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Alan Le, Ayesha Baig, Andrew Shin, Vadims Poukens, Joseph L Demer; Elastin Fibers Are Oriented Suitably for Uniform Composite Properties of the Human Optic Nerve Sheath (ONS). Invest. Ophthalmol. Vis. Sci. 2017;58(8):1736.
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© ARVO (1962-2015); The Authors (2016-present)
It has been suggested that tension exerted by the ONS on the globe optic-nerve junction in adduction may be an intraocular pressure-independent cause of optic neuropathy in glaucoma. We show elsewhere at this ARVO that the ONS has inner and outer layers (Baig et al, ARVO’17) having differing viscoelastic properties that are roughly isotropic during tensile loading (Shin et al, ARVO’17). Since individual elastin fibrils could not be isotropic, we examined elastin fiber abundance and orientation in the ONS to determine if their ensemble might influence overall tissue properties.
Human ONSs aged 26 to 82 years were obtained from eyebanks, formalin fixed, paraffin embedded, coronally sectioned at 10 μm thickness, and stained with van Giesson’s elastin method. Micrographs were analyzed using Photoshop and ImageJ for relative abundance and orientation of elastin fibers within the inner and outer ONS layers.
Three general orientations of elastin fibers were identified: longitudinal fibers (LF) running parallel to the length of the ONS, diagonal fibers (DF) crisscrossing at roughly ±45° to the LF, and circumferential fibers (CF) concentric to the ONS. DF were interwoven through the ON sheath at angles 30° to 80° relative to the longitudinal ONS axis, and at a wide range of orientations within the coronal plane clustering at 160°, -20° and -150° relative to the circumferential tangent. Fibers were uniformly distributed around the ONS circumference in both layers. The ONS inner layer exhibits significantly more DF and CF than the outer: 69% of total DF appeared in the inner layer vs. 31% in the outer (P<10-3), 88% of total CF in the inner vs. 12% in the outer layer (P<10-3). LF were evenly distributed between the two layers. The inner ONS layer contained 56±5%(SEM) LF, 32±4% DF, and 12±1% CF. The outer ONS layer contained 75±6% LF, 23±5% DF and 2±1% CF (P< 0.003, chi-square).
Elastin fibers are embedded in the collagen matrix of the ONS in a woven, 3-D pattern of longitudinal, circumferential, and oblique orientations that forms a mesh suitable to provide stiffness in all directions. This organization may explain why overall ONS biomechanical properties at the tissue level are isotropic despite construction from anisotropic constituents. Differences in fiber orientations between the ONS inner vs outer layers may reflect greater tensile loading in the inner layer.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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