Abnormalities of the Structure of the Superficial Lipid Layer on the in Vivo Dry-Eye Tear Film

Abstract

The complexity of the tear film covering the anterior surface of the human cornea has become increasingly evident with recent investigations into its structure and chemical composition. The three-layered structure proposed by Wolff¹, perhaps useful as an approximation, is a considerable simplification of reality. While the oily lipid portion of the tear film usually forms a thin layer on the anterior surface of the tear film, as proposed by Wolff, the distribution of other components, such as mucins, appear to be not as well stratified. For instance, there is increasing evidence that a significant portion the mucin component, in highly-hydrated form, is present throughout the aqueous phase as well as being adherent, in a rather thick coating, to the surface of the epithelial layer of the cornea. This mucin coating is much thicker than that proposed by early investigators, at least 1 micron, and perhaps very much greater. Most of this mucin is probably restricted in its ability to freely enter the aqueous phase of the tear film. Thus, the fluid portion of the tear film is now often described as having two layers, rather than three. These consist of the floating lipid phase, secreted primarily by the meibomian glands within the eyelids, and the aqueous phase, a watery solution containing “everything else”, the major components being secreted by the main and accessory lacrimal glands.