A rare pre-existing progenitor-like Primed SMC compartment is the dominant inferred source of SMC-derived cellularity in vascular injury and atherosclerosis

The cellular origin of smooth muscle cell (SMC)-derived populations in vascular lesions remains unresolved. Here we show, using single-cell transcriptomic analyses spanning carotid ligation injury, Myh11-CreERT{superscript 2}-traced aortic homeostasis, and LDLR- and ApoE-deficient atherosclerosis, that a rare progenitor-like "Primed" SMC compartment pre-exists at baseline in all models and in the healthy human aorta. Relative to contractile SMCs, Primed SMCs attenuate sarcomeric and contractile programmes while inducing matricellular, progenitor-niche and chondrogenic-poised developmental programmes, resolving into conserved niche/progenitor (Cd34, Fst, Tnfrsf11b) and matricellular (Vcam1, Thbs1, Timp1) cores overlaid by vessel-specific signatures, on a retained SMC identity. Multiple orthogonal computational lineage-inference approaches indicate that this compartment expands predominantly through autonomous self-renewal and is the dominant inferred source of cycling and lesion fibrochondrocyte populations, while contractile SMCs are consistently depleted as a feeder source. These findings reframe lesional SMC cellularity as expansion of a pre-existing Primed compartment rather than widespread phenotypic switching of contractile SMCs.