Abstract
Purpose: :
Tear film Meibomian lipid layer features were investigated and compared to artificial analogues.
Methods: :
Grazing Incidence X–Ray Diffraction resolved the 2D spatial organisation of the lipid layer. A pendant drop model measured evaporation rate, dilatational rheology and protein insertion by means of drop shape analysis. Neutron reflectivity produced an accurate description of the mass density profile of the lipid/protein layer.
Results: :
Despite its complex composition, Meibomian lipid layer showed an ordered 2D structure that is index of a remarkable molecular organisation. The d–spacings of the Meibomian and of an analogue comprising egg lecithin and soy bean oil systems are very close (4 Å). In the presence of such layer the evaporative flux of the underlying aqueous phase was significantly reduced. The evaporative flux j follows a physical model in which the driving force is the relative humidity and the total resistivity against evaporation is the sum of Rw (water) and Rm (lipid layer). The Rm of the Meibomian and lipid analogue are very close while DPPC was lower.
Rheological measurements showed that the meibomian film, weakly packed, is prevalently elastic where the viscous component of the dilatational modulus increases at high surface compression.
Another feature of meibomian layer is the ability to limit tear proteins adsorption at the interface and their consequent interference. The neutron reflectivity study showed that lactoferrin adsorbs at a free air/aqueous interface not retaining its native globular structure. A close packed lipid layer effectively prevented such event where a non–uniform or weakly packed lipid layer allowed protein coadsorption and the set up of a lipid protein mixed interface. The insertion of proteins at the interface and their progressive loss of ternary structure eventually impaired the stability of the whole interface.
Conclusions: :
The Meibomian layer is an effective evaporation retardation agent and its dilatational rheology is ideal for efficient respreading after a blink. Moreover, if densely packed, it prevents the undesired insertion of proteins. The lecithin–oil composition was found to significantly mimic the physical properties that characterise the natural tear lipid film. This was not an obvious result since pure phospholipids compositions and some commercial emulsions were found to produce a substantially dissimilar interface. The therapeutic approach of supplementing lipids to deficient tear films, may produce an effective restructuring of the pre–existing lipid layer in the case of lecithin–oil emulsions.
Keywords: extracellular matrix • lipids