zonular

The zonular apparatus, commonly known as the zonules or the Zonule of Zinn, is a network of delicate collagenous fibers that suspend the crystalline lens from the ciliary body in the eye. The zonules form a circular ring around the lens at its equator and extend from the ciliary processes to the lens capsule.

Structure and attachments: The zonules are composed of fibrillin-rich microfibrils and other extracellular matrix components. They originate primarily from the nonpigmented ciliary epithelium and ciliary processes, attaching to the lens capsule near the equator. Thousands of slender fibers create a flexible, tension-bearing mesh that distributes forces across the lens.

Function and mechanism: The zonules transmit tension from the ciliary muscle to the lens and play a crucial role in accommodation. When the ciliary muscle contracts for near vision, tension on the zonules decreases, allowing the lens to become more convex. When the ciliary muscle relaxes for distance vision, the zonules become taut and flatten the lens. In addition to accommodation, the zonules help maintain the lens’s position within the globe.

Clinical significance: Zonular weakness or rupture can lead to lens dislocation (ectopia lentis), a feature of conditions such as Marfan syndrome and homocystinuria, or result from trauma. Pseudoexfoliation syndrome and cataract surgery can compromise zonular integrity, complicating capsular support. In eye surgery, devices like capsular tension rings may be used to stabilize the lens when zonular support is inadequate.

Terminology: The term “zonule of Zinn” is historical; the broader term zonular apparatus is commonly used in contemporary anatomy.