intermediate zone is inner to the cortex and surrounds the center canal.

Vitreous Cortex

The vitreous cortex, also called the hyaloid surface, is the outer zone.69 It is 100 μm wide,2 and it is composed of tightly packed collagen fibrils, some of which run parallel and some perpendicular to the retinal surface.70,71 The anterior cortex lies anterior to the base and is adjacent to the ciliary body, posterior chamber, and lens. The posterior cortex extends posterior to the base and is in contact with the retina. It contains transvitreal channels that appear as holes—the prepapillary hole, the premacular hole, and prevascular fissures. The prepapillary hole can sometimes be seen clinically when the posterior vitreous detaches from the retina.60 The premacular hole, a weak area, may be a region of decreased density rather than an actual hole.60,71 The prevascular fissures provide the avenue by which fine fibers enter the retina and encircle retinal vessels.61

Intermediate Zone

The intermediate zone contains fine fibers that are continuous and unbranched and that run anteroposteriorly.60,70,72 These fibers arise at the region of the vitreous base and insert into the posterior cortex.73 The peripheral fibers parallel the cortex, whereas the more central fibers parallel Cloquet’s canal. Membranelike condensations, called vitreous tracts, may be differentiated as areas that have differing fiber densities (Figure 6-13).71

Cloquet’s Canal

Cloquet’s canal, also called the hyaloid channel or the retrolental tract, is located in the center of the vitreous body.2 It has an S shape, rotated 90 degrees with the center dip downward, and is the former site of the hyaloid artery system, which was formed during embryologic development (see Chapter 7). Cloquet’s canal arises at the retrolental space. Its anterior face is approximately 4 to 5 mm in diameter.2 It terminates at the area of ­Martegiani, a funnel-shaped space at the optic nerve head that extends forward into the vitreous to become continuous with the canal.2,60

COMPOSITION OF VITREOUS

The highly transparent vitreous is a dilute solution of salts, soluble proteins, and hyaluronic acid contained within a meshwork of the insoluble protein, collagen. Vitreous is 98.5% to 99.7% water and has been described as having connective tissue status and being an extracellular matrix.71,74 Because of its high water content, study

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FIGURE 6-13

Eisner’s interpretation71 of vitreous structures (according to slitlamp examinations of eyes obtained at autopsy). Vitreous body is divided into three zones: Externally, as far as retina extends, there is a relatively thick vitreous cortex (light orange). It has holes at characteristic locations: in front of papilla, in region of fovea centralis, in front of vessels, and in front of anomalies of ora serrata region (enclosed ora bays, meridional folds, zonular traction tufts). Intermediate zone (medium orange) contains vitreal tracts, membranelles that form funnels packed into one another and that diverge from region of papilla anteriorly. Central channel (dark orange) is space delimited by hyaloid tract. It is closed off anteriorly by a retrolental section of anterior vitreous membrane. It contains no typical tracts but only irregularly arranged vitreous fibers, part of which are residua of Cloquet’s canal. Outermost vitreous tract, the preretinal tract (1), separates intermediary substance from vitreous cortex. Innermost tract, the hyaloid tract (3), inserts at the edge of

the lens. Between these tracts extends the median tract (2) to median ligament of pars plana and the coronary tract to

coronary ligament. (From Sebag J: The vitreous. In Hart WM Jr, editor: Adler’s physiology of the eye, ed 9, St Louis, 1992, Mosby.)

of the vitreous is difficult. Attempts at tissue fixation often have dehydrating effects that introduce artifacts. Recent investigations suggest that the epithelium of the pars plana has a significant role in the production and secretion of several connective tissue macromolecules of the vitreous body.74

Collagen

The collagen content of the vitreous is highest in the vitreous base, next highest in the posterior cortex, next in the anterior cortex, and lowest in the center.60 A fine meshwork of uniform collagen fibrils, each 8 to 16 nm in diameter, is evident on electron microscopy and fills