CHICAGO, Sept. 15 (Xinhua) -- A study of Northwestern University (NU) found that blood proteins linked to macular degeneration crossed the blood-ocular barrier of new blood vessels in mouse models.

This indicated that newly formed blood vessels may be cracks in the barrier between the bloodstream and the eye.

In the study, NU researchers drew blood from nitrogen-15 mice and injected it into normal mice with one of two eyes induced for new blood vessel growth. Within hours, the researchers detected nitrogen-15-labeled proteins in both eyes, but only the induced eye let through certain immune suppressor proteins linked to age-related macular degeneration.

"These new vessels are different when compared to older or more typical vessels in the eye," said Jing Jin, assistant professor of Medicine in the Department of Nephrology and Hypertension and senior author of the study. "There are some pathological consequences associated with this."

"Newly formed blood vessels are involved in a number of human eye diseases," Jin said. "Our method characterizes the protein basis for that barrier permeability."

The study became possible through a method of marking mouse models' protein with nitrogen-15, a nitrogen isotope slightly heavier than normal nitrogen.

Mice are fed a diet containing nitrogen-15 and without normal nitrogen for three months. During the time the mice incorporate nitrogen-15 to make proteins. These "heavy" proteins are detectable by mass spectrometer, an instrument that detects molecules by weight.

The study has been published in the Proceedings of the National Academy of Sciences.