To retrace the history of an organism, researchers transformed its DNA into a true genealogical record, documenting everything from one generation to the next.
It all begins with a single cell: a mouse's fertilized egg. As it divides, it gives rise to cells that will form the brain, heart, skin, and all the other organs. But once these cells have specialized, it is very difficult to determine which ones descend from the same ancestor.
Illustration of DNA
Pixabay image
A team from the University of Washington designed a way to preserve this history. The researchers introduced a device called DNA Typewriter into the fertilized egg. It consists of specially designed DNA segments capable of receiving successive marks.
As the embryo develops, new marks are added in a specific order. The cells then pass these traces on to their descendants. Two cells carrying the same sequence of marks therefore probably shared the same ancestral cell at some point in their history.
After 13 and a half days of development, the researchers analyzed the cells of an embryo. They were able to read the traces recorded in 1.28 million of them, representing about 10% of its cells at this stage. From this information, they reconstructed a vast family tree reaching back to the fertilized egg.
The first divisions appear clearly in it. In particular, the researchers identified the separation between the embryo's first two cells. They then followed how their descendants were distributed among different tissues and organs.
The experiment also reveals that not all cells choose their future role at the same time. Some lineages destined to form blood or the retina become distinct quite early. Others, such as those that produce the outer layer of the skin, assume their identity later.
It should not be imagined that the researchers filmed each of these divisions for nearly two weeks. They read the marks at the end of the experiment, then reconstructed the cells' history from these clues. The result therefore remains a reconstruction, involving only some of the cells.
The significance extends beyond scientific curiosity. Understanding how a cell gives rise to an organism could help study malformations, certain genetic diseases, or cancers. The team has also made this family tree available on an interactive website so that other researchers can explore it.