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Renard with Marguerite: claims his research answers many of the criticisms of the Dolly scientists. Credit: AFP/PASCAL GUYOT

Controversy about the authenticity of Dolly, the lamb cloned from an adult udder cell, took a new turn last week with the release by a French group of preliminary data appearing to confirm that differentiated somatic mammalian cells can be reprogrammed to make them totipotent.

In experiments carried out by Jean-Paul Renard's team at the national agricultural research agency (INRA)'s centre at Jouy-en-Josas near Paris, 35 cows were implanted with 61 blastocysts created by somatic cell transfer into enucleated oocytes.

One calf, Marguerite, cloned from a muscle cell of a 60-day-old fetus, has already been born. Four other pregnancies have advanced beyond mid-term, including one obtained from a fetal skin cell containing a transgenic marker, and one from a skin cell from a two-week-old calf.

Renard says the group took the unusual step of releasing results before publication to contribute “supplementary arguments” to the debate triggered by a letter in Science by Norton Zinder, a bacterial geneticist at Rockefeller University in New York, and Vittorio Sgaramella, a scientist at the University of Calabria in Italy (see Nature 391, 825; 1998).

The letter described Dolly as an “anecdote not a result”, given that it was the only successful birth out of several hundred attempts. In particular, the authors criticized what they claimed was the poor characterization of the donor cells used, and the possibility that Dolly could have originated from a stem cell or perhaps even from a circulating fetal cell, as the donor ewe was pregnant when the cells were obtained.

The French experiments, designed to prove that cloning from adult cells is possible, answer many of these criticisms, says Renard. The possibility that the adult clone might be derived from a fetal cell is excluded, for example, because the cells used came from a two-week-old calf that was not pregnant.

Similarly, the donor cells were carefully characterized using marker antibodies against vimentin, which is expressed in fibroblasts, against cytokeratin 3/18, which is expressed only in epithelial cells, and against type A/C lamins, which are expressed only in the nuclei of differentiated cells, not in embryonic cells.

“We can exclude the possibility of any contamination from fetal cells; and the nuclei definitely come from somatic cells, not germinal or fetal ones,” says Renard. He adds that the definitive evidence they have cloned from an adult cell must await the birth of the calf, and a genetic fingerprinting comparison with the source animals.

The fact that clones have been obtained from a variety of cell types also suggests that the reprogramming of differentiated somatic cells to become totipotent is a widespread phenomenon, says Renard. This is consistent with the claims of supporters of Dolly's authenticity, who argue that Dolly is part of a progression from the production of clones from embryos and differentiated fetal cells to the routine generation of embryos from adult somatic cells.

Commenting on the French results, Zinder said last week that the researchers should not count their chickens until they hatch, as the pregnancies, although advanced, may yet fail. But he also says he has never questioned the possibility that cloning from adult cells may be a reality, and that his concerns focused only on the adequacy of the evidence presented in the Dolly paper.

Renard says he felt uncomfortable about releasing the data before having a paper accepted by a peer-reviewed journal. But he says he eventually felt it necessary to contribute to the controversy.