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S. R. Raju, D. L. Leiske, G. G. Fuller, T. J. Millar; Unusual Surface Properties of Meibomian Lipids From the Koala, Phascolarctos cinereus. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4252.
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Meibomian lipids are the major component of the lipid layer of the tear film. This layer lowers the surface tension of tears allowing them to spread across the ocular surface to form a stable film. An Australian marsupial, the koala, is unusual in that it has a blink rate of more than 10 minutes per blink. This suggests that there is something quite different about the structure of its tear film which allows such elongated stability. One possibility is that the Meibomian lipids have unusual physicochemical properties and hence these have been investigated.
Koala eyelids were obtained from a veterinary hospital where the animals had been euthanized for medical reasons. Meibomian lipids were extruded, dissolved in chloroform, and spread onto the surface of an artificial tear buffer in a Langmuir trough. Pressure was monitored as the surface area was repeatedly compressed and expanded (Π-A isocycles). Viscoelastic properties were measured on similar films using an interfacial stress rheometer. The appearance of the film was monitored using Brewster angle microscopy (BAM)
Π-A isocycles of the lipid film showed that maximum surface pressure (Πmax) continually increased from 30mN/m to 43mN/m on the 7th isocycle and then the film collapsed. This indicates that the koala Meibomian lipids are continually rearranging on the surface. Human Meibomian lipid films had a smaller Πmax, did not show a continuous increase in Πmax with isocycles, and did not collapse. Viscoelastic measurements of koala Meibomian lipid films varied with aging of the film which is consistent with a continuous rearrangement of the lipid molecules. In general, films at low surface pressures (5mN/m) are more fluid (viscous modulus only), and at higher surface pressures (10-25mN/m), viscous and elastic moduli are present (gel-like), but vary depending upon the history of the film and with the frequency of oscillation. Major rearrangements of the film were evident in BAM images at 5-10mN/m progressing from an open meshwork in the first compression to a sheet by the 4th isocycle. Human Meibomian lipid films showed continuous sheets on the first compression.
The physicochemical properties of koala Meibomian lipids are markedly different from human Meibomian lipids. They continually self-organise into a more viscoelastic gel with time. It is yet to be determined how this might relate to the unusual stability of the koala tear film, but an analysis of the lipid composition of koala Meibomian lipids may indicate if particular lipids are responsible.
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