Ewen Callaway in Nature:
Ritchie: The simple way of describing nuclear transfer is that you take an oocyte, an unfertilized egg, and you remove the chromosomes. You then take a complete cell which contains both male and female chromosomes — all of our cells do, apart from the gonads. You take that cell and fuse it to the enucleated egg, activate it — which starts it growing — and transfer it to a surrogate mother. Hopefully, with your fingers crossed, you will get a cloned offspring, a copy of the animal you've taken that cell from.
Walker: Tedious is absolutely the word. You're sitting, looking down a microscope and you've got both hands on the micromanipulators. It's kind of like the joysticks kids use nowadays on games. If your elbow slipped, you could wipe the whole dish out.
A year earlier, the team had produced twin sheep, named Megan and Morag, by cloning cultured embryonic cells in an effort spearheaded by Roslin developmental biologist Keith Campbell. But on this day in February 1996, problems with the fetal cell lines they had planned to use meant that they would need another nuclear donor.
Walker: My memory is of flapping like a chicken, thinking, 'What are we going to put in?' because the cells we were going to use aren't there. The last thing you want to do is waste those oocytes you've got. We wanted to try something, at least.
Angela Scott, cell-culture technician, PPL: I received word from Karen to say that the cells they were expecting had been contaminated. They asked me if I had any cells that they could use. The cells I had were ovine mammary epithelial cells: we were looking to increase expression of proteins in milk. These were adult cells.
More here.