Yet the time to consider the ethics of such research has arrived. Animal-human hybrids already exist in most laboratories, said Jonathan Moreno, director of the Center for Biomedical Ethics at the University of Virginia and a co-author of National Academy of Sciences (NAS) ethical guidelines for scientists working with stem cells.

Scientists often implant cells into research animals to study how genes behave and to help find cures for diseases such as cancer and diabetes, he said, but the number of cells is so small they don't humanize the animal. Moreno said many people can accept the transplantation of organ cells, but grow cautious about brain cell transfer.

"What if it wasn't heart or pancreas cells, but human brain cells to study neurological diseases? Would that create a talking mouse? It's an interesting thought experiment," Moreno said at the Eccles Institute of Human Genetics.

Moreno said some scientists believe they can grow a brain from human stem cells in animals, but ethical concerns need to be taken into consideration in all aspects of stem-cell research.

The NAS guidelines he helped write call on scientists not to pay stem cell donors. They also recommend obtaining consent from all donors and having a review board provide oversight.

Following such guidelines would help prevent ethical breeches.

Creating stem cell lines is a difficult process. Stem cells are taken from the blastocyst, or the stage when an embryo consists of only eight cells. Those cells have the ability to turn into about 130 different types of human tissues. Scientists believe they can take those basic cells and use them to research cures for genetic diseases and conditions, and regenerate broken down parts of the body. Only about 22 stem cell lines exist in the United States, with more than 150 worldwide, Moreno said. The federal government will not fund any new ones, but there is no law in the United States prohibiting scientists from finding private funding and conducting research on new lines.

One of the biggest ethical hurdles is the fact that the embryo, even at the blastocyst level, is destroyed during the research.

"We often think we can technologize our way around ethical quandaries, but that has not been successful," Moreno said.

While scientists may be able to create human tissues, he warns against cloning an entire human being.

He said that even if scientists tried to create clones, the clone probably would not be healthy or survive long because the genetic communication between an egg and a sperm would not occur. He also warns that many women trying to birth clones probably would have difficult pregnancies and have a much higher mortality rate.

"If people are concerned with the science fiction concerns of having an army of Hitlers or Saddam Husseins or realistic safety concerns, there is no good reason to clone a human," he said.

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Contact Sheena McFarland at smcfarland@sltrib.com or 801-257-8619. Send comments to religioneditor@sltrib.com.