Canal of

Schlemm

FIGURE 6-1

Periphery of the anterior segment. Structures of the anterior chamber angle are labeled.

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 consid­ ered 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

b

FIGURE 6-2

Drawing of the limbus.  Limbal conjunctiva (A) is formed by an epithelium (1) and a loose connective tissue stroma (2). Tenon’s capsule (B) forms a thin, poorly defined connective tissue layer over episclera (C). Limbal stroma occupies the area (D) and is composed of scleral and corneal tissues, which merge in this region. Conjunctival stromal vessels are seen at a; they form peripheral corneal arcades (b), which extend anteriorly

to termination of Bowman’s layer (arrow). Episcleral vessels (c) are cut in different planes. Vessels forming intrascleral plexus (d) and deep scleral plexus (e) are shown within limbal stroma. Scleral spur, with its coarse, dense collagen fibers, is shown

at f. Anterior part of longitudinal portion of ciliary muscle (g) merges with scleral spur and trabecular meshwork. Lumen of Schlemm’s canal (h) and loose tissues of its wall are clearly seen. Sheets of trabecular meshwork (i) are outer to cords of uveal meshwork (j). An iris process (k) arises from iris surface and joins trabecular meshwork at level of anterior portion of

scleral spur. Descemet’s membrane terminates (double arrows) within anterior portion of the triangle, outlining aqueous outflow system. (From Hogan MJ, Alvarado JA, Weddell JE: Histology of the human eye, Philadelphia, 1971, Saunders.)

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 micronsized 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