We identified endothelial ICs (Figure 2E) with strong contributions from genes with arterial (IC 5 and 20) and veinous (IC 3 and 12) expression, suggesting heterogeneity related to vessel type (Data S4G) (Vanlandewijck et al., 2018). These genes (e.g. Tm4sf1 and Slc38a5) showed continuous, reciprocal expression (Data S4G), consistent with the smooth molecular transitions described for endothelial cells associated with arteries or veins (Vanlandewijck et al., 2018). Other ICs identified genes with more bimodal expression, indicating specializations within this artery/vein gradient. Subcluster 6 exhibited exclusive expression of Cytl1 and enrichment for arterial marker Bmx, along with other genes implicated in growth-factor dependent vascular remodeling (Mgp, Fbln5, Eln, Igfbp4, and Clu) (Figure 2E and Data S4G)(Boström et al., 2004; Contois et al., 2012; Fu et al., 2013; Guadall et al., 2011; Karnik et al.; Vanlandewijck et al., 2018). Other subpopulations could represent specializations shared across vessel types. For example, IC 10 loads onto cells with both artery and vein markers and strongly contributing genes suggest a signal related to host immunity, including genes encoding interferons (Ifit3, Ifit1, Ifit2, and Ifitm3), GTPases
