Abstract
Purpose: :
Human meibomian gland secretion (hMGS), or meibum, is a complex mixture of lipids whose major role in vivo is believed to protect cornea by forming the outermost part of the tear film (TF), whose structure, composition, and physiology is still not completely understood. One of the approaches to studying human TF and hMGS is to look for the clues provided by nature trying to find a suitable animal model which would closely resemble biochemistry and physiology of human TF. Last year, we presented data on ten animal species whose MGS were compared side-by-side with hMGS. However, none of them was a close match for hMGS. Here, we report our findings on the comparative analysis of normal hMGS and MGS collected from a great ape - a 47 year old female lowland gorilla (Gorilla gorilla gorilla) (gMGS).
Methods: :
MGS were collected from healthy human volunteers and the gorilla from a local zoo using the soft expression technique. HPLC separation of the lipid analytes was performed in normal and reversed phase modes. Qualitative and quantitative mass-spectrometric (MS) analyses of MGS samples were conducted using ion trap ESI and APCI MSn in both positive and negative ion modes. Direct infusion and direct injection of the samples were used for shotgun lipidomic analyses. More than 100 lipids were "fingerprinted" and/or structurally characterized.
Results: :
The shotgun lipidomic MS analyses showed no major differences between hMGS and gMGS: when compared side-by-side, the samples were qualitatively and quantitatively similar to each other. The major lipid classes detected in gMGS by HPLC-MSn were oleic acid-based long chain wax esters, long-chain and very long-chain cholesteryl esters, triacylglycerols, di- and triesters (as defined by Nicolaides et al), and very long-chain (O-acyl)-omega-hydroxy fatty acids, all present in almost the same proportions in both hMGS and gMGS.
Conclusions: :
Among a dozen tested animals, the meibum sample from the lowland gorilla was the closest to the human one in terms of its lipid composition. This implies striking similarities in ocular physiology and biochemistry of the great apes and humans. Taking into account the remarkable anatomical similarities of human and gorilla eyes reported earlier (Liang et al, 2005 and Knapp et al., 2007), and our close genetic relatedness, one can predict that the mechanisms of TF stabilization and deterioration in both the species should be alike.
Keywords: cornea: tears/tear film/dry eye • cornea: basic science • lipids