Chapter 6 Aqueous and Vitreous Humors
Anterior Chamber
The anterior chamber is bounded anteriorly by the corneal endothelium; peripherally by the trabecular meshwork, a portion of the ciliary body, and the iris root; and posteriorly by the anterior iris surface and the pupillary area of the anterior lens (Figure 6-1). The center of the anterior chamber is deeper than the periphery. The anterior chamber angle is formed at the periphery of the chamber, where the corneoscleral and uveal coats meet. The aqueous humor exits the anterior chamber through the structures located in this angle.
Anterior Chamber Angle Structures
Scleral Spur
The scleral spur lies at the posterior edge of the internal scleral sulcus (see Chapter 2). The posterior portion of the scleral spur is the attachment site for the tendon of the longitudinal ciliary muscle fibers, whereas many of the trabecular meshwork sheets attach to the spur’s anterior aspect, such that the collagen of the spur is continuous with that of the trabeculae1 (Figure 6-2).
Trabecular Meshwork
The trabecular meshwork encircles the circumference of the anterior chamber, occupying most of the inner aspect of the internal scleral sulcus. In cross section it has a triangular shape, with its apex at the termination of Descemet’s membrane (Schwalbe’s line) and its base at the scleral spur (Figure 6-3). The inner face borders the anterior chamber, and the outer side lies against corneal stroma, sclera, and Schlemm’s canal. The meshwork is composed of flattened perforated sheets, with three to five sheets at the apex. These sheets branch into 15 to 20 sheets as they extend posteriorly from Schwalbe’s line to the scleral spur.2 The trabecular meshwork is an open latticework, the branches of which interlace. The intertrabecular spaces between the sheets are connected through pores, or openings within the sheets (historically called the “spaces of Fontana”).3 The openings are of varying sizes and become smaller near Schlemm’s canal. No apertures directly join the meshwork with the canal.2 A small aspect of the meshwork at the most anterior location is adjacent to connective tissue of the limbus and differs in structure from the filtering portion. Some believe that this is a niche where cells reside that have properties similar to stem cells. These cells may be capable of replacing the endothelial cells of the trabecular meshwork after injury.4
The meshwork can be separated into two anatomic divisions. The corneoscleral meshwork is the outer region; its sheets attach to the scleral spur. The inner sheets, which lie inner to the spur and attach to the ciliary stroma and longitudinal muscle fibers, make up the uveal meshwork; some of these sheets may attach to the iris root.3,5 The two portions differ slightly in structure; the corneoscleral meshwork is sheetlike, and the uveal meshwork is cordlike2 (Figure 6-4). The pores in the uveal meshwork are the largest, and pore size diminishes in the sheets closer to the canal. Projections from the surface layer of the iris, known as iris processes, connect to the trabeculae, usually projecting no farther forward than the midpoint of the meshwork.2
FIGURE 6-4 Drawing of aqueous outflow apparatus and adjacent tissues.
(From Hogan MJ, Alvarado JA, Weddell JE: Histology of the human eye, Philadelphia, 1971, Saunders.)
The meshwork trabeculae consist of an inner core of collagen and elastic fibers6 embedded in ground substance and covered by basement membrane and endothelium.7 The endothelial cells are a continuation of the corneal endothelium.5,8 The endothelial cells contain the cellular organelles for protein synthesis and apparently are capable of replacing the connective tissue components. These cells also contain lysosomes, which give them the capacity for phagocytosis.5 Gap junctions and short areas of tight junctions join the endothelial cells; no zonula occludens are found.9 Cytoplasmic projections connect cells of neighboring sheets.5,9,10
At the scleral spur, the trabecular sheets lose their endothelial covering, but the collagenous and elastic fibers continue into the connective tissue of the spur and ciliary body.2,5 Some connective tissue fibers of the ciliary muscle pass forward and merge with the inner sheets of the meshwork.11
Canal of Schlemm
The canal of Schlemm is a circular vessel and is considered to be a venous channel, although it normally contains aqueous humor rather than blood. It is outer to the trabecular meshwork and anterior to the scleral spur. The external wall of the canal lies against the limbal sclera, and the internal wall lies against the juxtacanalicular connective tissue and the scleral spur (see Figure 6-2). Thin tissue septa may bridge the lumen, dividing it into several channels.2,3
The lumen is lined with endothelial cells, many of which are joined by zonula occludens.2,9,12 The endothelial cells have an incomplete basement membrane.2,3,7 The continuous endothelial lining with cells joined by tight junctions make the canal similar to blood vessels, whereas the discontinuous basement membrane make it similar to lymph channels.13 The tight junctions of the inner wall restrict flow into the canal between the lateral walls of the cells. Pores and pinocytic vesicles in the cell membrane may be an avenue for passage of aqueous humor.2,3,14–16
Juxtacanalicular Connective Tissue
The region separating the endothelial cell lining of the canal from the trabecular meshwork is called the juxtacanalicular tissue5,11 or the cribriform layer.5,15,17 It consists of endothelial cells and fibroblasts embedded in a matrix of collagen, elastic-like fibers, and ground substance.15,18–20 The cells of this region have processes occasionally joined by adhering and gap junctions. The cells also form similar connections with the endothelium of the inner wall of Schlemm’s canal.21 There are micron-sized spaces within the juxtacanalicular tissue that appear to lack extracellular matrix (although these may be presumed spaces whose material is yet to be observed)22 and may provide a tortuous pathway for fluid to move toward the inner wall of the canal. The endothelium of Schlemm’s canal is anchored to the juxtacanalicular tissue by a network of elastic-containing fibrils that is also connected to the scleral spur and the tendon of the ciliary muscle.19 This connective network might help in modulating aqueous outflow.21
Clinical Comment: Gonioscopy
If the angle does not appear to be wide enough or if there is concern that aqueous exit is inadequate, a view of the chamber angle structures might be necessary. A direct view of the angle cannot be achieved because the limbus is opaque, and light directed obliquely through the cornea into the angle does not exit because of total internal reflection. Therefore a clinical procedure, gonioscopy, is performed that uses a special lens, a goniolens (Figure 6-5). A goniolens can overcome total internal reflection and contains mirrors in which the examiner views the angle. The image the examiner sees is as if he or she is facing the angle and sighting along the anterior surface of the iris. If all structures can be seen, they appear in the following order, beginning at the inferior aspect: iris root, ciliary body, scleral spur, trabecular meshwork, and Schwalbe’s line (Figure 6-6). (The iris root may or may not be visible.) Schlemm’s canal lies behind the trabecular meshwork in this view and appears as a thin red line within the meshwork if blood is backed up in the canal. Such pooling of blood occurs if the examiner exerts pressure on the goniolens, thereby compressing the episcleral veins and causing the episcleral venous pressure to exceed intraocular pressure.23
FIGURE 6-5 Optical principles of gonioscopy: n and n’, refractive index.
(From Kanski JJ: Clinical ophthalmology: a systematic approach, ed 5, Oxford, UK, 2003, Butterworth-Heinemann.)
Function of the Filtration Apparatus
The primary function of Schlemm’s canal and the trabecular meshwork is to provide an exit for the aqueous humor. In addition, with the movement of aqueous through these structures, nutrients can diffuse into surrounding tissue, thereby supplying nutrients to the nearby deep limbal and scleral tissue.8
Posterior Chamber
The posterior chamber is an annular area located behind the iris and bounded by the posterior iris surface, the equatorial zone of the lens, the anterior face of the vitreous, and the ciliary body. The ciliary processes that secrete the aqueous humor project into the posterior chamber. The zonule fibers arise from the internal limiting membrane of the nonpigmented epithelium of the ciliary body, pass through the posterior chamber, and insert into the lens capsule.1 The posterior chamber contains two regions: The area occupied by the zonules is the canal of Hannover, and the retrozonular space, the area from the most posterior zonules to the vitreal face, is the canal of Petit2 (Figure 6-7). The canal of Petit might be better described as a potential space.
Aqueous Dynamics
The aqueous humor provides necessary metabolites, primarily oxygen and glucose, to the avascular cornea and lens. It is produced in the pars plicata of the ciliary body and is secreted into the posterior chamber through the epithelium covering the ciliary processes. It passes between the iris and lens, entering the anterior chamber through the pupil (Figure 6-8). In the anterior chamber, the aqueous circulates in convection currents, moving down along the cooler cornea and up along the warmer iris and exiting through the periphery of the chamber.
There are two avenues by which aqueous exits the anterior chamber. It can pass through the spaces within the uveal meshwork; this pathway is called the unconventional outflow and accounts for a relatively small amount (variously reported in the literature as 5% to 35% of the total outflow24–26). The fluid then passes into the connective tissue spaces surrounding the ciliary muscle bundles,27 moves into the suprachoroidal space, and is absorbed into and through the sclera.28 Alternately, the fluid is absorbed into the anterior ciliary veins and vortex veins. Additional veins recently identified may represent suprachoroidal collector channels designed to accommodate this uveoscleral flow.24 There is some evidence that lymphatic channels exist in the ciliary stroma, which might provide an additional avenue for aqueous exit.29
The remainder of the aqueous follows the conventional outflow pathway and moves through the meshwork and into the narrower pores of the corneoscleral meshwork and through the juxtacanalicular tissue and the endothelial lining into Schlemm’s canal.18 In histologic sections, many of the endothelial cells lining the inner wall of the canal have been found to contain giant vacuoles,7,12,16,30–32 some of which exhibit openings into the lumen.10,33,34 The vacuoles apparently open and close intermittently, creating transient, transcellular, unidirectional channels that provide a means for transporting large molecules, such as proteins, across the endothelium. Flow occurs only into the canal.14,15,35 An indentation forms in the basal surface of the endothelial cell, gradually enlarges, and eventually opens onto the apical surface (Figure 6-9). Then the cytoplasm in the basal aspect of the cell moves to occlude the opening.35 The number of pores throughout the endothelium is uncertain because some may be artifacts caused by tissue preparation.36 Smaller pinocytic vesicles also provide a transport system for substances. However, the greatest volume of aqueous humor diffuses passively into Schlemm’s canal. Recent speculation suggests that the tight intercellular junctions may respond to changing physiologic conditions (i.e., effects of pharmacologic agents) by modifying their permeability and increasing the ease by which aqueous flows into the canal.9,12 The endothelial cells of the trabecular meshwork may actually release cellular factors that can increase the permeability of the inner wall of Schlemm’s canal.37
FIGURE 6-9 Formation of giant vacuoles in endothelial cells lining Schlemm’s canal.
(Modified from Bartlett JD, Jaanus SD: Clinical ocular pharmacology, ed 2, Boston, 1989, Butterworth-Heinemann.)
The internal wall of Schlemm’s canal contains a number of evaginations, or blind pouches, that extend into the juxtacanalicular tissue toward the trabecular meshwork. These internal collector channels (of Sondermann)38 can be fairly long and branching, and serve to increase the surface area of the canal2 (Figure 6-10). Their endothelium is always separated from the trabecular space by a sheet of connective tissue.
FIGURE 6-10 Drawing of Schlemm’s canal, an internal collector channel, and adjacent tissues.
(From Hogan MJ, Alvarado JA, Weddell JE: Histology of the human eye, Philadelphia, 1971, Saunders.)
The endothelial cells lining the external wall of Schlemm’s canal are joined by zonula occludens and contain no vacuoles. Approximately 25 to 35 external collector channels are distributed around the outer wall of Schlemm’s canal and branch from it to empty into either the deep scleral plexus or the intrascleral plexus of veins,5,39,40 which in turn drain into the episcleral and conjunctival veins.2 Aqueous veins (of Ascher), that pass from the outer wall of the canal directly to the episcleral veins, provide another route from the canal.41
