Metabolism and aging of the lens The lens is nourished by diffusion from

the aqueous humor. In this respect it resembles a tissue culture, with the aqueous humor as its substrate and the eyeball as the container that provides a constant temperature.

The metabolism and detailed biochemical processes involved in aging are complex and not completely understood. Because of this, it has not been possible to influence cataract development (see Cataract, p. 170) with medications.

The metabolism and growth of the lens cells are self-regulating. Metabolic activity is essential for the preservation of the integrity, transparency, and optical function of the lens. The epithelium of the lens helps to maintain the ion equilibrium and permit transportation of nutrients, minerals, and water into the lens. This type of transportation, referred to as a "pump-leak system," permits active transfer of sodium, potassium, calcium, and amino acids

Nucleus Lens

— Slit-lamp examination of the lens.

Slit beam of light

— Slit-lamp examination of the lens.

Slit beam of light

Vitreous Metabolism

Anterior chamber Anterior capsule Cortex

Discontinuity zones identifying the adult infantile ©, fetal ©, and embryonic @ nuclei

Vitreous chamber

Posterior lens capsule

Slit beam on the anterior surface of the iris

Cross section of cornea

Fig. 7.4 The various density zones (1 -4) created as the lens develops are discernible as discontinuity zones.

Cross section of cornea

Anterior chamber Anterior capsule Cortex

Discontinuity zones identifying the adult infantile ©, fetal ©, and embryonic @ nuclei

Vitreous chamber

Posterior lens capsule

Slit beam on the anterior surface of the iris

Fig. 7.4 The various density zones (1 -4) created as the lens develops are discernible as discontinuity zones.

from the aqueous humor into the lens as well as passive diffusion through the posterior lens capsule. Maintaining this equilibrium (homeostasis) is essential for the transparency of the lens and is closely related to the water balance. The water content of the lens is normally stable and in equilibrium with the surrounding aqueous humor. The water content of the lens decreases with age, whereas the content of insoluble lens proteins (albuminoid) increases. The lens becomes harder, less elastic (see Loss of accommodation), and less transparent. A decrease in the transparency of the lens with age is as unavoidable as wrinkles in the skin or gray hair. Manifestly reduced transparency is present in 95 % of all persons over the age of 65, although individual exceptions are not uncommon. The central portion or nucleus of the lens becomes sclerosed and slightly yellowish with age.

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