To investigate the details of CTC a

To investigate the details of CTC arrest, we next imaged the individual CTCs (251 events in 12 fish) every 6 hr in the blood circulation. We found that individual CTCs were elongated when they entered the capillaries (Supporting Information Fig. S4). Interestingly, some CTCs could perform a secondary morphology‐switch from a loaf to a sphere (Fig. 2a) and were then stably trapped in capillaries (81%, Supporting Information movie1). By measuring the diameter of arrested CTCs, we found that the diameters of CTCs were significantly increased after the ICMS, which made the size of CTCs far larger than the internal diameter of capillaries (∼20 vs. ∼10 μm) (Fig. 2b). Bitplane Imaris Surface analysis of the CTCs and blood vessels clearly showed the shape changes of single CTC during intravascular arrest and extravasation (Supporting Information Fig. S5). These results suggest that cell rounding might be related to the initial physical trapping (arrest) of individual CTCs. To further test our speculation, we then tracked a group of intravascular CTCs at different states (rounding or elongated) in the same zebrafish from 6 hr after cell seeding. Interestingly, we found that rounding CTCs were more likely to arrest and extravasate, compared to the elongated CTCs (Fig. 2d).