A tumor-microvessel on-a-chip reveals a mechanism for cancer cell cluster intravasation

Circulating tumor cell (CTC) clusters are often detected in blood samples of patients with high-grade tumor and are associated with tumor metastasis and poor prognosis. However, the underlying mechanisms by which cancer cell clusters are released from primary tumors beyond blood vessel barriers remain unclear. In this study, a three-dimensional (3D) <i>in vitro</i> culture system was developed to visualize tumor intravasation by positioning tumor organoids with distinct genetic backgrounds to surround microvessels. We visualized tumor intravasation in a cluster unit, including collective migration toward microvessels, vessel co-option, and the release of CTC clusters-an invasion mechanism not previously reported. Furthermore, elevated levels of transforming growth factor β (TGF-β) and activin expression in endothelial cells within the coculture microenvironment were pivotal for facilitating tumor cell intravasation, which was associated with endothelial-to-mesenchymal transition (EndoMT) in microvessels. Our 3D <i>in vitro</i> system can be used to develop therapeutic strategies for tumor metastasis by targeting the release of CTC clusters.