by various laboratories during the last few decades ( 1-8 ).In TF, meibum is believed to form its outermost layer, which retards evaporation of water from the bulk of TF and from the ocular surface beneath it ( 7 ). Yet another function of meibum is to form a hydrophobic barrier along the margins of the eyelids to contain TF at, and prevent it from leaking out of, the designated ocular surface area ( 2 ). These protective functions imply a very hydrophobic nature of meibum. Indeed, the major meibum components were identifi ed as various wax esters (WEs) and cholesteryl esters (CEs) with long-chain and very longchain FA ( 9-11 ). The information on acylglycerol content of meibum is more limited as no defi nite structural information on the putative meibomian acylglycerols is currently available ( 12, 13 ). Interestingly, lipidomic analyses of human meibum demonstrated that its composition is distinctively different from that of sebum. For example, free cholesterol, squalene, ceramides (Cers), and typical phospholipids were shown to be minor components of meibum ( 9-11, 14, 15 ), although they dominated in the skin ( 16,17 ).Meibum has been analyzed using various experimental techniques, discussion of which goes beyond the scope of this article. Recently, we published preliminary results on the lipid composition of human meibum and tears ( 9-11, 15 ). The major method employed in these studies was normal phase high performance liquid chromatography in combination with atmospheric pressure chemical ionization mass spectrometry (NP HPLC-MS). The main advantage of the NP HPLC is its ability to separate different classes of lipids. For example, mono-, di-, and triacylglycerols (MAGs, DAGs, and TAGs, respectively) elute as three distinctively different groups of HPLC peaks and so do other groups of lipids (free FAs and their amides and Meibum is a lipid-rich secretion that is produced by Meibomian glands located at the ocular lid margins of both upper and lower eyelids of various mammals and humans. Meibum is an intrinsic part of the human tear fi lm (TF), the main role of which is to protect the ocular surface from dehydration. Among other proposed functions of TF are antimicrobial, lubricatory, and nutritional ones. For general information on meibum and TF, the reader is advised to read earlier comprehensive reviews published

Lipids comprise the bulk of the Meibomian gland secretion (meibum) which is produced by meibocytes. Complex arrays of lipogenic reactions in Meibomian glands, which we collectively call meibogenesis, have not been explored on molecular level yet. Our goals were to elucidate the possible biosynthetic pathways that underlie generation of meibum, reveal similarities in, and differences between, lipid metabolism in Meibomian glands and other organs and tissues, and integrate Meibomian gland studies into the field of general metabolomics. Specifically, we have conducted detailed analyses of human and mouse specimens using genomic, immunohistochemical, and lipidomic approaches. Among equally highly expressed genes found in Meibomian glands of both species were those related to fatty acid elongation, branching, desaturation, esterification, reduction of fatty acids to alcohols, and cholesterol biosynthesis. Importantly, corresponding lipid products were detected in meibum of both species using lipidomic approaches. For the first time, a cohesive, unifying biosynthetic scheme that connects genomic, lipidomic, and immunohistochemical observations is outlined and discussed.

Dietary supplementation with omega-3 fatty acids in dry eye showed no significant effect in meibum lipid composition or aqueous tear evaporation rate. On the other hand, the average tear production and tear volume was increased in the omega-3 group as indicated by both Schirmer testing and fluorophotometry.

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Temperature critically influences MLF. Low temperature leads to stiffening of the film, which loses its ability to form continuous layers at the air-water interface. These effects were shown be of a cooperative nature, manifesting in relatively narrow concentration and temperature ranges.

Meibum is a lipid secretion that is produced by holocrine Meibomian glands (MGs). MGs are a specialized type of sebaceous glands that are embedded in the human eyelids. Chemically, meibum and sebum are different. A detailed characterization of lipidome and transcriptome of MG is required to deconvolute a complex and poorly characterized array of biosynthetic reactions (termed meibogenesis) that lead to formation of meibum. Changes in the composition and quality of meibum have been linked to various ocular disorders, some of which are more prevalent in males, while others in females. To establish the role of gender in meibogenesis in humans, we characterized MG transcriptomes and lipidomes of females and males, and identified signature genes of meibogenesis in both genders. Specimens of MG tissues were subjected to mRNA microarray analyses. Chemical composition of meibum samples was assessed chromatographically and mass spectrometrically. Both targeted and untargeted approaches were used. About 290 signature genes of meibogenesis were identified. The analyses of their expression patterns demonstrated no major differences between the genders. Lipid profiling of major classes of meibomian lipids, such as wax esters, cholesteryl esters, free cholesterol, (O)-acylated omega-hydroxy fatty acids (OAHFA), cholesteryl esters of OAHFA, and triacylglycerols, also demonstrated only minor (and random) differences in these lipids. The results of transcriptomic analyses correlated well with lipidomic data. Taken together, our data imply that in males and females, meibogenesis proceeds in a similar fashion, yielding secretions with similar, highly conserved, compositions. This finding is important for designing novel, gender-independent diagnostic and therapeutic approaches to various MG-related diseases and pathological conditions.

Classic KCS and KCS with MGD groups showed an increase in tear evaporation rates compared with the healthy group. Aqueous tear evaporation diminished in the healthy and KCS with MGD groups after expression of meibomian glands. However, this effect was transient and negligible after the second measurement.

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