THE OCEAN: The Eye
of the Earth

BY ROBERT E. BAIER, PH.D. WITH EVAN B. THOMAS, O.D., F.A.A.O.
JAN. 1996

Based on extensive studies of natural sea slicks and the human eye, a biophysicist challenges a widely accepted theory on tear film composition.T he "surface slick" on the earth's oceans -- wet, salty, and containing many biological secretions -- provides an excellent model for the superficial layer of the preocular tear film of the human eye.

As an environmental engineer, I've often studied the world's oceans, including natural sea slicks. Satellite photographs provide an amazing display of how well the earth, viewed from space, can be likened to the "blue eye of the universe." These pictures show that, from space, the thin film on the surface of the ocean looks like the eye's preocular tear film as viewed through the slit lamp microscope. Direct sampling and analysis of tear films and ocular surfaces provides data that indicate that the outermost layer is mucin and the innermost layer is lipid, contrary to the view developed from the pioneering observations of McDonald in 1969 that the slick floating over the surface of the tear film appeared to be oily (based solely on optical criteria.)

NATURAL FILM ON THE OCEAN'S SURFACE, SEEN FROM THE SPACE SHUTTLE, HAS A REMARKABLY SIMILAR APPEARANCE TO THE "LIPID" FILMS SEEN THROUGH THE SLIT LAMP MICROSCOPE DURING EYE EXAMINATIONS.

THE LONG, STRAIGHT LINE IN THIS SPACE SHUTTLE PHOTO IS ACTUALLY THE WAKE OF A SHIP AT SEA. A LIPID OR OILY FILM COULD NOT "STORE" THE SHIP'S SURFACE-INDUCED CHANGES SO WELL.

It's time to challenge the thesis that lipoidal matter from Meibomian gland secretions occupies the tear film's outer surface as an evaporation-retarding layer, especially because these wax esters have only small "spreading pressures" and the wrong molecular features to prevent water evaporation alone.

OPTICS VS. CHEMISTRY

Photographs of tear film show an iridescent layer that is conventionally called superficial lipid. Is there a chemical analysis to support this identification? No!

As optical experts know, this rainbow-like appearance of interference colors can result from any thin film of higher refractive index on a watery surface. Tear-film photographs of so-called superficial lipid actually illustrate substantial film thicknesses up to 0.1 micrometer. Surface chemists know that lipid films cannot spread to form such thick films. Only non-lipid mineral oils can do this, like motor oil on rain puddles in our driveways.

Other published images of the preocular tear film show mottled appearances that some scientists associate with aberrant properties of the eye's tear film chemistry. This may be correct, but it's not proper to presume that this mottling is lipid material. Lipid films are said to prevent water evaporation from the tear film layer, and certain lipids, namely the long straight-chain fatty alcohols with good "spreading pressures," do form evaporation-retarding films only one molecule thick. It was long assumed that such films also were responsible for the appearance of "slick" areas on natural bodies of water.

In many beautiful video images of the human tear film in motion, taken by researchers in Australia and Japan, these films move and flow with the movements of the eye and its lids. Photos of the natural ocean surface taken from space show similar colorful sea-surface films highlighted in the "sunglint" region from nearly vertical illumination, particularly when seen from the space shuttle. For more than 25 years, "science" understood and accepted that such surface films on the world's oceans were films of lipid that came from man's shoreline activities, from shipping, and from the natural fatty fractions of damaged plankton and fish.

When scientists learned that long "scars" through these natural sea slicks were actually wakes of ships, it became troublesome to maintain that the films were made of oil or lipid. Oil would not have sustained these scars so well or so long! It also was troubling that satellite measurements of the ocean could not always be usefully interpreted (for control of shipping, weather prediction, and even military purposes) on the basis of the assumed chemistry of these "slicks."

INTO THE "GARDEN OF EDEN"

Different shades of gray in satellite infrared photographs showed the edges of large and small slicks all over the globe. Some of these slicks were sampled near the Baja, Calif., coast, which I visited on an oceanographic cruise of the Scripps Oceanographic Institute under sponsorship of the U.S. National Science Foundation. Over many square miles, a scientific crew from the research ship R/V New Horizon viewed the same sea slicks that were monitored from overhead aircraft and satellites.

NOTE THE DIFFERENT SHADES OF GRAY IN THIS INFARED PHOTOGRAPH OF THE AREA AROUND BAJA, CALIF., SOME OF WHICH CORRELATE WITH THE PRESENCE OF UBIQUITOUS "SEA SLICKS" ON THE OCEAN SURFACE THA T HAVE A "TEAR FILM-LIKE" APPEARANCE.

Going forth into these unspoiled and natural ocean regions, unpolluted by man's activities, they had a marvelous opportunity to gather the truth about the chemical composition and architecture of these sea-surface films that look so much like the eye's preocular tear films. The surface-film sampling devices were very simple. In some instances, the scientist used a fishing pole with a small sampler at the end of the fishing line. The surface-film sampling device is a shiny, flat plate that will pick up a faithful specimen of the resident surface layer on both of its faces by a transfer process.

The sampling plates are most often made from transistor-grade materials like germanium and silicon. These materials are transparent to infrared radiation and can be used like "light pipes," or flat fiber optics, in a follow-up analytical method called internal reflection infrared (IR) spectroscopy. When one of the germanium plates is coated by a thin film of natural fatty tissue, for example, the resulting infrared spectrum shows distinct absorption bands associated with the presence of the lipid components. There is a simultaneous, noninterfering detection of many other substances, such as proteins and carbohydrates. When using the internal reflection analytical mode for the adsorbed films from the natural sea surface, infrared spectra show that protein components are dominant. This method reveals as little as 7 percent of lipid matter (if it is there) in natural mixed films of proteins and carbohydrates.

SPECTROSCOPY CONFIRMS COMPOSITION

Modern surface chemistry and surface physics laboratories combine many additional nondestructive thin-film analysis techniques with IR spectroscopy to confirm the film's compositions, thicknesses, refractive indexes and microstructures. Combining these techniques, which are sensitive to even the monomolecular level, has validated these conclusions for the surface films formed on natural water surfaces of all types. We also took similar samples from the eyes of human volunteers.

Natural sea-surface films, which most scientists accepted as layers of superficial lipids, proved to be long-chain carbohydrate-modified proteins (mucins) in various stages of "weathering." The most obvious "crumpled" regions in very thick natural sea slicks were mainly mechanically concentrated filaments and coagulated particles of these well-spread mucins. Similar structures were created in the tear films of the human eye during blinking.

This superficial cleaning process, collecting and aggregating particulate matter (dusts, for example) for elimination, does not readily occur with lipid films. In fact, the presence of lipid layers was not confirmed in most natural surface films studied. In addition, using miniaturized germanium plates to sample and collect the tear films of humans, we found that these measurements were nearly identical to those of the natural sea-surface films. The miniature plates allowed us to take pure samples directly from the Meibomian glands and the tear ducts, as well as mixed films from the tear "lake" itself in the lower sulcus. There were distinct differences between the Meibomian gland exudates and the ingredients of the actual distributed superficial tear films. Lipids are not prominent in the human tear film!

Using a classical technique to determine the spreading pressure of lipids, oils and any other substances, we inquired why Meibomian gland secretions were not dominant in the superficial tear layer. We discovered that the wax esters secreted by the Meibomian apparatus cannot spread over saline solutions at physiologic temperature.

Interestingly, a wiping action similar to when the moving upper eyelid depresses the eye globe, can distribute these soft waxes over the natural corneal epithelium (or the synthetic contact lens surface). However, we noted the Meibomian gland secretions do not exhibit the "explosive" spreading of lipids that have high spreading pressures. Thus, Meibomian gland secretions cannot compete well for a position at the tear film's anterior surface.

Floating black spots, often found in the tear films that are dominated by the mucin fraction uppermost, may represent lipid contaminants or air bubbles that can attach to damaged corneal areas and instigate "dry spot" formation. Waxy ester flakes and other aggregations may also be trapped within the mucous tear film surface layer. They do not spread out against the pressure of the high-molecular-weight glycoprotein structures.

COMPLETING THE ANALOGY

Even if the Meibomian lipids did spread completely over the tear film surface, they are of the wrong architecture to suppress water evaporation. It's more likely that the high surface viscosity that binds the tear film fluid in the mucous surface "gel" is keeping the tear film intact by suppressing mixing and convective heat transfer to the deeper layers. Returning to images of surface films at sea, when obvious harbor pollution is present, the analogy is complete. When there are floating aggregates and inclusions in the films, these also are analyzed as either oily patches or sewage that can destroy the natural health of the otherwise dominant layer of tangled, protein-carbohydrate strands. CLS

References are available upon request to Contact Lens Spectrum; to receive referances via fax, call 1-800-239-4684 and request Document #10. (Be sure to have a fax number ready.)

Dr. Baier is research professor of biophysical sciences, professor of biomaterials and director of the National Science Foundation's Industry/ University Cooperative Research Center for Biosurfaces at State University of New York, Buffalo.

Dr. Thomas is a diplomate in the Cornea and Contact Lens Section of the AAO. He is in private practice in Newport Beach, Calif.