November 4, 2008

experts comment on cloning of mice from frozen cells as published in the Proceedings of the National Academy of Sciences

Scientists in Japan have successfully cloned mice that had been frozen for 16 years, raising the possibility that extinct species could be brought back to life.

Prof John Armitage, Director of Tissue Banking at Bristol Eye Hospital, said:

“Mitochondrial and some nuclear DNA fragments have previously been isolated from mammoths frozen in permafrost, but the key question is whether sufficiently intact nuclei could be extracted from mammoth cells, which will have been frozen for at least 10,000 years at relatively high subzero temperatures. Such cloning would of course exclude mitochondrial DNA, which would come from the host (i.e., non-mammoth) oocyte. -20°C, the temperature at which the mice used in these experiments were stored, is insufficient to stop physical and chemical reactions of biological significance – even food in a domestic freezer has a limited storage time based on changing texture and taste. To achieve long-term storage of viable cells, including embryos, requires far lower temperatures of at least -140°C in the presence of cryoprotectants. Hence it is not surprising that the authors could find no viable cells. The structure of the ice crystal lattice would also change over time owing to recrystallization (grain growth) whereby larger ice crystals grow at the expense of small crystals. While it sounds impressive to have cloned mice from nuclei isolated from animals frozen for 16 years at -20°C, to extrapolate these findings to animals held for thousands of years, perhaps under varying temperature conditions, is still a big leap – extracting fragments of DNA, which has been done from mammoth bone is still some way from obtaining intact nuclei.”

Prof Malcolm Alison, Professor of Stem Cell Biology at Barts, said:

“Absolutely fascinating. The researchers obtained cell nuclei from mice that had been deep frozen for 16 years and then generated new mice by the same technology that created Dolly. While 16 years is not a long time for cells to be frozen (IVF clinics often have viable sperm frozen for longer periods), there are no scientific reasons why extinct animals like mammoths could not be similarly generated. A cell’s genetic apparatus, DNA, is remarkably stable at low temperatures, and provided the animal became frozen fairly quickly after its demise, then there is possibly a good chance of creating new replicas of formerly extinct animals.”