Researchers 'clone' human embryos.

For the first time, scientists have "cloned" human embryos, a step that has raised a host of ethical an scientific issues regarding the brave new world of reproductive research.

A team led by Robert J. Stillman and Jerry L. Hall of the George Washington University Medical Center in Washington, D.C., reported the research Oct. 13 at the annual meeting of the American Fertility Society. The duo appeared at a press conference this week to answer questions about their work.

Although it is not unusual for researchers to clone animal embryos, this marks the first known attempt to split a human embryo into individual cells, a technique more accurately described as "twinning." The practica application of the work is to boost the efficiency of in vitro fertilization, the procedure in which an egg and a sperm are united in the laboratory and the resulting embryo is placed in a woman's uterus.

Twinning could help women who produce very few eggs and thus have trouble getting pregnant, even with the aid of in vitro fertiization, Stillman says. By splitting an early embryo into its constituent cells, doctors could transfer more than one embryo, thus increasing the odds of a successful pregnancy. "Our research is one small step in that direction," Stillman says, adding that much more research remains before that vision becomes a reality.

The George Washington team began their experiment with 17 very young, flawed human embryos. These abnormal embryos result from the union between a single human egg and more than one human sperm. Such embryos contain too much genetic material and therefore are not viable.

First, the researchers used a chemical solution to strip the young embryos of their tough outer coating, called the zona pellucida. The shell-like zona pelucida protects the embryo, which at this stage has started to divide and consists of from two to eight cells. Next, the researchers carefully separate the individual cells and coat each with an artificial shell. The team created 48 embryos using this technique.

Cells split from a two-cell embryo appeared best able to divide, with some reaching the 32-cell stage of development, Hall said at the press conference. That finding suggests that researchers could create viable embryos with this process, although the abnormal embryos used in this experiment would not grow even if implanted. They were discarded after six days, Hall says.

If scientists go forward with this technique, it could be sus to split a normal human embryo, one in which a single sperm has fertilized the egg. Thus, a researcher could fertilize the egg, let it divide, and then separate the cells, thus creating two, three, or more embryos, all carrying identical gentic material.

An infertile couple who had two such embryos implanted could end up with identicaal twins, Stillman says.

This resarch proves that splitting human embryos can be done, comments Robert Visscher, executive director of the American Fertility Society, which is based in Birminham, Ala. "The question is, Should this research be done at all?"

Indeed, the research has sparked an ethical debate, with critics voicing many concerns. For example, couples could opt to implant one embryo and freeze the rest, notes Cynthia B. Cohen, executive diector of the Washington-based National Advisory Board on Ethics in Reproduction. If the child created from the implanted embryo develops a failing organ later in life, one of the genetically identical embryos could be used as a source of "spare parts," she warns.

Scientists wonder whether the technique would really improve a woman's chance of becoming pregant. Some women may produce eggs that appear normal but are somehoew unhealthy, says Lucinda L. Veeck of the Jones Institute for Reproductive Medicine in Norfolk, Va. Splitting may merely lead to a host of unsuitable embryos, she says.

For now, such questions remain unanswered. The George Washington team has no plans to forge ahead until ethical guidelines are in place.