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Steven Bassnett, Alicia De Maria, Yanrong Shi; Development, Composition, and Architecture of the Mouse Ciliary Zonule. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3061.
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Ocular manifestations of Marfan syndrome include myopia and ectopia lentis. Marfan syndrome is caused by mutations in fibrillin1, a glycoprotein enriched in force-bearing structures such as ciliary zonule. The use of mouse Marfan models for ocular studies has been limited because little is known about the nature of the murine zonule. The present study was designed to help fill this knowledge gap.
Oligonucleotide probes were raised against fbn1 and fbn2 and used for in situ hybridization. Antibodies against fibrillin isoforms and microfibril-associated glycoprotein-1 (Magp1) were used for immunofluorescence applications, in conjunction with volume-rendering techniques.
Fbn2 was the dominant fibrillin expressed in the embryonic eye. Fbn2 was transiently expressed in the vascular tunic, where it was found associated with Magp1 in microfibrils. By E16.5, Fbn2 was expressed strongly by non-pigmented ciliary epithelial (NPCE) cells. Zonular fibers were evident by P1, after which Fbn2 expression declined and Fbn1 expression increased. By P30, a well-organized ciliary zonule was present. Zonular fibers projected from the posterior portion of the pars plicata to anterior, posterior, and equatorial termination points on the lens capsule. The posterior fibers attached to a dense meshwork of radially-oriented microfibrils on the capsular surface. The microfibrils formed a 100 micrometer-wide band encircling the lens. 3D-reconstructions revealed that this “fibrillar girdle” was situated above the transition zone, a region of the epithelium where cells commit to terminal differentiation.
The organization/composition of the mouse ciliary zonule was grossly similar to that of humans, suggesting that mouse Marfan models may provide useful insights into human ocular disease. A spatial relationship between the zonular attachment and the transition zone of the lens epithelium was noted. In view of the known ability of microfibrils to modulate BMP signaling, the zonule could thus serve both a structural role and a role in lens growth. Finally, our data suggest a model for how the complex architecture of the zonule may arise. Connections between the ciliary epithelium and the lens capsule form during embryogenesis, when the two tissues are in intimate contact. The characteristic fanning of zonular fibers may be the consequence of differential growth rates in lens and NPCE surface areas.
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