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Tear film lipid layer and corneal oxygenation: a new function?

Abstract

The classic model of tear film is composed of mucin layer, aqueous layer and the outermost tear film lipid layer (TFLL). The complex mixture of different classes of lipids, mainly secreted by meibomian glands, gives the TFLL unique physicochemical properties. Based on these properties, several functions of TFLL have been found and/or proposed such as the resistance to evaporation and facilitating the formation of a thin film. However, the role of TFLL in the oxygenation of the cornea, a transparent avascular tissue, has never been discussed in the literature. The continuous metabolic activity of the corneal surface and the replenishment of atmospheric gas creates an O2 gradient in the tear film. The molecules of O2 must therefore be transferred from the gas phase to the liquid phase through the TFLL. This process is a function of the diffusion and solubility of the lipid layer as well as interface transfer, which is influenced by alterations in the physical state and lipid composition. In the absence of research on TFLL, the present paper aims to bring the topic into the spotlight for the first time based on existing knowledge on O2 permeability of the lipid membranes and evaporation resistance of the lipid layers. The oxidative stress generated in perturbed lipid layers and the consequent adverse effects are also covered. The function of the TFLL proposed here intends to encourage future research in both basic and clinical sciences, e.g., opening new avenues for the diagnosis and treatment of ocular surface conditions.

摘要

泪膜的经典模型由粘蛋白层、水液层和最外层的泪膜脂质层 (TFLL) 组成。不同类别脂质的复杂混合物, 主要由睑板腺分泌, 赋予了TFLL独特的理化性质。基于这些性质, 人们发现并/或提出了TFLL的一些功能, 例如防止蒸发和促进薄膜的形成。然而, TFLL在角膜 (一层透明无血管的组织) 的氧合中的作用在文献中从未被讨论过。角膜表面持续的代谢活动与空气环境使得泪膜产生了氧气梯度。因此, 氧分子必须通过TFLL从气相转移至液相。这个过程是脂质层扩散和溶解以及界面转移的功能, 这一功能受到物理状态和脂质成分变化的影响。在缺乏对TFLL研究的情况下, 本文基于脂质膜氧渗透和脂质层防止蒸发的现有知识, 首次将该主题进行探讨并引起大家的关注。本文讨论了紊乱的脂质层产生的氧化应激和后续的不良影响。本文在此提出TFLL的功能, 旨在鼓励进一步进行基础科学和临床方面的研究, 眼表疾病的诊疗开拓新途径。

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Fig. 1: The known and/or proposed functions for tear film lipid layer [7, 19, 27, 45].
Fig. 2: Cross-section of the ocular anterior segment (left), the cornea (middle) and the three-layered model of the tear film (right).
Fig. 3: The role of tear film lipid layer in corneal oxygenation and the adverse effects following its molecular and/or structural changes.

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Yazdani, M. Tear film lipid layer and corneal oxygenation: a new function?. Eye 37, 3534–3541 (2023). https://doi.org/10.1038/s41433-023-02557-1

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