EDITORIAL

LA PELÍCULA LAGRIMAL; UNA PARTE DEL OJO PEQUEÑA
PERO ALTAMENTE COMPLEJA

THE PREOCULAR TEAR FILM; A SMALL BUT HIGHLY COMPLEX PART OF THE EYE^*

FRANK J. HOLLY, Ph.D¹

The tear film is a minute but essential part of the anterior segment which only in the sixties began to receive the attention it deserves. The first Tear Film symposium in Lubbock, Texas was organized by the Dry Eye Institute in November, 1984. Over one hundred attendees from twelve countries participated in that meeting. Twenty years later in November, 2004, the fourth International Congress on the Tear Film and Ocular Surface was held in Puerto Rico with 400 researchers and clinicians attending from more than twenty countries all over the world.

One may rightfully wonder, why would such a small and fragile part of the cornea attract such a long lasting attention from scientists and clinicians?

The preocular tear film provides the most refractive and optically smooth surface for the cornea which is essential for a sharp visual image. It is resistant to gravitational forces. It has to be stable so that it remains continuous between consecutive blinks and it has to be able to repair itself. A continuous, normal tear film also plays an important role in protecting and maintaining the well being of the corneal surface and supplies the proper lubrication to the eyelids sans the superficial lipid layer.

In the late forties, Wolff (1) described the three-layered structure of the tear film as consisting of a mucus layer, an aqueous tear layer and a lipid layer. We now know much more about its composition, some about the energetics and the dynamics, but still speculate about its fine structure.

In the late sixties Holly and co-workers (2) following the first footsteps of N. Ehlers, started the pioneering work of establishing a sound scientific basis for tear film physiology. By the seventies it was established that the tear film is a fluid double film consisting of an aqueous and a lipid layer and supported by an underlying macromolecular layer that is loosely attached to the corneal epithelium. So the third layer of Wolff's tear film could be considered part of the solid phase (cornea) that supports the fluid film.

Langmuir (3) had shown earlier that the behavior of fluid films thinner than 100 micrometers is completely controlled by surface forces. There is no gravity-induced flow in such thin films. This also implies that a rigid gel structure is not required to keep the tear film in place even in a vertical position.

The most basic tenet of thin fluid film stability is that the surface energy of the film-coated solid has to be less than that of the solid surface in the absence of the film. This is a stringent requirement because the tear film in situ creates three new interfaces, the ocular surface/tear interface, the tear/lipid interface, and the lipid/air interface replacing the previous one, the ocular surface/air interface, each having interfacial energy.

This had been recognized thirty years ago. At the Congress of 2004 several authors presented results that reaffirmed the earlier view by Holly et al (4) who attributed tear film stability to protein-lipid interaction at the tear/lipid layer interface.

Perhaps here (at the tear-lipid interface) is where the research efforts on meibomian lipid composition led by James McCulley and co-workers will eventually pay off. They characterized the composition of meibomian gland secretion in both health and disease. Surface chemical characterization of the components should also be done. Thomas Millar and his group also reported at the Congress of 2004 some promising work on the surface chemistry of various tear lipids and proteins that could indicate a major role of lipid-protein interaction in tear film stability. This was first proposed thirty years ago (4), then ignored until recently.

The energy of the lipid / air interface is fairly low and would be difficult to decrease it any further. Only the remaining boundary surface, the ocular surface-tear interface can be minimized. Here is where adsorbed glycoprotein and possibly proteo-glycan working in conjunction with the epithelial glycocalyx (the mucus layer of old) can actually lower the interfacial free energy even below zero.

Now a few words on the assumed gel-like structure of the aqueous tear layer of the tear film. When the high viscosity for tear substitutes became in vogue, a considerable research effort was spent to suggest and demonstrate that under certain circumstances conjunctival mucin can be quite viscous and may even form gel structures in the tear film that would hopefully exhibit shear-thinning.

I suspect that many researchers find it difficult to accept the counterintuitive fact that the resistance the tear film demonstrates against gravity-induced flow is simply due to surfaces forces. So far, however, under physiologic conditions, no such flow-resistant gel or even high viscosity could be demonstrated.

Until the nineties, the mechanism of lid lubrication was assumed to be of boundary nature. In 1994, a more likely mechanism resulting in much less friction was proposed by Holly and Holly (5) who made a convincing argument for hydrodynamic lubrication for the lid motion in the healthy eye. The necessary condition for such a lubrication resulting in minimal wear is that the aqueous tear layer remains continuous under the lid and retains its low viscosity.

These examples strongly suggest that the preocular tear film physiology is strictly controlled by the laws and tenets of interface science as emphasized at the first Tear Film Symposium. It was unfortunate that during the nineties the surface chemical approach was abandoned and thus the basic directives lost.

At the Congress of 2004, there have been encouraging signs of a renaissance of surface chemical considerations. Such a basic approach could provide guidance and would promote progress in the future.

* This is basically the motto of the Dry Eye Institute since 1983: Multum in Parvo per Quod Videre.
1 Dry Eye Institute, Yantis, Texas, USA.
E-mail: fjholly@dry-eye-institute.org

REFERENCES

1. Wolff E. Anatomy of Eye and Orbit. IV Ed. New York: Ed. Blakiston Co; 1954; 207.

2. Holly FJ, Lemp MA. Wettability and wetting of corneal epithelium. Exp Eye Res 1971; 11: 239-250.

3. Langmuir I. Oil lenses on water and the nature of monomolecular extended films. J Chem Phys 1933; 1: 756-776.

4. Holly FJ. Surface chemistry of tear film component analogues. J Coll Interface Sci 1974; 49: 221-231.

5. Holly FJ, Holly TF. Advances in ocular tribology. Adv Exp Med Biol 1994; 350: 275-283.