inner wall of Schlemm’s canal as seen from the canal lumen. Arrowhead shows a pore and inset shows high magnification image of pore. Adapted from Allingham et al. (1992).

Figure 4

Enucleated human eye fixed by perfusion at 15 mm Hg. (A) Giant vacuoles (V) in the inner wall of Schlemm’s canal (SC) in tissue prepared for transmission electron microscopy using conventional methods; notice the large open spaces in the region of the JCT immediately under these giant vacuoles; (B) a similar region as seen in tissue prepared using quick-freeze/deep-etch; notice that while open spaces still exist under the giant vacuoles, a more complex and extensive extracellular matrix is seen. Note that in 4A, open spaces are light, while in 4B, open spaces are dark. (x4860) (Gong et al., 2002).

Figure 5

Pores of the inner wall endothelium. Left: Intracellular pores (I) and artifactual pore (A); Right: intercellular or border (B) pore (Ethier et al., 1998).

Figure 6

Schematic of changing configuration of inner wall endothelium, trabecular meshwork and Schlemm’s canal at low (A) and high (B) perfusion pressure (Johnstone and Grant, 1973).

Figure 7

Schematic showing funneling phenomenon. (A) Flow distribution with inner wall cells attached to substratum; (B) uniform flow that results when these attachments are broken (Overby et al., 2002).

Figure 8

Schematic of inner wall endothelium of monkey eye perfused with colloidal gold. (A) control eye; (B) eye perfused with H-7. Each panel shows an equivalent number of endothelial cells (Sabanay et al., 2000).

Figure 9

Light micrographs of trabecular meshwork (TM) and Schlemm’s canal (SC) in monkey eyes treated with H-7 (B) or vehicle (A). The JCT and intercellular spaces are extended following H-7 (arrow in B). Bars are 50 μm. Adapted from Sabanay et al. (2000).

Figure 10

Confocal microscopy of fluorescent tracer patterns within the trabecular meshwork (TM) of bovine eyes and along the inner wall of the aqueous plexus (AP; the bovine equivalent to Schlemm’s canal). The tracer distribution (pink) was more segmental in control eyes (A), and tended to concentrate near the ostia of collector channels (CC), while in Y-27 treated eyes (B) the tracer distribution was more uniform along the inner wall. Nuclear background stain is shown in blue. Adapted from Lu et al. (2008).

Figure 11

Schematic of contacts between inner wall cells and JCT cells. Crosshatched cells are inner wall cells while shaded cells are JCT cells. Process-to-process contacts (type 4) were most common (~40%) followed by process-to-cell associations (type 5 ~30%; type 2 ~25%). Percentages represent measurements from 1000 regions of contact in 9 rhesus monkey eyes fixed at 15 mmHg. Adapted from (Grierson et al., 1978).