雌激素对角膜生物力学特性的影响及其机制探讨

The eye is a complex organ in human body, whose primary function is to convert light into electrical signals that the brain can understand. The cornea provides two thirds of the focusing power and thus plays an important role in image formation. The cornea is also the outermost, and it protects the eye from foreign matters and resists intraocular pressure. The cornea is a viscoelastic tissue with both visco and elastic components. The corneal structure is the foundation of its biomechanical response, which is dominated by the stoma. 200 to 250 collagen lamellas run from limbus to limbus and are stacked with angular offsets. Collagen bundles interweave between neighboring lamellas. The elastic modulus is an intrinsic indicator of material stiffness and traditionally measured in excised tissue with an extensometer. The ocular response analyzer is a commercially available device designed to obtain in vivo measurements of corneal biomechanical properties, two core metrics of which are corneal hysteresis and corneal resistance factor. Less keratocytes density, decreased collagen synthesis and increased collagen degradation may contribute to viscoelastic failure and abnormal biomechanical behavior. Keratoconus and post-LASIK ectasia are disease processes in which the cornea deforms in association with biomechanical weakening. Although the pathogenesis of keratoconus and ectasia currently remains unclear, it seems that a primary event leads to loss of collagen fibrils. Estrogen deprivation has negative effects on connective tissues, including skin, bone, tendons, ligaments and intervertebral discs. Such an effect can be prevented with estrogen replacement therapy to some extent. The cornea is also connective tissue, and estrogen receptors have been found in keratocytes, which indicated that the cornea could response to the sex hormone. The prevalence of keratoconus in men is higher than that in women, while women have higher risk for developing ectasia after LASIK than men. The phenomenon suggests that estrogen play some role in the diseases. The aim of our study is to assess the impact of estrogen on corneal biomechanics in surgically induced menopause animals and in excimer laser induced different corneal thickness animals, and to investigate the possible mechanisms.

角膜是人眼的关键组成部分之一，不仅提供眼球2/3的屈光力，同时保护角膜免受外界损伤和抵抗眼内压，这种光学和屏障作用有赖于角膜自身硬度的对其形态的维持。以往采用离体实验测量角膜弹性模量描述其硬度，眼反应分析仪的问世，使活体测量角膜生物力学特性成为可能，越来越多的研究采用角膜迟滞性和阻力因子来描述角膜硬度。角膜的组织结构是角膜固有硬度的解剖基础，胶原数量、质量或空间构象的改变都可能以角膜硬度下降为特征的扩张性角膜疾病的发生，如圆锥角膜和角膜屈光手术后角膜扩张。到目前为止，尚不清楚何种因素可能影响角膜胶原的生成和降解。研究证实，雌激素能够增加结缔组织的强度和硬度，角膜属于结缔组织，存在雌激素受体，理论上也可以对雌激素产生反应。临床观察发现，扩张性角膜疾病具有明显的性别倾向性，提示可能与雌激素有关。本课题拟研究雌激素对角膜生物力学特性的影响，探讨其作用机制，同时比较离体和在体生物力学检测的相关性。

角膜的光学和屏障作用有赖于角膜生物力学特性的维持，而这种生物力学特性以角膜的结构为基础，其中角膜基质是关键结构。正常状态下，胶原蛋白由角膜细胞分泌，被分解代谢酶降解，其生成和降解处于动态平衡。角膜胶原生成减少和降解增加或角膜胶原交联减少均可导致局部角膜生物力学特性的改变。临床上最常见的以角膜生物力学特性异常为特征的疾病是扩张性角膜病变，包括圆锥角膜和角膜屈光手术后角膜扩张，这类疾病的最终治疗手段是角膜移植。然而供体材料的稀缺、移植手术后的并发症以及高昂的费用，使角膜移植的开展困境重重。到目前为止，何种因素启动胶原的异常分泌、降解和交联尚不清楚。对角膜以外其它结缔组织的研究发现女性绝经后雌激素缺乏对骨、韧带、皮肤、颈动脉和椎间盘有明显的不良作用，而雌激素的补充则有利于这些组织结构和功能的改善。角膜属于中层结缔组织，其上皮、基质和内皮细胞中均有雌激素受体的表达，因此，也可能对雌激素产生反应。一些研究结果提示雌激素可能是维持角膜生物力学特性的保护性因素，然而，另外一些研究结果则暗示雌激素可能减弱角膜生物力学特性，是一个危险因素。因此，明确雌激素对角膜生物力学特性的作用，对于扩张性角膜疾病的诊断、治疗和预防具有重要意义。. 本研究拟从细胞水平、动物实验和临床观察三个方面证实雌激素对角膜生物力学特性的影响及其可能的机制。细胞水平：体外培养的人角膜基质细胞经不同浓度雌二醇处理后，其增殖和凋亡均不受影响。且MMP-2的基因表达显著降低，A1PI显著增加， SP1被抑制。动物实验：卵巢切除术后1周，雌激素替代治疗组的兔眼角膜厚度明显高于未治疗组；两组的角膜迟滞性和角膜阻力因子均明显下降，但是后者的降幅明显高于前者。术后2月，两组的角膜最大刚度和最大强度没有差异，角膜基质中几乎检测不到MMP-2和TIMP-2，其主要分布于角膜上皮内，且两组之间没有明显差异。临床观察：健康育龄期女性排卵期的雌激素水平最高，而角膜迟滞性和角膜阻力因子最低。. 我们的研究提示，在角膜被机械削弱的早期，雌激素可能通过减少胶原酶，增加胶原酶抑制剂以及调节转录因子的表达来增强角膜的生物力学特性。这一发现证实了此前的一些研究结果，为此类疾病的早期干预提供了有一种途径。