Lens (vertebrate anatomy)

Lore & Background

The lens is composed of three main parts: the lens capsule, the lens epithelium, and the lens fibers. The capsule is a smooth, elastic basement membrane surrounding the lens, thickest near the equator and thinner near the posterior pole. The lens epithelium is a single layer of cells at the front that maintains homeostasis and divides into new lens fibers at the equator. Lens fibers are long, thin, transparent cells arranged in concentric layers like an onion, with mature fibers lacking organelles or nuclei.

Development begins when the embryo is about 4 mm long, derived from the outer skin rather than inner embryo layers. The lens placode forms, deepens, and buds off to become the lens vesicle, which separates from the skin by the time the embryo is about 10 mm long. Signals from the developing retina induce posterior cells to elongate and synthesize crystallins.

In humans, accommodation declines with age: by age 25 the lens's ability to alter light path reduces to 10 dioptres, continuing to decline. The lens lacks nerves, blood vessels, or connective tissue. Cell fusion and voids have been observed in living lenses, speculated to be involved in transport systems.

Reader's Guide

The vertebrate lens is essential for vision, providing about one-third of the eye's total refractive power in humans. Its ability to change shape—accommodation—allows focusing at different distances, analogous to a camera lens. The lens grows throughout life, with new fibers added from the epithelium, and its capsule maintains optical shape. Understanding lens structure and aging is critical for correcting sight with glasses or other means, as accommodation declines with age. The lens's development from embryonic skin and its lack of blood vessels or nerves make it a unique model for studying cell differentiation and transparency. Its suspensory ligaments and capsule must withstand forces from focusing, and its metabolic activity is concentrated at the equator to avoid scattering light.

Did You Know?

• The lens grows throughout life, with new layers of fiber cells recruited from a thin epithelium at the front.
• In humans, the lens's refractive power is about 18 dioptres in youth, roughly one-third of the eye's total power of about 60 dioptres.
• The lens capsule varies from 2 to 28 micrometres in thickness, being thickest near the equator.
• Mature lens fibers have no organelles or nuclei.