Abstract: The meibomian puzzle

Well, it's a puzzle alright. And for those of you who have often wondered, "Geez, with all of modern medical technology why can't they come up with a decent tear substitute?" perhaps this article will help explain why - bearing in mind of course that this is discussing meibum which is only a very small albeit critical part of the tear film.

I found the summary of this 'state-of-the-meibomian-knowledge-nation' review fascinating and those of you who are really into MGs may want to get a copy of the complete study. The phospholipid point (vs. long chain fatty acids, see my highlight below) was surprising to me and ought to have interesting implications for treatment research.

The Meibomian Puzzle: Combining pieces together.
Prog Retin Eye Res. 2009 Aug 4. [Epub ahead of print]
Butovich IA.
Department of Ophthalmology and Graduate School of Biomedical Sciences, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, TX 75390-9057, USA.

The purpose of this review was to summarize the available information on lipidomic analysis of human meibum and tear film, and critically evaluate the pertinent past and present analytical procedures and results obtained in various laboratories. Human meibum was shown to be a very complex mixture of lipids of various classes. For decades, their exact structures have remained elusive. Because of the limitations of the then-current techniques, most of the complex lipids that constitute meibum could not be analyzed as whole molecules and required prior hydrolysis and/or transesterification of the entire lipid pool. These procedures effectively made it very difficult, and often impossible, to reconstruct the complete structures of the original intact compounds, which prompted us to call this The Meibomian Puzzle. Modern techniques such as high-performance liquid chromatography in combination with mass spectrometry help in solving this puzzle by allowing a researcher to detect and analyze intact molecules of complex lipid compounds, even if present in extremely low concentrations. This current de-facto standard procedure in lipidomic analysis of natural lipids and their mixtures is compared with other experimental techniques such as nuclear magnetic resonance spectroscopy, infrared spectroscopy, gas chromatography, and thin layer chromatography, among the others. The results obtained by older techniques, and their limitations and deficiencies are discussed. It appears that some of the earlier findings did not withstand a scrupulous re-evaluation and need to be modified and/or corrected. The most intriguing development is the virtual absence in meibum of typical phospholipids - an important group of amphiphilic compounds whose role in the human tear film was thought to be to stabilize the entire tear film structure. Instead, another group of previously unidentified compounds, very long chain (O-acyl)-omega-hydroxy fatty acids, appears to be a stabilizing factor which might be related to tear film stability and deterioration. Thus, these compounds may become an important target in biochemistry and (patho)physiology of ocular surface and dry eye research.