Dynamic Wt1 expression in the gastrulation-stage mouse embryo specifies vascular and visceral smooth muscle cell fate independently from mesothelial fate

The Wilms’ Tumour protein (Wt1) had previously been shown to specify both the mesothelial lineage and vascular smooth muscle cell (vSMC) lineage in the mouse intestine, with evidence suggesting that intestinal vSMCs arise from the mesothelium. Here, we dissect the temporal relationship between these two mesodermal lineages, reporting that unexpectedly, Wt1 expression already in gastrulation stage embryos specifies a population of SMC precursor cells in the lateral plate mesoderm (LPM). Tamoxifen-induced genetic lineage tracing of Wt1-expressing cells revealed that vSMC and visceral smooth muscle cells (visSMCs) of the foetal mid-gut are labelled when Tamoxifen was administered at E7.5 or E8.5. By contrast, intestinal mesothelial cells were labelled after Tamoxifen dosing from E9.5 and later. Analysis of scRNAseq data of gastrulation stage mouse embryos and confocal microscopy demonstrated for the first time Wt1 expression in epiblast and primitive streak, emerging mesoderm and, from E7.5 onwards, in the LPM. In E8.5 mouse embryos, the co-expression of key smooth muscle fate markers Tagln , Acta2 , MrtfA/B and Pdgfrb in Wt1 -expressing cells revealed that vSMC and visSMC fate is specified independently from visceral mesothelium formation. Together, our study provides insight into the developmental lineage of smooth muscle specified by Wt1 expression at gastrulation stages.