Claire Ainsworth in New Scientist:
In a mammalian embryo, all cells are equal – or so biologists believed. But a series of studies suggest that the fate of individual cells might be determined much sooner after conception than previously thought.
In some non-mammals, such as fruit flies, there are different concentrations of certain molecules in different parts of the egg. When the egg cell divides, the “daughter” cells use this as a kind of grid reference to work out where in the egg cell they have come from and what they should become. This pattern is inflexible: split an insect egg by pinching it in the middle and you don’t get twins; you get a front end and a back end of the insect.
Mammalian embryos appear to be much more flexible. If you take a mouse embryo at the two-cell stage and destroy one of the cells, you still get a complete mouse. This originally led to the idea that cells in early mammal embryos are totipotent – able to form any cell type.
But recent studies have raised doubts. And now…