the anterior chamber and are referred to as the uveal meshwork. The next more superficial layer is the corneoscleral meshwork.The juxtacanalicular space is the next layer, which is between the cor­ neoscleral meshwork and Schlemm’s canal inner wall endothelium (see Fig. 3-2).

Uveal meshwork

The uveal meshwork is adjacent to the anterior chamber. Iris proc­ esses are fine strands of the innermost layer of the uveal meshwork present in many eyes.The processes arise from the anterior surface of the iris, bridge the angle recess, and insert into the deeper uveal trabeculae or Schwalbe’s line (see Fig. 3-1). The rest of the uveal meshwork has a ropeor cord-like character, with randomly ori­ ented interconnecting bands, and is only a few layers thick. The uveal meshwork inserts anteriorly into the region of Schwalbe’s line and posteriorly into the ciliary body and iris root. The inner layers are generally oriented radially although they branch and interconnect in multiple planes.

Corneoscleral meshwork

The corneoscleral meshwork consists of a series of 8–14 flattened, perforated parallel sheets or lamellae, each 5–12 microns thick.22 The sheets closer to the anterior chamber are anchored anteriorly to Schwalbe’s line.The sheets pass in a meridional fashion posteri­ orly to attach to the scleral spur.The anterior tendons of the longi­ tudinal ciliary muscle fibers insert on the posterior portion of the corneoscleral meshwork as well as on the scleral spur.23 The inner trabecular lamellae closer to the anterior chamber are considerably thicker than the outer ones closest to Schlemm’s canal (Fig. 3-4).22

Trabecular lamellae are attached to one another via cytoplasmic processes (Fig. 3-4).24–30 The cytoplasmic processes originate from

the surface of the endothelial cells covering the lamellae and meet in the intertrabecular space with a complex zone of apposition involving desmosomes and gap junctions.31,32 Intertrabecular col­ lagen beams are difficult to find.22

The trabecular sheets have a generally circumferential orienta­ tion parallel to the limbal circumference.22 The sheets are fused in such a manner that only two or three layers are seen anteriorly. The sheets separate in an anterior–posterior plane so that 12–20 layers are detectable posteriorly.

Sheets of the trabeculae are perforated by elliptical (transtrabec­ ular) openings with an equatorial orientation, with an average dimension of 12–30 microns. Perforations become progressively

smaller from the superficial layers of the uveal meshwork to the deep layers of the corneoscleral meshwork (see Fig. 3-2).22,33,34

The perforations are not aligned, so aqueous humor must follow a circuitous route to reach Schlemm’s canal (Fig. 3-5).

Uveal and corneoscleral meshwork ultrastructure

The composition of the trabecular meshwork tissues has been com­ pared to that of other highly compliant and resilient tissues, such as lung and other blood vessel walls.20 Ultrastructurally, the uveal and corneoscleral meshworks are the same, being composed of four concentric layers.35 A description of these layers follows.

First, trabecular sheets or lamellae have a central core of types

I and III collagen and elastin with a typical 64 nm periodicity (Fig. 3-6).20,35,36 Second, elastic fibers surround the core region

with a spiraling pattern and a 100 nm periodicity.37 The fibers may be wound loosely or tightly thus conferring elastic proper­ ties to the meshwork.38 Third, the cortical zone (also referred to as the glassy membrane)35 is a broad zone that contains collagen

types III, IV, andV; laminin; fibronectin; and heparin sulfate proteo­ glycan.20,37 Types VI and VIII collagen are also present.39–44 Fourth

is a continuous layer of endothelial cells that covers the trabecu­ lar lamellae; cells are joined by desmosomes as well as gap junc­ tions.31,45 Intercellular clefts allow aqueous to pass freely.38,45

Numerous cytoplasmic processes arise from the trabecular lamel­ lae endothelial cells.24–30 These cytoplasmic processes are attached to cytoplasmic processes of adjacent lamellae and to juxtacanalicular

Resisting tissue responses to IOP-induced tissue loading forces

Schlemm’s canal endothelium

Juxtacanalicular

cells

TLP

Trabecular lamellae

Baseline IOP Increase in IOP

SCP

JCP

cell cytoplasmic processes (see Fig. 3-4) by robust desmosomes.31 Endothelial cells lining the trabecular lamellae are anchored to a well-defined basement membrane by means of integrin attach­ ments;46–48 this is in contrast to Schlemm’s canal endothelium,

where a basement membrane is sparse or absent.26 Cytoskeletal elements include microfilaments (F-actin),23,49–51 intermediate

filaments (vimentin)52–58 and microtubules (alpha-tubulin).59–61 Endothelial cells lining the trabecular lamellae are responsi­ ble for maintaining the structural topography and extracellular

matrix composition of the lamellae in the face of constant oscil­ latory stresses.15 Tissue composition predicts anticipated tissue responses.62–64 Type I collagen provides tensile strength and type III collagen imparts resilience.62 Together these collagenous ele­ ments provide structural support in tension while elastin provides a recoverable response over wide excursions.62 The organization and distribution of collagen and elastin in the trabecular lamellae is like that of tendon,65 which provides a mechanism for reversible defor­ mation in response to hydrodynamic tissue loading.15