Early Stage of Metastasis (image) American Institute of Physics Caption This is an illustration of the early stage of metastasis, called "intravasation." Tumor cells (blue) in contact with the bloodstream experience shear forces that can be strong enough to exceed the adhesion forces keeping them attached to the primary tumor. Once released into the bloodstream, they become circulating tumor cells capable of spreading the disease to other sites. Credit The University of Southern California, Oregon Health and Science University, and the Scripps Research Institute Usage Restrictions With credit Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.