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Jacob Wallace, Ning-Jiun Jan, Alexandra Gogola, Michael Iasella, Kira L Lathrop, Andrew P Voorhees, Huong Tran, Ian A Sigal; Stretch-Induced Collagen Bundle Uncrimping and Recruitment are Independent of Depth in Equatorial Sclera. Invest. Ophthalmol. Vis. Sci. 2017;58(8):3162.
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The micro-scale response of the sclera to load is important in physiology and disease. However, studies of sclera mechanics are often limited to the superficial layers. Our goal was to measure micro-scale effects of macro-scale stretch on equatorial sclera collagen fiber bundles and determine if it varies with depth.
3 goat eyes were cryosectioned sagittally (30µm) without fixation. Equatorial sclera sections were mounted to a custom uniaxial stretcher and imaged with polarized light microscopy at various levels of macro-scale stretch (Jan ARVO 2016 E-Abstract 3566). Bundle depth, collagen fiber orientation (Jan BOE 2015), and micro-scale strain (percent change in length) were measured (Sigal IOVS 2014). Waviness (circular SD of fiber orientation) and average micro-scale strain were tracked. 75th percentile waviness values at the final stretch point were considered eye-specific cutoffs to determine fiber recruitment. Linear mixed effect models were used to determine the association of depth with waviness and micro-scale strain. Nonlinear mixed effect models were used to determine the associations of waviness to micro-scale strain and recruitment to macro-scale stretch.
An average of 18 bundles were tracked on each of 2 sections per eye. There was no significant relationship between depth and waviness (p=0.18) nor depth and micro-scale strain (p=0.43, Figure 1). Waviness decreased exponentially with micro-scale strain (p<0.001, Figure 2A) while bundle recruitment followed a sigmoidal curve with macro-scale stretch (p<0.001, Figure 2B). Waviness recruitment cutoffs averaged 3.6°. Within any given section, bundles varied in initial waviness (average range 12.3°), with this variability decreasing with increasing micro-scale strain (Figure 2A). Bundles also experienced variable micro-scale strain before sample failure (range 7-63%).
Within the equatorial sclera the micro-scale response of collagen bundles to macro-scale stretch was independent of depth. This suggests that the equatorial sclera might be a particularly useful portion of the eye in which to study scleral collagen mechanics for imaging techniques that can only access the more superficial layers.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
Fig 1: Bundle waviness and depth at initial and high strain.
Fig 2: Waviness of all bundles from eye 1 (A), and recruitment curves for all eyes (B).
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