It’s no secret why we shed tears. But exactly what our tears are made of has remained a mystery to scientists. A new study sheds some light on the complex design of tears.
What we think of as tears, scientists call tear film, which is made up of three distinct, microscopic layers.
The middle, watery layer — what we normally think of as tears when we cry — is sandwiched between a layer of mucus and an outer layer of fatty, oily substances collectively called meibum.
It’s in this outer layer that researchers describe, for the first time, a new class of lipids — a type of fat — that make up part of the film. They also identified one of these lipids, oleamide, which had not been known to be a part of tears before. With each blink, meibum spreads over the surface of the eye.
It keeps the watery middle layer in place, ensuring that our eyes stay moist. Finding these lipids may help scientists better understand the causes of eye-related disorders such as dry eye disease, which affects anywhere from 12 to 14 million Americans, said Kelly Nichols, the study’s lead author and an assistant professor of optometry at Ohio State University.
“The lack of certain compounds in the tear film may result in a number of different eye-related disorders, including dry eye,” she said. “The amount of oleamide and related lipids in tear film may be related to these disorders.”
Dry eye is really a collection of irritating symptoms that includes microscopic damage to the front of the eye. The eyes may ache, burn, feel extremely dry or excessively tear.
The researchers reported their findings in the current issue of the journal Investigative Ophthalmology and Visual Science. Oleamide was first identified as a lipid in the brain, where one of its roles is to induce sleep.
It also has other key functions throughout the central nervous system. But the current study is the first to find and describe oleamide and related lipids in tear film.
“The finding could give us more insight into the role of lipid activity in humans and may also indicate a new function for oleamide and related lipids in cellular signalling in the eye and in the maintenance of tear film,” said Nichols, adding that the researchers still don’t fully understand the function of oleamide, or the other lipids in the meibum and there is research still waiting to happen.