Although the research announced by Advanced Cell Technology of Worcester, Massachusetts, has yet to be published or confirmed, the company said the method eventually could be used to grow replacement body tissues.
Several experts in the field expressed skepticism about the company's claims.
Dr. Michael West, Advanced Cell Technology's chief executive, said the research could eventually solve one of the most difficult problems in human medicine -- transplantation of organs.
West said he took the unusual step of announcing the research before it was published to gauge its acceptability so that the privately held company can decide whether to commit money to developing the process.
"Our attempt was to ask for an open and ethical debate about the status of this research. These are issues people have never resolved," he said Thursday.
The company acknowledged there were difficult ethical questions involved in creating a cell that is part human and part cow.
The hybrid cell is made by removing the nucleus of a cow egg and inserting a human cell's nucleus. Only a small amount of the cow's genetic material -- that found in the mitochondria -- would remain.
West said that genetic material influences only the proteins in the mitochondria, which produce energy in cells, but has no effect on the physical traits of the tissue that eventually comes from the cell.
The research, which was conducted three years ago using cells scraped from the cheek of one of the scientists involved in the research, showed that the cow proteins were displaced by human proteins, the company said.
The hybrid cell theoretically could be transferred to a uterus and grown into a clone of the human donor, but the company said that would be unsafe and unethical. Instead, the cell could be guided into becoming human tissue.
West said his company had not yet managed to direct the cell growth into a particular kind of tissue such as a muscle, cartilage or nerve.
Despite the announcement, several experts in the field said they needed much more proof before they believed embryonic cells had been created. Some also doubted that the technique would work.
"It's hard to say this is a total sham, but I smell a sham here," Dr. Roger Pedersen of the University of California, San Francisco told The New York Times. He doubted that the cow-derived mitochondria would work well enough in human cells.
Last week, two published studies described how human stem cells, the foundation of cells that form all body parts, can be grown in a laboratory.
Dr. John Gearhart of Johns Hopkins University, who was involved in one of those studies, said of Advanced Cell Technology's claim: "It's not that I don't believe this iologically. I just think they could have given a little bit more assurance as to what was done here."
In January, two Advanced Cell Technology researchers announced they had succeeded in cloning a cow by making an adult cow cell revert to its embryonic state.
Written By Alison Fitzgerald