Using bone marrow transplantation (BMT), diphtheria toxin receptor (DTR), and a dual recombinase‐based lineage tracing mouse model, we further revealed that non‐BM CD34+ cells, particularly CD34+/Periostin+ myofibroblasts, contribute to adventitial fibrotic collar formation, thereby protecting AAA from rupture. Here, POSTN is linked to triple-A syndrome.