Scientists Trace a Mouse Embryo's Cells Back to the Egg They Came From

Researchers at the University of Washington have reconstructed the family tree of 1.28 million cells from a single mouse embryo, tracing each of them back to the fertilized egg the animal started as.

The authors say the tree lets them date the moments when the body's cell types first part company. In their own assessment, a lineage-anchored account of how a mammal develops, long out of reach, is coming into view rather than finished.

The study appeared on Oct. 8, 2026, in Science. The authors recorded the lineage with DNA Typewriter, which they call a sequential molecular recorder: it writes edits into a cell's own DNA in the order they occur, so the sequence of divisions can be read back later. The recording ran across nearly two weeks of development, and from that one embryo the team built a dated tree of those cells, each with its gene activity measured as well.

A burst of editing marked the daughter cells of the embryo's first division, giving the authors a built-in replicate pair inside one animal. They report that siblings on the tree shared a cell type far more often than chance allows, that sibling pairs ending in different cell types marked where an identity split had finished, and that during the stage when the embryo lays down its body plan a few early lineages came to supply a disproportionate share of cells.