Demand has risen for couples to donate excess embryos to science since the U.S. government lifted restrictions on embryonic stem cell research, researchers say.

The (Nashville) Tennessean reported Sunday that about 500,000 embryos are stored in U.S. fertility clinics, some of which will be transferred back to the mother. However, others will be discarded.

Researchers say they can use the embryos to help find cures for debilitating conditions, such as Alzheimer's disease and Parkinson's.

"The embryo is our best teacher," said Patricia Labosky, a stem cell biologist at Vanderbilt University Medical Center. "We need to understand the developmental process of a normal embryo, in the way Mother Nature intended."

In 2002, a study by the Rand Corp. found there were about 396,000 frozen embryos, with about 88 percent being saved for future family building.

"You've got to be careful that people don't get the impression that there are half a million babies in cold storage," said George Hill, medical director of the Nashville Fertility Center. "That's not the case.''

Hill said he does not like to discard embryos, but they belong to patients and not to the clinic.

"Any embryos being discarded is too many, if you have patients that would like to use those embryos," he said.