All five of the murine and human tumor lines tested for experimental brain metastasis in Fig. 1 were plated on live slices. Each of these cell lines was found to preferentially elongate upon the vasculature within 2 hours. Tumor cells not directly in contact with vessels did not spread. These results were directly 
verified with timelapse confocal microscopy. Thus, we concluded that Tubeimoside-I metastatic carcinoma cells, when given equal access to live vascular and neural substrates, interact preferentially with the vessels over the neural parenchymal elements. Based on morphological features of cells and microcolonies in histological sections and the rapid cell spreading on live slices observed above, we reasoned that there was likely an active adhesive interaction between the metastatic tumor cells and the exterior of blood vessels. To test the potential for tumor cell adhesion to elements of the brain parenchyma, either vascular or neural, we assayed adhesion of metastatic tumor cells plated on thawed slide-mounted snap frozen brain sections. 4T1-GFP cells were plated on normal murine brain and cultured for 2 h followed by washing off non-adherent cells. Only a small minority of plated cells adhered to the slices, however, 93% of these adherent cells were in contact with vessels. To test the adhesion of human tumor cells, we used the MDA-MB-231 breast carcinoma line in a parallel assay with human brain sections. Similarly 85% of the adherent human cells were associated with vessels. These results demonstrated that metastatic tumor cells adhere to the brain parenchyma and that their preferred substrate is vascular rather than neural. These studies complement the observations in live slices that showed tumor cell spreading to be associated with vascular, not parenchymal engagement. We have demonstrated that the primary soil for metastatic tumor cell attachment and growth in the brain is vascular rather than neural. This vascular cooption, or the utilization of preexisting vessels, has only been previously anecdotally reported as a form of vascularization in experimental brain metastasis. Here, we have quantitatively demonstrated it to be the predominant form of vessel use by tumor cells during early experimental brain metastasis establishment and in human clinical specimens reflecting early stages of the disease. We further show the interaction relies on b1 integrin-mediated tumor cell adhesion to the vascular basement Folinic acid calcium salt pentahydrate membrane of blood vessels. These findings exclude a requirement for de novo angiogenesis prior to microcolony formation. They also contrast with the classical seed and soil hypothesis for brain metastasis suggesting a neural substrate and reliance upon neural-derived trophic factors for growth. Importantly, they do not exclude vascular remodeling or contributions from the neural elements for later growth. This work thus describes in detail a major mechanism of brain metastasis formation in addition to identifying the mechanism of vessel cooption in the brain for the first time. The CNS parenchyma is largely devoid of non-vascular stromal basement membrane components which are necessary for epithelial and carcinoma cell adhesion and survival. Vascular cooption, therefore, supplies substrates for malignant growth of non-neural carcinoma cells not otherwise widely available in the neuropil. Proliferation by metastatic tumor cells is highly potentiated upon adhesion to a basement membrane substratum and is attenuated by inhibiting MEK in vitro. We found the vast majority of micrometastases to be in direct contact with the VBM of existing.