We may cry for sorrow or joy, but what are we dumping when crying?
An Ohio State University solved the mystery.
Tears are in fact made of tear film, a microscopically three layered structure.
The middle, watery layer (the proper tears) is wrapped by an inner layer of mucus and an outer layer of fatty substances called meibum.
The investigators discovered for the first time in meibum a new class of fats, named oleamide, previously known only from brain, where it induces sleep.
While we blink, meibum spreads over the surface of the eye, keeping the eye humidity by maintaining the watery layer in place.
"Finding these lipids (fats) 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.
"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 provokes microscopic damage to the eye's cornea (the front invisible membrane), accompanied by ache, burn, dryness or excessively tear.
Meibum collected from meibomian glands (situated on the rim of the upper and lower eyelids) of healthy volunteers was analyzed using electrospray mass spectrometry and chromatography to differentiate between the different lipid components.
"Other scientists used different techniques to try to determine the composition of meibum, but mass spectrometry is sensitive enough to detect individual lipid molecules, like oleamide," Nichols said.
Oleamide was known to have key functions throughout the central nervous system, but now it has been found for the first time 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 signaling in the eye and in the maintenance of tear film," said Nichols.
"Oleamide appears to be a predominate lipid in tear film," she said.
"It's there for a reason, but we're not sure yet what that reason is."
"Dry eye is really a disorder of symptoms that irritate the eyes," Nichols said.
"Not everyone with dry eye responds to the same treatment. If we could find individuals with varying oleamide levels, then we may be able discern one cause of dry eye, and specifically treat that."
"Even though two people with dry eye may have the same symptoms, there may be very different causes underlying those symptoms."