Aggregation and formation of lethal emboli with MSCs. Viable MSCs from passage 1/donor 1 were plated at 100 cells/cm² incubated for 5, 7, or 9 days. (A) Passage 2 MSCs from high and low of PODXL/CD49f were lifted with trypsin/EDTA, washed by centrifugation in HBBS, suspended in HBBS at a concentration of either 0.5 × 10⁴ cells/μL or 10⁴ cells/μL, and incubated at RT. Turbidity was assayed by absorbance at 590 nm. (B) Death rate of mice (n = 10 or 13) after infusion into a tail vein of passage 2 MSCs (donor 1) from high, medium, and low of PODXL/CD49f. (C) Photographs of lungs from euthanized control mouse and mouse that died within a few minutes of intravenous infusion of 2 × 10⁶ of the MSCs that were PODXL^lo/CD49f^lo in panel B. (D) Photomicrographs of sections from mice that died after infusion of PODXL^lo/CD49f^lo in panel B. Sections stained with hematoxylin and eosin. Bar = 100 μm. Insets: Regions containing large blood vessels.