Collagen crimp waviness, tortuosity and period have different patterns around the eye

Abstract

Purpose : Collagen undulations, or crimp, play a key role in determining the biomechanical properties of the eye. However, very little is known about the characteristics of crimp and how they vary throughout the globe. We measured three characteristics of collagen fiber crimp in different locations of the globe: period, waviness, and tortuosity.

Methods : Two lamb eyes and one sheep eye from a local abattoir were fixed at 0 mmHg, and cryosectioned axially at 30 μm. Sections were imaged with polarized microscopy and local fiber orientation determined using a recently described method. Period was measured manually by marking fiber bundles. Waviness and tortuosity were quantified along the markings using custom code. Waviness was defined as the normalized SD of fiber orientation, and tortuosity as the ratio between path length and end-to-end length of a fiber. Linear mixed effect models were used to determine regional differences (Fig 1), and a linear model to determine if high parameter values were associated with higher variability.

Results : More than 2,000 measurements were made. Period, waviness, and tortuosity varied substantially across the eye globe (Fig 2). Crimp period (mean±SD) was lowest in the cornea (14.2±3.4μm) and superior/inferior peripapillary sclera (PPS, 12.8±2.8μm) and highest at the equators (29.1±9.5μm), whereas waviness and tortuosity were lowest at equators (0.06±0.04 and 1.012±0.002, respectively) and highest in the cornea (0.17±0.13 and 1.018±0.011, respectively). Equatorial sclera and the rest of the sclera were not significantly different in period (P>0.05), but were highly significantly different in waviness and tortuosity (P<0.01). For all parameters, high values were associated with high variability (P<0.01).

Conclusions : Period, waviness, and tortuosity of the collagen varied across the globe. Spatial trends of waviness and tortuosity were similar, whereas period was significantly different, particularly when comparing equators to other parts of the sclera. Our results evidence specific spatial patterns in collagen crimp parameters, which likely reflect the mechanisms that determine local biomechanical properties. Waviness and tortuosity may relate to tissue stiffness while period may relate to the level of organization.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

 

Axial sections of sheep eyes were divided into 17 regions for analysis.

 

Mean (line) and SE (shading) of collagen parameters across the globe pooled over all eyes.