The human-cow hybrid did not survive beyond a few days but it developed to the stage of a 32-cell embryo in an experiment that has far- reaching ethical implications.
An American biotechnology company, Advanced Cell Technology (ACT), based in Worcester, Massachusetts, yesterday stunned the scientific community by announcing in a press statement that it had created the hybrid embryo three years ago from the cells of one of its own scientists.
The company's aim was to generate human embryonic "stem cells", which are the vital progenitor cells of all the body's many different tissues.
"This advance, based on fusing a human somatic [non- reproductive] cell with a bovine egg cell from which the nucleus has been removed, may enable the production of an unlimited supply of such stem cells for transplant medicine," the company said.
Although it is thought unlikely that a human-cow hybrid embryo would ever be able tobe implanted in a womb and develop normally, the research will raise fears that the company may be pioneering a form of human cloning.
However Michael West, ACT's president and chief executive officer, denied that the research would lead to the full cloning of an adult. "We will not use this technology to clone human beings," he said yesterday.
The research, which has not been published in a scientific journal, was performed by Jose Cibelli, an Argentine-born scientist at the University of Massachusetts, which has a commercial link with ACT.
Dr Cibelli took 52 of his own cells - either white blood cells or skin cells from the inside of his cheek - and fused each with a cow egg. Most failed to thrive, according to a New York Times report, but one embryo grew and divided five times.
Dr Cibelli and his university colleague James Robl, who is well known in the area of animal cloning, have filed patents on the process with ACT controlling the commercial rights.
Asked if he was concerned about destroying 52 potential twins of himself, Dr Cibelli told The New York Times: "I never thought about it. But if you use your own cells to treat a disease you may have, you are not taking cells from another person selfishly."