4. Incorporation of characteristic

4. Incorporation of characteristic cells and structures of the tissue into the tumor. Parenchymal epithelial cells (e.g., lung pneumocytes, liver hepatocytes, neuralglia) and larger intact structures (e.g., lung bronchioles and large vessels in the lungs, portal triads in the liver) are taken up into the expanding infiltrative tumor mass and get locked into the tumor.5. Maintained vessel morphology and marker expression. Co-opted vessels are known to be remodeled by surrounding tumor tissue and therefore may not in all cases appear “normal.” Newly recruited co-opted vessels in particular may maintain morphological and molecular characteristics of tissue endothelium (e.g., GLUT-1 and p-glycoprotein endothelial expression in brain tumors).6. Low rates of endothelial cell proliferation relative to angiogenic tumors. Specific molecular markers of coopted blood vessels are not currently known; thus, several criteria are used to distinguish between vessel subtypes and differential growth patterns of tumors. Vessel co-option was first identified in tumors from patients; however, experimental mouse models have also been shown to recapitulate the growth patterns observed in human tumors.