The cells used by the Dolly team at the Roslin Institute near Edinburgh had also been cultured and kept frozen for some three years. These problems are unlikely to affect Marguerite, says Renard, because he knows more about the donor cell. This came not from an adult, but from a fetus 60 days into a cow's nine-month gestation. It was a muscle cell, and as such had already differentiated. The donor was a red Limousin cow, while the egg cell came from a white Charollais.
Marguerite is Limousin.
Renard took muscle cells from the fetus, grew them in a dish, froze them and then cultured them again before fusing one with an egg cell that was stripped of its nucleus. He is confident that the cells did not become undifferentiated during culture, and that the process did not select for stray undifferentiated cells. There may be a chance that the donor nucleus happened to come from an undifferentiated cell in the culture, but it is very small,' says Renard. It's verifying that cells that are really differentiated can be compelled to form an embryo,' says James Robl of the University of Massachusetts at Amherst. He says that his group has cloned six calves from fibroblast cells taken from 40 to 60-day fetuses.