Citing the “tremendous potential” of research on human stem cells in nonhuman embryos, scientists and a bioethicist from the Stanford University School of Medicine have co-authored a letter urging the removal of funding restrictions recently imposed on such research by the National Institutes of Health.

The researchers believe that work on what are called chimeric embryos is vital to advance our understanding of early human development, further our ability to accurately model devastating diseases and facilitate drug testing to ensure that potential therapies are safe and effective.

The abrupt suspension of federal funding for this research "impedes scientific progress in regenerative medicine and should be lifted," said a letter from the Stanford scientists published Thursday in the journal Science.

“By eliminating federal funding for all aspects of this research, the NIH casts a shadow of negativity toward all experiments involving chimera studies regardless of whether human cells are involved,” said assistant professor of medicine Sean Wu, PhD, MD, one of the senior authors of the letter. Weissman is the other co-senior author. “The current NIH restriction serves as a significant impediment to major scientific progress in the fields of stem cell and developmental biology and regenerative medicine and should be lifted as soon as possible.”

However, such studies have prompted ethical concerns, and on Sept. 23 the NIH abruptly suspended its funding of “research in which human pluripotent cells are introduced into nonhuman vertebrate animal pre-gastrulation stage embryos while the agency considers a possible policy revision in this area.” The agency is hosting the Nov. 6 workshop of researchers and bioethicists from around the country to consider issuing new guidelines about the research.

“Much of the bioethical concern in regard to human/non-human chimerism arises from the possibility of chimeric animals harboring human neurons and germ cells. Can human neural cells co-exist with those from animals and establish ‘humanized’ cerebral anatomy and circuitries? Furthermore, would such chimeras be elevated to a higher metaphysical state and ‘think’ more like us? Current scientific data have not supported such possibilities, despite hundreds of xenotransplant studies introducing human neurons into the mouse brain,” they wrote.

The researchers also cited current restrictions by the National Academy of Medicine and the National Research Council against breeding animals in which human pluripotent stem cells were implanted during development. These rules aim to prevent any possible transmission of human genes via the sperm or egg of the animal. They also pointed to the stricture against using nonhuman primates, which are considered too similar to humans, for such studies as reasonable boundaries for chimeric research.

“Ultimately, we believe that human/nonhuman chimerism studies in pregastrulation embryos hold tremendous potential to improve our understanding of early development, enhance disease modeling, and promote therapeutic discovery,” they wrote.