It's basically another reminder to be cautious about oversimplifying the classic old "three layers to the tear film" model where we think of the meibomian glands as the source of the oily "evaporation-retarding" layer (incidentally, for those interested that broader role of retarding evaporation is also subject to some serious questioning if you dig deeper - click here for an interesting discussion by Dr. Holly). While most of the lipid clearly comes from the MGs the lipids in the tear film itself are different enough from the lipids in what the MGs put out that we can't assume healthy MGs will produce a healthy lipid component.
On the lipid composition of human meibum and tears: Comparative analysis of nonpolar lipids.
Invest Ophthalmol Vis Sci. 2008 May 16. [Epub ahead of print]
PURPOSE: To qualitatively compare the nonpolar lipids present in meibomian gland (MG) secretions (samples T1) with aqueous tears (AT) collected from the lower tear menisci of healthy, non-dry eye volunteers using either glass microcapillaries (samples T2) or Schirmer test strips (samples T3).
METHODS: Samples T1-T3 were analyzed using high pressure liquid chromatography/positive ion mode atmospheric pressure chemical ionization mass spectrometry. Where possible, the unknown lipids were compared with known standards.
RESULTS: Samples T1 had the simplest lipid composition among all the tested specimens. Samples T2 and T3 were very similar to each other, but remarkably different from samples T1. In addition to all the compounds detected in samples T1, there were large amounts of lower molecular weight wax esters and other compounds found in samples T2 and T3. No appreciable amounts of fatty acid amides (e.g. oleamide), ceramides, or monoacyl glycerols were routinely detected in any of the samples. The occasionally observed minor signals of oleamide (m/z 282), were attributed to the contamination of the samples with common plasticizers routinely found in plasticware extractives and/or organic solvents.
CONCLUSIONS: The MG is a prominent source of lipids for the tear film. However, it would have been a mistake to exclude from consideration other likely sources of lipids such as conjunctiva, cornea, and tears produced by the lacrimal glands. Our data showed that lipids in AT are fundamentally different from, and more complex than, MG secretions, which necessitates a more cautious interpretation of the functions of the latter in the tear film.