The endothelial cells, irregular in shape, had a cytoplasm with mitochondria, rough-surfaced endoplasmic reticulum, smooth-surfaced endoplasmic reticulum, Golgi apparatus, and pinocytotic vesicles. Endothelial cells interconnect through thin areas of more or less tortuous interendothelial clefts composed of adjacent cell membranes. The basement membrane, almost parallel to the outer contour of the endothelial cells, consists of one to several dense layers with a less dense zone Þlling the space between them. Pericytes are attached to the basement membrane, often with one or more dense zones between them and the endothelial cell; their cytoplasm contains endoplasmic reticulum, mitochondria, Golgi apparatus, pinocytotic vesicles, and Þne Þlaments. No smooth muscle cells are present. Bundles of collagen Þbrils adjacent to the vessel walls merge into the surrounding connective tissue.

Episcleral and conjunctival vessels in monkeys have an identical morphology [53]. Episcleral and conjunctival vessels are permeable to tracer molecules, such as ßuorescein-labeled dextrans of different molecular weight or horseradish peroxidase injected into the blood stream or into the anterior chamber; [53, 54] tracers escape from the vessel lumen by crossing the thin interendothelial clefts. It can equally be expected that the aqueous humor that reaches the episcleral and conjunctival vessels through the Schlemm«s canal and collector channels can diffuse freely into the episcleral loose connective tissues and the subconjunctival spaces across the walls of these permeable vessels.

1.4Biochemistry

Collagen forms 75% of the dry weight of the sclera, as determined by quantitation of 4-hydroxyproline in acid hydrolysates of the total tissue [55]. Collagen type I is the predominant type, whereas type III is found in smaller amounts [56Ð58]. There is no difference in the collagen content or collagen type in sclera between anterior and posterior segments of the eye [56].

Elastin content, estimated by desmosine and isodesmosine analysis, forms less than 2% of the dry weight of the sclera [59].

Proteoglycans, determined by uronic acid analysis, constitute 0.7Ð0.9% of the dry weight of the sclera [55]. Proteoglycans are composed of GAG chains linked to a protein core. The most abundant GAG in sclera is dermatan sulfate, followed by chondroitin sulfate and hyaluronic acid [60, 61]. Little heparan sulfate and no keratan sulfate is present. Regional analysis shows that the sclera around the optic nerve is the area richest in dermatan sulfate, the sclera around the equator is the richest in hyaluronic acid, and the sclera around the fovea is the richest in chondroitin sulfate [61].

Glycoproteins, such as Þbronectin and vitronectin, also can be detected in sclera. Laminin has not been found [58], except in vessels.

The water content of sclera ranges from 65 to 75%. The sclera appears opaque if the water content is maintained between 40 and 80% but becomes transparent if it falls below 40% or rises above 80% [55].

1.5Immunohistochemistry

We studied the immunolocalization of extracellular matrix components in human adult sclera using monoclonal antibodies against the collagens, proteoglycans, and glycoproteins.

Collagen types I, III, V, and VI stained intensely in extravascular sclera, whereas collagen type II and VII were not identiÞed. Collagen type IV was almost absent, except for its dramatic presence in blood vessels (see Chap. 5). Studies on tissue distribution of collagen type VIII in human sclera showed a linear or Þbrous pattern of intense staining in anterior and posterior human adult sclera [11].

The most abundant GAGs in sclera were dermatan sulfate and chondroitin sulfate; hyaluronic acid and heparan sulfate also were present, although in small amounts.

The glycoproteins, Þbronectin and vitronectin, were identiÞed in scleral specimens. Laminin was