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I. A. Butovich, T. J. Millar; In Search for a Better Animal Model of Human Tear Film: Comparative Lipidomic Analysis of Human and Animal Meibum. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2545.
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© ARVO (1962-2015); The Authors (2016-present)
Human meibomian gland (MG) secretions (meibum) are secreted onto the ocular surface (OS) to form a protective tear film lipid layer (TFLL). TFLL is believed to retard water evaporation from the OS, thus protecting it from drying. This function of TFLL is critical for maintaining healthy OS and, apparently, is a universal protective mechanism in mammals. The purpose of this study was to conduct a comparative lipidomic analyses of meibum samples collected from normal human volunteers and those from ten different species, namely cat, cow, rabbit, ferret, possum, wallaby, koala, kultarr, antechinus, and phascogale.
Meibum was hard-squeezed from MG of the corresponding animals and healthy human volunteers. Between 0.1 and 1mg of dry meibum/subject was collected. The samples were analyzed using HPLC-MS/MS and compared with authentic lipid standards where possible.
No two species had an identical mixture of lipids. The major lipid species found in their meibum were of extremely hydrophobic nature and consisted of wax esters, steryl esters, and triacylglycerols present in various ratios. Contrary to the previous reports, the rabbit samples were among the most different from human ones in terms of nonpolar lipids having squalene and related compounds as major components, and very little of cholesteryl esters. Also found were large amounts of phospholipids, absent in human meibum.
Different species have vastly different lipid makeup of their meibum. This diversity does not impact their ocular health, vision, and survivability in adverse dry and hot environment. Given the blink rate in many tested species is measured in minutes (e.g. once every 10min for koalas meaning an extremely stable TF) vs. few seconds for humans, species differences in meibum composition may provide information for enhancing healthy human TF, and repairing damaged human TF such as occurs in dry eye. A dramatic difference between the human and the rabbit samples came as a surprise as rabbits are routinely used in dry eye studies. However, the differences between their meibum lipidomes are too dramatic to be ignored.
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