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T. J. Millar, S. Khanal; Using Fluorescent Quantum Dots to Assess the Structure of the Tear Film. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1234.
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The interaction of the different layers of the tear film in vivo is dynamic and difficult to assess. The purpose of these experiments was to use different fluorescent quantum dots (QDs) to differentially assess the dynamics of the lipid and aqueous layers of the tear film in real time. QDs have a fluorescence lifetime about 10 times that of organic dyes such as fluorescein.
Lipophilic QDs (orange) were applied to the lower lid margin in the Meibomian gland orifice zone and hydrophilic carboxyl QDs (red) were instilled into the aqueous without touching the ocular surface. A video-slitlamp with a cobalt-blue filter was used to record the results.
In the aqueous, QDs dispersed immediately spreading from the concentrated centre to progressively less concentrated regions. They did not spread across the whole aqueous layer. They were stable in this position until a blink occurred and then they all disappeared into the upper and lower menisci. They did not redisperse onto the ocular surface during blinking, but were progressively removed from the ocular surface through the puncta. A viscoelastic layer could be seen above the aqueous layer containing the QDs and had a slower upward movement that could be readily traced by monitoring trapped debris. The dispersion of the organic QDs was more complex. Some dispersed quickly but patchily over the whole outer surface of the tear film. Shortly after, they could be seen to be part of the viscoelastic layer. They also strongly marked both eyelid margins and slowly dispersed onto the skin and eyelashes. They were not removed via the puncta. Some were trapped in the meniscus where they formed blobs that rolled along the meniscus. Their outflow through the puncta was not observed. Unexpectedly, when the organic QDs were placed between the Meibomian gland orifices and the eyelashes, they rapidly demarcated a continuous line on the lid margin between the Meibomian gland orifices and the eyelashes. This was not Marx’s line.
These data support a more traditional view of a distinct three layered tear film: an inner mucin layer attached to the epithelial cells; a very fluid aqueous layer; and an outer viscoelastic layer. The aqueous layer is separated from the meniscus (perched) and is replaced on every blink. Flow of aqueous is from the surface to the meniscus and not vice versa. The data suggest that lipids spread rapidly over the viscoelastic outer layer as a mosaic and then become incorporated into this layer. A linear barrier on the eyelid margin between the Meibomian gland orifices and the eyelashes probably prevents skin lipids moving to the ocular surface.
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