Chimera: sheep have 15 per cent human cells and 85 per cent animal cells

The sheep have 15 per cent human cells and 85 per cent animal cells - and their evolution brings the prospect of animal organs being transplanted into humans one step closer.

Professor Esmail Zanjani, of the University of Nevada, has spent seven years and £5million perfecting the technique, which involves injecting adult human cells into a sheep's foetus.

He has already created a sheep liver which has a large proportion of human cells and eventually hopes to precisely match a sheep to a transplant patient, using their own stem cells to create their own flock of sheep.

The process would involve extracting stem cells from the donor's bone marrow and injecting them into the peritoneum of a sheep's foetus. When the lamb is born, two months later, it would have a liver, heart, lungs and brain that are partly human and available for transplant.