Some evidence does not favor the juxtacanalicular space resist­ ance model. The model requires relatively unchanging juxtacanal­ icular space geometry so that the space may maintain its resistance characteristics. Studies of tissue biomechanics involving tissue load­ ing described later in this chapter demonstrate that the juxtacanal­ icular space undergoes a twoto three-fold enlargement in response to physiologic increases in pressure. The same pressure increases induce marked increases in resistance, making the juxtacanalicular space a less likely site of significant resistance in normal eyes.

Although spaces are present in the juxtacanalicular region, in hypotonous eyes that may be construed as being filled and held open by a syncytium of extracellular matrix material, studies of biomechanics that examine boundary conditions indicate that extracellular matrix material does not act as a space-occupying syn­ cytium.When IOP is reduced to zero in living eyes, blood refluxes into Schlemm’s canal in response to the episcleral venous pressure gradient of about 8–9 mmHg. Under these conditions, the juxtacanalicular space does not act as a space-occupying syncytium.

Rather, the juxtacanalicular space is almost completely obliter­ ated.27,29,79,89,140 A pressure gradient reversal of as little as 5 mmHg

causes obliteration of the space.168 The elimination of the juxtaca­ nalicular space in response to a modest pressure reversal is thought to indicate that the spaces within the juxtacanalicular region are dependent on hydrostatic pressure rather than on the rigidity induced by a syncytium of extracellular matrix material.79

Histochemical studies provide additional insights.66 For many years it was assumed that the apparently open spaces of the trabec­ ular meshwork were filled completely with a GAG gel that was washed out in conventional histologic processing.169 In recent years it has been possible to localize histochemically hyaluronan, and it is now clear that most of the open spaces are not filled with gel. Some hyaluronan is found in the juxtacanalicular region, but the amount decreases with age and no morphological study has

demonstrated extracellular matrix that could generate the meas­ ured outflow resistance.98,169,170

Schlemm’s canal endothelium resistance

Different lines of evidence support the idea of Schlemm’s canal endothelium as the major site of resistance to aqueous outflow.33

Tracer studies demonstrate accumulation of material at the inner wall of Schlemm’s canal.25,33,118 Other evidence offered in support

of Schlemm’s canal endothelium as the main resistance site comes from the improvement in outflow facility that follows experi­

mental infusion of certain substances, such as iodoacetic acid,171 N ethylmaleimide,172 cytochalasin B or D,168,173–175 EDTA,176,177

and colchicine.178 Histologic studies suggest that these agents alter the inner wall of Schlemm’s canal.168,174,176,177 However, disruption

of Schlemm’s canal causes washout of extracellular material in the juxtacanalicular space.29,168 Because of the contemporaneous loss of extracellular matrix material with the above studies, disruption of Schlemm’s canal endothelium is of unclear value in discrimi­ nating between the juxtacanalicular space and Schlemm’s canal endothelium as the primary resistance site.

Schlemm’s canal endothelium/ trabecular meshwork: a resistance unit

Studies of cellular biomechanics point to Schlemm’s canal inner wall endothelium and the trabecular meshwork acting as a unified resistance unit.

Principles of biomechanics as a methodology to identify tissue resistance

The principles of biomechanics require, in turn, study of tissue geometry, tissue composition, laboratory effects of tissue loading, boundary conditions, and, finally, in-vivo effects of tissue load­ ing.62,64 Tissue composition and geometry, issues discussed previ­ ously in this chapter, determine constraints and possible responses of the tissues to external forces.

Tissue loading studies subject tissues to normally encountered forces to determine force-induced responses. Load-bearing struc­ tural elements respond by characteristic changes in configuration. In other words, tissues causing the resistance are the ones that undergo configuration changes appropriate to the loading forces they experience.62 The applicable loading force in the aqueous outflow system is IOP. Boundary conditions define the maximum limits of tissue responses to induced forces. In-vivo tissue loading

responses are discussed in a later section and are the most crucial test of the validity of conclusions from the laboratory.62,64

Tissue loading studies

Tissue loading induced by IOP, both in vitro and in vivo, consist­

ently demonstrates progressive distention of Schlemm’s canal inner wall endothelium that correlates with IOP increases.15,27– 29,72,73,78,79,89,96,106,140 Evidence from these same studies follows,

To induce tissue loading, the pressure gradient is systematically raised above zero (in the enucleated eye, pressure above hypotony, or in living eyes, pressure above episcleral venous pressure 8–9 mm). Schlemm’s inner wall begins its outward distention when the pres­ sure gradient is as low as 5 mmHg.27 Distention of Schlemm’s canal inner wall continues progressively, both within the physiologic pres­

sure range and beyond. Concomitently, inner wall distention causes the juxtacanalicular space to enlarge,27–29,79,89,96,106 as much as twoto three-fold.27–29,90 Because of their anchoring attachments

to the distending wall of Schlemm’s canal endothelium, trabecu­ lar lamellae move progressively outward toward Schlemm’s canal

lumen, thus developing progressively increased spacing between lamellae.27–29,72,79,89,106,156 Cytoplasmic processes throughout the

meshwork undergo progressive changes from a parallel to a per­ pendicular orientation.28,29 The processes, initially short and stubby, undergo elongation and thinning28,29 both in the juxtacanalicular and intertrabecular spaces. A more pronounced longitudinal orien­ tation of the cytoskeletal filaments of the processes develops as IOP increases.28

At the cellular level, Schlemm’s canal endothelial cell membrane and cytoplasmic contents, as well as the nuclear envelope and its contents, change shape in a progressive fashion from a spherical

configuration in hypotony to an elongated plate-like configura­ tion.27–29,72,79,89,96,106,140 At cell process origins, the cytoplasm and

nucleus reorganize from a neutral to an elongated cone-shaped configuration in response to tension.72 Juxtacanalicular cells undergo a change in configuration involving the cell membrane, the cytoplasm, the nuclear envelope, and the nuclear contents, all of which develop a progressively more cone-shaped appearance directed toward cell process origins.27,72 Such cellular changes are

reflective of stresses induced by progressive tension that develops

between Schlemm’s canal endothelium and the restraining trabecu­

lar lamellae.27–29,31,32,72,79,89,96

Tissue loading by IOP thus provides evidence at both the tis­ sue and cellular levels, placing trabecular meshwork resistance to IOP at Schlemm’s canal endothelium. Attachments of Schlemm’s canal endothelium to the underlying trabecular meshwork pro­

vide a dynamic tensional integration between the endothelium and the underlying load-bearing trabecular tissues.15,27–29,31,32,72,79,89,96

In contrast, tissue loading studies provide no evidence of hydrau­ lic resistance in the juxtacanalicular space.15,27–29,31,32,72,79,89,96

Juxtacanalicular space enlargement and reduced compaction of both extracellular and cellular elements occurs.15,27–29,31,32,72,79,89,90,96,179

This juxtacanalicular space enlargement progressively reduces the ability of the juxtacanalicular space to act as a determinate of resist­

ance, yet as IOP increases, measured resistance to aqueous outflow increases,180–182 making this region an unlikely source of resistance.

Boundary conditions

High IOP induces trabecular meshwork distention and Schlemm’s canal lumen collapse (Fig. 3-10). As IOP progressively increases, Schlemm’s canal endothelium progressively distends into the lumen

of the canal.27,28,31,73,79,89,90,96 At higher IOPs, Schlemm’s canal endothelium becomes appositional to the external or corneoscle­

ral wall of Schlemm’s canal, effectively occluding much of the canal lumen.27,28,31,73,79,89,90,96 No further excursions can occur. Aqueous

cannot easily pass across Schlemm’s canal endothelium in these regions and circumferential flow to collector channel ostia is progres­ sively compromised.90 For example, in one report, one-third of sec­ tions had over 75% of the angle closed at 20 mmHg.90 This finding in enucleated eyes may result from an absence of ciliary body tone

and normal episceral back pressure. However, in living primates, the walls of the canal also become appositional,27–29,73 with fairly exten­

sive apposition present at a relatively low IOP of 20–25 mmHg.73,90

Low IOP induces trabecular meshwork collapse and Schlemm’s canal lumen dilation (see Fig. 3-10).27,29,78,79,89,96,140,168 When

IOP is reduced below episcleral venous pressure ( 4–8 mmHg), Schlemm’s canal endothelium moves inward toward the anterior chamber. The trabecular lamellae nearest Schlemm’s canal are com­

pressed together to form a uniform, solid-appearing sheet of tis­ sue.27,29,78,79,89,96,140,168 No further excursions of Schlemm’s canal

endothelium can occur. Additionally, no blood crosses this tissue, leading in the initial report of the behavior27 to the proposal that the configuration provides a means of assuring maintenance of the blood aqueous barrier.