Normal Crystalline Lens
The crystalline lens is a transparent, biconvex structure located posterior to the iris and anterior to the vitreous body (Fig 2-1). The lens is suspended by numerous fibers that together are called the zonule. Collectively, this ring of fibers (zonule of Zinn) attaches the lens to the ciliary body and can be considered a ligament. Components of the lens include the capsule, the epithelium, the cortex, and the nucleus (Fig 2-2).
An imaginary line called the optic axis joins the anterior and posterior poles of the lens, passing through them. Hypothetical lines on the lens surface that pass from one pole to the other are referred to as meridians. The equator of the lens is its greatest circumference.
The functions of the lens are
to maintain its own clarity
to refract light
to provide accommodation, in conjunction with the zonule and the ciliary body
Lacking a blood supply and innervation after fetal development, the lens depends entirely on the aqueous humor to meet its metabolic requirements and also to remove its wastes.
Figure 2-1 Cross section of the human crystalline lens, showing the relationship of the lens to surrounding ocular structures.
(Illustration by Christine Gralapp.)
The lens is able to refract light because its index of refraction—normally about 1.4 in the center and 1.36 in the periphery—is different from that of the aqueous and vitreous humors surrounding it. In its nonaccommodative state, the lens contributes approximately 20.00 D of the approximately 60.00 D of convergent refractive power of the average human eye; the air–cornea interface provides the rest, about 40.00–45.00 D.
The lens continues to grow throughout an individual’s life. At birth, it measures about 6.4 mm equatorially and 3.5 mm anteroposteriorly and weighs approximately 90 mg. The lens of an adult typically measures 9–10 mm equatorially and about 5 mm anteroposteriorly and weighs approximately 255 mg. With increasing age, the relative thickness of the cortex increases; the lens also adopts an increasingly curved shape, so that older lenses have more refractive power. However, the index of refraction of the lens decreases with increasing age, probably as a result of the increasing presence of insoluble protein particles. Thus, with increasing age, the eye may become either more hyperopic or more myopic, depending on the balance of these opposing changes.
The lens capsule is an elastic, transparent basement membrane that is composed of type IV collagen and other matrix proteins and laid down by the epithelial cells. The capsule contains the lens substance and is capable of molding it during accommodative changes. The outer layer of the lens capsule, the zonular lamella, also serves as the point of attachment for the zonular fibers. The lens capsule is thickest in the anterior and posterior preequatorial zones and thinnest at the central posterior pole, where it may measure only 2–4 μm (Fig 2-3). At birth, the anterior lens capsule is considerably thicker than the posterior capsule; its thickness increases throughout a person’s life.
Figure 2-2 Structure of the normal human lens.
(Illustration by Mark Miller.)
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.