Overview

The zonular fibres arise from the basal lamina of the non-pigmented ciliary epithelium of the pars plana and between the ciliary processes, and insert into the lens capsule just anterior and posterior to the equator. They are composed largely of fibrillin-1 microfibrils.

At rest (distance vision) the ciliary muscle is relaxed, the ciliary ring is wide, and the zonules are taut, pulling the lens capsule into a flatter shape (lower power). During accommodation the ciliary muscle contracts, reducing the diameter of the ring, so the zonules slacken and the elastic capsule moulds the lens into a more spherical, higher-power shape (Helmholtz theory).

The zonule also forms the anterior boundary of the vitreous and separates the posterior chamber from the vitreous cavity.

Location: Between the ciliary processes/pars plana and the equator of the lens, in the posterior chamber.

Anatomy

Histology: Bundles of 10–12 nm fibrillin microfibrils (elastic-fibre microfibrils without elastin).

Function

Suspend and centre the lens on the visual axis
Transmit ciliary muscle tension to the lens capsule (accommodation)

Clinical significance

Ectopia lentis (lens subluxation)

Weak zonules allow the lens to dislocate. Marfan syndrome (FBN1 mutation): classically upward/superotemporal displacement. Homocystinuria: classically downward/inferonasal displacement. Also Weill–Marchesani syndrome and trauma. Causes fluctuating vision, iridodonesis (tremulous iris) and may cause pupil-block glaucoma.

Pseudoexfoliation syndrome

Fibrillar material deposits on the lens and zonules, weakening them (risk during cataract surgery) and blocking the trabecular meshwork (secondary glaucoma).

Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.