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Review
. 2021 May:82:100902.
doi: 10.1016/j.preteyeres.2020.100902. Epub 2020 Sep 25.

Zinn's zonule

Steven Bassnett  1
Affiliations
Review

Zinn's zonule

Steven Bassnett. Prog Retin Eye Res. 2021 May.

Abstract

The Zonule of Zinn, or ciliary zonule, is the elaborate system of extracellular fibers that centers the lens in the eye. In humans, the fibers transmit forces that flatten the lens during the process of disaccommodation, thereby bringing distant objects into focus. Zonular fibers are composed almost entirely of 10-12 nm-wide microfibrils, of which polymerized fibrillin is the most abundant component. The thickest fibers have a fascicular organization, where hundreds or thousands of microfibrils are gathered into micrometer-wide bundles. Many such bundles are aggregated to form a fiber. Dozens of proteins comprise the zonule. Most are derived from cells of the non-pigmented ciliary epithelium in the pars plana region, although some are probably contributed by the lens and perhaps other tissues of the anterior segment. Zonular fibers are viscoelastic cables but their component microfibrils are rather stiff structures. Thus, the elastic properties of the fibers likely stem from lateral interactions between microfibrils. Rupture of zonular fibers and subsequent lens dislocation (ectopia lentis) can result from blunt force trauma or be a sequela of other eye diseases, notably exfoliation syndrome. Ectopia lentis is also a feature of syndromic conditions caused typically by mutations in microfibril-associated genes. The resulting ocular phenotypes raise the possibility that the zonule regulates lens size and shape, globe size, and even corneal topology, in addition to its well-recognized role in accommodation.

Keywords: Ectopia lentis; Elastic modulus; Fibrillin-1; LTBP-2; Luxation; Microfibril; Proteome; Zinn; Zonule.

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Figures

Figure 1.
Figure 1.
Hiding in plain sight. (A) Hematoxylin and eosin-stained section of an eye from a 3-week-old mouse. Note the large, centrally-located lens and the apparent absence of any suspensory system. (B, C) Slice preparation of 1 one-month-old mouse eye following incubation with anti-fibrillin-1 (green) to visualize the zonular fibers. Three dimensional reconstructions provide en face (B) or sagittal views (C) of the zonular fibers near the lens surface.
Figure 2.
Figure 2.
The relationship between the zonule and adjacent structures in the human eye.
Figure 3.
Figure 3.
The fifteen most abundant proteins in zonular samples, as detected by mass spectrometric analysis of the human or bovine zonule (De Maria et al., 2017).
Figure 4.
Figure 4.
Scanning electron micrograph of a portion of the ciliary zonule in the eye of a 1-month-old mouse. Scale bar = 5 μm.
Figure 5.
Figure 5.
Structure of the mouse zonule. From the posterior aspect (A), the zonular fibers (light blue, here immunolabeled for fibrillin-1) project radially from the pars plana region of the ciliary epithelium to the lens (green) equator. A higher magnification image from the anterior aspect (B) after removal of the iris, shows individual zonular fibers (red, here immunolabeled for LTBP-2) and the irregular organi2) and the irregular organization of the ciliary processes. Images from (Jones et al. 2019).
Figure 6.
Figure 6.
Branching patterns of zonular fibers in the mouse eye visualized in the yz (A), xy (B), and xz (C) planes. Longitudinal fibers of the fibrillar girdle indicated with * in B. Images from (Bassnett, 2019).
Figure 7.
Figure 7.
Organization of the human zonule. (A) Fibers of the pars plana zonule originate in the bays of the pars plana close to the ora serrata and pass forward between the major and minor ciliary processes (from McCulloch, 1954; reprinted with permission from The American Ophthalmological Society). (B) The zonular fibers change direction as they pass between the processes, separating into anterior and posterior tines (from Rohen, 1979). Z, zonule, CP, ciliary processes, CB, ciliary body, L, lens, S, Sclera, and SC, Schlemm’s canal.
Figure 8.
Figure 8.
Illustration showing the fascicular organization of a large diameter zonular fiber. Large fibers are formed from bundles of smaller diameter fibers, each of which contains many individual microfibrils. Bundles are shown surrounded by a glycan coat (blue, see section 2.1) and with a core of fibrillin-2-rich microfibrils (yellow) surrounded by a cortical layer of fibrillin-1-rich fibrils (red). Note that it has yet to be confirmed that the coaxial organization of fibrillins found in the mouse zonule is a universal feature.
Figure 9.
Figure 9.
Microfibril ultrastructure. (A) Faint transverse banding is observed in longitudinally sectioned zonular fibers. (B) Transverse sections through a zonular fiber reveal the presence of many microfibrils in cross section (arrows). The microfibrils have electron-lucent cores. (C) An individual microfibril isolated from the bovine ciliary zonule (Godwin et al., 2018). Arrow heads indicate the position of “beads”. (D) High resolution reconstruction of a microfibril in the interbead region. Attributions: A and B, with permission from Raviola 1971, C and D from Godwin et al., 2018 (licensed under a Creative Commons CC-BY license).
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