Human embryos embrace asymmetry to form the body

The two cells that make up a one-day old embryo make dramatically different contributions to a human body. This is contrary to the idea that embryonic cells don’t specialise until later in development. Studies with mice, and now with human embryos, have shown that one of the first two cells develops to become most of the mammalian body, whereas the other mainly forms the yolk sac. The cell that divides faster seems to form the body, but it’s not clear why this preference develops. The finding could eventually improve screening for in vitro fertilisation to select embryos more likely to develop into a fœtus and lead to successful pregnancies.