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-
germinal zone forming lens fiber lens capsule (basal lamina)
lens fiber subcapsular epithelium
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 fibers 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. 23.15). 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 /xm, and a thickness of 2 /xra. 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. 23.15). The high density of lens fibers makes it difficult to obtain routine histologic sections of the lens that are free from artifacts.
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 due to 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 significantly impair vision can usually be corrected surgically by removing the lens and replacing it with a plastic lens implanted in the posterior chamber.
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