Lens

Lens crystallina

Also known as: crystalline lens, lens capsule, lens fibres, crystallins

A transparent, avascular, biconvex structure suspended behind the iris by the zonule; it contributes ≈15–20 D of refraction and changes shape to focus on near objects (accommodation).

Show on body map Eye — Horizontal Section

Overview

Diameter≈9–10 mm

Axial thickness≈4 mm (increases with age)

Refractive power in situ≈15–20 D

Accommodative amplitude≈12–14 D (child) → ≈4 D (40 y) → ≈1 D (60 y)

The adult lens is ≈9–10 mm in diameter and ≈4 mm thick (increasing with age), with a flatter anterior and a more curved posterior surface. It has three components: the capsule — an elastic basement membrane (the thickest in the body), thickest anteriorly; the lens epithelium — a single layer of cuboidal cells on the anterior surface only, which divide at the equator (germinative zone) and elongate into lens fibres; and the lens substance of tightly packed fibres that lose their nuclei and organelles and are filled with crystallin proteins (α, β, γ) at very high concentration, giving a refractive index of ≈1.39–1.41.

New fibres are added throughout life; old fibres are never shed but compressed centrally, so the lens has a dense nucleus (containing the embryonic and fetal fibres) and a softer cortex. Fibre ends meet at Y-shaped sutures (upright anteriorly, inverted posteriorly). The lens has no blood vessels or nerves and relies on aqueous humour for nutrition; its metabolism is mainly anaerobic glycolysis.

Accommodation: for distance the ciliary muscle relaxes, zonules are taut and the lens is flat. For near, parasympathetic stimulation contracts the ciliary muscle, the zonules slacken and the elastic capsule makes the lens more spherical — especially its anterior surface — increasing its power by up to ≈12–14 D in childhood. The lens hardens progressively, so accommodative amplitude falls to ≈4 D by 40 and ≈1 D by 60 years (presbyopia).

Location: Behind the iris and pupil, in front of the vitreous body (sitting in the patellar fossa of the vitreous), suspended from the ciliary body by the zonule.

Parts & subdivisions

Capsule
Elastic, transparent type IV collagen basement membrane enveloping the lens; zonules insert into it; site of the "bag" that holds an intraocular lens implant.
Lens epithelium
Anterior single cuboidal layer; mitotically active at the equator; regulates ion transport (Na⁺/K⁺-ATPase).
Cortex
Younger, nucleated outer lens fibres.
Nucleus
Older compacted central fibres; becomes yellow-brown and hard with age (nuclear sclerosis).
Lens fibres and crystallins
Hexagonal, ribbon-like anucleate cells joined by interdigitations and gap junctions; crystallins remain stable for life.

Anatomy

Arterial supply: Avascular in the adult — nourished by aqueous humour. In the fetus the hyaloid artery supplies the tunica vasculosa lentis, which regresses before birth.
Histology: Capsule (PAS-positive basement membrane), anterior simple cuboidal epithelium, anucleate lens fibres packed with crystallins.
Development: Lens placode (surface ectoderm) induced by the optic vesicle → lens pit → lens vesicle (week 5). Posterior cells elongate to form primary lens fibres (embryonic nucleus); anterior cells remain as the lens epithelium.

Function

Fine focusing of the image on the retina (≈15–20 D)
Accommodation for near vision
Absorb UV light, protecting the retina (increasingly with age)

Physiology: Near point recedes from ≈7 cm at age 10 to ≈25 cm at 40 and ≈1 m by 60. Transparency depends on the regular packing of fibres, absence of organelles and short-range order of crystallins. The lens yellows with age, filtering blue light.

Clinical significance

Cataract

Any opacity of the lens: nuclear (age), cortical, posterior subcapsular (corticosteroids, diabetes, radiation), congenital (rubella, galactosaemia). In diabetes and galactosaemia, aldose reductase converts glucose/galactose to sorbitol/galactitol, which osmotically damages fibres. Treatment: phacoemulsification and intraocular lens.

Presbyopia

Universal age-related loss of accommodation due to lens hardening (and growth), noticed when reading at ≈40–45 years; corrected with plus (reading) lenses or multifocals.

Aphakia

Absence of the lens (surgical or traumatic) leaves the eye highly hyperopic (≈+10–12 D) and unable to accommodate.

Test yourself

Which change occurs during accommodation for near vision?

Ciliary muscle relaxes, zonules tighten, lens flattens
Ciliary muscle contracts, zonules slacken, lens becomes more convex
Ciliary muscle contracts, zonules tighten, lens becomes more convex
Dilator pupillae contracts and the eyes diverge

Answer: Ciliary muscle contracts, zonules slacken, lens becomes more convex

Parasympathetic (CN III → ciliary ganglion) contraction of the ciliary muscle reduces the diameter of the ciliary ring, relaxing zonular tension so the elastic capsule rounds the lens. This is accompanied by convergence and miosis (the near triad).