seen in senile nuclear sclerosis.92 The color changes that often accompany nuclear cataracts are usually seen as various hues of yellow or brown; this pigmentation is primarily protein-bound.91

Posterior Subcapsular Cataract (PSC)

The opacity in the posterior subcapsular region is formed by epithelial-like cells that migrate from the equatorial region. These cells accumulate at the posterior pole forming an opacity.96 It is speculated that radiation damage is one causative factor as patients undergoing radiation therapy for cancer treatments also develop PSCs and/or cortical cataracts.62 There are some sutural congenital cataracts that occur at this location, but they are caused by swelling of the basal ends of cortical fibers.62

Steroid-Induced Cataract

Steroid-induced cataracts are also located in the posterior subcapsular region. Dosage and the duration of steroid use appear to be controlling factors, although individuals may have varying levels of susceptibility. Children develop such cataracts at a faster rate than do adults. Reversal of the cataract can occur but this is rare.5,37,66 The opacity appears to be formed of undifferentiated epithelial cells at the interface of the posterior cortex and capsule. These misplaced cells (which should only be present in the lens epithelium) display aberrant behavior. The undifferentiated cells may have migrated from the preequatorial area, influenced by a change in the concentration of growth factors.66 The growth factors governing mitosis, migration, and differentiation are obtained from aqueous and reside in the lens capsule. If steroids influence production of these growth factors that are present in the aqueous, and the concentration and location in the capsule is altered, cellular processes can be affected.66

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The aqueous and vitreous are contained in three chambers within the eye. The anterior and posterior chambers contain aqueous humor, and the vitreous chamber contains the vitreous gel. A description of each chamber will be followed by an explanation of the formation, composition, and function of the aqueous and the vitreous.

A N T E R I O R C H A M B E R

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

The structures through which aqueous exits, collectively called the filtration apparatus, consist of the trabecular meshwork and Schlemm’s canal. These structures and the scleral spur occupy the excavated area located at the internal corneoscleral junction known as the internal scleral sulcus.

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

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