nuclear transfer A technique used in cloning animals in which a nucleus from a donor cell is inserted into an egg cell, which is then stimulated to develop as an embryo. The technique has been used successfully with various mammal species, most famously producing Dolly the sheep in 1997 (see clone). Previous to Dolly's birth, nuclear transfer had used cultured embryo cells in a relatively undifferentiated state. A single embryo cell is injected into an unfertilized egg cell, from which the chromosomes have been removed by micropipette. Fused recipient and donor cells are then stimulated by electrical pulses to begin dividing and form an embryo, like a normal fertilized egg cell. The embryo is then implanted into the uterus of a surrogate mother to continue its development. Dolly was the first mammal to be cloned from a fully differentiated adult body cell. She demonstrated that it is possible to ‘reprogram’ such cells so that they can direct the development of a new individual. In Dolly's case, the donor cells were taken from a culture of sheep udder cells and starved into a state of quiescence in a low-nutrient medium. This was done to switch off all but essential genes and better mimic a natural fertilization.

There are several advantages in using adult body cells: cultures are easier to obtain and maintain; also, there is greater scope for genetically engineering such cells and screening them to select successfully modified cells. Nuclear transfer, using embryo cells or body cells, is now used increasingly to replicate elite animals in the livestock industry, to produce genetically engineered mammals for commercial use (e.g. goats that secrete human proteins in their milk), and to replicate endangered species. However, the failure rate is generally high, and even the few live clones produced often have congenital defects that shorten their lives. This shows that ‘reprogramming’ differentiated body cells poses formidable technical obstacles.