The lens is a transparent, avascular, biconvex structure. It is suspended between the edges of the ciliary body by the zonular fbers. The pull of the zonular fibers keeps the lens in a flattened condition. Release of tension causes the lens to fatten or accommodate to bend light rays originating close to the eye so that they focus on the retina.
The lens has three principal components (Fig. 24.15):  The lens capsule is a thick basal lamina measuring ap proximately 10 m to 20 m produced by the anterior lens cells.  The Subcapsular epithelium is a cuboidal layer of cells present only on the anterior surface of the lens.  The lens fbers are structures derived from subcapsular epithelial cells.
The lens capsule, composed primarily of type IV collagen and proteoglycans, is elastic. It is thickest at the equator where the fibers of the zonule attach to it.
Gap junctions connect the cuboidal cells of the subcapsular epithelium. They have few cytoplasmic organelles and stain faintly. The apical region of the cell is directed toward the internal aspect of the lens and the lens fbers, with which they form junctional complexes. The lens increases in size during normal growth and then continues to produce new lens fibers at an ever-decreasing rate throughout life. The new lens fibers develop from the subcapsular epithelial cells located near the equator (see Fig. 24.15). Cells in this region increase in height and then differentiate into lens fibers.
As the lens fibers develop, they become highly elongated and appear as thin, flattened structures. They lose their nuclei and other organelles as they become filled with proteins called crystallins. Mature lens fibers attain a length of 7 to 10 mm, a width of 8 to 10 m, and a thickness of 2 m. Near the center of the lens, in the nucleus, the fibers are compressed and condensed to such a degree that individual fibers are impossible to recognize. Despite its density and protein content, the lens is normally transparent (see Fig. 24.15). The high density of lens fibers makes it difficult to obtain routine histologic sections of the lens that are free from artifacts.
Changes in the lens are associated with aging.
With increasing age, the lens gradually loses its elasticity and ability to accommodate. This condition, called presbyopia, usually occurs in the fourth decade of life. It is easily corrected by wearing reading glasses or using a magnifying lens.
Loss of transparency of the lens or its capsule is also a relatively common condition associated with aging. This condition, called cataract, may be caused by conformational changes or cross-linking of proteins. The development of a cataract may also be related to disease processes, metabolic or hereditary conditions, trauma, or exposure to a deleterious agent (such as ultraviolet radiation). Cataracts that signifcantly impair vision can usually be corrected surgically by removing the lens and replacing it with a plastic lens implanted in the posterior chamber.
The vitreous body is the transparent jellylike substance that fills the vitreous chamber in the posterior segment of the eye.
The vitreous body is loosely attached to the surrounding structures, including the inner limiting membrane of the
FIGURE 24.15 • Structure of the lens. a. This schematic drawing of the lens indicates its structural components. Note that the capsule of the lens is formed by the basal lamina of the lens fibers and the subcapsular epithelium located on the anterior surface of the lens. Also note the location of the germinal zone at the equatorial area of the lens. b. This high-magnification photomicrograph of the germinal zone of the lens (near its equator) shows the active process of lens fiber formation from the subcapsular epithelium. Note the thick lens capsule and the underlying layer of nuclei of lens fibers during their differentiation. The mature lens fibers do not possess nuclei. 570.