Department of Mechanical Engineering, Stanford University, Stanford, CA, USA.
J R Soc Interface. 2013 Jul 31;10(87):20130512. doi: 10.1098/rsif.2013.0512. Print 2013 Oct 6.
The transparency of the human cornea depends on the regular lattice arrangement of the collagen fibrils and on the maintenance of an optimal hydration--the achievement of both depends on the presence of stromal proteoglycans (PGs) and their linear sidechains of negatively charged glycosaminoglycans (GAGs). Although the GAGs produce osmotic pressure by the Donnan effect, the means by which they exert positional control of the lattice is less clear. In this study, a theoretical model based on equilibrium thermodynamics is used to describe restoring force mechanisms that may control and maintain the fibril lattice and underlie corneal transparency. Electrostatic-based restoring forces that result from local charge density changes induced by fibril motion, and entropic elastic restoring forces that arise from duplexed GAG structures that bridge neighbouring fibrils, are described. The model allows for the possibility that fibrils have a GAG-dense coating that adds an additional fibril force mechanism preventing fibril aggregation. Swelling pressure predictions are used to validate the model with results showing excellent agreement with experimental data over a range of hydration from 30 to 200% of normal. The model suggests that the electrostatic restoring force is dominant, with the entropic forces from GAG duplexes being an order or more smaller. The effect of a random GAG organization, as observed in recent imaging, is considered in a dynamic model of the lattice that incorporates randomness in both the spatial distribution of GAG charge and the topology of the GAG duplexes. A striking result is that the electrostatic restoring forces alone are able to reproduce the image-based lattice distribution function for the human cornea, and thus dynamically maintain the short-range order of the lattice.
人眼角膜的透明度取决于胶原纤维的规则晶格排列和维持最佳水合状态——这两个方面的实现都依赖于基质蛋白聚糖 (PGs) 及其线性侧链的负电荷糖胺聚糖 (GAGs)。尽管 GAGs 通过唐南效应产生渗透压力，但它们发挥晶格位置控制作用的方式尚不清楚。在这项研究中，使用基于平衡热力学的理论模型来描述可能控制和维持纤维晶格并构成角膜透明度基础的恢复力机制。描述了由纤维运动引起的局部电荷密度变化引起的基于静电的恢复力，以及由桥接相邻纤维的双 GAG 结构产生的熵弹性恢复力。该模型允许纤维具有 GAG 密集涂层的可能性，这增加了一种防止纤维聚集的额外纤维力机制。通过将预测的肿胀压力与实验数据进行比较来验证模型，结果显示在正常水合度的 30%至 200%范围内，模型与实验数据具有极好的一致性。该模型表明，静电恢复力占主导地位，GAG 双链的熵力小一个或多个数量级。考虑到最近成像中观察到的随机 GAG 组织的影响，在包含 GAG 电荷空间分布和 GAG 双链拓扑随机性的晶格动态模型中对其进行了考虑。一个显著的结果是，仅静电恢复力就能够再现基于图像的人眼角膜晶格分布函数，从而动态维持晶格的短程有序。
