p63 regulates the switch from multipotency to unipotency during embryonic MG development

The mammary gland (MG) is composed of basal cells (BCs) and luminal cells (LCs). While it is generally believed that MG arises from embryonic multipotent progenitors (EMPs), it remains unclear when lineage restriction occurs and what are the mechanisms responsible for the switch from multipotency to unipotency during MG morphogenesis. Here, we performed multicolor lineage tracing and assessed the fate of single embryonic progenitors during mouse MG development. We demonstrated the existence of a developmental switch from multipotency to unipotency during embryonic MG development. Molecular profiling and single cell RNA-seq revealed the hybrid gene expression of EMPs, and the gene trajectory and lineage segregation occurring during MG development. In situ characterization showed that one of the earliest signs of lineage segregation consists in the restricted expression of p63 in the future BCs. Sustained p63 expression during MG development promotes unipotent BC fate in EMPs. Altogether, this study identifies the timing and the mechanisms mediating the switch from multipotency to unipotency during MG development. To understand the molecular mechanisms regulating multipotency during embryonic development, we isolated EMPs by FACS at E14, performed their transcriptional profiling by microarray and compared their transcriptome to adult BCs and LCs Embryonic CD49f Hi Lgr5Hi mammary cells from E14 Lgr5-IRES-GFP female embryos were FACS isolated and RNA was extracted before being hybridized to Affymetrix microarrays. The three biological replicates samples were a pool of all female embryos from one litter.