Lens

At a glance

English

Lens

Latin (TA2)

Lens

System

Eye

Region

Head

Side

Paired, left and right

About the lens

The lens lies just behind the iris and pupil, between the aqueous humour in front and the vitreous body behind, where it sits in a shallow hollow on the vitreous face. It is held in place by the zonule, a ring of fine fibres that runs from the ciliary body to its rim. In the adult it is about ten millimetres across and four millimetres thick from front to back, though both figures change during focusing and the lens keeps growing slowly throughout life. Its back surface is more strongly curved than its front; the centres of the two surfaces are its anterior and posterior poles, and the rounded rim between them is the equator.

Three components build it. An elastic, transparent capsule, a thick basement membrane rich in type IV collagen, encloses the whole lens and gives the zonular fibres their attachment. Under the front of the capsule only lies a single layer of cuboidal epithelium, which controls the lens's fluid and ion balance. At the equator these epithelial cells keep dividing and stretch into long, ribbon-like lens fibres that lose their nuclei and organelles and fill with crystallin proteins. New fibres are laid down outside older ones, so the lens has a softer outer cortex and a denser central nucleus, and the fibre ends meet in Y-shaped sutures.

The lens develops from the surface ectoderm, which thickens into a lens placode, folds in and pinches off as a lens vesicle. It has no blood vessels, nerves or connective tissue; it is fed entirely by diffusion from the aqueous humour. Its refractive power is roughly eighteen dioptres, about a third of the eye's total, and unlike that of the cornea it can be varied. In accommodation the ciliary muscle contracts, the zonule slackens and the elastic capsule lets the lens round up to focus on near objects.

With age the lens stiffens and near focusing fails, which is presbyopia. Clouding of the lens, cataract, is a common cause of poor vision in older people and is also linked to diabetes, injury and some congenital conditions; it is treated by removing the lens contents and implanting an artificial lens. A lens displaced from its normal place is called ectopia lentis, and absence of the lens is aphakia.

Function and supply

Function

Refracts light onto the retina and changes its curvature, under the control of the ciliary muscle, to focus on objects at different distances.

Nerve supply

Has no nerve supply; the ciliary muscle that alters its shape is innervated by parasympathetic fibres of the oculomotor nerve.

Blood supply

Avascular; nourished by the aqueous humour. The fetal hyaloid artery that once supplied it regresses before birth.