THE OCULAR SURFACE / JULY 2009, VOL. 7, NO. 3 / www.theocularsurface.com 145 ABSTRACT Meibomian glands (MG) secrete an oily sub- stance, meibum, that spreads across the ocular surface and mixes with secretions produced by other ocular structures to create a thin film. The protective efficacy of the tear film is believed to be related to the chemical composition of the lipid layer. We reviewed the literature describing the composi- tion of human MG secretions and have provided an overview on methods of collecting meibum samples, methods of lipid analyses, and the results obtained in previous studies. The usefulness and quality of the data obtained about meibum depend on proper sampling and the analytical techniques used. Historically, several methods have been developed, which have yielded contradictory data regarding meibum sample collection and analytical techniques. Based on review of the literature, the major lipids present in meibum are of nonpolar origin: waxes, sterols, and sterol esters, followed by triacylglycerides and fatty acids. The amphiphilic lipids, diacylglycerides, were reported in fewer studies, and mono- acylglycerides were reported in only two. Information on the composition of the polar lipids is more controversial. Meibum phospholipids were found in small amounts (16% or less) in some studies, but not in others. Thus, meibum is a complex mixture of lipid molecules. Historical analytical inconsisten- cies may be partly explained by limitations of past analytical procedures and by the consideration that the tear film lipid layer may have contributing sources other than meibum. KEY WORDS human tear film, lipids, meibomian glands, meibum, polar lipids, nonpolar lipids I. INTRODUCTION he function of meibomian glands (MG) is of inter- est because MG play a critical role in the patho- genesis of various ocular surface disorders, such as chronic blepharitis and evaporative dry eye. These glands are located on the tarsal plate of the eyelid with openings to the outer margin of the eyelid. 1 They secrete an oily sub- stance, termed meibum, onto the surface of the tear film, which spreads across the surface of the eye. The composition of meibomian lipids has been an area of research interest for several laboratories. Meibum plays a critical role in the protection of the ocular surface, where lipids, due to their poor miscibility with water and lower density, tend to localize at the air/aqueous interface. There- fore, the protective efficacy of the tear film lipid is likely to be related to its chemical composition. 2,3 It is logical to presume that the lipid layer component of the tear film must have specific characteristics that ensure coverage of the preocular surface between blinks in order to provide the optical qualities necessary for good image formation by the eye. 4 Meibum chemistry investigations are complex and are affected by sampling and methods of analysis. Studies of mammalian meibum, including rabbit, 5-10 rodents, 11-13 bovine, 14-18 and human, indicate a species variability in its composition. A wide range of lipid classes are present in meibum, and these vary greatly in amount and type between species. 19 However, there are some similarities between them, especially with regard to wax and sterol esters. 15,17 Yet, the results obtained from animal studies cannot be extrapolated to human results, because, currently, there are no adequate animal models that can be substituted for human studies. 20 T Historical Brief on Composition of Human Meibum Lipids JADWIGA C. W OJTOWICZ, MD, IGOR A. BUTOVICH, PHD, AND JAMES P. MCCULLEY , MD, FACS, FRCOPHTH (UK) Clinical Science W. BRUCE JACKSON, MD, SECTION EDITOR ©2009 Ethis Communications, Inc. The Ocular Surface ISSN: 1542- 0124. Wojtowicz JC, Butovich IA, McCulley JP. Historical brief on composition of human meibum lipids. 2009;7(3):145-153. Accepted for publication April 2009. From the Department of Ophthalmology, The University of Texas South- western Medical Center at Dallas, Texas, USA. This work was supported by an unrestricted grant from Research to Prevent Blindness, New York, New York. The authors have no proprietary or commercial interest in any product or concept discussed in this article. Corresponding author: James P. McCulley (address below). James P. McCulley, Department of Ophthalmology, University of Texas Southwestern Medical Center at Dallas, 5323 Harry Hines Boulevard, Dallas, TX 75390-9057. Telephone: 214-648-2020. Fax: 214-648-9061. E-mail: james.mcculley@utsouthwestern.edu.