Meibomian Gland Dysfunction: A Clinical Scheme for Description, Diagnosis, Classification, and Grading

Mucins and TFF peptides of the tear film and lacrimal apparatus

The three-dimensional organization of the tear film, which is produced and drained by the different structures of the ocular adnexa, is essential for maintainance and protection of the ocular surface. This is facilitated by a class of large, highly glycosylated, hydrophilic glycoproteins, the mucins, which are usually expressed in association with a class of peptides having a well-defined, structurally conserved trefoil domain, the mammalian trefoil factor family (TFF) peptides. In this review, the latest information regarding mucin and TFF peptide function and regulation in the human lacrimal system, the tear film and the ocular surface is summarized with regard to mucous epithelia integrity, rheological and antimicrobial properties of the tear film and tear outflow, age-related changes and certain disease states such as dry eye, dacryostenosis and dacryolith formation.

Sex-related effect on gene expression in the mouse meibomian gland

PURPOSE

Sex-related differences have been identified in the anatomy and physiology of the meibomian gland. We hypothesize that these differences are due, at least in part, to variations in gene expression. This study's objective was to determine whether sex-related differences do exist in meibomian gland gene expression. We also sought to elucidate whether such differences, if any, might be (a) analogous to those known to occur in the lacrimal gland and (b) due to the effect of sex steroids.

METHODS

Meibomian glands were obtained from young adult male and female BALB/c mice (n=7 to 15 mice per sex per experiment), pooled according to sex and processed for the isolation of RNA. Samples were evaluated for differentially expressed mRNAs by using CodeLink Bioarrays and GEM 1 and 2 gene chips. Bioarray data were analyzed with GeneSifter. Net software and also compared with microarray data in GEO and GeneSifter databases.

RESULTS

Our results demonstrate that sex has a significant influence on the expression of 164 genes in the mouse meibomian gland. These genes are involved in a broad spectrum of biological processes, molecular functions, and cellular components, including such activities as metabolism, catalysis, cell growth and maintenance, membrane architecture, nucleic acid binding, transcription, and signal transduction. In addition, the nature of the sex-related variations in meibomian gland gene expression is quite different from those in the lacrimal gland and appear to be mediated in part by the action of androgens, but not estrogens or progestins.

CONCLUSIONS

These findings support our hypothesis that sex-related differences exist in gene expression of the meibomian gland.

Functional aspects of the tear film lipid layer

The lipid layer is an essential component of the tear film, providing a smooth optical surface for the cornea and retarding evaporation from the eye. The meibomian lipids which compose it are well adapted for this purpose. They form a thin, smooth film whose thickness, and probably composition, influences the rate of evaporation. Their melting range ensures sufficient fluidity for delivery to the tear film from the lid margin reservoirs, while the film itself may exhibit a higher viscosity at the cooler temperature of the ocular surface. The factors governing lipid film formation during the blink are not fully understood, but one view is that the polar lipids, interacting with the aqueous sub-phase of the tear film, spread in advance of the non-polar components, which form the bulk of the film. The meibomian lipids stabilise the tear film by lowering its free energy; they carry water into the film during its formation and interact with lipid-binding proteins in the aqueous phase, such as tear lipocalin. The lipocalins, complexed with other tear components, may also contribute to the high, non-Newtonian viscosity of the tear film and its low surface tension, features which are essential for tear film stability. Formation of the lipid film is a complex process. Lipid is delivered to the tear film in the up-phase of the blink, more from the lower than the upper reservoir. The lipid layer comes to a stop well after completion of the blink and remains relatively immobile until it is compressed in the down-phase of the blink that follows. Then, it either retains its structure in a series of subsequent blinks, or is completely re-constituted after mixing with the reservoir lipids. Delivery of meibomian lipid to the marginal reservoirs is mainly the result of continuous secretion, under neural and hormonal control, supplemented by lid action. The reservoirs provide a hydrophobic barrier to tear overspill and to contamination by skin lipids which might destabilise the tear film. They probably also provide the chief route for meibomian lipid excretion.

The Contribution of Meibomian Disease to Dry Eye

The tear film lipid layer is the major barrier to evaporation from the ocular surface. A decrease in its thickness or functional integrity may cause evaporative dry eye (EDE). Obstructive meibomian gland dysfunction (MGD) is the most common cause of EDE and occurs as a primary disorder or secondary to acne rosacea, seborrheic or atopic dermatitis, and with cicatrizing conjunctival disorders, such as trachoma, erythema multiforme, and cicatricial pemphigoid. MGD may be an incidental finding in asymptomatic eyes, or it may be responsible for irritative lid symptoms in the absence of dry eye. MGD-dependent EDE is diagnosed on the basis of a defined degree of MGD in a symptomatic patient showing typical ocular surface damage in the absence of an aqueous tear deficiency. When MGD occurs in a background of aqueous tear deficiency (ATD), then an additional evaporative component may assumed, depending on the extent of meibomian obstruction. However, definitive criteria are not yet established. The clinical severity of dry eye is greatest when ATD and EDE occur together, particularly in Sjogren syndrome. A hypothesis is proposed to explain the steps leading to primary, simple MGD and subsequent EDE.