Animal cloning has existed since 1952, when scientists first cloned a tadpole by removing the nucleus from a somatic cell — any cell in the body other than a sperm or egg — and transferring it into a frog egg to replace the original nucleus, a process known as somatic cell nuclear transfer.
But it took decades for scientists to work their way up to cloning mammals, finding their first success with Dolly the sheep in 1996. In the years since, scientists have cloned mice, pigs, cats, rabbits, goats and cattle, but the oddball reproduction of dogs presents unique challenges for cloning, according to the new study. Female dogs only come into heat once a year, and the ovulated eggs aren't ready to be fertilized immediately. Rather, the eggs mature in the oviduct as they move toward the uterus, which makes it harder for scientists to harvest them at just the right time, the study authors reported.