Scientists have created a model human embryo with a heartbeat and traces of blood in an advance that offers an extraordinary window into the first weeks of life.
The synthetic structure, created from human stem cells without the need for eggs, sperm or fertilisation, replicated some of the cells and structures that would typically appear in the third and fourth week of pregnancy. But it was specifically designed to lack the tissues that go on to form the placenta and yolk sac in a natural embryo, meaning that it did not have the theoretical potential of developing into a foetus.
