Lens (vertebrate anatomy)

The crystalline lens is a transparent, biconvex structure found in the eyes of most land vertebrates.

It is a vital component of the visual system because it helps refract light. Refraction is the process of bending light rays so they focus clearly onto the retina.

Along with the cornea and the aqueous and vitreous humours, the lens ensures that images are sharp. Without this precise focusing, the world would appear blurry to the animal.

The lens is composed of three primary anatomical parts: the lens capsule, the lens epithelium, and the lens fibers.

The lens capsule is the outermost layer, consisting of a thick, elastic basement membrane made largely of collagen. Inside this capsule, the lens fibers form the bulk of the structure. These fibers are long, thin, transparent cells that are tightly packed together. At the very front of the lens, between the capsule and the fibers, sits the lens epithelium. This single layer of cells is crucial for maintaining lens homeostasis by providing nutrients and removing waste.

To understand how the lens works, one must look at its unique growth mechanism. Unlike many other tissues, the vertebrate lens grows throughout an individual's entire life. New cells are recruited from the lens epithelium at the front of the lens. These cells migrate toward the equator, which is the outer edge of the lens. At the equator, these epithelial cells elongate and transform into new lens fibers. These new fibers wrap around the older ones in concentric layers, similar to the structure of an onion.

As these cells mature into fibers, they undergo a dramatic change where they lose their nuclei and organelles. This allows the lens to remain transparent for light to pass through.

Accommodation is the process by which an organism adjusts its focus to see objects at different distances. This mechanism is often compared to how a photographic camera changes its lenses to achieve focus. In many land vertebrates, the lens achieves accommodation by altering its physical shape. By changing its curvature, the lens changes its focal length. However, many fully aquatic vertebrates, such as fish, use a different strategy. Instead of changing the shape of the lens, they change the lens's position relative to the retina.

While land vertebrate lenses are typically ellipsoid, fish lenses are often nearly spherical.

In humans, the lens exhibits specific refractive properties that change over time. A young human lens has a refractive power of approximately 18 dioptres. This represents about one-third of the eye's total refractive power, which is roughly 60 dioptres. As humans age, the ability to perform accommodation declines. By age 25, the lens's ability to alter the light path has reduced to 10 dioptres. This natural decline in flexibility is why many people require glasses to supplement their focus as they grow older.

The development of the lens is a complex process that begins in the embryo. In humans, lens formation starts when the embryo is only about 4mm long. Interestingly, while most eye structures come from inner embryonic layers, the lens is derived from the outer skin. A patch of skin transforms into the lens placode, which then bows inward to form a lens vesicle.

Once the vesicle separates from the skin, signals from the developing retina induce the cells to elongate. These cells produce proteins called crystallins, which create a clear, highly refractive jelly that forms the nucleus of the lens.

The lens is held securely within the anterior segment of the eye. It is suspended by thousands of tiny suspensory ligaments, also known as the zonule of Zinn. These ligaments attach to the lens capsule at its equator and connect to the ciliary body.

The lens is also supported by the jelly-like vitreous body located behind it. At the front, the lens is bathed in the liquid aqueous humour, which provides necessary nutrients. Because the lens lacks its own blood vessels or nerves, it relies entirely on these surrounding fluids to function and survive.