Abstract
The structure and function of meibomian gland lipids in the tear film are highly complex. Evidence shows that the precorneal tear film consists of discrete layers: the inner mucin layer, the middle aqueous layer and the outer lipid layer. In this review we focus on the outer, biphasic lipid layer of the tear film which consists of a ‘thick’ outer, non-polar layer and a ‘thin’ inner, polar layer. We discuss the main composition of the polar and non-polar lipids within meibum (wax esters, cholesteryl esters, mono-, di- and tri-acylglycerols, ceramides, phospholipids et cetera). We address the composition of meibomian lipids in subjects suffering from various ocular diseases in comparison with the composition in healthy individuals. Further analysis is needed to determine whether a correlation exists between the etiology of various ocular diseases and the fluctuation on the lipids as well as to establish whether or not tear lipid analysis can be used as a diagnostic tool.
Highlights
The cornea does not have a blood supply to sustain the corneal epithelium
The precorneal tear film (PTF) covers the ocular surface in a more or less continuous structure, approximately 20 μm thick[2]. Exact details of this structure have not been confirmed experimentally but there is strong evidence supporting the existence of discrete layers within the PTF
Current models suggest that the polar glycocalyx directly anchors a continuous aqueousmucin layer in which salts and hydrophilic substances are dissolved[3]
Summary
The cornea does not have a blood supply to sustain the corneal epithelium. tears are needed to nourish and maintain the ocular surface[1]. The PTF covers the ocular surface in a more or less continuous structure, approximately 20 μm thick[2]. Exact details of this structure have not been confirmed experimentally but there is strong evidence supporting the existence of discrete layers within the PTF. The layer closest to the surface of the eye is the carbohydrate rich, polar glycocalyx (sometimes referred to as the mucin layer). The aqueous layer protects the ocular surface by creating a suitable environment for the cornea[5]. The outermost layer of the tear film is the biphasic tear film lipid layer (TFLL), consisting of a ‘thick’ outer non-polar
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