'I was very much afraid. Before this, we had been doing this work only on pigs.'
Professor Paolo Macchiarini at Universitat de Barcelona, Spain - the first surgeon to perform a tissue-engineered whole-organ transplant.
New surgical techniques are always developed and practiced on animals before they are used on humans. Pigs are often used for surgery because their organs are a similar size to those of humans.
"I was very much afraid. Before this, we had been doing this work only on pigs.
"But as soon as the donor trachea came out of the bioreactor it was a very positive surprise."
BBC News, quotation given during a press conference, November 2008.