Scientists have mapped the family tree of about 1.3 million cells in a single developing mouse embryo, tracing them back to the fertilized egg. A team led by Jay Shendure of the University of Washington used a tool called DNA Typewriter. The study was published Oct. 8 in Science, with Chengxiang Qiu of Dartmouth College as co-senior author.

The tool writes a short genetic mark into a stretch of DNA, called a tape, each time a cell divides. The marks are added in strict order, so each later cell carries a record of its history, Science and GEN reported, and cells that share identical marks share a common ancestor.

The team inserted the system into fertilized mouse eggs and allowed the embryos to develop for nearly 14 days, until their organs had formed. In one embryo, they rebuilt the relationships of about 1.3 million cells, roughly 10% of the embryo, Nature reported. GEN said the team tried 100 eggs and got 10 embryos to study.

The recording captured the very first cell division. One of the two founding cells made more descendants, but both made many cell types in nearly equal shares, GEN reported. Blood and retina cells settled on their jobs early, while cells of the skin's outer layer did so later.

A second team, led by Jonathan Weissman, published a companion study the same day in Cell. It used a related tool called PEtracer to map more than 1.4 million cells from 16 mouse embryos, GEN reported. Science said that work covered embryos 7.5 to 10 days old.

Independent researchers praised the work. "These are the most exciting papers in this field in the last 5 years, easy," Reza Kalhor of Johns Hopkins, who was not involved, told Science.

The goal goes back decades: in the 1970s and 1980s, biologist John Sulston mapped every cell of a tiny clear worm called C. elegans, Nature and Science reported. No one has matched that achievement in mammals, whose embryos develop hidden inside the womb and contain far more cells.

The researchers say detailed maps like this could help scientists study birth defects, cancer and future stem cell treatments. Jan Philipp Junker of Charité University Hospital in Berlin told Science the work "is the beginning, not the end."

What we know: In mice, DNA Typewriter recorded a family tree of about 1.3 million cells from one embryo, published Oct. 8 in Science.
What we don't: The full tree comes from a single mouse embryo, and the reports do not say when the method could run across many embryos or be used to study human development.