Body Parts: Tears are good for eyes

By: BRADLEY J. FIKES - Staff Writer | Saturday, December 2, 2006 7:28 PM PST ∞

The eyes are the windows to the soul, it is said. If that's true, tears are the cleaner that keep those windows bright and clear.

Tears moisten sensitive eye surfaces that would otherwise cloud up. They form a smooth surface that improves the eye's optics. They're infused with germicidal compounds that prevent infection. They wash away dust and grit.

Unless we're crying or our eyes are irritated, tears go unnoticed. But they're being produced all the time, coating the eye in a film. Oxygen from the air dissolves in this film and passes into the cells of the cornea, the transparent tissue at the front of the eye.

The tear film also contains other nutrients, mainly proteins. It also contains lysozyme, a bacteria-killing enzyme. This is important, because the cornea has no blood vessels, which supply nutrients to nearly all other parts of the body. If blood vessels were to grow into the cornea, which sometimes happens, they can interfere with vision.

Manatees, or sea cows, have blood vessels in their corneas. Manatees appear to have poor vision, although the blood vessels, present in low density, do not seem to be the cause. Manatees also have a very thick tear film, which researchers from the University of Florida suggest helps protect the eyes from infection in the turbid waters manatees frequent. Because the tear film is so thick, oxygen can't get through. Hence, the blood vessels in the cornea.

Back to humans: Riding on top of this mixture on the cornea is an extremely thin layer of oils called lipids. This layer slows down evaporation. The aqueous layer beneath it contains water, salts, proteins and lysozyme. Beneath those layers is a final layer of mucin, a sticky substance that tears adhere to.

Each component of this layered film is secreted by a different set of cells. Mucin is secreted by "goblet cells" on the sclera, the "white" of the eye. Goblet cells are found in other parts of the body to secrete mucin, the major component of mucus.

Goblet cells are also found parts of the body collectively known as "mucus membranes." These include the mouth, nose and rectum. Deficiencies in goblet cells on the eye have been treated by transplanting goblet cells from the latter two areas, according to medical journal articles.

The aqueous layer is secreted by glands in the eyelids, especially the "lacrimal" glands, which make most of the tears produced in crying or due to irritation.

Lipids for the top layer are produced by sebaceous glands, akin to those found on the skin near hair follicles. Some of these glands are located near eyelashes; when they are enlarged or infected, the result is a stye.

The tears produced by the glands are drained by tear ducts, tubes that stretch from the corners of the eye into the nose. When you cry profusely, tear drainage gives you a runny nose.

When these glands don't make enough tears, the result is dry eye. The eyes hurt, and may feel gritty. Dry eye can cause corneal ulceration. Dry eye symptoms can be temporarily relieved with "artificial tears," a lubricating solution given in eyedrops. There are many different formulations of artificial tears on the market.

An estimated 3.2 million American woman age 50 and over have dry eye syndrome, according to an article in the 2003 American Journal of Ophthalmology.

Women whose ovaries stop functioning prematurely are at greater risk for dry eye. In some case, dry eye is produced by a disease called Sjogren's syndrome, in which the body's immune system attacks the tear and saliva glands. This "autoimmune" disease causes dry mouth and dry eye.

Dry eye can also be caused by wearing contact lenses, especially if proper care is not followed. Contact lenses can also block oxygen from reaching the eye surface. This is not true of rigid gas permeable lenses, which contain pores that allow oxygen to pass through.

-- Contact staff writer Bradley J. Fikes at (760) 739-6641 or bfikes@nctimes.com.

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