As mentioned, the lens is supported by a system of fibers (the zonule) that originate from the basal lamina of the nonpigmented epithelium of the pars plana and pars plicata of the ciliary body. These zonular fibers, which are located in the valleys between the ciliary processes, consist of microfibrils composed of elastic tissue. They insert at discrete points on the lens capsule 1.5 mm anterior to the equator and 1.25 mm posterior to the equator (Fig 2-4). With increasing age, the equatorial zonular fibers regress, leaving separate anterior and posterior layers that appear in a triangular shape on cross section of the zonular ring. The fibers are 5–30 μm in diameter; on light microscopy, they are revealed as eosinophilic structures that have a positive periodic acid–Schiff (PAS) reaction. Ultrastructurally, the strands, or microfibrils, composing the fibers are 8–10 nm in diameter, with 12–14 nm of banding (Video 2-1).
Endoscopic view of ciliary body, zonular fibers, and lens capsule. Courtesy of Charles Cole, MD.
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Figure 2-3 Schematic of the adult lens capsule showing the relative thickness of the capsule in different zones.
(Illustration by Christine Gralapp.)
Figure 2-4 Scanning electron micrograph of a sagittally cut specimen of the ciliary body, zonular fibers, and lens (L) of the eye of a 4-year old rhesus monkey. The anterior (A) and posterior (P) zonules are attached to the zonular plexus (arrowhead) posteriorly in the valleys between the ciliary processes.
(Courtesy of Johannes W. Rohen, MD, PhD, and Cassandra Flügel-Koch, MD, PhD.)
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.