The embryonic hearts were dissociated using the Neonatal Heart Dissociation Kit (Miltenyi Biotec, 130-098-373) according to manufacturer's instructions in combination with the gentleMACSTM Octo Dissociator with Heaters (Miltenyi Biotec), including the red blood cell lysis steps. The resulting cell pellets were resuspended in 100µl of 3% FBS in PBS containing 1:100 anti-mouse-CD31 conjugated to FITC (BioLegend, 102406) and incubated on ice for 25 minutes. 100µl of 1µg/ml DAPI solution was then added to each sample and incubated on ice for a further 5 minutes before cell suspensions were centrifuged at 600x g for 5 minutes at 10°C. Cell pellets were then resuspended in 500µl of 2% FBS in PBS, transferred to FACS tubes and sorted using a Sony MA900 FACS Sorter. CD31-positive endothelial cells from each sample were sorted into 100µl 3% FBS in PBS, centrifuged at 600x g for 5 minutes at 10°C and the cell pellets resuspended in 30µl 0.04% BSA in PBS. Samples were processed for sequencing using the Chromium Next GEM Single Cell 3' GEM, Library & Gel Bead Kit v3.1 from 10X Genomics according to manufacturer's instructions. Each sample was processed in separate GEM well, and then samples were pooled to a single flow cell and run on a NextSeq2000 sequencer.