Comprehensive lineage tracing maps the landscape of cell fate decisions in mouse embryogenesis.

A comprehensive cell fate map of mammalian embryogenesis has remained out of reach given the scale, cellular diversity, and non-deterministic nature of development in utero. Here, we use PEtracer to continuously install heritable genetic marks as development progresses, reconstructing lineage trees that resolve ∼75% of cell divisions across >1.4 million cells from 16 replicate embryos. We pair these trees with deep transcriptional profiling to resolve cell fate biases, restriction timing, progenitor pool sizes, and lineage relationships throughout embryogenesis. Using this quantitative reference, we uncover strikingly reproducible lineage architecture between replicate embryos and chart the lineage dynamics driving fate specification across diverse tissues. Specific biological insights include the progressive restriction of neural crest fate, the relative contributions of distinct mesodermal origins to endothelium, and the dynamics of axial elongation. This work provides a foundation for a quantitative and predictive understanding of mammalian development.