A cellular hierarchy of Notch and Kras signaling controls cell fate specification in the developing mouse salivary gland

The development of the mouse salivary gland involves a tip-driven process of branching morphogenesis that takes place in concert with differentiation into acinar, myoepithelial and ductal (basal and luminal) sub-lineages. By combining clonal lineage tracing with 3D reconstruction of the branched epithelial network and single-cell RNA-seq analysis, we show that in tips a heterogeneous population of renewing progenitors transition from a Krt14+ multipotent state to unipotent states via two transcriptionally distinct bipotent states, one restricted to the Krt14+ basal and myoepithelial lineage, and the other to the Krt8+ acinar and luminal lineage. Using genetic perturbations, we show how differential expression of Notch signalling correlates with spatial segregation, exit from multipotency and promotion of the Krt8+ lineage, while Kras activation promotes proacinar fate. These findings provide a mechanistic basis for how positional cues within growing tips regulate the process of lineage segregation and ductal patterning.