AbstractMaternal cells can become engrafted in various fetal organs during pregnancy. The nature of the cells and the mechanisms of maternofetal cell trafficking are not clear. We demonstrate that human lineage-negative, CD34-negative (Lin−CD34−) multipotent mesenchymal stromal cells express α2, α4, α5, and β1 integrins, which mediate their adhesion to endothelium, and vascular endothelial growth factor receptor-1 (VEGFR-1), which mediates their response to vascular endothelial growth factor A (VEGF-A). A maternal-fetal VEGF-A concentration gradient exists across the placental barrier, and cord blood plasma induces transendothelial and trans-Matrigel migration of stem cells in vitro. Migration is inhibited by a VEGF-A-neutralizing antibody or antibodies against VEGFR-1 or integrin α2, α4, α5, or β1. When Lin−CD34− multipotent mesenchymal stromal cells are transferred to rat maternal venous blood, they traffic through the placenta, engraft in various fetal organs, and persist in offspring for at least 12 weeks. Cell proliferation ability is retained in the xenogeneic placenta. Maternofetal trafficking is significantly reduced by blocking antibodies against integrins α2, α4, α5, and β1 or VEGFR-1. These results suggest that maternal microchimerism arises by the trafficking of multipotent mesenchymal stromal cells via VEGF-A- and integrin-dependent pathways across the hemochorial placenta to fetal tissues.Disclosure of potential conflicts of interest is found at the end of this article.