At 14 days, a human embryo develops its first clear feature: a ridge of cells, called the primitive streak, which marks the body’s central axis.
In 1998, scientists isolated stem cells from early embryos and eventually figured out how to develop them into just about any tissue in the body, from heart muscle to nerves.
In 2007, scientists figured out how to reprogram adult cells into embryolike stem cells, a discovery that one day may lead to personalized treatments for degenerative diseases.
Scientists could keep human embryos alive for just over a week, without freezing them. Two teams of scientists determined how to grow human embryos for 13 days. While these artificial embryos developed from embryonic stem cells, it may soon become possible to build them from reprogrammed adult human cells.
A Harvard historian of science who was not involved in the new paper, said she did not think ethicists would have to start from scratch to find rules for these strange new Sheefs.